Adsorption monolayer adsorbed

The acid monolayers adsorb via physical forces [30] however, the interactions between the head group and the surface are very strong [29]. While chemisorption controls the SAMs created from alkylthiols or silanes, it is often preceded by a physical adsorption step [42]. This has been shown quantitatively by FTIR for siloxane polymers chemisorbing to alumina illustrated in Fig. XI-2. The fact that irreversible chemisorption is preceded by physical adsorption explains the utility of equilibrium adsorption models for these processes. 

A theoretical treatment of the effect caused by the competition between the sine-like angular-dependent component of the adsorption potential and dipole lateral interaction demonstrated that the values 6 are the same in the ground state and at the phase transition temperature.81 Study of the structure and dynamics for the CO monolayer adsorbed on the NaCl(lOO) surface using the molecular dynamics method has also led to the inference that angles 0j are practically equalized in a wide temperature range.82 That is why the following consideration of orientational structures and excitations in a system of adsorbed molecules will imply, for the sake of simplicity, the constant value of the inclination angle ty =0(see Fig. 2.14) which is due to the adsorption potential u pj,q>j). 

Fig. 17. Varying composition of mixed octadecyltrichlorosilane (OTS) + dye (D) monolayers adsorbed on glass as a function of adsorption time Rm and Rs are the molar ratios OTS/D in the monolayer and in solution, respectively [183]...

Both the adsorption velocity and the adsorbed amount in equilibrium rise with increasing concentration. The adsorption isotherm (adsorbed amount as a function of the concentration) shows a plateau in the vicinity of ceff. That was interpreted basing on a two-step model [27] as formation of a surfactant monolayer. From the adsorbed amount of about 1.2 0.3 pmol/m2 at ceff an area per molecule of 1.3 0.3 nm2 was calculated which is typical for a loose monolayer of the used type of surfactant. 

On the other hand, this procedure for the self-assembled monolayer formation has a drawback. A monolayer adsorbed on a silica plate surface is subjected to the detachment of CRA-CM molecules more or less in polar solvents such as water and alcohols,since the adsorption stems from the hydrogen bond formation. This problem was overcome by the pretreatment of a silica plate surface with an aminoalkylation silylating reagent to introduce amino residues on the surface. A photoresponsive monolayer of... 

Langmuir-style descriptions of adsorption at high relative humidities (several monolayers adsorbed) are somewhat strained. Nevertheless, they are probably reasonable for strongly hydrophilic substances like salt and glass. In these materials, the water/solid bonds are much... 

Under electrochemical conditions and T, P = constant, adsorption isotherms can be derived using standard statistical considerations to calculate the Gibbs energy of the adsorbate in the interphase and the equilibrium condition for the electrochemical potentials of the adsorbed species i in the electrolyte and in the adsorbed state (eq. (8.15) in Section 8.2). A model for the statistical considerations consists of a 2D lattice of arbitrary geometry with Ns adsorption sites per unit area. In the case of a 1/1 adsorption, each adsorbed particle can occupy only one adsorption site so that the maximal number of adsorbed particles per unit area in the compact monolayer is determined by A ax = Ng. Then, this model corresponds to the simple Ising model. The number of adsorbed particles, A ads< and the number of unoccupied adsorption sites, No, per unit area are given by... 

The previous part showed that inverse gas chromatography is a very useful tool in the investigation of long-chain aliphatic alcohol monolayers adsorbed on the surface of porous silica gel. Now a simple theoretical model of the adsorbed layer that can be used to analyse the experimental data obtained by inverse gas chromatography is considered. The model is based on the theory of adsorption of simple gases on solid surfaces and, initially restricted to fully localized adsorption [36-38], was extended to treat also long chain molecules [39]. 

Long-chain alkanoic acids adsorb from the gas phase onto the surface of acid-terminated monolayers (e.g. a monolayer of 3-mercaptopropionic acid) through formation of complementary H-bonds. This binding is weak only ca 25-35% of the monolayer adsorbs and no adsorption was observed for short-chain acids HS(CH2)wCOOH, n < 9407-408. Binding of acetic acid to different monolayer surfaces, HS(CH2)ioX X = H, COOH, CONH2, NH2, was studied by Okahata and coworkers. They found that CH3COOH binds most strongly... 

The thickness of the adsorbed phase depends upon the number of adsorbed molecular layers and upon the orientation of adsorbed molecules. A useful approximation for most liquid-solid chromatographic systems is the assumption of monolayer adsorption. This limits the possible volume of the adsorbed phase within narrow limits and leads to a mathematical basis for correlating relative adsorption with adsorbent surface area (see Section 6-2A). 

Perhaps the best way to illustrate the diverse surface-structure chemistry of organic monolayer adsorbates is to review the adsorption behavior of ethylene and benzene on various transition metal surfaces. These two molecules are described in detail below. 

The high sensitivity of the ER method benefits bioelectrochemists in the detection of the redox reaction of the electron transfer proteins. Even for an adsorption monolayer of proteins, the superficial density of the electroactive center is much smaller than that of small molecules, especially when the molecular weight of the protein is several kilo-Daltons. The redox reaction of adsorbed protein buried in the double-layer charging current in the voltammogram can be detected by the ER method. Ikeda and coworkers succeeded in the clear observation of the ER spectrum and ER voltammogram of a heme c in an adsorbed protein (alcohol dehydrogenase) of ca. 140 kD containing hemes and PQQ, while direct redox reaction could not be detected by cyclic voltammetry [90]. 

Figure 4 EDL formed in the vicinity of an adsorption monolayer of ionic surfactant. The diffuse layer contains free ions involved in Brownian motion, while the Stem layer consists of adsorbed (immobilized) counterions. Near the charged surface there is an accumulation of counterions and a depletion of coions.

The Langmuir theory describes the quantitatively formation a monolayer adsorbate on the outer surface of the adsorbent without further adsorption taken place [3]. This model assumes an adsorption homogeneity, which includes the equally available adsorption sites, monolayer surface coverage and no mutual interactions. The equation of Langmuir adsorption isotherm is expressed in Equation (11.1) ... 

FIGURE 9.7. Adsorption isotherms are generally divided into five main types depending on the degree of adsorption (monolayer or multilayer), the mechanism of adsorption (physical or chemisorption), the nature of the adsorbent surface (porous or nonporous), and the relative strengths of adsorbate-adsorbent interactions. 

Notice that quantum chemical calculations of the adsorption of a CO2 cluster onto a silicon surface using periodic boundary conditions showed that the density of the monolayer can be higher than the critical density of relatively rigid CO2 molecules. Consequently, the adsorption layer density of molecules such as TEA and DEA softer than CO2 can be relatively high. Thus, any strong interactions of adsorbates with the adsorbent surface cause compaction of, at least, the first adsorption monolayer (due to both orientation effects, e.g., for acetonitrile, and conformation changes, e.g., for DEA and TEA) that result in diminution of the effective surface area occupied by a molecule and an increase in the 5 bet,x/ bet,n2 value. 

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