Representative structures of synthetic and biosurfactants. (i) Anionic synthetic surface-active agent (SAA) sodium dodecyl sulphate (SDS); (ii) cationic synthetic SAA cetyltrimethyl ammonium bromide (CTAB); (iii) non-ionic synthetic SAA Tween 20 (polyoxyethylene sorbitan monolaurate) (Tween 40 = monopalmitate; Tween 60 = monostearate; Tween 80 = monooleate, the sum of alkyl chains a, b, c and d is 20 for all Tweens); (iv) Monorhamnolipid R1 with two C10 alkanoate chains; (v) Dirhamnolipid R2 with two C10 alkanoate chains; (vi) Acetylated lactonic sophorolipid; (vii) Acetylated acidic sophorolipid; (viii) Mannositylerithritol lipid (the length of the alkyl chains, n, can be between 6 and 10). For mannosityl erithritol lipid (MEL), A, R1 and R2 are acetyl groups; MEL B, R1 = acetyl, R2 = H; MEL C, R1 = H, R2 = acetyl.