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Although phospholipids or bile acids are of a biological origin, the term “biosurfactant” concerns those amphiphilic derivatives that are produced by microorganisms. In general, they have excellent surface‐active properties.

Different classifications have been purposed for biosurfactants [154–156]. For instance, Otzen indicates that, based on their overall structure, biosurfactants fall into four classes: glycolipids, lipopeptides, saponins, and all the rest. Glycolipids can be subdivided into rhamnolipids, sophorolipids, trehalolipids …, in which the head group are different saccharides (rhamnose, sophorose, trehalose, …). Similarly, lipopeptides can be divided into several families as surfactin, iturin or fengycin [157, 158], some of them having a peptide‐cycle structure. Gemini type biosurfactants are also common [159]. The hydrophobic part is commonly one or more unsaturated or saturated hydrocarbon chains. The saponin Aescin or glycyrrhic acid are examples of biosurfactant complex structures [159, 160].

The number of biosurfactants can enormously be enlarged by chemical modifications, which can be carried out in laboratories. Sugar surfactants are well‐known examples in which glucose, galactose, xylose, sucrose, or lactose are common hydrophilic heads [161]. They can easily be obtained chemically [162] or enzymatically [163]. Baccile et al. [26] have obtained and characterized the self‐assembly properties of a broad family of amino derivatives of sophorolipid biosurfactants, including asymmetric (sophorose–ammonium and sophorose–amine oxide) and symmetric (sophorose–sophorose bearing three and four hydrophilic centers) bolaamphiphiles.

Previous naturally found biosurfactants and similar derivatives will all together show a broad range of physicochemical properties. Let us analyze some significant examples.

The surface tension vs concentration curves of biosurfactants are similar to those of classical surfactants. The break points in these plots, corresponding to cmc values, are normally well defined, and the surface tension values at cmc, γ _cmc are frequently found to be <30 mN/m. However, differences in reported cmc values are not uncommon, probably due to the presence of impurities or the use of mixtures instead of pure samples. For instance, Saini et al. [164] reported a value of 54 mg/l for the cmc of viscosin (Figure 1.6), a cyclic oligopeptide lipid which contains nine amino acids: two (leu‐glu) are linked to the fatty acid tail, and the remaining seven [val‐leu‐ser‐leu‐ser‐ile‐allo(thr)] form a cycle. However, other values have also been reported for cmc, as Saini et al. have noticed, with values ranging from 4 to 9 mg/l [165] to 150 mg/l [166].

Figure 1.6 Structure of viscosin.

Source: Saini et al. [164]. Reproduced by permission of the American Chemical Society.

General rules observed in classical surfactants are also observed for biosurfactants. Figure 1.5 shows a linear relationship between the aggregation number and the alkyl chain volume in classical surfactants. This is just an example of linear relationships for different properties in a series of homologous surfactants. For instance, Garofalakis et al. [161] have observed a reduction in the critical aggregation concentration (cac) of the surfactants when increasing the carbon chain length for a series of monoesters of xylose, galactose, sucrose, and lactose with different hydrophobic chain lengths (C12–C16). These authors also observed that the more hydrophilic head groups, the higher is cac, though this trend was moderated by the alkyl chain length. Some observed differences between maltose and glucose derivatives have been ascribed to a higher degree of hydration of maltose compared to that of the glucose head group as molar heat capacities for these two sugars suggest [167]. Another example corresponds to the standard free energy change. For n _c‐alkyl‐D‐maltosides (n _c , number of carbon atoms of the alkyl chain, = 8, 10, 12, and 14), Varga et al. [168] have shown that the dependence of the standard free energy change (and from here the cmc as well) with the length of the alkyl chain for these sugar surfactants is parallel to the one for alkyltrimethylammonium bromides and sodium alkylsulfates.

Similarly, Ribeiro et al. [169] have obtained diacetylated lactonic sophorolipids with different hydrophobic chains (C18:0, C18:1, C18:2, and C18:3). The cmc values (in mg/l) increase linearly with the number of double bonds from 29.2 (C18:0) to 39.1 (C18:3), the slope being 3.35 (r ² = 0.979). The surface tension at cmc ( γ _cmc) also increases with that number (from 35.7 to 38.8 mN/m).

Sugar alkyl surfactants also display additivities with the number of carbon atoms of the alkyl chain. For instance, Angarten and Loh [170] studied two series of surfactants CncGx using ITC, where x = 1,2 is the number of glucoside units (G) of the hydrophilic head and n _c = 7–12 (the limits depending on x). Both families exhibit a linear ΔC _p vs n _c plot, the slope representing the contribution of each methylene group to the heat capacity for micellization. The obtained values for the slope are 63 ± 2 J/(K mol) and 59 ± 2 J/(K mol) for x = 1 and x = 2, respectively. Therefore, within experimental error, there are not significant differences between them, suggesting that the contributions of the mono or diglucoside head groups are essentially the same. Let us remember that a value of 33 J/(K mol) was found for the contribution of each hydrogen atom of each methylene group for the removal of alkanes from water to air [171]. This also has been related to the number of water molecules in the first solvation shell that contribute to the thermodynamics of hydrophobic solvation [102]. Other ITC measurements for alkyl sugar surfactants have been carried out by Blume et al. [92, 172]. Similar studies [173] were carried out for monorhamnose and dirhamnose rhamnolipids (R1, R2) (and their mixtures) (Figure 1.7). The cmc values are shown at Table 1.2.

Figure 1.7 Structure of biosurfactants monorhamnose and dirhamnose rhamnolipids (R1 ‐left‐ and R2 right).

Source: Chen et al. [174], p. 18281 .

Table 1.2 Examples of cmc values of biosurfactants.

Compound	Cmc/M	γ/mN/m	References
Sucrose hexadecyl	4.1 × 10−6	31.0–43.0	Garofalakis et al. [161]
Sucrose dodecyl	2.1 × 10−4
R1 monorhamnose rhamnolipid L‐rhamnosyl‐β‐hydroxydecanoyl‐β‐hydroxydecanoate (RhaC10C10).	(3.6 ± 0.2) × 10−4 30 °C	31.2 ± 0.2	Chen et al. [173]
R2 L‐rhamnosyl‐L‐rhamnosyl‐β‐hydroxydecanoyl‐ β‐hydroxydecanoate (Rha2C10C10); The surface tension, NR, and SANS measurements were all made at pH 9 (buffer consisted of 0.023 M borax and 0.008 M HCl).	(1.8 ± 0.2) × 10−4 30 °C	37.4 ± 0.2
Rhamnolipid A	6.22 × 10−5 pH 7.35		Ishigami et al. [175]
Rhamnolipid B	1.50 × 10−4 pH 7.35
Rhamnolipid	50 mg/l		Whang et al. [176]
Surfactin	4.72 × 10−5 pH 8.0¸ 20 mM phosphate buffer		Onaizi et al. [177]
Surfactin	45 mg/l	<30	Whang et al. [176]
Sophorolipid (lactonic form) C18:1 LS	2.8 × 10−5 potassium phosphate buffer (0.1 M, pH 7.4) at 25 °C	36.1	Otto et al. [178]
Diacetyl LS (Sophorolipid)	6 × 10−5	36	Chen et al. [179]
Diacetyl AS (Sophorolipid)	6.7 × 10−4	38.5
Nonacetyl AS (Sophorolipid)	6.2 × 10−4	39
LAS (Sophorolipid)	1.6 × 10−3
SL‐p (palmitic)	>200 mg/l	35	Ashby et al. [180]
SL‐s (stearic)	35 mg/l	35
SL‐o (oleic)	140 mg/l	36
SL‐l (linoleic)	250 mg/l	36
Glycyrrhizic acid	2.9 × 10−3 pH 5 5.3 × 10−3 pH 6 No clear cmc at pH 7	55.2 56.8	Matsuoka et al. [160]

Another example of the general behavior of biosurfactants corresponds to the kinetics of the micelle formation (see above). For instance, Haller and Kaatze [140] have studied the kinetics of micelle formation in aqueous solution of sugar surfactants as hexyl‐, heptyl‐, octyl‐, nonyl‐, and decyl‐β‐D‐maltopyranoside (C _x G₂, x = 6, 8–10) as well as of decyl‐β‐D‐maltopyranoside C₁₀G₂. As for other alkyl surfactants, there is a general tendency in the backward rate constant to increase with increasing cmc and with decreasing length of the alkyl chain.

From small angle neutron scattering (SANS) experiments, Chen et al. [174] found that, in dilute solution (<20 mM), R1 and R2 form small globular micelles (aggregation numbers being about 50 and 30, respectively), while at higher concentrations, R1 aggregates have a predominantly planar structure (unilamellar or bilamellar vesicles) whereas R2 remains globular, with an aggregation number that increases with increasing surfactant concentration. Over the concentration range measured, for both surfactants the mean thickness of the adsorbed monolayer is about 21 ± 1 Å. From neutron reflectivity (NR) data and the Langmuir isotherm, the molecular areas at the surface are 62 ± 2 Ų (R1) and 77 ± 2 Ų (R2).

Gouzy et al. [24] have obtained two series of asymmetric bipolar surfactants with lactose as one of the hydrophilic groups. Their structures resemble those of asymmetric gemini surfactants but without a second hydrophobic moiety (Figure 1.1). These surfactants are known as divalent [26]. They have a long hydrocarbon chain, a nonionic polar head (lactose), a hydrocarbon spacer (of length n _c ), and a second polar head (of length m _c) at the end of the spacer. At constant n _c , the results evidence a linear variation of log (cmc) with m _c but with a positive slope, i.e. the largest the hydrophobic alkyl side chain, the larger the cmc, which is the opposite trend observed for classical alkyl surfactants. These results are in line with those observed for gemini surfactants. Menger and Littau affirm that they are “counter to all previously reported trends in surfactant chemistry” [151], are anomalous as “in the protected duchies of academia, it is taught that a longer hydrocarbon tail always lowers the cmc” [152], while Rosen et al. [181] described this behavior as “aberrant.” These authors accept that this unconventional behavior is indicative of substantial premicellar aggregation.

As indicated, low critical aggregation values have been measured and they favorably compare with both ionic and nonionic surfactants. The values obtained for the sucrose hexadecyl and dodecyl derivatives by Garofalakis et al. [161] (see Table 1.2) canbe compared with the cmc values for sodium hexadecyl sulfate (4.5 × 10⁻⁴ M) and sodium dodecyl sulfate (8.2 × 10⁻³ M) [145], as well as with those for polyoxyethylenated nonionic surfactants of structure C _nc H _nc+1(OC₂H₄) _x OH (C _n E _x ). For instance, for the series n _c = 12, x = 2, Rosen et al. [182] have measured values in the interval 3.3 × 10⁻⁵ (x = 2) to 1.09 × 10⁻⁴ M (x = 12) (all data at 25 °C), where it is obvious that the larger the hydrophilic head, the higher the cmc. Similar values for other members of this type of surfactant have been published elsewhere [183–185].

Some observed differences between maltose and glucose derivatives [161] have been ascribed to a higher degree of hydration of maltose compared to that of the glucose head group, as molar heat capacities for these two sugars suggest [167]. Significative differences in the hydration shell of the uncharged head groups were observed by Tyrode et al. [186, 187]. These authors have studied the OH stretching region of water molecules in the vicinity of nonionic surfactant monolayers by using vibrational sum frequency spectroscopy, thus allowing the study of the hydration shell around the head groups. Compounds such as dodecanol, sugar surfactants (n‐decyl‐β‐D‐glucopyranoside and n‐decyl‐β‐D‐maltopyranoside), and polyoxyethylene surfactants (C₁₂E₄ and C₁₂E₈) were studied. For all the surfactants, it was detected that the water molecules located in proximity to the surfactant hydrocarbon tail phase have both hydrogen atoms free from forming hydrogen bonds. For the two sugar surfactants, the strength of the hydrogen bonds in the hydration shell was found to be similar to those observed for tetrahedrally coordinated water molecules in ice. Despite being itself disordered, the polyoxyethylene head group induces a significant ordering and structuring of water at the surface. The orientation of the C₁₂E₅ molecules changes with concentration from lying on the surface with their hydrocarbon tails close to the surface plane (the observed results being consistent with the formation of disk‐like “surface micelles” with a flat orientation of the amphiphiles at low surface concentrations) to a more upright configuration as the surface covering liquid layer is formed [187].

The formation of surface micelles was discussed in another paper [188] in which surface tension measurements were used to study the adsorption isotherms for sugar surfactants (n‐decyl‐β‐D‐glucopyranoside (Glu), n‐decyl β‐D‐maltopyranoside (Mal), and n‐decyl‐β‐D‐thiomaltopyranoside (S‐Mal)). A gradual change in molecular areas is observed when the surfactant concentration is increased. As the area/molecule is comparatively large, the resulting surface phase cannot be a coherent hydrocarbon film and should include a large portion of unperturbed air–water interface. The formation of surface micelles can account for this observation. A hard‐disk simulation allowed the calculation of the number of molecules per micelle as a function of bulk surfactant concentration for Mal (values in the interval 9–12) and Glu (values in the interval 10–14), the surfactant molecules strongly favoring an orientation in the plane of the surface.