Adsorbents hydrophilic

The above results proved the potential viability of the adsorbed hydrophilic macromolecules as bonded phases in chromatography of biopolymers but it must be admitted that additional crosslinking of previously adsorbed macromolecules is usually needed in order to obtain stable composites. The cross-linked bonded polymeric phases, however, may suffer from the restricted flexibility of the chain segment and their steric repellency may be diminished. Moreover, the conformational adaptivity of cross-linked chains for binding with solutes is poorer than that of grafted or chemically bound macromolecules. 

On hydrophilic surfaces, such as PVA or poly(HEMA), OH-groups of the materials are incorporated in the network structure of adsorbed water molecules (see Sect. 4.4). In consequence, the absolute value of Wj(3 — Wi1 is considered to become still smaller, where - owing to the stabilization of water molecules on the hydrophilic surface - the water-removing-process (reverse reaction of Eq. (2.6)) proceeds slowly. Many experiments were carried out with water-adsorbed hydrophilic surfaces, the behavior of which was time-dependent. In a similar way, the water removal from the proteins [Eq. (2.9)] is also considered to proceed slowly. Thus, we must be careful in considering experimental results in comparison with the data in Tables 3, 4 and 5. 

Figure 7.7 Enthalpic stabilisation representation of en-thalp ic stabilisation of particles with adsorbed hydrophilic chains. The hydrated chains of the polyoxyethylene molecules —(0CH2CH2) 0H protrude into the aqueous dispersing medium. On close approach of the particles to within 26 (twice the length of the stabilising chains), hydrating water is released, resulting in a positive enthalpy change which is energetically unfavourable.

A note to the terms hydrophobicity and hydrophilicity these are frequently employed in characterizing adsorbents. Hydrophilicity/hydrophobicity is a qualitative measure of an adsorbent, characterizing its behavior towards water. The term lipophilicity/lipophobicity is applied to characterize the polarity of a compound. Table 3.4 summarizes the different column packings and stationary phases used in preparative chromatography. 

If hair is further damaged to the point that the cuticle has split (Figure 10.3), the exposed hydrophilic cortex could strongly adsorb hydrophilic soils that would not bind as strongly to the intact hydrophobic cuticle layers. 

Proteins adsorbed at an oil-water interface may stabilize the oil droplets by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) interactions and/or the steric stabilization mechanism. The proteins may possess or be capable of adopting extended structures, which protrude into a solution for a considerable distance from the interface. This extended hydrated layer may form the basis for steric stabilization of the emulsion. Interactions between the adsorbed protein layers can involve a reduction in conhgurational entropy as molecular chains overlap (Darling and Birkett, 1987). In addition, hydration of adsorbed hydrophilic components can lead to an enthalpic repulsion when two particles are in close proximity. This tends to force the oil droplets apart (Darling and Birkett, 1987). 

Weakly adsorbing surfaces can be prepared that either preferentially adsorb (hydrophilic interface) or repel (hydrophobic) water. Hydroxylated silica surfaces and... 

On the amorphous carbon surface, such water adsorption sites are oxidized carbon atoms. Their concentration is usually much lower than that of hydroxyl groups on the silica surface, which should diminish the hydrophilic properties of the silica gel surface during carbonization. In contrast to it, however, there is an increase in the adsorbent hydrophilic properties for the CSG-5 and CSG-6 samples, which manifests itself in the increase of the Ys and C"w values. In the case of the CSG-6 sample, Ys is even higher than for the initial silica gel. The regularities of variations in the hydrophilic... 

Reduce membrane dehydration near the anode by the larger cation contaminant induced electroosmotic drag Removal of adsorbed hydrophilic NaCl from fuel cell carbon components Removal or redistribution of NaQ in the ionomer ... 

In some situations, the latex particles are covered with adsorbed hydrophilic polymer chains (either ionic or nonionic), which are swollen with water and expand in the aqueous phase. These polymers contribute to the steric stabilization of the particles through their osmotic pressure. In the absence of adsorption of the hydrophilic polymer at the particle surface, an opposite effect may take place, known as depletion attraaion. 

Case 4. In an emulsion system, emulsifiers may induce fat-emulsifier interaction. Acceleration of nucleation may occur at the interface of the emulsion, with adsorbed hydrophilic emulsifiers becoming templates for heterogeneous nucleation. 

Clearly, it is important that there be a large contact angle at the solid particle-solution-air interface. Some minerals, such as graphite and sulfur, are naturally hydrophobic, but even with these it has been advantageous to add materials to the system that will adsorb to give a hydrophobic film on the solid surface. (Effects can be complicated—sulfur notability oscillates with the number of preadsoibed monolayers of hydrocarbons such as n-heptane [76].) The use of surface modifiers or collectors is, of course, essential in the case of naturally hydrophilic minerals such as silica. 

An additional complicating factor in many carbons is the presence of ash. which is usually hydrophilic if present as MgO or CaO resulting from high-temperature treatment of the charcoal, the ash will of course adsorb water chemically as well as physically. 

Hydrophilic and Hydrophobic Surfaces. Water is a small, highly polar molecular and it is therefore strongly adsorbed on a polar surface as a result of the large contribution from the electrostatic forces. Polar adsorbents such as most zeoHtes, siUca gel, or activated alumina therefore adsorb water more strongly than they adsorb organic species, and, as a result, such adsorbents are commonly called hydrophilic. In contrast, on a nonpolar surface where there is no electrostatic interaction water is held only very weakly and is easily displaced by organics. Such adsorbents, which are the only practical choice for adsorption of organics from aqueous solutions, are termed hydrophobic. 

AH corrosion inhibitors in use as of this writing are oil-soluble surfactants (qv) which consist of a hydrophobic hydrocarbon backbone and a hydrophilic functional group. Oil-soluble surfactant-type additives were first used in 1946 by the Sinclair Oil Co. (38). Most corrosion inhibitors are carboxyhc acids (qv), amines, or amine salts (39), depending on the types of water bottoms encountered in the whole distribution system. The wrong choice of inhibitors can lead to unwanted reactions. Eor instance, use of an acidic corrosion inhibitor when the water bottoms are caustic can result in the formation of insoluble salts that can plug filters in the distribution system or in customers vehicles. Because these additives form a strongly adsorbed impervious film at the metal Hquid interface, low Hquid concentrations are usually adequate. Concentrations typically range up to 5 ppm. In many situations, pipeline companies add their own corrosion inhibitors on top of that added by refiners. 

For a polar surface and molecules with permanent dipole moments, attraction is strong, as for water adsorption on a hydrophilic adsorbent. Similarly, for a polar surface, a molecule with a permanent quadrupole moment vidll be attracted more strongly than a similar molecule with a weaker moment for example, nitrogen is adsorbed more strongly than oxygen on zeolites (Sherman and Yon, gen. refs.). 

For many proteins, especially glycoproteins, the physical characteristics, particularly the hydrophilic nature of Toyopearl HW resins, improve mass and activity recovery rates. Toyopearl HW media do not adsorb proteins, as conventional gels can, and thus do not interfere with sample recovery (39). 

Anionic and neutral polymers are usually analyzed successfully on Syn-Chropak GPC columns because they have minimal interaction with the appropriate mobile-phase selection however, cationic polymers adsorb to these columns, often irreversibly. Mobile-phase selection for hydrophilic polymers is similar to that for proteins but the solubilities are of primary importance. Organic solvents can be added to the mobile phase to increase solubility. In polymer analysis, ionic strength and pH can change the shape of the solute from mostly linear to globular therefore, it is very important to use the same conditions during calibration and analysis of unknowns (8). Many mobile phases have been used, but 0.05-0.2 M sodium sulfate or sodium nitrate is common. 

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