Gel layer surface

The mechanism of such UF can be explained by the following concentration polarization model (cf. Figure 8.3) [3, 4]. In the early stages of UF, the thickness of the gel layer increases with time. However, after the steady state has been reached, the solute diffuses back from the gel layer surface to the bulk of solution this... 

Determine the saturated albumin concentration at the gel-layer surface and the liquid film mass transfer coefficient. 

The mechanism of such UF can be explained by the following concentration polarization model (cf. Figure 8.3) [3,4], In the early stages of UF, the thickness of the gel layer increases with time. However, after the steady state has been reached, the solute diffuses back from the gel layer surface to the bulk of solution this occurs due to the difference between the saturated solute concentration at the gel layer surface and the solute concentration in the bulk of solution. A dynamic balance is attained, when the rate of back-diffusion of the solute has become equal to the rate of solute carried by the bulk flow of solution towards the membrane. This rate should be equal to the filtrate flux, and consequently the thickness of the gel layer should become constant. Thus, the following dimensionally consistent equation should hold ... 

Fig. 10. Composite hoUow-fiber membranes (a) polysulfone boUow fiber coated witb fiiran resin. A and B denote fiiran resin surface and porous support, respectively (b) cross section of composite boUow fiber (PEI/TDI coated on polysulfone matrix). C, D, and E denote tightly cross-linked surface, "gutter" gel layer, and porous support, respectively. Both fibers were developed for reverse osmosis appHcation (15).

The immersion of glass electrodes in strongly dehydrating media should be avoided. If the electrode is used in solvents of low water activity, frequent conditioning in water is advisable, as dehydration of the gel layer of the surface causes a progressive alteration in the electrode potential with a consequent drift of the measured pH. Slow dissolution of the pH-sensitive membrane is unavoidable, and it eventually leads to mechanical failure. Standardization of the electrode with two buffer solutions is the best means of early detection of incipient electrode failure. 

A key factor determining the performance of ultrafiltration membranes is concentration polarization due to macromolecules retained at the membrane surface. In ultrafiltration, both solvent and macromolecules are carried to the membrane surface by the solution permeating the membrane. Because only the solvent and small solutes permeate the membrane, macromolecular solutes accumulate at the membrane surface. The rate at which the rejected macromolecules can diffuse away from the membrane surface into the bulk solution is relatively low. This means that the concentration of macromolecules at the surface can increase to the point that a gel layer of rejected macromolecules forms on the membrane surface, becoming a secondary barrier to flow through the membrane. In most ultrafiltration appHcations this secondary barrier is the principal resistance to flow through the membrane and dominates the membrane performance. 

In ulttafUttation, the flux,/ through the membrane is large and the diffusion coefficient, D, is small, so the ratio cjcan teach a value of 10—100 or mote. The concentration of retained solute at the membrane surface, may then exceed the solubility limit of the solute, and a precipitated semisohd gel forms on the surface of the membrane. This gel layer is an additional battier to flow through the membrane. 

Eigute 31 also shows that the point at which the gel layer forms and the flux teaches a maximum depends on the concentration of the macromolecule in the solution. The mote concentrated the solution, the lower the flux at which the gel layer forms. The exact relationship between the maximum flux and macromolecule concentration can be obtained from equation 2, expressing the concentration at the membrane surface, as at which point /becomes giving equation 3. 

In a static system, the gel-layer thickness rapidly increases and flux drops to uneconomicaHy low values. In equation 6, however, iCis a function of the system hydrodynamics. Typically, high flux is sustained by moving the solution bulk tangentially to the membrane surface. This action decreases the gel thickness and increases the overall hydrauHc permeabiUty. For any given channel dimension, there is an optimum velocity which maximizes productivity (flux per energy input). 

In the course of mixture separation, the composition and properties of both mobile phase (MP) and stationary phase (SP) are purposefully altered by means of introduction of some active components into the MP, which are absorbed by it and then sorbed by the SP (e.g. on a silica gel layer). This procedure enables a new principle of control over chromatographic process to be implemented, which enhances the selectivity of separation. As a possible way of controlling the chromatographic system s properties in TLC, the pH of the mobile phase and sorbent surface may be changed by means of partial air replacement by ammonia (a basic gaseous component) or carbon dioxide (an acidic one). 

Let the concentration of solvent (B) in equilibrium with the silica gel surface be (c) g/ml. Let a fraction (a) of the surface be covered with a mono-layer of the polar solvent (B) and, of that fraction (a), let a fraction ( 3) be covered by a second layer of the polar solvent (B). The number of molecules striking and adhering to the surface covered with a mono-layer of polar solvent (A) and that covered with a mono-layer of solvent (B) per unit time will be (n ) and be (n") respectively. Furthermore, let the number of molecules of solvent (A) leaving the mono-layer surface and the bi-layer surface per unit time be ni and 2 respectively. Now, under conditions of equilibrium,... 

Cake layer formation builds on the membrane surface and extends outward into the feed channel. The constituents of the foulant layer may be smaller than the pores of the membrane. A gel layer can result from denaturation of some proteins. Internal pore fouling occurs inside the membrane. The size of the pore is reduced and pore flow is constricted. Internal pore fouling is usually difficult to clean. 

Epiphase Airway surface liquid gel layer composed of mucins in the form... 

Several cleaning methods are used to remove the densified gel layer of retained material from the membrane surface. Alkaline solutions followed by hot detergent solutions are indicated for organic polymer colloids and gelatinous materials fouling. Ferrous deposits, t3 pical in water treatments, are usually removed with a citric or hydrochloric wash. [35]. 

Some of these conclusions may require revision, since recent findings of Ellison Warrens (1987) on the related glass-ionomer cement (Section 5.9.6) suggest that acid attack occurs throughout the body of the glass particle and not just at the surface layer. In that case the silica gel layer is not a relict but a zone of gelation. This view is more in accord with ideas on the decomposition of aluminosihcate glasses. 

Generally, the effectiveness of the separation is determined not by the membrane itself, but rather by the formation of a secondary or dynamic membrane caused by interactions of the solutes and particles with the membrane. The buildup of a gel layer on the surface of an ultrafiltration membrane owing to rejection of macromolecules can provide the primary separation characteristics of the membrane. Similarly, with colloidal suspensions, pore blocking and bridging of... 

The latest innovation is the introduction of ultra-thin silica layers. These layers are only 10 xm thick (compared to 200-250 pm in conventional plates) and are not based on granular adsorbents but consist of monolithic silica. Ultra-thin layer chromatography (UTLC) plates offer a unique combination of short migration distances, fast development times and extremely low solvent consumption. The absence of silica particles allows UTLC silica gel layers to be manufactured without any sort of binders, that are normally needed to stabilise silica particles at the glass support surface. UTLC plates will significantly reduce analysis time, solvent consumption and increase sensitivity in both qualitative and quantitative applications (Table 4.35). Miniaturised planar chromatography will rival other microanalytical techniques. 

For trace analysis in fluids, some Raman sensors (try to) make use of the SERS effect to increase their sensitivity. While the basic sensor layout for SERS sensors is similar to non-enhanced Raman sensors, somehow the metal particles have to be added. Other than in the laboratory, where the necessary metal particles can be added as colloidal solution to the sample, for sensor applications the particles must be suitably immobilised. In most cases, this is achieved by depositing the metal particles onto the surfaces of the excitation waveguide or the interface window and covering them with a suitable protection layer. The additional layer is required as otherwise washout effects or chemical reactions between e.g. sulphur-compounds and the particles reduce the enhancement effect. Alternatively, it is also possible to disperse the metal particles in the layer material before coating and apply them in one step with the coating. Suitable protection or matrix materials for SERS substrates could be e.g. sol-gel layers or polymer coatings. In either... 

The bright rings surrounding lower intensity spots correspond to the additional fluorescence signal from the sol-gel layer on the surface of the cones. The isolated spots (i.e., those without a surrounding ring) correspond... 

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

The equilibrium is driven far to the right by this prolonged treatment, and results in the formation of hydrated gel layers at the inner and outer surfaces of the membrane. A diagrammatic representation of a cross-section of the membrane is shown in Figure 6.3. 

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