Abnormal Reverse Osmosis Phenomenon in Dipalmitoylphosphatidylcholine Bilayers Enabled by Terahertz Waves

Developing new technology in membrane desalination is crucial for addressing the global water crisis. Reverse osmosis (RO) membranes exhibit numerous advantages, such as high efficiency, cost-effectiveness, environmental sustainability, etc. In this work, we observe an abnormal RO phenomenon for the first time in dipalmitoylphosphatidylcholine (DPPC) bilayers under the stimuli of terahertz (THz) waves. Our RO model contains two DPPC bilayers that divide the saline and aqueous solutions. Surprisingly, under specific field strength and frequency, we observe considerable net water flow from the saline solution chamber, crossing the bilayers, to the aqueous solution chamber, which suggests a new RO phenomenon in a highly controllable fashion. The mechanism for this abnormal RO process is that in THz waves, some ions can strip off their hydration shells and directly adsorb onto the lipid heads, resulting in local aggregation of head groups. This creates large gaps between some lipids and loose membrane structures in the saline solution region, breaking the structural symmetry in bilayers that facilitates the RO permeation. The reduced potential of mean force (PMF) barriers, ion hydration number, ion density behavior, and membrane structure strongly support our explanation of the RO mechanism. Our findings shed light on a complete new mechanism of RO for biological membranes, and breaking the membrane structural symmetry provides a potential new pathway for the design of RO membranes.