Monoester sulphosuccinates

Monoester sulphosuccinates use a wider variety of alcohols than diesters and tend to use longer carbon chain alcohols to obtain the required HLB value from a single alkyl group. A typical product for cleansing applications would use a C12-14 alcohol derived from coconut or palm kernel oil. These materials are easily available due to their use as raw materials for sulphation (see later). Effective sulphosuccinates can also be prepared from petrochemical alcohols but these seem to be less popular. Ethoxylated alcohols (typically 3 mol of EO) are also used and can provide additional benefits in personal care applications. In some cases, alkanolamides or ethoxylated alkanolamides are used as the alcohol, such as ethoxylated cocomonoethanolamide, but they are relatively uncommon, since they are difficult to manufacture and are prone to colouration. 

Sulphosuccinic acid esters should be grouped into mono- and diesters. The first step of the monoester sulphosuccinate synthesis is the esterification of a hydroxyl-bearing stock material by maleic acid anhydride. As stock materials, long-chain primary alcohols, ethoxylated alcohols, ethoxylated alkylphenols, alkanolamides, ethoxylated alkanolamides, or monoglycerides are generally used [10, p. 405 ff 75, 76]. 

The group of monoester sulphosuccinates is synthesized in two steps. In the first step, the hydroxyl-carrying molecule and maleic anhydride form the monoester of maleic acid ... 

Sulphosuccinates are of particular interest not only for their technical properties but also because structurally they combine the two hydrophile functions described earlier - the sulphonate and carboxylate moieties - in a single molecule (9.18). The sulphosuccinate diesters, however, are probably of greater commercial importance in textile processing than are the monoesters. The most important example is sodium dioctylsulphosuccinate (9.19), but the dinonyl, dimethylamyl and di-isobutyl analogues are also used commercially. As usual, a wide choice of hydrophobes is available and includes alcohols, lightly ethoxylated alcohols, alkanolamides and combinations of these. 

As a class of surfactants, sulphosuccinates differ from most other sulphonates in their chemical stability and, due to the presence of the ester linkages, sulphosuccinates will hydrolyse at extremes of pH and with elevated temperature. Monoesters are more sensitive than diesters, with optimal stability of pH 6-8, whilst diesters are more stable and will tolerate pH of 1-10 at room temperature. This allows the use of diesters in a much wider range of environments, particularly under moderately acidic conditions. 

Sulphosuccinates are not particularly effective detergents but they are good wetting agents and the monoesters are favoured in personal care formulations because of their very low irritancy. 

Composition vs. performance. The greatest effect of the properties of a sulphosuccinate is undoubtedly the degree of esterification. All monoesters are water soluble, with diesters being very much less so. Variation of the alcohol used in the sulphosuccinate has the expected effect - higher molecular weights give reduced solubility and this is particularly so in diesters, where the dihexyl product is up to 30% soluble in water, but the dioctyl is only 1% soluble. 

Alkyl phenol ethoxylates can also react with P4O10 yielding alkyl phenol etherphosphates as a mixture of mono-/diesters or with maleic anhydride to yield maleic acid monoesters, which then react with NaHS03 to yield sulphosuccinate monoesters. Alkylphenolpolyglycolether sulphates, phosphates or sulphosuccinates are mainly used as primary anionic emulsifiers for the manufacturing of acrylic, styrene/acrylic or vinyl acetate co-polymer dispersions. Another type of non-ionic emulsifier is block copolymers of ethylene oxide with propylene oxide. 

Water content is adjusted to the total surfactant concentration of 30-42 % wt. The residual sulphite in the product may be oxidised to sulphate. The sulphonation proceeds also well when using partially hydrated crystalline sodium sulphite in a jacketed shear-stress reactor. This process modification is especially appropriate for manufacturing concentrated sulphosuccinate monoesters as flakes or vermicelli (often with plasticisers and fillers added in situ) suitable in mild synthetic soap bars [78]. The Cn-ig alcohols (I), ethoxylated (x2-4 mole EO) alcohols (II), and fatty monoethanolamides (III) esters of sulphosuccinic acid, mainly as sodium and alkanolamine salts, are of most practical importance as very mild high-foaming surfactants useful for personal care products and in wool, fur, and leather treatment. Very mild disodium PEG-5 laurylcitrate sulphosuccinate (in combination with sodium lauryl ethersulphate) serve for cosmetics produced by Witco as "Rewopol SB CS 50". 

Dealing with the sulphosuccinate diester synthesis, the esterification procedure is performed very similar to that described above for monoesters but the feed of hydroxyl-bearing material (generally Ce-u alcohols) should be twice more with respect to maleic anhydride. The first step is performed under a gradually increasing temperature to 120-150 °C and with continuous vacuum distillation of water from esterification. At that step esterification catalyst may be used, such as /7-toluenesulphonic acid or ABSA. The sulphonation with sodium bisulphite proceeds as described above. A representative example of diesters is sodium bis(2-ethylhexyl) sulphosuccinate known as Aerosol OT of the formula ... 

Sulphosuccinate surfactants are of two main types, namely monoesters of C12-C18 alcohols and diesters of C6-Cg alcohols. Monoesters of ethoxy-lated alcohols and monoalkanolamides also exist. Both types may be synthesised by addition of sodium sulphite or ammonium bisulphite across the double bond of a mono- or diester of maleic acid. In the latter case the product may contain small quantities of mono- or dialkyl aspartate. 

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