Lipophilic linkers

Graciaa A, Lachaise J, Cucuphat C, Bourrel M, Salager JL (1993) Improving Solubilization in microemulsion with aditives - Part 1 The Lipophilic Linker role. Langmuir 9 3371-3374... 

Lipophilic linkers (Salager, 1998) and hydrophilic linkers (Uchiyama, 2000 Acosta, 2002) are used to increase the value of SP and decrease y. Lipophilic linkers are long-chained alcohols (above C8) and their low oxyethylenation products that increase the surfactant-oil interaction. The most effective ones have hydrophobic chain lengths that are an average of the hydrophobic chain length of the surfactant and the chain length of the alkane oil. Hydrophilic linkers increase the surfactant-water interaction. Examples are mono- and dimethylnaphthalene sulfonates and sodium octanoate... 

Figure 3.9 Solubilisation improvement of a conventional surfactant (a) by a lipophilic linker (b), an amphiphilic linker (c), or an extended surfactant (d).

The same concept was transferred to the water side and the respective molecules were called hydrophilic linker [47, 48]. However, since the shape of the water molecules and their interactions are quite different from those of oils, hydrophilic linkers are no elongated molecules like the lipophilic linkers. Up to now the hydrophilic linker concept has been tested with alkyl naphtalene sulphonates only. Non-alkylated naphtalene sulphonate is a... 

Graciaa, A., Lachaise, J., Cucuphat, C., Bourrel, M. and Salager, J.L. (1993) Improving solubilization in microemulsion with aditives - Part 1 The lipophilic linker role. Langmuir, 9, 669-672. 

Supersolubilisation of NAPL can be achieved by adding lipophilic linkers to the system. Best results for chlorinated hydrocarbons were obtained with both hydrophilic and lipophilic linkers [71]. Lipophilic linkers increase the interaction between surfactant and oil [72], and hydrophilic linkers the interaction between surfactant and water. Systems with Aerosol MA as surfactant, sodium mono- and dimethyl naphthalate as hydrophilic linker, and dodecanol as lipophilic linker display the best performance regarding efficiency, economy and environmental aspects [65]. 

In some cases, long-chain alcohols have been found to behave as polar oils, with a tendency to place themselves not in the oil phase bulk but near the interface [60]. This means that the oil phase which is in contact with the interfacial C layer is richer in alcohol than the overall oil phase, with a consequence on the interactions. This alcohol role has been called a lipophilic linker effect because the alcohol molecules that are located near the interface are not randomly oriented like oil molecules in the bulk, but with their OH group pointing toward the interface (i.e., perpendicular to the interface), a conformation that produces an additional ordering in the oil molecules that ends up in a better interaction [83,84]. 

This approach is essentially similar to elongating the surfactant tail into the oil phase, but this time by means of two segments (i.e., the proper surfactant hydrophobe plus the lipophilic linker that might be viewed as some kind of extension that occurs only at the interface). This extension provides some additional polarity in the oil phase near the interface, and there is evidence that polar oils such as ethyl oleate are solubilized thanks to such characteristics. However, it has not been possible yet to solubilize triglyceride oils by this method and some mesophase formation can happen if all the precautions like the hydrophobe branching are not taken. 

At the light of this new approach, it is worth asking how it could be possible to increase the reach of the surfactant tail into the oil phase even more, without falling into the penalty of low solubility and precipitation. Very recently, it was proposed to insert an intermediate polarity segment in between the hydrophilic and lipophilic groups of the surfactant, so that it would be longer. In order to mimic the lipophilic linker effect, the intermediate chain was selected as polypropylene oxide, which is mostly water insoluble but somehow polar, and also has a branched chain that would help prevent the formation of liquid dystals, as known from the behavior of EO-PO block copolymers. 

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