Solubilised systems

Solubilisation is the process of preparation of a thermodynamically stable isotropic solution of a substance (normally insoluble or sparingly soluble in a given solvent) by incorporation of an additional amphiphilic component(s) [29]. It is the incorporation of the compound (referred to as solubilisate or substrate) within a micellar (Li phase) or reverse micellar (L2 phase) system. 

The aqueous micellar solution A solubihses some alcohol to form normal micelles (Li), whereas the alcohol solution B dissolves large amounts of water, forming inverse micelles (L2). These two phases are not in equihbrium, but are separated by a third region, namely the lamellar liquid crystaUine phase (L phase). 

Lipophilic (water-insoluble) substances become incorporated in the Li (normal micelle) phase. 

Hydrophilic (water-soluble) substances are incorporated in the L2 phase. The lamellar liquid crystalline phase can also incorporate solubilisates. This also applies to the hexagonal (middle phase) that may be present in concentrated nonionic surfactant systems. 

Since the above-mentioned liquid crystals are anisotropic, the above definition of solubilisation does not strictly apply. The site of incorporation of the solubUisate 

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For 10-fold 13C labelled retinal, it has been shown that the differences between chemical shifts for polyene chain carbons of the chromophore in its native environment and detergent-solubilised system were small67 Analysis of the environment of the Schiff base has supported the model of stabilisation based on the protonation by a complex counterion. Three factors were responsible for the excessive positive charge in polyene (i) electronegative nitrogen, (ii) protonation and (iii) counterion strength. 

On the other hand, if one starts from the oil phase with dissolved surfactant and add water, solubilisation of the latter takes place and hence solubilisation increases rapidly with reduction of temperature near the haze point of the surfactant. This is illustrated in Figure 2b which shows both the haze point and solubilisation curve. Between the two curves, an isotropic region of w/o solubilised system exists. At any given temperature, any increase in water weight fraction above the solubilisation limit results in water separation, i.e. w/o solubilised + water, whereas at a given surfactant concentration, any decrease in temperature below the haze point result in separation into water, oil and surfactant. 

A typical phase diagram of a solubilised system is shown in Fig. 6.38. In solutions of high water content the oil is solubilised in... 

Hgure 6.38 Partial phase diagram for the Brij 97 (CigHjj (OCH2CH2)ioOH)-water-mineral oil solubilised system. 

SAPPHO technology which combines a modular approach to synthesis with an innovative solubilisation system guarantees ... 

The patented solubilisation system enables the same solvent to be maintained throughout each coupling cycle, thus avoiding any need for interim purification, precipitation or difficult solvent exchange. 

Conditions can be deduced from the energy balance mentioned above under which the rewetting tension is positive, a precondition for the mechanism shown in Fig. 1.12. It should be said that re-wetting is the first step in the complex process of removal of hydrophobic layers from a solid. The oil droplets formed must also be sufficiently stabilised by the surfactant to prevent them coalescing. This takes place in many cleaning processes. When a water-insoluble substance is to be dispersed in water we can distinguish between thermodynamically stable and unstable dispersions. Thermodynamically unstable dispersions are the usual emulsions or dispersions of solids. Solubilisation systems and optically transparent emulsions, so-called micro-emulsions, are in a metastable state where drop growing by collision and coalescence cannot be completely suppressed. These systems are frequently called thermodynamically stable. 

The quantity of insoluble substance which can be solubilised in micelles depends to a considerable extent on the chemical structure of the surfactant and is influenced by the presence of other components, which may influence either the micelle formation concentration (CMC) or the micelle geometry (aggregation number, shape). The transition from solubilisation to another important phenomenon, the formation of a micro-emulsions, is continuous. Microemulsions form spontaneously, whereas typical solubilisation systems attain their equilibrium state often only after extreme long periods of intensive mixing of both phases. 

At low temperatures, the ethoxylated surfactant is soluble in water and at a given concentration is can solubilise a given amount of oil. The oil solubilisation increases rapidly with rising temperature near the cloud point of the surfactant -this is illustrated in Figure 10.6, which shows the solubilisation and cloud point curves of the surfactant. Between these two curves, an isotropic region of O/W solubilised system exists. 

Future research is required for Solubilising agents that increase bio-availability use of co-solvents effect of surfactants on properties of solubilised systems and interaction with components of the body and mixed micelle formation between surface active drugs and surfactants. 

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