Experimental methods of studying solubilization

The study of solubilized systems must obviously start with the determination of the concentration of solubilizate which can be incorporated into a given system with the maintenance of a single isotropic solution. This saturation concentration of solubilizate for a given concentration of surfactant is termed the maximum additive concentration (MAC). 

Basically the methods of measuring this concentration are the same as those for the determination of the solubility of any compound in a given solvent with the additional difficulties due to the presence of a colloidal solution. As these solubilization properties are temperature sensitive, it is important that all these methods should be subject to adequate temperature control a factor often missing in much experimental work. 

Apparatus for the study of solubilization from the vapour phase has been described by McBain and O Connor [14], simply measuring the volume of the gas which dissolves in the pure solvent and the solution, respectively. Determination of the MAC of liquids or solids in surfactant solutions involves the observation of a second phase. 

Detection of the appearance of a second phase gives a direct measure of the MAC, but methods based on this are not always suitable. Analytical determination of the amount solubilized is then required, spectrophotometry being very useful in certain cases [16-19]. 

An excess of solubilizate is shaken up with the surfactant solution until equilibrium is attained and the two phases separated. High-speed centrifugation has been used, particularly for liquid solubilizates or if one phase is liquid crystalline [20, 21]. Filtration through filters of 0.4-1.5/im pore size has been used when the solubilizate is solid [22,23]. Supersaturation is a potential source of trouble, especially as surfactants tend to inhibit crystallization, but can be minimized by adequate temperature control. 

The concentration of solubilizate that can be incorporated into a given system with the maintenance of a single isotropic solution (saturation concentration or maximum ad- 

For solubilizates with significant water solubility, it is of interest to know both the distribution ratio of solubilizate between micelles and water under saturation and unsaturation conditions. For measuring the distribution ratio under unsaturation conditions, a dialysis technique can be employed using membranes that are permeable to solubilizate but not to micelles. Ultrafiltration and gel filtration techniques can be applied to obtain the above information. The data are treated using the phase separation model of micellization (micelles are considered to be a separate phase in equilibrium with monomers). 

The partition coefficient, P, between micelles and solution is given by 

Equation (2.51) does not include the volumes of the micellar or aqueous phase, which can be obtained from the partial molar values of the surfactant. A better expression is 

An alternative expression that treats solubilization as a process of binding of solute molecules to binding sites on the surfactant is 

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