Solution depletion method

Adsorption isotherms are habitually obtained using the solution depletion method, which consists of comparing the solute concentrations before and after the attainment of adsorption equilibrium. Electrokinetic or zeta potentials are determined by two techniques microelectrophoresis [12,14,17] and streaming potential [13,58,59]. The former is employed to measure the mobility of small particles of chemically pure adsorbents, whereas the latter is adopted to investigate the electrophoretic behaviour of less pure coarser mineral particles. A correlation between the adsorption and electrophoretic results is usually examined with the aim of sheding light on the mechanism by means of which the surfactants are adsorbed at the solution-solid interface. This implies the necessity of maintaining the same experimental conditions in both experiments. For this purpose, the same initial operational procedure is applied. 

Besides direct surface analysis methods, indirect solution depletion methods can be applied to study protein adsorption. Solution depletion methods use solids of known large surface area (usually dispersed solids) that are placed into a solution of known protein concentration. After incubation, the concentration of the protein in the supernatant is measured. From the depletion, the adsorbed protein amount can be calculated. 

The solution depletion method for studies of adsorption in solution uses material in the form of powder or pellets with specific dimensions and surface areas. This material may be physically or chemically treated to produce hydrophobic, or positively or negatively charged hydrophilic surfaces which is exposed to the solution. The decrease in concentration of the adsorbate from the bulk solution is then measured. The precision of this method is dependent on the analytical technique used. However, for biological material, the technique does not allow measurement of subtle changes in surface denaturation. The technique has been used by a number of researchers. ... 

Figure 2. Adsorption isotherm for bovine serum albumin on polystyrene latex particles at 295 K, pH 7.4, obtained by solution depletion methods. (Adapted from Fair and Jamieson. )...

The quantity of adsorption is usually measured by means of the solution depletion method [6] in glass stoppered tubes or fiasks (Fig. 6.14). A known mass of the solid... 

Measurements of the adsorption of inhibitors on corroding metals are best carried out using the direct methods of radio-tracer detection and solution depletion measurements . These methods provide unambiguous information on uptake, whereas the corrosion reactions may interfere with the indirect methods of adsorption determination, such as double layer capacity measurements", coulometry", ellipsometry and reflectivity Nevertheless, double layer capacity measurements have been widely used for the determination of inhibitor adsorption on corroding metals, with apparently consistent results, though the interpretation may not be straightforward in some cases. 

Adsorption is determined by the depletion method using a Dohrmann DC 80 carbon analyzer. The mineral is contacted with the polymer solution and agitated with a mechanical tumbler for 24 hours, a time which has been verified to be sufficient for adsorption to be complete (9). A more detailed description of experimental procedures is given elsewhere (10). All the data reported in this study are taken in the plateau region of the adsorption isotherm. 

Much of the early studies of surfactant adsorption at the solid-solution interface were based on classical experimental techniques, such as solution depletion [1, 32], fluorescence spectroscopy [2], and measurements of the differential enthalpy of adsorption [2], Such methods have provided much of the basic initial understanding. However, they provide no direct structural information and are difficult to apply to mixtures [23, 34], However, when combined with other techniques, such as NMR and flow microcalorimetry, they provide some insight into the behaviour of mixtures. This was demonstrated by Thibaut et al. [33] on SDS/C10E5 mixtures adsorbed onto silica and by Colombie et al. [34] on the adsorption of SLS/Triton X-405 mixtures onto polystyrene particles. 

When adsorption from solution Is monitored by the depletion method. It Is very difficult to measure changes in bulk concentration over time Intervals down to milliseconds. Perhaps this Is the reason that such systematic studies are not abundant in the literature. Fast measurements require stopped-flow, pressure-jump or temperature-jump techniques. The method used to determine concentrations must also be fast suitable methods include certain spectroscopies and, for charged substances, conductivity. When adsorption on Fresnel surfaces Is studied, say by reflectometry, concentration measurements in the solution are not needed. 

In a variemt, known as thin layer wicking, the ascent or descent of the liquid is measured through a packed thin layer, supported on a glass microscopic slide. Constanzo et al. showed that for a number of model systems this method give results for cos a agreeing with those obtained from direct measurements on a deposited and dried lawn. We repeat the caveat that the dynamic methods fall when for surfactant solutions, depletion takes place. 

Methods. The diffusion experiments were performed at room temperature (23 C) utilizing a glass diffusion cell consisting of two compartments each with a volume of 175 ml. Each chamber was stirred at a constant rate to reduce boundary layer effects. Solute concentrations were monitored by h or C tracers, refractive index, or U.V. spectroscopy. Partition coefficients, defined as the ratio of the concentrations in the membrane and in the bulk aqueous phase were determined by solution depletion technique. 

One of the simplest experiments is to establish how much polymer is adsorbed onto the surface. This is known as the adsorbed amount and is determined from the adsorption isotherm (see Fig. 1). The adsorption isotherm relates the amount of polymer adsorbed at an interface to its equilibrium solution concentration. Determination of the adsorbed amount almost always involves measurement of the polymer concentration in solution before and after equilibration with a dispersion of known surface area. This usually requires separation of the dispersion particles from the supernatant by centrifugation. Separation of the bound and free polymer by dialysis can also be used but it is difficult to determine the amount of polymer that may be adsorbed onto the dialysis membrane. As these techniques require a knowledge of the amount of polymer removed from solution, they are known as depletion methods. 

We now have two terms, and an additional independent experiment is necessary to obtain the surfactant surface excess concentration. One way to obtain the surfactant surface excess concentration at a water-solid interface is by the depletion method. A known amount of a powder of known specific area solid is immersed in a known volume of an aqueous solution of the surfactant at a known concentration. After several hours of contact (required to reach adsorption equilibrium), the suspension is separated and the surfactant concentration is accurately determined. The difference between initial and final surfactant concentrations multiplied by the volume of liquid gives the amount of surfactant adsorbed on the solid. The surfactant surface excess concentration is obtained by dividing the amount of surfactant adsorbed by the surface of solid (specific area multiplied by weight of solid). The difficulty resides in the exact determination of the final surfactant concentration. In... 

The amounts of alkyl sulfosuccinate and of the cationic polymer adsorbed on the fillers and fibers were determined by the depletion method. 1 g of the filler or 0.5 g of the fibers were shaken with 100 ml of surfactant or polymer solution of known initial concentration at pH 8.5 in a glass tube for 60 min. The suspension was subsequently separated by centrifugation (labcentrifuge UJIIE Heraeus-Christ, FRG) at 4000 rpm for 60 min. Control experiments without the addition of solids did not indicate any change in the polymer concentration during centrifugation. 

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