The Kinetics of Formation and Breakdown of Pre-hexagonal Phase Aggregates in Fluid-isotropic Amphiphile Solutions

— When the composition of a fluid isotropic solution is altered so that it approaches a liquid crystal phase boundary a marked increase is often observed in the solution viscosity. Viscoelastic properties can occur also, and this suggests that some long range order or network is present in the system. From the line broadening observed in high-resolution NMR spectra [a, b], we have recently demonstrated that these changes are associated with the amphiphile molecules. The amphiphile molecules form long cylindrical micelles, and when these are packed in a hexagonal array the hexagonal liquid crystal phase is formed. The cylindrical micelles can be regarded as pre-hexagonal phase aggregates. In this paper we describe an investigation into the formation and breakdown kinetics and mechanism for these aggregates in solutions containing sodium dodecyl sulphate and hexadecyl dimethyl ammonio propane sulphonate. Visco-elastic solutions occur in solutions adjacent to the hexagonal phase boundary [b]. On dilution with pure amphiphile solutions or water the cylindrical micelles break up to form spherical micelles in a time varying from minutes to several days. The kinetics of the process can be measured by NMR line broadening, or light scattering ; a comparison of the results from the two techniques indicates that the cylindrical micelles dissociate in one step, and not by gradual loss of monomers. The formation of the cylindrical micelles can also be followed using NMR or light scattering. When viscoelastic solutions are prepared by mixing pure amphiphile solutions a slow build-up in the concentration of the cylindrical micelles is observed, again the changes occur over a period varying from minutes to weeks. The kinetics are complex and are dependent on amphiphile composition. In solutions at equilibrium two dynamic processes that exchange amphiphile monomers between the two types of micelle are present. One is the rapid exchange of monomers between the spherical micelles and the prehexagonal phase aggregates. The second is the continual build-up and breakdown of the pre-hexagonal phase aggregates themselves. [e] BARKER, C. A., SAUL, D., TIDDY, G. J. T., WHEELER, B. A. and WILLIS, E„ J. Chem. Soc. Faraday Trans. 1 70 (1974) 154. [b] SAUL, D., TIDDY, G. J. T., WHEELER, B. A., WHEELER, P. A. and WILLIS, E., / . Chem. Soc. Faraday Trans. 1 70 (1974) 163. JOURNAL DE PHYSIQUE Colloque Cl, supplément au n° 3, Tome 36, Mars 1975, page Cl-167 Résumé. — Quand on change la composition d'une solution liquide isotrope pour passer à une phase cristal liquide, on observe souvent une nette augmentation de la viscosité. Un comportement viscoélastique peut aussi apparaître, ce qui suggère l'existence d'un ordre à longue distance ou réseau dans le système. A partir de l'élargissement observé dans les spectres NMR haute résolution [a, b], on a démontré récemment que ces changements sont dus aux molécules amphiphiles. Les molécules amphiphiles forment des micelles en forme de longs cylindres et, quand ceux-ci s'organisent en réseau hexagonal, la phase cristal liquide hexagonal est formée. Les micelles cylindriques peuvent être considérées comme des agrégats prétransitionnels de la phase hexagonale. Dans ce papier, nous essayons de décrire le mécanisme et la cinétique de la formation et disparition de ces agrégats dans des solutions contenant du sodium dodécyl sulphate et de l'hexadécyl diméthyl ammonio propane sulphonate. Des solutions viscoélastiques apparaissent pour des solutions au voisinage de la forme hexagonale [b]. En diluant une des solutions d'un amphiphile pur ou de l'eau, les micelles cylindriques se défont pour former des micelles sphériques dans un temps variant de quelques minutes à plusieurs jours. La cinétique de cette transformation peut être mesurée par l'élargissement des raies RMN ou par diffusion de la lumière. Une comparaison des résultats obtenus par ces deux techniques montre que les micelles se dissocient en une seule étape et non par une perte progressive de ménomères. La formation des micelles cylindriques peut aussi être observée par RMN ou diffusion de la lumière. Quand les solutions viscoélastiques sont préparées en ajoutant des solutions d'amphiphile pure, on observe un accroissement lent de la concentration des micelles cylindriques qui se produit aussi en un temps allant de quelques minutes à une semaine. Les cinétiques sont complexes et dépendent de la composition en amphiphile. Dans les solutions à l'équilibre, il y a deux types de procédés dynamiques qui permettent l'échange de monomères amphiphiles entre les deux types de micelles. L'un est l'échange rapide de monomères entre les micelles sphériques et les agrégats prétransitionnels de la phase hexagonale. Le second est la continuelle formation et déformation de ces agrégats prétransitionnels. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975132 G. J. T. TIDDY AND P. A. WHEELER In liquid crystalline (1. C.) phases the molecules are arranged in an ordered structure which exists over very large distances compared to the molecular dimensions. However, the molecules themselves undergo rapid translational and rotational motions. In most normal isotropic liquids the molecules also undergo rapid motions, and no long range order exists. Medium or long range order does occur in a few isotropic liquids and these are normally liquids that can be changed into an 1. c. phase by small temperature or composition alterations. The existence of this order can be inferred from the fact that some of their properties are similar to those of the adjacent l. c. phase. For example, Hayward and Packer [ l ] have inferred the existence of weak long range order in liquid N-(p-methoxy-benzy1idine)-p'-butyl-aniline (MBBA) from the time dependence of NMR measurements of translational diffusion. This is presumably due to the presence of pre-l. c. aggregates. In most lyotropic 1. c. systems, and particularly those containing soaps or detergents, the isotropic solutions formed by adding solvent (water) to the 1. c. phases contain surfactant molecules aggregated in micelles. These micelles have a spherical shape at low concentrations, but many authors have now reported that a micelle conformational change occurs at concentrations well above the critical micelle concentration (c. m. C.), (for examples see ref. [2-61). This conformztional change is thought to involve an increase in the micellar aggregation number resulting in the formation of cylindrical or other shaped micelles and these can be regarded as pre-l. c. aggregates. In previous studies of mixed surfactant systems we have reported that isotropic solutions adjacent to phase boundaries can exhibit properties indicative of medium or long range order 17, 81. In these solutions small changes in surfactant composition can cause large changes in solution viscosity, and viscoelastic properties can be observed. The viscosity changes were also correlated with changes in surfactant molecular motions by n. m. r. measurements. The change in solution properties was attributed to the presence of large cylindrical micelles. In addition, the n. m. r. measurements could only be explained by the presence of a large number of small (spherical) micelles, and it was proposed that a dynamic equilibrium occurred between the two types of micelle 17, 81. The large cylindrical micelles can be regarded as the components of the hexagonal phase, and a study of their properties should contribute to a greater understanding of the properties of this phase. In the mixed surfactant system hexadecyl dimethyl ammonio propane sulphonate (HDPS-I)/Sodium dodecyl sulphate (SDS) the properties of the solutions were dependent on the method of preparation, and were time dependent [g, 91. CH3 l CH~(CH,)~ ,N@(cH,)~so? I . I CH3 Preparation of dilute surfactant solutions by the addition of distilled water to concentrated viscoelastic solutions (see Fig. 1) gave samples with initial viscoelasticity which decayed over a period varying from hours to weeks. The same composition prepared by mixing dilute surfactant solutions was not viscoelastic. The time dependent properties were attributed to the slow breakdown of the cylindrical micelles responsible for the viscoelastic behaviour. The presence of cylindrical micelles gave rise to a broadening of the surfactant n. m. r. resonances, and the change in this broadening was used to measure the kinetics of the process [9]. Similar changes were also measured using the time dependent intensity of the scattered incident light, and combination of the light scattering and n. m. r. data enable the mechanism of cylindrical micelle breakdown to be deduced (see below). For some samples with higher surfactant concentrations it was also observed that the viscosity and viscoelasticity increased with time after initial mixing. This is attributed to the the slow formation of the cylindrical micelles [9, 101. The build-up process is more complex than the reverse process and possibly involves two distinct stages [10]. Since a dynamic equilibrium exists between the small micelles and the large cylindrical micelles, information on the build-up process can be inferred from a knowledge of the breakdown rates and the equilibrium properties. In this paper we report measurements of the breakdown process at different temperatures. The rates were measured using the n. m. r. and light scattering techniques reported previously [9], and the results are used to develop further the model for the breakdown process. In addition, we report some initial measurements of the rate of formation of the large micelles. 1. Experimental. 1 . l MATERIALS. -HDPS was prepared by the method of Clunie et al. [I l ] ; after recrystallization from acetone and isopropanol the material was 99 % pure by elemental analysis and exhibited no minimum in the surface tension vs log concentration plot (cmc = 4.5 X 10-5 m01 dm-3 at 308 K). SDS was BDH specially pure grade with cmc = 8.3 X 10m3 m01 dm-3 at 298 K, and was used without further purification. D,O was P Scientific Chemicals 99.7 % pure grade. H,O was deionised and distilled. PRE-HEXAGONAL AGGREGATE IS IN AMPHIPHILE SOLUTIONS Cl-169 1.2 MEASUREMENTS. 1.2.1 N. M. R. Solutions of HDPS in D,O and SDS in D,O were prepared separately and mixed in appropriate volumes to give a range of samples within the viscoelastic region (see Fig. 1) from HDPSISDS = 3.2 : 1.8 to HDPS : SDS = 4.1 : 0.9 (weight ratio). In each case the SDS solution was added to the HDPS solution to eliminate any order of mixing disparities (later work verified that no such effects exist in this system). The initial solution concentration was 5.0 % (wt) total surfactant. Equilibrated samples were diluted to 1.0 % by addition of D,O.