Microstructural Study of Aqueous Solutions of Octadecylamide Oligo(oxyethylene)ether

JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.

181, 191–199 (1996)

0370

Microstructural Study of Aqueous Solutions of Octadecylamide Oligo(oxyethylene)ether ALI KHAN,* ,1 ALON KAPLUN,† YESHAYAHU TALMON,†

AND

MARTIN HELLSTEN *

*Physical Chemistry 1, Chemical Center, University of Lund, S-221 00 Lund, Sweden; and †Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel Received October 4, 1995; accepted January 24, 1996

The nonionic surfactant octadecylamide oligo(oxyethylene)ether is soluble with water, giving rise to an isotropic solution phase over the entire concentration range at 298 K. Solution properties of surfactants have been investigated by applying several experimental techniques. The existence of nonspherical micelles is indicated by a large broadening of the 1H NMR linewidths. The surfactant self-diffusion coefficient determined by the pulse gradient spin echo NMR method identifies the presence of two types of micellar aggregates—small spherical micelles in coexistence with large nonspherical micelles. The radii of spherical micelles (20–30 A˚ ) are almost independent of surfactant concentration. The nonspherical micelles formed at a very high dilute solution grow dramatically in size with surfactant concentration until a maximum (É20% surfactant) is reached, after which there is a slow decrease in micellar size. Direct imaging by the cryo-TEM technique also identifies a mixture of spheroidal- and thread-like micelles in dilute solutions. The solution exhibits high viscosity which increases rapidly with concentration, (maximum at É20%) and then decreases again. Stress vs strain flow curves show the characteristics non-Newtonian flow of the pseudoplastic type. High viscosity measured at low strain decreases to a concentrationindependent constant low value at high shear strain. q 1996 Academic Press, Inc.

Key Words: NMR diffusion; cryo-TEM; nonionic amide surfactants; micelles.

INTRODUCTION

Alkylamide oligo(oxyethylene)ether surfactants, also known commercially as ethoxylated fatty acid monoethanol amides, are synthesized from natural raw materials such as coconut, rapeseed, and linseed oils. The products are readily biodegradable and possess good ecological properties (1, 2). The surfactants also have good detergent, dispersant, and thickening properties. These materials were introduced on the market in the 1930s. Today, the pollution of the environment—air, water, and 1 To whom correspondence should be addressed. E-mail: ali.khan@ f kem1.lu.se.

soil—by chemicals is a serious problem with a rapid growth of the chemical technology. Consumers are becoming increasingly conscious about using materials that have relatively fewer harmful effects on the nature. With improved techniques of synthesizing the ethoxylated fatty acid amide surfactants and their good ecological properties, these surfactants are preferred, in several cases, to traditional surfactants based on petrochemicals. Recently (3), we found that the amide surfactants are effective drag-reducing additives in water up to 507C. These effects are of great interest, e.g., in district heating and cooling transmission systems as a means of reducing frictional losses and improving inherent energy carrying capacity. We have explained the effect by the formation of large rod-like micelles which are destroyed at high temperatures or at high shear rates. At present, very limited information is available regarding the physicochemical properties of ethoxylated fatty acid amide surfactants in solution. In this report, we present the solution properties of one such surfactant for which rapeseed oil was used as a raw material. We have studied the solution microstructure by a combination of methods. The cryogenic transmission electron microscopy (cryo-TEM) method is used for direct imaging of aggregates, their growth and morphology; the data obtained by the pulse gradient spin echo (PGSE) NMR technique are used to obtain quantitative information of the aggregate size and shape, and 1H NMR linewidth measurement gives the qualitative information on the growth of micelles in the system. Finally, the rheological properties of the solution phase are also presented. EXPERIMENTAL

Materials. Octadecylamide poly ( oxyethylene ) ether, C18H37CONH(CH2CH2O)9H (OAEO), was supplied by the Akzo Nobel Surface Chemistry, AB (Sweden). Distilled rapeseed oil fatty acid received from the Karlshamns AB (Sweden) was used as a starting material. The surfactant was prepared by the reaction of the fatty acid methyl ester, RCOOCH3 , with monoethanolamine, HOC2H4NH2 . The oc-

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tadecanemonoethanolamide formed was then ethoxylated to obtain the desired length of the EO (ethylene oxide) group in the molecule. By selecting proper conditions, the formation of the undesirable by-products can be minimized. The surfactant, OAEO, contains small amounts of ester amides as by-products as has been detected by the IR spectroscopy technique. Octadecylamidepoly(oxyethylene)ether used in this work is a polydisperse preparation in terms of both alkyl chain length (C16 –C20 ) as well as the length of the ethylene oxide chain (EO4 –EO12 ). However, C18 (C18:1 É 60%, C18:2 É 20%, C18:3 É 10%) of the alkyl chain and EO9 of the ethylene oxide chain are the main components in the preparation. Heavy water (99.7 atom% 2H) was purchased from Dr. Glaser AG, Basel. Sample preparation. The samples were prepared by weighing appropriate amounts of substances into 5 mm (i.d.) NMR tubes which were then flame sealed. The samples were mixed by continuous shaking in a bench-type electric shaker and then kept standing at 257C for several days before the first NMR diffusion measurements were made. Measurements were repeated over the period of 6 months for many representative samples. The diffusion data presented are the average between two and four independent measurements. Larger samples were prepared for the measurement of viscosity. Cryo-TEM. We prepared specimens for electron microscopy in the controlled environment vitrification system (CEVS), to assure fixed temperature and to avoid water loss from the solution during sample preparation. The technique and apparatus were described in detail by Bellare et al. (4). In brief, thin liquid films, about 0.25 mm thick, were prepared on perforated polymer/carbon films (‘‘holey carbon films’’), and thermally fixed in liquid ethane at its freezing point. The technique produces vitrified specimens; i.e., the water is supercooled and does not undergo crystallization during fixation. This way the original fluid microstructure is preserved, because component segregation and rearrangement are prevented. The vitrified specimens were stored under liquid nitrogen and then transferred to the electron microscope (JEOL 2000FX), using a Gatan 626 cryo-holder and its work station. Specimens were kept in the microscope and imaged at a temperature of about 01707C. We used 100 kV acceleration voltage with low elec˚ 2 to minimize tron exposures of less than 10 electrons per A electron-beam radiation damage. Images were recorded on Kodak SO-163 electron imaging film, developed for maximum electron speed. NMR self-diffusion measurements. Solution structures are studied by analyzing self-diffusion coefficients of water and surfactant molecules measured by the PGSE FT- 1H NMR technique at 257C (5). The NMR self-diffusion experiments were performed on a JEOL FX-60 FT NMR instrument operating at 60 MHz for protons. An external deuterium lock was used for all samples. The basic PGSE NMR

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experiment generates a spin echo by a 907 –1807 pulse sequence, with a magnetic field gradient G applied during a time d in the form of two pulses separated by a time D, one before and one after the 1807 pulse. Fourier transformation of the digitized second half of the echo separates the contributions to the echo from the different signals in the spectra. The amplitude I of a given NMR resonance is

S D H

I Å I1 f ( J)exp 0

S

2D d exp 0 g 2G 2d 2 D D 0 T2 3

DJ

, [1]

where f (J) is modulation effects of protons in question, g is the gyromagnetic ratio, and D is the self-diffusion coefficient of the molecule. T 2 is the transverse relaxation time for the nuclei. The first three factors of Eq. [1] can be collected into a constant I0 , which yield

H

S

I Å I0 exp 0 g 2G 2d 2 D D 0 or ln

S

I d Å 0g 2G 2d 2 D D 0 I0 3

D

d 3

DJ

.

[2]

[3]

In a typical FT-PGSE experiment, signal amplitudes were determined at constant D and G values for a series of d values between 10 and 90 ms. As soon as the molecular propagation is governed by the Gaussian distribution, a linear attenuation of the amplitude by Eq. [3] is obtained at constant G. Prior to each experimental series, the factor (gG) 2 was obtained by performing measurements on a sample with known self-diffusion coefficient (HDO in D2O) (6). Error limits in reported self-diffusion coefficients correspond to 80% statistical confidence intervals regarding random errors only. In studies of the water self-diffusion coefficient in the presence of a compound containing exchangeable hydrogens ( in this study OH in the surfactant ) , one needs to take into consideration that the water diffusion measures an average over the different environments of water. If the diffusion of the compound containing the exchangeable hydrogen is studied separately ( as, for example, using another peak in the spectrum) , the water diffusion can easily be corrected for the exchange. The observed selfdiffusion coefficient is ( 7 ) Dobs Å

2Xw Xs Dw / Ds, 2Xw / Xs 2Xw / Xs

[4]

where D w and D s are the self-diffusion coefficients of water and surfactant, respectively, and X denotes mole fraction. The values of water reported here have been corrected for

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this effect. The temperature was measured in a calibrated copper–constantan thermocouple that was placed in the NMR tube with glycerol. The temperature was measured immediately before and after each experiment. The temperatures given were accurate to within {0.57C. 1 H NMR linewidths. 1H NMR spectra for linewidth studies were recorded using a 907 pulse length at a resonance frequency of 60 MHz on the same spectrometer as that used for the diffusion measurements. The linewidths of the main methylene signals of the alkyl chain as well as the ethylene oxide chains were obtained in Hz at half-height of the recorded signals. The line broadening due to the magnetic inhomogeneity was estimated to be about 2 Hz. Rheology. Rheological properties of surfactant solution were investigated with freshly prepared samples as well as with samples that were few months old and no significant difference in experimental data was observed. Viscosity of dilute solutions ( õ1.4 wt% surfactant ) was measured by a capillary viscometer. Pure water was used as a reference liquid. For sample composition higher than 1.5 wt% surfactant, rheological properties of isotropic solution were examined by the Bohlin Visco 88 BV viscometer. This is a portable rotational viscometer of the Searle type. The inner cylinder rotates at a known speed, while the outer cylinder is fixed. The diameter of the measuring spindle can be varied between 14 and 30 mm, while that of the outer cylinder, between 15.4 and 33 mm. All measurements were carried out at 298 K. For the capillary method, the temperature variation was {0.17C and for the Bohlin viscometer, it was {0.57C. RESULTS AND DISCUSSION

Binary phase behavior. The pure surfactant is a viscous liquid at room temperature and easily soluble with water at all proportions, giving rise to a clear isotropic solution phase between its cloud point and the solidification temperatures. We have not detected the formation of any liquid crystalline phase for the system. This observation is in contradiction of the report published previously where the formation of liquid crystalline phases is reported for this type of surfactants (8). Cryo-TEM. It should be emphasized that our specimen preparation for electron microscopy did not include any addition of stains or fixatives that may have profound deleterious effects on the microstructure of the system studied. Specimen vitrification assured quenching of the original microstructure in the vitreous ice matrix. Vitreous ice can be easily identified visually, or by switching the microscope to the diffraction mode. At low concentrations, 0.5% (not shown) and 1.0% (Fig. 1a) cryo-TEM identifies a mixture of spheroidal micelles (arrowheads) and thread-like micelles (arrows). The latter are present in a fairly broad-length distribution. Some of the shorter ones are closed into rings.

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This situation is rather similar to the observations in other thread-like aqueous micellar systems, e.g., that of cetyltrimethylammonium chloride (CTAC)/sodium chloride/sodium salicylate (9) or dimeric surfactants (10, 11). At higher surfactant concentration, 5% (not shown) and 10% (Fig. 1b), the spheroidal micelles have disappeared. Only long thread-like micelles are visible. As is quite common in these vitrified specimens (12), suspended aggregates, such as micelles, are pushed toward the hole edges, where the liquid film prior to vitrification is thicker. In those thicker areas (T), one sees the projection of many overlapping micelles. In the thinner areas (t), individual micelles may be seen. The dark dots (arrowheads) are in all probability thread-like micelle ends aligned closely parallel to the electron beam. At 20% surfactant (Fig. 1c) the field of view is full with micelles. In the thicker areas (T) it is very difficult to make out individual micelles in the projection of many aggregates. As one goes toward the thinner areas, the threadlike nature of the micelles becomes quite clear (M). In thin areas, micelles pack themselves in single layers of ‘‘fingerprint’’ pattern. Similar packing of concentrated thread-like micelles had been seen before in sodium dodecyl sulfate (SDS)/NaCl solutions above room temperature (13). Surfactant self-diffusion. Surfactant self-diffusion is studied by following the spin-echo attenuation of the 1H signal of EO groups. The diffusion is a population-weighted average over all the states visited by the observed molecules within the NMR time scale (fast exchange). For surfactant systems with high critical micelle concentration (CMC), the monomers can have significant influence on the measured diffusion coefficient even at high surfactant concentration. The CMC of nonionic surfactants of the type CmEOn is very low in water as, for example, CMC Å 7.2 1 10 05 M for C12EO8 (14). The main determinant of CMC for the nonionic surfactants is the length of the alkyl chain; the temperature and the number of EO units have very little effect on the CMC. For our nonionic surfactant system with a long alkyl chain, the CMC (not measured) is expected to be lower than that of the C12EO8 . Therefore, the contribution of surfactant monomer diffusion to the measured surfactant diffusion is very small and is neglected. The semilogarithmic plot of log I/I0 vs d 2 ( D 0 d /3) (Eq. [3]), does not yield a straight line for dilute surfactant solutions (Fig. 2a). Instead, there is an initial rapid decrease of the signal intensity with increased gradient pulse, d, followed by a much slower reduction of the intensity at higher d values. The system shows the non-Gaussian diffusion for the surfactant concentration up to about 50 wt%, above which the curvature of the plot decreases due to the diminishing effect of fast moving species. For concentrated samples ( ú60 wt% surfactant), the rapid decrease of the attenuation is almost absent, and the spin-echo attenuation is close to a Gaussian distribution (Fig. 2b), which is also observed for

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FIG. 1. Cryo-TEM images of the surfactant solutions: (a) At 1.0%, a mixture of spheroidal micelles (arrowheads) and thread-like micelles (arrows) of a fairly broad length distribution; some of the shorter ones are closed into rings (R). (b) At 10% only long thread-like micelles are visible; in the thicker areas (T), one sees the projection of many overlapping micelles, while thinner areas (t), individual micelles may be seen (arrows). The dark dots (arrowheads) are in all probability thread-like micelles ends aligned closely parallel to the electron beam. (c) At 20%, in the thicker areas (T) it is very difficult to make out individual micelles; as one goes toward the thinner areas, the thread-like nature of the micelles becomes quite clear (M); in thin areas micelles pack themselves in single layers of ‘‘fingerprint’’ patterns, (bar Å 3D 100 nm).

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FIG. 2. Echo attenuation of methylene protons of surfactant EO groups vs gradient pulse (according to Eq. [3]) for aqueous solution of (a) 0.66 wt% and (b) 70.00 wt% surfactant at 298 K.

the neat liquid surfactant. The diffusion coefficient obtained for the surfactant molecules in aqueous solution with surfactant concentration ú60 wt% is not much different from that of the pure surfactant. In order to ensure that the diffusion coefficient, D s , obtained by Eq. [ 3 ] is not dependent on the D value used ( 140 ms ) in this study, we have obtained D s for one sample ( 22 wt% surfactant ) at D Å 140, 200, 300, and 400 ms and no significant variation in the diffusion value was observed. The self-diffusion coefficients of surfactant measured for the entire solution region are presented in Figs. 3a and 3b at 298 K. For aqueous solutions at concentrations below 50 wt% of surfactant, two diffusion coefficients—one fast, D sf ast , and the other slow, D sslow —are computed from the biexponential spin echo attenuation. Both types of aggregates have rapid exchange with surfactant monomers, but there is a slow exchange between the aggregates (within the NMR time scale). Between 50 and 60 wt% of surfactant, the fast-mov-

ing particles are still present in the solution as shown by the presence of the fast decaying component of the 1H signal. However, the signal decays very rapidly with the application of very small gradient pulses, thus making the determination of D sf ast difficult with any reasonable accuracy. At above 60 wt% of surfactant, the solution yields only one diffusion coefficient, D sslow , for the surfactant molecules. The fast-moving aggregates have a rather high diffusion coefficient (Fig. 3a) that decreases very slowly with increased surfactant concentration. In order to have some qualitative understanding regarding the size of the aggregates, we assume that the modified Stokes–Einstein equation is valid for the system and the hydrodynamic radius, R, of a micelle is calculated by the equation RÅ

kbT , 6phD

[5]

where kb is the Boltzmann constant, h is the solvent viscos-

FIG. 3. Surfactant self-diffusion coefficient (a) D s (fast) and (b) D s (slow) measured in aqueous solutions as a function of surfactant concentration at 298 K.

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ity, and D is the diffusion coefficient of surfactant. The ˚ at 0.6 wt% calculated micellar size varies between R Å 21 A ˚ at 40 wt% of surfactant. The of surfactant and R Å 30 A ˚ is the upper limit, since the obstruction effect radius of 30 A has not taken into account in Eq. [5]. Therefore, the results indicate that the fast diffusing particles are small surfactant aggregates which are spherical in shape where the alkyl chains are probably not fully extended. The aggregates do not undergo any substantial micellar growth even at a high surfactant concentration. It is to be noted that spherical micelles are not recorded by the cryo-TEM technique at concentration above 5 wt% of surfactant. It is quite possible that a small number of spheroidal micelles present at high surfactant concentration is totally obscured by the large number of thread-like micelles. Unlike the diffusion behavior of the fast-moving particles, the slow diffusion coefficient is strongly dependent on surfactant concentration (Fig. 3b). The D sslow values are found to decrease rapidly with increased surfactant concentration until a minimum value is attained. Thereafter there is a slow increase in the diffusion coefficient toward the value for the neat liquid surfactant. The results parallel the observations made previously for the Triton X-100–water (7) and C16EO7 water systems (15). We have explained the results as follows: the experimentally determined self-diffusion coefficient is dominated by the surfactant aggregate diffusion, i.e., diffusion of closed surfactant aggregates in a continuous aqueous medium at a low surfactant concentration, while at a high surfactant concentration the self-diffusion coefficient is dominated by molecular diffusion, i.e., the diffusion of surfactant molecules in surfactant-continuous domains. The explanation to the occurrence of a minimum in the selfdiffusion coefficient is that there is a gradual changeover in the diffusion mechanism. In the intermediate region, the situation is complex, but one can imagine that the monomer exchange between different aggregates becomes more and more important the higher the surfactant concentration, causing the different aggregates to gradually merge and form surfactant-continuous domains (16). ˚ is By Eq. [5], the hydrodynamic radius (R) of 115 A obtained for a micelle at 0.6 wt% of surfactant and the radius ˚ at 22 wt% of surfactant. Intermicellar increases to 626 A interactions can easily be neglected at high dilution (0.6%); ˚ obtained is much larger the hydrodynamic radius of 115 A than the total length of an extended surfactant molecule including the length of the hydrated water layer at the interface. The intermicellar interactions will affect the measured diffu˚ calculated at sion coefficient, but the micellar size of 626 A 22% of surfactant is much larger than can, reasonably, be accounted for by intermicellar interactions. The results strongly suggest that the slow moving particles that form at low surfactant concentration are nonspherical in shape, and that there is a substantial micellar growth with increased surfactant concentration.

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FIG. 4. Water self-diffusion coefficient, D w , measured in aqueous solutions as a function of surfactant concentration at 298 K.

The system appears to behave similarly to the C12EO3 , C12EO4 (17), C12EO5 (16), and C16EO7 (15) systems in addition to the Triton X-100 system mentioned earlier (7) where micellar growth, i.e., the formation of nonspherical micelles, has been reported. Water self-diffusion. Spin-echo attenuation of water protons follows the Gaussian distribution as in Fig. 2b over the entire concentration range. The diffusion coefficients of water, D w , obtained as a function of surfactant concentration are presented in Fig. 4. D w values are fast and decrease monotonically with increased surfactant concentration up to a high concentration, whereafter the self-diffusion coefficient remains almost constant with further increase of surfactant contents. The diffusion behavior of water is similar to that reported for solutions of several nonionic surfactant systems (17, 18). The results are adequately explained in terms of obstraction (by surfactant aggregates) (19) and hydration (of water by the EO head groups) effects (7, 19). 1 H NMR linewidths. Measurement of 1H NMR linewidths is a convenient way to monitor micellar growth and sphere-to-nonsphere transformation of micelles in surfactant micellar solutions (20). Thus, the sharp 1H NMR resonances ( q1 / 2 õ 10 Hz) of surfactant alkyl chains observed for spherical micelles become broad ( q1 / 2 ú 20 Hz) upon the formation of nonspherical micelles. This is due to the fact that as the surfactant aggregates increase in size it takes longer time for them to rotate, as well as for a surfactant molecule to diffuse around the curved micelle surface. The long correlation time for a large aggregate makes the 1H NMR linewidths larger than for simple solutions containing small colloidal particles (21). In addition to the correlation time, the actual linewidth is affected by the local-order parameter of the alkyl chains, S Å 1/2(3 cos 2u 0 1), where u is the angle between the normal to the micelle surface and a H-H vector. We have measured the 1H NMR linewidths of the main methylene ( 0CH20 ) signal of the surfactant alkyl chain, as

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1

TABLE 1 H NMR Linewidths of the Main -CH2- and -EO- Groups of the Surfactant of the Binary Surfactant–Water System at 300 K

Surfactant (wt%)

q1/2 (EO) Hz

q1/2 (CH2) Hz

8.8 8.1 7.4

20.6 26.1 CH3 and CH2 signals collapse to one broad signal CH3 and CH2 signals collapse to one broad signal 10.3 8.8

0.294 0.664 3.26 22.15

7.8

80 100

7.2 6.8

well as the methylene signal of the headgroup ethylene oxide (EO) chains for the isotropic solution phase. The results at 298 K are shown in Table 1. NMR signals of methylene protons of the both alkyl chain and the EO headgroup are narrow ( q1 / 2 É 7–9 Hz) for the neat liquid surfactant. But the addition of small amounts of surfactant (0.3%) to water leads to a large broadening of the CH2 signal of the alkyl chain ( q1 / 2 É 21 Hz). On increasing the surfactant content at above 2 wt%, the methylene and methyl signals of the alkyl chain overlap, resulting in one very broad signal and q1 / 2 value of the CH2 signal can no longer be measured accurately. At high surfactant contents, the linewidth again becomes narrower and approaches the value of the neat liquid surfactant. The broadening of the 1H NMR signals is an indication of micellar growth (nonspherical aggregates) in very dilute solutions. The nonspherical aggregates become very large with increased surfactant concentrations. The sharp proton signal (equivalent to that of neat surfactant) observed at very high surfactant concentrations is an indication that no appreciable number of surfactant aggregates are present in this concentration range. In contrast, the 1H signal of the methylene protons of EO chains is found to be narrow ( q1 / 2 É 7–9 Hz) in micellar solutions; the value is independent of surfactant concentration. Moreover, the linewidth is comparable to that measured in the neat surfactant. The observation that q1 / 2 values are small and concentration independent is in agreement with results reported for micellar solutions of several nonionic poly(oxyethylene) alkyl ether surfactant–water systems in which micellar growth is observed with increased surfactant concentration (16). In the liquid crystalline phases (16), ethylene oxide signals are also narrower than the alkyl methylene signals and even narrower than the methyl signal. The observations demonstrate that the order parameters of the EO chains are much smaller and the chain conformations are, on the average, less extended than the alkyl chains. Solution rheology. The viscosity of dilute aqueous solutions measured by a capillary viscometer is shown in Fig. 5

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as relative viscosity, ( hs / hw ), vs surfactant concentration in weight percent at 298 K. The relative viscosity is found to increase slowly at very high dilution. h / h0 É 10 at about 1 wt% surfactant, above which the increase is dramatic; for example, h / h0 É 60 at 1.2% surfactant. The viscosity data deviate significantly from the Einstein equation for hard spherical particles, h Å (1 / 2.5f ), h0

[6]

where f is the volume fraction of the spherical particles. In fact, the increase of h / h0 is much more drastic than predicted by the above equation. The rheological properties of viscous solutions at concentrations above É1 wt% of surfactant are examined by measuring shear stress, s, vs strain, g (Fig. 6). The dynamic viscosity, hr , at a shear rate g is obtained from the equation hr Å s / g. hr values measured by varying shear rate between 23.7 and 1210 s 01 are shown in Fig. 7. The dynamic viscosity, hr , calculated at a constant shear rate of 23.7 s 01 is also shown in Fig. 8 over the entire concentration range. The relation between shear stress and shear rate illustrated in Fig. 6 shows that there is an upward curvature for dilute samples and the upward curvature becomes steeper with increased surfactant concentration. However, the characteristics of the flow curve are changed for samples with concentration above about 30 wt% of surfactant. Here the curvature is reduced continuously with surfactant concentration, and at concentrations above 50 wt% the relation between shear stress and shear rate is linear passing through the origin. The upward curvature of flow curves is a characteristic of the non-Newtonian flow of the pseudoplastic type or the structural viscosity and the linearity of flow curves passing through the origin at zero shear rate is represented by the

FIG. 5. Relative viscosity ( hs / hw ), for dilute aqueous solutions of surfactant ( õ1.5 wt%) at 298 K.

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FIG. 8. Variation of dynamic viscosity at a constant shear of 23 s 01 measured for the entire composition range of the binary surfactant–water system at 298 K.

FIG. 6. Shear stress vs shear strain for aqueous solutions of surfactant at different constant surfactant concentration at 298 K. Surfactant concentration in wt% is shown.

Newtonian liquids. The neat liquid surfactant also behaves almost like a Newtonian liquid. The dynamic viscosity hr for samples illustrated in Fig. 7 shows that hr values decrease with increased s for samples between 2.5 and 25 wt% of surfactant and the decrease of viscosity is very fast at low shear rate. However, the viscosity attains a lower plateau value at high shear rate. Concentrated samples exhibit relatively low viscosity which is weakly dependent on shear rate. In the intermediate region ( õ É50 wt% surfactant), thread-like micelles overlap and entangle together like a network. The rheology of the entan-

gled chains in aqueous solutions is, of course, different from that of isolated chains. The large change of viscosity at a constant shear rate vs surfactant concentration is a clear indication that the system forms large aggregates in the intermediate concentration region. CONCLUDING REMARKS

Although no one technique can fully characterize a complex fluid system, a carefully selected combination of experimental techniques may give a correct and almost complete picture of the system. In this work we have combined a direct imaging technique, cryo-TEM, with indirect techniques— NMR self-diffusion and viscosity measurements. The former can give high-resolution images of the aggregates that form at a low to moderate concentration. As such it is excellent in showing the diversity of structure, in this case the coexistence of spheroidal and threadlike micelles, and their growth at higher concentrations. NMR has provided additional proof of the coexistence of two types of aggregates, and of the breakup of large aggregates at higher concentrations, where cryo-TEM is no longer practicable. Viscosity provided additional corroboration of the general picture that has emerged from the results of the former techniques. Moreover, the viscosity results have also pointed out to some interesting rheological properties of the surfactant solution that will, in all probability, have practical applications, as surfactants are based on renewable resources. ACKNOWLEDGMENTS I. Lind and C. Kang are acknowledged for technical assistance. This work is financed by the Swedish Natural Science Council (NFR).

REFERENCES FIG. 7. Dynamic viscosity measured for different constant concentration of surfactant in water as a function of shear rate at 298 K. Surfactant concentration is as in Fig. 6.

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1. Bock, K. J., Huber, L., and Scho¨berl, P., Tenside Deterg. 25, 86 (1988). 2. Janicke, W., Tenside Deterg. 25, 345 (1988).

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AQUEOUS OCTADECYLAMIDE OLIGO(OXYETHYLENE)ETHER 3. Harwigsson, I., Khan, A., and Hellsten, M., Tenside Surfactants Deterg. 30, 174 (1993). 4. Bellare, J. R., Davis, H. T., Scriven, L. E., and Talmon, Y., J. Electron Microsc. Tech. 10, 87 (1988). 5. Stilbs, P., Prog. Nucl. Magn. Reson. Spectrosc. 19, 1 (1987). 6. Mills, J. R., J. Phys. Chem. 77, 685 (1973). 7. Sadaghiani, A. S., and Khan, A., Langmuir 7, 898 (1991). 8. Heusch, R., Tenside Detergents 21, 298 (1984). 9. Clausen, T. M., Vinson, P. K., Minter, J. R., H. T. Davis, Talmon, Y., and Miller, W. G., J. Phys. Chem. 96, 474 (1992). 10. Zana, R., and Talmon, Y., Nature 362, 228 (1993). 11. Danino, D., Kaplun, A., Talmon, Y., and Zana, R., ‘‘ACS Symposium Series’’ (C. A. Herb and R. K. Prude´mme, Eds.), Vol. 578, chap. 6, p. 105. Am. Chem. Soc., Washington, DC, 1994.

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12. Talmon, Y., Colloids Surf. 19, 237 (1986). 13. Su¨ss, D., Cohen, Y., and Talmon, Y., in press (1995). 14. van Os, N. M., Haak, J. R., and Rupert, L. A. M., ‘‘Physico-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants,’’ p. 218. Elsevier, Amsterdam, 1993. 15. Kato, T., Terao, T., Tsukada, M., and Seimiya, T., J. Phys. Chem. 97, 3910 (1993). 16. Nilsson, P.-G., Wennerstro¨m, H., and Lindman, B., J. Phys. Chem. 87, 1377 (1983). 17. Nilsson, P.-G., and Lindman, B., J. Phys. Chem. 88, 4764 (1984). 18. Nilsson, P.-G., and Lindman, B., J. Phys. Chem. 87, 23 (1983). 19. Bell, G. M., Trans. Faraday Soc. 60, 1752 (1965). 20. Khan, A., Lindman, B., and Shinoda, K., J. Colloid Interface Sci. 128, 396 (1989). 21. Ulmius, J., and Wennerstro¨m, H., J. Magn. Reson. 28, 309 (1977).

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AP: Colloid