Hydrophilic barriers

Figure 2.8. Proposed mechanism of action of BPI (a) The external polysaccharide chain of lipopolysaccharide (LPS) is a hydrophilic barrier stabilised by Ca2+ and Mg2+, which cross-link the LPS chains, (b, facing) BPI is highly basic and initially interacts by displacing the cations that bind to charged sites on the surface. See text for details.

The close agreement between the experimental and calculated (Equation 9) ratios of 18 2/18 3 support exclusion of the 4-hydroxylphenyl analogue from the calculations. Examination of Equation 9 shows an interdependence between the biological activity and the hydrophobic properties of the chemical used, commonly found with many QSAR equations. This interdependent relationship is determined by the and terms, respectively. These terms control phenomena of hydrophobic interactions with receptors and phenomena of transport and distribution within the total biological systems. The occurrence of squared terms of the hydrophobic parameter in structure-activity correlations has been explained on the assumption that the compound has to penetrate several lipophilic-hydrophilic barriers or compartments on its way to the site of action (16, 17). This is consistent with the uptake of pyridazinones by roots and sbsequent translocation to the shoots (chloroplast) as the site of action (13). 

FIGURE 23.3 The corneal structure showing the sequential hydrophobic and hydrophilic barriers. 

The polarity of insecticides has been regarded as an important factor for cuticular penetration. As mentioned earlier, the typical insect cuticle should be considered a two-phase system, the outer layer (epicuticle) having hydrophobic properties and the inner layers (procuticle) having hydrophilic properties. Thus, whether the insecticide is lipid soluble or water soluble, its tendency to move through the cuticle as a whole depends on whether it can pass through the hydrophobic or hydrophilic barrier, whichever the case may be. The efficiency of such movement will probably depend on the oil-water partition coefficient of the insecticide, the nature of the surfactant or solvent—which may be a part of the insecticide formulation—and the nature of the cuticle itself (Terriere, 1982). 

Water flux through reverse osmosis membranes is considerably dependent on the hydrophilic character of the barrier layer. In the composite membrane approach, highly hydrophilic barrier layer compositions can be used, suitably insolubilized by crosslinking. To a large extent, water flux and salt rejection can be controlled by the type and extent of crosslinking. 

For in vivo data eq. 61 was extended to eq. 62 by including a parabolic lipophilicity term. The idea behind eq. 62 was that molecules which are too hydrophilic or too lipophilic will not be able to cross lipophilic or hydrophilic barriers, respectively. Therefore, they will have a lower probability to arrive at the receptor site than molecules with intermediate lipophilicity, being readily soluble in aqueous phases as well as in lipid phases. 

The oxygen permeability of both amylose and amylopectin rich starch films is reported to be as good as that from commercial EVOH at ambient humidity (relative humidity <15%), but above 20% such efficacy is lost [85]. Investigation of the gas permeability of potato starch films embedded with LDPE showed similar oxygen barrier characteristics to hydrophilic barriers, but with increasing water content this property was gradually lost [86]. 

Also, surface modification by UV or UV/electron beam grafting of monomers of such acrylates is well documented in the literature [25-27]. Photosubstitution at carbonyl sites with alcohols or amines, to form ester or amide groups, respectively, always has an advantage over pure processes of photo-oxidation in improving wettability. These surface modification processes seem to be very beneficial in facilitating the fabrication of composite membranes with hydrophilic barriers on top of hydrophobic support, such as PP, with applications in separation technology and medicine [25]. 

Wang X, Zhang K, Yang Y, Wang L, Zhou Z, Zhu M, Hsiao BS, Chu B (2010) Development of hydrophilic barrier layer on nanofibrous substrate as composite membrane via a facile route. J Membr Sci 356(1-2) 110-116. doi 10.1016/j.memsci.2010.03.039... 

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