Ionic emulsifiers

Before we examine the polymerization process itself, it is essential to understand the behavior of the emulsifier molecules. This class of substances is characterized by molecules which possess a polar or ionic group or head and a hydrocarbon chain or tail. The latter is often in the 10-20 carbon atom size range. Dodecyl sulfate ions, from sodium dodecyl sulfate, are typical ionic emulsifiers. These molecules have the following properties which are pertinent to the present discussion ... 

Over time, finish components tend to separate and migrate within the fiber and throughout the yam package. With nylon, the ionic emulsifiers and antistats tend toward the core of the fiber whereas the hydrocarbon lubricants remain on the surface. It is, therefore, essential to scour yams and fabrics at neutral to basic pH to reemulsify the lubricant and remove the finish emulsifier prior to dyeiag. In formulating any new finish, environmental issues such as biodegradabihty, water and air pollution must be considered (137). 

The reactions are carried out under first-order conditions, i.e., the stoichiometric concentration of the antioxidant, a-tocopherol, is in large excess over that of 16-ArN, such that the concentration of a-tocopherol does not change significantly throughout the time course of the reaction. The emulsion employed was prepared by mixing the non-ionic emulsifier Brij 30, octane and HCl (3 mM, pH = 2.5). The resulting emulsion is opaque, thus values were obtained electrochemically by employing Linear Sweep Voltammetry (LSV). 

Medvedev et al. [57] extensively studied the use of nonionic emulsifiers in emulsion polymerization. The emulsion polymerizations in the presence of nonionic emulsifiers exhibited some differences relative to those carried out with the ionic ones. Medvedev et al, [57] proposed that the size of latex particles remained constant during the reaction period, but their number increased continually with the increasing monomer conversion. The use of nonionic emulsifiers in emulsion polymerization usually results in larger sizes relative to those obtained by the ionic emulsifiers. It is possible to reach a final size value of 250 nm by the use of nonionic emulsifiers in the emulsion polymerization of styrene [58]. 

Table 2 Ionic Emulsifiers and Their Stability Ranges...

Ionic emulsifier type Stable latex Unstable latex... 

Sarac, A. Yildirim, H., 2003, Effect of Initiators and Ethoxylation Degree of Non-Ionic Emulsifiers on Vinyl Acetate and Butyl Acrylate Emulsion Copolymerization in the Loop Reactor. J. Appl. Polym. Sci., 90(2), 537-543. 

In order to more closely represent the volatilization environment that would be encountered in an evaporation pond, Triton X-100, a non-ionic emulsifier similar to those used in some pesticide formulations, was added to prepared pesticide solutions at 1000 ppm. The presence of this emulsifier caused a decrease in the percent pesticide volatilized in one day in all cases except for mevinphos (Table VI). Three mechanisms are probably in operation here. First, Triton X-100 micelles will exist in solution because its concentration of 1000 ppm is well above its critical micelle concentration of 194 ppm (30). Pesticide may partition into these micelles, reducing the free concentration in water available for volatilization, which will in turn reduce the Henry s law constant for the chemical (31). Second, the pesticides may exhibit an affinity for the thin film of Triton that exists on the water surface. One can no longer assume that equilibrium exists across the air-water interface, and a Triton X-100 surface film resistance... 

When ionic emulsifying agents are used, lateral electric double layer repulsion may prevent the formation of a close-packed film. This film-expanding effect can be minimised by using a mixed ionic plus non-ionic film220 (see above) and/or by increasing the electrolyte concentration in the aqueous phase221. 

In the present experiments greatly enhanced rates of thermal emulsion polymerization were observed when potassium octadecanoate or sodium dodecyl sulfate (at 0.12 mol dm ) whereas sodium dodecyl benzene sulfonate and Triton1 X-100 (Rohm Haas, a non-ionic emulsifier octylphenoxypoly(ethyleneoxy)-ethanol) did not enhance the rate. The conversion after 12 hr at 60 °C with potassium octadecanoate was 69 % whereas with sodium dodecyl benzene sulphonate it was only 29 % (Fig. 2). 

Initial Batch Reactor Studies. An agitated 2000 ml thick-walled glass reactor was blanketed with nitrogen and operated at 50°C. Vinyl acetate containing about 15 ppm hydroquinone was used without purification. The ionic emulsifier was Sipex EST-30, advertised as a sodium tridecyl ether sulfate, and the nonionic surfactant was Siponic L-25, a lauryl alcohol ethoxy-late. Table I shows the recipes and properties of the three seed latexes produced in the batch reactor. Essentially complete conversions were obtained in 30 to 45 minutes, but with a temperature rise of almost 50°C. 

Water Vinyl Acetate 2 1 and ionic emulsifier nonionic emulsifier 3 1... 

Alkylphenol ethoxylates Fatty alcohol ethoxylates Castor oil ethoxylates Ethylene oxide-propylene oxide co-polymers Polyarylphenol ethoxylates Alkylphenol ethoxylate-formaldehyde condensates Non-ionic emulsifiers... 

Generally speaking, for a stable emulsion a densely packed surfactant film is necessary at the interfaces of the water and the oil phase in order to reduce the interfacial tension to a minimum. To this end, the solubility of the surfactant must not be too high in both phases since, if it is increased, the interfacial activity is reduced and the stability of an emulsion breaks down. This process either can be undesirable or can be used specifically to separate an emulsion. The removal of surfactant from the interface can, for example, be achieved by raising the temperature. By this measure, the water solubility of ionic surfactants is increased, the water solubility of non-ionic emulsifiers is decreased whereas its solubility in oil increases. Thus, the packing density of the interfacial film is changed and this can result in a destabilisation of the emulsion. The same effect can happen in the presence of electrolyte which decreases the water solubility mainly of ionic surfactants due to the compression of the electric double layer the emulsion is salted out. Also, other processes can remove surfactant from the water-oil interface - for instance a precipitation of anionic surfactant by cationic surfactant or condensing counterions. 

In earlier work (Sherman, 1950), inversion of water-in-oil emulsions was achieved by increasing the proportion of the dispersed aqueous phase. Emulsions containing 2-5% non-ionic emulsifier and up to 50% water are Newtonian. Above this level of water, the rheological parameters increased significantly and inversion occurred at a water concentration of 75—80%. The... 

Volume of the dispersed phase. Below a concentration of 50% aqueous phase and with 2-5% non-ionic emulsifier, emulsions behave as Newtonian fluids above 50% aqueous phase, emulsions become increasingly non-Newtonian (i.e., become shear rate-dependent and develop a yield value). 

Effect of hexadecane as additive In a series of papers Hallworth and Carless (7,8,9,TO) have investigated the effect of the nature oT the internal phase on the stability of oil in water emulsions as well as the effect of addition of long chain fatty alcohols with sodium dodecyl sulphate or sodium hexadecyl sulphate as the ionic emulsifier. They found that light petroleum and chlorobenzene emulsions prepared only with sodium hexadecyl sulphate were much less stable than those produced using the longer chain paraffins, white spirit and light liquid paraffins. 

As shown by Ono et al, (1974, 1975) decrease of the HLB of a mixed emulsifier by use of an increasing proportion of a nonionic emulsifier increases the stability of the latex to coagulation by electrolyte addition despite an increase in its average particle size. This is because purely eledrostatic stabilization by adsorbed ionic emulsifier is supplemented by steric stabilization by the adsorbed nonionic emulsifier which effectively decreases the van der Waals attractive force between the latex particles (which causes them to coalesce), thereby increasing their stalrility. 

Nonionic surfactants were developed subsequent to the ionic types and are not normally used as the sole emulsifying agent in emulsion polymerizations. Consequently, the characteristics of emulsion polymerizations using only nonionic emulsifiers have received little attention apart from a series of papers from Medvedev s group in the Soviet Union, although an understanding of these is a prerequisite for the interpretation of their action in combination with ionic emulsifiers. The Bobalek Williams recipe produces... 

The Smith-Ewart theory was ccmceived with reference ouly to ionic emulsifiers. If the mode of action of ncmionic emulsifiers differs from that of ionic emulsifiers, the theory may be inapplicable either when nonionic... 

Another, more specific method for the preparation of emulsions of Z, involves the addition of Z to a preformed mixture of an ionic emulsifier, a long-chain fatty alcohol, and water. In this way, the rapid formation of a stable emulsion may he obtained at ordinary stirring with relatively modest amounts of emulsifier. The mechanism of Ais process is still not satisfactorily explained. Also, subsequent polymerization (in the case where Zi is a monomer) may lead to polymerization with initiation in monomer droplets. 

One of ibc methods described above for preparing stable emulsions of Zj involved homogenization of Z2 with water and emulsifier, to produce small droplets of Z2 which served as loci for tbe subsequent preparation of an emulsion of Z, by diffusion. A different method which makes the use of a homogenization process unnecessary involves the use of a mixture of ionic emulsifier and long-chain fatty alcohols- This procedure seems to have been known industrially for quite a long time. 

The fatty alcohol and ibe ionic emulsifier should be mixed with water at a temperature above the melting point of tbe fatty alcohol bdbre addition of monomer. If such small amounts of fatty alcohol are dissolved in ibe monomer bdbre addition to a mixture of water and ionic emulsifier, only very coarse and unstable emulsions are formed. 

The ratio of fatty alcohol to ionic emulsifier should be approximately in the range of 1 1 to 4 1 on a molar basis. 

The most stable emulsions were formed with a molar ratio of fatty alcohol to ionic emulsifier of about 3 1, whereas higher ratios gave more unstable emulsions. Similar results were obtained using sysirans with anio nic emulsifier (Hansen et al, 1973). 

As the mixed micelles are very small (10-20 nm) the initial swelling must be very limited. To be able to absorb more of Z, the initial small droplets must be furnished with more emulsifier, and even more importantly, with more fatty alcohol (Z,). This may be achieved by coalescence of initial droplets or by absorbtion of mixed micelles from the surroundings. The assumption that the emulsification takes place by a diffusion process seems to be supported by experiments with mixed systems of ionic emulsifier and fatty alcohol and various dispersed phases, showing that a necessary condition for a rapid emulsification is that the compound to be emulsified have slight water solubility. Furthermore, it has been observed that if even small amounts of Zj are added to Z, before addition to the water-mixed emulsifier system, the extent of emulsification is reduced and the resulting emulsion becomes less stable. 

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