Characterization, Production, and Applications of Lipopeptides - Biosurfactants: Research Trends and Applications - page 148

Surfactants are classified as cationic, anionic, zwitterionic, and nonionic and are
made synthetically from hydrocarbons, lignosulfonates, or triglycerides. Some com-
mon synthetic surfactants include linear alkyl benzenesulfonates, alcohol sulfates,
alcohol ether sulfates, alcohol glyceryl ether sulfonates, α-olefin sulfonates, alcohol
ethoxylates, and alkylphenol ethoxylates (Layman, 1985). Surfactants have many
applications industrially with multiphasic systems. Sodium dodecyl sulfate (SDS,
C 12 H 25 -SO 4 − Na + ) is a widely used anionic surfactant. The effectiveness of a surfac-
tant is determined by surface tension lowering, which is a measure of the surface free
energy per unit area or the work required to bring a molecule from the bulk phase to
the surface (Rosen, 1978). These amphiphilic compounds (containing hydrophobic
and hydrophilic portions) concentrate at solid-liquid, liquid-liquid, or vapor-liquid
interfaces. An interfacial boundary exists between two immiscible phases. The
hydrophobic portion concentrates at the surface while the hydrophilic is oriented
toward the solution. A good surfactant can lower the surface tension of water from 72
to 35 mN/m and the interfacial tension (tension between nonpolar and polar liquids)
for water against n-hexadecane from 40 to 1 mN/m. Efficient surfactants have a
low CMC (i.e., less surfactant is necessary to decrease the surface tension) as the
CMC is defined as the minimum concentration necessary to initiate micelle forma-
tion (Becher, 1965). In practice, the CMC is also the maximum concentration of sur-
factant monomers in water and is influenced by pH, temperature, and ionic strength.
An important factor in the choice of surfactant is the product cost (Mulligan and
Gibbs, 1993). In general, surfactants are used to save energy and consequently energy
costs (such as the energy required for pumping or mixing). Charge type, physico-
chemical behavior, solubility, and adsorption behavior are some important selection
criteria for surfactants.
Some surfactants, known as biosurfactants, are biologically produced from yeast
or bacteria (Lin, 1996). They can be potentially as effective with some distinct
advantages over the highly used synthetic surfactants due to high specificity, biode-
gradability, and biocompatibility (Cooper, 1986).
Biosurfactants are grouped as glycolipids, lipopeptides, phospholipids, fatty acids,
and neutral lipids (Bierman et al., 1987). Most of these compounds are either anionic
or neutral, with only a few cationic ones. The hydrophobic parts of the molecule are
based on long-chain fatty acids, hydroxy fatty acids, or α-alkyl-β-hydroxy fatty acids.
The hydrophilic portion can be a carbohydrate, amino acid, cyclic peptide, phosphate,
carboxylic acid, or alcohol. A wide variety of microorganisms can produce these
compounds. The CMCs of the biosurfactants generally range from 1 to 200 mg/L
and their molecular weights (MWs) from 500 to 1500 amu (Lang and Wagner, 1987).
LIPOPEPTIDE BIOSURFACTANTS
Lipopeptides are produced by a variety of microorganisms, including Bacillus ,
Lactobacillus , Streptomyces , Pseudomonas , and Serratia (Cameotra and Makkar,
2004; Georgiou et al., 1992). The lipopeptides are cyclic peptides with a fatty acyl
chain. Various lipopeptides include surfactin (Roongsawang et al., 2003; Youssef
et al., 2007), lichenysin A (Yakimov et al., 1995) or C (Jenny et al., 1991), B (Folmsbee
et al., 2006), D (Zhao et al., 2010), bacillomycin (Roongsawang et al., 2003), fengycin