Interfacial and nanoscale phase transitions are of fundamental importance for climate, industry and biological processes. On planar extended interfaces surface freezing occurs when an alkane/surfactant layer in contact with water reaches a temperature a few degrees above the bulk freezing point. This phenomenon entails the freezing of the surface alkyl chains, and is purely associated with the oil phase and not the water. Using second harmonic and sum frequency scattering techniques we investigate surface structural transitions on nanoscale hexadecane oil drops covered with a very dilute layer of alkyltrimethylammonium (CXTA+) ions in water. We find that on the nanoscale the behavior is significantly different from planar extended interfaces: Surface transitions are found that occur at temperatures a few degrees above the bulk super-cooled freezing point and not the bulk freezing point of the oil drops. The transitions involve all three interfacial constituents, the oil, the water and the surfactant, and are triggered by changes in the hydration structure of the alkyltrimethylammonium groups. Transitions do not occur for positively charged and not negatively charged. These results show remarkable difference related to the length scale of the system. Nanoscale droplets do not display the same interfacial structure and chemistry as extended planar interfaces of the same chemicals.

Hydration mediated interfacial transitions on mixed hydrophobic/hydrophilic nanodroplet interfaces

256th National Meeting (2018)

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