Thermodynamics of water under hydrophilic versus hydrophobic confinement: boron nitride nanotubes with different charge models

Abstract

Molecular dynamics simulations with two-phase thermodynamics free energy method can provide the quantitative results, which make possible to analyze observed phenomenon theoretically. In this study, we reported the hidden principle of spontaneous water filling inside the boron nitride nanotubes (BNNTs) from 0.69 to 2.21nm diameters, using previous method, and how this altered and differ from the case of CNTs depending on the existence and values of partial charges for boron and nitrogen (positive charge for boron and negative charge for nitrogen).. From MD simulation and 2PT method, we revealed that the free energy of water molecules inside the BNNTs is always lower than the bulk water. Free energy trend of water inside the BNNTs, without any charge, seemed to be equal with the trend of CNTs: BNNTs with 0.83nm and 1.24nm have the lowest free energy and converged to bulk water value for BNNTs larger than 1.38nm. The trend of free energy altered as the partial charge applied depending on their sizes. Absolute entropy and enthalpy were calculated for better understanding of its alteration. Due to the electrostatic interaction between water and BNNT walls originated from applied partial charges for B and N, both properties were generally decreased and the free energy altered from their compounded effect. To complement the lack of distinguishing thermodynamics of water inside the BNNTs with or without partial charge from CNTs, dominant component for negative free energy were calculated. In our calculation, the dominant factor for negative free energy of internal water among BNNTs with an without partial charge are different in absolute value, but similar in trend compared to CNTs’. The transition from entropy dominance to enthalpy dominance occurred under ±1.05 condition in BNNTs from 1.38nm to 1.93nm seems like offbeat, caused by exceedingly high partial charge values. These thermodynamic analysis offers us the reason for spontaneous water filling inside the BNN...