Hydroxylated surfaces

A pletliora of different SA systems have been reported in tire literature. Examples include organosilanes on hydroxylated surfaces, alkanetliiols on gold, silver, copper and platinum, dialkyl disulphides on gold, alcohols and amines on platinum and carboxyl acids on aluminium oxide and silver. Some examples and references can be found in [123]. More recently also phosphonic and phosphoric esters on aluminium oxides have been reported [124, 125]. Only a small selection out of tliis number of SA systems can be presented here and properties such as kinetics, tliennal, chemical and mechanical stability are briefly presented for alkanetliiols on gold as an example. 

Organosilanes, such as trichlorosilanes or trimethylsilanes, can establish SA monolayers on hydroxylated surfaces. Apart from their (covalent) binding to the surface these molecules can also establish a covalent intennolecular network, resulting in an enlranced mechanical stability of the films (figure C2.4.11). In 1980, work was published on the fonnation of SAMs of octadecyltrichlorosilane (OTS) 11171. Subsequently, the use of this material was extended to the fonnation of multilayers 11341. 

In Fig. 5.21, from Dawson s paper, the uptake at X for the 250°C-outgassed sample is dose to the calculated value for a monolayer of water with a (H20) = 101 A. Point X has therefore been ascribed to a close-packed monolayer of water on a hydroxylated surface of rutile. The fact that the differential entropy of adsorption relative to the liquid state (calculated from the isosteric heat of adsorption) changes sharply from negative to positive values in this region with A s 0 at X was regarded as supporting evidence. ... 

In choosing a SAM system for surface engineering, there are several options. Silane monolayers on hydroxylated surfaces are an option where transparent or nonconductive systems are needed. However, trichlorosilane compounds are moisture-sensitive and polymeri2e in solution. The resulting polymers contaminate the monolayer surface, which occasionally has to be cleaned mechanically. CarboxyUc acids adsorb on metal oxide, eg, AI2O2, AgO through acid—base interactions. These are not specific therefore, it would be impossible to adsorb a carboxyUc acid selectively in the presence of, for example, a terminal phosphonic acid group. In many studies SAMs of thiolates on Au(lll) are the system of choice. 

The most commonly used amphiphiles to build L-B hlms for tribological applications are the straight chain hydrocarbon compounds with simple functional groups such as the fatty acids, including stearic acids, arachidic acids, and behenic acids [32], but other amphiphilic molecules, e.g., 2,4-heneicosanedione and 2-docosylamina-5-nitropyridine, are also applied in some cases. There are two major systems of self-assembled monolayers, namely the alkylsilance derivatives (e.g., OTS, octadecyltrichlorosilane) on hydroxylated surfaces and the alkanethiols on metal substrates, which have been investigated extensively to examine their properties as solid lubricants and protective surface films [31 ]. 

In figure 3 and show that the relative thermal motion of the surface atoms is significantly greater than in the bulk metal over the range from 100 - 800 K, This result is expected considering the partial coordination, hence lack of constraint of the surface atoms. A similar result has been found from LEED measurements on a Pt surface. ( ) Significantly, the surface atom disorder when extrapolated to 0 K remains sizable. This static disorder or strain appears to be a result of the interaction of the Ft atoms with the support, a kind of epitaxy to the oxygen (or hydroxyl) surface of the support. 

It i.s easiest to get a picture of the chemical structure of the surface of aluminas by starting from a fully hydroxylated surface (Fig. 3.20). 

Varian (see Catalog, SPE products) Bond Elut Plexa Highly cross-linked polymer with hydroxylated surface Hydrophobic retention of small molecules and hydrophilic exclusion of proteins See catalog... 

Schluter and co-workers have reported dendronized polystyrene with dendrons having amine, ammonium and hydroxyl surface functionalities [29, 55]. Ammonium functionalized materials are soluble in water, methanol and... 

A sample of silica, dried at 150°, representing about 10 m /gm, is added to 25 ml of a methyl red solution in benzene containing 0.6-0.7 gm of the azo compound per liter. After agitating for about 2 hours at room temperature, the concentrations of equilibrium solution and original solution are compared by spectrophotometry at 4750 A. The specific hydroxylated surface area in m /gm is calculated from the formula ... 

Hydrothermal synthesis of the non-uniform silica gel used for preparative chromatography proceeds in a different fashion. Sodium silicate (IMa2Si03),1 obtained by alkaline fusion of very pure sand, is acidified to yield orthosilicic acid (Si(0H)4). This unstable acid initially dimerises then condenses further to yield a gel with a hydroxylated surface. Under conditions of controlled polymerisation, a hydrogel is obtained which is further calcinated to yield a very dense silica gel (xerogel). Some of the processes involved here are of the same type as those used to produce microspheres for analytical chromatography. 

Charging Hydroxylated Surfaces Definition of IEP(s). A hydroxyl-ated surface should be expected on all oxidic materials which have had a chance to come to equilibrium with an aqueous environment. Charge can develop on a hydroxylated surface through amphoteric dissociation of the surface hydroxide groups. Dissociation reactions can be written as follows, where underscored symbols refer to species forming part of the surface. Symbols not underscored refer to species assumed aqueous unless otherwise specified. 

The clay minerals are layer structures (38, 39, 86) incorporating, for example, sheets of interconnected Si04 tetrahedra cross-linked with sheets of A10<5 octahedra. Particles have broken bonds only on the edges of sheets. The broken bonds hydroxylate they and structural OH on the layer surfaces can dissociate to produce a pH-dependent charge in the same way as described for the surfaces of simpler oxides. Charge from this source may be referred to as originating in the hydroxylated surface. 

Zero Point of Charge. We are operating on the assumption that pH-dependent surface charge arises predominantly from H+ adsorption by reactions of the hydroxylated surface similar to Reactions 1 and 3. Figure 4 shows schematic H+ adsorption isotherms illustrating the pH variation of surface charge and H+ adsorption density, Y/, associated with this source. Curve a represents the total amount of adsorption on a given amount of surface area. Curve b represents more surface. In both... 

The IEP(s) characteristic of the hydroxylated surface is the pH at which the hydrogen ion adsorption density on the hydroxylated surface sites alone is zero. To achieve zero total charge when 07 5 0, however, Yh must be increased by adjusting the pH toward the acid side of the IEP(s) until eVH = —07 hence the ZPC is smaller than the IEP(s). 

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