Surface Affinity of Tetramethylammonium Iodide in Aqueous Solutions: A Combined Experimental and Computer Simulation Study

Thesurface affinity of tetramethylammonium iodide (TMAI) in aqueoussolutions is investigated by surface tension measurements and moleculardynamics computer simulations. Experiments, performed in the entirecomposition range of solubility using the pendant drop method withtwo different setups, clearly reveal that TMAI is a weakly capillaryactive salt. Computer simulations performed with the AMBER force fieldreproduce the experimental data very well, while two other major forcefields (i.e., CHARMM and OPLS) can still reproduce the experimentaltrend qualitatively; however, even qualitative reproduction of theexperimental trend requires scaling down the ion charges accordingto the Leontyev-Stuchebrukhov correction. On the other hand,the GROMOS force field fails in reproducing the experimentally confirmedcapillary activity of TMAI. Molecular dynamics simulation resultsshow that, among the two ions, iodide has a clearly larger surfaceaffinity than tetramethyl-ammonium (TMA(+)). Further,the adsorption of the I- anions is strictly limitedto the first molecular layer beneath the liquid-vapor interface,which is followed by several layers of their depletion. On the otherhand, the net negative charge of the surface layer, caused by theexcess amount of I- with respect to TMA(+), is compensated by a diffuse layer of adsorbed TMA(+) cations,extending to or beyond the fourth molecular layer beneath the liquidsurface.