Entropy 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. 

The cause of a positive entropy of micellisation is not entirely clear. A decrease in the amount of water structure as a result of micellisation may make some contribution. A more likely contribution, however, involves the configuration of the hydrocarbon chains, which probably have considerably more freedom of movement in the interior of the micelle than when in contact with the aqueous medium. 

This argument does not necessarily hold for processes such as adsorption from solution and micellisation, since a certain amount of destructuring (e.g. desolvation) may be involved and the net entropy change may be positive. 

The entropy of micellisation can then be calculated from the relationship between AG° and AH°, that is... 

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 second source of entropy increase on micellisation may arise from an increase in the flexibility of hydrocarbon chains on their transfer from an aqueous to a hydrocarbon medium [19]. The orientations and bendings of an organic chain... 

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... 

Detailed overviews on the micellisation of amphiphilic copolymers in organic solvents are provided in the reviews of Riess [1], Gohy [2], Hamley [7] and Chu et al. [101]. Apparently, a wide range of styrene, acrylate or methacrylate and diene-based block copolymers have been investigated, while AB diblock and ABA triblock architectures have been systematically compared. One of the main conclusions is that the formation of micelles in organic solvents can generally be considered as an entropy-driven process. 

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