Interfacial Composition, Structural and Thermodynamic Parameters of Water/(Surfactant+n‐Butanol)/n‐Heptane Water‐in‐Oil Microemulsion Formation in Relation to the Surfactant Chain Length

Abstract

Interfacial behavior, structural and thermodynamic parameters of a water/(surfactant?n-butanol)/ n-heptane water-in-oil (w/o) microemulsion have been investigated using the dilution technique at different temperatures, and [water]/[surfactant] mole ratios. The cationic surfactants used were alkyltrimethyl ammonium bromides (CnTAB, n = 10, 14 and 16) while the nonionic surfactants were polyoxyethylene (20) sorbitan monoalkanoates (polysorbate), viz., palmitate (PS 40), stearate (PS 60) and oleate (PS 80). The distribution of cosurfactant between the oil-water interface and the bulk oil at the threshold level of stability, and the thermodynamics of transfer of the cosurfactant from the bulk oil to the interface were evaluated. Structural parameters such as the dimensions, population density and effective water pool radius of the dispersed water droplets in the oil phase and the interfacial population of the surfactant and cosurfactant have been evaluated in terms of the surfactant chain length.

Physico‐Chemical Studies on Microemulsion: Effect of Cosurfactant Chain Length on the Phase Behavior, Formation Dynamics, Structural Parameters and Viscosity of Water/(Polysorbate‐20 + n‐Alkanol)/n‐Heptane Water‐in‐Oil Microemulsion

“…Increasing the temperature might lead to the opening up of the coiled surfactant head group, for which better efficacy of the system could have been achieved by Polysorbate-20 at a higher temperature [30,31]. A decrease in the negative value of DG t o with increasing [water]/[Polysorbate-20] mole ratio x was observed in general.…”

“…Experiments were then done under different conditions, viz., [water]/[Polysorbate-20] mole ratio, x, temperature and cosurfactant chain length in order to evaluate thermodynamic and structural parameters. Each set of experiments was carried out at least four times and the average of the values were used in obtaining the final result [30,31].…”

“…Varying amounts of oil (or water) were then added. Water (or oil) was progressively added by using a Hamilton (USA) microsyringe under constant stirring and the temperature was controlled using a cryogenic circulatory water bath (of precession ±0.1 K) at 303 K. Appearance of the state of turbidity was noted visually, which indicated the onset of phase separation or immiscibility [29][30][31]. The solutions were allowed to attain equilibrium for at least for 30 min (we found that equilibrium was attained within this time period in most of the systems).…”

“…Moulay et al [1] have studied the effect of alkanol chain length during the microemulsion breakdown by a pervaporation technique. Our research group has also studied the effect of cosurfactant chain length on different physicochemical parameters of AOT-based microemulsions [29] and other ionic/nonionic surfactants [30,31]. Moulik et al [32,33], Digout et al [34] and Wang et al [21] have systematically studied the effect of alkanol chain length on the interfacial composition and thermodynamic of formation of water-in-oil (w/o) microemulsions.…”

“…However, besides these above mentioned references, studies on microemulsions comprising polyoxyethylene sorbitan monolaurate and n-alkanols (n-butanol to n-octanol) of varying chain length are not plentiful in the literature to the best of our knowledge. In most of the cases, cosurfactants used include n-butanol, n-pentanol and n-hexanol for their better stabilizing efficiency in microemulsion formation [1,21,30,34]. However, the exact reasons for the better efficacy of these alkanols over the other homologs are still not exactly known.…”