A Aqueous Dispersions

The adsorption from aqueous solution of surfactants with two hydrophilic and two hydrophobic groups (gemini surfactants, Chapter 12) onto oppositely charged sites on solid particle surfaces—cationic geminis onto clay particles (Li, 2000), anionic geminis onto limestone particles (Rosen, 2001)—results in one hydrophilic group oriented toward the solid surface and the second oriented toward the aqueous phase. The solid particles are dispersed in both cases. 

Examples of commonly used dispersing agents containing multiple ionic groups and aromatic hydrophobic groups are p-naphthalene-sulfonic acid-formaldehyde condensates and ligninsulfonates (Chapter 1, Section IA2). 

Block and graft polymers are widely used as steric stabilizers. Since the two blocks are separated in the molecule, they can be designed chemically, by use of the proper functional groups and degree of polymerization, for optimum efficiency and effectiveness. One block should be designed to adsorb strongly onto the particle surface (and also have limited solubility in the liquid phase), the other block(s) to extend into the liquid phase (good compatibility with and/or interaction with the liquid phase). One commonly used type is the polyoxyethylene-polyoxypropylene 

When the particles to be dispersed are hydrophilic, then adsorption of conventional POE nonionic surfactants occurs with the POE chains oriented toward the hydrophilic particle surface and the hydrophobic chains oriented toward the aqueous phase (Glazman, 1986). Stabilization of the dispersion is then achieved by bilayer formation, with the hydrophobic groups of the two surfactant layers oriented toward and associated with each other, while the POE chains of the second layer are oriented toward the aqueous phase. Consistent with this explanation is the lack of any stabilization effect until the surfactant concentration in the aqueous phase is considerably above the concentration required for hydrophobic group association, i.e., the CMC. 

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MXXT/A—aqueous dispersion coating of PVdC, both sides... 

Zampetti et al. [126] developed PEDOT PSS-coated titania nanofibers for NO2 detection. The fabrication approach was rather easy and was based on (1) preparation of Ti02 fibers by sol-gel electrospinning and calcination of PVP and (2) -dip-coating of Ti02 fibers in a aqueous dispersion of PEDOT PSS. 

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