title: Why are water-hydrophobic interfaces charged?
abstract: We report ab initio molecular dynamics simulations of hydroxide and hydronium ions near a hydrophobic interface, indicating that both ions behave like amphiphilic surfactants that stick to a hydrophobic hydrocarbon surface with their hydrophobic side. We show that this behavior originates from the asymmetry of the molecular charge distribution which makes one end of the ions strongly hydrophobic while the other end is even more hydrophilic than the regular water (H2O) molecules. The effect is more pronounced for the hydroxide than for the hydronium. Our results are consistent with several experimental observations and explain why hydrophobic surfaces in contact with water acquire a net negative charge, a phenomenon that has important implications for biology and polymer science.
authors.affiliation: Department of Chemistry and Princeton Institute for Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: Konstantin N
authors.initials: KN
authors.lastname: Kudin
authors.forename: Roberto
authors.initials: R
authors.lastname: Car
mesh_terms: D003198:Computer Simulation; D057927:Hydrophobic and Hydrophilic Interactions; D006878:Hydroxides / Q000737:chemistry; D008956:Models, Chemical; D009861:Onium Compounds / Q000737:chemistry; D011789:Quantum Theory; D013499:Surface Properties; D014867:Water / Q000737:chemistry*
chemical_list: D006878:Hydroxides; D009861:Onium Compounds; D014867:Water; C027727:hydronium ion; C031356:hydroxide ion