Micellization enthalpy

Thus the logarithm of the erne can be plotted against inverse temperature to extract information on the micellization enthalpy. Equivalently, the logarithmic concentration can be plotted against the inverse critical micelle temperature (Alexandridis et al. 1994a Yang et al. 1994). 

Muller, N. Errors in micellization enthalpies from temperature dependence of critical micelle concentrations. In Micellization, solubilization, and microemulsions. Vol. l,p. 229, Mittal, K. L. (ed.). New York - London Plenum 1977... 

The lack of data reported in the literature prevents comparing thermodynamic measurements of micellization enthalpies. Although numerous calorimetric studies have been made, many times the measured enthalpies have been mathematically manipulated to give a value for the enthalpy of micellization. Although valuable in testing theories, such manipulation obscures the use of different methods for treating data. As a result, each method may have produced a different value. The methods briefly summarized are as follows ... 

Due to the d3mamic nature and to the small micellization enthalpies (25), micelles and other aggregates of amphiphilic molecules are sensitive in shape and size to various additives. It has been known for a long time that the addition of salt to solutions of ionic spherical micelles induces the formation of rodlike aggregates (144), but also the solubilization of alcohols, alkanes, and aromatic liquids (36,145-147) has consequences on aggregation numbers and on shapes of micelles. 

The micellization enthalpy was much greater than the CP enthalpy, demonstrating the dominance of the poly(propylene oxide)-water interactions. The strongly endothermic enthalpy indicates that the micellization is an entropy-driven process, since micellization is a spontaneous phenomenon. Although cooperative, the micellization process was rather long and spanned a temperature range of 10-20°C due to the temperature dependence of the amount of unimers in equilibrium with the micelles (as temperature increases, more copolymer molecules associate into micelles) [159,160] as well as to the size polydispersity inherent in copolymers. The CMT shifted to lower temperatures as the polymer concentration increased, but its width was not affected much. The CP increased with polymer concentration. 

Hecht et al. [166] found that the surfactants bind cooperatively on the block copolymer molecules, and then the hydrophobic block changes to hydrophilic. At very low SDS concentrations, the micellization peak of the copolymer was not influenced hence, no binding occurred, or this binding of SDS to the poloxamer was reversible. From the plot of the micellization enthalpy versus SDS concentration, it is possible to determine the concentration at which SDS starts to adsorb on the poloxamer as the intersection of two straight lines (Fig. 19 [166]). From the difference between this concentration and that at which becomes zero, the amount of bound SDS can be estimated. 

In the most recent ones. Booth and co-workers [79, 148] outlined the correlations between the molecular characteristics (composition, structure, molecular weight) and the corresponding micellar characteristics, such as the CMC and GMT, the micellization enthalpy, the micelle size and association number, etc. These characteristic features were studied by SLS, DLS, SAXS, SANS, NMR, etc. 

Attempt of correlating the molecular structures and experimental data, for example, cmc, and the thermodynamic parameters of micellization (enthalpy, entropy, and free energy), rests on the assumption that they have been calculated by a consistent procedure this point needs further consideration. At the outset, it should be noticed that there are systematic differences between the results, for example, the cmc, obtained by using distinct experimental techniques. The reason is that the function plotted (absorbance of micelle-solubilized dye, conductivity, surface tension, light scattering intensity, etc.) versus [surfactant] measures different averages of the various species in solution. Examples are surface tension that primarily depends on monomer concentration and solubilization of (water-insoluble) dye that depends mainly on the total amount of micelles present. The consequence is that cmc measured from surface tension will always be lower than cmc measured by dye solubilization [28]. In fact, values of the cmc of the same surfactant, determined by different groups, by the same technique show differences. For example, fifty-four erne s determined by the same technique for Cj NMe Br (measurements at 25°C) differ by 22% [29]. 

The effect of the counterion on aqueous micellar solutions of 1-(3,3,4,4,5,5,6,6-nonafluorohexyl)pyridinium halides has been examined by Fisi-caro et al. [174]. The cmc values and the values of micellization enthalpies of the surfactants decrease in the order Cl > Br > I . A comparison to hydrocarbon analogs suggests that fluorination of the alkyl group has restrained the strong effect the halides as counterions have on the micellar properties. 

The micelle formation enthalpies (Fig. 30) calculated according to a mass action approach by Burchfield and Woolley [52] (Table 19) become more exo-... 

TABLE 19 Enthalpies of Micelle Formation for Alkanesulfonates in Water... 

The small and positive values of enthalpy of solution of water in AOT-reversed micelles indicate that its energetic state is only slightly changed and that water solubilization (unfavorable from an enthalpic point of view) is driven mainly by a favorable change in entropy (the destructuration of the water at the interface and its dispersion as nanodroplets could be prominent contributions) [87],... 

In the case of Kryptofix 221D, a cryptand able to complex the alkali metal cations [141-143], it has been observed that it is solubilized mainly in the palisade layer of the AOT-reversed micelles. And from an analysis of the enthalpy of transfer of this solubilizate from the organic to the micellar phase it has been established that the driving force of the solubilization is the complexation of the sodium counterion. In addition, the enthalpy... 

Guillaume et al. [69] presented a high performance liquid chromatographic method for an association study of miconazole and other imidazole derivatives in surfactant micellar using a hydrophilic reagent, Montanox DF 80. The thermodynamic results obtained showed that imidazole association in the surfactant micelles was effective over a concentration of surfactant equal to 0.4 pM. In addition, an enthalpy-entropy compensation study revealed that the type of interaction between the solute and the RP-18 stationary phase was independent of the molecular structure. The thermodynamic variations observed were considered the result of equilibrium displacement between the solute and free ethanol (respectively free surfactant) and its clusters (respective to micelles) created in the mobile phase. 

With increasing temperature the CMC passes through a minimum (Fig. 15). The initial small decrease at low temperatures is due to a positive enthalpy of the micelle formation whereas the stronger increase of CMC towards higher temperatures is caused by a thermal perturbation of the emulsifier molecules in the micelles. The smaller influence of the temperature on the CMC in case of EUP indicates that these micelles are thermally more stable than SDS-micelles. 

Therefore, the physical meaning of the solubility curve of a surfactant is different from that of ordinary substances. Above the critical micelle concentration the thermodynamic functions, for example, the partial molar free energy, the activity, the enthalpy, remain more or less constant. For that reason, micelle formation can be considered as the formation of a new phase. Therefore, the Krafft Point depends on a complicated three phase equilibrium. 

Standard entropies and enthalpies of micellization, AS°. and AH°-c, can be calculated from the relationships ... 

From the solubility data of n-decane in water, the enthalpy for the process n-decane (H2O) - n-decane (pure) at 25°C has been estimated by Boddard et al. ( ) to be -5.85 kJ moT. Substracting this value from the calculated AH°(25°c) values for CioBMG and C12BMT, in Tables III and IV, the AH (-W) values for micellization and for adsorption at the aqueous solution/air interface at 25 C can be estimated. Values are shown in Table V. 

The tendency of apolar side chains of amino acids (or lipids) to reside in the interior nonaqueous environment of a protein (or membrane/micelle/vesicle). This process is accompanied by the release of water molecules from these apolar side-chain moieties. The effect is thermodynamically driven by the increased disorder (ie., AS > 0) of the system, thereby overcoming the unfavorable enthalpy change (ie., AH < 0) for water release from the apolar groups. 

Note the change of sign.) Likewise, we can define the standard entropy and enthalpy for micellization as follows ... 

In general, the standard enthalpy of micellization is large and negative, and an increase in temperature results in an increase in the c.m.c. the positive entropy of micellization relates to the increased mobility of hydrocarbon side chains deep within the micelle as well as the hydrophobic effect. Hoffmann and Ulbricht have provided a detailed account of the thermodynamics of micellization, and the interested reader will find that their tabulated thermodynamic values and treatment of models for micellar aggregation processes are especially worthwhile. 

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. 

The activation energies calculated for the two steps of the above reaction are + 160 kJ/mol for the ki step and -l- 78 kJ/mol for the k2 step [15]. The overall enthalpy of reaction is — 78 kJ/mol. It has been found that the half-life for the ki reaction is sensitive to the counterion concentration in case of SDS micelles. The effect of added counterion may be due to the charge neutralisation of the sulphate anion heads in the SDS micellar Stern layer, to facilitate approach and penetration of the CN- ions at the micelle-water interface. Hemin encapsulated in CTAB micelles reacts much faster with cyanide compared to that in SDS presumably because of the cationic Stern layer in CTAB. The... 

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