Water Adsorption and Characterization on Alkanethiol Self-assembled Monolayers on Gold

Open Access
Author:
Tu, Aimee
Area of Honors:
Interdisciplinary in Chemical Engineering and Engineering Science
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Seong Kim, Thesis Supervisor
  • Seong Han Kim, Thesis Supervisor
  • Christine Masters, Honors Advisor
  • Themis Matsoukas, Honors Advisor
  • Judith A Todd, Faculty Reader
Keywords:
  • carboxylic acid
  • water adsorption
  • self-assembled monolayers
  • hydroxyl group
Abstract:
The water adsorption isotherms on three different functional groups – methyl, carboxylic acid, and hydroxyl terminated alkanethiol SAMs on gold were studied. The water adsorption isotherms were measured with polarization modulation reflection absorption infrared spectroscopy (PM-RAIRS), which gives structural information, but cannot give absolute thickness; quartz crystal microbalance (QCM), which gives the mass gain that can be related to thickness; and ellipsometry, which gives thickness information. While it was desired for more quantitative information to be obtained from QCM and ellipsometry, because of the nature of these techniques, the small amount of molecules adsorbed, and the materials used, the results from these techniques must be mainly taken as qualitative. For QCM, the adsorption of water on the bare quartz surface not covered with SAM interferes with the accurate measurement of the water adsorbed by the SAM-modified surface on the QCM crystal. Meanwhile, the ellipsometer experienced some problems due to laser instability and electrical grounding, so the sensitivity was not good. The ellipsometer gave qualitative trends, but actual thickness could not be determined. However, these three techniques used together provided an idea of the structure and thickness of the water layer adsorbed and the behavior of water on these surfaces. The –CH3 terminated SAM did not adsorb water as the relative humidity was increased. The water on the –COOH terminated SAM has a broad OH stretching peak, indicating different hydrogen bonding behavior. Ionized carboxylic acid groups were also present on the surface, causing a stronger water-surface interaction. The –OH terminated SAM showed a narrower OH stretching peak, and also a large amount of water (~18 Å) that could not be removed in dry conditions.