Close-packed monolayers

Direct electron transfer (DET), as the name suggests, insinuates that electron transfer occurs between enzyme and electrode without the use of any third-party mediators (Figure 15.1). For DET to occur, several criteria must be met (i) the enzyme must be immobilized on the electrode with its active site within electron mnneling distance, (ii) the immobilization must be stable over time, and (iii) ideally, enzymes should be arranged in a monolayer (close-packed array) that eliminates residual enzymes on top of the initial monolayer that are not expected to be active and can, in fact, impede diffusion of reactants to the active enzymes. 

Consider the case of an emulsion of 1 liter of oil in 1 liter of water having oil droplets of 0.6 /rm diameter. If the oil-water interface contains a close-packed monolayer of surfactant of 18 per molecule, calculate how many moles of surfactant are present. 

Figure Bl.21.1 shows a number of other clean umeconstnicted low-Miller-index surfaces. Most surfaces studied in surface science have low Miller indices, like (111), (110) and (100). These planes correspond to relatively close-packed surfaces that are atomically rather smooth. With fee materials, the (111) surface is the densest and smoothest, followed by the (100) surface the (110) surface is somewhat more open , in the sense that an additional atom with the same or smaller diameter can bond directly to an atom in the second substrate layer. For the hexagonal close-packed (licp) materials, the (0001) surface is very similar to the fee (111) surface the difference only occurs deeper into the surface, namely in the fashion of stacking of the hexagonal close-packed monolayers onto each other (ABABAB.. . versus ABCABC.. ., in the convenient layerstacking notation). The hep (1010) surface resembles the fee (110) surface to some extent, in that it also...

The central idea underlying measurements of the area of powders with high surface areas is relatively simple. Adsorb a close-packed monolayer on the surface and measure the number A of these molecules adsorbed per unit mass of the material (usually per gram). If the specific area occupied by each molecule is A then the... 

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

In Table 5.3, is compared with the total hydroxyl concentration (Ni, + N ) of the corresponding fully hydroxylated, sample. The results clearly demonstrate that the physical adsorption is determined by the total hydroxyl content of the surface, showing the adsorption to be localized. It is useful to note that the BET monolayer capacity n JH2O) (= N ) of the water calculated from the water isotherm by the BET procedure corresponds to approximately 1 molecule of water per hydroxyl group, and so provides a convenient means of estimating the hydroxyl concentration on the surface. Since the adsorption is localized, n.(H20) does not, of course, denote a close-packed layer of water molecules. Indeed, the area occupied per molecule of water is determined by the structure of the silica, and is uJH2O) 20A ... 

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. ... 

The earlier interpretation of point X in terms of a close-packed monolayer of water would thus seem untenable. As has been clearly demonstrated, the total uptake at X, 327pmolg" , contains a contribution of ISOpmolg" from chemisorption thus physisorption accounts for only 177pmolg, which corresponds to 21 h per molecule of water. The fact that the total uptake at X corresponds to 11-2A, and is therefore close to the figure 10-5 for a close-packed monolayer, must be regarded as fortuitous. 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

High quahty SAMs of alkyltrichlorosilane derivatives are not simple to produce, mainly because of the need to carefully control the amount of water in solution (126,143,144). Whereas incomplete monolayers are formed in the absence of water (127,128), excess water results in facile polymerization in solution and polysiloxane deposition of the surface (133). Extraction of surface moisture, followed by OTS hydrolysis and subsequent surface adsorption, may be the mechanism of SAM formation (145). A moisture quantity of 0.15 mg/100 mL solvent has been suggested as the optimum condition for the formation of closely packed monolayers. X-ray photoelectron spectroscopy (xps) studies confirm the complete surface reaction of the —SiCl groups, upon the formation of a complete SAM (146). Infrared spectroscopy has been used to provide direct evidence for the hiU hydrolysis of methylchlorosilanes to methylsdanoles at the soHd/gas interface, by surface water on a hydrated siUca (147). 

Some thiophenolate monolayers also have been investigated. Thiophenolate, C H S—, forms ordered monolayers on Ag(lll) with a (/7 X v, 88°)R40.9°, and benzene rings closely packed in face-to-face stacked columns (294). Benzylthiolate (295),/xpyridinethiolate (296), and (9-pyridinethiolate (296), also form ordered monolayers on Ag(lll), but with fewer close-packed aromatic rings. 

The fdr studies reveal that the alkyl chains in SAMs of thiolates on Au(lll) usually are tilted 26-28° from the surface normal, and display 52-55° rotation about the molecular axis. This tilt is a result of the chains reestabUshing VDW contact in an assembly with - 0.5 nm S—S distance, larger than the distance of - 0.46 nm, usually quoted for perpendicular alkyl chains in a close-packed layer. On the other hand, thiolate monolayers on Ag(lll) are more densely packed owing to the shorter S—S distance. There were a number of different reports on chain tilt in SAMs on Ag(lll), probably owing to different amounts of oxide, formed on the clean metallic surface (229,230,296,297). In carefully prepared SAMs of alkanethiolates on a clean Ag(lll) surface, the alkyl chains are practically perpendicular to the surface. 

In a separate study using the JKR technique, Chaudhury and Owen [48,49] attempted to understand the correlation between the contact adhesion hysteresis and the phase state of the monolayers films. In these studies, Chaudhury and Owen prepared self-assembled layers of hydrolyzed hexadecyltrichlorosilane (HTS) on oxidized PDMS surfaces at varying degrees of coverage by vapor phase adsorption. The phase state of the monolayers changes from crystalline (solidlike) to amoiphous (liquid-like) as the surface coverage (0s) decreases. It was found that contact adhesion hysteresis was the highest for the most closely packed... 

The lowest wettable surface known = 6 dyne/cm) is a monolayer of perfluoro-launc acid on platinum, whose surface is made up of closely packed CF3 groups [20] Fluonnated graphite, (C2FJ and (CF) , also have surface tensions approachmg 6 dyne/cm [21] Perfluorinated materials, however, are not required tor low surface energies only the outermost surface groups must be perfluorinated [20, 22]... 

The integrated charge would correspond to 0.7 0.1 Cu monolayers. Thus, either a less closely packed Cu layer or an anion co-adsorption that can both lead to a Moire superstructure are probed in the solution investigated [Al2Cl7] is the predominant anion. At h-200 mV vs. Cu/CW the superstructure disappears and a completely closed Cu monolayer is observed, with a charge corresponding to 1.0 0.1 Cu monolayers. 

Dominguez, D.D.,Mowery, R. L., and Turner, N. H., Friction and Durabilities of Well-Ordered, Close Packed Carboxylic Acid Monolayers Deposited on Glass and Steel Surfaces by the Langmuir-Blodgett Technique, Tribal. Trans., yi. No. 1, 1994, pp. 59-66. 

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