Hydrotrope solution properties

Aqueous Solution Properties of a Fatty Dicarboxylic Acid Hydrotrope... 

The potential industrial use of hydrotropes was recognized in 1946 by McKee (2) due to the interesting characteristics that these materials displayed. By diluting a hydrotropic solution, the solute will precipitate and the hydrotrope can easily be recovered. The lack of flammability and (in general) no problems with emulsification, as might be the case for surfactants, are also properties which are valuable in industrial applications. The use of hydrotropes in various applications is described in refs (11) and (13). 

The use of hydrotrope solutions for the extraction of natural product such as anti oxidants and flavours, is also described in the scientific literature but is not an industrial reality at the moment. One major interest of hydrotropic solution for this application is the theoretical ability to recover both the substance extracted and the starting hydrotropic solution by simple dilution after the process, since the hydrotropic properties are annihilated at low concentrations. The development of switchable hydrotropes is of particular interest for this application. 

In recent studies, Friberg and co-workers (J, 2) showed that the 21 carbon dicarboxylic acid 5(6)-carboxyl-4-hexyl-2-cyclohexene-1-yl octanoic acid (C21-DA, see Figure 1) exhibited hydrotropic or solubilizing properties in the multicomponent system(s) sodium octanoate (decanoate)/n-octanol/C2i-DA aqueous disodium salt solutions. Hydrotropic action was observed in dilute solutions even at concentrations below the critical micelle concentration (CMC) of the alkanoate. Such action was also observed in concentrates containing pure nonionic and anionic surfactants and C21-DA salt. The function of the hydrotrope was to retard formation of a more ordered structure or mesophase (liquid crystalline phase). 

An inert SSE of great utility in cathodic reductions is a concentrated solution of a hydrotropic quaternary ammoniumsalt, e.g. tetrabutylammonium tosylate, in water 16 Such a solution has solvating properties approaching those of an organic solvent and hence enables one to obtain fairly concentrated homogeneous solutions of organic compounds in an aqueous medium. 

Because hydrotropic polymers can be diluted when introduced into aqueous solution, cross-linked hydrotropic polymers, i.e., hydrotropic hydrogels, were prepared. Hydrotropic hydrogels are prepared by polymerization of hydrotropic monomers in the presence of cross-linking agents. Despite the cross-linking of linear hydrotropic polymer chains, the hydrogels maintained the hydrotropic property. The hydrotropic hydrogel... 

The use of hydrotropes, substances which possess the property of selectively enhancing the aqueous solubility of sparingly-soluble compounds, can afford an opportunity for mixture separation. Colonia, Dixit and Tavare (1998) describe an example of the use of a hydrotrope for the separation of the 0- and j -isomers of chlorobenzoic acid from a 42/58% mixture, a typical ratio for the product from an industrial reactor. Sodium butyl monoglycol sulphate, used as a 50% aqueous solution, is a suitable hydrotrope in which the ortho-... 

Alkylbenzene sulfonates and alkylnaphthalene sulfonates act as hydrotropes to modify solubilities, viscosities, and other properties of surfactants and surfactant formulations. Sodium toluenesulfo-nate (STS) and sodium xylene sulfonate (SXS) are the best-known hydrotropes, however, alkylnaphthalene snlfonates also possess hydrotropic properties. Hydrotropicity is the amount of different hydrotropes required to achieve clear solutions. The amount of different hydrotropes, for example, SXS, STS, sodium cumene sulfonate (SCS), and others, in light-duty liquid (LDL) detergent formulations is given in Figure 14.1. There seems to be no trend different hydrotropes are (more or less) similarly efficient in different formulations. 

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.., ). ... 

Another important property of hydrotropes is their ability to interact with surfactants and modify their association in water. Particularly, hydrotropes increase the cloud point of ethojylated surfactants, i.e. the temperature at which the surfactant solution separates into two immiscible phases. Hydrotropes are also able to destabilise and eventually break down the liquid crystalline phases formed by the surfactants at higher concentrations and thus promote fluid formulations. ... 

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