Temperature effects micelles

An increase in the rate of radical production in emulsion polymerisation will reduce the molecular weight since it will increase the frequency of termination. An increase in the number of particles will, however, reduce the rate of entry of radicals into a specific micelle and increase molecular weight. Thus at constant initiator concentration and temperature an increase in micelles (in effect in soap concentration) will lead to an increase in molecular weight and in rate of conversion. 

Study 5.3. Water temperature effects on micelle formation... 

Last time, electron-transfer reactions were frequently performed in micellar media. Analyzing temperature effects on electron transfer from aromatic amines to coumarins in aqueous Trilon X-100 micelles, Kumbhakar et al. (2006) deduced that the two-dimensional electron-transfer (2DET) model is more suitable to explain the results obtained than the conventional electron-transfer theories. The model is detailed in the article by Kumbhakar et al. (2006) and references therein. 

Caution should be exercised when considering temperature effects on solubilization by micelles, since the aqueous solubility of the solute and thus its micelle/water partition coefLcient can also change in response to temperature changes. For example, it has been reported that although tt solubility of benzoic acid in a series of polyoxyethylene nonionic surfactants increases with temperature, the micelle/water partition coefLci rt, shows a minimum at 2C, presumably due to the increase in the aqueous solubility of benzoic acid (Humphreys and Rhodes, 1968). The increasr in Km with increasing temperature was attributed to an increase in micellar size, as the cloud point temperature of the surfactant is approached (Humphreys and Rhodes, 1968). 

SDS/NaCI Mixtures. The effect of temperature on the micelles formed in 70 mM SDS + NaCl solutions is presented below. Mazer et al. (14) have found that the aggregation number, N, is at a maximum for supercooled solutions below the critical micellization temperature (cmt), and decreases towards the value expected for a spherical micelle as the temperature is increased. The variations in N with temperature are dependent on the concentration of added electrolyte, with the rodlike micelles formed in high salt (0.6 M) showing large variations, and the spherical micelles formed in little (0.3 M) or no salt showing only small variations. 

Another study on these variegated cells depicting an amphiphile revealed a temperature effect on the critical micelle concentration (cmc) that was minimal at about PB(W) = 0.25. Experimentally, the minimal cmc value occurs at about 25 °C.64 The onset of the cmc was also modeled and shown to be dependent on a modestly polar fragment of the amphiphile. 

Figure 4.5 Solubility of an amphilic drug at different temperatures in a phosphate buffer, illustrating the effect on solubility at the micelle formation at the critical micelle temperature (CMT).

Visser, H. (1992) A new casein micelle model and its consequences for pH and temperature effects on the properties of milk, in Protein Interactions, (ed. H. Visser), VCH, Weinheim, pp. 135-65. 

Minimizing the temperature effects discussed above could be obtained with the use of polymer micelles or polymer surfactants [81-83], whose CMC is zero, and even in nonaqueous solvent, the micelle is stable. Although several polymer surfactants are commercially available, no such surfactant is widely accepted, probably because SDS, CTAB, or CTAC, and bile salts are superior to polymer surfactants as the pseudostationary phase in MEKC. Although microemulsion electrokinetic chromatography (MEEKC) is not discussed in this chapter but covered in Chapter 4 by Altria and colleagues, a similar optimization strategy to that in MEKC applies to MEEKC [84-86]. Since... 

Although most polymers tend to accumulate at the fluid interface, reports involving the transfer of polymeric micelles (micellar shuttle) between two immiscible phases have been pubHshed. Poly(N-isopropylacrylamide) (PNIPAM), a thermally responsive polymer, is insoluble and can undergo a conformation change above its lower critical solution temperature of 32 ° C. The thermo reversible miceUization—demicellization process and micellar shuttle of PNIPAM-PEO diblock copolymer at a water-IL interface were investigated by dissipative particle dynamics (DPD) simulations (Soto-Figueroa et al, 2012). Simulation results confirm that the phase transfer behavior of polymeric micelles is controlled by the temperature effect that changes the diblock copolymer from hydrophilic to hydrophobic (as shown in Fig. 33). 

M Okawauchi, M Hagio, Y Ikawa, G Sugihara, Y Murata, M Tanaka. A light-scattering study of the temperature effect on micelle formation of N-alkanoyl-N-methylglu-camines in aqueous solutions. BuU Chem Soc Jpn 60 2718-2725 (1987). 

Table 8.20. Effect of fatty acid residues on the critical micelle temperature Tc of lecithins...

In order to further discuss the temperature effect by a scaling theory in polymer science, the quantity of log CMC was converted to the standard Gibbs free-energy change of micellization AG by eq. (1). 

In summary, if one assumes that the hydrophobic core of a micelle is ellipsoidal, then the most consistent analysis of hydration is obtained if one assigns oblate shapes. This was found to be valid for Triton-X-100, at varying temperatures. However, it has recently been suggested [31] that temperature effect on nonionic micelles may also arise from other effects such as critical fluctuations of size. In these systems prolate shape considerations gave negative hydration values, which cannot be acceptable. The data of other nonionic micelles, e.g. NP-10, NP-13 and NP-18, were not as conclusive. However, the analyses of latter systems did provide an indication that useful hydration values were found for oblate ellipsoids, while prolate shapes were not acceptable. 

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