Monomeric and dimeric anionic surfactants: A comparative study of self-aggregation and mineralization

Abstract

A study on the phase behavior and structure of the alkanolamine salts of the dimeric amphiphile 3,4-bis-dodecyloxycarbonyl-hexanedioic acid (GS-H) is presented for the first time. Data are compared to those of the corresponding monomeric surfactant (lauric acid, LA). The alkanolamine salts of GS-H show very low Krafft points (<0 degrees C) and form hexagonal liquid crystals at concentrations lower than its monomeric counterpart, indicating that aggregation is favored for dimeric surfactants. The minimum concentration for liquid crystal formation increases for bulky alkanolamines with a structure-disrupting effect, such as triethanolamine (TEA). However, the specific surface areas per molecule in the liquid crystals derived from small-angle X-ray scattering (SAXS) are similar for monoethanolamine (MEA) and TEA salts; the same can be said when comparing monomeric (LA) and dimeric (GS-H) salts. GS-H can also form hexagonal and lamellar liquid crystals with organic aminosilanes acting as reactive counterions, as revealed by solvent penetration experiments with polarized optical microscopy (POM). Consequently, mineralization with silica and alumina was carried out by a sol-gel method using GS-H as a possible structure-directing agent. Both silica and alumina samples possessed a lamellar structure, which disappears on calcination; however, calcined alumina has indeed a high surface area coming mainly from micropores. It was found that the surfactant/aminosilane ratio is critical for obtaining structured silica before calcination.