Canonical Monte Carlo simulations of the fluctuating-charge molecular water between charged surfaces.

Monte Carlo simulations have been performed to study the arrangement of water molecules between charged surfaces. The molecular water used—the four- point transferable intermolecular potential (TIP4P), together with fluctuating charge (FQ)—constitutes the water model TIP4P-FQ. The TIP4P-FQ features the same molecular frame as the TIP4P model; however, with the former, charges on the molecular sites of the same molecule are allowed to fluctuate in response to their environment. Therefore, the induced dipole moment under an external field is inherently considered in the TIP4P-FQ water model. This feature makes it more useful than multipoint fixed-charge models, such as the TIP4P, in studying water arrangement between charged surfaces. The formation of a few ordering layers of water molecules adjacent to a charged surface is confirmed by (N,V,T) Monte Carlo simulations. The preferable orientation of water molecules in this study is found to be similar to previous findings in the literature, with the oxygen atoms pointing to the positively charged surface (oxygen-down) and the hydrogen atoms pointing to the negatively charged surface (oxygen-up). Modeling results also demonstrate that the dominant orientation at the negatively charged surface is one O-H bond pointing to the surface while another O-H bond is aligned parallel to the charged surface. In addition, simulation results using the TIP4P-FQ water model show that the water density in the interfacial region increases ∼8%-14%, which is lower than the density increase observed in experimental data discussed in the literature. [ABSTRACT FROM AUTHOR]