Molecular Association Behaviors of Anionic Surfactants in Aqueous Solutions (Micelles, Microemulsions, Light Scattering).

Quasi-elastic and static light scattering have been employed to investigate three aqueous solutions of anionic surfactants. These are: (1) Micellization of sodium dodecyl benzene sulfonate (SDBS). (2) Brine-sodium dodecyl sulfate (SDS)-n-pentanol-dodecane O/W microemulsions. (3) Mixed micelle formation of sodium octyl sulfate (SOS) and sodium hexadecyl sulfate (SHS). From measuring the intensity and autocorrelation function of scattered light, the average size, shape and aggregation number of SDBS micelles have been deduced as a function of temperature, NaCl concentration and SDBS concentration. The results showed that at high temperatures, SDBS micelles approach a minimum size of (')R(,h) = 22 (ANGSTROM) and an aggregation number n = 28 independent of NaCl and SDBS concentration. Based on a simple model calculation, the shape of the large micelles is close to a prolate ellipsoid of revo- lution. By assuming that the semiminor axis of the prolate micelles is fixed as 22 (ANGSTROM), the semimajor axis of the micelles increases strongly with NaCl concentration as the temperature is lowered. Using a thermodynamic model of micelle formation to analyze our data, we find that there is very good agreement between the theoretical predictions of micelle size and the experimental measurements. We also used the Derjaquin-L and au-Verwey-Overbeek (DLVO) theory of colloidal interaction to explain the salt concentration dependence of intermicellar interactions. From this analysis we determined the net charge on an SDBS micelle to be q (TURNEQ) 10 and the Hamaker constant to be A (TURNEQ) 5k(,B)T. Both of these values are in qualitative agreement with those found for other anionic surfactants, such as SDS system. The O/W microemulsions studied are brine-SDS-n-pentanol-dodecane. At 25(DEGREES)C, they were monodisperse and spherical in shape over the concentration range we investigated. Based on the DLVO theory, both individual particle parameters and interparticle pair interaction potential were deduced from the QLS measurements. From the analysis we determined the net charge on an O/W micro- emulsion to be (TURNEQ) 39 and the Hamaker constant to be A (TURNEQ) 2.6 k(,B)T. Aqueous solutions containing two homologous ionic surfactants: SOS and SHS above the phase boundary temperature and cmc were studied by QLS. The results indicate that the mixed micelles are prolate ellipsoids of revolution over the composition range explored.PhDChemical engineeringUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/160495/1/8512381.pd