Solvent polarity of a medium is an important physical property that impacts the interaction between the solvent and the solute molecule. The polarity of a solvent can be empirically determined through many techniques. However, the polarity at the surfaces of particles, dispersed in bulk liquid, is not well understood. Knowledge of particle surface polarity is important in the realm of nanotechnology, aerosol chemistry, and drug delivery because processes such as adsorption and reactions are influenced by surface polarity. We have applied the surface selective spectroscopic method of second-harmonic generation (SHG), with a solvatochromic dye molecule as a polarity indicator, to probe the polarity of aqueous-colloidal interface.  We demonstrated that 4-[[(5-methoxyphenyl)-2oxazoly] pyridinium methane sulfonate] (PyMPoMEMs), which adsorbs to the surface of polystyrene based particles and yield strong SHG signal, is a good polarity indicator by comparing its solvatochromic behavior with that of other well-known solvatochromic dyes that do not adsorb to the colloidal particles investigated. PyMPO-MeMs exhibits spectral shift with varying solvent polarity and thus from its surface SHG spectra the polarity of the aqueous-colloidal interface has been inferred for both charged and neutral polymer based particles. Our results suggest that interfaces of these colloids exhibit marked differences in polarity compared to the polarity of the corresponding bulk solvent. This study opens up the possibility to determine interfacial polarity of numerous colloidal particles omnipresent in the environment.

Determination of aqueous-colloidal interfacial polarity using surface selective second harmonic generation spectroscopy

246th National Meeting (2013)

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