Solute-membrane interaction

Membrane Morphology—Pores, Symmetric, Composite Only nucleopore and anodyne membranes have relatively uniform pores. Reverse osmosis, gas permeation, and pervaporation membranes have nonuniform angstrom-sized pores corresponding to spaces in between the rigid or agamic membrane molecules. Solute-membrane molecular interactions are very high. Ultrafiltration membranes have nonuniform nanometer sized pores with some solute-membrane interactions. For other microfiltration membranes with nonuniform pores on the submicrometer to micrometer range, solute-membrane interactions are small. 

With the exception of rather small polar molecules, the majority of compounds, including drugs, appear to penetrate biological membranes via a lipid route. As a result, the membrane permeability of most compounds is dependent on K0/w. The physicochemical interpretation of this general relationship is based on the atomic and molecular forces to which the solute molecules are exposed in the aqueous and lipid phases. Thus, the ability of a compound to partition from an aqueous to a lipid phase of a membrane involves the balance between solute-water and solute-membrane intermolecular forces. If the attractive forces of the solute-water interaction are greater than those of the solute-membrane interaction, membrane permeability will be relatively poor and vice versa. In examining the permeability of a homologous series of compounds... 

Figure 8 Localization of solute (propranolol) within the lipid bilayer. This solute-membrane interaction has been shown to influence the conformation and activity of a calcium-pump protein (X) embedded in the bilayer. (From Ref. 78.)... 

X-Y Liu, Q Yang, C Nakamura, J Miyake. Avidin-biotin-immobilized liposome column for chromatographic fluorescence on-line analysis of solute-membrane interactions. J Chromatogr B 750 51-60, 2001. 

Pervaporation is a membrane separation process in which a dense, non-porous membrane separates a liquid feed solution from a vapour permeate (Fig. 19.2c). The transport across the membrane barrier is therefore based, generally, on a solution-difliision mechanism with an intense solute-membrane interaction. It... 

Commercial as well as potential uses of inoiganic membranes multiply rapidly in recent years as a result of the continuous improvement and optimization of the manufacturing technologies and applications development for these membranes. Most of the industrially practiced or demonstrated applications fall in the domains of microfiltration or ultrafiltration. Microfiltration is applied mostly to cases where the liquid streams contain high levels of particulates while ultrafiltration usually does not involve particulates. While their principal separation mechanism is size exclusion, other secondary mechanisms reflecting the solution-membrane interactions such as adsorption are often operative. Still under extensive research and development is gas separation which will be treated in Chapter 7. 

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. 

This model has been successful in describing flux decline during dead-end filtration of particulate suspensions, but is not appropriate for application to crossflow filtration where the feed solution continuously recirculates [158]. Also, neither the occurrence of macromolecules and colloidal particles diffusion nor the influence of solute-solute and solute-membrane interactions on flux decline is considered in this model [42,59,159]. 

Table M8 MI-QSAR intermolecular solute-membrane interaction descriptors (set b).

Another solute-membrane interaction, formation of a precipitated gel at the solution-membrane Interface, is also not considered in our model. Gel formation during protein ultra-filtration is a major premise of many theoretical and experimental studies, but as is discussed later, there was no evidence for gel formation during the experiments with protein containing systems. All of our mathematical modeling and data analysis is for the pregel region of hemofiltration. 

Rejection of the solute (or dispersed colloid) is, together with permeate flux, one of the two key performance parameters of any ultrafiltration membrane. The values of rejection coefficients are of crucial Importance in many applications of ultrafiltration. The objective of this contribution is to consider and analyze the individual factors affecting rejection of polymer solutes by ultrafiltration membranes. The factors that will be considered include, sterlc rejection (sieving), solute velocity lag and solute-membrane Interaction. 

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.

Once the average pore size of the membrane has been determined using Eq. (21.1-7), the permeability of any other solute of known radius through the same membrane can be calculated from simultaneous solution of Eqs. (21.1-5) and (21.1-6). fn the absence of specific solute-membrane interactions, such as charge or hydrophobic bonding, ihis model is useful for predicting solute permeability coefficients through a characterized membrane, for values of q less ihan 0.6. 

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

UF membrane separation depends upon membrane pore size, solute-membrane interactions, shape and size of the macromolecule and CP. For maximum separation efficiency there should be a 10-fold difference in the sizes of the species to be separated. In addition, since all fiquid separation membranes have a certain, pore size distribution (e.g., there is a bi-modal distribution in the case of RO membranes predominantly small pores, < 10 A, and occasional large pores, > 100 A, that are attributable to the inevitable existence of defects in the skin layer) MWCO of UF membranes should be at least one-half that of the smallest solute to be removed. Characteristics of UF membranes based on pore size are fisted in Table 6.12. 

The causes of fouling vary depending on the nature of the solute and solute-membrane interactions. Fouling is often the result of a strong interaction between the membrane and the components in the feed stream for example, fouling by coUoids, iron and biomaterials can be especially severe. As a general rule, a reversible flux reduction is due to CP, whereas an irreversible flux reduction is due to fouHng. 

As mentioned, the permeate flux ean be eonsidered to be eontrolled by several hy-draulie resistance mechanisms. Initially, they were attributed only to the membrane itself [159]. It is now known that other time-dependent terms related to the solute— membrane interactions have to be considered. These can be attributed to several phenomena such as concentration polarization, gelation, deposition, adsorption, pore blocking, etc. Among these processes which increase membrane resistance, the most relevant ones (the irreversible ones) can be represented with only four kinetic models [160, 161] (1) complete blocking, (2) intermediate blocking, (3) standard blocking, and (4) cake nitration models. 

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