The role of charge in solvation at liquid/liquid interfaces

Abstract

This dissertation describes the development and characterization of new surfactants, dubbed "molecular rulers," that provide an upper limit to the dipolar width in aqueous/organic systems. Here, dipolar width describes the distance required for the dielectric properties of one phase to converge to those of the adjacent phase. Molecular rulers consist of a hydrophobic, solvatochromic chromophore and a charged headgroup connected via a variable length methylene chain. These surfactants are anchored to the aqueous phase by the ionic headgroup while the solvatochromic probe "floats" into the organic phase. The length of the alkyl chain controls the position of the chromophore within the interfacial region. Resonance-enhanced second harmonic generation (SHG) is used to profile the electronic excitation energy of the chromophore as a function of alkyl chain length. Since the solute's excitation energy depends on solvent polarity, we can infer interfacial dipolar width.

In previous work anionic molecular rulers were used to characterize the water/cyclohexane interface. Anionic rulers having two carbon alkyl chains sample a polarity between that of bulk water and bulk cyclohexane. Analogous cationic rulers described in this dissertation sample an environment equivalent to that of bulk cyclohexane. These results suggest that interfacial polarity may depend on surface charges having a close proximity to the adsorbed solute.

This idea was tested using cationic rulers adsorbed to the water/vapor surface of an electrolyte solution saturated with 1-octanol (a mimic of the water/alkane interface). As ionic strength increases, cationic ruler SHG behavior approaches that of the anionic species, suggesting that the ions in solution shield the cationic charge from a probe-headgroup interaction that was observed with NMR experiments for bulk aqueous solution samples.

A neutral organic molecule at the electrolyte solution/cyclohexane interface was employed to elucidate the role of charge in interfacial solvation. Observed shifts in SHG spectra from salt-free limits are similar to those of absorbance spectra for the solute in bulk electrolyte solutions. We conclude that, in the absence of direct charge-probe correlation, charges have a similar influence on interfacial solvation of neutral species as they do in bulk solution.