Solid/liquid adsorption layer

FIG. 3 Schematic picture of the heat evolution (enthalpy of displacement A21H) in the solid/liquid adsorption layer. After adding component 1 (Awi) to the liquid mixture, the composition of the bulk and the adsorption layer wiU be exchanged (notation with ). 

In the case of liquid adsorption layers, the deformation by shear occurs at any small load (Fig. 11-29, line 2), and the shear rate, e, is proportional to the applied stress, xs (the torque angle of the thread). The latter allows one to estimate the surface viscosity of the adsorption layer, which is strongly dependent on the nature of surfactants. The adsorption layers may also reveal more complex rheological behavior, intermediate between that of liquids and solids (Fig. 11-29, curve 3). 

Column chromatography is another form of solid-liquid adsorption chromatography and depends on the same fundamental principles as does thin-layer chromatography (TLC, Sec. 6.2), as you will see from the discussion that follows. It has an advantage over TLC in that multigram amounts of mixtures can be separated but has the disadvantage that this technique requires considerably more time... 

Adsorption may in principle occur at all surfaces its magnitude is particularly noticeable when porous solids, which have a high surface area, such as silica gel or charcoal are contacted with gases or liquids. Adsorption processes may involve either simple uni-molecular adsorbate layers or multilayers the forces which bind the adsorbate to the surface may be physical or chemical in nature. 

For the solid-liquid system changes of the state of interface on formation of surfactant adsorption layers are of special importance with respect to application aspects. When a liquid is in contact with a solid and surfactant is added, the solid-liquid interface tension will be reduced by the formation of a new solid-liquid interface created by adsorption of surfactant. This influences the wetting as demonstrated by the change of the contact angle between the liquid and the solid surface. The equilibrium at the three-phase contact solid-liquid-air or oil is described by the Young equation ... 

The importance of surface characterization in molecular architecture chemistry and engineering is obvious. Solid surfaces are becoming essential building blocks for constructing molecular architectures, as demonstrated in self-assembled monolayer formation [6] and alternate layer-by-layer adsorption [7]. Surface-induced structuring of liqnids is also well-known [8,9], which has implications for micro- and nano-technologies (i.e., liqnid crystal displays and micromachines). The virtue of the force measurement has been demonstrated, for example, in our report on novel molecular architectures (alcohol clusters) at solid-liquid interfaces [10]. 

Electrochemical reactions are driven by the potential difference at the solid liquid interface, which is established by the electrochemical double layer composed, in a simple case, of water and two types of counter ions. Thus, provided the electrochemical interface is preserved upon emersion and transfer, one always has to deal with a complex coadsorption experiment. In contrast to the solid/vacuum interface, where for instance metal adsorption can be studied by evaporating a metal onto the surface, electrochemical metal deposition is always a coadsorption of metal ions, counter ions, and probably water dipols, which together cause the potential difference at the surface. This complex situation has to be taken into account when interpreting XPS data of emersed electrode surfaces in terms of chemical shifts or binding energies. 

Modifications of surface layers due to lattice substitution or adsorption of other ions present in solution may change the course of the reactions taking place at the solid/liquid interface even though the uptake may be undetectable by normal solution analytical techniques. Thus it has been shown by electrophoretic mobility measurements, (f>,7) that suspension of synthetic HAP in a solution saturated with respect to calcite displaces the isoelectric point almost 3 pH units to the value (pH = 10) found for calcite crystallites. In practice, therefore, the presence of "inert" ions may markedly influence the behavior of precipitated minerals with respect to their rates of crystallization, adsorption of foreign ions, and electrokinetic properties. 

Valuable information can be obtained from thermal desorption spectra (TDS) spectra, despite the fact that electrochemists are somewhat cautious about the relevance of ultrahigh vacuum data to the solution situation, and the solid/liquid interface in particular. Their objections arise from the fact that properties of the double layer depend on the interaction of the electrode with ions in the solution. Experiments in which the electrode, after having been in contact with the solution, is evacuated and further investigated under high vacuum conditions, can hardly reflect the real situation at the metal/solution interface. However, the TDS spectra can provide valuable information about the energy of water adsorption on metals and its dependence on the surface structure. At low temperatures of 100 to 200 K, frozen molecules of water are fixed at the metal. This case is quite different from the adsorption at the electrode/solution interface, which usually involves a dynamic equilibrium with molecules in the bulk. 

Spreading may occur by a process of surface solution or by vaporisation from the lens and condensation on the water surface. This latter, indeed, is the only method of spreading on a solid. The adsorption of vapours from a liquid onto a second liquid surface to the point of equilibrium results in the formation of a primary (unimolecular) film and this is doubtless followed in many cases by secondary film formation or a banking up of the layers on the primary film to a thickness which may be several hundred molecules thick. The conditions which have to be fulfilled are two (1) the surface tension of the film whether primary or secondary o- must attain the value... 

Given these caveats, we first treat the case of adsorption to a solid surface, and then absorption into a liquid particle or liquid surface layer on a particle. As we shall see, the distribution of SOC between the gas and condensed phases can be used to infer the nature of the sorbent sites. 

If an SOC is absorbed into a liquid organic layer on the particle, a relationship between Kp and pL that is similar to that developed for adsorption onto a solid can be derived (Pankow et al., 1994a, 1994b). In this case, the gas-particle partitioning coefficient for the / th compound is defined as... 

Figure 1.5 shows a schematic representation of the double layer at a planar solid-liquid interface. The potential drop across the Helmholz layer is shown as linear (in the presence of specific adsorption, it will not be completely linear), followed by a tailing-off of the potential into the diffuse layer. For concentrated solutions (>0.1 M) the diffuse layer is typically a nanometer or less, while for dilute solutions it may be tens or even hundreds of nanometers. 

Extremely low interaction forces in monomolecular adsorption layers on liquids, Langmuir Blodgett films or in solid interfaces are the precondition for low energy surfaces. Perfluorocarbons or terminal fluorinated parts in amphiphilic molecules and fluoropolymers meet these demands. 

Figure 11.1. Schematic views of various ways in which an organic chemical, i, may sorb to natural inorganic solids (a) adsorption from air to surfaces with limited water presence, (b) partitioning from aqueous solutions to the layer of vicinal water adjacent to surfaces that serves as an absorbent liquid, (c) adsorption from aqueous solution to specific surface sites due to electron donor-acceptor interactions, (d) adsorption of charged molecules from aqueous solution to complementarily charged surfaces due to electrostatic attractions, and (e) chemisorption due to surface bonding or inner sphere complex formation.

It is postulated that one of the ions of the adsorbed 1 1 electrolyte is surface active and that it forms an ionized monolayer at the solid/liquid interface. All counterions are assumed located in the diffuse double layer (no specific adsorption). Similions are negatively adsorbed in the diffuse double layer. Since the surface-containing region must be electrically neutral, the total moles of electrolyte adsorbed, n2a, equals the total moles of counterions in the diffuse double layer which must be equal to the sum of the moles of similions in the diffuse double layer and the charged surface, A[Pg.158]

Altering the adsorption layer at the solid-liquid interface, therefore affecting the integration of growth units... 

FIGURE 17.8 Illustration of adsorption of a polymer at the solid-liquid interface, inhibiting particle agglomeration via a steric barrier, (a) Adsorbed polymer on the surface of two particles (b) interpenetration of the adsorbed layers as the particle surfaces approach is energetically unfavorable owing to osmotic and entropic phenomena. (Reprinted from Meyers, D. (19ffiljrfaces, Interfaces, and CollojctefCH Publishers, Inc.,... 

Interestingly, protein adsorption is also a field of biological interfacial chemistry which parallels that of synthetic materials at the solid - liquid interface. A number of spectroscopic advances have been made which allow FT-IR to be used in kinetic monitoring of protein adsorption on metals and "biocompatible" polymers. In addition to providing in - situ measurements of total adsorbed protein, FT-IR can also yield information about perturbation of protein secondary structure in adsorbed layers. 

Surfactants at Interfaces. Somewhat surprisingly, the successes described above in the in-situ studies of protein adsorption have not inspired extensive applications to the study of the adsorption of surfactants. The common materials used in the fabrication of IREs, thalliumbromoiodide, zinc selenide, germanium and silicon do, in fact, offer quite a range in adsorption substrate properties, and the potential of employing a thin layer of a substance as a modifier of the IRE surface which is presented to a surfactant solution has also been examined in the studies of proteins. Based on the appearance of the studies described below, and recent concerns about the kinetics of formation of self-assembled layers, (108) it seems likely that in-situ ATR studies of small molecules at solid - liquid interfaces ("wet" solids), will continue to expand in scope. 

Polymer Adsorption. A review of the theory and measurement of polymer adsorption points out succinctly the distinquishing features of the behavior of macromolecules at solid - liquid interfaces (118). Polymer adsoiption and desorption kinetics are more complex than those of small molecules, mainly because of the lower diffusion rates of polymer chains in solution and the "rearrangement" of adsorbed chains on a solid surface, characterized by slowly formed, multi-point attachments. The latter point is one which is of special interest in protein adsoiption from aqueous solutions. In the case of proteins, initial adsoiption kinetics may be quite rapid. However, the slow rearrangement step may be much more important in terms of the function of the adsorbed layer in natural processes, such as thrombogenesis or biocorrosion / biofouling caused by cell adhesion. 

Protein function at solid-liquid interfaces holds a structural and a dynamic perspective [31]. The structural perspective addresses macroscopic adsorption, molecular interactions between the protein and the surface, collective interactions between the individual adsorbed protein molecules, and changes in the conformational and hydration states of the protein molecules induced by these physical interactions. Interactions caused by protein adsorption are mostly non-covalent but strong enough to cause drastic functional transformations. All these features are, moreover, affected by the double layer and the electrode potential at electrochemical interfaces. Factors that determine protein adsorption patterns have been discussed in detail recently, both in the broad context of solute proteins at solid surfaces [31], and in specific contexts of interfacial metalloprotein electrochemistry [34]. Some important elements that can also be modelled in suitable detail would be ... 

---