Cation Effects on Interfacial Water Organization of Aqueous Chloride Solutions. I. Monovalent Cations: Li⁺, Na⁺, K⁺, and NH₄⁺

Abstract

The influence of monovalent cations on the interfacial water organization of alkali (LiCl, NaCl, and KCl) and ammonium chloride (NH4Cl) salt solutions was investigated using surface-sensitive conventional vibrational sum frequency generation (VSFG) and heterodyne-detected (HD-)VSFG spectroscopy. It was found in the conventional VSFG spectra that LiCl and NH4Cl significantly perturb water’s hydrogen-bonding network. In contrast, NaCl and KCl had little effect on the interfacial water structure and exhibited weak concentration dependency. The Im χs(2)(ωIR) spectra from HD-VSFG further revealed that, for all chloride solutions, the net transition dipole moments of hydrogen-bonded water molecules (O → H) are oriented more toward the vapor phase relative to neat water. This suggests the presence of an interfacial electric field generated from the formation of an ionic double layer in the interfacial region with a distribution of Cl(-) ions located above the countercations, in agreement with predictions from MD simulations. The magnitude of this electric field shows a small but definite cation specificity and follows the order Li(+) ≈ Na(+) > NH4(+) > K(+). The observed trend was found to be in good agreement with previously published surface potential data.

Specific cation effects at aqueous solution−vapor interfaces: Surfactant-like behavior of Li + revealed by experiments and simulations

“…Moreover, MD simulations have shown that, with few exceptions [e.g., SO 4 2− in (NH 4 ) 2 SO 4 (23)], anions adsorb more strongly to the solution−air interface than their counter cations, and, consequently, electrical double layers are formed near the interface, with the anions residing in or near the topmost layer of the solution, and the cations residing below the anions (14,24,25). Surface potentials (26), phase-sensitive vibrational sum frequency generation (PS-VSFG) spectra (22,27,28), and X-ray photoelectron spectroscopic (XPS) data (19,(29)(30)(31)(32) are consistent with the double layer picture.Compared with anion-specific effects, cation-specific effects at the solution−air interface are generally observed to be relatively weak. For example, the concentration dependence of the STIs of LiCl, NaCl, and KCl are very similar (33).…”

“…In one of the few studies that directly determined cation-specific effects on ion distributions in the interfacial region, XPS spectra and MD simulations revealed that Na + approaches the solution−air interface more closely than Rb + , and that the interfacial population of Cl − is greater in NaCl vs. RbCl solutions (31). PS-VSFG measurements, which provide indirect information on interfacial ion distributions via surface electric fields inferred from the imaginary part of the nonlinear susceptibility, have provided evidence of cation-specific effects on the strength of the electric double layer at the solution−air interfaces of nitrate, sulfate, and halide salt solutions (22,27,28).In almost all aqueous salt solutions of salts containing alkali metal cations, the cations are excluded from the topmost layer of the solution (14). It has been suggested, based on MD simulations (34)(35)(36), that Li + may be an exception.…”

Ion Spatial Distributions at the Air– and Vacuum–Aqueous K2CO3 Interfaces

“…However, before model predictions can be generalized the input parameters and assumptions must be validated by analytical measurements that can provide molecular level insight at the water interface. In this regard, three prominent analytical techniques have recently proved noteworthy complements to the well established macroscopic techniques of surface tension [8][9] and surface potential [10][11] for the study of air (vacuum)-water interfaces: second harmonic generation (SHG), [12][13][14][15][16][17][18][19][20][21][22] sum-frequency generation (SFG) spectroscopy [23][24][25][26][27][28][29][30][31] and liquid based X-ray photoelectron spectroscopy (XPS). [32][33][34][35][36][37][38][39][40][41][42][43][44][45] All three of these methods are capable of interrogating the microscopic structure of the air (vacuum)-water interface, and often provide complementary information due to the different properties probed.…”

Communication: Salt-induced water orientation at a surface of non-ionic surfactant in relation to a mechanism of Hofmeister effect

“…25,26 The ion dependence was also reported. However, the reported difference is much smaller than that observed in the present study between surfaces of the surfactant-pure water and the surfactant-saline solutions, inspite of the salt concentrations four times larger than the present study.…”

“…Such a heterogeneous distribution is similar to the case of the airwater interface. 25,27,28 The heterogeneous distribution of anions and cations supposedly induces the downward orientation of water at the surface of monomyristolein monolayers with their hydrogen atoms pointing to the bulk, as schematically shown in Fig. 4.…”