Most membranes designed for molecular separations in liquids, namely Reverse Osmosis (RO) and Nanofiltration (NF) membranes, comprise a tight separating film on a more open support. Understanding liquid transport through membranes at the molecular scale remains challenging due to (1) the lack of resolution of the membrane material at the molecular scale and (2) description of molecular flow through the resulting structure. Membrane pores (free volume fluctuations) are commensurate with permeate molecules and therefore describing transport is challenging. Our group employs Molecular Dynamics (MD) simulations to predict the flux through membranes. The aim of this exploratory proposal is to investigate the feasibility of BRISP measurements to evaluate the nature of collective excitations of water confined in two model membranes, with different nanoporosity and transport performance. Our ambition is to create routes to manufacture polymeric thin membranes for pressure-driven molecular separations in liquids, with fine control of throughput and selectivity, through the fundamental understanding of the coupling between membrane structure and function.