Adsorption-desorption water interface

Kakiuehi et al. [84] studied the adsorption properties of two types of nonionic surfactants, sorbitan fatty acid esters and sucrose alkanoate, at the water-nitrobenzene interface. These surfactants lower the interfacial capacity in the range of the applied potential with no sign of desorption. On the other hand, the remarkable adsorption-desorption capacity peak analogous to the adsorption peak seen for organic molecules at the mercury-electrolyte interface can be observed in the presence of ionic surfactants, such as triazine dye ligands for proteins [85]. 

Figure 19. Schematic representation of the cooperative adsorption and desorption of DOPC molecules between an air/water interface and a sublayer.

Hayes and Leckie (1986) postulate on the basis of their pressure jump relaxation experiments on the adsorption-desorption of Pb2+ at the goethite-water interface the following mechanism ... 

Comans, R. N. J. (1990), "Adsorption, Desorption and Isotopic Exchange of Cadmium on lllite Evidence for Complete Reversibility", in Sorption of Cadmium and Cesium at Mineral/Water Interfaces, Ph. D. Thesis, Rijksuniversiteit Utrecht, Netherlands. 

Due to the fast kinetics of adsorption/desorption reactions of inorganic ions at the oxide/aqueous interface, few mechanistic studies have been completed that allow a description of the elementary processes occurring (half lives < 1 sec). Over the past five years, relaxation techniques have been utilized in studying fast reactions taking place at electrified interfaces (1-7). In this paper we illustrate the type of information that can be obtained by the pressure-jump method, using as an example a study of Pb2+ adsorption/desorption at the goethite/water interface. 

Based on the pressure-jump relaxation results reported here, the following mechanism is postulated for the adsorption/desorption of Pb2+ ion at the goethite/water interface (8) ... 

Chemical relaxation methods can be used to determine mechanisms of reactions of ions at the mineral/water interface. In this paper, a review of chemical relaxation studies of adsorption/desorption kinetics of inorganic ions at the metal oxide/aqueous interface is presented. Plausible mechanisms based on the triple layer surface complexation model are discussed. Relaxation kinetic studies of the intercalation/ deintercalation of organic and inorganic ions in layered, cage-structured, and channel-structured minerals are also reviewed. In the intercalation studies, plausible mechanisms based on ion-exchange and adsorption/desorption reactions are presented steric and chemical properties of the solute and interlayered compounds are shown to influence the reaction rates. We also discuss the elementary reaction steps which are important in the stereoselective and reactive properties of interlayered compounds. 

Zhang, P.C. Sparks, D.L. (1990) Kinetics and mechanism of sulfate adsorption and desorption on goethite using pressure jump relaxation. Soil Sci. Soc. Am. J. 54 1266-1273 Zhang, P.C. Sparks, D.L. (1990) Kinetics of selenate and selenite adsorption/desorption at the goethite/water interface. Environ. Sci. Technol. 24 1848-1856... 

Interfacial transfer is the transport of a chemical across an interface. The most studied form of interfacial transfer is absorption and volatilization, or condensation and evaporation, which is the transport of a chemical across the air-water interface. Another form of interfacial transfer would be adsorption and desorption, generally from water or air to the surface of a particle of soil, sediment, or dust. Illustration of both of these forms of interfacial transfer will be given in Section l.D. 

Adsorption of FeCp-PrOH on the droplet/water interface influences the MT processes. If the MT rate of FeCp-PrOH is determined by the saturated amount of the adsorbed molecules on the interface and successive desorption to the droplet interior, the rate is given by a sum of two exponentials with the fast and slow components corresponding to the adsorption and desorption rates, respectively. Using rcc = 2 x 10 11 mol cm-2, however, the amount of FeCp-PrOH adsorbed on the droplet surface (r = 4.3 /im and C0 = 0.047 M) is calculated to be 4.6 x 10 17 mol, and this corresponds to 4.5 pC as electric charge. The calculated electric charge is 170 times smaller than the observed saturated Q t) value (750 pC), indicating that the consecutive-reaction-type kinetics cannot explain the present results. Therefore, Q(t) should be analyzed on the basis of simultaneous-reaction-type kinetics. 

Radium may be transported in the atmosphere in association with particulate matter. It exists primarily as a divalent ion in water, and its concentration is usually controlled by adsorption-desorption mechanisms at solid-liquid interfaces and by the solubility of radium-containing minerals. Radium does not degrade in water other than by radioactive decay at rates that are specific to each isotope. Radium may be readily adsorbed by earth materials consequently, it is usually not a mobile constituent in the environment. It may be bioconcentrated and bioaccumulated by plants and animals, and it is transferred in food chains from lower trophic levels to humans. 

Figure 7.3 pH/solubility curves for anthropogeni-cally derived Zn, Pb and Cu in marine precipitation (Lim et al., 1994). In all cases a classical pH adsorption edge is seen indicating that pH dependent adsorption/desorption processes at the particle-water interface control the solubilities of these metals in rainwater. 

Zhang, R-C. and Sparks, D.L. (1990) Kinetics of selenate and selenite adsorption/ desorption at the goethite/water interface. Environ. Sci. Technol., 24, 1848-1856. 

Polysaccharides interfaced with water act as adsorbents on which surface accumulations of solute lower the interfacial tension. The polysaccharide-water interface is a dynamic site of competing forces. Water retains heat longer than most other solvents. The rate of accumulation of micromolecules and microions on the solid surface is directly proportional to their solution concentration and inversely proportional to temperature. As adsorbates, micromolecules and microions ordinarily adsorb to an equilibrium concentration in a monolayer (positive adsorption) process they desorb into the outer volume in a negative adsorption process. The adsorption-desorption response to temperature of macromolecules—including polysaccharides —is opposite that of micromolecules and microions. As adsorbate, polysaccharides undergo a nonequilibrium, multilayer accumulation of like macromolecules. 

Even if some polymers might be hydrophobic in the bulk, concerns about possible water adsorption effects in thin films are justified, because of the preponderant role of the interface in confinement. For example, negligible water adsorption is reported in the handbook of polymers for polystyrene in the bulk 0.05% at 23 °C and 50% relative humidity. In spite of this, strong water adsorption-desorption effects were observed in thin polystyrene films the dielectric loss decreased with more than one decade when a thin film was measured in a dry nitrogen atmosphere and after 2 horns of annealing at 135 °C (Fig. 9, example for a film thickness of 223 nm). A pronounced decrease of s ( 20% at 1 Hz) was detected as well. 

Ion adsorption and desorption at the mineral-water interface are important processes in soils, sediments, surface waters, and groundwater. By capturing or releasing ions, mineral surfaces play key roles in soil fertility, soil aggregation, chemical speciation, weathering, and the transport and fate of nutrients and pollutants in the environment. Proton adsorption is a very specific form of ion adsorption. This area is so important... 

For comparison, the measured adsorption rate for PNP to the air/water interface gave a rate constant of it = 4.4 0.2 x 10 s but a desorption rate of 6 2 s [52]. The rapid adsorption to the air/water interface is quite consistent with the very rapid establishment of the SHG signal from PNP at the dodecane/water interface observed in these flow cell experiments. However, the observed decay rate constant in the presence of TBP of ca. 0.5 s is much faster than the desorption rate constant that would be implied from the air/water experiments. This further implicates a reorganization process involving bonding between TBP and PNP as the cause of the loss of SHG intensity, which results in an overall loss of orientational order. 

Figure 3.7 shows the simultaneous measurement of the time courses of the electric potential across the interface and the interfacial tension at a chemical oscillation induced at the W/NB interface by successive introduction of SDS into the water phase. Clearly, the interfacial tension and the electric potential changed simultaneously. No change in the interfacial tension was observed before the first electric potential generation. This result indicated that the electric potential was induced not by desorption of the surfactants from the interface, but by their sudden and corrective adsorption onto the interface. The absolute value of the electric potential at the peaks was almost constant at about 200 mV under our experimental conditions. In contrast, the baseline of the electric potential gradually increased. Corresponding to the increase of the electric potential, a... 

S6). It depended on the variation of the number of latex particles formed iV with temperature. Unfortunately, they have overlooked the fact that the particle growth rate fi which appears to the power —f in the Smith-Ewart expression for the number of latex particles formed coitains the propa gation rate constant which is temperature dependent. It has also recently been realized that another factor on which JV depends, the area occupied by a surfactant molecule at the polymer-water interface Og, is also temperature dependent- Dunn et al. (1981) observed that the temperature dependence of N in the thermal polymerization of styrene differed from different emulsifiers. It seems unlikely that the differences ran be wholly explained by differing enthalpies of adsorption of the emulsifiers and, if not, this observation implies that the energy of activation for thermal initiation of styrene in emulsion depends on the emulsifier used. Participation of emulsifiers in thermal initiation (and probsbly also in initiation by oil-soluble initiators) is most probably attributable to transfer to emulsifier and desorption of the emulsifier radical frcan the micelle x>r latex particle into the aqueous phase the rates of these processes are likely to differ with the emulsifier. 

Understanding chemical reactivity at liquid interfaces is important because in many systems the interesting and relevant chemistry occurs at the interface between two immiscible liquids, at the liquid/solid interface and at the free liquid (liquid/vapor) interface. Examples are reactions of atmospheric pollutants at the surface of water droplets[6], phase transfer catalysis[7] at the organic liquid/water interface, electrochemical electron and ion transfer reactions at liquidAiquid interfaces[8] and liquid/metal and liquid/semiconductor Interfaces. Interfacial chemical reactions give rise to changes in the concentration of surface species, but so do adsorption and desorption. Thus, understanding the dynamics and thermodynamics of adsorption and desorption is an important subject as well. 

Often a rather slow adsorption at the air-water interface has been observed. Whether this is due to electrical potential barriers, or whether a particular orientation is required of the arriving molecule before it can enter the monolayer has not yet been clearly demonstrated. For small ions taking part in reactions at interfaces, such as hydroxyl and permanganate, the latter effect has never been observed, although Alexander (29) claims that ion exchange below monolayers of amines is a slow process. The present author considers that this may be explained in terms of a slow desorption of one ionic species rather than as a slow approach of the other. A gradual change in the structure of the amine film is also a possibility. 

The nature of the silica-water interface is determined by adsorption/desorption of the species in the water. When a silicon oxide, e.g., quartz, is fractured, the initial surface is composed of dangling silicon and oxygen bonds (Fig. 4.30a) which are not stable and hydroxylate easily with available waterThe hydroxylated surface is dominated by SiOH groups (Fig. 4.30b). The initial adsorbed water adjacent to the surface is oriented and has properties different from the bulk water. As this adsorbed water layer increases to more than three monolayers, its properties become more like bulk water. The surface potential changes as a result of the adsorption of the ionic species in the water. °... 

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