Study on the synthesis and characterization of surface activities of hydrophilic derivatives of β‐sitosterol

Three kinds of hydrophilic derivatives of β‐sitosterol (HPS) were synthesized from poly(ethylene glycol) (PEG) with molecular weight 1000, 2000, and 4000 by two‐step reactions. The chemical structure and the distribution of isomers in HPSs were investigated by 1 H‐NMR and Maldi‐Tof, respectively. The solubility of HPS in water increased as the molecular weight of the parent PEG increased. However, when the solubility of each HPS was calculated based on the weight ratio of the β‐sitosterol moiety in HPS, HPS‐1000 prepared from PEG 1000 showed the highest solubility. Both the CMC and surface tension of HPS surfactant system have been found to increase with an increase in molecular weight of the parent PEG. Dynamic surface tension measurement using a maximum bubble pressure tensiometer showed that much longer time was required to reach an equilibrium value presumably due to the lower mobility of a surfactant molecule with high molecular weight. The interfacial tensions measured between HPS system and n ‐decane at 25°C were in the same order of magnitude as those exhibited between micellar solutions and nonpolar hydrocarbon oils and also found to decrease with a decrease in molecular weight of the parent PEG. The most stable foams were observed with HPS‐1000 system which is consistent with that of surface tension measurement where HPS‐1000 system exhibited a lower surface tension value at CMC than other two systems.