The reaction of an organofunctional silane with a surface bearing hydroxyl groups results in a substitution reaction at silicon and the formation of the silylated surface where the silicon is covalently attached to the surface via an oxygen linkage. This connection may be formed directly or in the presence of water through a reactive silanol intermediate. In general the reactivity of hydroxylated surfaces with organofunctional silanes decreases in the order:
Si-NR2 > Si-Cl > Si-NH-Si > Si-O2CCH3 > Si-OCH3 > Si-OCH2CH3.
An analysis of the relevant bond energies indicates that the formation of the Si-O-surface bond is the driving force for the reaction under dry, aprotic conditions. Secondary factors contributing to the reactivity of organofunctional silanes with a surface are the volatility of the byproducts, the ability of the byproduct to hydrogen bond with the hydroxyls on the surface, the ability of the byproduct to catalyze further reactions (e.g. HCl or acetic acid), and the steric bulk of the groups on the silicon atom.
| Bond | Dissociation Energy (kcal/mol) |
|---|---|
| Me3Si-NMe2 | 98 |
| Me3Si-N(SiMe3)2 | 109 |
| Me3Si-Cl | 117 |
| Me3Si-OMe | 123 |
| Me3Si-OEt | 122 |
| Me3Si-OSiMe3 | 136 |
Methoxy and ethoxysilanes are the most widely used organofunctional silanes for surface modification since they are easily handled and the alcohol byproducts are non-corrosive and volatile. Methoxysilanes are capable of reacting with substrates under dry, aprotic conditions, while the less reactive ethoxysilanes require catalysis for suitable reactivity. The low toxicity of ethanol as a byproduct of the reaction favors the ethoxysilanes in many commercial applications. The vast majority of organofunctional silane surface treatments are performed under conditions in which water is a part of the reaction medium, either directly added or contributed by adsorbed water on the substrate or by atmospheric moisture.
| Type | Byproduct | Advantage | Disadvantage |
|---|---|---|---|
| dimethylaminosilane | dimethylamine | reactive, volatile byproduct | toxic |
| chlorosilane | hydrochloric acid | reactive, volatile byproduct | corrosive |
| silazane | ammonia | volatile | limited availability |
| methoxysilane | methanol | moderate reactivity, neutral byproduct | moderate toxicity |
| ethoxysilane | ethanol | low toxicity | lower reactivity |
| cyclic azasilane | none | no byproducts | water-sensitivity |
The majority of surface modifications are affected by the hydrolytic deposition of trialkoxysilanes. Specific Wetting Surface (SWS) is a value determined empirically for the amount of silane required to obtain minimum uniform multilayer coverage on a substrate. This calculator uses SWS numbers for different silanes to tell you the suggested amount of silane to use on a substrate.
Minimum Uniform Coverage
Monolayer deposition is a widely used term, but the definition of a monolayer is usually contextual. The simplest definition is that there is an attachment of a surface treatment molecule to every surface atom, however, this is highly unlikely. In general, monolayer coverage refers to the reaction of the surface treatment molecule with available hydroxyl groups on the surface, but this is also rarely achieved. For example, hydrated fumed silica has 4.4-4.6 –OH/nm2. A high surface fumed silica has a surface area of 3.25 x 1020 nm2/g and thus 1.5 x 1021 hydroxyls. If this is divided by Avogadro’s number (6.02 x 1023), 2.4 x 10-3 moles of silane are required to provide coverage on 1 g of fumed silica. Monolayer bonding of a silane with a molecular weight of 200 would deposit 0.5 g silane per g of silica. In fact, most monolayer depositions of silanes result in about 10% of the calculated requirement, i.e. 0.5 g silane per gram of fumed silica.
| Substrate | m2/g |
|---|---|
| E-glass | 0.10-0.12 |
| Silica, ground | 1-2 |
| Silica, diatomaceous | 1-3.5 |
| Calcium silicate | 2.6 |
| Clay, kaolin | 7 |
| Talc | 7 |
| Silica, fumed | 150-250 |