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.
1.2 Micelle Formation
The necessity of a quantitative measurement of the surface tension of soap solutions was soon evident. By the time that I. Traube published his earliest paper in 1884, significant theories of capillarity from La Place, Poisson, or Gauss were known [31]. Early measurements of the surface tension only imply inorganic salts, acids, and bases. In 1864 Guthrie [32, 33] measured some organic liquids. At the same time, Musculus [34] studied the capillarity of aqueous solution of alcohol observing that “the capillarity of the water decreases considerably with the addition of the least amount of alcohol, in the beginning, much faster than in the presence of more alcohol.” He also noticed that “all derivatives of ethyl alcohol which are soluble in water (as acetic acid) behave like this, and probably this is also the case with the other alcohols,” but substances such as “sugars, and salts if they are not present in a great amount, almost do not influence the capillarity of water.” He proposed the use of capillarity for measuring the concentration of alcohol and acetic acid in water, among other reasons, because “it offers the advantage that one needs only very little fluid for analysis, one drop being enough.” He continued that, as “the animal fluids, such as blood serum, urine, have a capillarity which is equal to that of water, it is possible to detect and quantify substances in the urine,” making reference, for instance, to bile.
Traube started the measurement of the influence of many organic substances on the surface tension of water in the period 1884–1885 [31] and observed that “the surface tension of capillary‐active compounds belonging to one homologous series decreased with each additional CH2 group in a constant ratio which is approximately 3:1,” leading him to propose Traube's Rule.
A nice historical paper was published by Traube [31] in