Minimum hydrotrope concentration

Hydrotropes have many features in common with micelles. The most important is the presence of a minimum hydrotrope concentration (CHC) analogous to the minimum micellar concentration (CMC) described earlier (Balasubramanian et al., 1989). The most important difference is that in hydrotropes, the dissolved solute is precipitated on dilution, whereas with surfactants dilution leads to emulsification with consequent problems of separation. Another difference is that surfactants show solubility enhancements at low concentrations, usually in the millimolar range, whereas hydrotropic solubilization occurs in the molar concentration range. Yet another difference is that, unlike micellar solubilization which is general and nonselective, hydrotropes do not solubilize all hydrotropic substances and are hence selective. This is obviously an advantage where reactant selectivity is important. 

The ability to solubilise hydrophobic compounds in water starts at a concentration named Minimum Hydrotropic Concentration (MHC), ° which is usually high, up to more than 1 mol/L, depending on the nature of the lipophilic part of the hydrotrope (see Table 2). This concentration at which hydrotropic properties starts expressing is usually also the concentration at which several properties in solution change (surface tension, viscosity, self-diffusion, etc.., ). ... 

Table 2 Minimum Hydrotropic Concentrations (MHO of various hydrotropes determined by solubilisation of a hydrophobic dye in water or surface tension measurements (indicated by ). For the solvo-surfactants (liquids at room temperature), T9o% determined by ATG (see text) is also given. ...

Fig. 10 Schematic representation the solubilisation of a hydrophobic compound in water by hydrotrope and surfactant solutions. CMC = Critical Micellar Concentration, MHC = Minimum Hydrotropic Concentration.

Table 2 gives the Minimum Hydrotropic Concentrations (MHC) of various classical and sugar-based hydrotropes and show that the MHC value depends primarily on the nature and length of the hydrophobic chain. 

Pulverized roots of C. forskohlii were suspended in aqueous hydrotrope solution of sodium cumene sulfonate and agitated vigorously. After the extraction, the solution was subsequently filtered under vacuum. A clear brown color solution was obtained as filtrate, while the insoluble sticky solid portion was collected as residue. The filtrate was diluted with water to the respective minimum hydrotrope concentration (MHC) of the hydrotrope. Solid brown color crystals of forskolin that precipitated out from the hydrotrope solutions were isolated by centrifugation or filtration. The purity of isolated forskolin was 85 % w/w with maximum extraction of 70 % w/w [34]. 

The recent investigation [71] of a nonionic system, hexaoxyethylene dodecyl ether and water, showed a hydrotrope molecule to be introduced into the micelle first at concentrations at which the hydrotrope self-associates.This increase of the minimum concentration at which the hydrotrope molecule enters the micelle from the values in ionic systems [61-66] is in all probability due to electrostatic effects. One essential result of the investigations into nonionic systems [71] is that the presence of the hydrotrope reduces the size of the micelle i.e., the radius of the curvature toward the hydrophobic region is reduced and, hence, the cloud point is enhanced in accordance with the views of Shinoda and Arai [70], Investigations of block copolymer systems [72-76] may now be interpreted in a similar manner and the coupling or linking action of a hydrotrope in a nonionic system is given a simple explanation in the form of a modified micellar structure. 

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