12 March 1999 Ž . Chemical Physics Letters 302 1999 157–163 The structure of water on HCl solutions studied with sum frequency generation Steve Baldelli, Cheryl Schnitzer, Mary Jane Shultz ) Department of Chemistry, Tufts UniÕersity, Medford, MA 02155, USA Received 1 August 1998; in final form 11 January 1999 Abstract Ž . The influence of dissolved HCl on the surface of water has been investigated with sum frequency generation SFG spectroscopy. Ions in solution cause water on the surface to be reoriented relative to pure water, with hydrogen atoms directed toward the bulk solution. There is no signal due to molecular HCl suggesting that oriented water molecules, not molecular HCl, dominate the surface. A model is proposed to account for the reported SFG results as well as surface tension and surface potential measurements. The result suggests that application of the Gibbs equation to determine surface excess may need to be reevaluated. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction The structure of the surface of water has been studied since the early days of physical chemistry. The majority of these investigations involve systems of surface-active organic monolayers on liquid water. One area where there is scarce information is the influence of dissolved inorganic ions on the surface despite their frequent occurrence in nature. Methods such as surface tension and surface potential have been applied to these systems with very important results but are limited due to lack of molecular sensitivity of these techniques. In the reported work, sum frequency generation Ž . SFG is utilized to investigate the effect of HCl on ) Corresponding author. E-mail: mshultz1@emerald.tufts.edu the water surface. A previous SFG study indicates that molecular HCl is not present on the surface of wx HClrwater solutions 1 . The results presented here focus on the structure of water in these solutions and suggest that ionic HCl influences water through a sub-surface electric double-layer. These results are discussed in relation to previous surface tension and surface potential experiments, theoretical calcula- tions, Raman and infrared spectroscopic results. SFG is a nonlinear, vibrational spectroscopy which, in the electric dipole approximation, is sensi- tive to the surface region between two centrosym- metric phases. Since the bulk phase is isotropic, the probed region extends only to a depth as long as the isotropic symmetry is broken, i.e. where there is a net polar orientation of molecules. A vibrational SFG spectrum is obtained from the spatial and temporal overlap of two laser beams at the surface, one visible and the other tunable infrared. As the frequency of 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 00094-9