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. 2016 Mar 1;40(3):2028-2035.
doi: 10.1039/C5NJ02108A. Epub 2015 Dec 7.

Aggregate-based sub-CMC Solubilization of n-Alkanes by Monorhamnolipid Biosurfactant

Hua Zhong  1 , Xin Yang  2 , Fei Tan  2 , Mark L Brusseau  3 , Lei Yang  2 , Zhifeng Liu  2 , Guangming Zeng  2 , Xingzhong Yuan  2
Affiliations

Aggregate-based sub-CMC Solubilization of n-Alkanes by Monorhamnolipid Biosurfactant

Hua Zhong et al. New J Chem. .

Abstract

Solubilization of n-decane, dodecane, tetradecane and hexadecane by monorhamnolipid biosurfactant (monoRL) at concentrations near the critical micelle concentration (CMC) was investigated. The apparent solubility of all the four alkanes increases linearly with increasing monoRL concentration either below or above CMC. The capacity of solubilization presented by the molar solubilization ratio (MSR), however, is stronger at monoRL concentrations below CMC than above CMC. The MSR decreases following the order dodecane > decane > tetradecane > hexadecane at monoRL concentration below CMC. Formation of aggregates at sub-CMC monoRL concentrations was demonstrated by dynamic light scattering (DLS) and cryo-transmission electron microscopy examination. DLS-based size (d) and zeta potential of the aggregates decrease with increasing monoRL concentration. The surface excess (Γ) of monoRL calculated based on alkane solubility and aggregate size data increases rapidly with increasing bulk monoRL concentration, and then asymptotically approaches the maximum surface excess (Γmax). Relation between Γ and d indicates that the excess of monoRL molecules at the aggregate surface greatly impacts the surface curvature. The results demonstrate formation of aggregates for alkane solubilization at monoRL concentrations below CMC, indicating the potential of employing low-concentration rhamnolipid for enhanced solubilization of hydrophobic organic compounds.

Keywords: aggregation; biosurfactant; critical micelle concentration; monorhamnolipid; n-alkane; solubilization.

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Figures

Figure 1
Figure 1
Molecular structure of monoRL and the four n-alkanes.
Figure 2
Figure 2
(a) The air-PBS and n-alkanes/PBS interfacial tension as a function of monoRL concentration. (b) Interfacial tension-concentration relation regression at monoRL concentrations below CMC using Szyszkowski equation (Equation (3) in ref.18).
Figure 3
Figure 3
(a) Apparent n-alkanes solubility (Calk) versus monoRL total concentration (C0). Two sets of regressions represent data for below and above the CMC. (b) Zoom-in for Calk-C0 relation for C0 lower than CMC. Error bars show mean ± standard deviation.
Figure 4
Figure 4
Number distribution of aggregate particles for solubilization of dodecane and hexadecane by monoRL at concentration of 30 μM and 750 μM.
Figure 5
Figure 5
Cryogenic-transmission electron microscopy (cryo-TEM) images showing aggregates for the solubilization of hexadecane by monoRL at monoRL concentration of 30μM (below CMC) (a) and 750μM (above CMC) (b).
Figure 6
Figure 6
DLS aggregate size (diameter, d) versus the total monoRL concentration (C0) for the n-alkanes solubilization. Error bars show mean ± standard deviation.
Figure 7
Figure 7
Zeta potential of aggregates versus the monoRL total concentration (C0) for the n-alkanes solubilization. Error bars show mean ± standard deviation.
Figure 8
Figure 8
(a) Apparent solubility of n-alkanes (Calk) versus the monoRL bulk concentration (Cw) at Cw below CMC. (b) Surface excess (Γ) and molecule area (A) of monoRL on the aggregates surface versus monoRL bulk concentration (Cw). Error bars show mean ± standard deviation.
Figure 8
Figure 8
(a) Apparent solubility of n-alkanes (Calk) versus the monoRL bulk concentration (Cw) at Cw below CMC. (b) Surface excess (Γ) and molecule area (A) of monoRL on the aggregates surface versus monoRL bulk concentration (Cw). Error bars show mean ± standard deviation.
Figure 9
Figure 9
Aggregates diameter (d) versus surface excess of monoRL (Γ) at monoRL bulk concentration (Cw) below CMC. Error bars show mean ± standard deviation.
Figure 10
Figure 10
Schematic diagram of alkane-monoRL aggregate formation at monoRL concentration below CMC.
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