Surfactant Monographie

Fundamentals 1 Fundamentals 1.1 What is a surfactant? 1.1.1 Physical and chemical properties «Surfactants, what on earth are they? Yes, Ive heard of them, arent they found in washing powders?» «What are they called again ?» An alternative term for a surfactant, «tenside», goes back to the Latin «tensio», which means tension and refers to the alteration in surface tension under the influence of surfactants1. This is why surfactants are also called «surface-active substances». Such a surface could be, for example, a water surface forming the boundary between water, the liquid medium and air, the gaseous medium. The surface of a potato in a cooking pot can also be the boundary surface between the solid medium (potato) and the liquid medium (water). Naturally there are also boundary surfaces between two liquids such as fat droplets floating in a soup. If vegetable oil and water are brought together then the two immiscible liquids separate out and a boundary surface forms between them. It is exactly this boundary surface on which the surface-active substances, the surfactants, act. If a suitable surfactant is added then products such as mayonnaise or margarine can be formed from the immis- Fig. 1: Molecular structure of dodecyl sulphate (schematic) cible components vegetable oil and water. All typical surfactant properties can be traced back to their amphiphilic structure, i.e. to the presence of hydrophilic (water-loving) and hydrophobic (water-repellent) groups in the molecule. This structure has the ability to lower the surface tension. An example of this is the molecular structure of dodecyl sulphate in Fig. 1. In the literature a simplified representation of the so-called surfactant pins or surfactant matches (Fig. 2) can often be Fig. 2: Surfactant symbol: the «surfactant pin» seen. 1.1.2 Surfactants and foam Once you start foaming , and who doesnt think about foam when reading or hearing the term «surfactant»: foam in the washing machine or the dishwasher, foam in the bubble bath or under the shower. In all these cases the foam has something to do with the presence of surfactants. Read- ers who lived in the Federal Republic of Germany (FRG) during the hot summer of 1959 may remember the giant mountains of foam that could be seen on lakes and rivers, particularly near barrages. Above and below weirs these foam mountains were often as high as houses. In those days the foam was caused by the surfactant tetrapropylenebenzene sulphonate (TPS) (Fig. 3), a «hard», i.e. non- degradable surfactant. Industry and government reacted quickly and in 1964 detergent regulations were introduced Fig. 3: Structural formula of tetrapropylenebenzene sulphonate in the FRG that banned the use of non-degradable (TPS) surfactants. The industry developed the linear alkylbenzene sulphonate (LAS) (Fig. 4), a soft, i.e. biodegradable surfactant. Since 1st January 1990 the new law on the biodegradability of detergents has been in effect in the FRG. It contains minimum requirements regarding the biodegradability of surfactants contained in aqueous cleaning agents, washing agents and detergents. According to the Council Directive of the of the European Union dated 23rd November 1973, only those detergents that are eliminated from wastewaters to at least 90% within 28 days may be sold. In this way the analytical circle was closed right from the very start, as whether a surfactant is biodegradable or not Fig. 4: Structural formula of an alkylbenzene sulphonate (LAS) is a question that can only be solved by analytical means. A small laboratory sewage treatment plant is used and the surfactant concentration in the inflow and outflow of this plant is determined under specified conditions and applying defined procedures. From the analytical results obtained the biodegradability can be determined. At least three sewage disposal plants are required; one for the surfactant under investigation, a further one with the «hard» standard TPS and the final one with the «soft» standard LAS. Titrimetric determination of surfactants and pharmaceuticals 11 Fundamentals 1.1.3 Equilibria in surfactant solutions (micelle formation) Up to now we have concerned ourselves primarily with the effect of surfactants on the boundary surfaces and their ability to reduce the interfacial tension. Let us now have a look at the behaviour of surfactants in aqueous solutions. They tend to accumulate at the boundary surfaces, i.e. the walls of the vessel containing the surfactant solution, and at the interfacial boundary water/air. As the surfaces of most vessels are negatively charged, the orientation of the surfactant molecules at the surface depends on the type of surfactant, as shown in Fig. 5. In this respect surfactants clearly differ from all other substances, as neither common salt nor alcohol, etc., will ori- ent themselves with respect to a surface. This unusual behaviour compared to other substances is even more pronounced within the solution. Surfactants form micelles. Micelle formation describes the interesting property of the surfactant to form different molecular aggregates depend- ing on its concentration. Only surfactants in the strict sense of the term have this distinctive feature. However, surface- active compounds are also known in the fields of biology and biochemistry. These are structurally similar to surfactants but will not be discussed any further here. The type of molecular aggregate formed by the surfactant depends on its concentration and the structure of the surfactant molecule. At lower concentrations real solutions are formed in addition to enrichment at the surface. Above a defined point, the so-called CMC (Critical Micelle Concen- tration) the tendency of the hydrophobic components of the surfactant to no longer interact with the water becomes so pronounced that they join together to form micelles (Fig. 6). Fig. 5: Orientation of surfactant molecules at a negatively In the example shown a sphere is formed in which all hydrophilic (water-loving) heads are oriented towards the aqueous phase. In the interior of this sphere the hydrophobic alkyl groups of the surfactant are found. The number of surfactant molecules which join together to form a micelle depends on the type of surfactant and starts at about 50 for ionic surfactants of low molecular mass. Micelles need not be spherical; they can also assume other shapes. A micelle should not be thought of as being just a static aggregate. The average lifespan is just a few milliseconds. Then it dissociates and another aggregate forms with different surfactant molecules. The CMC is an important feature for characterising a surfactant. The CMC of a pure- chain dodecyl sulphate is about 700 ± 100 mg/L. 1.1.4 History2 The German chemist Justus von Liebig once said «The Fig. 6: Schematic representation of micelle formation more educated and well-off the people, the greater their soap consumption». Soaps also belong to the large surfactant family and form their own sub-group within it. Other personalities have also pointed out that the standard of living and cultural performance of a people develop in parallel to their soap consumption. In the second century AD the Romans already manufactured soap from oils and ashes (potash) and used it chiefly for body cleansing purposes, but also for washing clothes. Industrial soap manufacture only started after the French chemist Nicolaus Leblanc (1742 to 1806) developed a method of manufacturing soda. For a long time soap remained the only surfactant available. Only the scarcity of raw materials during the Second World War, which also affected vegetable oils, which were used for soap manufacture, caused the German industry to look for alternatives. The use for cleaning purposes of an emulsifier normally used in the emulsion polymerisation of buna (synthetic rubber) produced promising results. A practical application to the requirements of the population was not permitted by those then in power, as buna production was declared to be more important. Immediately after the war development was taken up again and pursued more strongly; in a short time marketable products were introduced. The manufacturers received whole sacks of thank-you letters from the population. In the Ruhr region in particular it was again possible to clean oneself and ones dirty working clothes properly. A special advantage was that the new surfactants, as opposed to soap, were not rationed and could be bought in almost any quantity. Chemically speaking the first surfactants used were alkylbenzene sulphonates, fatty alcohol ethoxylates, fatty alcohol sulphonate ethoxylates and alkylphenol ethoxylates. The first alkylbenzene sulphonate in the world was sold in 1936 by the National Aniline Chemical Company, New York, under the name «Nacconol». The successors to IG Farben brought it onto the market shortly after the war under the 12 Titrimetric determination of surfactants and pharmaceuticals Fundamentals name «Igepal NA» and in 1948 Hüls introduced it under the trade name «Marlon», which is still in use today. Alkylnaphthalene sulphonate was produced in parallel to it. In those days the raw materials benzene and naphthalene were still raw products from the coking process. Alkylbenzene sulphonate established itself very rapidly against alkylnaphthalene sulphonate, because its washing activity was clearly greater. The first generation of alkylbenzene sulphonates consisted exclusively of products with a strongly branching alkyl group, mainly tetrapropylene benzene. Only the problems encountered during the hot summer of 1959 with its very low water levels in some places led to the development of the linear alkylbenzene sulphonates, which were available from 1963 onwards. On 1st October 1964 the new Law on Detergents came into effect and banned the use of non-biodegradable surfactants (detergents). In the fifties and sixties pride was still taken in the fact that the new generation of surfactants were manufactured solely from synthetic raw materials and thus the natural fat resources were conserved.

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