Hydrophilic centre

Gillis-D Hamers explains this shift by the interaction of the free silanols with the surrounding hydrophilic centres (adsorbed water, bridged and free silanols). The free silanol peak shift is considered as a secondary effect although the free silanols are in no direct interaction with the other silanols, still their presence causes slight changes in the electronic environment of the free O-H bond, resulting in a small peak shift. 

The amount of water, which is sorbed on zeolites, can be very significant, up to 25 wt% for Na and H-FAU samples with low Si/Al ratios. This amount depends on the partial pressure of water and on various characteristics of the zeolite, such as the pore system, which determines the micropore volume accessible to water, the framework Si/Al ratio, the nature of the cations and the crystallite size. Thus, a linear decrease was found in the amount of water adsorbed at low pressure (that is, strongly adsorbed) over various protonic high silica zeolite materials MOR,[28] bea[29] or MFI,[30] with a decrease in the framework A1 content. This can be attributed to the decrease in the number of partially ionic, hydrophilic centres associated with the tetrahedrally coordinated A1 atoms at the profit of nearly homopolar (hydrophobic) =Si-0-Si= bonds. A stoichiometry of four water molecules per H(A1) was found, suggesting the formation of H9O4 species. 

Hydration of HPP. All factors mentioned above affect the T i value, showing the change in the state of water to be the result of a change in the composition of the water and the surface properties of the disperse phase. Aluminium hydrolysis products are mostly particles of Al(OH)3 with aluminium hydroxyl complexes adsorbed on them, so the change in T can be associated with an altered nature and number of hydrophilic centres. It has been shown5,6 that in all cases the spin-spin relaxation time of water protons decreases with increasing OH/A1 ratio in the coagulant molecule. This is an evidence of increased hydration of particles surface in this direction. 

The hydrophilic centres in PUDs are basically of three types (Klein and Schwab, 1993) ... 

Changes in the structure and hydrophilic properties of the wood cell wall m specimens after the impregnation and ageing procedures samples were studied by the water vapours sorption method. The sorption-desoqjtion isotherms measured on a vacuum sorption balance at 295 K were analysed by the comparative method in combination with the BET method (8). The accessible specific area A (m /g), reflecting swelling capacity, the mass hydrophilicity a (mM/g) and the surface concentration of Ihc hydrophilic centres a (groups/nm ) was determined. 

After the exposure of the impregnated wood specimens to C. puteana and L. lepideua, the hydrophilic properties of wood increase, i.e. A and a values increase by 6.9% and 9.4% in the case of C. puteana, respectively, and by 15.7% and 18.8% in the case of L lepideus, respectively. The increase in a values in both cases indicates the appearance of new hydrophilic centres in wood. Obviously, it occurs owing to a partial destruction of wood substance by the microorganisms. 

The impregnation of wood with pyrolysis oils (by EN 113) decreases its hydrophilic properties mainly owing to the decrease in its swelling ability- The surface concentration of hydrophilic centres decreases negligibly. 

L lepidius has a destructive action upon oil-impregnated wood, which is twice as high as that in the case of C. puteana, i.e. in particular, specific surface is increased by 7% and 16%, respectively. In this case, the hydrophilic properties of the surface are changed not only at the expense of the loosening of the structure, but also owing to the increase in the surface concentration of the hydrophilic centres. 

The activity of C. puteana relative to the sample after the leaching procedure tends to increase dramatically, i.e. specific surface and mass hydropbility tend to increase by 40% at some simultaneous increase in the concentration of the surface hydrophilic centres. At the same time, the activity of L lepideus relative to this sample is negligible, i.e. specific surface is increased only by 2%. 

The same situation can also be found in arch-shaped or concave deoxycholic acid (= DC Figure 7.17) derivatives, in which rings A and B are cw-fused. As an example, a bilayer was observed in monoclinic crystals of the rubidium salt of DC (= RbDC). Hexagonal crystals of NaDC and RbDC hydrates, on the other hand, produced interesting helical structures (Figure 7.17) with hydrophilic centres. They constitute the prototype of a well defined micellar structure,... 

This hydrophilic centre is large enough to accommodate an ion and it is found that a naked potassium ion (i.e. no surrounding water molecules) fits the space and is complexed by the amide carboxyl groups (Fig. 10.65). 

A secondary or tertiary amine that could be either cyclic or an open-chain analogue usually affords a marked and pronounced hydrophilic centre ,... 

A hydrophilic centre may be suitably linked to either an amide frmction or an ester moiety either by S, N, 0-atoms or apporpriately by a short hydrocarbon rmit. However, it has been observed that the latter i.e., short hydrocarbon unit) happens to be the most preferred choice in majority of synthesized local anaesthetics,... 

The water-solubility characteristics are predominantly provided by the corresponding hydrophilic centre of the molecule. Undoubtedly, this constitutes a cardinal factor in the transportation of the drug substance i.e., local anaesthetic) to the membrane and once slipping inside the cell, subsequently moves on to the desired receptor site. Besides, hydrophilicity also aids towards the binding of the drug molecule ultimately to the receptor. 

Most of the water-soluble monomers, such as the acrylic and methacrylic acids, are functional monomers and are covered in Section 6.2.3. In Table 6.1 the most water-soluble monomers are acrylamide, acrylonitrile, methyl acrylate and vinyl acetate. Acrylamide contains two reactive centres. The amide group undergoes the reactions characteristic to aliphatic amides. For this reason, acrylamide may be considered as a functional monomer. Copolymerization of acrylamide with other monomers is often done to incorporate hydrophilic centres in oleophylic polymers to promote adhesion and dye acceptance. The monomer is available either as a solid or as a 50% aqueous solution. The latter is the preferred form, since it eliminates handling of a solid. The monomer is a neurotoxin and exposure to skin or inhalation must be prevented. Acrylamide solution is stabilized with cupric ions. Cupric ion availability is pH dependent and the pH must be between 5.2 and 6. Storage temperature should be between 16 and 32 °C... 

Tarasevich, Y.I. Zhukova, A.I., and Aksenenko, E.V., Interaction of water and other polar substances with hydrophilic centres on the surface of hydrophobic adsorbents, Adsorpt. Sci. Technol., 15(7), 497-506 (1997). 

The constancy of the Y contributions is an important, although indirect, test that in the saturated compounds the interactions solute-water is a sum of localized interactions between water and the groups constituting the molecule of the solute. This does not occur when the solute molecules possess more than one hydrophilic centres distant from each other less than four bond distances. The... 

The interaction between hydrophilic centres in saturated compounds usually does not produce important effect on the volumetric properties, but it is important in determining the values of the... 

When a hydrophilic group is introduced in hydrocarbons or in monofunctional compounds the Cp decreases in both the cases but the decrease for the introduction of a second hydrophilic centre depends remarkably on the type of the hydrophilic group already present and on the distance between the two functional centres(42, 50,97). Effects of interactions between hydrophilic centres are also reflected in the different values the of many isomeric polyols and carbohydrates(42,72,73,98) show. 

AX (R) = 3xA(R). For the methyl group bonded to a hydrophilic centre Y, a correction has been introduced to assign the same value to the AX (Y) contribution for the Y group, whether Y is bonded to the methyl or to other saturated alkyl groups (see ref. 30). 

As far as the interaction parameters between the hydrophilic centres are concerned. Figure 1 gives a picture of the situation. 

A procedure similar to that above described, i.e. the choice of hydrocarbon compounds as reference molecules in order to evaluate contemporaneously the effects connected to the formation of a cavity and to the hydrophobic hydration, was first applied to the partial molar volumes by Terasawa et al. (61). In this case, of course, in equation 2 the intrinsic volumes V, calculated according to Bondi (101), of the molecules are to be considered instead of surface areas and AX (R) identifies itself with Comparison of hydrophilic solutes with imaginary hydrocarbons with exactly the same dimension allows to deduce that the interaction water-hydrophilic centres produces a shrinkage a(Y) in the volume, f.i. the partial molar volume of solutes containing one hydroxyl group is lower by 4.3 ml mol than that of the hypothetical hydrocarbon with the same intrinsic volume. Some other values of the shrinkage in volume due to some polar centres are (in ml mol ) ... 

Until now simulation methods have not been applied to the study of molecules containing a hydrophilic centre, it will be very interesting to see what structural information these methods will give on the state of water around the hydrophilic centres. 

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