Hydration and Dispersion Forces in Hydroxypropylcellulose Phase Behavior.

Area of Science:

• Materials Science
• Computational Chemistry
• Physical Chemistry

Background:

• Many-body polarization and hydration forces significantly influence mixed phase structures and energetics.
• Accurately modeling these forces is challenging for force fields, requiring balance between short-range hydrogen bonding and long-range interactions.

Purpose of the Study:

• To compare experimental pressure-area isotherms of hydroxypropylcellulose (HPC) with molecular dynamics simulations using various force field models.
• To evaluate the ability of different force fields to reproduce the heat-induced polymer aggregation observed in HPC.

Main Methods:

• Molecular dynamics simulations were performed using four force field models: united-atom, OPLS, CHARMM, and Drude oscillator.
• Simulations were compared against experimental pressure-area isotherm data for HPC.

Main Results:

• All force fields replicated short-range repulsive forces.
• Nonpolarizable force fields failed to show the critical point and exhibited biphasic behavior, indicating overestimated attractive forces.
• The Drude polarizable force field correctly predicted a single, homogeneous phase and positive osmotic pressure across tested conditions.

Conclusions:

• Polarizable force fields are crucial for accurately modeling hydration forces and polymer aggregation phenomena.
• Nonpolarizable, pairwise additive models tend to overestimate long-range dispersive attractions.
• Hydration forces are coupled to polymer coordination via local structural water molecules.