Impact of Salt Purity on Interfacial Water Organization Revealed by Conventional and Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Abstract

Water organization and ion distribution at air/ aqueous interfaces investigated by nonlinear vibrational spectroscopy as well as by other surface-sensitive techniques depend critically on the purity grade and purification processing of the chosen salts and their solutions. This is true not only for the ACS grade salts but also for the ultrapure (UP) grade, even though both have <1% impurities. It is shown here by means of conventional vibrational sum frequency generation (VSFG) and heterodyne-detected VSFG (HD-VSFG) spectroscopy that salt purity grade and pretreatment has a tremendous impact on the interfacial water spectrum of aqueous salt solutions. Our spectroscopic results indicate that salt solutions prepared from ACS and UP grade salts (NaCl, Na 2 CO 3 , Na 2 SO 4 , and (NH 4 ) 2 SO 4 ) display highly similar conventional VSFG and HD-VSFG spectra, provided that their solutions undergo pretreatment, that is, either salt baking followed by solution filtration or simply filtration for low melting point salts. In addition, untreated (NH 4 ) 2 SO 4 and NaCl salt solutions prepared from ACS grade salts show no significant spectral difference compared with pretreated ones. However, the corresponding untreated solutions made from UP grade salts show a remarkably higher content in organic impurities, which perturb greatly the conventional VSFG spectra in the water OH stretching region. Only in the case of high melting point halide salts such as chlorides, was it found that salt baking has an effect comparable to solution filtration. However, this was not always true for all salts, particularly those having oxyanions. Overall, to avoid spectral distortion introduced by the presence of organic contamination, it is strongly recommended that metal halide salts should be baked, yet all other molecular ion-based salts should be first baked (depending on the salt melting point), followed by filtration of their solutions before being used by VSFG spectroscopy and, most likely, in any other surface-sensitive spectroscopic studies of water organization at air/aqueous interfaces.

Effect of cation enrichment on dipalmitoylphosphatidylcholine (DPPC) monolayers at the air-water interface

“…For example Lee et al reported a large expansion ($15 Å 2 /molecule) in the LE phase for a 0.2 mM CaCl 2 solution [54], while other studies showed only small MMA increases (61 Å 2 /molecule) for 5 mM and 0.1 M solutions [53,56]. More recent studies done in our group have demonstrated the impact of salt purity and purification method on interfacial measurements [28], especially when investigating ion binding to lipids [57]. Furthermore, studies done by Kewalramani et al on dimyris toylphosphatidylcholine (DMPC) monolayers found that aqueous solutions made from unbaked salts impacted the surface pressure at which the DMPC LE-LC coexistence phase occurred [58].…”

“…Sodium chloride (NaCl) (P99%, ACS certified, Fisher Scientific), potassium chloride (KCl) (P99%, ACS certified, Fisher Scientific), calcium chloride dihydrate (CaCl 2 Á2H 2 O) (P99%, FCC grade, Fisher Scientific), and magnesium chloride hexahydrate (MgCl 2 Á6H 2 O) (P99%, ACS certified, Fisher Scientific) were dissolved in water with a resistivity of 18.2 MX cm and a measured pH of 5.6 (NANOpure Analytical Deionization System, model D4741, Barnstead/ Thermolyne Corporation) to prepare stock solutions. These solutions were subsequently purified by a method that has been previously described [28]. Briefly, trace organic and inorganic impurities were removed by several filtrations through activated carbon filters (Whatman Carbon-Cap 75, Fisher Scientific).…”

Surface Electric Fields of Aqueous Solutions of NH4NO3, Mg(NO3)2, NaNO3, and LiNO3: Implications for Atmospheric Aerosol Chemistry

“…Stock solutions for VSFG measurements were prepared by dissolving salts in ultrapure water and then by filtering them four to six times using activated carbon filters (Whatman Carbon Cap 75, Fisher Scientific) to completely eliminate organic impurities. 56 Raman calibration curves were generated for each nitrate salt solution in order to determine the concentration following filtration (Supporting Information). The measured pH of 2.0 M LiNO 3 , 1.7 M NaNO 3 , 1.6 M NH 4 NO 3 , and 1.0 M Mg(NO 3 ) 2 ranged between 5 and 7.…”

“…In addition, previous VSFG spectra from carbonate, sulfate, and chloride salt solutions demonstrated that for the same chemical species ultrapure (>99.99%) versus high purity (∼99%) grade salts display highly similar conventional VSFG and HD-VSFG spectra in the OH region after appropriate pretreatment of salts and salt solutions. 56 Methods. Conventional and Heterodyne-Detected VSFG Spectroscopy.…”

Study of Water Interfaces With Phase-Sensitive Sum Frequency Vibrational Spectroscopy

“…[83][84][85][86] The differential Imχ (2) S spectra for different salts with respect to that of neat water are presented in Fig. 12.…”