Ionic Binding of Na⁺ versus K⁺ to the Carboxylic Acid Headgroup of Palmitic Acid Monolayers Studied by Vibrational Sum Frequency Generation Spectroscopy

Abstract

Ionic binding of alkali ions Na+ and K+ to the carboxylic acid headgroups of fatty acid monolayers is studied as a proxy toward understanding the fundamental chemistry in cell biology. In this study, we used broad-bandwidth sum frequency generation (BBSFG) vibrational spectroscopy to investigate the ionic binding event that leads to deprotonation and complex formation of fatty acid headgroups. Palmitic acid (C15H31COOH) exists as a monolayer on aqueous surfaces. Surface vibrational stretch modes of palmitic acid from 1400 cm−1 to 3700 cm−1 were observed (νs-COO−, ν-CO, ν-CH, ν-OH of COOH, free OH). Palmitic acid is mostly protonated at the aqueous surface at neutral pH (∼6). However, various degrees of deprotonation are initiated by the introduction of Na+ and K+ that results in the complexation of K+:COO− and solvent separated Na+:COO−. Evidence in several spectral regions indicates that K+ exhibits stronger ionic binding affinity to the carboxylate anion relative to Na+