Influence of hydrotrope on micellization behaviour of zwitterionic dimeric surfactants carrying ammonium and phosphodiester headgroups at different temperatures in aqueous medium

Abstract

Analogous novel zwitterionic dimeric surfactants in the sequence of 2-dodecylphosphate,1-(N,N-dimethyl,N-dodecylammonium)ethane abbreviated as C12(-)–2–C12(+); 2-dodecylphosphate,1-(N,N-dimethyl,N-tetradecylammonium)ethane abbreviated as C12(-)–2–C14(+); and 2-dodecylphosphate,1-(N,N-dimethyl,N-hexadecylammonium)ethane abbreviated as C12(-)–2–C16(+); with phosphodiester anion and quaternary ammonium salt as hydrophilic and the methylene chains as hydrophobic groups, two methylene groups separate the two headgroups, were prepared, and the effect of aromatic hydrotropes aniline hydrochloride (AHC) and sodium salicylate (NaSal) on the mode of micellization at four unlike temperatures with a difference of 5 K in aqueous medium by the method of conductivity were studied. The decrease in cmc of zwitterionic surfactants with the addition/increase in bulk mole fraction of NaSal and AHC is almost linear. But anionic NaSal is more productive as compared to the cationic hydrotrope AHC in reducing the critical micelle concentration (cmc). Negative values of interaction parameter (βm) were found to decline with extension in one of the hydrophobic chain lengths keeping the other hydrophobic chain length identical. Symmetrical zwitterionic surfactants interact more strongly via headgroups and hydrocarbon tails with aromatic hydrotropes and decrease the cmc effectively by concealing of headgroups and hydrophobic forces operating mutually thus promotes micellar shape transition by alteration of micelle surface curvature. The enthalpy of micellization ΔmicH° suggest an endothermic process and the negative value of free energy of micellization ΔmicG° due to encouraging augmentation in entropy of micellization ΔmicS°. The doping of surfactants with organic salts or hydrotropes amplifies the solubilizing power, viscosity and subdue the cmc value. Such systems promise to improve the performance used in separation methods, drug permeation, and delivery as these surfactants are less toxic and less sensitive to changes in pH and ionic strength.