Phase Structure and Rheological Properties of a Mixed Zwit terionic/ Anionic Surfact ant Sys tern BY DOROTHY SAUL, GORDON J. T. TIDDY, BARBARA A. WHEELER, PHILLIP A. WHEELER* AND EDWIN WILLIS Unilever Research, Port Sunlight Laboratory, Port Sunlight, Wirral, Cheshire L62 4XN Received 5th July, 1973 The phase diagram of the mixed zwitterionic/anionic surfactant system hexadecyldimethyl- ammoniopropanesulphonate/sodium dodecyl sulphate/water has been determined in the aqueous region (> 90.0 % water) by optical microscopy and low angle X-ray scattering ; the phases observed were an isotropic surfactant solution and a hexagonal liquid crystalline phase. Some aqueous solutions were found to be viscoelastic and the composition boundaries of solutions with these properties were parallel to the phase boundaries.N.m.r. was used additionally to study the structure of the viscoelastic solutions and the results are interpreted using a model which involves the existence of both normal spherical niicelles and cylindrical micelles in equilibrium. In this laboratory the rheology and phase equilibria of various types of nixed surfactant solutions have been studied. Notably a mixed cationic/anionic surfactant system was investigated with particular emphasis on the aqueous region of the three component (cationic/anionic/water) phase diagram. In this system viscoelastic behaviour was observed for compositions close to the anionic side of a two liquid coexistence phase boundary and such samples showed line broadening in the high resolution nuclear magnetic resonance (n.m.r.) spectra.The n.m.r. data were interpreted as showing the existence of two types of micellar species, a normal spher- ical micelle with rapid motion and a second type with restricted molecular mobility, which is asymmetric, probably cylindrical. In this present study the phase diagram of the mixed zwitterionic/anionic surfactant system hexadecyldimethylammoniopropanesulphonate (HDPS)(I)/sodium dodecyl sulphate (SDS)/water has been determined in the aqueous region (> 90 % water) by optical microscopy and low angle X-ray scattering. The relationship between phase boundaries and the extent of viscoelasticity in the system has been examined. Addi- tionally, both high resolution and pulsed n.m.r. were used to study the structure of the viscoelastic solutions.CH3 I I CH3 C16H33Nf-CH2CH2CH20SO; (1) EXPERIMENTAL MATERIALS HDPS was prepared by the method of Clunie et aL3 and after recrystallisation from acetone+isopropanol was >99 % pure by elemental analysis and exhibited no minimum in the surface tension against log concentration plot (c.m.c. = 4.5 x mol dmV3 at 163164 VISCOELASTIC MIXED SURFACTANT SOLUTIONS 308 K). SDS was B.D.H. specially pure grade with c.m.c. = 8.3 x mol dm-j (at 298 K) and was used without further purification. DzO was p Chemicals 99.7 % pure grade. HzO was deionised and distilled. MEASUREMENTS Since HDPS is not soluble to the extent of 10 % by weight at temperatures below - 303 K all the experiments were done at 308 K. Mixtures containing up to 10 % total surfactant were prepared by adding water to the solids and leaving them to equilibrate for 2-6 days at 308 K.Phase studies were performed using a Reichert-Neopan polarising microscope fitted with a hot stage, and low angle X-ray measurements were obtained using Rigaku-Denki equip- ment. Rheological measurements under conditions of steady shear were obtained using a Haake Rotovisko viscometer fitted with the NV cup and bob assembly, and under conditions of both steady and oscillatory shear using a Weissenberg rheogoniometer 4* fitted with a 5.0 cm diameter cone and plate. The range of compositions where mixtures showed viscoelastic properties was determined visually by the titration technique described previously.2 The order of addition of the separate surfactant solutions was found to be important. The boundary of the viscoelastic region on the SDS side was approached from the SDS axis and vice versa on the HDPS side.Nuclear magnetic resonance measurements were made using a Bruker B-KR 322s 4-62 MHz pulse spectrometer and a Perkin-Elmer R12A 60 MHz high resolution spectro- meter. RESULTS AND DISCUSSION 1. OPTICAL OBSERVATIONS AND X - R A Y ANALYSIS The phase diagram of the aqueous region is shown in fig. 1. Samples in the liquid crystal (LC) and liquid+ LC regions were optically transparent when viewed through the microscope with unpolarised light but striations in incipient geometric texture (Rosevear’s classification 6, indicative of hexagonal phase structure were visible when the samples were viewed through crossed polarising lenses.SDS FIG. 1.-The dilute region of the HDPS/SDS/water phase diagram.TABLE l.-X-RAY SPACINGS AND STRUCTURE PARAMETERS FOR THE LIQUID CRYSTAL PHASE IN HDPS/SDS SYSTEM sample 6 % HDPS 4 % SDS 90 % H2O 7 % HDPS 3 % SDS 90 % H2O 10.5 % HDPS 4.5 % SDS 85 % H2O 14 % HDPS 6 % SDS 80 % H2O observed spacings dolnm (do/t/3)/nm (d0P)lnm 10.82 6.05 5.36 k0.15 k0.06 j--0.05 10.82 6.71 5.51 k0.15 k0.07 k0.06 9.95 5.66 4.75 10.14 k0.06 k0.05 8.81 4.94 4.32 k0.12 k0.05 kO.04 (do/ +/ 7 )/nm 3.98 & 0.03 4.23 & 0.03 3.57 rt 0.03 3.43 0.03 volume fraction of surfactan t Y O 0.108 k 0.002 0.108 & 0.002 0.172 k0.15 0.244 f 0.004 lattice parameter+ dpbm 12.29 k0.15 12.89 k0.15 11.18 k0.003 10.10 k0.15 * calculated according to the method described in ref. (9) cylinder diameter+ dclm 4.23 - + 0.10 4.45 & 0.10 4.87 - +0.12 5.24 k0.13 water spacing* dinterlm 8.06 0.25 8.44 f 0.25 6.31 f 0.27 4.86 0.28 surface hydrophilic area per group SL/nrn2 0.58 & 0.04 0.57 & 0.04 0.56 & 0.04 0.55 & 0.04166 VISCOELASTIC MIXED SURFACTANT SOLUTIONS X-ray diffraction data were obtained for composition ratios of HDPS : SDS of 7 : 3 and 6 : 4 at 10 % total concentration and for the ratio 7 : 3 at concentrations of 15 and 20 %.The spacings (table 1) show that the LC has hexagonal phase structure i.e. hexagonally packed cylinders. Table 1 also shows values of the lattice parameters d,,, the diameter of the surfactant cylinders do the resulting inter cylinder water spacing tiinter and the surface area available for each polar head group, S.For a weight ratio of HDPS : SDS of 7 : 3 (mole ratio 1.7 : 1) the cylinder diameter increases from 4.55 nm to 5.25 nm when the concentration is increased from 10 to 20 %. This 16 % increase in diameter is probably due to an increase in ‘‘ trans ” chain conformations, or to a decrease in the aqueous content of the head group environment. It is noteable that for the potassium oleate/water system Ekwall et aL7 found that doubling the potassium oleate concentration in the hexagonal mesophase region increased the cylinder diameter by less than 1 %. The average area per polar head group was, within experimental error, invariant with concentration. 15- 10- pc 1 F 5- 2. VISCOELASTIC PROPERTIES The boundary of viscoelastic behaviour determined by observing recoil in swirled solutions is shown in fig.1 and extends beyond the mesophase regions. All samples within the LC and liquid+LC boundaries were viscoelastic. As mentioned earlier the method of solution preparation had an important bearing on the viscoelastic behaviour. Dilution of a 5 % mixture with a weight ratio of HDPS : SDS of 7 : 3 to a concentration of 1 % gave a viscoelastic solution, however, the viscoelasticity slowly decayed, finally disappearing after several days. A 1 % mixture of identical composition prepared by mixing separate 1 % solutions of the surfactants did not show viscoelastic properties. The lowest concentration at which viscoelasticity was observed on mixing separate solutions was about 2 %. 2ol 1:- ISOTROPIC PHASE 5 K L+LC + LC - ~ L + L C X I R O ~ I C YHDPSi 4 a 3 A 4 R S D S 9 8 7 6 5 4 3 2 1 0 composition FIG.2.-Viscosity, 9, against composition for HDPS/SDS samples at 10.0 % total surfactant con- centration at shear rate = 17.7 s-’ : 0, original results ; A, repeated results.D. SAUL, G . TIDDY, B . WHEELER, P . WHEELER AND E. WILLIS 167 50 3. RHEOLOGICAL MEASUREMENTS Samples whose compositions lay within the LC and liquid + LC regions were shear thinning, viscoelastic and exhibited normal forces (i.e. when sheared they exerted a force in the direction normal to the direction of flow). Values of the first normal stress difference (a,) were calculated from measurements of normal force. Plots of viscosity (q) and of o1 against shear rate (9) were obtained from measurements made under steady shear conditions.The plots of viscosity against composition across sections of the phase diagram illustrated in fig. 2-6 were obtained from the former. - ISOTROPIC "T 0 b composition FIG. 3.-Viscosity, 77, against composition for HDPS/SDS samples at 10.0 % total surfactant con- centration : 0, shear rate = 1.12 s-' ; A, shear rate = 7.06 s-l. Plots of dynamic viscosity (q') and of dynamic rigidity (G') against angular fre- quency (0) and against 2w were obtained from measurements made under oscillating shear. The plots of q against 9 and of q' against w or 2w and those of crl against w or 2co and those of c1 against 9 and of G' against w or 2w were expected to superimpose. For the 1 % and 5 % concentrations the position of the maximum in the viscosity against composition curves is independent of shear rate, however, at a concentration of 8 % the maximum shifts to a higher HDPS : SDS ratio as the shear rate is increased.At 10 % concentration three maxima were observed, that corresponding to the LC region of the phase diagram was independent of shear rate and reproducible. The other two maxima which roughly correspond to the liquid + LC/isotropic solution phase boundaries on each side of the LC region were only apparent at the higher shear rates and were difficult to reproduce. The oscillatory and the steady shear data were168 VISCOELASTIC MIXED SURFACTANT SOLUTIONS not superimposable in these regions which together with the difficulties of repro- ducibility suggests that the equilibrium of the systems was being disturbed by shearing.Pulsed n.m.r. measurements on a solution of composition 3.5 % HDPS, 1.5 % SDS, 95 % D20 gave the value for the spin-lattice relaxation time, TI of ca. 5 x 10-l s and a spin-spin relaxation time, T2 of ca. 2 x s. High resolution spectra of this system showed broad lines (10-20 Hz) whereas a sample of concentration 1 %, prepared by mixing 1 % solutions of the individual surfactants showed sharp reson- ances. A sample of identical composition to the latter, prepared by diluting the viscoelastic 5 % sample initially gave broad n.m.r. lines which gradually became sharper over a period of several days. composition FIG. 4.-Viscosity, 7 against composition for HDPs/SDS samples at 8.0 % total surfactant concen- tration; 0, shear rate = 2.81 s-' ; A, shear rate = 7.06 s-l; 0, shear rate = 17.74 s-l.N.m.r. measurements were also made on samples of 5 % concentration but of varying composition. Fig. 5 illustrates how n.m.r. line width, viscosity and first normal stress difference vary with composition. It is noticeable that each of the curves exhibits a maximum at approximately the same composition, indicating that the entities responsible for the rheological properties are also responsible for the broadening of the n.m.r. lines. Also the plot of linewidth of the hydrocarbon chain -CH2-- peak against conceptration (fig. 7) shows a definite break in the region of 2.0-2.5 % surfactant above which the slope is linear. The discontinuity appears to represent a c.m,c. for the micelles with restricted molecular motion and the positionD.SAUL, G. TIDDY, B . WHEELER, P. WHEELER AND E . WILLIS 169 of the break corresponds to the minimum concentration at which viscoelasticity is observed. The slow decay of viscoelasticity on dilution from 5.0 % total surfactant shows that the units responsible for the viscoelastic behaviour have breakdown times which are much longer than the time scales usually associated with micellar equilibria. composition hydrocarbon chain (0) against composition of 5 % HDPs/SDS solutions. FIG. 5.-Viscosity (a), first normal stress difference ( x ) at = 2.81 s-I and n.m.r. line width of the I I I I 1 4.0 728 1 .o HDPS 3.2 3.4 3.6 SDS 1 8 16 1 *4 composition (%) FIG. 6.-Viscosity, 7, against composition for HDPSlSDS samples at 1.0 % total surfactant con- centration : 0, shear rate = 2570 s-' ; A, shear rate = 1235 s-' ; 0, shear rate = 428 s-l.170 VISCOELASTIC MIXED SURFACTANT SOLUTIONS 2og O' ; 4 i k 4 k-" % HDPSISDS FIG.7.-N.m.r. line width (hydrocarbon chain -CH2-- peak) against surfactant concentration of HDPS/SDS mixtures (ratio HDPS : SDS = 3.5 : 1.5). The n.m.r. spin-lattice (TI) and spin-spin (Tz) relaxation times recorded for a visco- elastic 5.0 % surfactant solution were not equal as would be the case for normal liquid motions which have correlation times (2,) of the order of 10-l' s. Non-equality of TI and T2 is observed for slower molecular motions but for any particular system there are fixed pairs of values of Tl and T2 which correspond to a given correlation time. The experimental results found for this system do not correspond to such a pair and by similar arguments to those used for the cationic/anionic/water system more than one micellar species may be present one of which has slow molecular motion (correlation time s).It seems likely that the micelles with restricted molecular motion are cylindrical and could be regarded as precursors of the hexagonal LC phase. Such micelles would be expected to give rise to the observed rheological properties. Further, the slow decay of viscoelasticity on dilution is probably associated with the breakdown of the cylindrical micelles present at 5 % concentration into the spherical micelles which are present in 1 % concentrations at equilibrium. Studies of the kinetics of this process are continuing.H. A. Barnes, A. R. Eastwood and B. Yates, RheoZogica Acta, in press. C. A. Barker, D. Saul, G. J. T. Tiddy, B. A. Wheeler, E. Willis, J.C.S. Faraday I, 1974,70,154. J. S. Clunie, J. M. Corkhill, T. F. Goodman and C. P. Ogden, Trans. Faraday Soc., 1967, 63, 505. K. Walters, Basic Concepts and Formulae for the Rheogoniometer (Sangamo Controls Ltd., 1969). K. Walters and R. A. Kemp, Deformation and Flow of High Polymers, ed. R. Wetton and R. Whorlow (McMillan, London, 1968), chap. 18, p. 237. F. B. Rosevear, J. Amer. Oil Chem. SOC., 1954, 31, 635 (fig. 24). ' P. Ekwall, L. Maodell and K. Fontell, J. CoZZoid Interface Sci., 1969, 31, 508. Phase Structure and Rheological Properties of a Mixed Zwit terionic/ Anionic Surfact ant Sys tern BY DOROTHY SAUL, GORDON J.T. TIDDY, BARBARA A. WHEELER, PHILLIP A. WHEELER* AND EDWIN WILLIS Unilever Research, Port Sunlight Laboratory, Port Sunlight, Wirral, Cheshire L62 4XN Received 5th July, 1973 The phase diagram of the mixed zwitterionic/anionic surfactant system hexadecyldimethyl- ammoniopropanesulphonate/sodium dodecyl sulphate/water has been determined in the aqueous region (> 90.0 % water) by optical microscopy and low angle X-ray scattering ; the phases observed were an isotropic surfactant solution and a hexagonal liquid crystalline phase. Some aqueous solutions were found to be viscoelastic and the composition boundaries of solutions with these properties were parallel to the phase boundaries. N.m.r. was used additionally to study the structure of the viscoelastic solutions and the results are interpreted using a model which involves the existence of both normal spherical niicelles and cylindrical micelles in equilibrium.In this laboratory the rheology and phase equilibria of various types of nixed surfactant solutions have been studied. Notably a mixed cationic/anionic surfactant system was investigated with particular emphasis on the aqueous region of the three component (cationic/anionic/water) phase diagram. In this system viscoelastic behaviour was observed for compositions close to the anionic side of a two liquid coexistence phase boundary and such samples showed line broadening in the high resolution nuclear magnetic resonance (n.m.r.) spectra. The n.m.r. data were interpreted as showing the existence of two types of micellar species, a normal spher- ical micelle with rapid motion and a second type with restricted molecular mobility, which is asymmetric, probably cylindrical.In this present study the phase diagram of the mixed zwitterionic/anionic surfactant system hexadecyldimethylammoniopropanesulphonate (HDPS)(I)/sodium dodecyl sulphate (SDS)/water has been determined in the aqueous region (> 90 % water) by optical microscopy and low angle X-ray scattering. The relationship between phase boundaries and the extent of viscoelasticity in the system has been examined. Addi- tionally, both high resolution and pulsed n.m.r. were used to study the structure of the viscoelastic solutions. CH3 I I CH3 C16H33Nf-CH2CH2CH20SO; (1) EXPERIMENTAL MATERIALS HDPS was prepared by the method of Clunie et aL3 and after recrystallisation from acetone+isopropanol was >99 % pure by elemental analysis and exhibited no minimum in the surface tension against log concentration plot (c.m.c.= 4.5 x mol dmV3 at 163164 VISCOELASTIC MIXED SURFACTANT SOLUTIONS 308 K). SDS was B.D.H. specially pure grade with c.m.c. = 8.3 x mol dm-j (at 298 K) and was used without further purification. DzO was p Chemicals 99.7 % pure grade. HzO was deionised and distilled. MEASUREMENTS Since HDPS is not soluble to the extent of 10 % by weight at temperatures below - 303 K all the experiments were done at 308 K. Mixtures containing up to 10 % total surfactant were prepared by adding water to the solids and leaving them to equilibrate for 2-6 days at 308 K.Phase studies were performed using a Reichert-Neopan polarising microscope fitted with a hot stage, and low angle X-ray measurements were obtained using Rigaku-Denki equip- ment. Rheological measurements under conditions of steady shear were obtained using a Haake Rotovisko viscometer fitted with the NV cup and bob assembly, and under conditions of both steady and oscillatory shear using a Weissenberg rheogoniometer 4* fitted with a 5.0 cm diameter cone and plate. The range of compositions where mixtures showed viscoelastic properties was determined visually by the titration technique described previously.2 The order of addition of the separate surfactant solutions was found to be important. The boundary of the viscoelastic region on the SDS side was approached from the SDS axis and vice versa on the HDPS side.Nuclear magnetic resonance measurements were made using a Bruker B-KR 322s 4-62 MHz pulse spectrometer and a Perkin-Elmer R12A 60 MHz high resolution spectro- meter. RESULTS AND DISCUSSION 1. OPTICAL OBSERVATIONS AND X - R A Y ANALYSIS The phase diagram of the aqueous region is shown in fig. 1. Samples in the liquid crystal (LC) and liquid+ LC regions were optically transparent when viewed through the microscope with unpolarised light but striations in incipient geometric texture (Rosevear’s classification 6, indicative of hexagonal phase structure were visible when the samples were viewed through crossed polarising lenses. SDS FIG. 1.-The dilute region of the HDPS/SDS/water phase diagram.TABLE l.-X-RAY SPACINGS AND STRUCTURE PARAMETERS FOR THE LIQUID CRYSTAL PHASE IN HDPS/SDS SYSTEM sample 6 % HDPS 4 % SDS 90 % H2O 7 % HDPS 3 % SDS 90 % H2O 10.5 % HDPS 4.5 % SDS 85 % H2O 14 % HDPS 6 % SDS 80 % H2O observed spacings dolnm (do/t/3)/nm (d0P)lnm 10.82 6.05 5.36 k0.15 k0.06 j--0.05 10.82 6.71 5.51 k0.15 k0.07 k0.06 9.95 5.66 4.75 10.14 k0.06 k0.05 8.81 4.94 4.32 k0.12 k0.05 kO.04 (do/ +/ 7 )/nm 3.98 & 0.03 4.23 & 0.03 3.57 rt 0.03 3.43 0.03 volume fraction of surfactan t Y O 0.108 k 0.002 0.108 & 0.002 0.172 k0.15 0.244 f 0.004 lattice parameter+ dpbm 12.29 k0.15 12.89 k0.15 11.18 k0.003 10.10 k0.15 * calculated according to the method described in ref.(9) cylinder diameter+ dclm 4.23 - + 0.10 4.45 & 0.10 4.87 - +0.12 5.24 k0.13 water spacing* dinterlm 8.06 0.25 8.44 f 0.25 6.31 f 0.27 4.86 0.28 surface hydrophilic area per group SL/nrn2 0.58 & 0.04 0.57 & 0.04 0.56 & 0.04 0.55 & 0.04166 VISCOELASTIC MIXED SURFACTANT SOLUTIONS X-ray diffraction data were obtained for composition ratios of HDPS : SDS of 7 : 3 and 6 : 4 at 10 % total concentration and for the ratio 7 : 3 at concentrations of 15 and 20 %.The spacings (table 1) show that the LC has hexagonal phase structure i.e. hexagonally packed cylinders. Table 1 also shows values of the lattice parameters d,,, the diameter of the surfactant cylinders do the resulting inter cylinder water spacing tiinter and the surface area available for each polar head group, S. For a weight ratio of HDPS : SDS of 7 : 3 (mole ratio 1.7 : 1) the cylinder diameter increases from 4.55 nm to 5.25 nm when the concentration is increased from 10 to 20 %.This 16 % increase in diameter is probably due to an increase in ‘‘ trans ” chain conformations, or to a decrease in the aqueous content of the head group environment. It is noteable that for the potassium oleate/water system Ekwall et aL7 found that doubling the potassium oleate concentration in the hexagonal mesophase region increased the cylinder diameter by less than 1 %. The average area per polar head group was, within experimental error, invariant with concentration. 15- 10- pc 1 F 5- 2. VISCOELASTIC PROPERTIES The boundary of viscoelastic behaviour determined by observing recoil in swirled solutions is shown in fig. 1 and extends beyond the mesophase regions. All samples within the LC and liquid+LC boundaries were viscoelastic.As mentioned earlier the method of solution preparation had an important bearing on the viscoelastic behaviour. Dilution of a 5 % mixture with a weight ratio of HDPS : SDS of 7 : 3 to a concentration of 1 % gave a viscoelastic solution, however, the viscoelasticity slowly decayed, finally disappearing after several days. A 1 % mixture of identical composition prepared by mixing separate 1 % solutions of the surfactants did not show viscoelastic properties. The lowest concentration at which viscoelasticity was observed on mixing separate solutions was about 2 %. 2ol 1:- ISOTROPIC PHASE 5 K L+LC + LC - ~ L + L C X I R O ~ I C YHDPSi 4 a 3 A 4 R S D S 9 8 7 6 5 4 3 2 1 0 composition FIG. 2.-Viscosity, 9, against composition for HDPS/SDS samples at 10.0 % total surfactant con- centration at shear rate = 17.7 s-’ : 0, original results ; A, repeated results.D.SAUL, G . TIDDY, B . WHEELER, P . WHEELER AND E. WILLIS 167 50 3. RHEOLOGICAL MEASUREMENTS Samples whose compositions lay within the LC and liquid + LC regions were shear thinning, viscoelastic and exhibited normal forces (i.e. when sheared they exerted a force in the direction normal to the direction of flow). Values of the first normal stress difference (a,) were calculated from measurements of normal force. Plots of viscosity (q) and of o1 against shear rate (9) were obtained from measurements made under steady shear conditions. The plots of viscosity against composition across sections of the phase diagram illustrated in fig.2-6 were obtained from the former. - ISOTROPIC "T 0 b composition FIG. 3.-Viscosity, 77, against composition for HDPS/SDS samples at 10.0 % total surfactant con- centration : 0, shear rate = 1.12 s-' ; A, shear rate = 7.06 s-l. Plots of dynamic viscosity (q') and of dynamic rigidity (G') against angular fre- quency (0) and against 2w were obtained from measurements made under oscillating shear. The plots of q against 9 and of q' against w or 2w and those of crl against w or 2co and those of c1 against 9 and of G' against w or 2w were expected to superimpose. For the 1 % and 5 % concentrations the position of the maximum in the viscosity against composition curves is independent of shear rate, however, at a concentration of 8 % the maximum shifts to a higher HDPS : SDS ratio as the shear rate is increased.At 10 % concentration three maxima were observed, that corresponding to the LC region of the phase diagram was independent of shear rate and reproducible. The other two maxima which roughly correspond to the liquid + LC/isotropic solution phase boundaries on each side of the LC region were only apparent at the higher shear rates and were difficult to reproduce. The oscillatory and the steady shear data were168 VISCOELASTIC MIXED SURFACTANT SOLUTIONS not superimposable in these regions which together with the difficulties of repro- ducibility suggests that the equilibrium of the systems was being disturbed by shearing. Pulsed n.m.r. measurements on a solution of composition 3.5 % HDPS, 1.5 % SDS, 95 % D20 gave the value for the spin-lattice relaxation time, TI of ca.5 x 10-l s and a spin-spin relaxation time, T2 of ca. 2 x s. High resolution spectra of this system showed broad lines (10-20 Hz) whereas a sample of concentration 1 %, prepared by mixing 1 % solutions of the individual surfactants showed sharp reson- ances. A sample of identical composition to the latter, prepared by diluting the viscoelastic 5 % sample initially gave broad n.m.r. lines which gradually became sharper over a period of several days. composition FIG. 4.-Viscosity, 7 against composition for HDPs/SDS samples at 8.0 % total surfactant concen- tration; 0, shear rate = 2.81 s-' ; A, shear rate = 7.06 s-l; 0, shear rate = 17.74 s-l. N.m.r. measurements were also made on samples of 5 % concentration but of varying composition.Fig. 5 illustrates how n.m.r. line width, viscosity and first normal stress difference vary with composition. It is noticeable that each of the curves exhibits a maximum at approximately the same composition, indicating that the entities responsible for the rheological properties are also responsible for the broadening of the n.m.r. lines. Also the plot of linewidth of the hydrocarbon chain -CH2-- peak against conceptration (fig. 7) shows a definite break in the region of 2.0-2.5 % surfactant above which the slope is linear. The discontinuity appears to represent a c.m,c. for the micelles with restricted molecular motion and the positionD. SAUL, G. TIDDY, B . WHEELER, P. WHEELER AND E . WILLIS 169 of the break corresponds to the minimum concentration at which viscoelasticity is observed.The slow decay of viscoelasticity on dilution from 5.0 % total surfactant shows that the units responsible for the viscoelastic behaviour have breakdown times which are much longer than the time scales usually associated with micellar equilibria. composition hydrocarbon chain (0) against composition of 5 % HDPs/SDS solutions. FIG. 5.-Viscosity (a), first normal stress difference ( x ) at = 2.81 s-I and n.m.r. line width of the I I I I 1 4.0 728 1 .o HDPS 3.2 3.4 3.6 SDS 1 8 16 1 *4 composition (%) FIG. 6.-Viscosity, 7, against composition for HDPSlSDS samples at 1.0 % total surfactant con- centration : 0, shear rate = 2570 s-' ; A, shear rate = 1235 s-' ; 0, shear rate = 428 s-l.170 VISCOELASTIC MIXED SURFACTANT SOLUTIONS 2og O' ; 4 i k 4 k-" % HDPSISDS FIG.7.-N.m.r. line width (hydrocarbon chain -CH2-- peak) against surfactant concentration of HDPS/SDS mixtures (ratio HDPS : SDS = 3.5 : 1.5). The n.m.r. spin-lattice (TI) and spin-spin (Tz) relaxation times recorded for a visco- elastic 5.0 % surfactant solution were not equal as would be the case for normal liquid motions which have correlation times (2,) of the order of 10-l' s. Non-equality of TI and T2 is observed for slower molecular motions but for any particular system there are fixed pairs of values of Tl and T2 which correspond to a given correlation time. The experimental results found for this system do not correspond to such a pair and by similar arguments to those used for the cationic/anionic/water system more than one micellar species may be present one of which has slow molecular motion (correlation time s). It seems likely that the micelles with restricted molecular motion are cylindrical and could be regarded as precursors of the hexagonal LC phase. Such micelles would be expected to give rise to the observed rheological properties. Further, the slow decay of viscoelasticity on dilution is probably associated with the breakdown of the cylindrical micelles present at 5 % concentration into the spherical micelles which are present in 1 % concentrations at equilibrium. Studies of the kinetics of this process are continuing. H. A. Barnes, A. R. Eastwood and B. Yates, RheoZogica Acta, in press. C. A. Barker, D. Saul, G. J. T. Tiddy, B. A. Wheeler, E. Willis, J.C.S. Faraday I, 1974,70,154. J. S. Clunie, J. M. Corkhill, T. F. Goodman and C. P. Ogden, Trans. Faraday Soc., 1967, 63, 505. K. Walters, Basic Concepts and Formulae for the Rheogoniometer (Sangamo Controls Ltd., 1969). K. Walters and R. A. Kemp, Deformation and Flow of High Polymers, ed. R. Wetton and R. Whorlow (McMillan, London, 1968), chap. 18, p. 237. F. B. Rosevear, J. Amer. Oil Chem. SOC., 1954, 31, 635 (fig. 24). ' P. Ekwall, L. Maodell and K. Fontell, J. CoZZoid Interface Sci., 1969, 31, 508.