Engelhardt K, Weichsel U, Kraft E, Segets D, Peukert W, Braunschweig B (2014)
Publication Type: Journal article
Publication year: 2014
Book Volume: 118
Pages Range: 4098-4105
Journal Issue: 15
DOI: 10.1021/jp501541q
Mixtures of \textgreekb-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.
APA:
Engelhardt, K., Weichsel, U., Kraft, E., Segets, D., Peukert, W., & Braunschweig, B. (2014). Mixed Layers of β-Lactoglobulin and SDS at Air-Water Interfaces with Tunable Intermolecular Interactions. Journal of Physical Chemistry B, 118(15), 4098-4105. https://dx.doi.org/10.1021/jp501541q
MLA:
Engelhardt, Kathrin, et al. "Mixed Layers of β-Lactoglobulin and SDS at Air-Water Interfaces with Tunable Intermolecular Interactions." Journal of Physical Chemistry B 118.15 (2014): 4098-4105.
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