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726

Surfactant gelsHeinz Hoffmann and Werner Ulbricht*

AddressesLehrstuhl fUr Physikalische Chemie I der Universitat Bayreuth,Universitatsstrape 30, D-95447 Bayreuth, Germany"e-rnail; [email protected] Opinion in Colloid & Interface Science 1996, 1:726-739 Current Chemistry Ltd ISSN 1359-0294Abbreviationscat-HEC HEC modified with cationic groupscat-HMHEC HEC modified with cationic andhydrophobicalkyl groups

Gel-like systems containing surfactants either in polar orin nonpolar solvents consist of cubic phases of globularmicelles, of lamellar phases with vesicles, of networks ofrod-like micelles or of polymers which arc cross-linked bysurfactant molecules. Recent advances in the understandingof the rheological properties of gel-like systems involvemodels that have been developed to allow a qualitative and,in some cases, a quantitative description of the systems andtheir viscoelastic properties.

IntroductionSurfactants can reversibly aggregate, both in s tronglypolar and in nonpolar solvents, into micelles. Suchsolut ions usually are lowly viscous Newtonian liquids.The aggregates can also organize into superstructuresthat can give the systems gel-like properties with yieldstress values. According to the Encyclopedia Britannica,a gel is an elastic coherent mass consisting of a liquidin which ultramicroscopic particles are either dispersed orarranged in a fine network extending throughout the mass.Such surfactant gels can occur in binary surfactant/solventsystems as well as in mulricornponenr systems. They cancontain cubic phases of globular micell es, lamelIar phaseswith vesicles, networks of rod-like micelIes or polymermolecules which are crosslinked by surfactant molecules.Especially in the last case, gela tion can take place at lowtotal concentrations (below 1wt%), and such systems areused in the pharmaceutical, cosmetic, agricultural and foodindustries.

CSOPCOPPCOSCEHECFHMHECHEC .O/WPEGPOEPOPSANSSAXSW/O

cationic starchdipalmitoylglyceroldipalrnitoylphosphatidylchclinedifferential scanning calorimetryethyl(hydroxyethyl)celluloseHEC modified with hydrophobic perlluoro groupshydroxyelhyl celluloseoil-in-waterpolyethylene glycolpolyoxyethylenepolypxypropylenesmall angle neutron scatteringsmall angle X-ray scatteringwater-in-oil

Due to the great interest in the gelation of surfactantsystems, numerous investigations have been made in orderto elucidate the relation between the' const itut ion of thecomponents and the resulting superstructures, and theirinfluence on the rheological properties of the systems.The aim of the article is to summarize the most recentpapers, to present the obtained results and point out theprogress in understanding the gelation and the rheology ofthe various surfactant gels. The subject wilI be restricted tothermodynamicalIy stable systems with polar and nonpolarsolvents in which gelarion is caused by the surfactantcomponents.

OrganogelsThe preparation of gelatin-containing microernulsionbased organogels was first described in 1986 [I ,2]. Figure1 shows the structure of i:hese gels schematically, In 1988,Scarrazzini and Luisi [3] observed that hydrocarbons withabout 1% of soybean lecithin jelI on addition of traces ofwater. This effect was attributed to a transition of globularreversed micelles inro cylindrical micelles that form anentangled network. The authors showed that enzymes andbacteria can be entrapped within these gels without lossof their activity. Numerous papers on organogels basedon different amphiphiles, their structures and rheologicalproperties have appeared since.Equimolar mixtures of 2,4,6triamino-l,3,S-triazine andbarbituric acid derivatives with long or branched alkylchains cause esters, such as soybean oil or triolein, to jellby the formation of intermolecular hydrogen bonds [4].Hcxadecane forms an anhydrous gel with nonionic sugarsurfacrants with a sorbitan or a polyoxyethylene headgroupand an alkyl chain [5]. The system shows a reversibletransition into a solon heating. The gel contains clustersof tubul i and fibrils dispersed in the hydrocarbon. Onadd ition of water at temperatures around 60'C, water-in-oil(W/O) emuls ions with vesicles are formed which jell oncooling to form fibril structures with continuous aqueouschannels.Robinson and coworkers [6] studied gelatin conrauungW/O microernulsion-based organogels with sulfonate surfactants such as Aerosol OT (AOT). The continuous oilphase was a hydrocarbon, such as decane or cyclohexane.The authors proposed a mechanism for the gelationinvolving droplet percolation coinciding with a helix-coiltransition of gelat in. The structures of the gels aredependent on the composition of the microemulsions,and range from fractal-like aggregates of droplets toa network of gelatin and water rods coexisting withmicroemulsion droplets, all stabilized by a monolayer ofsurfactant molecules.


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Figure 1

Surfactant gels Hoffmann and Ulbricht 727

Proposed model structure ofgelatin-water-in-oil 01110) microemulsiongels with AOT as surfactant. Publishedwith permission from [2].water pool

surfactant

\\

Figure 2

gelatinrandom coils

----apolar medium

helical gelatincrosslinks

-----

Phase behaviour and rheologicalproperties of organogels of the ternarysystem lecithin-water-propylene asa function of the pressure. (a) Phasebehaviour of the ternary systemlecithin-water-propylene, at two constantlecithin concentrations and 30'C as afunction of the water: lecithin ratio, W,and the pressure. (b ) The viscosity oflecithin-water-propylene microemulsionsat a constant lecithin concentration of33 mM and 30'C for two water: lecithinratios as a function of the pressure.1 and 29 denote one and two phaseregions. respectively. Published withpermission from [7].

(a)'" (b ) . IT ~ 3 0 ' C I o ~ IltdthinJ=l3l1lMp- '0 T..30.0C14> " 1 I 0P 0 I I 0 I ,e /00 I h " ' h i " J ' " \ l m ~ ~ ~ 0t I 2l1 , '$0

\;/ s-:;; I

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728 Food colloids, emulsions. gels and foams

Figure 3(a ) (b) (c)G' < .

I. ,10' B 10 ' , ..G". ,G' ,i '. \i 10 '

. .J Gi Gi.. . 1 '.. b C, G' \ '... . C 410' ,C e C' 10 .o o .G" ,o 0 - ,. '.lD' o 0 0 Gil ~ ~ O C l l - -, , ,0 ~ ~ o o o 0.1 15 2S 3S 4S0.01 0.1 1 10 ' 102 100 stress (Pa)1000 TOe)frequency (Hz)

The storage modulus G' and the loss modulus G" for a gel phase of 2,3-bis-n-decyloxyanthracene (concentration=O.98wtOfo) in octanol (a) as afunction of the frequency u (strain,y= l % ) , (b) as a function of the shear stress (J (u=l Hz), and (c) as a function of the temperature T (y=l0f0,u= l Hz); the sample was heated and cooled again in a cycle. Published with permission from [12'].

Figure 4Hypothetical phase diagram of a binarysystem of surfactant and water showingthe positions of cubic phases (HII:reverse hexagonal phase; La: lamellarphase; HI: hexagonal phase; a. b, c, d:cubic phases). Published with permissionfrom [17]. { ; @ ., 0-JJ r r r- r-2'"Jc.EOJ

InverseMicellar H" b La H, d Micellara cSolution Solution

I....a Water content 1'1,' 100

the increase of the density of the oil. The microstructuresin the gel phase consist of long, flexible rods with a water.core and a lecithin shell .Shchipunov er 01. [SO,9] reported the gelation of hydrocarbons with lecithins on addition of nonaqueous,stronglypolar solvents such as glycols, glycerol or formamide.They attribute the gelation to the formation of cylindricalreversed micelles which form a network by hydrogenbonds between the polar solvent and the phosphategroups. Shchipunov determined the rheological behaviourof lecithin-based organogels with water [10].The systemswere found to show electrorheological properties [11].

The hydrocarbon with 1% lecithin jells in the range ofone to two water molecules per lecithin, and becomesbiphasic at higher water contents. The viscosity of theorganogel decreases with increasing electric field strengthdue to the orientation of the anisometric particles, whereasthe two-phase systems show a strong increase in viscosity,more than two orders of magnitude, under the influenceof an electric field above 10kVm-I . Polarizing microscopyshows that under the influence of the electric field atransparent birefringent phase with a columnar order ofsolid particles is formed. If the field is applied fora longertime, the viscosity decreases and a transparent isotropicmetastable gel is formed.


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Octanol

Figure 5

SOS, above a character is tic ' SOS concentration. Themelting in this case is due to a suppression of th e blockcopolymer micel les by the interactions of the surfactantan d the block copolymer molecules [19-].

AOTater

NMR,SANS.DSC-

Other studies concentrate on the structure of th e variouscub ic phases. Lindb lom and coworkers [ZZ-] describea cubic phase, besides a reversed hexagonal and alamellar one.' in th e t ernary sys tem of a phospholipid, I,Z-dioleoylglycerol and heavy water. From X-raydiffraction and Nf\lR self-diffusion measurements, astructure built by reversed mice lles is concluded forthis cub ic phase . Quinn and coworkers [23-] report,in a similar system of dipalrnitoylphosphatidylcholine(OPPC), dipalrnitoylglycerol (OPC) and water, a cubicphase between the lamellar and the reverse hexagonalphase at intermediate temperatures, which belongs to

Richtering and coworkers [ZO-] studied the rheologicalproperties of the isotropic micellar solution, th e cubicphase and the hexagonal phase in aqueous solutions of abranched nonionic surfactant. Whereas th e micellar phaseshows Newtonian behaviour, th e cubic phase behaveslike an e lasti c solid with a high plateau modulus andalso shows shear melting at high shea r stresses. Thehexagonal phase has a complex rheological behaviour andth e rod-like aggregates can orien t both perpendicular toth e flow direction and in-shear-plane parallel to th e flowdirection; in th e latter case th e moduli decrease. Jonesand Mcl.eish [ZI-] proposed a model for th e response ofsurfactant cubic phases to oscillatory shear strain abovea cri tical s tress with th e formation of 'slip planes' underthose conditions.

Phaso diagram of the ternary system AOT-octanol-water at 25'C(in wt%) (L l : isotropic water continuous phase; 0: lamellar phase;12: bicontinuous cubic phase; L2: isotropic oil continuous pMse;F: reverse hexagonal phase; the dashed lines mark the sampleson which measurements have been carried out with the indicatedmethods: NMR , SANS, differential scanning calorimetry losel. staticlight scallering [SLS], dynamic fight scallering [OLSJ). Published withpermiss ion from [31"].

This is true, for example, for a cubic phase of th e(POE)97(POP)69(POE)97-block copolymer which .forrnsfor concentrations above ZOWt% in water [18]. ' Thiscubic phase consists o f spherical micel les of the blockcopolymer and shows reversed melting on cooling belowa characteristic temperature. The melting is a result ofth e disappearance of th e micelles due to the increasedhydrophilicity of th e hydrophobic POP-block with de creasing temperatures. The cubic phase also 'melts' to anisotropic Lj-phase on addition of the anionic surfactant

Cubic gel phasesCubic mesophases can be formed in binary surfactant/water systems or in multicomponcnt surfactant/watersystems containing hydrocarbons, fats or waxes. Seddon ( I01. [17] established a schematic phase diagram (Fig . 4) inwhich th e typical positions of cubic phases are shown. Forphases which border on th e isotropic micellar or reversedmicellar solution, structures with globular panicles in cubicorder arc usual ly found, whereas th e phases betweenth e lamellar and the hexagonal phases normally showbicontinuous structures. All cubic phases are stronglyviscoelastic gels with high yield stress values; theirrheological propert ies can usually be explained on th ebasis of a hard sphere model.

The same author describes rheological measurements onan organogel of decalin with the binuclear copper(II)-tetraocranoate complex, Cuz(OzCsHls)- t [1S-]. This systemshows also a reversible sol-gel transition at a characteristic'temperature, T trans. The system can be described, belowT trans, by th e theory of 'living' polymers of Cates [16],in thi s system physical junctions are liable to behave likeentanglements . Terech and coworkers also state in theirarticles that, in spite of me progress in understandingstructure.and properties of organogels, ic is not yet possibleto predict whether an amphiphile will act as a gelator fora given solvent from only its constitution.

Tcrech ( I 01. [IZ--14-] report th e gelation of organicsoh'ents with a few percent of strongly lipophilic amphiphiles containing steroids or alkyl chains connected toester or ether groups (polar links). These amphiphiles canbring about gelation of both hydrocarbons and aliphaticn-alcohols with intermediate chainlengths. SAXS andSANS measurements showed long and rigid fibres of th egelator interconnected with swollen lyotropic .domains.The fibres consist of hexagonal-like arrangements ofcylindrical aggregates which transform into lamellar-likest ructur es with increa sing alcohol content, ' and havea rectangular cross-sectional shape ( ribbons) in purealcohols. This packing appears to be influenced by th epolarity ' o L ~ h e solvent and the nature of th e polar links,whereas th e' a romatic pa n determines the aggregationleading to the organogels. Rheological measurements showhighly viscoelastic gels with high yield stress values(Fig. 3) which exhibit a reversible 'melting' and re-gelationduring a heating and cooling c y . l [1Z-].


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730 Food colloids. emulsions. gels and foams

Figure 6Two-dimensionally detected SANSintensities for tbe cubic phase in theternary system AOT -octanol-water atvarious compositions and a constantmass ratio of 0 20 :octanol=4: 1.(a ) 32.5 wtOJo AOT. 13.5 wtOJo octanol,54wtOJo water; (b) 37.5wtOJo AOT,12.5wtOJo octanol, 50wt% water; (c)43.75 wtOJo AOT, 11.25 wtOJo octanol,45 wtOJo water; (d ) 47.5 wtOJo AOT,10.5 wtOJo octanol, 42 wtOJo water}.Published with permission from [31""].

,-- - - - - - - - - - - - - - - - - - - - ~ - - - - - - ~ -(a)

(d )

the body-centred space group Im3m at a rmxing ratiooppe: ope = 1:1 .and the face-centred group Fd3m at amixing ratio oppe: ope = 1:2. Also, the binary systemmonoacyl-glycerol/wfter (acyl means oleoyl or s tearoy\)develops two cubic phases between the reverse hexagonaland the lamelIar phase [24,25]. The systems [24,25]also show a the rmotrop ic behaviour: a transition of thelamelIar phase to the cubic phases and a transition ofth e cubic phases to the reverse hexagonal phase canbe observed above characteristic temperatures at cer ta inconcentrations. Bartels and coworkers [26] s tudied theinfluence of temperature and pressure on the phasebehaviour of the binary system monoacylglycerol/water(acyl means olein or elaidin) using SANS and SAXSmeasurements. The olein system forms a bicontinuouscubic phase with the space group Pn3m over a largefield in the p,T-plane which is t ransformed into a reversehexagonal phase at 9SOe, and a lamelIar phase at lowtemperatures and high pressures. The elaidin systemshows, besides lamellar phases, two cubic phases with thespace groups Im3m and Pn3m and also several metastablephases with cubic structures.Luzzati and coworkers [27] also describe two cubic phases(Q224 and Q227) in a quarternary system of phospholipid,cholesterol, diacy1cholesterol and water. Q224 is waterand lipid permeable, whereas Q227 is lipid permeablebu t water impermeable. This selective permeability isassumed to control the membrane fusion. Luzzati et 01.[28] also report the existence of seven cubic phasesin lipid-containing systems: tWO bicontinuous and fourphases of g lobu lar micelles, while the structure of the

Figure 7

.::: : "::.:::.:::.: ~ . : : . : ~ : . . : :.: : . . : ::... .. .. .. . . .. .. . . . .

1 1i. --L _Schematic drawing of the structure of the amphiphile bilayer inthe gel phase (Lp) and in the liquid crystalline lamellar phase (La)'Reproduced with permission from [43"].

seventh phase is not given. From these phases, threeare of the oil-in-water type: the Q223_phase consistsof two ' types of micelles, the Q225_ and Q229_phasescontain spherical micelIes that are closely packed in eitherface-centred or body-centred structures. The structuresand the properties of these phases are discussed in analogyto foams. The same phases were also found by X-rayand freeze-fracture electron microscopy studies in aqueoussolutions of gangliosides [29]. These systems also formhexagonal and lamellar phases which border directly withthe isotropic micellar phase.


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Sur factant gels Hoffmann and Ulbricht 731

Figure 8

La/yield stress vclJe

\ \ \ \ \ \ \ \ \ \I,".-.-"-/

II

IIII

Ln

HLj~ L

Lj ,shear. induced birefringenceI III ,I II\ \\ \\ \ 2

Lj \ ILl"/LaI I

/ II I

I II II III II\120

40

60

Phase diagram of the binary systemCxF2x+1C2H4E 4_7-water with x=6-12(Ll " ; isotropic, lowly viscous phase withunilamellarvesicles; La: birefringentphase with multilamellarvesicles; L2:isotropic, lowly viscous phase withreversed aggregates); 29 denotes a twophase region; cPF-surf: concentration ofthe perfluoro surfactant. Reproduced withpermission from [56"].

To I10 I20 I30 o, CPF-Surf Iwt%I I90 100In contrast, Quinn and coworkers [30-] report tWO cubicphases with the space groups Im3m and Pn3m insynthetic, stereochernically pure glycoglycerolipids, whichdevelop at a constant lipid concentration' of 20wt%on heat ing and cooling between the lamellar and thereverse hexagonal phase. These cubic phases form onlyin a system containing two dodecyl chains, whereas thesystems with longer alkyl chains show a direct transit ionfrom the lamellar-to the reverse hexagonal phases.Hoffmann and coworkers [31--] present studies on the cubic phase in the ternary system Aerosol OT/IOctanol/water, which lies between a reverse hexagonalarid a lamellar phase and borders on a Lg-phase (Fig.S). The transition of the cubic phase of the Lg-phaseoccurs at a characteristic t emperature on heating, andat lower AOT concentrations, also on cooling of thesamples; the transition is reversible and is first order. Theelectric conductivity and the self-diffusion coefficients donot show a break at this transit ion. SANS measurementsshow polycrystallinity for the gel-like cubic phase withdecreasing sizes of the structural units with increasingAOT content (Fig. 6). The shear modulus rises withincreasing AOT concentration, and this can be explainedusing a simple network theory where each structuralunit acts as network point. The data clearly p ~ o v e abicontinuous structure, both for the cubic and for theLz-phase. With this in mind, it is interesting that Holmesand coworkers [32-] have found an 'intermediate' phasebetween the hexagonal and the lamellar phase in thebinary system of a nonionic surfactant' C30E9 having abranched alkyl chain. This phase has a complicated meshstructure with probably rhombohedral symmetry and is

also birefringent. The stability of this phase is attributedto the length of the hydrophobic chain.In the ternary system ceryltrimerhylarnrnonium bromidesodium desoxycholate-warer, a cubic phase as well as a gelphase has been found in the isotropic micellar solution byAlmgren and coworkers [33-]. It borders on the normalhexagonal and the Lj-phase. The gel phase consists ofentangled thread-like micelles and is highly viscoelasticwithout a yield stress value. The cubic phase is consideredto be bicontinuous consisting of interpenetrating networksof connected rods; it develops from the hexagonal phaseby branching of the rods on addition of the bile sale.In a theoretical paper, Gelbart and coworkers [34--]simulate the phase evolution in concentrated surfactantsolutions by Monte Carlo and mean field studies startingfrom three types of micellar aggregates. A solution. ofdisclike micelles undergoes a first order phase transitionto a continuous bilayer, rod-like micelles grow and showa continuous transition to long 'strips' which fuse intocontinuous sheets at higher concentrations, and solutionswith branched cylindrical micelles form gel phases withconnected linear micelles linked by 'T-like' junctions.Templer [35-] gave a mathematical formula co determinethe area neutral surface from X-ray structural measurements of inverse bicontinuous cubic phases. With otherauthors [36-], he developed a simple curvature elasticmodel for such phases which allows one co determine thecurvature elastic moduli from structural data. Engblom andHyde [37-] analyzed the swelling of bicontinuous lyotropicmesophases in water as a function of the dilution. With


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732 Food colloids, emuls ions, gels and foams

Figure 9TEM-micrographs of a freezefracturedgel in a binary system containingdodecylsemicarbazonemaltose-waterat different magnifications (the barsrepresent 200 nrn). Reproduced withpermission from [66'1.

the derived swelling laws, bicontinuous oil-in-water (O/W)aggregates can be readily distinguished from rods, sheetsor globules, whereas the morphology of W/O aggregatescan be less easily deduced from swelling data.Vesicle phasesWhereas lyotropic hexagonal phases exhibit strong gelproperties (such systems usually have strongly anisotropicand nonlinear rheological properties and shall not bediscussed here), lamell ar phases with flat surfactantbilayers normally do not jell because the bilayers canbe oriented in the flow and can pass each other readily.Systems with curved bilayers which form closed unilarnellar or multilamellar vesicles can show gelation. In such

situations, the vesicles can be very large (radius r> 1urn)and are densely packed in the phase. The gelation of suchphases is determined by the stiffness of the bilayer and theattractive or repulsive interactions between the vesicles.While the formation of thermodynamically unstable vesicles by sonification of aqueous pho spholipid dispersionshas been known for a long time, more recently, spontaneous vesiculation was observed in ternary systems ofzwitrerionic surfactanrs, cosurfactants and water (38), inbinary systems of perfluorosurfactanrs [39] or double chainsurfactants [40] and water, and in aqueous ~ i x t u r e s ofanionic and cationic surfactants [41]; reverse vesicles canalso form spontaneously in water-free systems [42]. In the


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Sur factant gels Hoffmann and Ulbricht 733

Figure 10

reported period, numerous papers on vesicles systems andtheir properties have been published; we restrict thereforeto articles which refer to gelation in vesicle phases.

Ishiwatari el 01. [54] brought about gelation of vesiclesby a different mechanism. They entrapped a silane in th ebilayers of th e vesicles and polymerized it by using a basiccatalyst. This procedure leads to silicone-coated vesicleswhich can aggregate with each other by th e adhesion ofsilicone. Warriner el 01. [55] obtained a lamellar biogelby adding small amounts of polyethyleneglycol-derivedpolymer lipids (PEG-lipids) to vesicles with fluid mem-branes consisting of lipid and surfactant molecules. Thesebirefringent biogels form on addition of water to th econcentrated lamellar phase, and are transformed againto lowly viscous l iquids on addit ion of more water. T hePEG-lipids segregate to regions with high curvature in th efluid membranes and connect th e membranes.

The influence of addit ives on th e L13-La transition wasalso invest igated by several groups. Klose a 01. [48]found that addition of a nonionic surfactant (C1ZE.t) toth e gel phase of a phospholipid, leads to formation ofth e La phase above a characteristic concentration; th esame effect can be brought about by dilution of th e gelwith water. It is attributed to solvation of th e hydrophilicgroup of th e lipid by water and/or th e retraoxyethylenegroup. The. influence of shear and branched n-alkanols[49] an d of ionic surfacrants [50,51] on th e gel-liquidcrystal transit ion of vesicle phases .was investigated byvarious groups. They studied th e incorporation of additiveinto th e vesicle bilayers, which depends both on thechain l en gt h a nd on th e constitution of th e headgroupsof th e additive. Berlepsch and coworkers [Szo,53] described a metastable, bu t long lived, gel phase in th esystem sodium-sulfopropyl-octadecylmaleate/water belowth e Krafft temperature consisting of lamellar bilayers ofdensely p ac ke d a nd interdigitated surfactant molecules.

(Fig. 7). It was shown by cryo-transrmssron electronmicroscopy [44] that the vesicles adopt faceted shapeor are decomposed to open bilayer fragments below T e.In a review, Engberts and coworkers [4sooJ summarizedth e influence of the amphiphile constitution (alkyl chainlength, counterion etc) on th e gel-liquid crystal transition.T he same authors also described th e influence of the chainflexibility on this transition process [46]. In a theoreticalpaper [47), hydrophobic attraction was used to providesimulations of th e gel and liquid crystal phases of th ebilayer:

.. '..."", -,

\'I.

\ @@ 'I.

(b )

,

(a)

Schematic illustration of the gelation mechanism in aqueous solutionsof hydrophobically modified sodium polyacrylate and a nonionicsurfactant (C12E41 below a characteristic temperature Tg. (a ) TTg. Published with permission from [68J.

Vesicle phases formed by phospholipids show a rev;.[sibletransition from a gel phase (L13-phase) to a liquidcrystalline phase (La-phase) on heating at a characteristictemperature T e. Cheng and Caffrey [43J concluded,from time-resolved X-ray diffraction studies, such systems,under the influence of small pressure oscillations coveringfour decades in frequency, that th e transition is du e to a'melting' of the ordered alkyl chains at th e T o leading toa decrease of th e thickness of the amphiphile membranes

WUrtz an d Hoffmann [56] described gelation in vesiclephases of a binary system of a nonionic perfluorosurfacranr(CxF Zx+1CZH.tE+-7 with x =6-1Z) and water. This systemshows an extended region of vesicle phases (Fig. 8)which are uni lamellar at low surfactant concentra tionsand multilamellar at high concentrations; their diametersare in th e range of 1011m. Rheological measurementsshow that th e vesicle phase has a yield stress value atsurfactant concentrations above ZOwt%, this is explainedby th e rigidity of th e vesicle bilayers, which hindersth e tightly packed vesicles passing each other. Charging


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734 Food colloids, emulsions, gels and foams

FIgure 11 th e ionic charge which induces both stiffer bilayers andelectrostatic repulsion between the vesicles.

(c ) Re on

Schematic illustration 'of the mechanism of gelation and redissolutionin aqueous solutions of EHEC and SOS. (a)-(c) illustrate increasing50S concentration. Published with permission from [76'].

Similar observations have been made by Hoffmann ( I 01.when they charged th e bilayers of vesicles in th e ternarysystem alkyldimerhylaminoxide, cosurfactant and waterby adding an ionic surfactant or HCI [57--]. Rheologicalmeasurements indicate th e existence of a yield stress valuein th e ves ic le phases. Both th e storage modulus G' andth e yield stress increase with th e charge density on thebilayers to a plateau value, and decrease on addition ofelectrolyte . The rheological data of th e vesicle phases canbe explained with the model ofVan der Linden [58) whichis based on th e deformation of vesicles, and calculates theshear modulus from th e compression and bending energyof the system. The bending constant can be derived, froman expression by Lekkerkerker [59], as a function of thecharge density. The bulk compression modulus can becalculated according to Palberg (101. [60-]. With this inmind, it is worth mentioning that Yuct and Blankschtcin[61-] .developed expressions which allow a calculation ofthe surface potentials of charged vesicles without solvingthe nonlinear Poisson-Boltzmann equation numerically.Furthermore, a method has been introduced by Libchaberand coworkers [62-] which allows a direct measurement ofthe bending energy of the vesicle membranes by observinga single vesicle under th e microscope and determiningth e pressure for its deformation. It must be pointedout, however, t hat t he available models do not take intoaccount a change of th e vesicular structures by increasingthe charge density or the electrolyte concentration. Thus,an exact theory for a quantitative and exact interpretationof the experimental rheological data of vesicle phases doesnor exist at present.

.-:?

(b) cuion II

(a) ~ c i

th e bilayers by incorporation of an anionic perfluorosurfactant (CsF I7COzLi) leads to yield stress values atmuch lower surfactant concentrations. This is due to

NetworksNetworks in surfactant gels can consist of long rod-likeor thread-like micelles ; in these systems the crosslinksare mostly entanglements of th e aggregates. They canalso consist of polymers or polyelectrolyres which are

Figure 12(a) (b) (c)I :.. " 1JOOI O / .\I . j ,...-0 O"f. ~ ulf.. ..

.... \- . .< " " " .. " .. .. .. O 01 . . I I " {,,dC.ru'XJ,..) . cls,)S1e-1- - - - - -- - - -The zero shear viscosity 11 0 of aqueous solutions of 1wt% of modified hydroxyethylcellulose derivatives as a function of the concentration ofadded surfaclants. (a) F-HMHEC with different substitution degrees and 11 at a shear rate of 0.1 s- I with added CaFI 7C02Li; (b) cat-HMHECwith different subst itution degrees with added SOS; (c) cat-HEC with different substitution degrees and different molar weights with addedSOS (cry marks the presence of a yield stress value). Reproduced with permission from [81"].


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connected by surfactant molecules or aggregates. In thiscase the crosslinks can also come about by hydrophobicor electrostat ic attraction be tween the surfactant andthe macromolecules. Network systems usually can jellat very low surfactant concent ra tions and are thereforeused for many applications in the pharmaceutic, cosmetic,agricultural and food industries.In the referred period, numerous papers on networksolutions containing surfactants have appeared. Thereforewe rest ricted the review on selected papers whichdescribed sys tems in which gela tion was brought aboutby the' surfactant component. Papers on the interaction ofgels with surfacranrs or on network systems with covalentcrosslinks \ ~ i I l not be discussed.Systems with entanglement networks of rod-like orthread- like micelles can show a strong viscoelast icity,even at low surfactant concentrations of around 1wt%.Such sys tems do not have a yield stress value. Theycan often be described by a Maxwell model with oneshear modulus and one structural relaxation time. Modelsfor. the understanding of the rheology of such systemswere developed before 1994 [63]. Nernoto et 01. [64-)confirmed the Maxwell model for mixtures of CTABand sodium salicylate, and found an agreement of thedecay of concentration fluctuations with the mechanicalrelaxation time according to the Maxwell model. Thismodel was also confirmed., by Balzer et 01. [65-] formixtures of fatty alcohol ether sulfates and also ofcarboxymethylated fatty alcohol ethoxylates with non ionicsugar surfactants in the presence of excess electrolyte;the Maxwell model, however, cannot be applied tosurfactant solutions \vith polymer thickeners. A new classof saccharide-based 4-alkyl-semicarbazone surfactanrs wassynthesized by Demharter er al. [66-); the sugar groupsare bound to the hydrazine unit. These surfactants canjell at concen tr at ions below 1wt% and both cmc andgel concentration decrease with increasing chain length.The gels consist of twisted rope- like s tructures (Fig . 9)which are stable for long times. The stabilization of theseaggregates is explained by hydrogen bonds between theurea units . Rheological measurements for these systemsare not reported in the paper. Fibrous aggregates werealso found by Imae and Kidoaki [67-] in gel-like aqueoussolutions of N-acyl-amino acids. These aggregates arest abi lized by add it ion of 2-ethylhexylamine. At hightemperatures, g lobula r particles are formed which aretransformed into fibrous aggregates at room temperature.Networks in solutions of hydrophobically modified polyelectrolytes (sodium polyacrylate with n-alkyl sidechains)and nonionic surfactants CIZEn have been studied byAudebert and coworkers [68-,69-]. These systems jell,reversibly, on heating above a characteristic temperatureT g' This effect is explained by an aggregation of the surfactant around the hydrophobic chains and by a transit ionof the small spher ical micel les at low temperatures into


large vesicles above T g; the vesicles are crosslinked by thepolyelectrolyte chains, as shown in F igure 10. T g increaseswith increasing number, n, of ethylene ox ide groups inthe nonionic surfactant, due to the corresporiding shiftof the micelle-bilayer transition. The gel strength alsoincreases with n and with t he numberof alkyl chains inthe polyelectrolyte; the stronger gels exhibit a yield stressvalue. Crosslinking of the modified polyelectrolyte chainscan also be achieved by the addition of small amphiphiliccationic or anionic globular proteins [70-).A thermoreversible gelat ion on heating above T g at lowtotal concentrations has also been reported by Lindmanand coworkers [71--74-] for aqueous solut ions of thenonionic polymer. ethyl(hydroxyethyl)cellulose (EHEC)derivatives with both the anionic surfactant SDS and thecationic surfactant CTAB. Both surfactants bind to thepolymer, bu t the degree of binding, which is larger for SOSthan for CTAB, is a lmost independent of temperaturewhen T g is approached. The gelation' is not due toa change of the polymer-bound surfactant aggregates,bu t is attributed to surfactant-induced associations andenhanced polymer-polymer interactions resulting froman increased hydrophobicity of the hydroxyethyl groupsat higher temperatures. Phase diagrams of pure EHECderivatives show clouding due to separation of a polymerrich phase above a characteristic temperature due to theseenhanced polymer-polymer interactions. A microphaseseparation model in which the polymer-rich phase isseparated into microscopic 'lumps' by the bound surfactantmolecules is introduced [75--] to expla in the gelation ofthe polymer-surfactant systems. The mechanical rigidityof the gels which is larger for the EHEC/SOS systemoriginates from the association of macromolecular strandsthrough these lumps.Bloor et al. [76-) have also studied the system EHEC/SOS.Their observations for increasing SOS concentrations areshown in Figure 11. At SOS concentrations below a characteristic concentration c j , no binding of SOS to EHECis concluded from the experiments (Fig.11, Region 1). Atc j , micelles tha t b ind to the polymer are formed and startto crosslink the polymers and give rise to gelation (Fig.11,Reg ion II). At further increasing SOS concentration, thecrosslinks are detached again; micelles bound to onepolymer molecule form (Fig. 11, Region III ) and the gelsbreak down again.Merta and Stenius [77-] have investigated aqueoussolutions of cationic starch (CS), modified by differen t numbers of 2-hydroxy-3-trimethylammoniumpropylgroups, and various anionic surfactants, They observedassociation between CS and the surfactant at a critical surfactant concentration which is mostly well below the cmcof the su rfac tan t. At a round charge neutrality, phaseseparation of a gel consist ing of CS and surfactant and alowly viscous supernatant is observed. The gels are highlyviscous and their rheological behaviour is determined


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73 6 Food colloids, emulsions, gels and foams

by the constitution of the CS and of the surfactant.At further increasing surfactant concentrations, the gelsredisperse according to a charge reversal mechanism. Asimilar phase behaviour has been reported by Ranganathanand Kwak [78] for aqueous solutions of the anionicpolyelectrolyte sodium polyfacrylate-co-acrylamide) andthe cationic surfactant OTAB. The experimental datacan be described by a Flory-Huggins model, with thesurfactant micelles as second polymer component , andare consistent with an associative nature of the phaseseparation. Similar results have also been presentedby Moren and Khan [79] on mixtures of the anionic-surfactant $OS and the cationic protein lysozyme. Thedifference of this system to the ones described previouslyis tharprecipitation starts on addition of only very smallamounts of SOS to the lysozyme solutions, and is maximalat charge neutralization; . further increasing amounts ofSOS lead again to redissolurion. A bluish transparent gelphase appears between the precipitation region and theredissolved isotropic liquid...A reversible sol-gel transition has been published byEliassen and Kim [80] on complexes of natural andmodified starch and various lipids. In these systems; thegel phase exists at low temperatures and melts at acharacteristic temperature. This transition is due to a helixcoil transition of the amylose lipid complex. Both thestability of the complexes, the transition temperature andthe strength and elasticity of the gels can be varied in awide range by changing the constitution of the starch andof the lipid..Hoffmann and coworkers [81,8Z] reported gelationof aqueous solutions of modified hydroxyethyl cellulose(HEC) derivatives and surfacrants. The polymers are modified with hydrophobic perfluoro groups (F-Ht-.1HEC),with cationic groups (cat-HEC) or with both cationic andhydrophobic alkyl groups (cat-Ht-.1HEC). The modifiedHEC entangles in aqueous solutions at a character-istic concentration c* which decreases with increasinghydrophobic substitut ion. F-Ht-.IHEC and surfactantsinteract, leading to a strong increase of the viscoelasticity of the systems; at higher surfactant concentrationsprecipitation occurs. The gel strength and the yieldstress values increase with increasing substitution ofthe HEC-derivate. Cat-HEC interacts only with anionicsurfactants. The viscosity of these systems increaseswith increasing surfactant concentration; at approximatelycharge neutralization precipitation takes place. At higheramounts of surfactant, redissolution starts and the viscosities of the systems decreases steadily with increasingsurfactant concentration. Cat-Hl\IHEC interacts both withcationic and with anionic surfactants, bur an increaseof the viscosity and gelation is observed only with theanionic surfactant. The behaviour of these systems isanalogous to that of the cat-HEC and anionic surfactantsystem. Figure lZ shows the behaviour of the different

systems. The increase of the viscoelasticity and thegelation are explained by the formation of intermolecularnetworks between the hydrophobic groups of the HECderivatives and of the surfactant molecules; these networksare destroyed at higher surfactant concentrations and arereplaced by polymer bound micelles.ConclusionsGelation of two- and multicomponent systems containingsurfactants can take place both in polar solvents such aswater and in nonpolar solvents. The gelation is due tothe formation of superstructures in the systems which canconsist of densely packed spherical micelles, of lamellaraggregates having vesicles with stiff bilayers, of networksof entangled rod-like micelles or polymer molecules whichare crosslinked by the surfactant molecules. Progress hasbeen achieved in the determination of the superstructuresin th e different gel systems, for example, cubic phases,lamellar phases such as vesicles or networks, and in measurements of their rheological properties. Furthermore,theoretical models have been developed which can predictthe conditions for the formation of the superstructuresor establish relations between the present structuresand the rheological properties of the samples. At leastqualitative and, in some cases, also quantitative relationsbetween the present structures and the rheology of thegels could be confirmed for several systems, for example,the hard sphere model for cubic phases or the modelof ' living' polymers for solutions with entangled rod-likeaggregates. At present , however, a model which can givea quantitative description of the rheological properties ofthe various surfactant gels from only the constitution of thecomponents and the solvent is still missing. The researchfocuses on the development of a theory which allows oneto predict the structure and the rheology of gels frommolecular parameters of the components.

References and recommended readingPapers of particular interest, published within the annualperiod of review,have been highlighted as:

of special interest of outstanding interest

1. Haering G, Luisi PL: Hydrocarbon gels from water-In-oi lmicroemulsions. J Phys Chem 1986, 90:5892-5895.2. Ouellet C, Eicke HF: Mutual gelation of gelatin and water-in-oilmicroemulsions. Chimia 1986,40:233-238.3. ScartazziniR, Luisi PL: Organogels from leci th ins. J Phys Chem1988, 92:829-833.4. Hanabusa K, Watanabe Y,KimuraM, KoyamaT,Shirai H:Two-component organogel-forming agents working byintermolecular hydrogen bonding. Sen'i Gakkaishi 1996,52:129-136.Gelation of esters, such as triolein,on addition of substituted triazinesmixedwith barbituric acid derivativeswith long or branched alkyl chains.5. Murdan S, Gregoriadis G, FlorenceAT:Nonionic surfactantbased organogels Incorporating niosomes. STP PharmaPrat

1996, 6:44-48.Gelation of hydrocarbonswith sugar surfactantswith andwithout water; thegels show a reversible transition to sols at a characteristic temperature.


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6. Atkinson PJ, Robinson BH, Howe AM, Pill AR: Characterisationof water-in-oil microemulsions and organogels based onsulphonate surfactants. Colloid Surf A 1995, 94:231-242.Studies of the gel structures and themechanismof gelation in a systemconsisting of hydrocarbons,water, gelatin and dialkylsulfosuccinatesurfactantsusingSANS.7. Zhang J,White GL, Fulton JL: Spectroscopic investigation of anL-u-phosphatidylcholine gel formed in near-critical propylene.J Phys Chem 1995, 99:5540-5547.Studies of the influence of the pressure on the gelation of temary systemsof propylene, phospholipids and water.8. Shchipunov YuA, ShumilinaEV: lecithin organogels: role ofpolar solvents and nature of intermolecular interactions.Colloid J 1996, 58:117-125.Gelation in ternary systems of hydrocarbons, phospholipids and a stronglypolar nonaqueoussolvent.9. Shchipunov YuA, Shumilina EV: lecithin bridging by hydrogen bonds in.the organogel. Mater Sci Eng C 1995, 3:53-50.Mechanism of the"gelation and gel structures in ternary systems of hydrocarbons, phospholipids and a strongly polar nonaqueous solvent.1O. Shchipunov YuA: lecithin organogels: rheological properties ofpolymerlike micelles formed in the presence of water. Colloid J1995, 57:556-560.Rheologicalmeasurementson organogelsconsisting of hydrocarbons,phospholipids and water.11. Shchipunov YuA,Schmiedel P: Electrorheological phenomenain lecithin-decane-water mixtures. J Colloid Interface Sci 1996,179:201-206.Description of electrorheologicalphenomenain ternarysystemsof hydrocarbons, phospholipids and water in the two-phase region.12. Terech P, Bouas-laurent H, Desvergne JP: Small molecularluminescerit gelling agent 2,3-bis-n-decyloxyanthracene:rheological arid structural study. J Colloid Interface Sci 1995,174:259-263.Formation of stiff gels of hydrocarbons or aliphatic alcohols with stronglylipophilic amphiphilescontaining anaromaticgroup with aliphaticsidechains.13. Terech P, FurmanI, Weiss RG t Structures of organogels basedupon cholesteryl-4(2-anthryloxy)butanoate, a highly efficientluminescent geJator: neutron and X-ray small-angle scattering

investigations. J Phys Chem 1995, 99:9558-9566.Structural studies on gels formed from hydrocarbons or aliphatic alcoholswith a few percent of steroid derivatives.14. TerechP,Ostuni E,Weiss RG: Structural study of cholesterylanthraquinone2-carboxylate (CAQ) physical organogels byneutron and X-ray small angle scattering. J Phys Chem 1996,100:3759-3766.Rheological measurements on gels from hydrocarbonsor aliphatic alcoholswith steroid derivatives,reversible gel-sol transition at a characteristic temperature.15. DammerC, Maldivi P.TerechP.Guenet JM: Rheological studyof a bicopper tetracarboxylate/decalin jelly. Langmuir 1995,11:1500-1506.Description of the properties of gels from a hydrocarbon with a binuclearcopper-II complex with the theory of 'living' polymers.16. Cates ME: Reptation in living polymers: dynamics of entangled

polymers in the presence of reversible chain-scissionreactions. Macromolecules 1987, 20:2289-2296.17. Seddon KM, Hogan Jl , Warrender NA, Pebay-Peyroula E:Structural studies of phospholipid cubic phases. Prog ColloidPolym Sc i 1990, 81:189-197.18. Wanka G, Hoffmann H, Ulbricht W: The aggregation behaviourof poly-(oxyethylene)-poly(oxypropylene)-poly-(oxyethylene)'block copolymers in aqueous solutions. Colloid Polym Sci1990,268:101-117.19. Hecht E,MortensenK, Gradzielski M, Hoffmann H: Interactionof ABA block copolymers with ionic surfactants: influence onmicellisation and gelation. J Phys Chem 1995, 99:4866-4874.'Melting' of a cubic gel phase in the binarysystem of an ABA-block copolymer and water on addition of surfactants, due to a suppression of the micelles.20. LinemannR, Uiuger J, Schmidt G, Kratzat K, RichteringW: Linearand nonlinear rheology of micellar solutions in the isotropic,cubic and hexagonal phase probed by rheo-small-angle lightscattering. RheolActa 1995, 34:440-449.


Rheological studies on cubic phases: systems with strong elasticities andshearmelting at high shear stresses.21. Jones JL,Mcleish TCB: Rheo!Jgical response of surfactantcubic phases. Langmuir 1995, 11:785-792.Rheological properties of cubic phases above a critical shear stress.22. Oraedd G, Lindblom G, Fontell K, l jusberg-Wahren H: Phasediagram of soybean phosphatidylcholine-diacylglycerol-water

studied by X-ray diffraction and 31p. and pulsed field gradientlH-NMR: evidence fo r reversed micelles in the cubic phase.Biophys J 1995, 68:1856-1863.Description of a cubic phase consisting of reversed spherical micelles.23. TakahashiH, Hatta I, Quinn PJ:Cubic phases in hydrated 1:1and 1:2 dipalmitoylphosphatidylcholine-dipalmitoylglycerolmixtures. Biophys J 1996, 70:1407-1411.Structural studies on cubic phases in aqueous solutions of lipid mixtures.24. Aomori H, Ishiguro T,Kuwata K, KanekoT,Ogino K: Study onthermal and structural behavior of monoacylglycerol-watersystems. I. Phase behavior of monostearoylglycerol-watersystems. Yukagaku 1995, 44:997-1003.Formationof cubic phasesin binarysystemsconsisting of glycerol-estersoffatty acids and water.25. Aomori H, Ishiguro T, Kuwata K, KanekoT,Ogino K: Study onthermal and structural behavior of monoacylglycerol-watersystems. II. Phase behavior of monooleoylglycerol-watersystems. Yukagaku 1995, 44:1004-1011.Structural studies.and thermotropic phase transitions in cubic phases inbinary systems of glycerol-esters of fatty acids and water.26. Czeslik C, Winter R, Bartels K: Temperature and pressuredependent phase behaviour of monoacylglycerides monooleinand monoelaidin. Biophys J 1995, 68:1423-1429.Influenceof temperature and pressure on the phase behaviourof the binarysystem of glycerol-esters of fatty acids and water.27. Nieva Jl , Alonso A, BasanezG, Goni FM, Gulik A, VargasR,luzzati V:Topological propert ies of two cubic phases of aphospholipid:cholesterol: diacylglycerol aqueous system andtheir possible implications in the phospholipase C-inducedIiposome fusion. FEBS Lett 1995,368:143-147.Cubic phaseswith differentpermeabilitiesfor water and lipids in quarternarysystems of a lipid, two steroid derivatives and water.28. LuzzatiV,DelacroixH, Gulik A: The micel lar cubic phases oflipid-containing systems: analogies with foams, relationswith the inf inite periodic minimal surfaces, sharpness of thepolar/apolar partition. J Phys 1/1996, 6:405-418.Description of the structures and the properties of seven different cubicphases in lipid-containing aqueous systems. .29. Gul ik A, DelacroixH, KirschnerG, luzzati V: Polymorphism ofganglioside-water systems: a new class of micellar cubicphases. Freeze-fracture electron microscopy and X-rayscattering studies. J Phys 1/1995, 5:445-464.Cubic phaseswith different structures in aqueous solutions of a ganglioside.30. TenchovaR, TenchovB, Hinz HJ, Quinn PJ:lamellar-non-Iamellarphase transitions in synthetic glycoglycerolipids studied bytime-resolved X-ray diffraction. LiqCryst 1996, 20:469-482.Inf luence of the chainlength of lipids on the formation of a cubic phasebetween the lamellarand the reversed hexagonalphase.31. Gradzielski M, Hoffmann H, PanitzJC, WokaunA: Investigations on l2-phase and cubic phase in the system AOT!loctanollwater. J Colloid Interface Sci 1995, 169:103-118.The structure of a bicontinuous cubic phase in the ternary systemAOT-octanol-water; description of the rheological properties of this cubicphase with a simple network theory.32. BurgoyneJ, HolmesMC, Tiddy GJT:An extensive mesh phasel iquid crystal jn aqueous mixtures of a long chain nonionicsurfactant. J Phys Chern 1995, 99:6054-6063.An 'intermediate' liquid crystalline phase between the hexagonal and thelamellarphase that has a complicated mesh structure in aqueous solutionsof a nonionic surfactant with a long branched alkyl chain.33. VethamuthaMS, AlmgrenM, Brown W, Mukhtar E:Aggregatestructure, gelling, and coacervation within the ll-phase ofthe quasi-ternary system alkyltrimethylammoniumbromidesodium desoxycholatewater. J Colloid Interface Sc i 1995,

174:461-479.A bicontinuous cubic phase and a 'gel'-phase with entangled thread-likemicelles in a ternary system containing a cationic surfactant, a sodium saltof a bile acid and water.


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73 8 Fo o d c o ll o id s , e mu ls io ns , g el s a nd f o am s

34. Bohbot Y, Ben-ShaulA, Granek R, Gelbart WM: Monte Carlo and mean field studies of phase evolution in concentratedsurfactant solutions. J Chem Phys 1995, 103:8764-8781.Monte Carlo and mean field simulations of the phase evolution in concentrated surfactant solutionsstarting from three types of micelleswith disc-like,rod-like and branched cylindrical structures.35. TemplerRH: On the area neut ral surface of inversebicontinuous cubic phases of lyotropic liquid crystals. Langmuir

1995, 11:334-340.A formalismfor the determinationof the areaneutralsurfacefrom experimental data of inversebicontinuous cubic phases.36. TemplerRH, TurnerDC, Harper P,Seddon JM:Correctionsto some models of the curvature elastic energy of inversebicontinuous cubic phases. J Phys 111995, 5:1053-1065.A method for the determination of the curvature elastic moduli of bicontinuous phases from structural data

-_37_ Engblom J,Hyde ST: On the swelling of bicontinuous lyotropic _ mesophases. J Phys 111995, 5:171-190.Swelling laws for bicontinuous lyotropic mesophases consisting of aW/O-and arr0.y.J-structure.38. HoffmannH, Thunig C, Munkert U, Meyer HW, Richter W: Fromvesicles to the L3 (sponge) phase in alkyldimethylamineoxide/heptanol systems. Langmuir 1992, 8:2629-2638.39. Szonyi S, Watzke HJ, CambonA: Perfluoralkyl bilayer

membranes prepared from saturated amphiphiles withfluorocarbon chains. Prog Colloid Polym Sc i 1992, 89:149-155.40. Ninham BW, EvansDF,Wei GJ: The cur ious wor ld of hydroxidesurfactants. Spontaneous vesicles and anomalous micelles.J Phys Chem 1983, 87:5020-5025.41. Kaler EW, Herrington KL, ZasadzinskiJAN: Spontaneous vesiclesand other solution structures in cationic mixtures. Mat Res SocSymp Proc 1992, 248:3-10.42. Kunieda H, NakamuraK, Olsson U,LindmanB: Spontaneousformation of reverse vesicles. J Phys Chem 1993,97:9525-9531.43. Cheng A, Caffrey M: Interlamellar transit ion mechanism inmodel membranes. J Phys Chem 1996, 100:5608-5610.Mechanism of the transition flOma gel phase to a liquid crystalline lamellarphase and 'melting' of the ordered alkyl chains of the gel phase on heating.44. Andersson M, Harnmarstrorn L, Edwards K: Effect of bilayerphase transitions on vesicle structures and its influenceon the kinetics of viologen reduction. J Phys Chem 1995,99:14531-14538;,Electronmicroscopy studies on vesicles in thegel phasebelow the transitiontemperature.45. BlandamerMJ, Griggs B, Cullis PM, Engberts JBFN: Gel-to-liquidcrystal transitions in synthetic amphiphile vesicles. Chem SocRev 1995, 24:251-257.A review about studies on the influence of the amphiphile constitution onthe gel-liquid crystal transition.46. Smits E,Blandamer MJ,Griggs B, Cul lis PM, Engberts JBFN: Theeffects of chain f lexibi li ty on the propert ies of vesicles formedfrom di-n-alkyl phosphates. Reel Trav Chim Pays Bas 1996,115:37-43.Dependence of the gel-liquid crystal transition on the flexibility of the alkylchains.47. Rusling JF, KumosinskiTF:An approximation to hydrophobicattraction for molecular dynamics of self-assembled surfactantaggregates. J Phys Chem 1995,99:9241-9247.Simulation of the gel and the liquid crystalline phases of bilayers includinghydrophobic allractions.48. Klose G, Eisenblaller S, Konig B: Ternary phase diagramof mixtures of palmitoyl-oleoyl-phosphatidylcholine,tetraoxyethylene dodecyl ether and heavy water as seen by31Pand 2HNMR.J Colloid Interface Sc i 1995,172:438-446.Transitionof thegel phase into a liquid crystallinelamellarphase.on additionof a nonionic surfactant or on di lut ion with water, due to solvation of thehydrophilic groups of the lipid.49. .Agui larLF,Sotomayor CP, Ussi EA: Main phase transit iondepression by incorporation of alkanols in DPPCvesicles inthe gel state: inf luence of the solute topology. Colloid Surf A1996, 108:287-293. .Influenceof the structure of cosurfactants (alcohols) on the gel-liquid-crystaltransition of vesicle phases.50. Kacperska A: The effect of alkyl tr imethylammonium bromideson the thermal stability of dioctadecyldimethylammonium

bromide (DOAB) vesicles In aqueous solutions. J Therm Anal1995,45:703-714.Influence of added ionic surfactants on the gel-liquid-crystal transition ofvesicle phases.51. Blandamer MJ, Griggs B, Cullis PM, Engberts JBFN, Kacperska A:Vesicle-surfactant interactions: effects of added surfactants onthe gel to liquid-crystal transition for two vesicular systems.J Chem Soc Faraday Trans1995, 91:4275-4278.Influence of added ionic surfactants on the gel-liquid crystal transition ofvesicle phases.52. Berlepsch Hv: Sodium sul fopropyl octadecyl maleat e- along chain surfactant with unusual aggregation behaviour inaqueous solution. Langmuir 1995, 11:3667-3675.A metastable gel phase of lamellarbilayers of densely packed and interdigitated surfactant molecules below the Krafft-temperature.53. Berlepsch Hv, Hofmann D, Ganster J: Order in the bilayers ofgel-phase sodium sulfopropyl octadecyl maleate. Wide angleX-ray scattering and molecular modeling. Langmuir 1995,11:3676-3684.Structural studies on themetastable gel phaseof lamellarbilayersin a binarysystemof a long chain ionic surfactant and water below the Krafft-temperature.54. IshiwatariT,ShimizuI,Mitsuishi M: Formation of sil icone coatedvesicle by sol-gel method. TEM observation. Chern Lett 1996,1:33-34.Gelation of vesicles by coating the vesicle surface with 'sticky' silicones.55. Warriner HE, ldziak SHJ, Slack NL, D a ~ i d s o n P,SafinyaCR:Lamellar biogels: fluid-membrane-based hydrogels containingpolymer lipids. Science 1996, 271 :969-973.Gelation of fluid membranesof lipids and surfactants by the addition of smallamounts of PEG-derived polymer lipids.56. Wurtz J, Hoffmann H: Vesicles from ethoxylatedperfluorocarbon alcohols. J Colloid Interface Sc i 1995,175:304-317.Rheological measurementson a vesicle phaseof a nonionic perfluorosurfactant: effect of charging the bilayerswith added anionic perfluorosurfactantson the gelation and on the rheological properties of the systems.57. Hoffmann H,Thunig C, Schmiedel P,Munkert U: Gels from surfactant solutions with densely packed multilamellarvesicles. FaradayDiscuss 1995, 101 :319-333.Description of the rheological properties of vesicle phases and of the influence of charging the bilayerswith ionic surfactants using theoreticalmodels.58. Van der Linden E, Droge JHM: Deformabi li ty of lamel lardroplets. Physica A 1993, 193:439-447.59. LekkerkerkerHNW: Contr ibut ion of the electric double layer tothe curvature elasticity of charged amphiphilic monolayers.Physica A 1989, 159:319-328.60. Palberg T,Kol lal J, Bitzer F,Simon R,Wurth M, Leiderer P: Shearmodulus titration in crystalline colloidal suspensions. J ColloidInterface Sci 1995, 169:85-89.A model developed for the calculation of the bulk compression modulus ofthe phases using the expressions for the interaction between two chargedparticles.61. Yuet PK,Blankschtein 0: Approximate expressions for thesurface potentials of charged vesicles. Langmuir 1995,11:1925-1933.Expressionswhich allow the calculation of the surface potentials of chargedvesicles without numericalsolvation of the Poisson-Boltzmann equation.62. Elbaum M, Fygenson DK, Libchaber A: Buckl ing microtubules invesicles. Phys Rev Lett 1996, 76:4078-4081.Description of a method for a direct measurement of the bending energyof vesicle membranes by microscopic observation of single vesicles andmeasuringthe pressure for their deformation.63. Hoffmann H,Thunig C, Schmiedel P,Munkert U, Ulbricht W:The rheological behaviour of different viscoelastic surfactantsclutlons, Tenside Surf Det 1994, 31:389-400.64. Nemoto N, KuwaharaM, Yao ML, Osaki K: Dynamic lightscattering of CTAB/NaSal threadlike micelles in a semidiluteregime. 3. Dynamical coupling between concentrationfluctuation and stress. Langmuir 1995, 11:30-36.Agreement between the decay of concentration fluctuations and mechanical relaxation in solutions of CTAB and sodium salicylate with thread-likemicelles.65. BalzerD, VarwigS, Weihrauch M: Viscoelastici ty of personalcare products. Colloid Surf A 1995, 99:233-246.Confirmation of the Maxwell model for aqueous solutions of fally alcoholethoxylateswith nonionic surfactants and excess electrolyte.


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66. DemharterS, Frey H, DrechslerM, MUlhauptR: Synthesis andgel formation of amphiphilic semicarbazones containingsaccharide units. Colloid Polym Sc i 1995,273:661-674.Electronmicroscopic studies on aqueous solutions of a new semicarbazonesugar surfactant in the qel-stete, formation of networks of twisted lope-likestructures.67. ImaeT,Kidoaki S: Solut ion propert ies of f ibrous aggregates ofamphiphilic molecules. Yukagaku 1995,44:301-308.Fibrous aggregates in gel-like aqueous solutions of N-acyl-amino acids, reoversible transition into globular particles abovea characteristic temperature.68. Loyen K, IIiopoulosI, Audebert R, Olsson U: Reversiblethermalgelation in polymer/surfactant systems. Control of the gelationtemperature. Langmuir 1995, 11:1053-1056.Mechanismof the gelationof aqueous solutions of hydrophobicallymodifiedsodiumpolyacrylatewith a nonionic surfactantabovea characteristic temper'ature.69. LoyenK, l Iiopoulos I, Olsson U,Audebert R: Association betweenhyarophobic polyelectrolytes and nonionic surfactants. Phasebehavior and rheology. Prog Colloid Polym Sc i 1995, 98:42-46.Phase behaviour'and the rheological properties of aqueous solutions ofhydrophobicallymodified sodium polyacrylateswith nonionic surfactants.70. Petit F,Audebert R, IIiopoulosI: Interactions of hydrophobicallymodified poly(sodium acrylate) with globular proteins. ColloidPolym Sc i 1995, 273:777-781.Gelation of aqueous solutions of hydrophobicallymodified sodiumpolyacrylates on addition of small cationic or anionic amphiphilic globular proteins.71. NystromB, LindmanB: Dynamic and viscoelastic propert iesduring the thermal gelat ion process of a nonionic cel luloseether dissolved in water in the presence of ionic surfactants.Macromolecules 1995, 28:967-974.Therrnoreversible gelation of aqueous solutions of ethyl-hydroxyethyl-cellulose in the presence of ionic surfactantsabovea characteristic temperature.'72. Walderhaug H, Nystrom B, Hansen FK, LindmanB: Interaction ofionic surfactants with a nonionic cellulose ether in solutionand in the gel state studied by pulsed field gradient NMR.

J Phys Chem 1995, 99:4672-4678.Studies on the gelating aqueous solutions of ethy1(hydroxyethyl) cellulosewith ionic surfactants with NMR selfdiffusionmeasurements.73. Walderhaug H, Nystrom B, Hansen FK, Lindman B: Surfactantpolymer interaction in thermoinduced gelling systems ofethyl(hydroxyethyl)cellulose (EHEC) studied by NMRself

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98:57-62.Gelation, precipitation, redissolution and the rheological properties ofaqueous solutions of various hydrophobically or cationically modifiedhydroxyethyl-cellulose derivatives, on addition of increasingamounts of surfactants82. Hoffmann H: Polymere und Tenside. Tenside Surf Det 1995,32:462-469 . [Title translation: Polymersand Surfactants.]The interaction of various modified polymers and polyelectrolyles with surfactants in aqueous solutions, rheological properties of the systems.

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