Particles hydrophilic

Hydrophobic mineral particles Hydrophilic mineral particles... 

As described above, the circulation time of a particulate carrier in the blood can be prolonged using stealth technology to enhance particle hydrophilicity. If the circulation time is sufficiently prolonged and the particle size does not exceed, say, 0.2 urn, then accumulation at tumor and inflammation sites (EPR-effect) can be observed. 

Figure 5.43. Elementary act of separation by flotation hydrophobic particles, hydrophilic particles. The former adhere to ascending air bubbles.

Ions in solution may affect the particle hydrophilicity according to their position in the lyotropic (Hofmeister) series. As a result, particles are salted-out and aggregate. 

Figure 526. Viscosity and thixotropy efTects of different types of silica in difTerent liquids. Discrete particles hydrophilic, partly hydrophobic, and completely hydrophobic. Small milled aggregates originally hydrophilic, partly hydrophilic, and completely hydrophobic, now with additional hydrophilic spots.

Figure 2.15 shows a comparison between the interaction of two nonpolar particles and the interaction of a nonpolar particle and a partially lyophilized particle (hydrophilized with acetyl cellulose). In the first case, V2 F reaches a limiting value corresponding to low but constant attraction, while in the second case, V2 F drops below zero, which is the real, experimentally observed change in the sign of the disjoining pressure, that is, a transition from attraction to repulsion. 

The methods and quantitative characteristics used in the analysis of the contact interactions between the particles can be applied to the processes involving the formation of sediments. This can be verified experimentally in sedimentation experiments with hydrophobic spherical particles (e.g., methylated glass or fluoropolymers) in a medium of a different polarity. The sedimentation of hydrophobic particles in low-polarity media, such as propanol or butanol, is of lyophilic type. The sedimentation of such particles in polar media, such as water or ethylene glycol, follows the lyophobic scheme. Similar to the case of experimental contact interaction studies, one can also observe a gradual transition from one type of sedimentation to the other. One particnlar example is the sedimentation of hydrophobic particles in aqueous solutions of alcohols. As the system becomes more lyophilic with the shift to alcohols with lower polarity or with an increase in the alcohol concentration, a transition from lyophobic sedimentation to lyophilic sedimentation takes place. A gradual decrease in the volume of the sediment is observed. Small additives of surfactants that make the surface of the particles hydrophilic have a similar effect. The concentration of an alcohol... 

An often-quoted mechanism for mixtures of hydrophobic oils and particles is that due to Kulkarni et al. [53, 207]. These authors claimed that the oil spreads over the air-water surface exposing the particles to the aqueous solution. Adsorption of surfactant onto the surface of the particles is then supposed to occur, rendering the particle hydrophilic so that particles are progressively extracted from the oil into the aqueous phase. Rapid local depletion of surfactant in the aqueous film is then, in turn, supposed to produce a surface stress that renders the foam film unstable so that rupture occurs. 

As is clear from the three hydrophilic items shown at the bottom of Table 8.1, (and also from the other items shown in that table), iJy its very nature, the strong attractive free energy of the hydrophobic effect is always present in all interactions taking place in water. What makes a compound or particle hydrophilic (i.e., repulsive in water), is its ability to achieve a Lewis acid-base repulsion that is significantly larger than the underlying hydrophobic attraction. (It should be remembered that usually a fairly small, but non-negligible Lifshitz-van der Waals attraction must also be surmounted, cf. Tables 8.1 and 8.2. 

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. 

As early as 1969, Wlieeler and Widom [73] fomuilated a simple lattice model to describe ternary mixtures. The bonds between lattice sites are conceived as particles. A bond between two positive spins corresponds to water, a bond between two negative spins corresponds to oil and a bond coimecting opposite spins is identified with an amphiphile. The contact between hydrophilic and hydrophobic units is made infinitely repulsive hence each lattice site is occupied by eitlier hydrophilic or hydrophobic units. These two states of a site are described by a spin variable s., which can take the values +1 and -1. Obviously, oil/water interfaces are always completely covered by amphiphilic molecules. The Hamiltonian of this Widom model takes the form... 

Fibrillated Fibers. Instead of extmding cellulose acetate into a continuous fiber, discrete, pulp-like agglomerates of fine, individual fibrils, called fibrets or fibrids, can be produced by rapid precipitation with an attenuating coagulation fluid. The individual fibers have diameters of 0.5 to 5.0 ]lni and lengths of 20 to 200 )Jm (Fig. 10). The surface area of the fibrillated fibers are about 20 m /g, about 60—80 times that of standard textile fibers. These materials are very hydrophilic an 85% moisture content has the appearance of a dry soHd (72). One appHcation is in a paper stmcture where their fine fiber size and branched stmcture allows mechanical entrapment of small particles. The fibers can also be loaded with particles to enhance some desired performance such as enhanced opacity for papers. When filled with metal particles it was suggested they be used as a radar screen in aerial warfare (73). 

The Smith-Ewart expression (eq. 1) accurately predicts the particle number for hydrophobic monomers like styrene and butadiene (21), but fails to predict the particle number (22) for more hydrophilic monomers like methyl methacrylate and vinyl acetate. A new theory based on homogeneous particle... 

Partially hydrolyzed poly(vinyl alcohol) grades are preferred because they have a hydrophobic /hydrophilic balance that make them uniquely suited for emulsion polymerization. The compatibUity of the residual acetate units with the poly(vinyl acetate) latex particles partly explains the observed stabilization effect. The amount of PVA employed is normally 4—10% on the weight of vinyl acetate monomer. The viscosity of the resulting latex increases with increasing molecular weight and decreasing hydrolysis of the PVA (318). 

Fig. 3. Air-sparged hydrocyclone, where A represents the tangential feed that estabHshes swid flow B, the area of small bubbles formed by high shear at the porous wall and C, the outlet for the (D) hydrophilic particles rejected by the swid flow. The (B) hydrophobic particles are in the axial froth flow.

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