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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. Kastens and Ayo [12] and Kosswig [13] reviewed the main achievements of these decades. The first result of this search was Nekal, an alkyl naphthalene sulfonate, although it probably was a mixture of various homologs [14]. Other pioneer compounds were Avirol series (sulfuric acid esters of butyl ricinoleic acid), Igepon A series (fatty acid esters of hydroxyethanesulfonic acid), Igepon T series (amide‐derivatives of taurine). All these products represented different approaches to the elimination of the carboxylic group of soaps. IUPAC defines a surfactant as a substance that lowers the surface tension of the medium in which it is dissolved and/or the interfacial tension with other phases, and, accordingly, is positively adsorbed at the liquid/vapor and/or at other interfaces. By detergent, IUPAC refers to a surfactant (or a mixture containing one or more surfactants) having cleaning properties in dilute solutions. Thus, soaps are surfactants and detergents.

It is not easy to whom the use of the word surfactant should be ascribed for the first time. A search in SciFinder^® suggests that the word was first used by Bellon and LeTellier in a French patent (1943) [15]. The SciFinder abstract of this patent indicates that “Surfactants such as wetting agents, detergents, emulsifiers, and stickers are prepared by treating by‐product materials containing starches, cellulose, amino acids, and smaller quantities of inedible fats with NaOH and neutralizing the reaction product.”

Because of their physicochemical properties, surfactants have found applications in almost any kind of industry. A list of the relevant ISO and DIN regulations for a utility evaluation of surfactants has been provided by Kosswig [13]. For instance, in 1950 Lucas and Brown [16] measured the wetting power of 13 surfactants to find a wetting agent that would enable sulfuric acid to wet peaches quickly and uniformly so as to permit acid peeling. Anionic, cationic, and neutral surfactants were tested. In the Application Guide appendix of the book Chemistry and Technology of Surfactants [17] there is a list that illustrates the variety of surfactants and their versatility in a wide range of applications. Among others the following are mentioned: Agrochemical formulations, Civil engineering, Cosmetics and toiletries, Detergents, Household products, Miscellaneous industrial applications, Leather, Metal and engineering, Paints, inks, coatings, and adhesives, Paper and pulp, Petroleum and oil, Plastics, rubber, and resins, and Textiles and fibers. For instance, their wetting properties have been early used in food technology. We have already mentioned the early connection of soap and medicine and correspondingly the use of surfactants in pharmacy in the formulation (as emulsifying agents, solubilizers, dispersants, for suspensions) and as wetting agents, which cannot be a surprise [18]. Nursing care makes a continuous use of surface‐active agents.

The soaps of Scheme 1.1 show the most important structural characteristic of surfactants: the coexistence of one lyophilic group (alkyl chain) and one lyophobic group (carboxylate ion). In aqueous solutions, it is more frequent to use the terms hydrophilic and hydrophobic. A graphical representation head–tail (hydrophobic group–hydrophilic group) is widely used, the alkyl chain being the tail and the carboxylate group the head (Figure 1.1). This structure gives the amphiphile character to surfactant compounds.

Figure 1.1 Schematic representation of the structure of some surfactants.

More generally, the head can be any polar group and the tail any apolar group, leading to a wide range of structures and types of surfactants. Among anionic heads, typical groups are carboxylate, sulfate, sulfonate, and phosphate, while the most frequent counterions are monovalent and divalent cations. Polycharged heads are also common, EDTA derivatives being well‐known examples [19]. Cyclopeptides constitute another important group [20]. Among cationic heads, typical groups are tetralkylammonium, N,N‐dialkylimidazolinium and N‐alkylpyridinium ions, while chloride and bromide are the most common counterions. Among neutral heads, polyethylene glycol ethers, polyglycol ethers, and carbohydrates can be mentioned. Zwitterionic heads are very important as phospholipids belong to this group, as well as sulfobetaines and trialkylamine oxides. Many examples can be found elsewhere [13].

However, the structures of surfactants may be more complex than the head–tail model suggests. For instance, the number of polar and non‐polar groups can be higher than one, the phospholipid phosphatidylcholine with two alkyl–allyl chains and a zwitterion as the head being an example. Gemini surfactants are dimeric surfactants [21] carrying two charged groups and two alkyl groups. The two amphiphilic moieties are connected at the level of the head groups, which are separated by a spacer group. They are characterized by critical micelle concentrations that are one to two orders of magnitude lower than those corresponding to conventional (monomeric) surfactants [22].

Bolaamphiphilic molecules contain a hydrophobic skeleton (e.g. one, two, or three alkyl chains, a steroid, or a porphyrin) and two water‐soluble groups on both ends [23]. They can be symmetric or asymmetric [24, 25]. Recent examples of bolaamphiphilic, Y‐shaped and divalent surfactants have been published by Baccile et al. [26] (Figure 1.1).

Some surfactants, instead of the mentioned head–tail structure, present a bifacial polarity with the hydrophilic and hydrophobic characteristics at two opposite sides of the molecule. The best‐known examples are bile salts (see Figure 1.2) [27, 28]. Many membrane‐active compounds are facial amphiphiles including cationic peptide antibiotics [29]. The facial amphiphilic conformation adopted by these peptides is a consequence of their secondary and tertiary structures, allowing that one face of the molecule presents cationic groups (protonated amines or guanidines) and the other face contains hydrophobic groups. An example may be magainin I [30]. Among other surfactant structures, diblock copolymers and polymeric surfactants, fluorosurfactants and silicone‐based surfactants can be mentioned [13].

Figure 1.2 Bifacial structure of cholic acid.