Sum frequency generation spectroscopy of the aqueous interface: ionic and soluble molecular solutions

The liquid interface of aqueous solutions is of central importance to numer ous phenomena from cloud processing of combustion generated oxides to corro sion degradation of structural materials to transport across cell membranes . Despite the importance of this interface, little molecular-level informat ion was known about it prior to the last decade-and-a-half. Molecular-level information is important not only for a fundamental understanding of proce sses at interfaces, but also for predicting methods for diminishing deleter ious effects. Recently, the nonlinear spectroscopic method, sum frequency g eneration (SFG), has been applied to the investigation of the structure of the liquid interface. This review focuses on the liquid-air interface of aq ueous solutions containing soluble, ionic species-H2SO4, HNO3, HCl, alkali sulphates and bisulphates, NaCl and NaNO3-as well as soluble molecular spec ies-glycerol, sulphuric acid and ammonia. Ionic materials influence the str ucture of water at the interface through an electric double layer which ari ses from the differential distribution of anions and cations near the inter face. Due to the extreme size of the proton, the strongest field is generat ed by acidic materials. As the concentration of these ionic materials incre ases, ion pairs form diminishing the strength of the double layer. This ena bles the ion-pair complex to penetrate to the interface and either displace water or bind it into hydrated complexes. Soluble materials of lower surfa ce tension partition to the interface and either displace water from the in terface or bind water into hydrated complexes. In particular, the conjectur ed ammonia-water complex on aqueous solutions is observed and it is determi ned to tilt 34-38 degrees from the normal.