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The head hydrophilic group of sophorolipids is the disaccharide sophorose (2‐O‐β‐D‐glucopyranosyl‐β‐D‐glucopyranose). It is linked to a hydroxy fatty acyl moiety by a glycosidic bond between the 1′‐hydroxy group of the sophorose‐sugar and the ω or (ω−1) carbon atom of the fatty acid. Sophorolipids were first isolated by Gorin et al. [189] from the oil formed during fermentation by a strain of Torulopsis magnoliae. It has been demonstrated that they have taste‐sensory properties [190]. Sophorolipids may have a free carboxylic acid (AS) structure or a lactone (LS) one. Figure 1.8 shows both structures for the derivative with a C18:1 (oleic) chain.

Figure 1.8 Chemical structures of the acidic (AS) and lactonic (LS) C18:1 sophorolipids.

Ashby et al. [180] have obtained other derivatives by fed‐batch fermentation of Candida bombicola on glucose and several fatty acids as palmitic acid (SL‐p), stearic acid (SL‐s), oleic acid (SL‐o), and linoleic acid (SL‐l). The cmc values obtained by these authors are shown in Table 1.2. The exact composition can vary with the type of hydrocarbon substrate used in the sophorolipid production and the production conditions [178], and correspondingly different cmc values have been published. For a pure diacetylated C18:1 LS, Otto et al. [178] have reported a cmc value of 2.8 × 10⁻⁵ M (Table 1.2). Higher values have been published by Chen et al. [179] for diacetyl LS, diacetyl AS, and nonacetyl AS.

Penfold et al. [191] have studied sophorolipids by SANS. At low surfactant concentrations (0.2–3 mM), data for LS are consistent with the formation of small unilamellar vesicles, with inner and outer radii increasing with concentration. The shell thickness also increases from about 15 to 24 Å. At high concentrations (30 mM), dynamic light scattering measurements are consistent with large aggregates (~300 nm). The solutions of AS with one and two acetyl groups have a hazy appearance, indicating the presence of large aggregates, while the solution of AS with no acetyl groups are consistent with small micellar structures, the aggregation number increasing steadily from 28 to 40 at the concentration range 5–50 mM. These results are consistent with predictions from the packing parameter.

At the air–solution interface, NR measurements were also carried out by Chen et al. for the deuterated surfactants (d‐LS and d‐AS) [179]. This technique provides different parameters as adsorbed amounts, composition, thickness of the adsorbed layer, and structure at the surface. The adsorption obtained values are consistent with a Langmuir isotherm (Eq. (1.38))

(1.38)

where Γ and Γ _max are the adsorbed amounts and the maximum adsorption, C is the surfactant concentration, and k is the adsorption coefficient. AS and LS have similar k values (2.2 × 10⁻⁶), suggesting that both sophorolipids have similar affinities for the air–water interface. Above cmc, the thickness is around 23 Å while the area/molecule is around 74 Ų. For the less hydrophobic AS, the authors obtained a value of 85 Ų. These results for the adsorbed amount are in good agreement with the values obtained from surface tension data.

Studies by Manet et al. [192] have shown that the micellar morphology of no acetylated C18:1 AS is a prolate ellipsoid. Depending on experimental conditions (the salts cause an increase of the aggregation number and an elongation of the micellar aggregates), the equatorial radius of the ellipsoid varies between 6.1 and 8.0 Å, the axial core ratio varies between 4.7 and 9.4, and the aggregation number between 24 and 73. The fraction of CH₂ groups inserted in the dry core of the micelle is in the interval 0.5–0.7, meaning that the core/shell interface is located far from the sugar head group. However, the equatorial shell thickness is almost constant (12.0 ± 0.5 Å). The shell thickness that best describes the sophorolipid micelles is a variable one from the equatorial value given above to zero, i.e. the hydrophilic shell has a nonhomogeneous distribution of matter containing carboxylic groups, sophorose, salt, water, and part of the aliphatic chain. This is an atypical result since most of surfactant systems are described by a homogeneous shell thickness. The area per sophorolipid between the alkyl chain and the sugar/carboxylate head group has been estimated between 102 and 141 Ų for the most ionized micelles. For nonacetyl AS, Chen et al. [179] reported a value of 104 ± 8 Ų for the area at the air−water interface.

Previously, Cecutti et al. [193] had noticed that the sugar rings represent a major part of the molecular volume for glycolipids and, consequently, they differentiate between the micelle‐solvent interface and the hydrophobic core‐sugar head group interface. The best result for interpreting neutron and X‐ray small‐angle scattering intensity curves for ‐dodecyl maltoside in water (6% w/v, 310 K) is by a short ellipsoid model with an ellipticity of 1.2. The difference between the total short radius of the micelle (24 Å), and the short radius of the apolar hydrophobic core (18 Å) allows enough space for the sugar head groups. Other obtained parameters are the areas per surfactant head at the water‐micelles interface (87 Ų), and at the chain‐head group interface (50 Ų), the aggregation number (82) and the number of water molecules per surfactant molecules (10).

AS has a bolaamphiphile asymmetrical structure with cis‐9‐octadecenoic chain linking the sophorose disaccharide and the carboxylic acid groups. Zhou et al. [194] have observed that, at a concentration of AS of 2.0 mg/ml and pH 2.0, ribbon formation may be noticed by a light microscope. The length and the width of the ribbons grow with time, and after two hours, twisted ribbons with lengths of a few hundred micrometers and a width of ~5 m had formed. Ribbon formation is slowed by increasing the pH. After a time, which depends on pH, precipitates were observed. After 28 hours, at pH 4.1 dynamic light scattering measurements of the solution showed that the large aggregates coexisted with small micelles. Small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray scattering (WAXS) measurements were carried out in aqueous solutions and dried solid ribbons formed at pH 4.1. All WAXS diffractograms indicate high crystallinity of the hydrocarbon chains and the disaccharide head groups inside the ribbons. At pH 5.1 individual ribbons were rarely found. At 0.02 mg/ml and pH 7.8 (at which the carboxylic groups are in their negative carboxylate form), small aggregates are formed, the hydrodynamic radius R _h being 37 nm (measured at a scattering angle of 15°). At concentrations of 0.97–1.78 mg/ml, nearly monodisperse micellar aggregates were formed with R _h about 100 nm. An apparent radius of gyration R _g of 175 nm was estimated for the radius of gyration by measuring the scattering intensity at the angle range of 15–35° at a surfactin concentration of 1.40 mg/ml. The R _g /R _h ratio is about 1.75, a rather large value which indicates that the large micellar aggregates have a very anisotropic geometry.

As described in the previous paragraph, the carboxylic acid group of C18:1 AS makes that its aggregation behavior is sensitive to pH. Baccile et al. [195] have studied the system by SANS at different amounts of NaOH and other bases (NH₃, KOH, and Ca(OH)₂). A core–shell prolate micelle structure with an interaction potential (which combines hard‐sphere and screened Coulomb potentials) was used to fit SANS data. The total effective cross‐radius of the micelle (core radius + shell thickness) decreases (from 21.8 to 18.2 Å) with increasing NaOH, the core radius contributing the most to this reduction as the shell is practically constant (~8 Å). The core size and the length of the oleic acid chain suggest that the chain is partially folded and that the formation of more carboxylate/Na⁺ pairs favors such a bending. The eccentricity of the prolate also reduces (from 3.3 to 2.6) and micelles become more negatively charged as the effective micellar charge varies from about −0.5 to −5.3 with increasing NaOH concentration. Baccile et al. have also noticed that in the presence of Ca(OH)₂, the system evolves toward a better surface charge screening, which has the effect of reducing the repulsive potential (between micelles) and the effective surface charge. The micellar length is also elongated.

In a nice piece of work, Baccile et al. [26] prepared a broad range (38 new compounds) of amino derivatives of sophorolipid biosurfactant, comprising quaternary ammonium salts, amine oxides, and symmetric and asymmetric bolaamphiphiles with three or four hydrophilic centers, as well as divalent and Y‐shaped derivatives. The compounds are constituted by at least two hydrophilic head groups, different in nature and charge since sophorose is neutral and the nitrogen atom is charged, being separated by a spacer. SAXS experiments were used for the estimation of the aggregation number.

Bolaform derivatives with two (sophorose and ammonium) hydrophilic groups either do not form aggregated or exhibit a poor tendency to self‐aggregate. Correspondingly, the aggregation numbers are small (<20) and the aggregates are highly hydrated. The behavior of compounds with three hydrophilic groups (sophorose–ammonium–sophorose) depends on the nature of the ammonium group. If this is small, the compounds behave as the bolaform derivatives with two groups. If the group is charged and bulky, small hydrated aggregates are observed, coexisting with fibrilary systems. Tetra‐center compounds (sophorose–ammonium–ammonium–sophorose) with small and/or charged ammonium groups, have a poor tendency for self‐aggregation, and again small aggregation numbers and highly hydrated micelles are observed. However, divalent and Y‐shaped surfactants tend to form larger micellar aggregates as a function of the size of the hydrophobic chain and spacers between sophorose and nitrogen, the aggregation numbers usually being >50. From these experiments Baccile et al. concluded that modification of acidic sophorolipids does not necessarily improve their aggregation behavior in water.