Determination of Structure and Energetics for Gibbs Surface Adsorption Layers of Binary Liquid Mixture 2. Methanol + Water

Abstract

Vapor/methanol and vapor/methanol-water mixture interfaces have been among the benchmark liquid interfaces under extensive experimental and theoretical investigation. In this report, we studied the orientation, structure and energetics of the vapor/methanol-water interface with newly developed techniques in sum frequency generation vibrational spectroscopy (SFG-VS). Different from the interpretations in previous SFG-VS studies for a more disordered interface at higher bulk methanol concentrations, we found that the methanol-water mixture interface is well ordered in the whole concentration region. We are able to do so because direct polarization null angle (PNA) measurement allowed us to accurately determine the CH3 orientation at the interface and to separate the orientational and interface density contributions to the SFG-VS signal. We found that the CH3 groups at the interface pointed out almost perpendicularly from the interface. We further found that this well-ordered vapor/methanol-water mixture interface has an antiparallel structure. With the double layer adsorption model (DAM) and Langmuir isotherm, the adsorption free energies for the first and second layer are obtained as -1.7 +/- 0.1 kcal/mol and 0.5 +/- 0.4 kcal/mol, respectively. Therefore, the second layer adsorption is slightly negative, and this means that replacement of the second layer water molecule with methanol molecule is energetically unfavorable. Comparing this interface with the vapor/acetone-water mixture interface reported previously, we are able to correlate the second layer adsorption free energy with the work of self-association using the pairwise self- and mutual interaction energies between the water and solute molecules. These results provided detailed microscopic structural evidences for understanding of liquid interfaces.

A combined droplet train and ambient pressure photoemission spectrometer for the investigation of liquid/vapor interfaces

“…The goal of these experiments is to determine the composition of the methanol solution/vapor interface, in particular addressing the possibility of segregation of methanol to the vapor/solution interface, as suggested from Molecular Dynamics simulations 39,40 and SFG measurements. 41,42,43,44 Since the probing depth in XPS depends on the kinetic energy of the photoelectrons, which in turn is determined by the incident photon energy, depth-profiles of the stoichiometry at the solution/vapor interface can be measured, in the case described here up to a depth of about 2 nm.…”

Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

“…47 The real difficulty lies on the fact that quantitative analysis and interpretation of the SFG-VS, as well as SHG, data had been not as well developed and widely performed until recently. 25,35,48,49,50,51,52,53,54,55,56,57,58 Therefore, conclusions in many previous reports on the investigations of air/water interface, as well as other liquid interfaces, with SFG-VS are subjected to different interpretations.…”

“…49,50,51,52,53,54,55,56,57,58 In these works, we not only developed methodology for quantitative polarization and experimental configuration analysis in SFG-VS and SHG, we also tested accuracy and sensitivity of some of the methodology. We have applied them to elucidated the anti-parallel double layered structure and thermodynamics of some organic liquid aqueous solution interfaces.…”

Reconsideration of second-harmonic generation from isotropic liquid interface: Broken Kleinman symmetry of neat air/water interface from dipolar contribution

“…71,94 As the experimental analyses have consistently given relatively narrower orientational distribution of the liquid interfacial molecules, simulation results have always accounted for a much broader orientational distribution. 82,86,87,89,90,91,95,96,97 Such issue has to be answered in the future studies.…”

“…Detailed analysis of the SHG and SFG-VS experimental data has been significantly improved and systematically demonstrated in recent years. 43,44,55,67,68,83,84,85,86,87,88,89,90,91 Even though simulation results have provided a great deal of detailed understandings of the liquid interfaces, these disagreements between experimental and theoretical results suggest that the physical picture provided by the simulations may need to be systematically reexamined as discussed below.…”