Enthalpy of micelle formation

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

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... 

If the aggregation number, m, is assumed to be independent of temperature, then the enthalpy of micelle formation, aH, can be estimated by using the Clausius-Clapeyron relationship ... 

Considering again the case of a nonionic surfactant, the standard molar enthalpy of micelle formation is then... 

In calorimetric studies of micelle formation it is often difficult to relate the measured enthalpy changes to specified steps in the aggregation process. Instead one perferably determines the partial molar enthalpy hA of the amphiphile as a function of concentration12). The ideal case of the phase separation model predicts that hA is constant up to the CMC where it discontinuously jumps to another constant value. 

The free energy of micelle formation has been found to be more dependent on entropy than on enthalpy factors (Kavanau, 1965 Elworthy, 1968). Micelle formation has been treated theoretically either... 

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... 

The enthalpy of micellization of many surfactants in aqueous solution has been determined in the past, using mostly cell type and flow microcalorimeters [6-8]. These determinations were based on measurements of the excess heat associated with dilution of a surfactant from a concentration above the cmc to a concentration below the cmc, which results in demicellization of the preexisting micelles. One diffleulty with these determinations relates to the dependence of the heat evolution (AQ) on the initial and final concentrations, probably due to secondary self-aggregations of the surfactants at high concentrations and/or pre-micellar dimer formation at low surfactant concentrations [6,9], These difficulties are at least partially responsible for the lack of consistent data on the thermodynamics of micelle formation [6]. 

The value of the partition coefficient increases with decreasing surfactant concentration. Furthermore, similar to the heat of micelle formation, transfer of OG molecules into the bilayers is endothermic at room temperature but exothermic at high temperature (Table 9). The enthalpy at any given temperature depends on the composition and size of the vesicle bilayers (Table 9). Thus, at room temperature, the introduction of OG into POPC bilayers appears to become more endothermic as the size of the vesicles increases as well as when either POPG or cholesterol is included in the POPC vesicles. However, even when the bilayers contain relatively high cholesterol concentrations, is only a factor of up... 

The enthalpies of micellization, AH, , can be calculated indirectly by use of the van t Hoff treatment or directly by isothermal titration calorimetry (ITC). Except for few cases (e.g., some nonionic surfactants), the results of these methods do not agree [38]. The main reason is that there is no provision in van t Hoff equation for factors that are important for micelle formation of ionic surfactants, in particular, the dependence of micellar geometry, surface-charge density, and extent of hydration on temperature T) [38]. On the other hand, the effects of (T) on the aforementioned micellar parameters are included in the direct (i.e., calorimetric) determination of AH, . From Gibbs free energy relationship, any uncertainty introduced in the calculation of Ai7, j will be carried over to so that A5 rK > AS, Where available, therefore, we compare the thermodynamic quantities of micellization, based on experimental data of the same technique. 

S.2.2.3 Thermodynamic Parameters The CMC value dependence on temperature is used for the determination of thermodynamic parameters applying models [60]. The mass action model, apparent and partial model, and phase separation model [14,59,60] are apphed to estimate the thermodynamic parameters Gibbs energy, enthalpy, and entropy of micelle formation. The enthalpy and entropy change for the miceUization can be determined using the Gibbs-Helmholtz equation [55]. 

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. 

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