Detection and characterization of liquid|solid and liquid|liquid|solid interfacial gradients of water nanodroplets in wet N-octyl-2-pyrrolidone.

We report on the rotational diffusion dynamics and fluorescence lifetime of lissamine rhodamine B sulfonyl chloride (LRSC) in two thin-film experimental configurations. These are liquid|solid interfaces, where N-octyl-2-pyrrolidone (NOP) containing water and ethylene glycol (EG) thin films are each supported on glass, and a liquid|liquid|solid interface where thin films of water and NOP, both supported on glass, are in contact with one another, forming an NOP|water interface. The reorientation dynamics and fluorescence lifetime of LRSC are measured as a function of distance from the NOP|glass and EG|glass interfaces and from the NOP|water and NOP|glass interfaces in the liquid|liquid|solid experimental configuration. Fluorescence anisotropy decay data from the liquid|solid systems reveal a liquid film depth-dependent gradient spanning tens of micrometers from the NOP|glass interface into the wet NOP phase, while this gradient is absent in EG. We interpret these findings in the context of a compositional gradient in the NOP phase. The spatially resolved fluorescence lifetime and anisotropy decay data for an NOP|water|glass interfacial structure exhibits the absence of a gradient in the anisotropy decay profile normal to the NOP|water interface and the presence of a fluorescence lifetime gradient as a function of distance from the NOP|water interface. The compositional heterogeneity for both interfacial systems is in the form of water nanodroplets in the NOP phase. We understand this compositional gradient in the context of the relative surface energies of the water, NOP, and glass components.