Monomer free micelles

Application of Activity at cmc. The above consideration suggested us to propose a new treatment for ionic micelle formation. According to thermodynamics, the micelle-monomer equilibrium is achieved when the chemical potential of surfactant in the micelle is equal to that in the bulk solution. The free energy of micelle formation can be represented by the use of the critical micelle activity, cma, which is the activity of surfactant at the cmc, as... 

In contrast to this is the addition of oleate surfactant - in the form of micelles or free monomer - to oleate or to POPC vesicles. In this case, the ratio of the two competitive rates is such that a considerable binding of the added fresh surfactant to the pre-existing vesicles takes place. The efficient uptake of oleate molecules by POPC liposomes (Lonchin et al., 1999) as well as to oleate vesicles (Blochiger et al., 1998) is well documented in the literature. 

Strictly speaking, if the size of the micelle, is to play a role in emulsion polymerization, it should be the size of the monomer-swollen surfactant micelle, and not the monomer free one. However, these two sets of "sizes" should be proportional in the present case... 

Since the x-value in the relationship of Rp oc Cg depends on the size of monomer-swollen micelles and the latter is, in turn, related to the solubilizing power of the monomer-free micelles and the hydrophobic properties of the monomers, the "micellar size effect" should predict the following ... 

These equations allow the volume change during the micelle-monomer exchange process, the dissociation rate constant for a salt-free system, the effect of added salts on the micelle distribution width a and the dissociation rate constant for the system upon addition of a salt to be calculated [77]. 

Vitrac H., Hauville C., Collin F., Couturier M., Therond P., Delaforge M., Remita S., Jore D., Gardes-Albert M., Hydroperoxide characterisation as a signature of the micelle/monomer balance in radiation-induced peroxidation of arachidonate. Free Radical Research, 2005,39,519-528. 

The surface tension of the continuous phase of a polymer emulsion may be used as a measure of the free onulsifier concentration. ITie term free onulsifier is used here to denote surfactant which is dissolved in the aqueous phase rather than being adsorbed on to polymer particles or monomer droplets, or aggregated into micelles. The free emulsifier concentration is widely considered to be a critical variable in the phenomenon of steady-state oscillation in a CSTR and in preventing coagulation during polymoization. 

The system initially consists of water, a practically water-insoluble monomer, an emulsifier, and a water-soluble initiator (see Figure 20-11). The emulsifier forms a great number of micelles above the critical micelle concentration. The micelles solubilize monomer, which causes them to swell. Another fraction of the monomer forms monomer droplets of about 1000-nm diameter. The initiator dissociates into free radicals, which can in some circumstances react with monomers really dissolved in water. A polymerization in the micelle is much more favorable, since a much greater monomer concentration is available there. If such a free radical encounters a monomer loaded emulsifier micelle, the polymerization proceeds in this micelle. The free radical can easily penetrate the micelle because of the loose micellar structure. According to another theory due to Medvedev, a soap micelle free radical is formed by transfer in the aqueous emulsion by an initator free radical, and the transfer free radical then starts the polymerization in the micelle. 

In contrast, the free radicals should be produced in the aqueous phase in the case of partially soluble monomers. These free radicals, or oligoradicals, can diffuse into and out of the micelle or latex particles. Micelle-penetrating free radicals can either escape to the aqueous phase, react by propagation with... 

A detailed physicochemical model of the micelle-monomer equilibria was proposed [136], which is based on a full system of equations that express (1) chemical equilibria between micelles and monomers, (2) mass balances with respect to each component, and (3) the mechanical balance equation by Mitchell and Ninham [137], which states that the electrostatic repulsion between the headgroups of the ionic surfactant is counterbalanced by attractive forces between the surfactant molecules in the micelle. Because of this balance between repulsion and attraction, the equilibrium micelles are in tension free state (relative to the surface of charges), like the phospholipid bilayers [136,138]. The model is applicable to ionic and nonionic surfactants and to their mixtures and agrees very well with the experiment. It predicts various properties of single-component and mixed micellar solutions, such as the compositions of the monomers and the micelles, concentration of counterions, micelle aggregation number, surface electric charge and potential, effect of added salt on the CMC of ionic surfactant solutions, electrolytic conductivity of micellar solutions, etc. [136,139]. 

Resolution at tire atomic level of surfactant packing in micelles is difficult to obtain experimentally. This difficulty is based on tire fundamentally amoriDhous packing tliat is obtained as a result of tire surfactants being driven into a spheroidal assembly in order to minimize surface or interfacial free energy. It is also based upon tire dynamical nature of micelles and tire fact tliat tliey have relatively short lifetimes, often of tire order of microseconds to milliseconds, and tliat individual surfactant monomers are coming and going at relatively rapid rates. 

Mesoscale simulations model a material as a collection of units, called beads. Each bead might represent a substructure, molecule, monomer, micelle, micro-crystalline domain, solid particle, or an arbitrary region of a fluid. Multiple beads might be connected, typically by a harmonic potential, in order to model a polymer. A simulation is then conducted in which there is an interaction potential between beads and sometimes dynamical equations of motion. This is very hard to do with extremely large molecular dynamics calculations because they would have to be very accurate to correctly reflect the small free energy differences between microstates. There are algorithms for determining an appropriate bead size from molecular dynamics and Monte Carlo simulations. 

Polymerization begins in the aqueous phase with the decomposition of the initiator. The free radicals produced initiate polymerization by reacting with the monomers dissolved in the water. The resulting polymer radicals grow very slowly because of the low concentration of monomer, but as they grow they acquire surface active properties and eventually enter micelles. There is a possibility that they become adsorbed at the oil-water interface of the monomer... 

The free styrene monomer is restrained within the gel and further reaction with fumarate groups is determined by the spacial arrangement the styrene polymerizes in homopolymer blocks as it intercepts fumarate reaction sites. As individual micelles expand and deplete available fumarate sites in the short polymer chains, the remaining styrene forms homopolymer blocks that terminate at the boundaries between overlapping micelles (Fig. 4). 

Monomer molecules, which have a low but finite solubility in water, diffuse through the water and drift into the soap micelles and swell them. The initiator decomposes into free radicals which also find their way into the micelles and activate polymerisation of a chain within the micelle. Chain growth proceeds until a second radical enters the micelle and starts the growth of a second chain. From kinetic considerations it can be shown that two growing radicals can survive in the same micelle for a few thousandths of a second only before mutual termination occurs. The micelles then remain inactive until a third radical enters the micelle, initiating growth of another chain which continues until a fourth radical comes into the micelle. It is thus seen that statistically the micelle is active for half the time, and as a corollary, at any one time half the micelles contain growing chains. 

In some cases, due to the highly polar character of the sulfate radicals, peroxydisulfate initiators can provide slow polymerization rates with some apolar monomers since the polar sulfate radicals cannot easily penetrate into the swollen micelle structures containing apolar monomers. The use of mercaptans together with the peroxydisulfate type initiators is another method to obtain higher polymerization rates [43]. The mercaptyl radicals are more apolar relative to the free sulfate radicals and can easily interact with the apolar monomers to provide higher polymerization rates. 

Water is extensively used to produce emulsion polymers with a sodium stearate emulsifrer. The emulsion concentration should allow micelles of large surface areas to form. The micelles absorb the monomer molecules activated by an initiator (such as a sulfate ion radical 80 4 ). X-ray and light scattering techniques show that the micelles start to increase in size by absorbing the macromolecules. For example, in the free radical polymerization of styrene, the micelles increased to 250 times their original size. 

The cmc is a key property, because it is related to the free energy difference between monomer and micelles. The onset of micellization is detected by marked changes in such properties as surface tension, refractive index and... 

We can summarize the principal properties of these aggregates, saying they form spontaneously at a well-defined concentration, the CMC (see p. 516) and adding more monomer to the solution yields more micelles, each colloidal particle having the same size, and ensuring the concentration of free monomer does not change. 

The monomers get absorbed in micelles resulting in their swelling. Water soluble initiators are used which form free radicals. Inorganic persulphates are commonly used as initiators. The initiator diffuses into a micelle and polymerisation proceeds. As more monomer is polymerised monomers from outside the micelle diffuse inside and the process continues when another radical enters the micelle the polymerisation stops. This technique can give high Molecular weight polymers. 

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