Molecular dynamics exploration for the adsorption of benzoic acid derivatives on charged silica surfaces

Abstract

Molecular dynamics (MD) simulations provide molecular level information about the interaction of organic molecules with the solid surfaces. There has been much development of this method to simulate the interaction of organic molecules with neutral surfaces but relatively less information is available regarding the interactions with charged surfaces. In this study we have developed MD model for the charged silica surface and have investigated the interactions of different benzoic acid derivatives with the charged silica surface in pure and saline water at acidic, neutral and basic pH. The investigated molecules were 2,3-dihydroxybenzoic acid (23DHBA), 3,4-dihydroxybenzoic acid (34DHBA), 1,2,4-benzenetricarboxylic acid (BTCA) and phthalic acid (PHTHA). To simplify the analysis of results, three different simulation systems were considered. Clustering simulations showed how molecules aggregate in solution, pulling simulations provided quantitate information regarding the interactions of single molecule with the silica surface, whereas adsorption simulations focused on the adsorption of multiple molecules on the surface. In general, at pH 2–3, all investigated molecules were clustering and were attracted towards the surface. At pH 7, due to arising electrostatic repulsion, the interactions became weaker which prevented 23DHBA, 34DHBA and BTCA from clustering, however, they continued to adsorb on the silica surface. The adsorption of 23DHBA and 34DHBA decreased significantly at pH 9–10 due to electrostatic repulsion between the molecules and charged silica surface, while BTCA adsorbed slightly stronger due to interactions with ions close to the surface. PHTHA molecules behaved differently by clustering stronger and adsorbing weaker at higher pH. The results of MD simulations presented in this work by using pulling and adsorption approaches provide possibility to compare the results with experimental data.