Effect solute-membrane interaction

Tan et al. [4] proposed to use a thin silica membrane (prepared by them), which presents hydrophilic properties. They obtained very interesting results, i.e., arejection rate and a permeability of 0.98 and 4.4 x 10 mol s Pa , respectively (see Table 7.1). To underline the effects of adsorption, the same membrane was turned hydrophobic by chemical vapor deposition (CVD) treatment and tested. A very low rejection was obtained and no significant selectivity was given by this membrane. On the basis of the fact that caffeine has more affinity with hydrophilic surfaces, these results gave evidence that adsorption was the major mechanism. Therefore, adsorption phenomenon should be taken into account for membrane choice to control solute/membrane interactions. 

Figure 8. Effect of solute-membrane interaction on rejection. Rejection curves calculated for Hub = 0, Hu, - 0.8 X erg, and H,u = 2.0 X erg.

Equation 7 shows that as AP — oo, P — 1. The principal advantage of the solution—diffusion (SD) model is that only two parameters are needed to characterize the membrane system. As a result, this model has been widely appHed to both inorganic salt and organic solute systems. However, it has been indicated (26) that the SD model is limited to membranes having low water content. Also, for many RO membranes and solutes, particularly organics, the SD model does not adequately describe water or solute flux (27). Possible causes for these deviations include imperfections in the membrane barrier layer, pore flow (convection effects), and solute—solvent—membrane interactions. 

Other modifications to the theory of Anderson and Quinn [142] have been reviewed by Deen [146]. Malone and Quinn [147] modified the above theory to include the effect of electrostatic interactions on transport in microporous membranes. Smith and Deen [148] have also looked at these electrostatic or double layer interactions. More recently, Kim and Anderson [149] investigated the hindrance of solute transport in polymer lined micropores. Also, as briefly mentioned above, an excellent review of the theories presented for transport in microporous membranes has been given by Deen [146]. 

The effectiveness of the method is most probably based on the fact that alkyl hypochlorite is formed at the oil/water interface where the cosurfactant alcohol resides. The oxidation that follows takes place either inside or on the surface of oil droplet. The rate of the reaction can result from a large hydrocarbon/water contact area permitting interaction between oil-soluble sulfide with interfacial cosurfactant that served as an intermediary. An extension ofthis procedure to mustard deactivation has also been proposed [20b]. Such systems could be also applied to the degradation of several environmentally contaminating materials The formation of microemulsions, micelles and vesicles is promoted by unfavourable interactions at the end sections of simple bilayer membranes. There is no simple theory of solute-solvent interactions. However, the formation of... 

It has already been stated that a simple way to enhance the resolution of an FFF measurement is to reduce the channel thickness. This however can lead to other problems as the effects of surface irregularities are enhanced, due to the increase of the surface-to-volume ratio of the channel, and lead to increasing, unpredictable solute-wall interactions, etc. Furthermore, non-uniformities in the channel planarity will also be a problem with very small channel thicknesses, especially in Fl-FFF where the accumulation wall is a membrane. Another possibility for the reduction of H is to reduce the flow velocity of the carrier liquid which, in turn, leads to longer retention times and slower separation. However, in Fl-FFF, one has the possibility to increase the flow rate with cost to resolution but simultaneously increase also the cross-flow rate. Nevertheless, this enhances sample absorption onto the membrane. The third possibility for the reduction of H is to increase the solute diffusion. This can be done by decreasing the solvent viscosity by increasing the temperature. 

There are two different levels where fouling phenomena and related effects may interfere with performance of composite inorganic or hybrid membranes. The first and the more classically reported in literature is the one of the separation process itself, which through various interactions between solution and material (adsorption, surface deposits, pore plugging) generally leads to reduced fluxes and increased retentions. The second, much more less described by authors but of the same nature and with analogous effects, concerns membrane preparation, and the possible interactions between deposited layers. Theses two aspects are linked up with the so-called formed-in-place membranes, obtained by deposition of species onto a ceramic support through cross-flow filtration. In what follows, they will be described in a unified approach. 

In contrast to lipid bilayer membranes, it has been found [4] that the permeability coefficient of the human red-cell membrane to water did not change when the free cholesterol content in the membrane was varied from 0.84 to 1.87 mg/ml cells. Furthermore, the permeability of the human red-cell membrane to sulfate and some nonelectrolytes remained constant when membrane cholesterol was partially removed (for review, see [6]). These results, however, should not be taken as evidence that water transport in human red cells is independent of membrane cholesterol, since this degree of variation may be insufficient to produce alteration. In fact, extensive depletion of membrane cholesterol induces a marked increase in nonelectrolyte permeability. The effect of membrane cholesterol on the transport of water is also found in other membrane systems. For example, the polyene antibiotic. Amphotericin B, which interacts specifically with sterol-containing membranes, increases the permeability of the mucosal but not the serosal membrane of toad bladder to water and other solutes [32]. It is possible that membrane cholesterol only effects the movements through the lipid bilayer pathway. This may explain the findings that the permeabiUty coefficient of the human red cell membrane to water... 

Transport in OSN membranes occurs by mechanisms similar to those in membranes used for aqueous separations. Most theoretical analyses rely on either irreversible thermodynamics, the pore-flow model and the extended Nemst-Planck equation, or the solution-diffusion model [135]. To account for coupling between solute and solvent transport (i.e., convective mass transfer effects), the Stefan-Maxwell equations commonly are used. The solution-diffusion model appears to provide a better description of mixed-solvent transport and allow prediction of mixture transport rates from pure component measurements [136]. Experimental transport measurements may depend significantly on membrane preconditioning due to strong solvent-membrane interactions that lead to swelling or solvent phase separation in the membrane pore structure [137]. 

The Dq values also depend on the amount of dissolved compounds (influencing their aggregation), pH value, and salinity of the aqueous solutions (affecting the shape of protein molecules). These effects and protein interactions with the CG membrane can result in broadening of the D value range. This range corresponds to a certain f(D) distribution that depends on individual protein types. 

---