To determine its appropriate application, the surfactant is being ranked by its
hydrophilic-lipophilic balance (HLB) value which is a common term used in industry and reflects the degree to which a surfactant is hydrophilic or lipophilic.
Surfactants with low HLB (< 10) are more lipophilic, thus more oil (fat) soluble,
and create water-in-oil emulsions, whereas HLB values over 10 indicate predominantly hydrophilic molecules with better water solubility that stabilize oil-in-water
emulsions. A value of 10 corresponds to a compound with equal affinity for water
and oil; complete water solubility of a surfactant occurs at HLB of ~7.3 (Burch et al.
2010; Rami et al. 2017). In cleaning applications, high HLB scale valued surfactants
are often used to emulsify oily soils (Király 2013).
Based on their molecular weight, biosurfactants can be classified as (1) low
molecular weight (low-mass) molecules including glycolipids, phospholipids and
lipopeptides efficient in lowering surface and interfacial tension and (2) high molecular weight (high-mass) polymers such as amphipathic polysaccharides, proteins,
lipopolysaccharides, lipoproteins or their complex mixtures that are more effective at
stabilizing oil-in-water emulsions (Rosenberg and Ron 1999; Pacwa-Płociniczak
et al. 2011).
5.4.1 Rhamnolipids
Rhamnose lipids, or rhamnolipids, are glycolipids often described as ‘the beststudied’ representatives of bacterial surfactants (e.g. Müller et al. 2012; Thiengmag
et al. 2016; Liu et al. 2018). The evolution of sensitive analytical techniques has
enabled great progress to be made in the identification of a wide diversity of
rhamnolipid congeners and homologues (about 60) that are produced at different
concentrations by bacterial species mainly from strains of Pseudomonas and
Burkholderia (Abdel-Mawgoud et al. 2010). Because of their anionic structure,
rhamnolipids have been proposed as a suitable material for the removal of some
heavy metals present in their cationic form, such as Cd
2+ , Pb
2+ , Zn
2+ , Ni
2+ , Ba
2+ and
Sr
2+ from soil and water (Miller 1995; Elouzi et al. 2012). Due to their excellent
emulsification properties, rhamnolipids also boost remediation of crude
oil-contaminated sites. Their addition in the soil enhances bacterial colonization,
growth, survival and metabolic activity (Rahman et al. 2003; Costa et al. 2010;
Tahseen et al. 2016) and helps microorganisms with uptake and assimilation of
many aliphatic hydrocarbons such as linear alkanes, which are poorly soluble in
water but provide a good nutrient source for P. aeruginosa (Hommel 1994).
Promising results have also been reported for the removal of PAHs, which are
recognized as being carcinogenic and hard to degrade environmental pollutants
(Makkar and Rockne 2003; Yu et al. 2011). PAHs make up about 85% of the
composition of creosote, which is besides copper chromium arsenate and pentachlorophenol a major chemical used in the wood-preserving industry worldwide.
Bruguera et al. (2017) focused their research on areas around wood preservation
facilities that are often highly contaminated due to practices such as the use of
unlined wood treatment cells and the disposal of wastes in unlined lagoons, and
150
L. Nemček and I. Hagarová
hydrophilic-lipophilic balance (HLB) value which is a common term used in industry and reflects the degree to which a surfactant is hydrophilic or lipophilic.
Surfactants with low HLB (< 10) are more lipophilic, thus more oil (fat) soluble,
and create water-in-oil emulsions, whereas HLB values over 10 indicate predominantly hydrophilic molecules with better water solubility that stabilize oil-in-water
emulsions. A value of 10 corresponds to a compound with equal affinity for water
and oil; complete water solubility of a surfactant occurs at HLB of ~7.3 (Burch et al.
2010; Rami et al. 2017). In cleaning applications, high HLB scale valued surfactants
are often used to emulsify oily soils (Király 2013).
Based on their molecular weight, biosurfactants can be classified as (1) low
molecular weight (low-mass) molecules including glycolipids, phospholipids and
lipopeptides efficient in lowering surface and interfacial tension and (2) high molecular weight (high-mass) polymers such as amphipathic polysaccharides, proteins,
lipopolysaccharides, lipoproteins or their complex mixtures that are more effective at
stabilizing oil-in-water emulsions (Rosenberg and Ron 1999; Pacwa-Płociniczak
et al. 2011).
5.4.1 Rhamnolipids
Rhamnose lipids, or rhamnolipids, are glycolipids often described as ‘the beststudied’ representatives of bacterial surfactants (e.g. Müller et al. 2012; Thiengmag
et al. 2016; Liu et al. 2018). The evolution of sensitive analytical techniques has
enabled great progress to be made in the identification of a wide diversity of
rhamnolipid congeners and homologues (about 60) that are produced at different
concentrations by bacterial species mainly from strains of Pseudomonas and
Burkholderia (Abdel-Mawgoud et al. 2010). Because of their anionic structure,
rhamnolipids have been proposed as a suitable material for the removal of some
heavy metals present in their cationic form, such as Cd
2+ , Pb
2+ , Zn
2+ , Ni
2+ , Ba
2+ and
Sr
2+ from soil and water (Miller 1995; Elouzi et al. 2012). Due to their excellent
emulsification properties, rhamnolipids also boost remediation of crude
oil-contaminated sites. Their addition in the soil enhances bacterial colonization,
growth, survival and metabolic activity (Rahman et al. 2003; Costa et al. 2010;
Tahseen et al. 2016) and helps microorganisms with uptake and assimilation of
many aliphatic hydrocarbons such as linear alkanes, which are poorly soluble in
water but provide a good nutrient source for P. aeruginosa (Hommel 1994).
Promising results have also been reported for the removal of PAHs, which are
recognized as being carcinogenic and hard to degrade environmental pollutants
(Makkar and Rockne 2003; Yu et al. 2011). PAHs make up about 85% of the
composition of creosote, which is besides copper chromium arsenate and pentachlorophenol a major chemical used in the wood-preserving industry worldwide.
Bruguera et al. (2017) focused their research on areas around wood preservation
facilities that are often highly contaminated due to practices such as the use of
unlined wood treatment cells and the disposal of wastes in unlined lagoons, and
150
L. Nemček and I. Hagarová