Molecular dynamics simulations have previously described how the physical properties across immiscible liquid-liquid interfaces should converge from aqueous to organic limits, but these predictions have largely gone untested, owing to difficulties associated with probing buried interfaces. X-ray and neutron scattering experiments have created detailed pictures of molecular structure at these boundaries, but such scattering studies cannot probe how surface-altered solvent structures affect interfacial solvating properties. Given that surface-mediated solvent properties control interfacial solute concentrations and reactivities, identifying the characteristic dimensions of interfacial solvation is essential for formulating predictive models of solution phase surface chemistry. Here we use specially synthesized solvatochromic surfactants that act as 'molecular rulers' and resonance-enhanced second-harmonic generation to measure the dipolar width of weakly and strongly associating liquid-liquid interfaces. Dipolar width describes the distance required for a dielectric environment to change from one phase to another. Our results show that polarity converges to a nonpolar limit on subnanometre length scales across a water-cyclohexane interface. However, polarity across the strongly associating, water-1-octanol interface is dominated by a nonpolar, alkane-like region. These data call into question the use of continuum descriptions of liquids to characterize interfacial solvation, and demonstrate that interfacial environments can vary in a non-additive manner from bulk solution limits.

Resonance-enhanced second harmonic generation (SHG) has been used to probe the solvatochromic behavior of two small, aromatic chromophores adsorbed to the aqueous/cyclohexane, liquid/liquid interface. SHG spectra of p-nitrophenol (PNP) and 2,6-dimethyl-PNP (dmPNP) indicate that these two chromophores sample markedly different environments. PNP sees a polar, waterlike environment, whereas solvent polarity surrounding dmPNP is dominated by the nonpolar, organic phase. Results suggest that subtle changes in solute structure can change the distribution of solutes across an interface and thus change a solute's local solvation environment.

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.

Homologous series of solvatochromic neutral alcohols and ionic sulfates are synthesized and characterized. Each surfactant series consists of hydrophobic, p-nitroanisole-based chromophores attached to polar or ionic headgroups by n-alkyl spacers. UV absorption measurements show that the optical properties of surfactant chromophores closely track those of the parent chromophore. Interfacial tension measurements are used to calculate surface excess concentrations of ionic surfactants adsorbed to an aqueous-cyclohexane interface. With a hydrophobic chromophore, a hydrophilic headgroup, and a variable-length, alkyl spacer, these surfactants have the potential to function as molecular rulers: probes of molecular-scale variation in solvation forces across condensed-phase interfaces. Changing the separation between the hydrophobic, solvatochromic probe and the hydrophilic headgroup should enable different members of a homologous series to span different interfacial widths, thus exposing the chromophore to different chemical environments. This idea is explored by using surface-specific, nonlinear optical spectroscopy. Resonant second harmonic spectra of p-nitroanisole and the surfactant product 4a adsorbed to an aqueous-cyclohexane interface show the surfactant spectrum blue-shifted 9 nm relative to the spectrum of adsorbed p-nitroanisole. On the basis of chromophore solvatochromism, these results are consistent with a less polar environment surrounding the surfactant chromophore. Significant differences in interfacial solvation resulting from a approximately 5 A separation between the surfactant headgroup and chromophore support recently proposed models of molecularly sharp, microscopically flat aqueous-alkane interfaces.

Solvatochromic surfactantsalso referred to as “molecular rulers”have been used to examine changes in polarity across strongly associating liquid/liquid interfaces. The water/alcohol interfaces were formed between an aqueous subphase and either linear (1-octanol and 1-decanol) or branched (3-octanol and 2,6-dimethyl-4-heptanol) alcohols. Resonance-enhanced second harmonic generation was used to collect effective excitation spectra of species adsorbed to these interfaces. Data suggest that all four water/alcohol interfaces contain a region of reduced polarity between the polar water phase and the bulk alcohol. We attribute this region to the alignment of the alkyl chains of the interfacial alcohol molecules. Polarity across the interfaces formed with linear alcohols changes gradually over the distances spanned by ruler surfactants, and interfacial width scales with the length of the alcohol solvent. In contrast, transitions at the interfaces between water and the two branched alcohols are more abrupt. These differences appear to correlate well with the free volume accessible to the solutes within the alcohol solvents adsorbed to the interface. Results are interpreted on the basis of recent studies examining vibrational structure and order in monolayers of alcohols adsorbed to water and alcohol-vapor interfaces.

A new family of neutral, solvatochromic surfactants has been used to probe solvent polarity across strongly associating solid/liquid interfaces. The surfactants consist of p-nitroanisole-based chromophores and polar -OH groups separated by alkyl chains of different lengths. The solid substrate is silanol-terminated, hydrophilic silica, and the strongly associating solvents are 1-butanol and 1-octanol. To understand how the chromophore itself interacts with the hydrophilic substrate, we acquire resonance enhanced second harmonic (SH) spectra of the bare chromophore adsorbed to the silica/cyclohexane, solid/liquid interface. Spectra show two features despite the chromophore having only a single electronic resonance in the wavelength region (∼300 nm) examined. These features are assigned to the chromophore adsorbed to the surface in two different orientations. Second harmonic spectra of the bare chromophore adsorbed to the hydrophilic/butanol and hydrophilic/octanol interfaces again show two features with the less polar, shorter wavelength feature being more pronounced in the octanol spectrum. Despite exhibiting pNAs-like behavior in bulk solution, molecular rulers present a very different picture of interfacial polarity. SH spectra from the hydrophilic/butanol interface indicate that interfacial polarity is enhanced over bulk solution limits. Furthermore, spectra lose their bimodal appearance, indicating that local polarity is rather uniform across the interfacial region. In contrast, the hydrophilic/octanol interface continues to show bimodal behavior for all of the molecular rulers studied, suggesting that surface-induced changes in solvent structure partitions solvent polarity into regions that are more polar and less polar than bulk solution. Data from the hydrophilic/octanol system suggest that variations in solvent polarity extend no more than ∼1.2 nm into solution or approximately the length of a fully extended, 1-octanol molecule.

A new family of cationic, solvent-sensitive surfactants is synthesized and characterized. The surfactants consist of a solvatochromic, hydrophobic chromophore connected to a quaternized ammonium headgroup by short alkyl spacers of varying lengths. The chromophore is based on p-nitroanisole (pNAs), a simple aromatic probe whose first electronic excitation wavelength shifts more than 20 nm as the local dielectric environment varies between nonpolar and polar limits. Given that pNAs is 20 times more soluble in cyclohexane than in water, lengthening the separation between the chromophore and the charged headgroup enables the chromophore to extend further into the organic phase when surfactants adsorb to an aqueous/organic interface. Thus, in principle, these surfactants can function as molecular rulers by measuring the distance required for solvent polarity to converge from an aqueous to an organic limit across different liquid/liquid boundaries. Preliminary resonance-enhanced second harmonic generation spectra of surfactants adsorbed to an aqueous/cyclohexane interface support this idea. In bulk solution, short-chain surfactants appear to self-associate, forming ringlike structures. This behavior manifests itself as diminished solvatochromic activity as well as clear, through-space coupling in nuclear Overhauser enhancement spectra. The strength of intramolecular association diminishes with increasing alkyl spacer length and disappears completely with alkyl chains having six methylene groups.

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.