An exception to linearity in EACN framework: Twin-tail lipophiles and n-alkanes interactions

Microemulsion phase behaviors were measured using a binary surfactant mixture consisting of twin-tail C28(O)-propoxy-epoxy-carboxylate and C19–23 internal olefin sulfonate at temperatures from 55 °C to 90 °C. Pure n-alkanes and their binary mixtures were used for the oil phase to investigate the effect of oil. By salinity scans of the model oils while keeping all other formulation variables constant, it was found that only n-tetradecane showed a remarkable negative deviation from a linear relationship between optimum salinity and alkane carbon number (ACN) besides a positive deviation in solubilization ratios at optimum compared to the neighboring n-alkanes. To our knowledge, this is the first reporting of the mismatch between ACN and apparent EACN for n-alkanes larger than n-hexane. The unexpected deviations could be attributed to a similarity in sizes between n-tetradecane and the two fourteen-atom twin-tail strands in the C28(O) lipophile, i.e. C14 and C12-O-C. Unlike other surfactants with branched lipophiles, the symmetric nature of the two strands in the C28(O) lipophile seems to play a key role. The experimental observations of the study can provide valuable insights into surfactants and microemulsions research such as practical implications on classical equivalent alkane carbon number (EACN) framework; and how to improve or optimize lipophile-oil interaction by tailoring the molecular structure of the lipophiles. [Display omitted] •An exception to the classical EACN framework was found for the first time in pure n-alkane.•A significant deviation of optimum salinity was observed for n-tetradecane from the linear optimum salinity vs ACN trend.•This deviation could be due to a favorable interaction between n-C14 and twin-tail lipophile with two 14-atom strands.