Invited Paper SS1-MoM3
Composition and Chemistry at the Liquid/Vapor Interface of Aqueous Solutions: Liquid-Jet XPS Experiments Coupled with MD Simulations

Monday, October 31, 2011, 9:00 am, Room 109

In spite of the importance of liquid/vapor interfaces to many real world problems our understanding of the composition and chemistry of liquid/vapor interfaces is limited. We have employed liquid micro-jet methods to obtain x-ray photoelectron spectra of the liquid/vapor interface of aqueous solutions at the BESSY II synchrotron. The variable photon energy characteristic of synchrotron light sources allows us to carry out experiments over a range of photoelectron kinetic energies—thus varying the probe depth of the experiment. The result is a depth profile of the composition of the liquid/vapor interface. We have focused our experiments on aqueous solutions. In this talk I will describe results from recent experiments on the following systems: (1) aqueous nitric acid solutions, in which we demonstrate a large difference in the degree of dissociation at the surface compared to bulk solution. We have also been able to determine the degree of dissociation at the interface and in the bulk solution for a range of temperatures sufficient to determine the enthalpy of the dissociation reaction for the interface and the bulk, (2) aqueous solutions of monoethanolamine and monoethanolamine reacted with CO₂---aqueous monoethanolamine is widely proposed for CO₂ capture processes, (3) aqueous solutions of the organosulfur compounds DMS, DMSO, DMSO₂, DMSO₃---DMS is a major source of environmental sulfur compounds including in the atmosphere. In these systems we have been able to determine the relative propensity of each molecule for the liquid/vapor interface, (4) carboxylic acid solutions where we have been able to determine the relative surface activity of the dissociated and undissociated acid. For each of the above systems we have carried out molecular dynamics simulations. These simulations combined with our experimental depth dependent measurements have provided molecular level insight into the behavior of molecular solutes in aqueous solution.