Layer gel-like

For many years, it was thought that the macro solute forms a new phase near the membrane—that of a gel or gel-like layer. The model provided good correlations of experimental data and has been widely used. It does not fit known experimental facts. An explanation that fits the known data well is based on osmotic pressure. The van t Hoff equation [Eq. (22-75)] is hopelessly inadequate to predict the osmotic pressure of a macromolecular solution. Using the empirical expression... 

Enzyme electrodes. Guilbault52 was the first to introduce enzyme electrodes. The bulb of a glass electrode was covered with a homogeneous enzyme-containing gel-like layer (e.g., urease in polyacrylamide) and the layer was protected with nylon gauze or Cellophane foil when placed in a substrate solution (e.g., urea) an enzymatic conversion took place via diffusion of substrate into the enzymatic layer. 

The establishment of an exact quantitative relationship between the thermodynamic potential, (p0, or the potential of the adsorption layer (the Stern layer) potential, (pd, and the electrokinetic potential, , is an important and at present unsolved problem. Depending on the thickness of the layer with increased viscosity near the solid surface, the electrokinetic potential may either approach the value of the Stem layer potential or be lower than the latter. In some cases (e.g. for quartz), as shown in studies by D.A. Fridrikhsberg and M.P. Sidorova [10,11], the difference between the electrokinetic and thermodynamic potentials may be related to the hydration (swelling) of the solid surface and the formation of a gel-like layer resistant to deformation, within which a partial potential drop takes place. The difference between (pdand C, may also be related to microscopic surface roughness of the solids, i.e. to the presence of growth steps, dislocations and other defects (see Chapter IV). 

The dissolution of a polymer in a penetrant involves two transport processes, namely penetration of the solvent into the polymer, followed by disentanglement of the macromolecular chains. When an uncrosslinked, amorphous, glassy polymer is in contact with a thermodynamically compatible liquid (solvent), the latter diffuses into the polymer. A gel-like layer is formed adjacent to the solvent-polymer interface due to plasticization of the polymer by the solvent. After an induction time, the polymer is dissolved. A schematic diagram of solvent diffusion and polymer dissolution is shown in Fig. 1. However, there also exist cases where a polymer cracks when placed in a solvent. 

A periodic and chaotic membrane oscillations have been observed in case of membrane doped by DOPH [19, 20]. After 30 min, a thick opaque di-oleyl phosphate (DOPH/oleyl alcohol/water (D/OAV) emulsion appears on the low-pressure surface of the membrane. The growth of this gel-like layer triggers a dramatic rise in the electrical resistance of the membrane and consequently oscillation in membrane potential. Chaotic behaviour appears as a part of periodic-chaotic sequence, also quite commonly associated with chaos. However fair amount of noise appears during membrane oscillations (Fig. 11.5) creating doubt regarding the existence of deterministic chaos. It has been shown that aperiodic behaviour in the system by way of a period-doubling sequence of bifurcation [19]. 

Experimental results with low concentration feeds or under conditions where M is close to 1.0 are in good agreement with the theoretical predictions. However, when the wall concentration becomes high, the solvent flux often cannot be controlled by adjusting the pressure difference. Thus, Eq. 117-401 no longer holds Some other phenomenon must be controlling the solvent flux. Careful examination of the membrane surface after these experiments shows a gel-like layer covering the membrane surface. This gel layer alters the flux-pressure drop relationship and controls the solvent flow rate. 

Membrane fouling is caused by capture of particles or formation of a gel-like layer on the membrane and the resultant decrease in the area available for fluid flow. Prefiltration, pH adjustment, or other means may be used to prevent fording. 

The most extremum behavior of all the characteristics is observed at low content of cells in the suspension (Figure 7.11). A minimum of Ys and C and a maximum of CZ are at Ch o = 98.4 wt% or Cy=C<-eii+Cicw=6.1 wt%. Notice that there is the extreme dependence of the Ys value on the total concentration of water in the aqueous suspensions of nanooxides (see Section 1.1.6) at a minimum at Chjo 93 wt%. This boundary concentration corresponds to transition from diluted suspensions to concentrated ones characterized by different particle-particle interactions. In the diluted suspensions, the systems can separate into a gel-like layer and upper layer with bulk, almost pure water. In the concentrated suspensions, the systems represent a continuous gel-like structure without separation of bulk water. With increasing size of particles, the critical concentration (CJ should increase. Therefore, one could expect a larger Q value for yeast S. cerevisiae cells (5-10 pm) than for nanosilica (primary particles 10 nm). However, the C<. values for yeast cells and nanosilica are relatively close due to the formation of silica nanoparticles aggregates 0.5-l pm and agglomerates >1 pm, which have sizes close to sizes of cells. Therefore, at Cy< 10 wt% (Cycolloidal dispersion with relatively weak intercell interactions. At these Cy values, the adhesion... 

Dissolution is an equilibrium reaction. That is, the dissolution of the surface is accompanied by precipitation of the solute oxide species. Some authors argue that this results in transformation of the surface morphology. For instance. Vigil et al. (1994) proposed the formation of a gel-like layer on a silica surface, which results from re-precipitation of dissolved silica and that consists of polymeric -Si(0H)2-0-Si(0H)2-0H chains. The thickness of this layer is estimated with 10 A. A similar concept was stated by Adamczyk et al. (2004). 

Such transitions have been observed at the interfacial layers formed by the gelatin-lecithin complexes during their adsorption from an aqueous phase in the present work too. In the begiiuiing, complexes are concentrated in the gel-like layer (that is observed during first 4 hours). Then the gel-Uke structure transfonns into flaky sediment. In the investigated systems, they become observed by sight even under small magnification. 

In the first case, the interfacial adsorption layers are characterized by clearly expressed viscoelastic properties and plasticity reflected by the yield stress Fig. 5a presents the interfacial shear viscosity, t], yield stress xys and surface elastic modules Gj. One can see that the elastic modules and the viscosity for gelatin layers without a surfactant (Ciec = 0) are equal to 1.2 mN/m and 8.3 mN s/m, respectively. Upon addition of phosphoUpids at low lecithin-to-gelatin ratio these parameters do not change. However some threshold component ratio exists and the elastic modules and the yield stress sharply increase beyond this threshold. This is the evidence of the increase in the number of contacts between strucmre elements in a gel-like layer. 

Fig. 13 Schematic mechanism of a conversion reaction based on a metal oxide MO. a The first cycle, showing the lithiation step yielding metal nanoparticles embedded in a Ii20 matrix and the delithiation step leaving nanosized metal oxide particles, b The ovtaxiU reaction illustrated with cobalt oxide (CoO). Starting with large CoO particles, the first lithiation yields Co nanoparticles embedded in Li20 (horizontal), the Co nanoparticles catalyze Li20 decomposition and Li extraction when the polarization is reversed, which results in nanosized CoO. Thanks to the nanosized particles, the electrode can be cycled between CoO and Li20 + Co as lithium is added or removed (vertical). Note however that the electrolyte decomposition (gel-like layer) is favored by the nanosized metal particles b is reprinted with permission from [157]...

We investigated the deposited material using cryo-SEM to reveal the formation of a porous gel-like layer with relatively uniform pores (Fig. 13c-e). The structure was strikingly reminiscent of that of filtration membranes, motivating us to investigate the system as a size-selective separation membrane. Assuming that it possesses... 

Nevertheless, hydrocolloids play an important role in stabilizing emulsions when used in conjunction with proteins. The two biopolymers form water-soluble amphiphilic molecules (steric and electrostatic stabilization) or gel-like layers (mechanical barriers) at interfaces. 

---