Self-Assembled Alkanethiolate Monolayers on Copper, Silver, and Gold
Abstract
Long-chain alkanethiols, HS(CH_2) _{rm n}X, adsorb from solution onto the surfaces of copper, silver,and gold and form densely packed, oriented monolayers. These assemblies were characterized by wetting, and X-ray photoelectron and infrared spectroscopies. The structures that form on the three metals are related but differ in details of structure. On the three metals, the "self-assembled" monolayers contain trans-extended hydrocarbon chains and are bound to the metal as thiolates. On gold, the hydrocarbon chain cants ~30 ^circ from the surface normal; on copper and silver, the chain is oriented closer to the surface normal (cant angle ~12 ^circ) suggesting that the density of thiolates is higher on these metals than on gold. At room temperature, n-alkanethiolate monolayers on copper and silver have a lower population of gauche conformers than those formed on gold. Using these monolayers as scattering layers, attenuation lengths (lambda ) ranging from 22 to 40 A were measured for electrons having kinetic energies 500 to 1500 eV, respectively. In the assembly of the monolayers, a variety of polar and non-polar tail groups (X) can be accommodated allowing formation of organic surfaces that cover the range of wettabilities: theta_{rm a}(H _2O) = <10 ^circ to ~115 ^circ. The differences that exist in structure between the monolayers formed on the different metals do not result in differences in their wetting properties. Monolayers composed of more than one alkanethiolate are prepared by exposing the metal to solutions containing mixtures of alkanethiols. The contact angle of water on monolayers on the three metals derived from mixtures of HS(CH_2)_{11} CH_3 and HS(CH_2) _{11}OH is related to surface composition better by Cassie's expression than by the Israelachvili -Gee equation. Exposure of gold and silver to solutions containing mixtures of thiols of different chain lengths (C_{11} and C _{22}) results in monolayers having different surface compositions: the monolayers formed on silver are closer to the kinetic product than those formed under similar conditions on gold. The "mixed" monolayers on silver exhibit similar wettabilities to those having the same composition as those on gold. On both metals, the two components in the monolayers do not segregate to form macroscopic islands.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1991
- Bibcode:
- 1991PhDT.......126L
- Keywords:
-
- ALKANETHIOLS;
- Chemistry: Organic; Chemistry: Physical; Physics: Condensed Matter