Dispersion surfactant selection

Surfactants enable the polymer particles to disperse effectively without coagulation in the mortar and concrete. Thus, mechanical and chemical stabilities of latexes are improved with an increase in the content of the surfactants selected as stabilizers. An excess of surfactant, however, may have an adverse effect on the strength because of the reduced latex film strength, the delayed cement hydration and excess air entrainment. Consequently, the latexes used as cement modifiers should have an optimum surfactant content (from 5 to 30% of the weight of total solids) to provide adequate strength. Suitable antifoamers are usually added to the latexes to prevent excess air entrainment increased dosages causes a drastic reduction in the air content and a concurrent increase in compressive strength [87, 92-94]. 

Recent developments in the dispersion technology such as the technique of surfactant selection, the optimisation of various parameters in the dispersion process and the enhancement to dispersion stability have made it possible to disperse a variety of curable oligomers such as acrylated EP, acrylated PU, acrylated polyester resins in water. Films based on the dispersions can have faster UV cure speed, better surface hardness and better flexibility compared with films based on undispersed resin systems [102]. UV curing offers many advantages including 100% solids with no solvents present in formulation and polymerisation is instantaneous leading to a large MW polymer [103]. 

Some talcs may be subjected to beneficiation by froth flotation. The talc is ground, dispersed in water, subjected to froth flotation in which the talc may be floated from the impurities or vice versa using a surfactant, selective for the appropriate surface, to transfer it to a foam and effect separation. The talc may then be subjected to grinding, classification or both to achieve the desired particle size distribution. 

Since commercial surfactant electrodes are available, there is no reason for the analyst to prepare membranes and construct electrodes. Considerable know-how is involved in producing durable electrodes which behave reliably, so we should leave this to the instrument manufacturers. To illustrate this point, we may mention a study in which six electrodes were compared for use in end point detection of a titration of a cationic with an anionic. Two commercial surfactant-selective electrodes, two tetrafluoroborate selective electrodes, a nitrate selective electrode, and a homemade PVC-membrane electrode incorporating a tetraphenylborate salt were tested (135). Only one electrode, one of the commercial models, was traly suited for routine use, giving smooth potentiometric curves without reconditioning for over 100 titrations. The relative standard deviation of the end point was about 0.5%, while that for the other electrodes was 1.3-2.3%. The standard deviation of the end point potential was 3 mV for this electrode, compared to 8 mV or more for the other electrodes. Besides this, those electrodes not designed as surfactant electrodes required reconditioning (i.e., soaking in a dispersion of a surfactant ion pair) after 25 titrations in order to remain usable. 

The selection of an optimum surfactant, or combination of surfactants, was necessary primarily to prevent re-agglomeration of the dispersed sample while settling. In addition, it was demonstrated that the use of a blender (Waring) was an effective aid in dispersing these two components. The violent action of the blender did not cause a change in the concn of coarse particles in the dispersion with increasing blending times up to 17 minutes... 

The cost/performance factor of individual surfactants will always be considered in determining which surfactants are blended in a mixed active formulation. However, with the recent advent of compact powders and concentrated liquids, other factors, such as processing, density, powder flowability, water content, stabilization of additives, dispersibility in nonaqueous solvents, dispersion of builders, and liquid crystalline phase behavior, have become important in determining the selection of individual surfactants. 

Surfactants are prepared which contain carboxylic acid ester or amide chains and terminal acid groups selected from phosphoric acid, carboxymethyl, sulfuric acid, sulfonic acid, and phosphonic acid. These surfactants can be obtained by reaction of phosphoric acid or phosphorus pentoxide with polyhydroxystearic acid or polycaprolactone at 180-190°C under an inert gas. They are useful as polymerization catalysts and as dispersing agents for fuel, diesel, and paraffin oils [69]. 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

Weatherley (1998) has discussed all the relevant aspects of the separation of low molecular weight biologically produced molecules by solvent extraction. A high degree of selectivity can be realized by careful selection of the solvent. Problems associated with the rheology of the broth, the presence of surfactants and solid materials needs to be recognized. There is a scope to consider intensified electrostatic contact for broth dispersion and separation. Examples covered in this treatise include penicillin G and cA-dihydrodiols. 

Surfactants are useful in formulating a wide variety of disperse systems. They are required not only during manufacture but also for maintaining an acceptable physical stability of these thermodynamically unstable systems. Besides the stabilizing efficiency, the criteria influencing the selection of surfactants for pharmaceutical or cosmetic products include safety, odor, color, and purity. 

Chemically, the preparation of a "stable" foam or emulsion requires the use of a surfactant to aid in dispersion of the internal phase and prevent the collapse of the foam (or emulsion) into separate bulk phases. The selection of a surfactant is made on the basis of severity of conditions to be encountered, the gas to be entrained (N2, C02, LPG, CH, or air), the continuous phase liquid (water, alcohol, or oil), and half-life of foam stability desired. 

CNT-coated electrodes can be constructed easily as long as stable CNT suspensions are obtained. However, one needs to choose a suitable surfactant dispersant for the fabrication of CNT/surfactant-modihed electrodes with high catalytic and good selectivity toward the analytes. 

An alternative efficient approach to disperse CNTs relies on the use of synthetic peptides. Peptides were designed to coat and solubilise the CNTs by exploiting a noncovalent interaction between the hydrophobic face of amphiphilic helical peptides and the graphitic surface of CNTs (Dieckmann et al., 2003 Zoibas et al., 2004 Dalton et al., 2004 Arnold et al., 2005). Peptides showed also selective affinity for CNTs and therefore may provide them with specifically labelled chemical handles (Wang et al., 2003). Other biomolecules, such as Gum Arabic (GA) (Bandyopadhyaya et al., 2002), salmon sperm DNA, chondroitin sulphate sodium salt and chitosan (Zhang et al., 2004 Moulton et al., 2005), were selected as surfactants to disperse CNTs (Scheme 2.1). 

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