Particle-solution interface

Several different approaches can be used to model the interaction of solutes with reactive mineral surfaces. The conceptual approaches differ in the degree to which they account for observed or postulated solution and surface reactions. Whatever the approach, the description of interactions at the particle/solution interface must inevitably take into account the effect of pH on solute adsorption. 

Table IV Techniques for Measurement of Properties of Mineral/ Particle Solution Interfaces...

Recent advances in the development of non-invasive, in situ spectroscopic scanned-probe and microscopy techniques have been applied successfully to study mineral particles in aqueous suspension (Hawthorne, 1988 Hochella and White, 1990). In situ spectroscopic methods often utilise molecular probes that have diagnostic properties sensitive to changes in short-range molecular environments. At the particle-solution interface, the molecular environment around a probe species is perturbed, and the diagnostic properties of the probe, which can be either optical or magnetic, then report back on surface molecular structure. Examples of in situ probe approaches that have been used fruitfully include electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin-probe studies perturbed vibrational probe (Raman and Fourier-transform IR) studies and X-ray absorption (Hawthorne, 1988 Hochella and White, 1990 Charletand Manceau, 1993 Johnston et al., 1993). 

The purpose of this chapter is to get a better insight of the first problem listed above, i.e., the polarization of interfaces (colloidal particles) during their interaction. Because of tutorial reasons, the electrolyte solution will be described using a rather simple, mean field approximation, that, however, allows to obtain an analytical solution of the problem. It is clear that this elaboration can easily be followed, and one can extend our model on more advanced situations. This model is identical with so-called weak-coupling theory for point ions treated in a course of the Debye-Hiickel approximation. Before going to make an elaboration for two interacting macrobodies immersed into an electrolyte solution, we would like to introduce a method, which is usually used to model this polarization, and to compute the electrical field next to a polarized medium. Then we will also discuss consequences of the polarization for the ion distribution at the particle-solution interface. 

The mathematical solution to equation 6.10 is also available for the more general case of finite volume or limited bath conditions, where equilibrium at the particle-solution interface is assumed at all times and the macro-concentration of ions A in the external solution is time-dependent. An analogous situation arises in the theory of heat transfer where the mathematical solution also serves the case of ion exchange ... 

We again stress that we are not considering polymer chains that are attached to the surface of the particle. Rather, we are considering adsorption in its broader manifestation relating to the presence of Gibbs surface layers at the particle/solution interface. Closer approach results in greater negative adsorption of the polymer chains, which is accompanied by a repulsion. 

The sedimentation potential method has been used only to a limited extent to determine the surface charge characteristics of blood cells. In this method, the colloidal particles (e.g., blood cells) are allowed to fall through a vertical column and the potential difference between two electrodes, vertically separated, is measured. An apparatus, used for measurements on erythrocytes, is shown in Figure 19. The zeta potential across the colloidal particle-solution interface is calculated from the sedimentation potential (E) according to the equation ... 

A probable cause for this variation in A is that the electrolyte solution theory used to calculate electrical potentials in the double layer is only valid for dilute solutions of monovalent ions. Whereas, in this study, the ionic concentrations were high and divalent ions were also used. An approach which partially compensates for this deficiency in the electrolyte theory is the introduction of a Stern layer of adsorbed counterions at the particle/solution interface. These counterions void a portion of the charges due to the surfactant. 

Clearly, it is important that there be a large contact angle at the solid particle-solution-air interface. Some minerals, such as graphite and sulfur, are naturally hydrophobic, but even with these it has been advantageous to add materials to the system that will adsorb to give a hydrophobic film on the solid surface. (Effects can be complicated—sulfur notability oscillates with the number of preadsoibed monolayers of hydrocarbons such as n-heptane [76].) The use of surface modifiers or collectors is, of course, essential in the case of naturally hydrophilic minerals such as silica. 

Schematic model of the solid-solution interface at a particle of AgCI in a solution containing excess AgNOa.

The particle-beam interface (LINC) works by separating unwanted solvent molecules from wanted solute molecules in a liquid stream that has been broken down into droplets. Differential evaporation of solvent leaves a beam of solute molecules that is directed into an ion source. 

The particle-beam interface is used to remove solvent from a liquid stream without, at the same time, removing the solute (or substrate). 

A stream of a liquid solution can be broken up into a spray of fine drops from which, under the action of aligned nozzles (skimmers) and vacuum regions, the solvent is removed to leave a beam of solute molecules, ready for ionization. The collimation of the initial spray into a linearly directed assembly of droplets, which become clusters and then single molecules, gives rise to the term particle beam interface. 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

The potential difference across the mobile part of the diffuse-charge layer is frequently called the zeta potential, = E(0) — E(oo). Its value depends on the composition of the electrolytic solution as well as on the nature of the particle-hquid interface. 

In 1997, a Chinese research group [78] used the colloidal solution of 70-nm-sized carboxylated latex particles as a subphase and spread mixtures of cationic and other surfactants at the air-solution interface. If the pH was sufficiently low (1.5-3.0), the electrostatic interaction between the polar headgroups of the monolayer and the surface groups of the latex particles was strong enough to attract the latex to the surface. A fairly densely packed array of particles could be obtained if a 2 1 mixture of octadecylamine and stearic acid was spread at the interface. The particle films could be transferred onto solid substrates using the LB technique. The structure was studied using transmission electron microscopy. 

Cl and El are both limited to materials that can be transferred to the ion source of a mass spectrometer without significant degradation prior to ionisation. This is accomplished either directly in the high vacuum of the mass spectrometer, or with heating of the material in the high vacuum. Sample introduction into the Cl source thus may take place by a direct insertion probe (including those of the desorption chemical ionisation type) for solid samples a GC interface for reasonably volatile samples in solution a reference inlet for calibration materials or a particle-beam interface for more polar organic molecules. This is not unlike the options for El operation. 

In the previous sections, we described the overall features of the heat-induced phase transition of neutral polymers in water and placed the phenomenon within the context of the general understanding of the temperature dependence of polymer solutions. We emphasised one of the characteristic features of thermally responsive polymers in water, namely their increased hydropho-bicity at elevated temperature, which can, in turn, cause coagulation and macroscopic phase separation. We noted also, that in order to circumvent this macroscopic event, polymer chemists have devised a number of routes to enhance the colloidal stability of neutral globules at elevated temperature by adjusting the properties of the particle-water interface. 

Sigg, L., W. Stumm, and B. Zinder (1984), "Chemical Processes at the Particle/Water Interface Implications Concerning the Form of Occurrence of Solute and Adsorbed Species", in C. J. H. Kramer and J. Duinker, Eds., Complexation of Trace Metals in Natural Waters, Dr. W. Junk Publishers, The Hague, Netherlands, 251-266. 

General Observations About x. its Relationship to the Overall Partitioning Coefficient and to the Concept of Surface-Site Heterogeneity. One approach to metal/particle surface interactions which has been developed, historically, in a variety of forms, is a conceptual model that assumes only two conditions for surface sites occupied by an adsorbate or unoccupied. In applying this approach to the solid/aqueous solution interface, the adsorption... 

We examine an electron transfer of hydrated redox particles (outer-sphere electron transfer) on metal electrodes covered with a thick film, as shown in Fig. 8-41, with an electron-depleted space charge layer on the film side of the film/solution interface and an ohmic contact at the metal/film interface. It appears that no electron transfer may take place at electron levels in the band gap of the film, since the film is sufficiently thick. Instead, electron transfer takes place at electron levels in the conduction and valence bands of the film. 

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