Monomer solutions, surfactants

Microemulsion Polymerization. Polyacrylamide microemulsions are low viscosity, non settling, clear, thermodynamically stable water-in-od emulsions with particle sizes less than about 100 nm (98—100). They were developed to try to overcome the inherent settling problems of the larger particle size, conventional inverse emulsion polyacrylamides. To achieve the smaller microemulsion particle size, increased surfactant levels are required, making this system more expensive than inverse emulsions. Acrylamide microemulsions form spontaneously when the correct combinations and types of oils, surfactants, and aqueous monomer solutions are combined. Consequendy, no homogenization is required. Polymerization of acrylamide microemulsions is conducted similarly to conventional acrylamide inverse emulsions. To date, polyacrylamide microemulsions have not been commercialized, although work has continued in an effort to exploit the unique features of this technology (100). 

The inverse emulsion form is made by emulsifying an aqueous monomer solution in a light hydrocarbon oil to form an oil-continuous emulsion stabilized by a surfactant system (21). This is polymerized to form an emulsion of aqueous polymer particle ranging in size from 1.0 to about 10 pm dispersed in oil. By addition of appropriate surfactants, the emulsion is made self-inverting, which means that when it is added to water with agitation, the oil is emulsified and the polymer goes into solution in a few minutes. Alternatively, a surfactant can be added to the water before addition of the inverse polymer emulsion (see Emulsions). 

Monomer Solutions. At surfactant concentrations less than the critical micelle concentration (cmc), all the surfactant is in monomeric form and the equilibrium between the protonated and neutral species of an alkyldimethyl amine oxide can be described by a classical dissociation constant, Ka ... 

Spectra of Monomeric Surfactant Solutions. The next step was to study alkyldimethylamine oxides having methylene chains of sufficient length such that the molecule is surface active. The FT-IR spectra of monomer solutions of CgAO at various degrees of protonation (Z) are shown in Figure 3. These spectra are much more complex than those of C AO due to the introduction of deformation modes of the CH2 and the terminal CH3 groups. 

The Monomer-to-Micelle Transition. Surfactants having relatively high cmcs facilitate FT-IR investigations of the monomer to micelle transition, since monomer solutions can be prepared at concentrations that readily provide acceptable signal/noise ratios. However, if the cmc is too high, it is not always possible to prepare concentrated, isotropic micellar solutions for comparison with those of the monomer. Of the amine oxide surfactants studied here, C AO is the best choice for examining this transition since its cmc (0.11 M to 0.15 M) is in an appropriate range. 

The general solubilization curve for surfactants is given in Fig. 14. If the monomers of surfactant in solution do not affect the solubility of the solute, then the solute concentration will remain constant (at the intrinsic solubility, S ) until the CMC. After the CMC the solute concentration will increase linearly with increasing surfactant (micelle) concentration. A simple mathematical representation for a solute s total solubility, x, in a surfactant system is... 

Inside the column, solutes are affected by the presence of micelles in the mobile phase and by the nature of the alkyl-bonded stationary phase, which is coated with monomers of surfactant (Fig. 1). As a consequence, at least two partition equilibria can affect the retention behavior. In the mobile phase, solutes can remain in the bulk water, be associated to the free surfactant monomers or micelle surface, be inserted into the micelle palisade layer, or penetrate into the micelle core. The surface of the surfactant-modified stationary phase is micelle-like and can give rise to similar interactions with the solutes, which are mainly hydrophobic in nature. With ionic surfactants, the charged heads of the surfactant in micelles and monomers adsorbed on the stationary phase are in contact with the polar solution, producing additional electrostatic interactions with charged solutes. Finally, the association of solutes with the nonmodified bonded stationary phase and free silanol groups still exists. 

In sufficiently dilute aqueous solutions surfactants are present as monomeric particles or ions. Above critical micellization concentration CMC, monomers are in equilibrium with micelles. In this chapter the term micelle is used to denote spherical aggregates, each containing a few dozens of monomeric units, whose structure is illustrated in Fig. 4.64. The CMC of common surfactants are on the order of 10 " -10 mol dm . The CMC is not sharply defined and different methods (e.g. breakpoints in the curves expressing the conductivity, surface tension, viscosity and turbidity of surfactant solutions as the function of concentration) lead to somewhat different values. Moreover, CMC depends on the experimental conditions (temperature, presence of other solutes), thus the CMC relevant for the expierimental system of interest is not necessarily readily available from the literature. For example, the CMC is depressed in the presence of inert electrolytes and in the presence of apolar solutes, and it increases when the temperature increases. These shifts in the CMC reflect the effect of cosolutes on the activity of monomer species in surfactant solution, and consequently the factors affecting the CMC (e.g. salinity) affect also the surfactant adsorption. 

The peculiarities of the aggregation behaviour of surfactant molecules in non-aqueous media. It is admitted that the aggregates exhibit disc-like structures with small aggregation numbos (<20) [39,40]. Such aggregates would not swell with monomer solutions, but on the other hand, would be able to capture initiator radicals, thus decreasing the overall radical efficiency. 

These are of two kinds related to each other by the difference in association structure as illustrated by the temperature variation of surfactant solubility and association. Figure 6 provides a schematic description of the interdependence. At low temperatures the solubility limit of the xmimers (s, solid line. Fig. 6) is lower than the limit for amphiphilic association (cmc, dashed line. Fig. 6), and, hence, the latter is not reached and a two-phase equilibrium, aqueous solution of monomers—hydrated surfactant, is established. At temperatures in excess of the Krafft point, Tj (Fig. 6), the association concentration (cmc, solid line, Fig. 6), is now beneath the solubility limit (s, dashed line. Fig. 6). Association takes place and the total solubility (ts. Fig. 6) is drastically increased. Hence, the water—siufactant phase diagram shows a large solubility range for the isotropic liquid solution (unimers plus micelles. Fig. 6) because the association structure, the micelle, is soluble in water. This behavior is characteristic of smfactants with Ninham R values less than 0.5. 

Micellar mobile phases of anionic, cationic, nonionic and zwitterionic surfactants are used in conjunction with different bonded stationary phases (including C8, Cl8 and cyano). Considerably less surfactant (usually <0.2 M) is used compared to the organic modifier content in an analogous traditional separation. A variation in the concentration of surfactant is translated into an increase in the concentration of micelles in the solution, whereas the number of monomers of surfactant remain constant. As a consequence, the characteristics of the stationary phase modified by the adsorption of surfactant are very stable, and usually, a regular retention behavior is observed as a function of the concentration of surfactant. 

According to this model, four equations were formulated corresponding to different situations. The first coincides with eq. 5.8, which is valid for solutes that are not excluded from both stationary phase and micelles. For solutes repelled by the adsorbed monomers of surfactant on the stationary phase, but associated with the micelles ... 

Reverse micelle-mediated polymer syntheses are carried out in water-insolvent systems with solvents such as isooctane, benzene, and chloroform in the presence of a detergent. The monomer is dissolved in a solvent containing a surfactant (bis(2-ethylhexyl) sodium sulfosuccinate (AOT)], and to this monomer solution the required volume of a buffer containing horseradish peroxidase is added... 

The nature of the radical within the latex particle determines its fate ie, its propensity to desorp, propagate, chain transfer, or terminate. It seems reasonable that an ionic radical will not penetrate deeply into a latex particle but rather anchor its ionic head on the surface or palisade of the latex particle, much the way a surfactant molecule does. Once anchored, the nonpolar tail containing the radical will penetrate into the particle, and reactively diffuse throughout the poljmier and monomer solution imtil either the ionic radical desorbs back into the aqueous phase, the ionic radical terminates with another radical within the particle, or the ionic radical undergoes a chain transfer event with either the monomer, polymer, or a chain-transfer agent within the latex particle. Once a chain transfer event occurs, the new radical becomes nonionic and has a markedly different solublity in the particle and aqueous phases. As the nonionic radical grows in chain length within the particle, it becomes even less soluble in the aqueous phase and becomes less likely to desorb. Such a qualitative description of radical fate was quantified... 

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