Phase and Rheological Behavior of Surfactant/NovelAlkanolamide/Water Systems

Carlos Rodriguez,† Durga P. Acharya,‡ Koheita Hattori,‡ Takaya Sakai,§ andHironobu Kunieda*,‡

Escuela de Ingenierıa Quımica, Universidad de Los Andes, Merida, Venezuela,Graduate School of Environment and Information Sciences, Yokohama National University,

Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan, andMaterials Development Research Laboratories, Kao Corporation, 1334, Minato, Wakayama-shi,

Wakayama 640-8580, Japan

Received May 23, 2003. In Final Form: August 1, 2003

Phase diagrams of water; sodium dodecyl sulfate (SDS); and a new foam booster, alkanoyl-N-methylethanolamide (C8, NMEA-8; C12, NMEA-12; C16, NMEA-16), were constructed at 25 °C. NMEA ishardly soluble in water: liquid-liquid phase separation occurs with NMEA-8, lamellar-phase formationoccurs with NMEA-12, and solid precipitation occurs with NMEA-16. In the presence of a small amountof NMEA, the surfactant hexagonal phase (H1) is extended to the dilute region. In the aqueous micellarsolution beyond the H1 phase, the viscosity dramatically increases and a viscoelastic solution is formedin NMEA-12 and NMEA-16 systems. The viscoelastic micellar solution formed in SDS-NMEA-12 systemsfollows the Maxwell model typical of wormlike micellar systems at low shear frequencies. In the SDS-NMEA-16 system, a gel-like highly viscoelastic solution is formed in the maximum-viscosity region.Rheological measurements show that the ability of NMEA to induce micellar growth increases in thefollowing order: NMEA-8 , NMEA-12 < NMEA-16. In agreement with this result, dynamic light scatteringmeasurements show that with an increasing mixing fraction of NMEA-12 or NMEA-16 in SDS-NMEAsystems the micellar size increases, leading to the formation of wormlike micelles.

Introduction

Solutions of elongated micelles have attracted muchinterest because of their peculiar properties and potentialapplications as structured fluids. These solutions showrheological properties similar to those of polymers in agood solvent;1,2 the main difference is that micellar chainscontinuously break and recombine, and, therefore, theyare referred to as “living polymers”.3

In charged micelles, there are two contributions to theenergy: the end-cap energy that promotes micellar growthand a repulsive contribution due to charges along themicelle that favors the breaking of micelles.4 Hence,micellar growth occurs as a consequence of the reductionin the repulsion between surfactant headgroups, whichcan be induced by adding salts, strongly binding coun-terions, or cosurfactants.5

Three concentration-dependent regimes can be identi-fied for micellar growth in charged systems:6 a diluteregime in which the micellar length increases slowly withconcentration, a semidilute regime corresponding to rapidgrowth, and a concentrated regime in which, again,micelles grow only slightly with concentration. The

transition between the dilute and the semidilute regimescorresponds to the overlap concentration in which the end-cap energy equals the repulsive energy. At this concen-tration, a sharp increase in the viscosity is usually found.

The local structure created by entanglement betweenelongated micelles is perturbed by shear, and viscoelasticbehavior appears; namely, there is a superposition of theviscous and elastic forces.1 This behavior can be inter-preted in terms of stress relaxation processes. The modelof Cates7 considers two relaxation mechanisms: micellebreaking and reforming and reptation, the last related tocurvilinear diffusion above the overlap concentration c*.8When the relaxation time for breaking (τb) is shorter thanthe relaxation time for reptation (τr), the viscoelasticityof micellar solutions can be described by a simple Maxwellmodel2,9 at low frequencies, and the two-components ofthe complex elastic modulus G*(ω) ) G′(ω) + iG′′(ω) aregiven by10

where G′, G′′, and G0 are the storage, loss, and plateaumoduli, respectively, ω is the frequency of oscillatory shearflow, and τ is the relaxation time, given by 1/ωc, where ωcis the frequency at which G′ ) G′′. If G′′ is plotted as a

* Author to whom correspondence should be addressed. E-mail:kunieda@ynu.ac.jp. Phone and fax: +81-45-339 4190.

† Universidad de Los Andes.‡ Yokohama National University.§ Kao Corporation.(1) Hoffmann, H.; Rehage, H.; Schorr, W.; Thurn, H. In Surfactants

in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York,1984.

(2) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081.(3) Granek, R.; Cates, M. E. J. Chem. Phys. 1992, 96, 4758.(4) Kern, F.; Lequeux, F.; Zana, R.; Candau, S. J. Langmuir 1994,

10, 1714(5) Evans, D. F.; Wennerstrom, H. The Colloidal Domain: Where

Physics, Chemistry and Biology Meet; Wiley-VCH: New York, 1999.(6) Mackintosh, F. C.; Safran, S. A.; Pincus, P. A. Europhys. Lett.

1990, 12, 697.

(7) Cates, M. E. Macromolecules 1987, 20, 2289.(8) De Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell

University Press: Ithaca, NY, 1979.(9) Cates, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2,

5869.(10) Larson, R. G. The Structure and Rheology of Complex Fluids;

Oxford University Press: Oxford, 1999.

G′(ω) ) ω2τ2

1 + ω2τ2G0 (1)

G′′(ω) ) ωτ1 + ω2τ2

G0 (2)

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10.1021/la0348923 CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 09/12/2003

function G′ in the so-called Cole-Cole plot, a semicircleis obtained for pure Maxwellian behavior. The complexviscosity |η*| in terms of G′ and G′′ is as follows:

For viscoelastic gels, the complex viscosity is related tothe zero shear rate viscosity (η0) by11

This equation allows us to estimate the η0 by extrapolatingthe experimental points of viscosity to zero shear frequencyin oscillatory-shear measurements.

Viscoelastic properties are sometimes desired in ap-plications such as health care products. Among otheradditives, alkanoylethanolamides are widely used insurfactants formulations as viscosity enhancers.12,13 How-ever, only few studies have been reported on the propertiesof these mixed systems.14 Recently, we reported on thephase behavior and microstructure of a new kind ofalkanoylethanolamide.15 The aim of this paper is to extendthat study to other homologues, focusing on the relation-ship between phase diagrams, rheological behavior, andmicrostructure.

Experimental SectionMaterials. A series of alkanoyl-N-methylethanolamides (N-

hydroxyethyl-N-methylalkanamides, 99.4%) designated as NMEA-n, where n is the alkanoyl chain length, were kindly supplied byKao Corp., Japan. Sodium dodecyl sulfate (SDS, 98.9%) was alsoreceived from Kao Corp. Deionized (Millipore filtered) water wasused for preparing the samples.

Phase Behavior. For the study of the phase behavior, sealedampules containing required amount of reagents were homog-enized and left in a water bath at 25 °C for a few days (for theWm phase) to several weeks (for the liquid-crystal phase) forequilibration. The liquid-crystal phases were identified bypolarizing microscopy and from small-angle X-ray scatteringperformed on a small-angle scattering goniometer with a 15 kWRigaku Rotating Anode generator (RINT 2500).

Rheological Measurements. After mixing the components,the samples were left in a water bath for at least 24 h before therheological measurements, which were performed in an ARES7Test Station (Rheometric Scientific) at 25 °C using a Couettefixture with a 33.3-mm-long bob for samples of a low viscosityand a cone-plate fixture (diameter ) 25 mm, cone angle ) 0.04rad) for viscous samples and gels. Dynamic rheological measure-ments were carried out in the linear viscoelastic regime.

Dynamic Light Scattering (DLS). DLS measurements wereperformed on DLS-7000 equipment (Otsuka Electronics, Japan)with a 75-mW Ar laser source (488 nm) and a digital real-timecorrelator ALV-5000/EPP (ALV, Germany). Cylindrical cells ofa 12-mm path length were used. Samples were filtered through0.2-µm Millipore filters directly into the clean, dry cell. DLSmeasurements were carried out at 25 °C. Diffusion coefficientswere calculated from the intensity correlation function usingCONTIN analysis.

Results and DiscussionPhase Behavior of Water/SDS/Alkanolamide Sys-

tems. Figure 1 shows the partial phase diagrams of water/SDS/NMEA systems. Only in the case of the dodecanoylchain, a liquid-crystal phase (lamellar) appears in thebinary system/NMEA. The octanoyl chain seems to be tooflexible, and only liquid phases are found in aqueous

(11) Fischer, P.; Rehage, H. Rheol. Acta 1997, 36, 13.(12) Barker, G. In Surfactants in Cosmetics; Rieger, M., Ed.; Marcel

Dekker: New York, 1985.(13) Rieger, M. In Foams: Theory, Measurements and Applications;

Prud’homme, R. K., Khan, S. A., Eds.; Marcel Dekker: New York, 1996.(14) Herb, C.; Chen, L. B.; Sun, W. M. In Structure and Flow in

Surfactant Solutions; Herb, C. A., Prud’homme, R. K., Eds.; ACSSymposium Series 578; American Chemical Society: Washington, DC,1994.

(15) Rodrıguez, C.; Sakai, T.; Fujiyama, R.; Kunieda, H. J. ColloidInterface Sci., submitted for publication.

Figure 1. Partial phase diagrams of water/SDS/NMEA-nsystems at 25 °C. (a) n ) 8; (b) n ) 12, from ref (15); and (c)n ) 16. Wm, Om, and W are the micellar, reverse micellar, andexcess water phases, respectively. LR and H1 are lamellar andhexagonal liquid crystals, respectively. Note that part c is shownin a different scale. The shaded area inside the Wm phase regionis the region of the highly viscous micellar solution. Samplesfor rheological measurements were prepared by keeping theSDS concentration fixed and varying the NMEA concentration(composition indicated by circles) along the arrow originatingfrom the SDS/water binary axis in the water-rich region.

|η*| )(G′2 + G′′2)1/2

ω(3)

|η*| )η0

(1 + ω2τ2)1/2(4)

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mixtures whereas the hexadecanoyl chain is too rigid,and crystalline solid precipitates in water. As reported inour previous article, NMEA-12 is in the liquid state atroom temperature (melting point ) 20 °C), in contrast toother homologues that show a much higher melting point.It is attributed to the presence of one additional methylgroup in the molecule.15

Upon addition of SDS, lamellar and hexagonal liquidcrystals are formed in all the systems. The ionic SDSincreases the repulsion between headgroups in micelles,making the curvature more positive and inducing alamellar-hexagonal phase transition. SDS also promotesthe solubilization of the alkanolamide in the dilute region.The liquid-crystalline domain expands as the alkanoylchain increases: in the case of NMEA-16, the hexagonalphase (H1) is formed at considerable low surfactantconcentration (about 5 wt % SDS). Moreover, in the vicinityof this diluted H1 phase, a gel-like, isotropic phase wasfound. Because the H1 phase is formed by a packing ofcylindrical micelles, it is possible that the micellar shapeis similar in the adjacent isotropic solution. The mixingof SDS and alkanolamide allows for a balance of the chargeon the surface of micelles and, hence, makes micellargrowth possible. It can be observed in the phase diagramsfor the NMEA-12 and NMEA-16 systems that the isotropicphase Wm is divided into two legs by an intrudinghexagonal liquid-crystal region. This kind of phasebehavior has been related to the existence of maxima inmicelle relaxation times and viscosity.14

Rheological Behavior. To further investigate theproperties of mixed surfactant solutions, steady- andoscillatory-shear viscosity measurements were performedin a dilute solution, keeping the SDS concentration fixedat 0.15 M (∼4.3 wt %) and changing the concentration ofalkanolamide along the direction of the arrows in Figure1. The results of the steady-shear-rate measurements areshown in Figures 2-3.

Figure 2 shows the data for water/SDS/NMEA-12systems. Samples with low NMEA-12 content showNewtonian behavior; namely, the viscosity is constant inthe entire shear range. However, with an increasingNMEA-12 concentration, shear thinning appears at 0.20M, which is typical of systems containing wormlikemicelles.16-19 The decrease in the viscosity with the shear

rate might be attributed to structural changes in micellarentanglements16,20

The constant-viscosity plateau is changed when thealkanolamide content is increased from 0.20 to 0.29 MNMEA, and, however, Newtonian behavior is recoveredabove 0.29 M.

Shear thinning is also observed in Figure 3 for water/SDS/NMEA-16 systems above a certain NMEA-16 con-centration (0.18 M). No viscosity plateau is found between0.18 and 0.20 M NMEA-16, indicating highly non-Newtonian behavior.

The change in the zero shear viscosity (η0) with thealkanolamide concentration is presented in Figure 4. Theη0 values have been calculated by extrapolating theviscosity data at a low shear rate back to a zero shear ratein the steady- (for low-viscosity samples) or oscillatory-shear measurements (see eq 4). With increasing NMEA-12 and NMEA-16 concentrations, the zero shear viscosityincreases at first, attains a maximum, and then decreases,suggesting structural changes in the system with in-creasing NMEA concentration. However, with NMEA-8,no significant structural change occurs in the micellarshape and size, as suggested by a small increase in theviscosity over a wide span of NMEA concentrations.Samples with no added alkanolamide have a low viscosity,similar to water. Upon addition of NMEA-12 or NMEA-

(16) Raghavan, S.; Kahler, E. Langmuir 2001, 17, 300.(17) Cappelaere, E.; Cressely, R. Colloid Polym. Sci. 1998, 276, 1050.(18) Kim, W.; Yang, S. J. Colloid Interface Sci. 2000, 232, 225.

(19) Lortie, F.; Boileau, S.; Bouteiller, L.; Chassenieux, C.; Deme, B.;Ducouret, G.; Jalabert, M.; Laupretre, F.; Terech, P. Langmuir 2002,18, 7218.

(20) Keller, S. L.; Boltenhagen, P.; Pine, D. J.; Zasadzinski, J. A.Phys. Rev. Lett. 1998, 80, 2725.

Figure 2. Steady-shear-rate viscosity measurements of the0.15 M SDS/NMEA-12 system with different concentrations ofNMEA-12 at 25 °C. The NMEA-12 concentrations are (a) 0, (b)0.08, (c) 0.16, (d) 0.20, (e) 0.25, (f) 0.29, (g) 0.34, and (h) 0.38M.

Figure 3. Steady-shear-rate viscosity measurements of the0.15 M SDS/NMEA-16 system with different concentrations ofNMEA-16 at 25 °C. The NMEA-16 concentrations are (a) 0.08,(b) 0.16, (c) 0.18, (d) 0.20, and (e) 0.23 M.

Figure 4. Variation of the zero shear viscosity (η0) of 0.15 MSDS/NMEA systems as a function of NMEA concentrations at25 °C.

8694 Langmuir, Vol. 19, No. 21, 2003 Rodriguez et al.

16, a great increase in the zero shear viscosity is obtained.This increase is larger and sharper in the case of NMEA-16: a concentration increase from 0.10 to 0.18 M causesa change in the viscosity of nearly 5 orders of magnitude.The most viscous samples in NMEA-16 systems (∼0.20 MNMEA-16) are gel-like, and they do not flow even whenupside down (η0 > 1000 Pa‚s). Some of the samples areflow birefringent; that is, they are isotropic at rest butoptically anisotropic when observed through crossedpolarizers while some stress is applied, for example, byshaking the sample. Flow birefringence is typical forsolutions containing wormlike micelles, and this phe-nomenon isattributed toashear-induced first-orderphase-transition isotropic solution-nematic liquid crystal.21,22

The concentration corresponding to a sharp increase inthe viscosity is related to the overlap concentration c* atwhich the contact between polymerlike micelles starts.Above c*, entanglement occurs and a network is formed;therefore, the viscosity increases as a result of hindereddiffusion between entangled micelles. Figure 4 suggeststhat the long-chain NMEA-16 is more prone to entangle-ment than NMEA-12.

As mentioned previously, the viscosity decreases againat high alkanolamide concentrations. This decrease mightbe caused by changes in the structure of micelles, forexample, breaking or branching;16 in the latter case, slidingof connections along the micelles can occur, which causesa decrease in the viscosity of the system.23,24 A maximumin the viscosity has also been related to a maximum in themicellar contour length.4,25 Further increase in the al-kanolamide concentration causes liquid-crystal formation

in the case of NMEA-12 or phase separation in the caseof NMEA-16.

Dynamic shear measurements were also carried out,and the results are presented in Figures 5 and 6. Figure5 corresponds to the NMEA-12 sample with the highestviscosity. The storage modulus G′ is smaller than the lossmodulus G′′ at low frequency, and the system behaves asa liquid. With an increasing frequency, G′ exceeds G′′,suggesting solidlike behavior. As can be seen in the fittingsof Figure 5a and more clearly in the Cole-Cole plot (Figure5b), this sample follows the Maxwell model, with a singlerelaxation time in the low-frequency region. This suggeststhat τb e τr.14 Values of ∼185 Pa‚s for the plateau modulus(G0) and ∼0.37 s for the relaxation time are obtained fromeqs 1 and 2. The existence of G0 is a consequence ofentanglements between wormlike aggregates. Experi-mental data for the complex viscosity also show a good fitto eq 4.

At high frequency, G′′ deviates from Maxwell behavior.The deviation from Maxwell behavior observed in theCole-Cole plot at high frequencies is probably caused bythe Rouse mode of stress relaxation, which occurs atfrequencies on the order of the inverse of the breakingtime of micelles.

Figure 6 shows the data for NMEA-16 systems. In thecase of the 0.18 M NMEA-16 sample (Figure 6a), withincreasing frequency, G′ increases more steeply than G′′,and this increase continues in the entire frequency range,with no plateau of G′ at the higher frequency. It suggeststhat there is no well-defined relaxation time (τb > τr), and,therefore, the behavior does not fit the Maxwell model.14

(21) Olmsted, P. D. Curr. Opin. Colloid Interface Sci. 1999, 4, 95.(22) Pujolle-Robic, C.; Olmsted, P. D.; Noirez, L. Europhys. Lett. 2002,

59, 364.(23) Drye, T. J.; Cates, M. E. J. Chem. Phys. 1992, 96, 1367.(24) Lequeux, F. Europhys. Lett. 1992, 19, 675.(25) Magid, L. J. J. Phys. Chem. 1998, 102, 4064.

Figure 5. (a) Variation of the storage modulus G′ (circles),loss modulus G′′ (squares), and complex viscosity |η*| (triangles)as a function of the oscillatory shear frequency (ω) for the 0.15M SDS/0.25 M NMEA-12 solution system at 25 °C. The linesshow the best fitting to eqs 1, 2, and 4. (b) Cole-Cole plot ofthe values shown in part a.

Figure 6. Variation of G′, G′′, and |η*| as a function of ω forthe 0.15 M SDS/NMEA-16 solution system at 25 °C for differentNMEA-16 concentrations. (a) 0.19, (b) 0.20, and (c) 0.23 M.Notations are the same as those in Figure 5.

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This is similar to some polymer systems where manyconfigurational motions of the flexible chains contributeto the decay of shear stress and, therefore, have acontinuous spectrum of relaxation processes, such asreptation and local motions of the chains (Rouse modes).3,19

Figure 6b corresponds to the sample with the highestviscosity. G′ > G′′ in the whole frequency range studied;namely, there is no crossover of G′ and G′′, suggestingvery long relaxation times (low ωc values) and solidlikebehavior in the wide range of frequencies. The gel-likebehavior of this sample may be a consequence of the longrelaxation times of wormlike micelles, associated withreptation.16

Figure 6c shows a behavior similar to that in Figure 6a;the viscous component G′′ prevails in the low-frequencyrange. Therefore, structural changes in micelles seem tooccur when the surfactant concentration varies, as alreadyshown in Figure 4. Again, no plateau is found for G′, andthe behavior departs from the Maxwell model as a resultof multiple relaxation times.

It should be pointed out that for the NMEA-16 samplesabove 0.18 M NMEA, the values of the steady-stateviscosity (Figure 3) and the complex viscosity (Figure 6)deviate from one another, indicating that a structure(network) is destroyed by shearing. On the other hand,there is an obvious variation of G′ with the NMEA-16concentration. This can be correlated with changes in thenumber of effective chains between cross-links. The samplewith highest viscosity (Figure 6b) seems also to show thelongest relaxation time.

DLS. To carry out a preliminary study on microscopicstructural changes, DLS measurements were performedon some of the systems for different concentrations.

As can be seen in Figure 7a, the diffusion coefficient (D)tends todecreaseas theNMEA-12concentration increases,suggesting an increase in the size of the aggregates. Forconcentrations below 0.12 M, the diffusion coefficientpractically does not change with the scattering vector, q,indicating weak interactions between aggregates. Thistendency is not clear for the 0.15 M SDS/NMEA-12 sample.

DLS data for NMEA-16 systems are shown in Figure7b. The diffusion coefficient first decreases with theNMEA-16 concentration and then tends to become con-stant; namely, micellar growth is masked by intermicellar

interactions.1,26 The existence of these interactions is alsosuggested by the change in the diffusion coefficient withthe scattering vector for NMEA-16 concentrations above0.15 M, which also indicates the existence of more thanone relaxation time.27,28 Diffusion coefficients for the mostviscous samples are of the same order of magnitude ofthat reported for other systems containing wormlikemicelles.27

ConclusionPhase diagrams of water/SDS/NMEA-8, NMEA-12, and

NMEA-16 show that NMEA-12 can form a lamellar (LR)phase in the NMEA/water binary system, whereas NMEA-16 forms a solid precipitate and NMEA-8 shows a liquid-liquid phase separation. Upon addition of SDS, lamellarand hexagonal (H1) liquid-crystal phases form in all thesystems. The liquid-crystal region expands as the alkylchain increases, and the H1 phase is formed at lowerconcentration of SDS. Upon adding a small amount ofNMEA to the aqueous micellar solution of the SDS phase,the viscosity dramatically increases and a viscoelasticsolution is formed in the NMEA-12 and NMEA-16 systems,whereas no such increase is observed in the NMEA-8system. Oscillatory shear rheology of the viscoelasticsolution of the SDS/NMEA-12 system can be described bythe Maxwell model typical of wormlike micelles. Consis-tent with the rheological measurements, DLS also sug-gests that NMEA-16 is more efficient than NMEA-12 ininducing the micellar growth.

Acknowledgment. The authors are grateful to Dr.Kenji Aramaki (Yokohama National University) for helpin the experimental work and preparation of the manu-script. C.R. thanks Comision de Intercambio Cientıficode la Universidad de Los Andes and FUNDACITE,Venezuela, for financial support during his stay atYokohama National University. D.P.A. is thankful toKathmandu University, Nepal, for providing study leave.

LA0348923

(26) Nicoli, D. F.; Dorshow, R. B.; Bunton, C. A. In Surfactants inSolution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York,1984.

(27) Imae, T. J. Phys. Chem. 1990, 94, 5953.(28) Claire, K.; Pecora, R. J. Phys. Chem. B. 1997, 101, 746.

Figure 7. DLS results showing the variation of the diffusion coefficient, D, with the scattering vector, q, for the (a) 0.15 MSDS/NMEA-12 and (b) 0.15 M SDS/NMEA-16 systems with different concentrations of NMEA at 25 °C.

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