Introduction

Sugar-based surfactants are very interesting nonionicsurfactants as they are biodegradable, nontoxic, andhave dermatological properties. In recent times, severalstudies about their physicochemical properties and abook [1] have been published. Von Rybinski hassummarized the general properties of alkyl glycosidesand compared them with the properties of fatty alcoholethoxylate[2]. The sugar-based surfactants presentinteresting applications because of the structure andrheological parameters of the different phases. Theeffects of chain length [3], headgroup polymerization [3],and stereochemistry [4] on phase behavior have alsobeen studied. Increasing the alkyl chain length increasesthe hydrophobicity and favors the formation of reversemicelles. The increase in the hydrophobicity of the sys-tem has also been observed when D2O is substituted forH2O [5]. The phase behavior of alkyl polyglucosides incombination with alcohols has also been described.Different lamellar phases have been found for commer-cial alkyl polyglucosides [6] and dilute commercial alkylpolyglucoside solutions with small amounts of alkanols

[7]. We have chosen the octyl-b-D-glucoside (C8G1) tostudy its influence on the CTAB/glycerol/water system.The octyl-b-D-glucoside consists of one glucose moleculeattached to the octyl chain via a glucoside b-linkage. Ingeneral, the alkyl glucosides with a b-linkage betweenthe hydrophilic and hydrophobic parts are much moresoluble in water, have higher cmc values, lower Kraftpoints, and form smaller aggregates at lower concen-trations than alkyl glucosides with an a-linkage [8, 9, 10].The binary phase diagram of b-C8G1/water shows alarge extension of the isotropic region and the hexago-nal, cubic, and lamellar liquid crystalline phases [11, 12,13, 14]. The large isotropic phase implies [11] that thesurfactant aggregates remain relatively small and do notgrow into extended rods and that the aggregate-aggregate interactions are relatively short-ranged. Uponaddition of 5 wt% of butanol to a 5 wt% aqueousmicellar solution of b-C8G1, the micelles change fromalmost spherical to prolate ellipsoids of revolution[15, 16].

The present paper outlines how the phase behavior ofCTAB/glycerol/water and CTAB/glyceraldehyde/watersystems is influenced by the addition of a minimum

ORIGINAL CONTRIBUTIONColloid Polym Sci (2003) 281: 319–324DOI 10.1007/s00396-002-0779-8

Ana Belen Cortes

Mercedes ValienteThe effect of a minimum amountof octyl-b-D-glucoside on micellar, nematic,and hexagonal phases of theCTAB/glycerol/water system

Received: 26 March 2002Accepted: 6 August 2002Published online: 22 October 2002� Springer-Verlag 2002

A.B. Cortes Æ M. Valiente (&)Dpto. Quımica Fısica,Universidad de Alcala,Madrid, 28871 SpainE-mail: mercedes.valiente@uah.es

Abstract The incorporation of aminimum amount of octyl-b-D-glucoside (1 wt%) in the ternaryCTAB/glycerol/water and CTAB/glyceraldehyde/water systems helpsthe formation of a nematic liquidcrystal at 30 �C. Moreover, thepresence of octyl-b-D-glucosideenlarges the micellar and the hexag-onal phases. The nematic phase ap-pears between the micellar and thehexagonal liquid crystal phases. Itchanges to an isotropic phase at

34 �C. The rheological study of thedifferent phases is shown. Thepresence of octyl-b-D-glucosideaffects the structure of the micelles.The incorporation of octyl-b-D-glucoside leads to micelles athighest amounts of CTAB andglycerol, which change theirstructures under shearing.

Keywords Nematic phase Æ Octyl-b-D-glucoside Æ Micelle Æ CTAB ÆShear induced structure

amount of octyl-b-D-glucoside. The ternary systemswithout octyl-b-D-glucoside have already been studied[17]. Both systems show micellar and hexagonal phases.No lamellar phase and no reverse micelles were found.Systems with other polyalcohols such as the TTAB/1,2-ethanediol/water system [18] show similar phasebehavior. In this paper, we present the phase diagramsof the pseudoternary CTAB+(1%) b-C8G1/glycerol/water and CTAB+(1%) b-C8G1/glyceraldehyde/watersystems together with the viscoelastic study of theformer system.

Experimental section

Cetyltrimethylammonium bromide was purchased from Aldrich.DL-glyceraldehyde (>95%), glycerol (99.5%), and octyl-b-D-glucoside, C8G1 (>98%), were purchased from Sigma. Allmaterials were used as supplied.

The phase diagrams were constructed using macroscopicsamples prepared by weighing the components. These samples wereprepared in two different ways: with fixed wt% surfactant/wt%alcohol ratio and with fixed wt% surfactant/wt% water ratio,where the wt% surfactant is the wt% CTAB plus 1 wt% of C8G1.The difference in composition between the two samples was 2% inweight in the third component. To homogenize the samples, theywere shaken in a Heidolph REAX 2000 vibrator, heated, stored at30.0±0.1 �C and subsequently visually checked through crossedpolarizers.

The liquid-crystalline behavior was investigated using aLaborlux S Leitz optical microscope between crossed polarizerswith a camera to photograph the samples. A Linkam MS100controller controlled the temperature.

The rheological study was performed using a Carri-MedCSL-100 rheometer in which stress is controlled, with a cone andplate configuration (40 mm and 1 deg.) at 30.0±0.1 �C. Twodifferent experiments were carried out: steady flow (shear stresssweep mode) and oscillation. In the oscillation experiments, thestorage modulus G’ and the loss modulus G’’ were measured as afunction of stress to obtain the linear viscoelastic region. When thelinear viscoelastic region was established, measurements werecarried out as a function of frequency at a constant stress.

It is important to know that there were some problems when itcame to measuring the rheological properties of the samples. Theshort time used to manipulate them at room temperature beforemeasurement was sufficient for precipitation of the CTAB.

Results and discussion

Pseudoternary phase diagrams

The phase diagrams of the CTAB+(1%) b-C8G1/glycerol/water and CTAB+(1%) b-C8G1/glyceralde-hyde/water determined at 30.0±0.1 �C are plotted inFigs. 1 and 2. Both systems show a micellar phase andtwo anisotropic phases, the nematic and hexagonalliquid crystals. These liquid crystals were characterizedby optical microscope with crossed polarizers.

The incorporation of b-C8G1 extends the isotropicphase of micelles from water corner to smaller amounts

of water for both systems containing glycerol andglyceraldehyde. The change in the extension of thehexagonal phase is bigger. All the boundaries of thehexagonal phase shift to higher surfactant and watercontents and the region becomes much greater than inabsence of glucoside [17].

Without b-C8G1 [17], the L and H phases are veryclosely located. A new phase appears between micellarand hexagonal phases with b-C8G1. This phase is an-isotropic, transparent, of low viscosity, and it aligns withits director parallel to the magnetic field (Fig. 3), ascorresponds to the nematic phase of rod-like micelles(NC). This thermotropic liquid crystal is very sensitive tothe variation in temperature. When the temperatureincreases to 34 �C, the nematic phase converts into anisotropic phase, while the hexagonal phase (lyotropic

Fig. 1 Phase diagram for CTAB+(1%) b-C8G1/glycerol/watersystem at 30.0±0.1 �C (dotted lines without b-C8G1)

Fig. 2 Phase diagram for CTAB+(1%) b-C8G1/glyceraldehide/water system at 30.0±0.1 �C (dotted lines without b-C8G1)

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liquid crystal) does not change. The presence of anematic liquid crystal in ionic surfactant systems is notcommon. This phase, located between L and hexagonalphases, has been found for ionic systems in the presenceof salts such as cetylpiridinium chloride, sodium salicy-late, and brine [19] and also for zwitterionic surfactants[20]. For quaternary ammonium surfactant/water sys-tems, the nematic phase has also been found by someauthors [21, 22] in narrow ranges of concentration andtemperature. However, most of the phase diagrams withCTAB do not show the nematic phase. Among thesephase diagrams are the ternary CTAB/alcohol/watersystems at 25.0±0.1 �C, published by our researchgroup [17, 23, 24]. The difficulty in finding this nematicphase is due to problems with the preparation of thesample. The Kraft point is close to room temperatureand it is necessary to work at high temperatures tosolubilize highly concentrated samples. Local waterlosses that take place with highly viscous samples do notenable homogenous samples to be obtained easily. Thesurfactant arranges in some local positions in thehexagonal phase. Other local places are less concen-trated in surfactant and the isotropic phase forms. Thepresence of b-C8G1 favors the formation of the nematicphase and so it is easier to observe than in the absence ofb-C8G1. As we will see later, the samples are less viscousin this pseudoternary system. This helps when obtaininghomogenous samples by heating and shaking. More-over, the molecular structure of b-C8G1 can permit theformation of a hydrogen-bond network between thesugar units that stabilizes [12] the nematic phase. As you

can see in phase diagrams, the phase boundaries of thenematic phase are parallel with water. This suggests thatthe presence of glycerol or glyceraldehyde does not favorthe solubilization of surfactant in the nematic phase, asit requires the same % CTAB/% C8G1 ratio of 25, withor without polyalcohols. In both cases, the nematicphase extends under the solubility of the polyalcohol inwater.

Rheological study

The micellar phase of the CTAB/glycerol/water system[17] behaves like a Newtonian fluid except at the highestCTAB content (near the hexagonal liquid crystal). Thesemore concentrated samples with a shear thinning be-havior, consistent with rod shape micelles, showed aNewtonian region at smaller shear rates. When we addb-C8G1(Figs. 1 and 2), the L phase extends widely andwe can analyze this more interesting region with higherglycerol contents than in the absence of b-C8G1. Firstly,we present the viscous behavior in the same range ofglycerol content that we had analyzed for the ternarysystem [17]. The flow curves for the samples at 20%surfactant (CTAB+1% b-C8G1) and different amountsof glycerol are plotted in Fig. 4. The viscosity data showthat the mixed CTAB+1% b-C8G1 micelles are lessviscous than without b-C8G1 and that the Newtonianpart of the curves becomes greater. By increasingglycerol content, the flow behavior becomes Newtonian.

Fig. 3 Light micrographs between crossed polarizers for thenematic phase under a magnetic field of 2 Tesla for 3 h Fig. 4 Viscosity vs shear rate for micelles with 20% of surfactant

and different amounts of glycerol: squares 0%, inverted triangles3%, circles 5%, triangles 8%, X 10%, + 15%

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In order to corroborate the decrease in viscosity due tothe incorporation of b-C8G1, measurements were carriedout varying the b-C8G1 content, but keeping the totalsurfactant content constant at 20%.

The zero-shear rate viscosities as a function of theamount of glycerol are plotted in Fig. 5. The rod-shapedmicelleswithout b-C8G1 growwith the amount of glyceroluntil reaching the maximum viscosity. Above the maxi-mum, the viscosity decreases because the micelles breakup into smaller ones. These small rods may further breakdown into spherical micelles at still higher amounts ofglycerol. The maximum viscosity with the amount ofglycerol disappears progressively by increasing theb-C8G1 content. In contrast, the viscosity reaches similarvalues at glycerol contents higher than 10%. The smallervalues of viscosity and the greater Newtonian part of thecurves with b-C8G1 suggest smaller mixedCTAB+b-C8G1micelles by the incorporation of b-C8G1.The b-C8G1 is located between CTAB molecules. Itreduces the electrostatic repulsions between the cationicheadgroups of CTAB molecules and it favors sphericalcurvature. Thus, the viscous behavior of the mixedCTAB+ b-C8G1micelles, with respect to CTABmicelles,is more different with smaller amounts of glycerol andwithout glycerol. The addition of b-C8G1 changes theshape of the micelles from rod to spherical.

We now pay attention to the mixed micelles atamounts of glycerol that are so high that glycerol cannotbe solubilized into micelles without b-C8G. The flowbehavior of the L phase at higher amounts of glycerol

can be seen in Fig. 6. For concentrations up to 35% ofglycerol, the viscosity begins to increase from a criticalshear rate. This increase in viscosity is direct evidence ofa structural transition induced by shear from sphericalto rod-like micelles. The increase in the shear stressinduces a change in the structure. Therefore, when weapply a preshear stress of 30 Pa on the sample (corre-sponding to the value of the critical shear rate), after thestructural transition has already been produced, theviscosity decreases with the shear rate in the whole rangeof shear rate. This means that the application of thepreshear stress causes the same change in the viscosity asthe increase in the glycerol content (Fig. 6) for 50% ofthe glycerol content, where the viscosity decreases withthe shear rate in the whole range. The size and flexibilityof these micelles are determined by the glycerol content,as the surfactant content is constant. Of course, theviscosity has to increase with the glycerol content, butthe flow behavior should not change from Newtonian tonon-Newtonian. Besides, the increase in viscosity due tothe incorporation of glycerol is only significant from60% glycerol content (see Table 1).

There are several proposals about shear-inducedtransitions in micellar solutions. Some papers havereported isotropic-to-nematic transition under shear[19, 25]. Lequeux et al. [26] have reported that shearinduces a phase separation in wormlike micelles ofgemini surfactants. Probably, our system behaves in asimilar way and the aggregation of small micelles occursin the sheared solution with the formation of rod

Fig. 5 Zero-shear viscosity vs glycerol content at 20% of surfac-tant content: squares without b-C8G1, circles 0.25% b-C8G1,triangles 1.0% b-C8G1, inverted triangles 1.25% b-C8G1

Fig. 6 Viscosity vs shear rate for micelles with 20% of surfactantand different amount of glycerol: squares 35%, circles 40%,triangles 45%, inverted triangles 47%, diamonds 50%

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micelles, which could explain the higher viscosity. Theincrease in glycerol content (Fig. 6) could have the sameeffect on the micelles, an aggregation of micelles. Wemust not forget that the nematic phase is built up of rodmicelles so that both explanations could be unified inthis case. In any case, samples after shearing werechecked with an optical microscope with polarizers andthey did not show birefringence. To confirm the kind ofstructural change produced, some technique such as reo-SANS or reo-SALS, that permit simultaneous mea-surements of viscosity of small angle light or neutronscattering would be necessary.

The flow behavior of the nematic phase at differentglycerol contents can be observed in Fig. 7. At low shearrates, a Newtonian region appears in the flow curve withlower amounts of glycerol, while this plateau disappearsby increasing the glycerol content. After this, the flowcurves are similar to the ones for the L phase at thehighest amount of glycerol. The flow curves are consis-tent with rod-like micelles that orientate in the flowdirection. There is no difference in the flow behaviorbetween both phases at the highest amount of glycerol.With respect to the elastic behavior of the nematicphase, oscillatory measurements were carried out(Fig. 8). Both moduli, viscous modulus (G’) and elastic

Table 1 Viscosity of water-glyc-erol mixtures at 30 �C % Glycerol 0 5 10 20 30 40 50 60 70 80 90

g0 (mPa s) 0.80 1.26 1.51 2.02 2.73 3.91 5.74 9.76 20.73 53.81

Fig. 7 Viscosity vs shear rate for nematic phase at 25% ofsurfactant and different amount of glycerol: squares 0%, circles3%, triangles 6%, inverted triangles 20%, diamonds 25%

Fig. 8 Frequency dependencies of the elastic, viscous and complexviscosity moduli for nematic phase: squares g*, circles G’’, trianglesG’

Fig. 9 Relative viscosity vs shear rate for samples with 27% ofsurfactant and different amounts of glycerol: squares micelles (45%glycerol), circles nematic phase(33% glycerol), triangles hexagonalphase (3% glycerol)

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modulus (G’’), crossed at around a frequency of 20 s–1.The elastic behavior is only important for higher angularfrequency.

Finally, to compare the viscosity for the differentphases at the same amount of surfactant, we compiledthe relative viscosity for samples with 27% surfactantcontent and different amounts of glycerol (Fig. 9). Thehexagonal phase is the most viscous phase because of thestructure of this phase that is built up of cylindricalsurfactant aggregates in a hexagonal array [18]. We havefound that the viscosity strongly depends on surfactantcontent and it is nearly independent of the glycerolcontent in the hexagonal phase.

Conclusions

The incorporation of a minimum amount of the octyl-b-D-glucoside (1%) in CTAB surfactant systems favors

the formation of a nematic phase between micellar andhexagonal phases. The sequence of the phases withsurfactant content is L1, Nc, and H1. The nematicphase is formed by rod-like micelles with a long-orientational order. This phase is more stable in thepresence of glycerol or glyceraldehyde up to contentsnear 35%. The mixed CTAB+b-C8G1 micelles aresmaller than CTAB micelles but a structural transitioninduced by shearing of the mixed CTAB+b-C8G1

micelles takes place at higher amounts of glycerol andCTAB.

Acknowledgements Financial support of this work by CYCITthrough the PB95–0322-C02–01 program, Ministerio de Ciencia yTecnologıa through the MAT 2001–1188-C02–01 program and thegrant of the Ministerio de Educacion y Cultura of the SpanishGovernment (A. B. Cortes) are gratefully acknowledged. We areindebted to Prof. Hoffmann and coworkers from the BayreuthUniversity for making it possible to obtain the micrographs undermagnetic field.

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