Enthalpy, of micellization

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

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

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. 

Abbreviations A H Huggins coefficient M, molar mass R, radius of gyration RD, core radius p, association number AmcH°, standard enthalpy of micellization, AmlcG°, standard Gibbs energy of micellization A2, second virial coefficient. Ru, hydrodynamic radius. 

The values of the CMC or CMT collected as a function of temperature or concentration can be used to extract the enthalpic and entropic contributions to the association process. For a closed association mechanism with relatively large aggregation number and a narrow distribution, the standard free energy and standard enthalpy of micelle formaMi nd AH°, per mole of the solute in the micelle) are related to the CMC and its temperature dependence in the form (Lindman and V fennerstrom, 1980 Zhou and Chu, 1994). 

One of the most frequently varied parameters is the temperature of the system The obvious reason for such measurements was to determine thermodynamic properties of the system, for example, the enthalpy of micellization from the temperature dependence of the critical micelle concentration (CMC) using the well-known relation... 

The enthalpy of micellization A H° can be measured either from the variation of cmc with temperature or directly by microcalorimetry. From AG° and AH0, one can obtain the entropy of micellization AS0,... 

Enthalpy of Micelle Formation of Mixed Sodium Dodecyl Sulfate and Sodium Deoxycholate Systems in Aqueous Media... 

The enthalpy of micelle formation of various mixed sodium dodecylsulfate (NaDDS) and sodium deoxycholate (NaDOC) systems was measured by calorimeter In aqueous systems. The heat of micelle formation, AH, showed a maximum around NaDDS NaDOC molar ratio 1. These data are analyzed In comparison to the aggregation number of mixed micelles and the second virial coefficient, Bg. 

This study is a continuation of our previous investigations, in which the aggregation phenomena of surfactant molecules (amphiphiles) in aqueous media to form micelles above the critical micelle concentration (c.m.c.) has been described based on different physical methods (11-15). In the current literature, the number of studies where mixed micelles have been investigated is scarcer than for pure micelles (i.e., mono-component). Further, in this study we report various themodynamlc data on the mixed micelle system, e.g., ci H25soi4Na (NaDDS) and sodium deoxycholate (NaDOC), enthalpy of micelle formation (by calorimetry), and aggregation number and second virial coefficient (by membrane osmometry) (1 6). 

The mixed NaDDS-NaDOC systems gave the enthalpy of micelle formation,AH C, which varies with composition as shown in Figure... 

The present study reports the variation of enthalpy of micelle formation of mixed NaDDS-NaDOC systems. Our current enthalpy... 

Since surfactant molecules can aggregate into micelle-like structures on a solid surface, the enthalpies of micellization in the bulk phase are also listed in this table. They will... 

Critical micelle concentration, cmc, area per molecule at surface saturation, io, and molar enthalpy of micellization, Dmich, for the surfactant molecules (in deionized water)... 

Jones MN, Agg G, Pilcher G. Enthalpy of micellization IV. n-dodecylpyridi-nium iodide in aqueous potassium iodide and aqueous urea. J Chem Thermodynamics 1971 3 801-809. 

The initial work at Bartlesville has concentrated on measurements of enthalpy changes from dilution and adsorption for surfactant systems. From the observed dilution enthalpy changes, critical micelle concentrations have been determined, and standard state enthalpies of micel lization have been calculated. In the studies on adsorption, several properties are of interest the enthalpy of adsorption, the amount of surfactant adsorbed, the surface area of the solid and determining whether the adsorption is reversible. The kinetics of adsorption and desorption are also of interest. 

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 ... 

TENTATIVE VALUES FOR THE ENTHALPY OF MICELLIZATION IN H20 AT 35° C FROM EQUATION 6... 

In general, but not always, micelle formation is found to be an exothermic process, favored by a decrease in temperature. The enthalpy of micellization, given by... 

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