Experimental determination of micelle shape and size in aqueous solutions of dodecyl ethoxy sulfates

Alkyl ethoxy sulfates represent an interesting class of ionic surfactants w hose micellar properties depend on the number of ethoxy (EO) groups in thei r hydrophilic heads. A systematic experimental investigation was carried ou t on pure, monodisperse samples of dodecyl ethoxy sulfates having one, two, four, and six EO groups. Specifically, the effects of salt concentration ( 0.1-1.0 M NaCl) and temperature (25-45 degrees C) on micelle shape and size were investigated through dynamic and static light scattering and viscosit y measurements. The classical DLVO potential was utilized to quantitatively analyze the effect of intermicellar interactions. In addition, quantitativ e consistency between the light scattering and viscosity results was demons trated. It was found that the micelle shape and size of these surfactants a re determined through an interplay of electrostatic interactions between th e charged sulfate groups and steric interactions between the hydrated EO gr oups. This interplay can be tuned by varying the number of EO groups, the t emperature, or the salt concentration. Specifically, measurements of the av erage micelle hydrodynamic radius, the average micelle aggregation number, and the micellar solution relative viscosity indicate that, for the surfact ants containing one or two EO groups, increasing salt concentration screens electrostatic repulsions between the charged sulfate groups and permits th e micelles to undergo one-dimensional growth beyond a threshold salt concen tration. In contrast, the micelles formed by the surfactants with the large r heads (containing four or six EO groups) remain spherical over the entire salt concentration and temperature ranges studied. It is proposed that the stronger steric interactions between these larger heads inhibit micellar g rowth.