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.
1.6 Sophorolipids
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))
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 Å2. For the less hydrophobic AS, the authors obtained a value of 85 Å2. 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.
