Solvent Polarity across Weakly Associating Interfaces

Abstract

Molecular ruler surfactants, solvatochromic probes of solvent polarity, have been used to examine changes in solvent polarity across weakly associating liquid/liquid interfaces. The water/alkane interfaces were formed between an aqueous subphase and either cyclic (cyclohexane and methylcyclohexane) or linear (octane and hexadecane) alkanes. Resonance-enhanced second-harmonic generation was used to collect effective excitation spectra of species adsorbed to these interfaces. As surfactants lengthened, the surfactant probe sampled an increasingly nonpolar environment as evidenced by an excitation wavelength that shifted toward the alkane limit. Data suggest that all four water/alkane interfaces are molecularly sharp (<9 Å), but that differences in the solvent molecular structure alter the transition from aqueous to organic solvation across the interface. Polarity across two interfaces (cyclohexane and hexadecane) changes gradually over the distance spanned by ruler surfactants. In contrast, the transitions at the interfaces between water and methylcyclohexane and octane appear much more abrupt. These findings appear to correlate with each organic solvent's ability to pack and associated free volume. More free volume in the organic phase leads to a more abrupt water/alkane interface. Results are interpreted on the basis of recent molecular dynamics simulations examining polarity at different water/monolayer interfaces.

A study on orientation and absorption spectrum of interfacial molecules by using continuum model

“…11 The selected transitions with large oscillator strength are given in Table 1. It is seen that the transitions are assigned S 0 ?…”

“…11 The solvation energies and spectral shifts of C 2 H and C 2 at cyclohexane/water interface are given in Figures 8e-8h. In the paper by Steel and Walker, 11 the notation C 2 indicates that two methylene groups separate the sulfate headgroup from the PNAS-based chromophore, and the PNAS-based chromophore is set in the organic solvent. Therefore, we choose the charge distribution of excited state in cyclohexane to perform the interfacial property calculation for both.…”

Reactivity and Dynamics at Liquid Interfaces

“…The application of SHG to electronic spectra at liquid interfaces has been mainly limited to measuring the peak spectrum for insight into the polarity of the interface region. 23,[355][356][357][358][359][360][361] By comparing the peak position of the SHG spectrum at the interface with the peak UV absorption spectra of the same dye molecule in the bulk of different solvents, one can place the interface polarity on an established polarity scale. For example, the gas phase intramolecular…”

“…The SHG spectra of these surfactants were measured at different water/liquid alkane interfaces. 23,360,361 As the length of the alkyl spacer changes, the polarity "reported" by the PNA probe varies from the polarity of bulk water to that of bulk alkane. This is direct evidence that the interface is sharp -the width is no greater than the size of a C 6 "ruler", i.e., about 1nm.…”

“…More importantly, the |χ (2) | 2 spectra cannot be compared directly with UV absorption spectra, and thus lineshape information is not readily available, although qualitative correlations between SHG spectral width and the heterogeneity of the system can be made. 23,360,361 Recently, Tahara and coworkers used heterodyne-detected electronic sum frequency generation (HD-ESFG) spectroscopy to measure the imaginary χ (2) spectra, which, for the first time, can be compared with the bulk absorption lineshape. 371 The effective polarity of the air/water interface, which was determined by the peak position, depended on the type of dye molecule used.…”