Monolayer adsorption

Cyclic voltammetry (adsorption, monolayers) Potentiodynamic polarisation (passivation, activation) Cathodic reduction (thickness) Frequency response analysis (electrical properties, heterogeneity) Chronopotentiometry (kinetics)... 

An optical three-layer model has proved superior to a one-layer model for the interpretation of the ellipsometric data. The refractive indices of the film and surface layers are determined and it is found that the index for the surface is higher than that for the film core. A Lorenz-Lorentz type treatment of NBF reveals that there are approximately seven water molecules per molecule of surfactants in both NaDoS and NaDoBS films. The optical data obtained by the three-layer model for NBF from NaDoS indicate that the thickness of the aqueous core is zero while that of the adsorption monolayers of surfactant molecules with refractive index 1.365 is 1.8 nm, i.e. the thickness of NBF is 3.6 nm. 

Because of the strong t(C) dependence at and below a given surfactant concentration Cc the foam bilayer cannot be observed experimentally, as it ruptures instantaneously. Hence, Cc is the experimental limit of bilayer metastability and is determined by the resolution of the measuring equipment. For NaDoS foam bilayers Cc - 1.2-10 4 mol dm 3, a value which coincides with that of the lowest bulk surfactant concentration at which maximum packing of the adsorption monolayer at the solution/air interface is attained [332,366]. 

The critical concentration Cc for formation of foam and emulsion bilayers of Do(EO)22 are 4-10 6 mol dm 3 and 1.6 10 5 mol dm 3, respectively, and are in good correlation with the lowest concentrations, 2-31 O 6 mol dm 3 and 10 5 mol dm 3 [421] at which maximum filling of the surfactant adsorption monolayer is attained. It should also be noted that in the case of the emulsion bilayers, CMC < Ce which implies that it is not possible to obtain infinitely stable (i.e. with r = °°) bilayers of Do(EO)22 between two droplets of nonane under the described conditions. For this reason, it may be thought that thermodynamically stable nonane-in-water emulsions stabilised with Do(EO)22 do not exist. 

In systems with area of II(/i) isotherms within the range of Id < 0 greater than the area within the range of n > 0 but without other region in the II(/i) > 0, except at h < ho (h < h0, Fig. 3.118), the region of existence of stable films is restricted by the values of A0 to the right of point f (curve 4, Fig. 3.119). The initially formed thick film (s> s > 0) stratifies into lenses and thin films (most often adsorption monolayers). Hence, rf B/A > s > 0, se < 0. 

Solute retention in reversed-phase HPLC is dependent on the different distribution coefficients established between a polar mobile and a nonpolar stationary phase by the peptidic components of a mixture. Although there are many similarities between reversed-phase HPLC separations of peptides and the classical liquid-liquid partition chromatographic methods, it is debatable whether the sorption process in reversed-phase HPLC arises due to partition or adsorption events, i.e., whether the nonpolar stationary phase functions as a bulk liquid or as an adsorptive monolayer. These aspects and the theoretical models for reversed-phase HPLC are discussed in a subsequent section. 

Amount of grafted modifier a (mg/m ), specific surface area with respect to nitrogen Sn (m /g), BET adsorption monolayer values for various substances am (mmol/g), water molecular area whjO (nm ) and heat of wetting q (mj/m ) for the initial and modified silicas [38]... 

The derivative d In c ldT is calculated for each adsorption isotherm, and then the integration in Equation 5.5 is carried out analytically. The obtained expressions for J are listed in Table 5.2. Each surface tension isotherm, oCEi), has the meaning of a two-dimensional equation of state of the adsorption monolayer, which can be applied to both soluble and insoluble surfactants. ... 

An important thermodynamic property of a surfactant adsorption monolayer is its Gibbs (surface) elasticity ... 

If the surface of an equilibrium surfactant solution is disturbed (expanded, compressed, renewed, etc.), the system will try to restore the equilibrium by exchange of surfactant between the surface and the subsurface layer (adsorption-desorption). The change of the surfactant concentration in the subsurface layer triggers a diffusion flux in the solution. In other words, the process of equilibration (relaxation) of an expanded adsorption monolayer involves two consecutive stages ... 

The surfaces of fluid particles can be treated as tangentially immobile when they are covered by dense surfactant adsorption monolayers that can resist tangential stresses.In such a case, the bubbles or droplets behave as flexible balls with immobile surfaces. When the fluid particles are rather small (say, microemulsion droplets), they can behave like hard spheres therefore, some relations considered below, which were originally derived for solid particles, can be also applied to fluid particles. 

The presence of surfactant adsorption monolayers decreases the mobility of the droplet (bubble) surfaces. This is due to the Marangoni effect (see Equation 5.282). From a general viewpoint, we may expect that the interfacial mobility will decrease with the increase of surfactant concentration until eventually the interfaces become immobile at high surfactant concentrations (see Section 5.5.2, above) therefore, a pronounced effect of surfactant concentration on the velocity of film drainage should be expected. This effect really exists (see Equation 5.286, below), but in the case of emulsions it is present only when the surfactant is predominantly soluble in the continuous phase. 

Carbon Gasification Rates. Because the reforming rates we observed during this work were often controlled by diffusion, it was not possible to. determine individual reaction rates and rate constants. However, from the TPSR measurements we were able to estimate rate constants for the gasification of catalyst and noncatalyst carbons. These rates are listed in Table VII along with selected results taken from the literature (29, 30, 31). We found that the catalyst carbon gasification rates were first order in carbon amounts up to equivalent (CO adsorption) monolayer... 

Most atoms and molecules in the adsorption monolayer form ordered surface structures at appropriate temperature and coverage regimes. 

Earlier, the author showed [2 8 ] that the dewetting adsorption amoimts to a chemical adsorption by cation exchange between the solid and the adsorption monolayer at the solution-air interface. 

The data from [78] are consistent with the group of best experimental results characterised by an experimental error of 1.5mN/m. Moreover, the location of the critical point in the Ci2S04Na isotherm at 0.1 M NaCl for the data from [78] (approximately at c = 10" mol/1), and the inflection-like agglomeration of the experimental data from other publications (indicated by an arrow at c = 8-10 mol/1 in Fig. 3.48) occur both at similar surface pressures (ca. 8 mN/m). Note that the results indicated by the arrow in Fig. 3.48 correspond to an electrolyte concentration of 0.1 to 0.5 MNaCl. To summarise, the supposition that Ci2S04Na undergoes a condensation in the adsorption monolayers at sufficiently high NaCl concentration can be regarded to as a reliable hypothesis, which however, still requires further experimental verification. 

The first one is an adsorption monolayer of MB on a basal plane pyrolytic graphite (BPG) electrode surface [9, 24, 25]. Eig. 2.7 a shows the experimental set-up and Eig. 2.7b shows the plot of the change ARic(E) at constant electrode potentials. The experimental ARi E) can actually fit to a Nernst equation ... 

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]. 

Haladjian s group studied these same cytochrome C3 molecules by differential pulse polarography. An adsorption prepeak was linear with increasing concentration up to a concentration of 4 fiM and 7 fiM for D. vulgaris and D. desulfuricans, respectively, at an 0.5-s drop time. Above these concentrations the amplitude of the prepeak leveled off, indicating that a primary adsorption monolayer had been formed. 

As mentioned in the Sec. 1, an important thermo-dynamic parameter of a surfactant adsorption monolayer is its Gibbs (surface) elasticity. The physical concept of surface elasticity is the most transparent for monolayers of insoluble surfactants, for which it was initially introduced by Gibbs (18, 19). The increments A a and AT in the definition of Gibbs elasticity ... 

The Gibbs elasticity characterizes the lateral fluidity of the surfactant adsorption monolayer. For high values of the Gibbs elasticity the adsorption monolayer at a fluid interface behaves as tangentially immobile. Then, if two oil drops approach each other, the hydro-dynamic flow pattern, and the hydrodynamic interaction as well, is the same as if the drops were solid particles, with the only differenee that under some conditions they could deform in the zone of contact. For lower values of the Gibbs elastieity the... 

Table 2 Expressions for the Gibbs Elasticity of Adsorption Monolayers (Valid for Roth Nnnionic and Ionic Siirfaciants). Which Correspond to the Various Types of Isotherms in Table I...

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 definition of Gibbs elasticity given by Eq. (19) corresponds to an instantaneous (Aft t ) dilatation of the adsorption layer (that contributes to o ) without affecting the diffuse layer and o. The dependence of o on Ty for nonionic surfactants is the same as the dependence of o on Ty for ionic surfactants, cf Eqs (7) and (19). Equations (8) and (20) then show that the expressions for Eq in Table 2 are valid for both nonionic and ionic siufactants. The effect of the surface electric potential on the Gibbs elasticity Eq of an ionic adsorption monolayer is implicit, through the equilibrium siufactant adsorption T y which depends on the electric properties of the interface. To illustrate this let us consider the case of Langmuir adsorption isotherm for an ionic surfactant (17) ... 

The characteristic time of surfactant adsorption at a fluid interface is an important parameter for suriaetant-stabilized dynamic systems sueh as emulsions. Sutherland (22) derived an expression deseribing flie relaxation of a small dilatation of an initially equilibrium adsorption monolayer... 

The solution to the problem of hydrodynamie interaction between two rigid spherieal partieles, approaehing each other aeross a viseous fluid, was obtained by Taylor (122). Two spherieal emulsion drops of tangentially immobile sur-faees (due to the presenee of dense surfactant adsorption monolayers) are hydrodynamically equivalent to the two rigid partieles eonsidered by Taylor. The hydrodynamic interaction is due to the dissipation of kinetic energy when the liquid is expelled from the gap between the two spheres. The resulting friction force decreases the velocity of the two spherical drops proportionally to the decrease in the surface-to-surface distance h in accordance with the Taylor (122) equation ... 

Here, is the so called foam parameter, and t is the viscosity m the surfactant-containing phase (Liquid 1 in Fig. 15) the influence of the transition zone film - bulk liquid is not accounted for in Eq. (76). Note that the bulk and surface diffusion fluxes (see the terms with and Z) in the latter equation), which tend to damp the surface tension gradients and to restore the uniformity of the adsorption monolayers, accelerate the film thinning (Fig. 1). Moreover, since Din Eq. (76) is divided by the film thickness h, the effect of surface diflhsion dominates that of bulk diffusion for small values of the film thickness. On the other hand, the Gibbs elasticity Eq (the Marangoni effect) decelerates the thinning. Equation (76) predicts that the rate of... 

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