Localization of n-Alcohols and Structural Effects in Aqueous Solutions of Sodium Dodecyl Sulfate

Abstract

Small-angle neutron scattering measurements on sodium dodecyl sulfate aqueous solutions have been performed in the presence of n-alcohols, from methanol to octanol, at different alcohol concentrations. By modeling the experimental intensities, it was possible to obtain structural information and to derive simultaneously the distribution of the alcohols between the aqueous and the micellar phases. It was found that short chain alcohols tend to remain in the aqueous phase and, by altering the solvent properties, induce a decrease in the aggregation number of sodium dodecyl sulfate micelles. On the other hand, alcohols with longer hydrocarbon chains were found to be present in both phases though favoring the micellar phase the longer the alkyl chain and the larger the concentration; this could be rationalized by assuming that the insertion of alcohol molecules in the micelle produced weaker repulsive interactions between the charged head groups of the surfactant molecules. For long chain alcohols, appreciably localized in the micellar phase, screening of the interaction among head groups leads to bigger micelles than those observed in the absence of alcohol: in these cases the alcohol/surfactant molar ratio reaches the value of 0.86, and hence the aggregates can be considered as mixed micelles. Sodium dodecyl sulfate micelles, at the examined concentration, were found to deviate from spherical symmetry and, when added with heptanol or octanol, assumed an ellipsoidal shape growing preferentially along the rotation axis.

Trapping Metal Nanoclusters in “Soap and Water” Soft Crystals

“…[13] One such system is the direct hexagonal phase that self-assembles from water/surfactant mixtures. [14,15] High concentrations of surfactants in water yield spherical micelles that, in the presence of an alcohol cosurfactant, elongate to form long tubes in hexagonal packing. The result is a stable ™soft crystal∫ gel (Figure 1 a).…”

“…When we increased the cluster concentrations we observed a higher overall scatter intensity. [14] The nanoclusters remained inside the hexagonal phase and were not extracted into water. Moreover, no phase separation and no aggregation was observed even after centrifuging the gels for 5 min at 200 g. The cluster-containing gels can be heated to 55 8C and cooled back to 20 8C to break and reform their crystalline structure.…”

Elongational Flow of Solutions Containing Poly(ethylene oxide)/Sodium Dodecyl Sulfate Complexes in the Presence of n-Alkanols

“…For the series of n-alkanols, at a fixed alcohol concentration, the SDS micellar aggregation number decreases as the alkyl chain length increases, reaching a minimum for pentanol, and then it increases from pentanol to octanol (17). The minimum becomes more pronounced as the alcohol concentration is increased.…”

“…The minimum becomes more pronounced as the alcohol concentration is increased. At the same time, the number of alcohol molecules within a micelle increases uniformly with alcohol chain length (16,17). At low alcohol concentrations, trends in the micelle volume are controlled by the SDS aggregation number: appreciable swelling of the micelles is only observed for long-chain alcohols (n-heptanol and n-octanol).…”

“…In the micelle, short-chain alcohols are located near the ionic shell, which reduces the electrostatic repulsion and lowers the CMC at low alcohol contents (1,14). However, relatively high concentrations of short-chain alcohols increase the solvent hydrophobicity, which results in an increase of surfactant solubility (as monomer) and an increase of the CMC (14): adding high concentrations of a short-chain alcohol to a micellar solution decreases the micellar aggregation number (15)(16)(17) and ultimately dissolves the micelles (18,19). On the other hand, long-chain alcohols, whose solubility in water is relatively small, partition favorably inside the micelles and always lower the CMC (14,17).…”