Monomer-type surfactants, surface

Scheme 9.11. Concept of surface imprinting using monomer-type surfactants.

The surfactant should be polymerised with the matrix monomers in the form of a metal complex. In order to make surface imprinting easy and reliable, the monomer-type surfactant 10-(p-vinylphenyl)decanoic acid has been prepared and applied to an emulsifier for the preparation of surface-template resins [37]. The preparation of metal-imprinted resins is illustrated in Scheme 9.11. 

More than two surfactants can be put together to form tri,- tetra- or polymeric surfactants. Trimeric or even tetrameric surfactants show properties often superior to monomeric surfactants. Besides, they are intermediate between conventional surfactants and polymeric surfactants. In a normal polymeric surfactant each monomer unit is amphiphilic. Another type of polymeric surfactant, called block copolymer [522], consists of at least two parts. One part is made of monomer type A, the other part is made of monomer B. If A is polar and B nonpolar, the blockcopolymer will be strongly surface active and show many properties of a conventional surfactant. If there are two different blocks we talk about a diblock copolymer. In the following part of this chapter we concentrate on conventional surfactants. 

Mention has been made of the fact that the polar character of polymer surfaces is strongly affected by the ionic polymer end groups that are residues of initiator-derived ion radicals, when persulfates are used in emulsion polymerizations. Variation of the initiator type between those that yield ionic and nonionic end groups is an effective way to control particle stability and avoid complications due to migration of surfactant from one polymer surface to another [25]. This method can also be supplemented by copolymerization with polar monomers to affect surface hydrophilicity. 

Emulsion polymerization is known to be a method of carrying out polymerization in a disperse system in which water is usually the dispersion medium. In order to ensure the stability of an emulsion containing 30-60% of the monomer, emulsifiers are used. They are compounds of diphilic type surfactants which decrease the surface tension at the hydrocarbon-water interface. This decrease facilitates the emulsification of the monomer in water and favours the stabilization of the emulsion. 

The interfacial tension between the various polymer phases in the composite latex particles is of paramount importance in determining their final morphology. Other factors, such as the mode of monomer addition, the surface polarity of the polymer particles, initiator type, surfactant type, the presence or absence of crosslinking of one or more of the polymer phases, and the presence of chain transfer agents will also strongly influence the final particle structure. In addition, a survey of methods used to characterize the composite latex particles has been given. 

Several components of the organic phase contribute greatly to the character of the final product. The pore size of the gel is chiefly determined by the amount and type of the nonsolvent used. Dodecane, dodecanol, isoamyl alcohol, and odorless paint thinner have all been used successfully as nonsolvents for the polymerization of a GPC/SEC gel. Surfactants are also very important because they balance the surface tension and interfacial tension of the monomer droplets. They allow the initiator molecules to diffuse in and out of the droplets. For this reason a small amount of surfactant is crucial. Normally the amount of surfactant in the formula should be from 0.1 to 1.0 weight percent of the monomers, as large amounts tend to emulsify and produce particles less than 1 yam in size. 

While CMC is assumed to be an observable and definite value in the case of surfactant monomers, there are frequent reports in the literature of the formation of aggregates or micelle-like associations in solutions of organic solutes so dilute as to preclude apparently the formation of micelles [208, 267-269, 272, 275,278]. Work with different types of commercial surfactants has indicated that molecularly non-homogeneous surfactants do not display the sharp inflection in surface tension associated with CMC in molecularly homogeneous monomers, but rather the onset of aggregation is broad and indistinct [253,267,268]. The lack of well-defined CMCs for non-homogeneous surfactants is speculated to result from the successive micellization of the heterogeneous monomers at different stoichiometric concentrations of the surfactant, which results in a breadth of the monomeric-micelle transition zone. 

The largest portion of the monomer (>95%) is dispersed as monomer droplets whose size depends on the stirring rate. The monomer droplets are stabilized by surfactant molecules absorbed on their surfaces. Monomer droplets have diameters in the range 1-100 pm (103-105 nm). Thus, in a typical emulsion polymerization system, the monomer droplets are much larger than the monomer-containing micelles. Consequently, while the concentration of micelles is 1019-1021 the concentration of monomer droplets is at most 1012-1014 L 1. A further difference between micelles and monomer droplets is that the total surface area of the micelles is larger than that of the droplets by more than two orders of magnitude. The size, shape, and concentration of each of the various types of particles in the... 

For a successful incorporation of a pigment into the latex particles, both type and amount of surfactant systems have to be adjusted to yield monomer particles, which have the appropriate size and chemistry to incorporate the pigment by its lateral dimension and surface chemistry. For the preparation of the miniemulsions, two steps have to be controlled (see Fig. 14). First, the already hydrophobic or hydrophobized particulate pigment with a size up to 100 nm has to be dispersed in the monomer phase. Hydrophilic pigments require a hydro-phobic surface to be dispersed into the hydrophobic monomer phase, which is usually promoted by a surfactant system 1 with low HLB value. Then, this common mixture is miniemulsified in the water phase employing a surfactant system 2 with high HLB, which has a higher tendency to stabilize the monomer (polymer)/water interface. 

Perhaps the simplest type of a polymeric surfactant is a homopolymer, that is formed from the same repeating units, such as PEO or poly(vinyl pyrrolidone). These homopolymers have minimal surface activity at the O/W interface, as the homopolymer segments (e.g., ethylene oxide or vinylpyrroUdone) are highly water-soluble and have little affinity to the interface. However, such homopolymers may adsorb significantly at the solid/liquid (S/L) interface. Even if the adsorption energy per monomer segment to the surface is small (fraction of kT, where k is the Boltzmann constant and T is absolute temperature), the total adsorption energy per molecule may be sufficient to overcome the unfavourable entropy loss of the molecule at the S/L interface. 

When surface active agents are considered, a further complication may be encountered. Because of their surface active nature, the surfactants not only emich at the surfaces, but also form extended structures themselves. At low concentrations, the surfactants remain as dissolved monomers or asssociate to oligomers. However, when the critical micellization concentration (cmc) is surpassed, a cooperative association is activated to micelles (1 to 10 nm) consisting typically of some 50 to 100 monomers. At stiU higher concentrations, or in the presence of cosurfactants (alcohols, amines, fatty acids, etc.), liquid crystalline phases may separate. These phases have an infinite order on the x-ray scale, but may remain as powders on the NMR (nuclear magnetic resonance) scale. When the lamellar liquid crystalline phase is in equilibrium with the liquid micellar phase the conditions are optimal for emulsions to form. The interface of the emulsion droplets (1 to 100 pm) are stabilized by the lamellar liquid crystal. Both the micelles and the emulsions may be of the oil in water (o/w) or water in oil (w/o) type. Obviously, substances that otherwise are insoluble in the dispersion medium may be solubilized in the micelles or emulsified in the emulsions. For a more thorough analysis, the reader is directed to pertinent references in the literature. ... 

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