Solubilization hydrophobic monomers with surfactant

Synthesis of high molecular weight random copolymers of acrylamide and alkylacrylamides required a novel aqueous surfactant micellar solution polymerization. The surfactant type and concentration were chosen to provide solubilization of the hydrophobic monomer with preferably one or at most a few hydrophobic monomer groups per micelle. 

When nonionic surfactant is applied to a soil-aqueous system, the surfactant can exist as dissolved monomers, sorbed molecules on the soil, or aggregated groups of molecules called micelles. Molecules of HOCs in such a system can be solubilized in surfactant micelles, dissolved in the surrounding solution, sorbed directly on the soil, or sorbed in association with sorbed surfactant. The presence of nonionic surfactant micelles in the bulk solution of the system results in the partitioning of the HOC between two bulk solution compartments, commonly referred to as pseudophases. The micellar pseudophase consists of the hydrophobic interiors of surfactant micelles, whereas the aqueous pseudophase consists mainly of dissolved surfactant monomers and water. Micelles form when the bulk solution concentration exceeds the surfactant CMC. 

Tween 60) times of improvement compared with using water alone. Theoretically, the solubilization of hydrophobic compound by surfactant is initiated by surfactant monomer, but much greater and efficient solubilization is observed as the surfactant concentration increases beyond its critical micellar concentration, (CMC). The characteristics of surfactant and its ability to solubilize hydrophobic compounds beyond cmc can be quantified by an analogous micelle-water partition coefficient, Km ... 

Novel polymerization techniques were used to synthesize new macro-molecules that consisted of a water-soluble backbone unth small amounts of hydrophobic functionality. Micellar polymerization is based on the capability of surfactant micelles to solubilize hydro-phobic molecules into an aqueous medium it was used to copolymerize acrylamide and hydrophohically substituted acrylamide monomers. A critical aspect of these polymerizations was the incorporation of the hydrophobic monomer into the water-soluble polymers. A method that used the UV chromophore of newly synthesized N-aryl substituted acrylamides was developed to quantify incorporation at the low levels of hydrophobe normally used about 1 mol %). The synthesis of the substituted acrylamides, the UV technique, and results obtained with it are discussed. 

Hydrophobically associating polymers consist primarily of water-soluble monomer units with a small number of water-insoluble monomer units. Synthesis of high-molecular-weight random copolymers of acrylamide and alkylacrylamides required a novel aqueous surfactant micellar solution polymerization (2-4) because of the mutual immiscibility of the water-soluble and hydrophobic monomers. The use of surfactant micelles enabled solubilization of the hydrophobic monomer (alkylacrylamide [R]) into the aqueous phase containing the water-soluble monomer (acrylamide [AM]). The resulting RAM polymer after isolation provided homogeneous aqueous solutions. 

The RAM polymers were synthesized with the micellar polymerization technique, in which the hydrophobic monomer was solubilized into the aqueous medium with a surfactant SDS. The polymerizations were initiated with potassium persulfate at 50 C. In some cases, a sample of the reaction... 

Such surfomers can be easily copolymerized with acrylamide (AM) to form water soluble copolymers, i.e. poly(AM-co-R-EO-Ac), which contain low levels ( < 5 mol %) of surfactant group. Unlike the copolymerization of acrylamide with other hydrophobic monomers (e.g. long chain alkylacrylamides), large amounts of external surfactant are not needed to solubilize the hydrophobic monomer. Rather, conventional free radical solution methods can be used to form high molecular weight copolymers [13]. 

The cyclodextrins are stable bodies in aqueous solution, unlike the micelles, which are transitory and are in a state of dynamic equilibrium with the monomer surfactants. However, in many aspects the inclusion of analytes in the cyclodextrin cavity is reminiscent of the solubilization of hydrophobic molecules in micelles in aqueous solution. 

A typical emulsion polymerization recipe includes specific proportions of the added ingredients, e.g. (in wt%) monomer, 100 water, 150 initiator, 0.5 surfactant, 5. Because the monomer has low water solubility, it is clear that there will be two separate phases referred to as the monomer phase and the aqueous phase. The aqueous phase, containing the surfactant in the form of micelles, can be considered as consisting of two phases, the micellar phase and the true aqueous phase. The emulsifier helps disperse the monomer in the aqueous phase with droplets in the order of a few micrometers in size. The hydrophobic interior of the micelles contains solubilized monomer, which is apportioned by diffusion out of the emulsified monomer droplets and through the aqueous phase. 

One of the most relevant properties of aqueous micelles is their ability to solubilize compounds whose water solubility is limited. Because the size of the hydrophobic core of micelle is about 15 — 30 A, this solubilization process allows the organization of solutes on a molecular scaie. Hazardous materials solubilzed by micelles undergo similar processes as those of the surfactant monomers, i.e. rapid exchange takes place between micelles and the bulk solution. The solute, on the average, may spend most of its time within a micelle, with residence times approaching 10 3 sec being reported. Water penetration of two to four carbon atoms from the head group or surface of the micelle is often cited (Thomas J.K., 1980). 

Hydrophobic regions can be one or two small, well-defined blocks of pendant hydrophobic moieties in an otherwise water-soluble polymer (2-4). An example is a water-soluble sulfonated BAB triblock copolymer where B is hydrophobic f-butylstyrene and A is vinyltoluene (2). However, hydro-phobic regions can also be less well-defined as well as more numerous in a polymer molecule than is the case for a triblock copolymer (5-22). For example, pendant alkyl esters appear to have been randomly incorporated in styrene-maleic anhydride (5) and vinyl benzyl ether-styrene-maleic anhydride (6-ii) copolymers. Also, alkyl polyoxyethylene acrylate monomers can be copolymerized with acrylamide to yield copolymers with pendant hydrophobic chains (12-15). More recently it was found (16-22) that small amounts of water-insoluble monomers that are solubilized by surfactants into aqueous solutions of a hydrophilic monomer produce copolymers with pendant hydrophobic chains, but the size, number, and nature of the hydro-phobic regions has not been determined. 

The copolymers of acrylamide and alkylacrylamide were prepared according to a micellar process in which sodium dodecyl sulfate was used to solubilize the water-insoluble monomer to enable copolymerization with the water-soluble monomer to occur (i). The mixture before the polymerization was clear and free of any emulsion particles or undispersed monomer. Potassium persulfate was used to initiate polymerization, which was taken to complete conversion at 50 °C for 24 h. The polymers were purified from the surfactant by acetone precipitation-redissolution and then dried in a vacuum oven at 25 °C. The amount of hydrophobe actually incorporated was taken to be equal to that used in the polymerization. 

In this type of extraction, micellar structures are retained by correctly selecting the ultrafiltration (UF) membrane (Scamehorn et al., 1988). Hydrophobic species are solubilized within the micelles, but surfactant monomers in equilibrium with the micelles can penetrate the membrane along with the free solutes in equilibrium with those solubilized in the micelles. Whereas several uses for this technique have been suggested, such as the collection of radioactive uranium and plutonium present in acid wastes during nuclear plant decommissioning, from our point of view its principal use is in enantiomeric separation (Overdevest et al., 1998). 

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