Temperature and pressure dependence of the interfacial free energy against a hard surface in contact with water and decane

Abstract

Theoretical descriptions of molecular-scale solvation frequently invoke contributions proportional to the solvent exposed area, under the tacit expectation that those contributions are tied to a surface tension for macroscopic surfaces. Here we examine the application of revised scaled-particle theory (RSPT) to extrapolate molecular simulation results for the wetting of molecular-to-meso-scale repulsive solutes in liquid water and decane to determine the interfacial free energies of hard, flat surfaces. We show that the RSPT yields interfacial free energies at ambient pressures that are consistently greater than that obtained from the liquid-vapor surface tensions of water and decane by ∼4%. Nevertheless, the hard surface and liquid-vapor interfacial free energies are parallel over a broad temperature range at 1 bar indicating similar entropic contributions. With increasing pressure, the hard, flat interfacial free energies exhibit a maximum in the vicinity of ∼1000 bars. This non-monotonic behavior in both water and decane reflects solvent dewetting at low pressures, followed by wetting at higher pressures as the solvents are pushed onto the solute. By comparing the results of RSPT against classic scaled-particle theory (CSPT), we show that CSPT systematically predicts greater entropic penalties for interface formation and makes inconsistent predictions between the pressure dependence of the interfacial free energy and solvent contact density with the solute surface.

Temperature, Pressure, and Length-Scale Dependence of Solvation in Water-like Solvents. II. Large Solvophovic Solutes

“…The monotonically decreasing μ ̅ *(T) curves in Figure 4 are actually mirroring the decrease in γ ̅ ∞ with T. 40 In turn, γ ∞ itself decreases with T and is known to parallel the behavior of solvent's liquid−vapor surface tension, 41 which is nevertheless systematically smaller by ca. 4%.…”

“…4%. 40 Then, since γ ̅ ∞ and −T(∂γ ∞ /∂T) p are both positive, eqs 13 and 14 predict that h̅ p * > s ̅ p * > 0 for p > 0. Simulation data in Figure 4 certify such an expectation while they furthermore show that v p increases smoothly with T and departs relatively little from its constant cavity-volume 4/3πR 3 contribution in eq 15, in accordance with the weak pressure dependence of γ ∞ noted above.…”

“…To this end, we will use TIP4P/ 2005 γ ∞ literature data. 40 In doing so, we adopt γ ∞ at p ≃ 1 bar for our calculations at all pressures because the dependence of γ ∞ on p is mild. 40 Figure 4 shows for p ≃ 1 bar and p ≃ 800 bar that μ ̅ * values computed from eq 12 compare favorably with their simulated counterparts.…”

“…40 In doing so, we adopt γ ∞ at p ≃ 1 bar for our calculations at all pressures because the dependence of γ ∞ on p is mild. 40 Figure 4 shows for p ≃ 1 bar and p ≃ 800 bar that μ ̅ * values computed from eq 12 compare favorably with their simulated counterparts. At p ≃ 1 bar, we have found that the 4/3πR 3 p ̅ bulk contribution is less than 0.2% of the 4πR 2 γ ̅ ∞ interfacial one, such a negligible character of the 4/ 3πR 3 p ̅ term for nanometer-sized solutes at the p = 0 nominal pressure being pointed out previously.…”

Excess Volumes from the Pressure Derivative of the Excess Chemical Potential: Testing Simple Models for Cavity Formation in Water

“…The pitfalls of the approximate model based on the scaled particle theory (SPT) in computing excess volumes at low 23 and high pressures are well known, 25 whereas the revised theory (RSPT) seems to provide satisfactory results. 34,35 Alternative models 30−32 formulated within the thermodynamics of surfaces have not yet been tested on volumetric quantities.…”