Solubilization of alcohols in SDS and TTAB from isentropic partial molar compressibilities and solubilities

Partition coefficients of some medium chain alcohols between water and micelles have been determined at infinite alcohol dilution from measured isentropic partial molar compressibilities, at intermediate alcohol concentrations by NMR self-diffusion measurements and at the phase boundary from total solubilities. The surfactants studied were sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB). The agreement between the different methods of measurement is poor. Inspection of the data show, however, that the fraction of alcohol solubilized is constant with respect to alcohol concentration at constant surfactant concentration, and this means that the partition coefficient must be dependent on the alcohol concentration. The partial molar volumes and compressibilities of the alcohols in the solubilized state have been evaluated at infinite dilution, showing increased volumes and compressibilities compared to the aqueous state, as is normal for alcohols in non-aquoeus environments. As alcohol is added at constant surfactant concentration, a significant increase in the partial molar compressibility is observed above a certain alcohol concentration. This happens when the alcohol to surfactant ratio is in the region of 1.7–2.0. The increased compressibility suggests that the alcohol molecules, on average, experience a more hydrophobic environment. A reasonable interpretation is that some of the solubilized alcohol molecules will be solubilized in the micellar interior, once the alcohol to surfactant ratio exceeds a critical value.