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.

Tensiometry and dilational rheology of mixed β-lactoglobulin/ionic surfactant adsorption layers at water/air and water/hexane interfaces

“…1a, and A2a show a progressive increase in the surface pressure with increasing concentration C SDS > 5 Â 10 À5 M (R S/P > 50) which is consistent with a continuous surface accumulation of SDS molecules and replacement of protein material from the surface [15,16,27,36]. In this C SDS range the competitive co-adsorption of free SDS and SDS/BLG complexes is well pronounced even in presence of higher protein concentrations, such as C BLG = 1.5 Â 10 À5 M (R S/P > 3.3) [42], C BLG = 5.45 Â 10 À5 M (R S/P > 0.9) [39] and C BLG P 7 Â 10 À4 M (R S/P > 0.07) [40]. Thus complexes with different degrees of increased hydrophilicity [41], respectively lower surface activity, are replaced by SDS.…”

“…Binding studies revealed a dynamic equilibrium between complexes and free surfactant molecules in the bulk solution which has been estimated by measuring the concentration of the free surfactant molecules as a function of the binding ratio C S /C BLG , and for SDS/BLG complexes at pH 7 the maximum ratio was estimated to be about 1 in presence of 6 mM free surfactant molecules [40]. Note, that appearance of SDS molecules at the W/A surface of a mixed SDS/BLG solution at pH 6.7 has been detected by SFG spectroscopy for R S/P > 1.7, C SDS > 2.55 Â 10 À5 M at constant C BLG = 1.5 Â 10 À5 M [42]. The competitive adsorption of free surfactant molecules and complexes is evidenced by the fact that increasing R S/P in the range 1-50 leads to (i) a continuous decrease of the dilational modulus |E| as demonstrated by the data in Figs.…”

“…For SDS/BLG solutions at pH < pI the protein net charge is positive and the electrostatic interaction with SDS anions leads to stronger binding of surfactant and formation of hydrophobic complexes [41] of higher surface activity [40,42]. Surface charge reversal and maximum ellipsometric layer thickness have been detected in the range of R S/P between 2 and 10, C BLG = 1.5 Â 10 À5 M [42] which can be attributed to multilayer adsorption of neutral SDS/ BLG complexes.…”

“…1a and A2a, the tensiometry detected a slight increase of P of about 2 mN/m in the region C SDS < 5 Â 10 À5 M [15,27] which corresponds to surfactant/protein ratios R S/P < 50 (in the present work a constant protein concentration of 10 À6 M was maintained for all experiments). In this R S/P range the SDS molecules bind to BLG in the solution bulk inducing protein conformational changes (partial unfolding) and formation of SDS/BLG complexes as detected by binding studies [40], spectroscopy and calorimetry measurements [40][41][42]. Two conformational changes of the complex at pH 6.3 have been revealed by UV-vis and fluorescence spectroscopy at R S/P of 10 and 60, respectively [41].…”

“…Two conformational changes of the complex at pH 6.3 have been revealed by UV-vis and fluorescence spectroscopy at R S/P of 10 and 60, respectively [41]. At pH > pI (pI % 5.2 is the isoelectric point of BLG in aqueous solution) the BLG globules carry a negative net charge and the interaction with the anionic SDS is dominated by hydrophobic forces rather than electrostatic coupling [41,42]. Therefore, the surface activity of the SDS/BLG complex decreases due to hydrophilization by hydrophobically bound surfactant anions.…”