Microstructure in n-alkane-water-electrolyte mixtures with small ethoxylated alcohol amphiphiles

Abstract

The ability of an amphiphilic molecule to form topologically ordered, surfactant-like aggregates is probed by studying the ethoxylated alcohols 3-oxaheptanol (C4 E1) and 3,6-dioxadodecanol (C6 E2). Density, refractive index, and NMR measurements indicated C4 E1-water-(NaCl salt) solutions are ideal dilute solutions with no solute association up to a C4 E1 mole fraction of about 0.018 (ca. 11 wt%). Above this concentration, the amphiphile forms aggregates which appear to be at least weakly cooperative. At high C4E1 concentrations, 23Na NMR and quasielastic light scattering (QLS) measurements indicated the existence of small brine-rich domains (<10A˚) which though probably of short life (⩽1 μs) can interact and exchange with the amphiphile. Addition of n-decane to C4E1-water-salt and of n-dodecane to C6E2-water-salt mixtures leads to strong solution nonidealities evidenced by both brine-rich and oil-rich critical points. The spectroscopic and QLS data obtained from amphiphile-oil-brine mixtures at low brine content (1–13 wt% brine) indicated larger brine-rich domain sizes (25–40A˚) than those in the oil-free mixtures. The 23Na NMR data in the oil-containing mixtures were very sensitive to nearness to the binodal separating one- and two-phase samples, which indicates that the domain size was largest when there was just enough amphiphile present to completely solubilize oil and brine. 13C NMR measurements suggested that both amphiphiles were oriented in the oil-containing mixtures with ethylene oxide groups toward water and tail groups toward oil. From the magnitude of the chemical shifts observed, C6E2 appeared to be more oriented than C4E1. Hence, even with these simplest ethoxylated alcohols, there are indications of surfactant-like aggregation, i.e., topological ordering, between oil and brine domains.