Solvation of p-nitrophenol at a water/alkane interface: the role of ionic strength and salt identity

Second harmonic generation (SHG), a surface specific, nonlinear optical spectroscopy, was used to study the interfacial solvation of a neutral surfactant, p-nitrophenol (PNP), adsorbed to the water/cyclohexane interface in the presence of simple salts at varying salt concentrations. The purpose of this work was to determine what relationship (if any) exists between interfacial polarity and bulk solution ionic strength. Data show an apparent red shift in SHG spectra with an increase in salt anion size from fluoride to chloride to bromide at 1 M salt concentrations. A spectral red shift of the PNP electronic excitation implies an increase in local polarity. Within experimental limits, however, these observed interfacial spectral shifts mimic shifts in absorbance spectra observed for PNP in bulk electrolyte solutions. Given the similarities between bulk and surface behavior, we conclude that observed shifts in SHG spectra may be attributed to effects similar to those found in bulk solution. Additionally, the surface adsorption of PNP to the water/cyclohexane interface was studied to determine the surface distribution of PNP and the conjugate base, p-nitrophenoxide (PNP(-)), for a 10 mM PNP solution. PNP adsorption is favored over PNP(-) adsorption by a factor of 10, giving rise to an equilibrium surface distribution that is an order of magnitude greater than that found in bulk solution. These findings indicate that the amount of PNP(-) at the surface in an aqueous solution of 10 mM PNP is negligible.