title: Self-assembly of a surfactin nanolayer at solid-liquid and air-liquid interfaces.
abstract: Surfactin, a sustainable and environmentally friendly surface active agent, is used as a model to study the adsorption of biosurfactants at hydrophobic and hydrophilic solid-liquid interfaces as well as the air-liquid interface. Surfactin adsorption was monitored as a function of time and concentration using surface plasmon resonance (SPR) technique in the case of the solid-liquid interfaces or the drop shape analysis (DSA) technique in the case of the air-liquid interface. The results obtained in this study showed that surfactin adsorption at the "hard" hydrophobic (functionalized with octadecanethiol) solid-liquid and the "soft" air-liquid interface were 1.12 ± 0.01 mg m(-2) (area per molecule of 157 ± 2 Å(2)) and 1.11 ± 0.05 mg m(-2) (area per molecule of 159 ± 7 Å(2)), respectively, demonstrating the negligible effect of the interface "hardness" on surfactin adsorption. The adsorption of surfactin at the hydrophilic (functionalized with β-mercaptoethanol) solid-liquid interface was about threefold lower than its adsorption at the hydrophobic-liquid interfaces, revealing the importance of hydrophobic interaction in surfactin adsorption process. The affinity constant of surfactin for the investigated interfaces follows the following order: air > octadecanethiol > β-mercaptoethanol. Biosurfactants, such as surfactin, are expected to replace the conventional fossil-based surfactants in several applications, and therefore the current study is a contribution towards the fundamental understanding of biosurfactant behavior, on a molecular level, at hydrophobic and hydrophilic solid-liquid interfaces in addition to the air-liquid interface. Such understanding might aid further optimization of the utilization of surfactin in a number of industrial applications such as enhanced oil recovery, bioremediation, and detergency.
authors.affiliation: School of Chemical Engineering and Advanced Materials, Newcastle University, 537 Clementi Road, #06-01, Singapore, 599493, Singapore. sagheer.onaizi@ncl.ac.uk.|School of Chemical Engineering and Advanced Materials, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK. sagheer.onaizi@ncl.ac.uk.
authors.forename: Sagheer A
authors.initials: SA
authors.lastname: Onaizi
authors.affiliation: Gas Processing Center, College of Engineering, Qatar University, PO Box 2713, Doha, Qatar.
authors.forename: M S
authors.initials: MS
authors.lastname: Nasser
authors.affiliation: School of Chemical Engineering and Advanced Materials, Newcastle University, 537 Clementi Road, #06-01, Singapore, 599493, Singapore.|School of Chemical Engineering and Advanced Materials, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK.
authors.forename: Nasir M A
authors.initials: NM
authors.lastname: Al-Lagtah
mesh_terms: D000327:Adsorption; D000388:Air; D057927:Hydrophobic and Hydrophilic Interactions; D055666:Lipopeptides / Q000737:chemistry*; D010456:Peptides, Cyclic / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D055666:Lipopeptides; D010456:Peptides, Cyclic; C009365:surfactin peptide
keywords: Adsorption; Hydrophilic; Hydrophobic; Surface plasmon resonance (SPR); Surface tension; Surfactin