Surface group concentration

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

Surface properties are generally considered to be controlled by the outermost 0.5—1.0 nm at a polymer film (344). A logical solution, therefore, is to use self-assembled monolayers (SAMs) as model polymer surfaces. To understand fully the breadth of surface interactions, a portfoHo of chemical functionahties is needed. SAMs are especially suited for the studies of interfacial phenomena owing to the fine control of surface functional group concentration. 

In addition, carbon-hydrogen bonds are present, particularly in carbonaceous materials obtained by carbonizing polymers at low temperatures, typically <1000 °C. Detailed discussions on the types of surface groups and their surface concentrations are presented by Boehm (14] and Rivin [15],... 

The initial step of the adsorption of cyclic sulfides on a Mo(100) surface is also the formation of adsorbed thiolate groups.395-397 Adsorbed alkyl thiolates decompose to adsorbed sulfur, carbon, and hydrogen on the clean Mo surface, but once the surface is deactivated by adsorbed sulfur, alkanes and alkenes evolve from the surface. Tetrahydrothiophene (34) and trimethylene sulfide decompose on Mo(110) to alkanes and alkenes by way of a common intermediate, which is proposed to be a surface thiolate (33). The thiolate undergoes hydrogenation or dehydrogenation, depending on the surface hydrogen concentration (Scheme 4.115).398 399... 

Attempts have been made to And correlations between the types and concentrations of the various surface groups and titanium oxo complexes, on the one hand,... 

Full generation poly(amidoamine) (PAMAM) dendrimers possessing amino surface groups are easily spread by spin coating on a mica surface to form films. Interdendrimer assembly events appears to influence these operations. Regardless of the generation levels examined, the film uniformity is determined primarily by the concentration of the dendrimer solution. 

Figure 13.10 Concentration of PAMAM dendrimers in surface groups at the minimum optical absorption (666 nm) as a function of generation. Adapted from ref. 23...

Estimate the variation of surface charge of a hematite suspension (same charac-teristics as that used in Example 7.2) to which various concentrations of a ligand H2U (that forms bidentate surface complexes with the Fe(III) surface groups, FelT such a ligand could be oxalate, phtalate, salicylate or serve as a simplified model for a humic acid we assume acidity constants and surface complex formation constants representative for such ligands. The problem is essentially the same as that discussed in Example 5.1. We recalculate here for pH = 6.5. 

Different modifications of hydrous oxides, even if present in solution with the same surface area concentrations, are characterized by significantly different reactivities (e.g., dissolution rate). This depends above all on the different coordination geometry of the surface groups. For a given pH (on surface protonation) the reactivity of a Fem-center is likely to increase with the number of terminal ligands (Wehrli et al., 1990), i.e., groups such as -Fe-OH are less acid and react faster than... 

Distribution coefficients based on adsorption equilibria are independent of the total concentrations of metal ions and suspended solids, as long as the metal concentrations are small compared with the concentration of surface groups. Examples of the Kd obtained from calculations for model surfaces are presented in Fig. 11.1. A strong pH dependence of these Kd values is observed. The pH range of natural lake and river waters (7 - 8.5) is in a favorable range for the adsorption of metal ions on hydrous oxides. 

Figure 6. CIR-FTIR spectra of SAL and iron SAL complex in solution and SAL on goethite. (upper) 0.1 M SAL in 1 M KC1 at pH 5.5 (middle) SAL on goethite in D.O with goethite surface groups subtracted at pD 4.5, 0.01 M KC1 and 100 g/L solid concentration (lower) aqueous iron SAL complex at pH 1.6.

The hydrothermal carbons obtained in the end from soluble, non-structural carbohydrates are micrometer sized, spherically shaped particle dispersions, containing a sp2 hybridized backbone (also responsible for the brown to black color) decorated with a dense layer of polar oxygenated functionalities still remaining from the original carbohydrate. The presence of these surface groups offers the possibility of further functionalization and makes the materials more hydrophilic and well-dispersible in water. The size of the final particles depends mainly on the carbonization time and precursor concentration inside the autoclave, as well as additives and stabilizers potentially added to the primary reaction recipe. An SEM image of a model reaction illustrating this dispersion state is shown in Fig. 7.1. 

---