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

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, Na2CO3, Na2SO4, and (NH4)(2)SO4) 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 (NH4)(2)SO4 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.