Rhamnolipids are class of glycolipid. They are constructed of rhamnose combined with beta-hydroxy fatty acids. Rhamnose is a sugar. Fatty acids are ubiquitous in animals and plants. The carboxyl end of the fatty acid end is connected to the rhamnose. Rhamnolipids are compounds of only 3 common elements; carbon, hydrogen, and oxygen. They are a crystalline acid.
There are two major groups of rhamnolipids; mono-rhamnolipids and di-rhamnolipids.
Mono-rhamnolipids have a single rhamnose sugar ring.
The basic formula (which is most often produced by P. aeruginosa) is:
It is often referred to as Rha-C10-C10 with a formula of C26H48O9
Di-rhamnolipids have two rhamnose sugar rings.
The basic formula for the most common form is:
It is often referred to as Rha-Rha-C10-C10 with a formula of C32H58O13
A more exhaustive discussion of the formula and naming conventions for rhamnolipids is on the rhamnolipids chemistry page.
One property of rhamnolipids is that they are surfactants. One end of the molecule is hydrophobic and lipophilic. The other end is hydrophilic. The rhamnose end is hydrophilic which means it will attract water and other polar molecules. The fatty acid will attract other oils and fats and non-polar solvents. This ability to attract both polar and non-polar molecules allows the mixing of two immiscible liquids such as oil and water. Such compounds are called amphiphilic compounds or amphiphiles.
For rhamnolipids, the carboxylate makes it anionic, having a negative charge. The negative charge contributes to increasing the rhamnolipids effectiveness as a surfactant.
A test for the presence of anionic surfactants is called the CTAB methylene blue test, which uses the cationic tenside cetyltrimethylammonium bromide and the basic dye methylene blue and an agar growth medium.
Surface Tension is the measure of the cohesion of a liquid to itself at the surface. At the surface where the liquid is in contact in another solid, liquid, or gas this surface tension can be measured. The unit of measure for surface tension is dyne/cm or equivalently milliNewton/meter. It is a measure of force/length
The term surfactant comes from the phrase ‘surface acting agent’ because the surfactant alters the physics of the surface of the liquid containing it. It increases the absorption of the other materials at the surface. This absorption facilitates the mixing of the two materials. The lower the surface tension the faster the other material is absorbed.
Rhamnolipids reduce the surface tension significantly, more than cutting it in half. With rhamnolipids, the surface tension of water can be reduced from a normal value of around 72 mN/m to about 30 mN/m or less.
Rhamnolipids are good at mixing fats and water to form an emulsion. In this case it acts as an emulsifier. Because rhamnolipids have both hydrophilic and hydrophobic (lipophilic) ends and are anionic, they tend to migrate to the surface of the liquid, where they are effective at drawing in lipids. Experiments suggest that the rhamnolipids with two shorter fatty acids are more active in reducing surface tension and as an emulsifier. Those rare rhamnolipids with a single fatty acid chain are not as effective.
An indicator of how effective a compound is at being an emulsifier is thehydrophilic-lipophilic balance (HLB), which measures the relative strength of the hydrophilic or lipophilic attractions. The general calculation is HLB = 20 x Mh / M where Mh is the molecular mass of the hydrophilic part of the molecule and M is the total molecular mass of the molecule. Unfortunately this formula is not a good predictor for ionic surfactants such as the anionic rhamnolipids. More advance formulae take into consideration the strength and numbers of the hydrophilic and lipophilic groups in the molecule by summing the strength of the hydrophilic groups in the molecule and subtracting the sum of the strengths of the lipophilic groups in the molecule.
Because rhamnolipids are a surfactant with both hydrophilic end and a lipophilic end, the one end is repulsed and the molecule migrates to the surface to reduce the energy state. When the concentration increases to a certain level, the rhamnolipids join together inside the liquid in a micelle. In the case of water, the rhamnolipid micelle is formed such that hydrophilic ends on the outside of the micelle protect the hydrophobic ends on the inside, thereby reducing the overall energy state.
In order to let the non-polar molecules freely distribute in a polar liquid, the amphiphile (e.g. rhamnolipid) needs to surround the non-polar modules aligned in such a manner that the lipophilic end is toward the non-polar molecules in the center and the hydrophilic end is on the outside. This clump of molecules allows the clump to behave in a polar manner. This clump is called a liposome, which is much like a micelle, but contains lipids in the middle.
The same behavior is also observed in a non-polar liquid containing polar molecules. Such micelles are called inverse micelles.
The minimum concentration of surfactant or amphiphile in the liquid that allows micelles to be formed is called the critical micelle concentration(CMC). The CMC is a measure of how effective the surfactant is in emulsifying and the minimum amount of surfactant necessary to form micelles in an emulsion. At the CMC the surface tension generally reaches a minimum and relatively constant.
Because the bonds forming the micelles are weak, the micelles are constantly forming and reforming. They can be affected by temperature, pressure, and even mechanical stresses as they form, break apart, and reform.
The formation of micelles affects the liquid’s properties. The formation of micelles can be detected by the change in color, transparency and pH of the liquid as well as slight changes in temperature as energy is given of or used by the formation and breaking apart of the micelles. In some studies the CMC of rhamnolipids has been measured in the range of 24-65 mg/L
The ability to mix the two immiscible materials is quantified as the solubilization capacity. The solubilization capacity is sometimes expressed as a ratio called the molar solubilization ratio (MSR) or rarely mass solubilization ratio. It is the ratio of the number of moles of the material being solubilized over the number of moles of the surfactant used. It is determined easiest at the CMC. Empirically it has been determined that the MSR for rhamnolipids is primarily dependent on the chemical being emulsified and the pH. There are a large number of studies that have determine the MSR for various rhamnolipids and various chemicals, with the general result that rhamnolipids are excellent solubiliziers.
In general surfactants such as rhamnolipids can emulsify or solubilize oil, meaning hydrocarbons, in water (O/W) or water in hydrocarbons (W/O). The HLB can be measured and therefore indicate for each rhamnolipid compound whether is it is more thermodynamically stable at O/W or W/O solubilization.
This emulsifying or solubilizing ability has tremendous commercial application such as soaps and detergents.
The migration of the rhamnolipids to the surface where they orient themselves with their hydrophobic end to the surface helps them attach to the surface of the adjacent material. This is called adhesion. This means that rhamnolipids can be used as a wetting agent to distribute other chemicals on the surface of another object such as a plant.
Surfactants are foaming agents because they tend to inhibit the coalescence of bubbles. Foams are only generated when there is enough excess surfactant at the surface of the liquid.
For instance, surfactant molecules emulsifying fats will not be available to foam. This is the reason that people add more soap when the foam is gone while washing dishes. If there is no foam then it is very likely there is more fat and oil that needs to be emulsified, or in the vernacular, dissolved.
Because of the rhamnolipids are produced in various chemical formulas, each with a different HLB, rhamnolipids can be produced or mixed to have a range of various foaming properties.