Molecular Structure and Surface Organization: A Study of Liquid/Vapor Interfaces Using Newly Developed Sum Frequency Methods

Abstract

Vibrational sum frequency spectroscopy (VSFS) is a surface specific nonlinear technique that provides vibrational spectra of molecules at interfaces. Studies presented in this thesis use VSFS to examine surface structure of common liquids at liquid/vapor interfaces. The goal of this work is to correlate molecular structure and molecules functionality with the structure adopted by molecules at liquid surfaces.

The first part of this work describes the instrumental design and components of the newly developed VSFS. New methods are developed to overcome experimental difficulties associated with data collection for an entire spectral region of interest. The methods for data analysis and spectral post processing are also presented.

Surface vibrational studies of linear alkanes ranging in length from 9 to 17 carbon atoms investigated dependence of surface order on chain length. The linear alkane liquid surfaces evince a surprising degree of conformational order and this surface structure becomes more disordered as chain length increases. Halogenated alkane studies showed that replacing one terminal methyl group of linear alkanes with CH2X (X=Cl or Br) for chain lengths 10, 14 and 16 leads to a mixed surface layer primarily occupied by CH2X. These results are attributed to dipole-dipole interactions between the CH2X and are indicative of a higher surface activity of the halogenated ends compared to the unsubstituted ends.

Additional experiments have investigated the effect of stronger interactions on the surface structure of 1-, 3-, and 5-nonanols and their ketone analogs namely 1-, 3-, and 5-nonanones. Results showed an increasing order as the OH or =O groups advances toward the center of the molecule. A surprising result is a slightly lower surface order for nonanones compared to nonanols hints at the relative importance of simple dipolar interactions and highly directional hydrogen bonding in determining the surface structure. In addition, vibrational studies are carried out for 1- and 3-octanol at liquid/vapor interfaces. The results confirm observations from the nonanols by having a higher order for the 3-octanol. However, monolayer of 1-octanol on aqueous surface have a much higher surface order compared to 3-octanol. The increased order is attributed to the strong hydrogen bonding with water molecules.