Surfactant–surfactant and surfactant–solute interactions in SLES–Brij35 mixed micelles: Effect of the degree of ethoxylation on pyrene solubilization enhancement in water

Abstract

We investigated the effect of the degree of ethoxylation on surfactant–surfactant (βM) and surfactant–solute (B) interactions toward pyrene solubilization in water using sodium lauryl ether sulfate (SLES) containing 1–3 OE groups per molecule and Brij35 mixed micelles. In single SLES micelles, pyrene MSR (and Km) increased with increasing degree of ethoxylation, but remained invariant in SLES–Brij35 mixed micelles at any given Brij35 molar fraction composition in the bulk (α). Upon increasing α, a direct correlation between the variation of the CMCs and MSRs was observed, suggesting that the negative deviation of the CMCs from ideal mixing behavior and the increase in Km12 are two influencing factors in pyrene solubilization in mixed micelles. The increase in Km12 was attributed to the opening up of mixed micelle from a random coil structure to a more stretched one with the increase in Brij35 molar fraction. The invariance of CMCs, MSRs, and Km12 with SLES degree of ethoxylation (at fixed α) was reflected by similar invariance in βavgM (−2.01) and X1M, suggesting identical mixed micelle structure and aggregation properties. On the other hand, although Km12 values were independent on the degree of ethoxylation at fixed composition α, B decreased with increasing degree of ethoxylation. This was considered as an evidence that pyrene–SLES interaction plays an insignificant role in the positive synergism observed in this study, compared to SLES–Brij35 interaction.