Multimodal confined water dynamics in reverse osmosis polyamide membranes

Area of Science:

• Materials Science
• Physical Chemistry
• Chemical Engineering

Background:

• Polyamide (PA) membranes are vital for water purification and desalination via reverse osmosis.
• A detailed molecular-level understanding of water diffusion and polymer dynamics within PA membranes is lacking.
• Previous research indicated water diffusion in PA membranes is similar to bulk water, despite their complex structure.

Purpose of the Study:

• To investigate water transport and polymer relaxation dynamics in PA membranes at the molecular and nanoscale.
• To elucidate the mechanisms of water diffusion within the confined environment of PA membranes.
• To establish a comprehensive 'diffusion map' for PA membranes to guide future simulations and design.

Main Methods:

• Utilized neutron spectroscopy with isotopic contrasts to study PA membranes under controlled hydration.
• Analyzed water diffusion and polymer relaxation over picosecond to nanosecond timescales and Angstrom to nanometer lengthscales.
• Experimental resolution of multimodal water diffusion and polymer matrix rotational relaxations.

Main Results:

• Experimentally resolved multimodal water diffusion in PA membranes, including slowed translational jump-diffusion, a long-range mode, and a localized mode.
• Quantified the geometry and timescales associated with these distinct water diffusion modes.
• Observed rotational relaxations in the PA polymer matrix that correlate with the nanoscale confinement of water diffusion.

Conclusions:

• The study provides the first experimental resolution of complex, multimodal water diffusion in PA membranes.
• The findings offer a molecular-level understanding of water transport and polymer dynamics, crucial for membrane performance.
• This comprehensive 'diffusion map' will aid in anchoring simulations and enabling predictive design of advanced PA membranes with tailored properties.