Sulphosuccinic acid diesters

Sulphosuccinic acid diesters play a role above all in American polymerization formulations. They are rarely used as principal emulsifiers, but rather to control secondary properties, for example, for the production of highly concentrated low viscosity acrylate dispersions. The branched sodium di-2-ethyl hexyl sulphosuccinate is widely used, combining favourable emulsifier properties with excellent wetting power. Dicyclohexyl sulphosuccinate has a particularly high CMC and a particularly high surface tension [48]. 

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]. 

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. 

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. 

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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