Influence of pH and salt on the ionization state and phase behavior of some aromatic acid derivatives of cholesterol at the air–aqueous interface

Abstract

For ionizable amphiphiles, changing the pH can profoundly influence the self-assembly and also the associated phases. Here, we systematically investigate the distinct roles of monovalent (Na⁺) and divalent (Mg²⁺, Ca²⁺) cations in modulating the pH-dependent self-assembly of two representative cholesterol-based aromatic acid derivatives, ChBA-C4 (even) and ChBA-C5 (odd), employing various interface-sensitive techniques. Surface pressure–area per molecule (π–A_m) isotherm studies of these derivatives reveal that both the headgroup deprotonation (pH > 10) and valency of ions are essential for obtaining unique monolayer phases, with both mono- and divalent cations promoting film expansion in the area per molecule. Based on the magnitude of compressional modulus, we assign two distinct surface phases that occur with an increase in surface density: ChBA-C4 exhibited phases L₁′ and L₁″ (L₁ and L₁′ for Na⁺ ions), whereas ChBA-C5 shows L₁ and L₂′ (L₁ and L₂ for Na⁺ ions) phases on Mg²⁺ and Ca²⁺ ion-enriched subphases. Hysteresis and relaxation studies conducted at pH 11 reveal that divalent cations significantly reduce material loss by more than half compared to Na⁺ ions, confirming the formation of stable, robust, and cohesive films. Brewster angle and atomic force microscopy complement the inference drawn from π–A_m isotherm studies. The apparent surface pK_a is obtained by fitting the collapse pressure as a function of pH and shows distinct behaviour for monovalent (10.1–10.3 ± 0.2) and divalent ions (9.3–9.7 ± 0.2) for both derivatives. The experimentally obtained degree of dissociation with pH is compared with the Gouy–Chapman theory (assuming an intrinsic pK_a of 6.8) and found to be in good agreement. The highlights of our work show that, by promoting electrostatic interactions, the cation identity precisely dictates the phase behaviour and stability of ChBA derivatives, offering critical insights for engineering responsive soft-matter interfaces.