Structure of Rhamnolipid
Rhamnolipids(RLs) are examples of glycolipids (non-ionic) that contain rhamnose (a pentose monosaccharide or 6-deoxy-L-mannose) bonded to a fatty acid chain length ranging from 8 to 16 carbons. There are two type of rhamnolipids, mono- (one rhamnose group) and di- (two rhamnose groups).
Figure 1: Types of rhamnolipids
Figure 2: Manufacturing process for rhamnolipids
Method of production of Rhamnolipid
The most common method for production of rhamnolipid use Pseudomonas aeruginosa, an example of gram negative bacteria which turn red under a chemical process known as gram staining. The process involves fermentation of oil (carbon source) using the bacteria, extraction and purification of RLs shown in the Figure . In the past few years there have been a wide variety of methods to generate RLs using different culture compounds, conditions and extraction procedures.
In the past, it had been difficult to mass produce RLs due to low yield. Evonik industries have been the first company to mass produce RLs by using recombinant Pseudomonas putida and butane.
Properties of rhamnolipids
General properties of biosurfactants include:
- Lower surface tension
To acts as a biosurfactant, the molecule need to lower tension by > 8mN/m. RLs are able to reduce surface tension of water from 72mN/m to 30mN/m. This was lower than synthetic surfactants such as sodium dodecyl sulphate (SDS).
- Lower critical micelle concentration (CMC)
CMC is the concentration at which micelles are formed. The micelles form at (~ 10-100 times) lower concentration than SDS.
- Effect of temperature, pH and concentration variations
RLs are stable up to a temperature of 353K.
RLs are sensitive to changes in pH as characterised by changes in the surface tension. At lower pH, CMC decreases, however this is not the case at high pH. These trends are also observed for di-RLs. This is due to the presence of the carboxylic acid moiety in the RLs which causes aggregation in solution
The ability of RLs to self assemble into micelles, vesicles and lamella depend on solution pH, concentration and presence of electrolytes. At concentrations higher than the CMC, these surfactants are known to form spherical vesicles. Results from dynamic light scattering measurements show that at all pHs, large aggregates of RLs are formed.
- Emulsion stability
In emulsions with a 50:50 hydrocarbon to water ratio, the performance of RLs at acting as surfactants to stabilise emulsions was characterised using laser profiling. The results are as follows (in decreasing order of emulsion stability) emulsion methyl methacrylate > emulsions of castor oil > emulsion n-heptane > emulsion toluene > emulsion hexadecane > octane emulsion. Emulsions with methyl methacrylate was the most stable with no phase separation after 24 hours but a slight change in the backscattering profile at the bottom of container indicating creaming.
- Ecotoxicity
Biosurfactants are beneficial in industrial, food, pharmaceutical, and cosmetic goods because of their low toxicity profile. Nevertheless, elevated levels of biosurfactants may pose a hazard to microorganisms, impeding their growth and influencing the effectiveness of biodegradation. This has been shown in aquatic environments where a naturally occurring mono rhamnolipid and a synthetic mono rhamnolipid both obtained EC50 level scores that were 'somewhat hazardous'.
Figure 3: Surface tension measurement of RLs extract in aqueous solution with varying pH measured using Wilhemy plate method on an tensiometer.
Figure 4: Transmission and backscattering profile of an emulsion with methyl methacrylate, water and 0.15 g/L of RL extract.
Applications of rhamnolipids
There have been many applications of RLs in industry. A small subset of this has been mentioned below in Table 1. Although rhamnolipid has several industrial uses, its use is restricted because of its high cost of manufacture.
Table 1: The examples of RLs used in industry currently ranging from antibiotics in medicine to bio remediation in waste management removal.
| Field | Example |
|---|---|
| Medicine | Used in drug delivery, has anti-bacterial properties, |
| Soil remediation | Mobilization of hydrocarbons (ie hexadecane, phenanthrene) in soil |
| Pollutant removal | Improved the extraction of Cr(III) from kaolinite-contaminated soils |
| Waste water treatment | Used in dispersing and dissolving materials clogging CW plants |
Remarks
There are numerous benefits and applications RLs have, however, cost of manufacturing and the potential health risk necessitates further research into this molecule to determine optimum fermentation conditions and genetic strain. It is also apparent RLs performance tends to be better than some commercial surfactants such as SDS in properties such as surface tensions and CMC.
References
- Green Surfactants (Biosurfactants): A Petroleum-Free Substitute for Sustainability─Comparison, Applications, Market, and Future ProspectsVaishnavi S. Nagtode, Clive Cardoza, Haya Khader Ahmad Yasin, Suraj N. Mali, Srushti M. Tambe, Pritish Roy, Kartikeya Singh, Antriksh Goel, Purnima D. Amin, Bapu R. Thorat, Jorddy N. Cruz, and Amit P. PratapACS Omega 2023 8 (13), 11674-11699DOI: 10.1021/acsomega.3c00591
- Eslami, Parisa & Hajfarajollah, Hamidreza & Bazsefidpar, Shayesteh. (2020). Recent advancements in the production of rhamnolipid biosurfactants by Pseudomonas aeruginosa. RSC Advances. 10. 34014. 10.1039/d0ra04953k.
- Mendes, A. , Filgueiras, L. , Pinto, J. and Nele, M. (2015) Physicochemical Properties of Rhamnolipid Biosurfactant from Pseudomonas aeruginosa PA1 to Applications in Microemulsions. Journal of Biomaterials and Nanobiotechnology, 6, 64-79. 10.4236/jbnb.2015.61007.
- Lavanya, M. (2024). Rhamnolipids: an insight to the overall characteristics of these extraordinary biomolecules. Green Chemistry Letters and Reviews, 17(1). https://doi.org/10.1080/17518253.2024.2371012
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