Self-Assembled Silane Monolayers: Conversion of Cyano to Carboxylic Termination
Read full article onlineFull Article
In recent years, there has been an increasing interest in the study and development of molecularly thin films such as self-assembled monolayers (SAMs) and Langmuir–Blodgett (LB) films. SAMs provide a unique way of altering just the surface properties of a material, by modifying the terminal functional groups displayed on the surface without altering or affecting the bulk properties of the material. The ability to modify just the surface properties gives rise to several technological applications such as protective coatings, lubrication layers, membranes for chemical and biochemical sensors, templates for crystal growth, fabrication of nanoparticles, micropatterning, and molecularly thin transistors, to name a few.
SAMs are molecular assemblies that are formed by spontaneous adsorption of surfactant molecules (amphiphiles), present either in solution or vapor phase, onto the surface of an appropriate substrate. The three classical types of SAMs are: 1) silanes on hydroxylated surfaces; 2) alkanethiols on gold, copper, or silver surfaces; and 3) long-chain carboxylic acids on aluminum oxide surfaces. The quality of the SAM depends on the head group, the hydrocarbon chain length, the terminal functional group, the solvent used, the amount of water present in the solvent (specifically for silane SAMs), and the preparation temperature.
In this paper, we present the results of the modification of the CN terminal functional group of 11-cyanoundecyltrimetjoxysilane (CUTMS) monolayer to the COOH terminal group by a simple hydrolysis reaction. SAMs with a COOH terminal group are technologically interesting, as the ionization state of these groups is sensitive to the pH of the surrounding environment. By varying the pH of the surface, the terminal groups are transformed from all COOH (at low pH) to all COO− (at high pH), or a mixture of COOH and COO− (at intermediate pH values). By controlling the pH, the ratio of the number densities of the COOH group to the COO− groups is controlled, which in turn determines the local structure and arrangement of the surface groups. Thus in an indirect way, the structure and arrangement of the COOH and COO− groups on the surface are fine-tuned by just controlling the pH of the system. We also demonstrate this interconvertibility of COOH↔COO− by controlling the pH using both modified CUTMS monolayer and LB film of stearic acid. From quantitative analyses of the infrared spectra of COOH and COO− groups, the number densities of these groups present on the surface have been independently determined. This also provides a tool to determine the extent of reaction or conversion of the cyano groups to carboxyl groups.