title: Mixed layers of β-lactoglobulin and SDS at air-water interfaces with tunable intermolecular interactions.
abstract: Mixtures of β-lactoglobulin (BLG) and sodium dodecyl sulfate (SDS) were studied at pH 3.8 and 6.7 under equilibrium conditions. At these pH conditions, BLG carries either a positive or a negative net charge, respectively, which enables tunable electrostatic interactions between anionic SDS surfactants and BLG proteins. For pH 3.8, vibrational sum-frequency generation (SFG) and ellipsometry indicate strong BLG-SDS complex formation at air-water interfaces that is caused by attractive electrostatic interactions. The latter complexes are already formed in the bulk solution which was confirmed by a thermodynamic study of BLG-SDS mixtures using isothermal titration calorimetry (ITC). For acidic conditions we determine from our ITC data an exothermal binding enthalpy of -40 kJ mol(-1). Increasing SDS/BLG molar ratios above 10 leads to a surface excess of SDS and thus to a charge reversal from a positive net charge with BLG as the dominating surface adsorbed species to a negatively charged layer with SDS as the dominating surface species. The latter is evidenced by a pronounced minimum in SFG intensities that is also accompanied by a phase change of O-H stretching bands due to a reorientation of H2O within the local electric field. This phase change which occurs at SDS/BLG molar ratio between 1 and 10 causes a polarity change in SFG intensities from BLG aromatic C-H stretching vibrations. Conclusions from SFG spectra are corroborated by ellipsometry which shows a dramatic increase in layer thicknesses at molar ratios where a charge reversal occurs. The formation of interfacial multilayers comprising SDS-BLG complexes is, thus, caused by cancellation of electrostatic interactions which leads to agglomeration at the interface. In contrast to pH 3.8, behavior of BLG-SDS mixtures at pH 6.7 is different due to repulsive electrostatic interactions between SDS and BLG which lead to a significantly reduced binding enthalpy of -17 kJ mol(-1). Finally, it has to be mentioned that SFG spectra show a coexistence of BLG and SDS molecules at the interface for BLG-SDS molar ratios > 2.
authors.affiliation: Institute of Particle Technology (LFG), University of Erlangen-Nuremberg , Cauerstrasse 4, 91058 Erlangen, Germany.
authors.forename: Kathrin
authors.initials: K
authors.lastname: Engelhardt
authors.forename: Ulrike
authors.initials: U
authors.lastname: Weichsel
authors.forename: Elena
authors.initials: E
authors.lastname: Kraft
authors.forename: Doris
authors.initials: D
authors.lastname: Segets
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Peukert
authors.forename: Björn
authors.initials: B
authors.lastname: Braunschweig
mesh_terms: D000388:Air; D006863:Hydrogen-Ion Concentration; D007782:Lactoglobulins / Q000737:chemistry*; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry*; D013500:Surface Tension; D014867:Water / Q000737:chemistry*
chemical_list: D007782:Lactoglobulins; D014867:Water; D012967:Sodium Dodecyl Sulfate