Cloud point phenomena in aqueous solutions of an anionic gemini surfactant with a dibenzene spacer in the presence of tetra-n-propyl-ammonium bromide

Abstract

The cloud point (CP) behaviour of a carboxylate gemini surfactant, O,O′-bis(sodium 2-dodecylcarboxylate)-p-dibenzenediol (referred to as C₁₂ϕ₂C₁₂), in aqueous solution has been examined in the presence of tetra-propyl-ammonium bromide (Pr₄NBr), an organic salt with less hydrophobicity than that of commonly used species. The aggregates of C₁₂ϕ₂C₁₂ in the solution were investigated by dynamic light scattering (DLS), steady-state fluorescence and rheological measurements. Compared with traditional single chain surfactants, C₁₂ϕ₂C₁₂ showed the CP phenomena at very low surfactant concentrations. In the presence of Pr₄NBr, C₁₂ϕ₂C₁₂ in aqueous solution formed several types of aggregates with different sizes. As the temperature was raised to the CP, the micelles rapidly associated, resulting in a cloud-like appearance of the solution. The mechanism has been attributed to C₁₂ϕ₂C₁₂, which can provide an attractive force for the micelle–micelle contact. Namely, with increasing temperature, some of the C₁₂ϕ₂C₁₂ molecules within the micelles changed their configuration from cis- to trans-form, leading to some tails of C₁₂ϕ₂C₁₂ in aggregates stretching towards the solution. When such micelles approached each other, these projecting tails interacted hydrophobically, and this resulted in the association of the micelles.