Partitioning of surfactants into bilayers

Starting at a certain concentration of lipids, the addition of surfactant first leads to a partitioning of the surfactant molecules into the lipid bilayers. Tliis equilibrium can be described by a partition coefficient P =, Vb xw, the mole fraction ratio of surfactant in the bilayers jfb and in water. tw- 

Further increase of the surfactant concentration leads to disruption of some vesicles at a concentration called D and first mixed micelles are fonned. The mole fraction at saturation of the vesicles is called and the mole fraction of surfactant in the micelles is. r . Therefore, we find a coexistence region of mixed vesicles of composition detennined by and mixed micelles with composition jr . 

Still further increase of surfactant concentration leads to a disappearance of vesicles till finally above the line called all vesicles have been transformed into micelles. The two lines enclosing the coexistence region are described by the equation 

From the titration curves, the relevant parameters of the partition coefficient and the transfer enthalpy A// can be extracted using the appropriate models. These have been described in detail in the original publications as well as in a recent review [138-141]. Here we will only give a shortened version of the derivation. The definition of the partition coefficient in mole fraction unit is  

With /7w = 1 - Dh with l)x being the total surfactant and W the water concentration and using the approximation Dw + IV fV one finds [138,139]  

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