Surfactant monomers, homogeneous

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 components in a simple penetration experiment consist of a surfactant, water-soluble herbicide, and water. Since the surfactant is at a concentration of 0.5 to 1%, it interacts with water and forms micelles. Since micelles are formed, these could solubilize some of the herbicide inside the micelle. Now we have five components, (1) water, (2) surfactant monomer, (3) surfactant micelle, (4) micelle with solubilized herbicide, and (5) an herbicide in anhydrous or hydrated form which all come in contact with the plant. Which one or more of these components has the greatest effect on the plant Before a thorough understanding of this phenomenon can be achieved, the interaction of each of these components with a plant must be investigated separately, and perhaps the plant is too complex for initial study. Perhaps a homogeneous semipermeable membrane could be used instead. 

If the surfactant concentration is just slightly above the CMC, homogeneous nucleation could compete effectively with the micellar nucleation depending on the water solubility of the monomer. Homogeneous nucleation becomes more important as the surfactant concentration approaches the CMC. The experimental and theoretical results of Nomura et al. [127] on the emulsion polymerization of vinyl acetate (a partially water-soluble monomer) are also consistent with this idea. 

This is a very common method for the preparation of graphene/conjugated polymer nanocomposites. In a typical synthetic procedure, surface-modified graphene or GO can be dispersed in acidic water and /or surfactant solution followed by the addition of monomer. It was then stirred obeying a certain conditions to disperse filler in the solvent and monomer homogeneously. Finally, the initiator (generally peroxides are used as initiator) is added to initiate the polymerization reaction at a certain temperature. Aniline, pyrrole, thiophene, 3,4-ethylenedioxythiophene, etc. can be polymerized by this method [73-80]. 

In recent years, there has been a rapid growth in the number of publications that report the use of surfactant monomers or micelles to improve the analytical perfommice of various spectroscopic (UV-visible spectrophotometry, fluorimetry, phosphorimetry, chemiluminescence and atomic spectroscopy), and electrochemical (especially amperometry) methods [1]. The unique properties of surfactants have been recognized as being very helpful to overcome many problems associated with the use of organic solvents in these methods. Surfactant-modified procedures yield sensitivity and/or selectivity improvements in determinations commonly performed in homogeneous solution, whereas certain analytic methods (such as room-temperature phosphorescence in solution) can be exclusively conducted in organized media. 

Copolymerization of a water-soluble monomer and a surfactant monomer can be performed in organic solution or in aqueous media in the presence or absence of noiqrolymeriz le surfactants (19,38-41). In aqueous media, micelle or emulsion polymerization may occur, by which copolymers with a blocky sequence distribution of tte surfactant comonomer unit may be obtained (19,22,38-40,42). In contrast, homogeneous polymerization in organic solution may yield copolymers with a random distribution. The association behavior of the copolymer depends strongly on the sequence distribution of surfactant comonomer units (19,20,26,39,43,44). To obtain copolymers with a random distribution of and DEmMA (Chart 1), homogeneous solution polymerization... 

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 debate as to which mechanism controls particle nucleation continues. There is strong evidence the HUFT and coagulation theories hold tme for the more water-soluble monomers. What remains at issue are the relative rates of micellar entry, homogeneous particle nucleation, and coagulative nucleation when surfactant is present at concentrations above its CMC. It is reasonable to assume each mechanism plays a role, depending on the nature and conditions of the polymerization (26). 

This paper presents the physical mechanism and the structure of a comprehensive dynamic Emulsion Polymerization Model (EPM). EPM combines the theory of coagulative nucleation of homogeneously nucleated precursors with detailed species material and energy balances to calculate the time evolution of the concentration, size, and colloidal characteristics of latex particles, the monomer conversions, the copolymer composition, and molecular weight in an emulsion system. The capabilities of EPM are demonstrated by comparisons of its predictions with experimental data from the literature covering styrene and styrene/methyl methacrylate polymerizations. EPM can successfully simulate continuous and batch reactors over a wide range of initiator and added surfactant concentrations. 

As an even more explicit example of this effect Figure 6 shows that EPM is able to reproduce fairly well the experimentally observed dependence of the particle number on surfactant concentration for a different monomer, namely methyl methacrylate (MMA). The polymerization was carried at 80°C at a fixed concentration of ammonium persulfate initiator (0.00635 mol dm 3). Because methyl methacrylate is much more water soluble than styrene, the drop off in particle number is not as steep around the critical micelle concentration (22.) In this instance the experimental data do show a leveling off of the particle number at high and low surfactant concentrations as expected from the theory of particle formation by coagulative nucleation of precursor particles formed by homogeneous nucleation, which has been incorporated into EPM. 

It has been reported that molecularly non-homogeneous surfactants are able to enhance the solubility of very hydrophobic chemicals, e. g., DDT, at surfactant concentrations well below the CMC. This is attributed to the successive micellization of the heterogeneous monomer species [271, 273, 274,276,278]. Examination of the solubility enhancement with different types of commercial surfactants reveals that molecularly homogeneous surfactants show relatively insignificant (but linear) solubility enhancement below CMC. Molecularly non-homogeneous surfactants, on the other hand, show a much greater solubility enhancement at concentrations below the CMC. 

The number of polymer particles is the prime determinant of the rate and degree of polymerization since it appears as the first power in both Eqs. 4-5 and 4-7. The formation (and stabilization) of polymer particles by both micellar nucleation and homogeneous nucleation involves the adsorption of surfactant from the micelles, solution, and monomer droplets. The number of polymer particles that can be stabilized is dependent on the total surface area of surfactant present in the system asS, where as is the interfacial surface area occupied by a surfactant molecule and S is the total concentration of surfactant in the system (micelles, solution, monomer droplets). However, N is also directly dependent on the rate of radical generation. The quantitative dependence of N on asS and R,- has been derived as... 

The size of the monomer droplets plays the key role in determining the locus of particle nucleation in emulsion and miniemulsion polymerizations. The competitive position of monomer droplets for capture of free radicals during miniemulsion polymerization is enhanced by both the increase in total droplet surface area and the decrease in the available surfactant for micelle formation or stabilization of precursors in homogeneous nucleation. 

Dispersion polymerization differs from emulsion polymerization in that the reaction mixture, consisting of monomer, initiator, and solvent (aqueous or nonaque-ous), is usually homogeneous. As polymerization proceeds, polymer separates out and the reaction continues in a heterogeneous manner. A polymeric surfactant of the block or graft type (referred to as protective colloid ) is added to stabilize the particles once formed. 

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