Free-disperse systems hydrophobic surfaces

The cohesion between the hydrophobic part of the interfacial adsorption layer and the adjacent nonpolar phase can be modeled nsing the cohesion between model hydrophobic snrfaces in the same liqnid. In snch a simnlation, the hydrophobic solid snrfaces represent the hydrophobic tails of the snrfactant molecnles. This approach allows one to overcome the difficnlties associated with the mutual solubility of the components (see Chapter 1). For the solid/liqnid/solid interface, the main parameter characterizing the interactions is the free energy of interaction, F (or Aoj), which can be established experimentally nsing Derjagnin s theorem, that is, p = %RF, where p is the cohesive force in a direct contact between two spherical particles immersed in a liqnid medinm. Snitable model systems include spherical molecularly smooth glass beads with a radius R 1-1.5 mm and hydrophobized surfaces of different natures, namely, HS and HL, immersed into the hydrocarbon and fluorocarbon liquids, HL and FL. Only dispersion forces are present in such systems, which makes the quantitative description of their interaction well defined and not complicated by the presence of various polar components. 

Part of the motivation behind so straightforward an approach derives from its ready application to certain simple systems, such as the solvation of alkanes in water. Figure 11.8 illustrates the remarkably good linear relationship between alkane solvation free energies and their exposed surface area. Insofar as the alkane data reflect cavitation, dispersion, and the hydrophobic effect, this seems to provide some support for the notion that these various terms, or at least their sum, can indeed be assumed to contribute in a manner proportional to solvent-accessible surface area (SASA). 

It is believed that polymerization of hydrophobic monomers is initiated by free radicals in the aqueous phase and that the surface-active oligomers produced migrate to the interior of the emulsifier micelles where propagation continues. Monomer molecules dispersed in the water phase also solubilize by diffusing —to the expanding lamellar micelles. These micelles disappear as the polymerization continues and the rate may be measured by noting the increase in surface tension of llie system. 

Nanocapsules act like a reservoir, which are called vesicular systems. They carry the active substance entrapped in the solid polymeric membrane or on their surfaces. The cavily inside contains either oil or water. A schematic diagram of Polymer Nanocapsules is shown in Fig. 9.2 [5], There are different methods that are used nowadays to prepare polymeric nanoparticles, such as nanoprecipitation (also termed as the solvent diffusion and solvent displacement method), solvent evaporation, dialysis, microemulsion, surfactant-free emulsion, salling-out, supercritical fluid technology, and interfacial polymerization [2]. Among these methods, nanoprecipitation is a fast and simple process, which does not require a pre-prepared polymer emulsion before the nanoparticle preparation. It produces a dispersion of nanoparticles by precipitation of preformed hydrophobic polymer solution. Under... 

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