Synthesis Of Surfactants

Various chemical reactions are utilised in the preparation of surfactants and an extensive description of them is not feasible within the limited scope of this book. Only the most important ones in this industry, sulfonation-sulfation and ethoxylation, used to generate a broad variety of sulfonated, sulfated and ethoxysulfated surfactants, will be described here. These key processes provide the fundamental change in the chemical structure of the molecule to convert it into a surface-active product.

1.5.1 Sulfonation and sulfation

LAS is produced by sulfonating LAB with either sulphuric acid, oleum or SO3 gas (Fig. 1.15). The first two methods were predominant in the market until the 1970s and, although still utilised in some places, they have been largely replaced by the most modern film sulfonation techniques using SO3 gas under various design possibilities [30].

LAB is sulfonated in the 'para' position and the resulting sulfonic acid is then neutralised with caustic soda to produce the sodium salt, which is the most widely used form of LAS worldwide. Other salts such as ammonia and triethanol amine are also used, although in substantially lower amounts compared with NaLAS.



Fig. 1.15. Schematic pathway for the synthesis of LAS out of LAB.



Fig. 1.15. Schematic pathway for the synthesis of LAS out of LAB.

LAS generally consists of a mixture of homologues and isomers. Individual components are identified by the length of the alkyl substituent and by the position of attachment of the benzene ring. In formulations, the alkyl chain length varies from C10 to C13 with an average approximate distribution of C12. Isomer configurations range from the 2-phenyl position to the 7-phenyl position depending on the alkyl chain length. Commercial blends usually contain some sulfonated impurities (< 1%), namely dialkyl tetralinsulfonates (DATSs) and slightly branched alkylbenzene sulfonates (ABSs). The relative amounts of these impurities depend on the synthetic route used. Alkylation of benzene in the presence of AlCl3 tends to yield higher DATSs than either HF or fixed bed alkylation processes [31,32]. The amount of ABS formed is similar in all alkylation technologies.

LAB sulfonic acid, also known as HLAS, is a stable material and it can be stored and transported for further neutralisation. Final neutralised LAS, usually as the sodium salt, is easily obtained as a white paste, which does not require any additional post-treatment.

This is not the case when sulfating alcohols (AS), ethoxylated alcohols (AES), AOS or MES, as the corresponding sulfonic acids are unstable and they can be easily decomposed. In these cases the neutralisation is done simultaneously with the sulfonation-sulfation, in order to produce the final sulfated, sulfonated and/or ethoxysulfated derivative.

ASs are made up of alkyl chains numbering 12 to 18 carbon atoms which are bound to a sulfate group. ASs are obtained by sulfation of fatty alcohols derived from palm oil, kernel oil and coconut oil, as well as from petrochemical raw materials.

Commercial AESs are produced via sulfation of AEs, which are also directly used as non-ionic surfactants (see below). AES preparations typically also contain some level of alkyl sulfates. The majority of technical AES blends are obtained from AE feed stocks that have alkyl chains in the range of 12-15 and an average degree of ethoxylation of three.

AOS and MES are also produced using sulfonation methodologies similar to that for LAS (falling film sulfonation) although they require some additional post-treatment steps. When sulfonating a-olefins to produce AOS, a final hydrolysis step is required in order to eliminate sulfone by-products formed during the sulfonation.

In the case of methyl ester sulfonation to produce MES, the colour of the final sulfonated product is usually very dark, and the process thus includes a bleaching step with hydrogen peroxide in order to obtain an acceptable quality.

In all the cases described above, the sulfonating agent, SO3 gas, is usually prepared on site by burning sulphur to SO2, followed by subsequent conversion to SO3 using a standard V2O5 catalytic converter.

Sulfonation of n-paraffins to produce SAS is a totally different process involving ultraviolet (UV) catalysed reactors and much more complex processing steps than the standard sulfonation processes documented above. This has limited the availability and accessibility of this technology and consequently its broader use. Sulfonated paraffins are only produced in Europe in limited amounts.

A special sulfonation technique is also used for the production of sulfosuccinates (Fig. 1.16). These are derived from maleate esters or half esters using a suitable sulfonation agent, most commonly sodium metabisulfite.

1.5.2 Ethoxylation

Ethoxylation of alcohols, alkylphenols, fatty acids, and many other organic raw materials is also a very important reaction for the surfactant industry, used to produce a broad variety of surfactants, most of which belong to the non-ionics group. The reaction with propoxylation (PO) is also practised, although to a lesser degree. The product used for ethoxylation is EO, or PO in the case of propoxylation, and is conducted using alkaline catalysts such as NaOH or NaOCH3.

Because of the difficulty and dangers of using EO in a continuous process, ethoxylation is usually conducted in batch operations. The required amount of the EO and alcohols are measured and pumped into the reactor in order to complete the reaction.

There is a great variety of ethylene oxide derivatives with differing numbers ofEO oligomers produced, depending on the desired properties of the final AEs used in the detergent or cleaning preparation. When the final product is transformed into an ethoxy sulfate, the AES usually

The Complete Coconut Oil Handbook

The Complete Coconut Oil Handbook

The coconut tree is one of the most versatile plants in existence. Whilst we are all familiar with the coconut as a food source not many of us know the myriad of other benefits the coconut holds.

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