Micellization thermodynamic parameters

Exchange of unimers between two different types of block copolymer micelles has often been referred to as hybridization. This situation is more complex than for the case described above because thermodynamic parameters now come into play in addition to the kinetic ones. A typical example of such hybridization is related to the mixing of micelles formed by two different copolymers of the same chemical nature but with different composition and/or length for the constituent blocks. Tuzar et al. [41] studied the mixing of PS-PMAA micelles with different sizes in water-dioxane mixtures by sedimentation velocity measurements. These authors concluded that the different chains were mixing with time, the driving force being to reach the maximum entropy. 

They have studied also the N dependence of the thermodynamic parameters of micellization, calculated from the cmc values and their temperature dependence, as shown in Figure 1, where AGm is... 

Table II. Thermodynamic Parameters of Micellization for Nonionic of n-C j2H250(EO)jj H at 25°C, data from Rosen et at.(3 ) ...

Table IX shows the thermodynamic parameters of micellization, calculated from the cmcs and their temperature dependence, for two homologues of each of these types of nonionics. It can be seen that AHm and ASm progressively decrease in the order usual-type > S > L-A > G type for both homologous series. This suggests that the high polarity of the amido linkage causes either less dehydration or exothermic association of the hydrophilic part on micelle formation.

In contrast to this, there is little information available (11) on the thermodynamics of adsorption of alkyl betaines and no data on the thermodynamic parameters of adsorption or micellization for sulfobetaines. 

Standard Thermodynamic Parameters of Micellization. Standard free energies of micellization were calculated by the relationship ... 

Table III. Standard Thermodynamic Parameters of Micellization for R-N+(CH2C6Hs)(CH3)CH2C00-...

MICELLAR CATALYSIS. Chemical reactions can be accelerated by concentrating reactants on a micelle surface or by creating a favorable interfacial electrostatic environment that increases reactivity. This phenomenon is generally referred to as micellar catalysis. As pointed out by Bunton, the term micellar catalysis is used loosely because enhancement of reactivity may actually result from a change in the equilibrium constant for a reversible reaction. Because catalysis is strictly viewed as an enhancement of rate without change in a reaction s thermodynamic parameters, one must exercise special care to distinguish between kinetic and equilibrium effects. This is particularly warranted when there is evidence of differential interactions of substrate and product with the micelle. Micelles composed of optically active detergent molecules can also display stereochemical action on substrates. ... 

Part II starts with the possibilities of ACE for characterizing the relevant physicochemical properties of drugs such as lipophilicity/hydrophilicity as well as thermodynamic parameters such as enthalpy of solubilization. This part also characterizes interactions between pharmaceutical excipients such as amphiphilic substances (below CMC) and cyclodextrins, which are of interest for influencing the bioavailability of drugs from pharmaceutical formulations. The same holds for interactions of drugs with pharmaceutical vehicle systems such as micelles, microemulsions, and liposomes. 

Lin and Yang (1987) also calculated the thermodynamic parameters of diazepam for micellar solubilization in Pluronic surfactant solutions at different temperatures (Table 13.4). For all systems, AG was negative, indicating micellar solubilization was spontaneous. The sign of entropy has been associated with the location of solubilized molecules within the micelles. Positive values have been observed for molecules embedded in the micelle center and negative values for adsorption of the molecules on the micelle surface. The results in this paper indicate that in the F-108 and F-88 Pluronics, diazepam molecules can penetrate into the micelle interior, whereas for F-68 and lower concentrations of F-88, diazepam is adsorbed on the micelle surface without penetration into the micellar core. 

Kinetic investigations of the effects of urea and similar denaturing agents on rates and thermodynamic parameters of micelle catalyzed reactions have been suggested to be more sensitive probes for the nature and extent of hydrophobic interactions than CMC determinations. Thus, Monger and Portnoy (1968) have taken advantage of the base catalyzed hydrolysis of micellar -nitrophenyl dodecanoate. The rate constant for the hydrolysis of -nitrophenyl dodecanoate decreases rapidly with increasing initial concentration of the ester (10 to 10 m), and the second-order rate constant for the base-catalyzed hydrolysis of this ester... 

As is evident from the previous discussion, a clear understanding of the process of micellization is necessary for rational explanation of the effects of structural and environmental factors on the value of the CMC and for predicting the effects on it of new structural and environmental variations. The determination of thermodynamic parameters of micellization AG c, A//mio and A,S mic has played an important role in developing such an understanding. 

The variations of the cmc with temperature have been used to estimate the thermodynamic parameters for micellization, and more rigorous direct measurements from heats of solution have also been made [1,2,7-9,34], Tanford [18] estimates a contribution of ca. 700 cal/mole per methylene group to the free energy of micellization, which is consistent with hydrophobic interactions of hydrocarbons in water. 

The study of thermodynamic parameters of mixing showed that the oligomer—siufactant system is thermodynamically stable even when the surfactant concentration is considerably above CCMF. This is a surprise and seems to be related to the fact that the surfactant molecules aggregate into the micelles in such a way that those parts of the surfactant molecules are directed at the oligomeric solvent that have maximum affinity to it. 

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