The molecular structure of zwitterionic and charged monolayers on small oil droplets in aqueous solutions is determined using a combined second harmonic and sum frequency study. From the interfacial vibrational signature of the acyl chains and phosphate head groups, as well as the response of the hydrating water, we find that zwitterionic and charged lipids with identical acyl chains form remarkably different monolayers. Zwitterionic phospholipids form a closely packed monolayer with highly ordered acyl tails. In contrast, the charged phospholipids form a monolayer with a low number density and disordered acyl tails. The charged head groups are oriented perpendicular to the monolayer rather than parallel as it is the case of zwitterionic lipids. These significant differences between the two types of phospholipids indicate important roles of phospholipid head groups in the determination of properties of cellular membranes and lipid droplets. The observed behavior of charged phospholipids is different from expectations based on studies performed on extended planar interfaces, at which condensed monolayer are readily formed. The difference can be explained by nanoscale related changes in charge condensation behavior which has its origin in a different balance of interfacial intermolecular interactions.<br/>

Zwitterionic and charged lipids form remarkably different structures on nanoscale oil droplets in aqueous solution

255th National Meeting (2018)

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