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Introduction to Biosurfactants
José Vázquez Tato1, Julio A. Seijas2, M. Pilar Vázquez‐Tato2, Francisco Meijide1, Santiago de Frutos1, Aida Jover1, Francisco Fraga3, and Victor H. Soto4
1 Departamento de Química Física, Facultad de Ciencias, Universidad de Santiago de Compostela, Avda, Lugo, Spain
2 Departamento de Química Orgánica, Facultad de Ciencias, Universidad de Santiago de Compostela, Avda, Lugo, Spain
3 Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Santiago de Compostela, Avda, Lugo, Spain
4 Escuela de Química, Centro de Investigación en Electroquímica y Energía Química (CELEQ), Universidad de Costa Rica, San José, Costa Rica
CHAPTER MENU
1 1.1 Introduction and Historical Perspective
3 1.3 Average Aggregation Numbers
4 1.4 Packing Properties of Amphiphiles
10 References
1.1 Introduction and Historical Perspective
Surface tension is a property that involves the common frontier (boundary surface) between two media or phases. Strictly speaking, the surface tension of a liquid should mean the surface tension of the liquid in contact and equilibrium with its own vapor. However, as the gas phase has normally a small influence on the surface, the term is generally applied to the liquid–air boundary. The phases can also be two liquids (interfacial tension) or a liquid and solid. According to IUPAC, the surface tension is the work required to increase a surface area divided by that area [1]. This is the reversible work required to carry the molecules or ions from the bulk phase into the surface implying its enlargement and corresponds to the increase in Gibbs free energy (G) of the system per unit surface area (A),
(1.1)
where γ is the interfacial tension. Therefore, the units of γ are J/m2 or N/m, but it is normally recorded in mN/m (because it coincides with the value in dyn/cm of the cgs system). In 1944, Taylor and Alexander [2] collected some representative published (1885–1939) values for the surface tension of water at 20 °C. Their own value was 72.70 ± 0.07 mN/m (calculated by extrapolation) in agreement with more recent determinations, the accepted value being 71.99 ± 0.36 mN/m at 25 °C [3]. This is a rather high value when it is compared with those of other common solvents as ethanol (22.39 ± 0.06 mN/m), acetic acid (27.59 ± 0.09 mN/m), or acetone (29.26 ± 0.05 mN/m) (values from [4]) at 20 °C.
The decrease in the surface tension of water has been traditionally achieved by using soaps or soap‐like compounds. According to IUPAC a “soap is a salt of a fatty acid, saturated or unsaturated, containing at least eight carbon atoms or a mixture of such salts. A neat soap is a lamellar structure containing much (e.g. 75%) soap and little (e.g. 25%) water. Soaps have the property of reducing the surface tension of water when they are dissolved in soap‐like compounds in water.” This reduction facilitates personal care, washing of clothes and other fabrics, etc. The early documents with descriptions of soaps and their uses are typically related with medicinal aspects, and nowadays there is almost a specific type of soap for each requirement. Levey [5] has reviewed the early history of “soaps” used in medicine, cleansing, and personal care. For instance, he mentions that “in a prescription of the seventh century BC, soap made from castor oil (source of ricinoleic [12‐hydroxy‐9‐cis‐octadecenoic] acid) and horned alkali is used… as a mouth cleanser, in enemata, and also to wash the head.” However, Levey concludes that a true soap using caustic alkali was probably not produced in antiquity but “evidence has been adduced to indicate that salting out was in use in early Sumerian times.” In his Naturalis Historia, Pliny the Elder [6] refers to soap (sapo) as prodest et sapo, Galliarum hoc inventum rutilandis capillis. fit ex sebo et cinere, optimus fagino et caprino, duobus modis, spissus ac liquidus, uterque apud Germanos maiore in usu viris quam feminis, which may be translated as “There is also soap, an invention of the Gauls for making their hair shiny (or glossy). It is made from suet and ashes, the best from beechwood ash and goat suet, and exists in two forms, thick and liquid, both being used among the Germans, more by men than by women.”
Hunt [7] indicates that centers of soap production by the end of the first millennium were in Marseilles (France) and Savona (Italy), while in Britain some references appear in the literature around 1000 AD. For instance, in 1192 the monk Richard of Devizes referred to the number of “soap makers in Bristol and the unpleasant smells which their activities produced.” Hunt also resumed other aspects as the chemistry of soap, the British alkali industry, the expansion of soap production, soap manufacturers, and manufacturing methods. As early as 1858, Campbell presented a USA patent [8] for the production of soaps. He described the process as consisting in “the use of powdered carbonate of soda for saponifying the fatty acids generally, and more particularly the red oil or ‘red (oleic) acid oil’ and converting them, by direct combination, into soap in open pans or kettles, at temperatures between 32 and 500 °F.” Mitchell [9] revised the Jabón de Castilla or Castile soap (named from the central region of Spain), probably the first white hard soap. It was an olive oil‐based soap and soaps with this name can still be bought today. Traditional recipes and videos can be easily found on the Internet. In the paper “Literature of Soaps and Synthetic Detergents”, Schulze [10] recorded the literature (including books, periodicals, abstracts, indexes, information services, patent publications, association publications, conference proceedings) on soaps, surfactants, and synthetic detergents up to 1966.
Nowadays descriptions for soap‐making from fats and oils are frequent for teaching purposes. For instance, Phanstiel et al. [11] have described the saponification process (basic hydrolysis of fats). It involves heating either animal fat or vegetable oil in an alkaline solution. The alkaline solution hydrolyses the triglyceride into glycerol and salts of the long‐chain carboxylic acids (Scheme 1.1).
Scheme 1.1 Alkaline hydrolysis of a triglyceride to obtain soaps.
To overcome the shortcomings of the carboxylic group of soaps, during the first decades of the twentieth century, new surface‐active agents were obtained in chemistry laboratories.