An Optical Voltmeter for Studying Cetyltrimethylammonium Interacting with Fused Silica/Aqueous Interfaces at High Ionic Strength

Abstract

Electrostatics and counterions play important roles in many supramolecular processes, including surfactant adsorption and aggregation at interfaces. Here, we assess their influence on how the common surfactant cetyltrimethylammonium (CTA) interacts with fused silica/aqueous interfaces by determining thermodynamic, kinetic, and electrostatic parameters for CTA adsorption across a range of NaCl concentrations (10-500 mM NaCl) using second harmonic generation (SHG). Using vibrational sum frequency generation (SFG), we demonstrate that vibrationally resonant contributions and nonresonant background contributions to the SFG signal intensity that depend on the interfacial potential can be quantified simultaneously during the adsorption process, which provides insight into the nonequilibrium dynamics of CTA adsorption. By analyzing the adsorption free energies as a function of interfacial potential at these four salt concentrations, the charge density per adsorbate is determined, indicating that CTA coadsorbs with counterions at a ratio of approximately 4 to 3 (i.e., 4 CTA(+) ions for every 3 Cl(-) ion). The chemical (i.e., non-Coulombic) portion of the free energy is found to dominate the overall free energy of adsorption, indicating that CTA adsorption at these ionic strengths is primarily driven by the favorable hydrophobic interactions between interdigitated surfactant hydrocarbon chains in the adsorbed aggregate. By applying Gouy-Chapman-Stern theory to our data, an average charge density of 2.8(3) x 10(13) charges/cm(2), which corresponds to 0.7 to 1.7 molecules/nm(2), was obtained for the four NaCl concentrations.