The Effect of Hydrotropic Salts on Phase Relationships Involving Hydrocarbons, Water, and Alcohols

Hydrotropic salts, which can increase the solubility of organic materials in aqueous solutions, are of interest in a variety of industries including those related to tertiary oil recovery. We have examined effects of solubility of hydrocarbons in water (with and without alcohols) by adding inorganic hydrotropic salts such as perchlorates, thiocyanates, and iodides (high in the usual Hofmeister series), organic salts such as short chain alkyl benzene sulfonates, and other salts based on substituted benzene derivatives. Although the inorganic salts are relatively ineffective in increasing solubility of hydrocarbons in water, many of the organic salts are excellent hydrotropic agents for hydrocarbons. We have examined the phase relationships for several series of aromatic salts such as sulfonates, carboxylates, and hydrocarboxylates as a function of alkyl-carbon substitution in three-component (hydrocarbon, salt, water) and in four-component (hydrocarbon, salt, alcohol, water) systems. We also have examined miscibility relationships for a given hydrotropic salt as the chain length of alkanes and alkyl benzenes is varied systematically. While miscibilities decrease with increase in chain length of the hydrocarbon, the hydrotropic properties in these systems increase rapidly with the number of alkyl carbons on the benzene ring of the salts, and they are relatively insensitive to the type of charged group (sulfonate vs. carboxylate) attached to the benzene ring. However, there are significant increases in hydrotropy as one goes from equally substituted sulfonates or carboxylates to salicylates. A number of salts have been identified that have much greater hydrotropic properties for hydrocarbons than well-known hydrotropic materials such as toluene and xylene sulfonates. Introduction Hydrotropic materials are compounds that in aqueous solutions tend to increase the solubility of materials normally slightly soluble in water. This phenomenon also commonly is referred to as "salting-in". Salting-in was noted by Hofmeister in 1888; Neuberg introduced the term "hydrotropic" in 1916. Hydrotropic salts may be organic or inorganic. Typical organic materials are alkali metal or alkaline earth salts of sulfonic acids, carboxylic acids, and salicylic acids: typical inorganic salts are alkali metal or alkaline earth perchlorates, ammonium thiocyanate, and iodides. Such materials have been proposed for use in many industries, including pulp and paper, soap and detergent, leather, dye and textile, and cosmetics. In the past few years we have published several papers on the effect of organic hydrotropic salts on solubility of hydrocarbons in either water or aqueous/alcohol systems. Much of the work was with low-molecular-weight alkyl benzene sulfonates. These are chemically similar to components of practical surfactants used in enhanced oil recovery, except that their alkyl chains are shorter than usually is thought necessary for formation of micelles. A series of alkyl benzoates and of alkyl salicylates were investigated also. We believe that the miscibilities of systems containing these classes of compounds, which we call "protosurfactants." will prove helpful in understanding the much more complicated hydrocarbon/water/inorganic salt/surfactant/cosurfactant systems of micellar flooding. These simpler systems can be regarded as limiting cases of practical micellar-flood chemistry and may help establish rules for mutual hydrocarbon/water solubility. Studies of phase behavior of simple systems comprised of hydrocarbon/alcohol/water/salt have been carried out at the U. of Minnesota, apparently from similar motivations. In addition, low-molecular-weight organic salts probably are components of practical oil-recovery surfactants or may be added advantageously, as they frequently are in conventional detergent formulations. SPEJ P. 363^