Hydrotropes

Hydrotropes are small, highly water soluble additives that increase markedly the solubility of other components, including surfactants, in water. They are employed very widely in industry. In fact, they only work when the insoluble phase is a mesophase with high molecular mobility (e.g. polymer coacervate or surfactant mesophase). They include weakly self-associating elec- 

Ethanol solubilizes the hexagonal phase, shifting the boundary from =40% to =45% although it initially stabilizes it. There is almost no increase in maximum solubility of crystalline surfactant. Where me-sophases occur over a wider concentration range (say 10-40%) then hydrotropes can sharply increase the isotropic solution range by removing the mesophase. 

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All lnaphthalenesulfonic Acids. The aLkyLnaphthalenesulfonic acids can be made by sulfonation of aLkyLnaphthalenes, eg, with sulfuric acid at 160°C, or by alkylation of naphthalenesulfonic acids with alcohols or olefins. These products, as the acids or their sodium salts, are commercially important as textile auxiUaries, surfactants (qv), wetting agents, dispersants (qv), and emulsifying aids, eg, for dyes (qv), wettable powder pesticides, tars, clays (qv), and hydrotropes. 

M low cost, easily handable reactions not stoichio-metric generally hydrotrope sulfonation of aromatics using ... 

In the last two methods, hydrotropes may be added to enhance the solubiUty of the starting material. The hydrotropes improve yield and quaUty of the final dyestuff. 

Liquids. Some Hquid dyes are made directly from the thionation melt by additions of caustic soda and sodium hydrosulfide. Hydrotropic substances are sometimes added, either at the initial thionation stage or after the polysulfide melt is finished in order to keep the reduced dye in solution. Pardy reduced Hquids are also available. They are usually more concentrated than fully reduced Hquids, thus saving packaging and transportation costs. However, they require a further addition of reducing agent to the dyebath in order to obtain full color value. On the other hand, fully reduced Hquids are... 

Two sohd organic peracids have been utilized ia textile bleaching products. Diperoxydodecanedioic acid, (16), [66280-55-5] a hydrotropic peracid, and the magnesium salt [78948-87-5] of monoperoxyphthaUc acid, (17), [2311-91-3] a hydrophilic peracid, were contained in bleaching products for a short period of time (142). 

Neutral cleaners have a soap-type hydrotroping base, with additions of surfactant (to improve cleaning, wetting, penetration and defoaming), inhibitors (which may be nitrite or organic) and a bactericide. The bactericide was often formaldehyde, but this is now being superseded by formaldehyde-free materials, based on quaternary ammonium salts. 

Coupling agents such as hydroxymethyl benzene or a polyoxyeth-ylene-polyoxypropylene ether [polypropylene glycol-polyethylene glycol ether, POE-POP ether, e.g., Ucon synthetic lubricant from Union Carbide Corp.)] and hydrotropes, such as an imidazoline car-boxylate may also be needed to hold the formulation together. 

Sodium xylenesulfonate (or its ammonium cousin) are hydrotropes— organic compounds that increase the ability of water to dissolve other molecules. 

The surfactant ammonium xylenesulfonate is used as both a thickener and a hydrotrope, a compound that makes it easier for water to dissolve other molecules. It helps keep other ingredients in solution, including some of the odd substances that are added for marketing effect, such as perfumes. Glycerol stearate is another emulsifier used for this purpose. 

Sodium LAS synthesized via aluminum chloride catalysis dissolves better than sodium LAS from the hydrogen fluoride route. The main difference is the tetralin content. Dialkyltetralinsulfonates (DATs) function as hydrotropes and this influence can be larger than that of the 2-phenylalkane content. For a homolog (equal alkyl side chains), the higher the DAT content, the lower the... 

C. Tanford, in The Hydrotropic Effect Formation of Micelles and Biological Membranes, 2nd Ed., John Wiley and Sons, New York, 1980, pp. 72-78. 

The information presented in this chapter is intended to provide a brief overview of the composition, performance, and formulation properties of LAS by itself and in combination with other surfactants. The particular performance synergies and processing characteristics of certain combinations of surfactants are discussed briefly. The examples of mixed active formulations provided herein represent to the best of the author s knowledge the approximate levels of major surfactants in actual household detergent products both past and present. This does not imply that these formulations are complete because many additives, such as bleaches, enzymes, builders, hydrotropes, thickeners, perfumes, and coloring agents, may also be present in varying amounts. 

The DATs present in LAB will readily sulfonate to form dialkyltetralin-sulfonate or DATS. The foam and detergency performance properties of individual C DATS homologs are very similar to that of the corresponding C j LAS homologs [21]. Thus, even at a level of 10% DATS in the LAB, no decrease in foam or detergency performance is observed. In some liquid formulations, the presence of DATS can provide a beneficial hydrotropic effect to LAS [22]. Figure 8 illustrates that DATS are indeed surfactants, as evidenced by their surface tension vs. concentration plot. 

In formulating liquid detergent products with LAS, the carbon chain distribution, phenyl isomer distribution, and DATS level can all contribute to the solubility and viscosity characteristics. Hydrotrope requirements for isotropic liquid detergents can vary widely for different types of commercial LAS. 

Table 6 shows a comparison of commercially produced C, 4 LAS samples in a current North American light-duty liquid (LDL) formulation containing more than 20% LAS together with alcohol ether sulfate (AES), amide, and hydrotrope. The highest viscosity is observed with the high 2-phenyl/low DATS sample, whereas the high 2-phenyl/high DATS sample had the lowest viscosity. The DATS provides the dual function of surfactant and hydrotrope. 

Typically, the saponification is run with 10% sodium hydroxide solution in a reactor cascade at 95-98°C under stringent pH control. The saponification mixture is separated in a settler. The upper phase consists of alkanes with a small proportion of chloroalkanes, which is removed by oleum refining or dehydrochlorination and high-pressure hydrogenation. The refined alkanes can be recycled to the reactor. In the aqueous lower phase are alkanesulfonates, sodium chloride, and between 4 and 8 wt % hydrotropically dissolved alkanes. An optimal separation can be approached at 95 °C, and residence times of less than 60 min if Fe(III) ions are added and pH values of 3-5 are maintained. 

After cooling of the aqueous mixture to 5-10°C an upper viscous phase is separated, which contains 45-47% alkanesulfonates and 1.0-1.3% sodium chloride, while the lower phase is a 7-8% brine with a small quantity of alkane-monosulfonates but 1.5-2.0 wt % di- and polysulfonates. The hydrotropically dissolved alkanes (neutral oil) are found entirely in the upper phase. Because of the small density differences, the separation of the two phases needs 15-20 h. The lower phase can be separated by membrane technology [13]. 

The applications of alcohol sulfates in consumer products depend on the alkyl chain and in some cases on the cation. Alcohol sulfates with alkyl chains 8 C1() are seldom used in consumer products except occasionally as hydrotropes in liquid detergent formulations. However, alcohol sulfates in the range C10-C18 are used in many commonly used formulations although other surfactants are generally added to enhance their properties. In some of these applications, particularly in shampoos, they compete with alcohol ether sulfates of the same alkyl chain distribution. The pattern of use of alcohol sulfates or alcohol ether sulfates in formulations varies with consumer personal care and laundry washing preferences in different cultural areas of the world. 

Foam low high Alkali stab Electrolyte stab Acid stab Chlorine stab Surface tension Detergent effect Hydrotropic effect Solubilizing effect Biodegrad- ability... 

A special application of a-sulfo fatty acid esters is for highly concentrated hard surface cleaners which are used after dilution in a pail of water. Normally hydrotropes are needed to increase the solubility of the surfactants in water and assure clear, homogeneous, and storage-stable products. If the LAS, which are typical surfactants in hard surface cleaners, are replaced by ester sulfonates the hydrotrope can be deleted from the formula [62]. 

Normally in the production of diesters great effort is spent in obtaining high yields. Catalytic support of the esterification reaction and azeotropic distillation to remove reaction water yields diesters near 100% purity. The amount of unreacted educt material is usually very small. Following sulfation, in the presence of a hydrotrope to reduce viscosity, a 65% active content product with virtually no byproducts is obtained. 

Although only the types of surfactants presently used in main detergent formulation have been indicated, the system allows for processing of a wider range of product while still complying with the maintenance of the quality level of the feed. Moreover, products like hydrotropes or blends of surfactants and inorganic salts can be conveniently processed to yield preformulated surfactant bases ready for incorporation in formulated detergents. 

The Ballestra Neutrex SV system (Fig. 10) has been developed to obtain, in a single process step, products with high AM at high purity. Neither solvents nor hydrotropes (sometime used to overcome viscosity and consequent high-pressure problems in the loop) are required to operate with these highly viscous products. 

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