Enthalpy, micellisation

In general, micellisation is an exothermic process and the c.m.c. increases with increasing temperature (see page 86). This, however, is not universally the case for example, the c.m.c, of sodium dodecyl sulphate in water shows a shallow minimum between about 20°C and 25°C. At lower temperature the enthalpy of micellisation given from equation (4.28) is positive (endothermic), and micellisation is entirely entropy-directed. 

Determine an enthalpy of micellisation in accord with the phase separation model. 

These surfactants can form micelles provided that the salt concentration is low. The cmc of 6-SLABS was readily determined by spectrophotometry at 262 nm, and the discontinuity in the plot is clearly shown in Fig. 19.2(a) from which the cmc can be obtained as 0.0014 mol dm-3. It should be noted that the cmc is found to be essentially independent of temperature over the experimentally-measured range of 15-30°C. This is generally found for micellisation involving ionic surfactants in water, so that the enthalpy change on transferring monomer from aqueous solution to the micelle is approximately zero. There is a 20% decrease in extinction coefficient of the benzene ring chromophore on transfer from an aqueous... 

The enthalpy of micellisation can be calculated from the variation of cmc with temperature. This follows from. 

It can be seen from Table 3.2 that AG° is large and negative, whereas AH° is positive, indicating that the process is endothermic. In addition, TAS° is large and positive, which impHes that in the micellisation process there is a net increase in entropy. This positive enthalpy and entropy points to a different driving force for micellisation from that encountered in many aggregation processes. 

However, as mentioned above, experimental results have shown clearly that micelle formation involves only a small enthalpy change, and is often endothermic. The negative free energy of micellisation is the result of a large positive entropy, and this led to the conclusion that micelle formation must be predominantly an entropy-driven process. 

The injection of a surfactant into a liquid can allow the calculation of the critical micelle concentration and the enthalpy of micellisation (after correction for dilution effects) (e.g., 39). The marginal disadvantage of these experiments is the long... 

Taking into account now the fundamental Eqs. (5.17) and Eq. (5.71) for 1 mole of the main component, one can get the standard entropy, volume and enthalpy of micellisation... 

Fig. 6.21 Cumulative enthalpy of dilution of a 0.009 mol kg aqueous solution of hexade-cyltrimethylammonium bromide (HTAB) at 303 K as a function of (a) the solute molality and (b) the injection number [88]. The solution is regarded as a binary system the solute ions HTA+ and Br form together the mean solute. Plot a the intersection of the two linear portions provides estimate of the critical micelle concentration for HTAB Plot b the enthalpy of HTAB micellisation is determined directly from the difference between the slopes of the two linear regression segments...

The molar change in partial molal enthalpy of the surfactant when monomers associate into a micelle at the cmc represents the standard enthalpy of micellisation rnich" per mole of surfactant monomers [98]. In accordance with the variations of /12 as a function of m in Fig. 6.30a, the enthalpy of micellisation for C12N3C at 298 K is positive, indicating that the micellisation process is endothermic. 

Table 6.5 Critical micelle concentrations, CMC, and standard enthalpies of micellisation,Amiclt°i per mole of surfactant monomers for selected quaternary ammonium surfactants in pure water at 298 K [73, 74, 88, 97, 103, 104]...

On the addition of phenol to the aqueous phase, the enthalpy of micellisation per mole of the surfactant becomes more negative (and the CMC value is decreased), thereby indicating more favourable phenol-surfactant interactions after the transfer... 

Fig. 6.32 Molar enthalpies of micellisation, A ich, for selected quaternary ammonium surfactants in pure water and in the presence of phenol molecules in the aqueous phaseat 298K [88, 114, 115]. The overall phenol (PhOH) content (in mmol kg ) is reported on the X-axis. The surfactant acronyms are explained in Table 6.5. For hexadecyltrimethylammonium bromide (HTAB), the calorimetry measurements were carried out at 303 K safely above the Krafft point...

---