^{1}and Niharendu Choudhury

^{1,a)}

### Abstract

We use molecular dynamics (MD) simulations of water near nanoscopic surfaces to characterize hydrophobic solute-water interfaces. By using nanoscopic paraffin like plates as model solutes, MD simulations in isothermal-isobaric ensemble have been employed to identify characteristic features of such an interface. Enhanced water correlation, density fluctuations, and position dependent compressibility apart from surface specific hydrogen bond distribution and molecular orientations have been identified as characteristic features of such interfaces. Tetrahedral order parameter that quantifies the degree of tetrahedrality in the water structure and an orientational order parameter, which quantifies the orientational preferences of the second solvation shell water around a central water molecule, have also been calculated as a function of distance from the plate surface. In the vicinity of the surface these two order parameters too show considerable sensitivity to the surfacehydrophobicity. The potential of mean force (PMF) between water and the surface as a function of the distance from the surface has also been analyzed in terms of direct interaction and induced contribution, which shows unusual effect of plate hydrophobicity on the solvent induced PMF. In order to investigate hydrophobic nature of these plates, we have also investigated interplate dewetting when two such plates are immersed in water.

The authors are thankful to Dr. S. K. Ghosh and Dr. T. Mukherjee for their kind interest and encouragement. It is a pleasure to thank Computer Division, B.A.R.C., Mumbai, for providing Anupam supercomputating facilities and support.

I. INTRODUCTION

II. MODELS AND METHODS

III. RESULTS AND DISCUSSIONS

A. Hydration characteristics of single plate

B. Interplate dewetting

IV. CONCLUDING REMARKS

### Key Topics

- Hydrophobic interactions
- 99.0
- Hydrogen bonding
- 28.0
- Hydrophilic interactions
- 18.0
- Interface structure
- 8.0
- Solvents
- 8.0

## Figures

The normalized single particle density *g* _{ SO }(*z*) of water oxygen as a function of distance *z* from the model paraffin plate for different values of λ.

The normalized single particle density *g* _{ SO }(*z*) of water oxygen as a function of distance *z* from the model paraffin plate for different values of λ.

(a) Solute-water potential of mean force *W* _{ SO }(*z*) of a water molecule as a function of distance *z* from the model paraffin solute (plate) for values λ = 0, 0.5, and 1. (b) Solvent induced PMF ω_{ ind }(*z*) for values of λ = 0, 0.25, 0.5, 0.75, and 1.

(a) Solute-water potential of mean force *W* _{ SO }(*z*) of a water molecule as a function of distance *z* from the model paraffin solute (plate) for values λ = 0, 0.5, and 1. (b) Solvent induced PMF ω_{ ind }(*z*) for values of λ = 0, 0.25, 0.5, 0.75, and 1.

Orientational distribution functions of the (a) dipole moment vector P_{μ}(cos θ) vs. cos θ(b) OH bond vector P_{ OH }(cos ψ) vs. cos ψ, and (c) P_{⊥}(cos ϕ) vs. cos ϕ of the water molecules for different values of λ.

Orientational distribution functions of the (a) dipole moment vector P_{μ}(cos θ) vs. cos θ(b) OH bond vector P_{ OH }(cos ψ) vs. cos ψ, and (c) P_{⊥}(cos ϕ) vs. cos ϕ of the water molecules for different values of λ.

Average values of the cosines of the angles made by (a) dipole moment vectors, (b) OH bond vectors, and (c) plane-perpendicular vectors of the water molecules with outward normal to the solute plate as a function of perpendicular distance from the solute plate with different values of λ.

Average values of the cosines of the angles made by (a) dipole moment vectors, (b) OH bond vectors, and (c) plane-perpendicular vectors of the water molecules with outward normal to the solute plate as a function of perpendicular distance from the solute plate with different values of λ.

Histograms of water molecules with *n* hydrogen bonds for water in plate-water systems with λ = 0, 0.25, 0.5, 0.75, 1, and bulk.

Histograms of water molecules with *n* hydrogen bonds for water in plate-water systems with λ = 0, 0.25, 0.5, 0.75, 1, and bulk.

(a) Average coordination number (⟨*n* _{ CN }⟩ ) of water, (b) average number of hydrogen bonds (⟨*n* _{ HB }⟩) per water molecules, and (c) fraction (⟨*n* _{ HB }/⟨ *n* _{ CN }⟩) of nearest neighbors that are hydrogen bonded as a function of perpendicular distance *z* from the solute plate for different paraffin-like plates (with different values of λ).

(a) Average coordination number (⟨*n* _{ CN }⟩ ) of water, (b) average number of hydrogen bonds (⟨*n* _{ HB }⟩) per water molecules, and (c) fraction (⟨*n* _{ HB }/⟨ *n* _{ CN }⟩) of nearest neighbors that are hydrogen bonded as a function of perpendicular distance *z* from the solute plate for different paraffin-like plates (with different values of λ).

Local compressibility χ_{ T }(*z*) of a slab of water of width 1 Å at different locations along the perpendicular direction of the plate for different values of λ.

Local compressibility χ_{ T }(*z*) of a slab of water of width 1 Å at different locations along the perpendicular direction of the plate for different values of λ.

Normalized fluctuations *K* _{ N }(*z*) of a slab of water of width 1 Å at different locations along the perpendicular direction of the plate for different values of λ and for bulk water.

Normalized fluctuations *K* _{ N }(*z*) of a slab of water of width 1 Å at different locations along the perpendicular direction of the plate for different values of λ and for bulk water.

Water-water correlations at different interfaces with different λ values as calculated through intermolecular water (a) oxygen-oxygen pair correlation function *g* _{ OO }(*r*) and (b) oxygen-hydrogen pair correlation function *g* _{ OH }(*r*) near the paraffin plate measured in a 1 Å thick slab placed parallel to the interface located at the half density plane of water.

Water-water correlations at different interfaces with different λ values as calculated through intermolecular water (a) oxygen-oxygen pair correlation function *g* _{ OO }(*r*) and (b) oxygen-hydrogen pair correlation function *g* _{ OH }(*r*) near the paraffin plate measured in a 1 Å thick slab placed parallel to the interface located at the half density plane of water.

(a) First shell tetrahedral order parameter, *q* _{4} (Bottom to top: λ = 0.0, 0.25, 0.50, 0.75, 1.00) and (b) second shell orientational order parameter *Q* _{6} for water at different slabs parallel to the surfaces as a function of distance from the plate for different plates of varying λ values. In the inset of panel (a), *q* _{4} as a function of λ is shown.

(a) First shell tetrahedral order parameter, *q* _{4} (Bottom to top: λ = 0.0, 0.25, 0.50, 0.75, 1.00) and (b) second shell orientational order parameter *Q* _{6} for water at different slabs parallel to the surfaces as a function of distance from the plate for different plates of varying λ values. In the inset of panel (a), *q* _{4} as a function of λ is shown.

Normalized one-particle density profile, ρ(*z*)/ρ_{0} as a function of perpendicular distance from the plate, *z* (a) for λ = 1.0 and (b) for λ = 0.75 plate-water systems. The red lines in (b) is for plates with interplate distance, r_{0} = 12 Å and the black line is for the same with r_{0} = 10 Å.

Normalized one-particle density profile, ρ(*z*)/ρ_{0} as a function of perpendicular distance from the plate, *z* (a) for λ = 1.0 and (b) for λ = 0.75 plate-water systems. The red lines in (b) is for plates with interplate distance, r_{0} = 12 Å and the black line is for the same with r_{0} = 10 Å.

Normalized one-particle density profile, ρ(*z*)/ρ_{0} as a function of perpendicular distance from the plate, *z* for (a) λ = 0.5 and for (b) λ = 0.75 plate-water systems. The red lines are for plates with interplate distance, r_{0} = 14 Å and the black line is for the same with r_{0} = 12 Å.

Normalized one-particle density profile, ρ(*z*)/ρ_{0} as a function of perpendicular distance from the plate, *z* for (a) λ = 0.5 and for (b) λ = 0.75 plate-water systems. The red lines are for plates with interplate distance, r_{0} = 14 Å and the black line is for the same with r_{0} = 12 Å.

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