Characterizing Anion Adsorption to Aqueous Interfaces: Toluene–Water versus Air–Water

Abstract

We continue our investigation of the behavior of simple ions at aqueous interfaces, employing the combination of two surface-sensitive nonlinear spectroscopy tools, broadband deep UV electronic sum-frequency generation and UV second harmonic generation, to characterize the adsorption of thiocyanate to the interface of water with toluenea prototypical hydrophobe. We find that both the interfacial spectrum and the Gibbs free energy of adsorption closely match results previously reported for the air–water interface. We observe no relative spectral shift in the higher-energy CTTS transition of thiocyanate, implying similar solvation environments for the two interfaces. Similarly, the Gibbs free energies of adsorption agree within error; however, we expect the respective enthalpic and entropic contributions to differ between the two interfaces, similar to our earlier findings for the air–water versus graphene–water interfaces. Further experiments and theoretical modeling are necessary to quantify the mechanistic differences.

Structure, Thermodynamics, and Dynamics of Thiocyanate Ion Adsorption and Transfer across the Water/Toluene Interface

“…The free energy profile for the transfer of the ion across the water/toluene interface is shown in the bottom panel of Figure . The profile exhibits a small local minimum near the GDS that corresponds to a free energy of adsorption (relative to bulk water) of about −1.4 kcal/mol (in reasonable agreement with the experimental value of −1.7 ± 0.2 kcal/mol) . As the ion crosses the interface, the free energy increases monotonically and reaches a plateau significantly deep into the organic phase, around Z = 20 Å.…”

“…The profile exhibits a small local minimum near the GDS that corresponds to a free energy of adsorption (relative to bulk water) of about −1.4 kcal/mol (in reasonable agreement with the experimental value of −1.7 ± 0.2 kcal/mol). 21 As the ion crosses the interface, the free energy increases monotonically and reaches a plateau significantly deep into the organic phase, around Z = 20 Å. The free energy of transfer of about 27 kcal/ mol is significantly smaller than the difference between the solvation free energy of the "naked" ion in toluene (estimated as −6 kcal/mol) and the hydration free energy (−70 kcal/mol 14 ), suggesting that some co-transfer of first hydration water molecules is in play, as will be discussed at length below.…”

“…This seems to be consistent with recent second harmonic generations experiments. 21 The sharp reversal (increase) in the order parameter near Z I ≈ 18 Å can be traced to an unusually stable and long-lived water finger just before breakup (see Figure 5) in a lamella that is one layer deeper into the organic phase.…”

“…Compared with previous studies on ion transfer of small ions and highly charged ions, the SCN – ion is relatively large and not strongly hydrated (Δ G hyd = −70 kcal/mol). This ion plays a significant role in several important interfacial processes including the extraction of rare earth metals from aqueous to organic media and several important biochemical pathways and has been the focus of many surface electrochemical experiments. − Our work is also motivated by new experimental data on the adsorption of this ion at a number of water interfaces, − including the application of broad-band deep UV electronic sum-frequency generation and UV second harmonic generation to the adsorption of thiocyanate at the water/toluene interface, following an earlier work utilizing resonant UV SHG to study the adsorption of this anion at the water/dodecanol interface . The measured adsorption free energy of this ion and some information about its orientation serve as important validation of the molecular model we employ.…”

On the mechanisms of ion adsorption to aqueous interfaces: air-water vs. oil-water

“…Most commonly, the water structure adjacent to a hydrophobic phase is studied via vibrational sum-frequency generation (SFG), and chemical information is inferred by monitoring changes in the OH spectrum ( 8 – 13 ), as has been demonstrated by the Richmond, Allen, Roke, and Shen groups, among others. An alternative approach is to directly probe a suitable resonant ionic chromophore that resides in the interfacial layer, which can yield fundamental thermodynamic information regarding ion adsorption and solvation structure ( 14 – 20 ).…”