Adsorbed layer thickness surfaces

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

Busscher, H. J., H. M. Uyen, G. A. M. ICip, and J. Arends. 1987. Adsorption of aminefluorides onto glass and the determination of surface free energy, zeta potential and adsorbed layer thickness. Colloids and Surfaces 22 161-69. 

On nonporous surfaces it has been shown that when W /W is plotted versus P/Po the data all approximately fit a common type II curve above a relative pressure of 0.3. This implies that when WJW = 3, for example, the adsorbed layer thickness t will be 10.62 A regardless of the adsorbent. The common curve is described closely by the Halsey equation which for nitrogen can be written as... 

Assume that a surface is covered by adsorbates with a distribution of layer thicknesses shown schematically in Fig. 11.6. A given surface site might be covered by 0, 1, or m layers of adsorbed molecules, with the adsorbed layer thickness on adjacent sites randomly distributed. An adsorption/desorption equilibrium will be assumed. 

It is clear that as [A] approaches [A]sat, x approaches 1, and the surface-adsorbed layer thickness Eq. 11.60 goes to infinity that is, there is an infinite reservoir of liquid in equilibrium with the vapor. This is the desired limiting behavior for the model. 

Surface layers (adsorbed, solvated, ionic) are of considerable importance in controlling the stability and rheological properties of colloidal systems. Sedimentation methods have proven effective in the measurement of adsorbed layer thickness using equations similar to Equation 1 when the density of the layer could be estimated ( 7,8). The equation can be considerably simplified if the density... 

Fig. 3.31 Schematic picture of average adsorbed chain conformations in extremely dilute solution (isolated chains on the surface), dilute and semidilute solutions, and the polymer melt. The adsorbed layer thickness increases sharply with increasing concentration, mainly due to the contribution of tails. Significant tail formation occurs as soon as the chains begin to compete for surface sites. (From ref. [144])...

As already indicated, by applying the Kelvin equation (assuming hemispherical meniscus formation) and correcting for the adsorbed layer thickness, we are able to calculate the ranges of apparent pore width recorded in Table 12.5. The values of mean pore diameter, w, are obtained from the volume/surface ratio, i.e. by applying the principle of hydraulic radius (see Chapter 7) and assuming the pores to be non-intersecting cylindrical capillaries and that the BET area is confined to the pore walls. 

Adsorption is the process of analyte accumulation on the surface under the influence of the surface forces. Determination of the total amount of the analyte adsorbed on the surface requires the definition of the volume where this accumulation is observed, usually called the adsorbed layer volume (U ). In chromatographic systems, adsorbents have large surface area, and even very small variation in the adsorbed layer thickness lead to a significant variation on the adsorbed layer volume. There is no uniform approach to the definition of this volume or adsorbed layer thickness in the literature [14,21,22]. 

The analysis of experimental excess adsorption isotherms using equation (2-50) had shown unusual results [22]. The adsorbed layer thickness of acetonitrile adsorbed from water on different types of reversed-phase adsorbents calculated as the ratio of adsorbed layer volume and adsorbent surface area appears to be on average equal to 14 A, which is equivalent to approximately five monolayers of acetonitrile molecules adsorbed on the hydrophobic surface. At the same time, the adsorbed layer thickness of methanol adsorbed from water on the same adsorbents is equal to only 2.5 A, which is equivalent to the monolayer-type adsorption. 

From the above discussion it is logical to assume that adsorbed layer thickness T, calculated as the ratio of VJS, is the maximum distance of the influence of the surface forces. The sum of the excess adsorption value and the product of the equilibrium concentration and adsorbed layer volume represent the total amount of the adsorbate in that layer for any given equilibrium concentration... 

Figure 2-9. Adsorbed layer thickness for THF, acetonitrile, and methanol on adsorbents of different surface chemistry. (Reprinted from reference 22, with permission.)...

Fig. 4 Thickness of the adsorbed /i-propanol layer measured with QCM as a function of n-propanol partial pressure. It should be noted that the QCM measures the thickness on a gold surface. The 100% saturation data point contains large error because a slight fluctuation in the substrate temperature causes a large change in the condensed layer thickness. The saturation vapor pressure on n-propanol is 21.2 Torr at room temperature. The inset shows the adsorbed layer thickness of different alcohols at partial pressures 90 10% to the saturation. (View this art in color at www.dekker.com.)...

As mentioned above, in order to fully characterize polymeric surfactant adsorption, three parameters must be determined (i) the adsorbed amount F (mgm or mol m ) as a function of the equihbrium concentration that is, the adsorption isotherm (ii) the fraction of segments in direct contact with the surface p (the number of segments in trains relative to the total number of segments) and (iii) the segment density distribution p(z) or the hydrodynamic adsorbed layer thickness 5. ... 

A schematic representation of the variation of G j, G p Ga, and Gj with surface-surface separation distance h is shown in Figure 8.4. G j increases very sharply with a decrease in h, when h<28 likewise, G i increases very sharply with a decrease in h, when h<8 and Gj versus h shows a minimum, G j , at separation distances comparable to 28. When h < 28, Gj shows a rapid increase with decrease in h. The depth of the minimum depends on the Hamaker constant A, the particle radius R, and the adsorbed layer thickness 8. G p increases with increases of A and R and, at a given A and R, also increases with a decrease in 8 (i.e., with decrease in the molecular weight, M, of the stabiliser). This is illustrated in Figure 8.5, which shows the energy-distance curves as a function of S/R. The larger the value of 5/R, the smaller the value of G j in this case, the system may approach thermodynamic stability, as occurs with nanodispersions. 

As discussed in Chapter 6, complete information on polymer adsorption may be obtained if the segment density distribution can be determined - that is, the segment concentration in all layers parallel to the surface. However, such information is generally unavailable, and therefore three main parameters must be determined, namely the amount of adsorption F per unit area, the fraction p of segments in direct contact with the surface (i.e., in trains), and the adsorbed layer thickness 5. 

As a last remark on polymer adsorption, let s consider Fig. 6.3. If a polymer with a hydrodynamic radius Rg is present at low concentration, its configuration at the interface will be relatively flat with trains on the interface. After more polymer is added, the irreversible adsorption on the surface will produce an adsorbed layer thickness on the... 

Fig. 6.3. a Polymer adsorption on interface showing the surface adsorption at low concentration of polymer (q) and at higher concentration of polymer (c2). 6 is adsorbed layer thickness, b Full coverage of the interface by the adsorbed copolymer. The hydrophobic part of the copolymer is shown at the interface (A) and the hydrophilic part protruding in H20 (B). c Schematic showing the segment density distribution for polymer segments attached to the interface... 

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