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 Å2 (R1) and 77 ± 2 Å2 (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
