Water Structuring Induces Nonuniversal Hydration Repulsion between Polar Surfaces: Quantitative Comparison between...

Area of Science:

• Physical Chemistry
• Surface Science
• Biophysics

Background:

• Polar surfaces in water exhibit repulsion at close distances, forming a stable nanometric water layer.
• This hydration repulsion is vital for concentrated suspensions but its molecular origin remains unclear.
• The prevailing hypothesis attributes repulsion to surface-induced water structuring.

Purpose of the Study:

• To quantitatively investigate the mechanism behind hydration repulsion between polar surfaces.
• To validate molecular simulation models against experimental data for hydration forces.
• To elucidate the role of water structuring in mediating surface interactions.

Main Methods:

• Performing molecular dynamics simulations of various planar polar surfaces immersed in water.
• Calculating total surface interaction forces and decomposing them into direct and water-mediated contributions.
• Analyzing water ordering profiles near the surfaces and comparing simulation results with theoretical predictions.

Main Results:

• Simulated hydration forces between phospholipid bilayers closely matched experimental results.
• Indirect hydration forces and water ordering profiles aligned with theoretical models of surface-induced water ordering.
• Different surface headgroup architectures led to varied decay lengths in water-mediated forces, indicating non-universal behavior.

Conclusions:

• The water-structuring hypothesis for hydration repulsion is strongly supported by simulation and theoretical agreement.
• Hydration repulsion is primarily caused by surface-induced water structuring.
• The specific nature of surface structures dictates the water ordering and thus shapes a surface-specific, non-universal hydration repulsion.