pH-Driven adsorption and desorption of fatty acid at the liquid crystal–water interface

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.

GRAPHICAL ABSTRACT

KEYWORDS:

• pH-driven
• fatty acid
• liquid crystal
• adsorption
• desorption

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was supported by a grant of the Korean Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea [grant number HI13C0891], and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning [grant number NRF- 2013R1A1A1A05008333].