Colloids with physically adsorbed surfactant

For colloids with a physically adsorbed surfactant or cca, the adsorption isotherm is important. The adsorbant concentration on the particle surface can be measured by infrared spectroscopy using diffuse reflectance and by ESCA. Absolute concentrations are difficult to determine with ESCA on "rough" surfaces, and a calibration point is required with other techniques. The change of the concentration of adsorbant in solution after adsorption on the colloid surfaces can be detected by elemental analysis of supernatant with plasma emission or atomic absorption if adsorbant contains specific element(s). When colloids are sterically stabilized, the effectiveness of the stabilization can be evaluated with solvent-nonsolvent techniques and with temperature studies ( 25,26). 

In the common procedure extremely large oil-water interfacial area is generated and the particle nuclei grow in size with the progress of the polymerization. Thus, effective stabilizers such as ionic and non-ionic surfactants and protective colloids e.g. hydroxyethyl cellulose and polyvinyl alcohol), which can be physically adsorbed or chemically incorporated onto the particle surface, are often required to prevent the interactive latex particles from coagulation. Under the circumstances, satisfactory colloidal stability can be achieved via the electrostatic stabilization mechanism [268], the steric stabilization mechanism [269] or both. 

Most of the other studies, describing methods based on the use of a third component to facilitate the incorporation of CNTs into a polymer matrix, report on the use of surfactants to reach the mentioned goal. The use of surfactant is based on the physics of colloidal systems. Bundles of CNTs are sonicated in the presence of a surfactant in an aqueous medium. During sonication, the provided mechanical energy overcomes the van der Waals interactions in the CNT bundles and leads to CNT exfoliation, as shown in Figure 2.11, whereas, at the same time, surfactant molecules adsorb onto the surface of the CNT walls. The colloidal stability of the dispersion of CNTs with adsorbed surfactant molecules on their surface is guaranteed by electrostatic, and/or steric repulsion. 

Colloidal systems with a cationic poly(lactide-co-glycolide) (PLGA) surface containing microparticles coated with a cationic cetyltrimethylammonium bromide surfactant by a solvent evaporation process were also reported to deliver DNA vaccine [122]. However, the microparticles attached to DNA complexes were weak as the cationic surfactant was physically adsorbed onto the microparticle surface [123],... 

One of the most important parameters in the S-E theory is the rate coefficient for radical entry. When a water-soluble initiator such as potassium persulfate (KPS) is used in emulsion polymerization, the initiating free radicals are generated entirely in the aqueous phase. Since the polymerization proceeds exclusively inside the polymer particles, the free radical activity must be transferred from the aqueous phase into the interiors of the polymer particles, which are the major loci of polymerization. Radical entry is defined as the transfer of free radical activity from the aqueous phase into the interiors of the polymer particles, whatever the mechanism is. It is beheved that the radical entry event consists of several chemical and physical steps. In order for an initiator-derived radical to enter a particle, it must first become hydrophobic by the addition of several monomer units in the aqueous phase. The hydrophobic ohgomer radical produced in this way arrives at the surface of a polymer particle by molecular diffusion. It can then diffuse (enter) into the polymer particle, or its radical activity can be transferred into the polymer particle via a propagation reaction at its penetrated active site with monomer in the particle surface layer, while it stays adsorbed on the particle surface. A number of entry models have been proposed (1) the surfactant displacement model (2) the colhsional model (3) the diffusion-controlled model (4) the colloidal entry model, and (5) the propagation-controlled model. The dependence of each entry model on particle diameter is shown in Table 1 [12]. 

Although this book significantly differs from the earlier Colloid Chemistry textbook, it nevertheless focuses on the specifics of educational and research work carried out at the Colloid Chemistry Division at the Chemistry Department of MSU. Many results presented in this book represent the art developed in the laboratories of the Colloid Chemistry Division, in the Laboratory of Physical-Chemical Mechanics (headed by E.D. Shchukin since 1967) of the Institute of Physical Chemistry of the Russian Academy of Science, and in other research institutions and industrial laboratories under the guidance of the authors and with their direct participation. Special attention is devoted in the book to the broad capabilities that the use of surfactants offers for controlling the properties and behavior of disperse systems and various materials due to the specific physico-chemical interactions taking place at interfaces. At the same time the authors made every effort to avoid duplication of material traditionally covered in textbooks on physical chemistry, electrochemistry, polymer chemistry, etc. These include adsorption from the gas phase on solid surfaces (by microporous adsorbents), the structure of the dense part of the electrical double layer, electrocapillary phenomena, specific properties of polymer colloids, and some other areas. 

Flotation processes are an important part of water treatment technologies in modern water treatment plants. Flotation is based on the principle of adhesion of insoluble particles to air bubbles and adsorption of dissolved surfactants at the surface of air bubbles. Flotation allows for different kinds of admixtures to be removed from water bulk in a physical and chemical manner. In this way, suspended and colloidal particles, emulsions of oils and fats, the separate surfactant molecules and their micelles, complexes of surfactants with colloid rust, and multivalent ions of heavy metals can be removed. At present, the flotation processes and equipment for their realization are widely described in the literature [12]. Flotation involves the injection of small bubbles of air or other gas into the water bulk. Surface-active impurities are adsorbed at the bubble surface and transferred through the water bulk to its surface. As a result, the foam concentrate is formed on the surface of bubbling water. It contains surfactants, suspended solid particles (water impurities), emulsified substances, bacterial cells, etc. This foam is evacuated from the surface by means of special scrapers and other devices. 

Nanoemulsions are only kinetically stable. They have to be distinguished from microemulsions (that cover the size range 5-50 nm) which are mostly transparent and thermodynamically stable. The long-term physical stability of nanoemulsions (with no apparent flocculation or coalescence) makes them unique and they are sometimes referred to as approaching thermodynamic stability . The inherently high colloid stability of nanoemulsions can be well understood from a consideration of their steric stabilization (when using nonionic surfactants and/or polymers) and how this is affected by the ratio of the adsorbed layer thickness to droplet radius as will be discussed below. 

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