Adapting SAFT-γ perturbation theory to site-based molecular dynamics simulation. II. Confined fluids and vapor-liquid interfaces

Accurate thermodynamics of simple fluids and chain fluids based on first-order perturbation theory and second virial coefficients: uv-theory

We develop a simplification of our recently proposed uf-theory for describing the thermodynamics of simple fluids and fluids comprising short chain molecules. In its original form, the uf-theory interpolates the Helmholtz energy between a first-order f-expansion and first-order u-expansion as (effective) lower and upper bounds. We here replace the f-bound by a new, tighter (effective) lower bound. The resulting equation of state interpolates between a first-order u-expansion at high densities and another first-order u-expansion that is modified to recover the exact second virial coefficient at low densities. The theory merely requires the Helmholtz energy of the reference fluid, the first-order u-perturbation term, and the total perturbation contribution to the second virial coefficient as input. The revised theory-referred to as uv-theory-is thus simpler than the uf-theory but leads to similar accuracy, as we show for fluids with intermolecular pair interactions governed by a Mie potential. The uv-theory is thereby easier to extend to fluid mixtures and provides more flexibility in extending the model to non-spherical or chain-like molecules. The usefulness of the uv-theory for developing equation-of-state models of non-spherical molecules is here exemplified by developing an equation of state for Lennard-Jones dimers.

Effect of Poly(vinyl butyral) Comonomer Sequence on Adhesion to Amorphous Silica: A Coarse-Grained Molecular Dynamics Study

Modulating a comonomer sequence, in addition to the overall chemical composition, is the key to unlocking the true potential of many existing commercial copolymers. We employ coarse-grained molecular dynamics (MD) simulations to study the behavior of random-blocky poly(vinyl butyral-co-vinyl alcohol) (PVB) melts in contact with an amorphous silica surface, representing the interface found in laminated safety glass. Our two-pronged coarse-graining approach utilizes both macroscopic thermophysical data and all-atom MD simulation data. Polymer-polymer nonbonded interactions are described by the fused-sphere SAFT-γ Mie equation of state, while bonded interactions are derived using Boltzmann inversion to match the bond and angle distributions from all-atom PVB chains. Spatially dependent polymer-surface interactions are mapped from a hydroxylated all-atom amorphous silica slab model and all-atom monomers to an external potential acting on the coarse-grained sites. We discovered an unexpected complex relationship between the blockiness parameter and the adhesion energy. The adhesion strength between PVB copolymers with intermediate VA content and silica was found to be maximal for random-blocky copolymers with a moderately high degree of blockiness rather than for diblock copolymers. We attribute this to two main factors: (1) changes in morphology, which dramatically alter the number of VA beads interacting with the surface and (2) a non-negligible contribution of vinyl butyral (VB) monomers to adhesion energy because of their preference to adsorb to zones with low hydroxyl density on the silica surface.