Equilibrium Aspects Thermodynamics of Micellization

As mentioned above, the process of micellization is one of the most important characteristics of surfactant solution and hence it is essential to understand its mechanism (the driving force for micelle formation). This requires analysis of the dynamics of the process (i.e. the kinetic aspects) as well as the equilibrium aspects whereby the laws of thermodynamics may be applied to obtain the free energy, enthalpy and entropy of micellization. Below a brief description of both aspects will be given and this will be followed by a picture of the driving force for micelle formation. 

Micellization is a dynamic phenomenon in which n monomeric surfactant molecules associate to form a micelle S , i.e.. 

Hartley [8] envisaged a dynamic equilibrium whereby surface active agent molecules are constantly leaving and entering, from solution, the micelles. The same applies to the counter ions with ionic surfactants, which can exchange between the micelle surface and bulk solution. 

Experimental investigations using fast kinetic methods such as stopped-flow, temperature and pressure jumps, and ultrasonic relaxation measurements have shown that there are two relaxation processes for micellar equilibrium [12-18], characterized by relaxation times ti and T2- The first, ri, is of the order of 10 s (10 to 10 s) and represents the life-time of a surface active molecule in a micelle, i.e. it represents the association and dissociation rate for a single molecule entering and leaving the micelle, which may be represented by Eq. (2.7). 

The above discussion emphasizes the dynamic nature of micelles and it is important to realise that these molecules are in continuous motion and that there is a constant interchange between micelles and solution. The dynamic nature also applies to the counter ions which exchange rapidly with life times in the range s. Furthermore, the counter ions appear to be laterally mobile and not to be associated with (single) specific groups on the micelle surfaces [2]. 

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