title: Effect of hydration of sugar groups on adsorption of Quillaja bark saponin at air/water and Si/water interfaces.
abstract: Adsorption of a natural glycoside surfactant Quillaja bark saponin ("QBS", Sigma Aldrich 84510) was studied at the air/water and Si/water interfaces using a combination of surface pressure (SP), surface dilatational rheology, neutron reflectivity (NR), Infra-Red Attenuated Total Reflection Spectroscopy (IR ATR) and Quartz Crystal Microbalance (QCM). The adsorbed layers formed at the air/water interface are predominantly elastic, with the dilatational surface storage modulus reaching the maximum value of E'=184 mN/m. The NR results point to a strong hydration of the adsorbed layers (about 65% hydration, corresponding to about 60 molecules of water per one QBS molecule), most likely related to the presence of multiple sugar groups constituting the glycone part of the QBS molecules. With a layer thickness of 19 Å, the adsorbed amount obtained from NR seems largely underestimated in comparison to the value obtained from the surface tension isotherm. While this high extent of hydration does not prevent formation of dense and highly elastic layers at the air-water surface, QBS adsorption at the Si/water interface is much weaker. The adsorption isotherm of QBS on Si obtained from the QCM study reflects much lower affinity of highly hydrated and negatively charged saponin molecules to the Si/water interface. We postulate that at the air/water interface, QBS adsorbs through the triterpene aglycone moiety. In contrast, weak hydrogen bonding between the glycone part and the surface silanol groups of Si is responsible for QBS adsorption on more polar Si/water interface.
authors.affiliation: Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland. Electronic address: kamil.wojciechowski@ch.pw.edu.pl.
authors.forename: Kamil
authors.initials: K
authors.lastname: Wojciechowski
authors.affiliation: Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland.
authors.forename: Marta
authors.initials: M
authors.lastname: Orczyk
authors.affiliation: Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland.
authors.forename: Kuba
authors.initials: K
authors.lastname: Marcinkowski
authors.affiliation: Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland.
authors.forename: Tomasz
authors.initials: T
authors.lastname: Kobiela
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Marcus
authors.initials: M
authors.lastname: Trapp
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Gutberlet
authors.affiliation: Laboratory for Neutron Scattering, Paul Scherrer Institute, WHGA/110, 5232 Villigen - PSI, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Geue
mesh_terms: D000327:Adsorption; D000388:Air; D002241:Carbohydrates / Q000737:chemistry*; D009502:Neutrons; D024301:Plant Bark / Q000737:chemistry*; D011312:Pressure; D058525:Quartz Crystal Microbalance Techniques; D031990:Quillaja / Q000737:chemistry*; D012212:Rheology; D012503:Saponins / Q000737:chemistry*; D012825:Silicon / Q000737:chemistry*; D012996:Solutions; D013499:Surface Properties; D014867:Water / Q000737:chemistry*
chemical_list: D002241:Carbohydrates; D012503:Saponins; D012996:Solutions; D014867:Water; D012825:Silicon
keywords: Biosurfactant; Dilatational surface rheology; Neutron reflectivity; QBS; Quartz crystal microbalance; Surface pressure