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Unformatted text preview: SAMs Revised 11/22/10 1 SELF-ASSEMBLED MONOLAYERS (SAMs) Theresa M. McIntire, Sergey A. Nizkordov, and Kimberly Edwards Department of Chemistry, University of California, Irvine, Irvine, CA 92697-2025 OBJECTIVE The goal of this laboratory experiment is to form models for the organic films present on surfaces in the environment (e.g. atmospheric aerosols, building materials, glass, vegetation, etc.) and measure the contact angle of these surfaces. INTRODUCTION Water vapor is a vital component of the Earths atmosphere. The fraction of water molecules in the air rarely exceeds a few percent by weight, but water has a significant effect on the Earths climate and global air circulation. The presence of water is critical to life on Earth, and the physical state of water changes at various stages in the Earths water cycle. As a gas, atmospheric water acts as a major natural greenhouse gas, helping to warm the Earths surface. In contrast, liquid water in the form of aerosols, fogs, and clouds has an opposite effect on the climate as it scatters as much as 30% of incoming solar radiation back into space. In environmental and biological processes, water plays an essential role, yet, despite this importance, little is known about the fundamental molecular properties of water interacting with the organic films present on surfaces in the environment (e.g. atmospheric aerosols) or water interacting with organic surfaces in biosystems (e.g. membranes). Chemical and physical properties of many materials of interest to the environment are significantly affected by water in the air. Under normal ambient conditions, most surfaces in the environment have a very thin film of water adsorbed on them. As the relative humidity increases, the water film grows thicker, and water eventually condenses in droplets under the saturation conditions, i.e., when the relative humidity reaches 100%. The growth of cloud droplets occurs in a similar way with the exception that the growth takes place on tiny aerosol particles instead of surfaces. Materials strongly differ in their ability to adsorb water on their surfaces. Water films form easily on surfaces of hydrophilic materials such as glass or rust. Alternatively, hydrophobic materials such as Teflon or wax will inhibit water adsorption. One convenient way to classify materials by their ability to adsorb water is a measurement called the contact angle the angle a liquid droplet makes with the surface it is sitting on, as illustrated in Figure 1. Water droplets spread on hydro philic surfaces resulting in a small contact angle, typically 10-30 . Water minimizes its contact with a hydro phobic surface resulting in large contact angles, sometimes in excess of 90 . SAMs Revised 11/22/10 2 Figure 1. Left : A contact angle is the angle that the liquid/vapor interface makes with the solid surface....
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This note was uploaded on 11/22/2010 for the course CHEMISTRY Chem 1LB taught by Professor Dr.kimberlyedwards during the Fall '10 term at UC Irvine.
- Fall '10