LOCALIZATION OF N-ALCOHOLS AND STRUCTURAL EFFECTS IN AQUEOUS-SOLUTIONS OF SODIUM DODECYL-SULFATE

Small-angle neutron scattering measurements on sodium dodecyl sulfate aqueous solutions have been performed in the presence of n-alcohols, f rom methanol to octanol, at different alcohol concentrations. By model ing the experimental intensities, it was possible to obtain structural information and to derive simultaneously the distribution of the alco hols between the aqueous and the micellar phases. It was found that sh ort chain alcohols tend to remain in the aqueous phase and, by alterin g the solvent properties, induce a decrease in the aggregation number of sodium dodecyl sulfate micelles. On the other hand, alcohols with l onger hydrocarbon chains were found to be present in both phases thoug h favoring the micellar phase the longer the alkyl chain and the large r the concentration; this could be rationalized by assuming that the i nsertion of alcohol molecules in the micelle produced weaker repulsive interactions between the charged head groups of the surfactant molecu les. For long chain alcohols, appreciably localized in the micellar ph ase, screening of the interaction among head groups leads to bigger mi celles than those observed in the absence of alcohol: in these cases t he alcohol/surfactant molar ratio reaches the value of 0.86, and hence the aggregates can be considered as mixed micelles. Sodium dodecyl su lfate micelles, at the examined concentration, were found to deviate f rom spherical symmetry and, when added with heptanol or octanol, assum ed an ellipsoidal shape growing preferentially along the rotation axis .