Abstract
The pH-driven adsorption and desorption of fatty acid monolayers at the liquid crystal (LC)-water interface were studied. We doped fatty acids (stearic acid, palmitic acid, myristic acid, dodecanoic acid, and decanoic acid) into 4-cyano-4 '-pentylbiphenyl (5CB), and employed sessile LC droplets as our experimental platform. Under a crossed polariser, the LC droplets displayed a bright flower bud-shaped texture at low pH, whereas at high pH, they exhibited a bright four-brush appearance due to desorption of the adsorbed fatty acids at the LC-water interface. Furthermore, we identified the critical transition pH of various concentrations of stearic acid and other fatty acids featuring distinct tail lengths. Based on the interfacial behaviour, we propose a new method to estimate the pK(a) of fatty acids, which opens up new possibilities for simple, precise, and reliable measurement of the pK(a) of other carboxylic acids. The findings presented herein will greatly facilitate the understanding of the interfacial behaviour of amphiphiles at the oil-water interface.