Surface charge effects on solvation across liquid/liquid and model liquid/liquid interfaces

Abstract

Resonance enhanced second harmonic generation (SHG) coupled with novel solvatochromic surfactants has been used to explore solvation across liquid/liquid interfaces in the presence of excess surface charge in the aqueous phase. The surfactants—dubbed “molecular rulers”—consist of hydrophobic, solvatochromic chromophores connected to charged headgroups via variable length alkyl spacers. Interfacial dipolar width is monitored as a function of chromophore/headgroup separation. Data show that cationic and anionic surfactants of equivalent lengths sample very different environments across a water/cyclohexane interface. The effective excitation wavelengths of cationic surfactants are shifted persistently to higher energies than in bulk cyclohexane while anionic surfactants solvation converges smoothly from the aqueous to the organic limit with increasing spacer length. To further evaluate the effect of surface charge on interfacial solvation, SHG was used to probe the environment surrounding molecular ruler chromophores adsorbed to the aqueous liquid/vapor interface in the presence of densely packed monolayers of 1-octanol. These monolayer systems are shown to reproduce qualitatively many of the features associated with bulk water/alkane interfaces and are logistically easier to assemble. Changing the ionic strength of the underlying aqueous sub-phase suggests that the headgroup of cationic molecular ruler surfactants alters the electronic structure of the chromophore rather than properties of the surrounding nonpolar environment.