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Drop profile analysis tensiometry is applied to measure the dilational visco-elasticity of BLG at the buffered aqueous solution/tetradecane (W/TD) interface using oscillating drops of TD immersed in W at frequencies between 0.01–0.2 Hz. The buffered solutions were investigated at pH 3, pH 5 (isoelectric point) and pH 7 at different buffer concentrations (1 mM, 10 mM and 100 mM). The real part of the complex visco-elasticity shows a maximum when plotted as a function of the interfacial pressure Π. In contrast to the water/air surface (W/A) where we observe maximum elasticity values between 15 and 20 mN/m, at the W/TD interface these maximum values are up to 65–70 mM/m, which is in parallel with the much higher interfacial pressure values at the W/TD interface when compared to the W/A surface. Refereed/Peer-reviewed

The adsorption kinetics of beta-lactoglobulin (BLG) at the water/tetradecane (W/TD) interface as studied by drop profile analysis tensiometry is significantly controlled by the diffusional transport in the aqueous solution bulk. However, due to the contact with the hydrophobic oil phase the protein molecules change their conformation in order to adapt to the interfacial environment. This conformational change can be expressed via the adsorption activity constant. The analysis of the dynamic interfacial tensions leads to much lower activities at short adsorption times and low surface coverages. This allows to conclude that in the early stage of the adsorption layer formation the structure of the BLG adsorption layer at the W/TD interface is similar to that at the water/air (W/A) interface. Refereed/Peer-reviewed

The dynamic surface pressure II of air bubbles aging in buffered beta-lactoglobulin (BLG) solutions containing various protein concentrations C-BLG (10(-9)-10(-4)M), pH (3-7) and buffer concentrations C-buff (1, 10 and 100 mM) was measured as a function of time t by bubble profile analysis tensiometry. Adsorption kinetics was studied by recording II(t) data for 80 000 s and the final II-values were used to construct the surface pressure isotherm II(C-BLG) for the pH values of 3, 5, 7 (C-buff = 10 mM) and 6.3 (in pure water). On the basis of obtained kinetic and concentration dependencies the effect of pH on the protein surface activity was qualitatively analyzed. At constant Cbuff =10 mM and relatively low protein concentrations (C-BLG10(-6) M) BLG is the most surface active at pH 5 (negligible net charge in the vicinity of the isoelectric point, pI approximate to 5.1). At constant C-buff, BLG solutions with pH 3 show the lowest surface activity. The influence of the ionic strength (buffer concentration) of the solution on the dynamic II was studied at a selected protein concentration of C-BLG =10(-5) M. The kinetics of adsorption is weakly affected by C-buff for solutions with pH 5 and significantly enhanced by increasing C-buff for solutions with pH not equal pI. The effect of the chemical nature of the electrolyte was obtained for non-buffered aqueous BLG solutions (natural pH similar to 6.3) at different concentrations of NaCI or CaCI2. It is demonstrated that the adsorption kinetics and the surface pressure isotherm of BLG at the water/air surface depend strongly on the protein effective charge, which is dictated by the solution pH and ionic strength. Refereed/Peer-reviewed

The local velocity profile (LVP) of a rising bubble can serve as a fingerprint for the dynamic behavior of the adsorption/desorption processes going on at a solution/air interface. The LVP of bubbles in β-lactoglobulin (BLG) solutions proves to be extremely sensitive for the adsorption of BLG at low bulk concentrations. In addition, it can show the impact of pH and ionic strength on the dynamic surface properties of BLG at the solution/air interface. It is observed that the time for establishing an immobile rigid surface layer at the rising bubble surface becomes shorter with increasing pH from 3 to 7. A peculiar behavior is observed at the isoelectric point (IEP) where the LVPs show irregularities. Under dynamic conditions BLG does not exhibit highest surface active at the IEP within the range of BLG concentrations as measured by the rising bubble technique. Refereed/Peer-reviewed

The recently described new experimental tool called Drop and Bubble Micro Manipulator (DBMM) has been applied to studies of the stability of protein covered bubbles against coalescence. Among the available protocols the time interval between the moment of bubble contact to the moment of coalescence of two contacting bubble is determined for different β-Lactoglobulin (BLG) concentrations and different ageing times. A threshold concentration of BLG is needed to prevent immediate coalescence. The longer the ageing time above this threshold concentration of BLG is the longer is the lifetime of the liquid film between the bubbles. The adsorbed amount of BLG was determined as a function of time and concentration and correlates with the observed coalescence behaviour of the contacting bubbles. Refereed/Peer-reviewed

Liquid interfaces are met everywhere in our daily life. The corresponding interfacial properties and their modification play an important role in many modern technologies. Most prominent examples are all processes involved in the formation of foams and emulsions, as they are based on a fast creation of new surfaces, often of an immense extension. During the formation of an emulsion, for example, all freshly created and already existing interfaces are permanently subject to all types of deformation. This clearly entails the need of a quantitative knowledge on relevant dynamic interfacial properties and their changes under conditions pertinent to the technological processes. We report on the state of the art of interfacial layer characterization, including the determination of thermodynamic quantities as base line for a further quantitative analysis of the more important dynamic interfacial characteristics. Main focus of the presented work is on the experimental possibilities available at present to gain dynamic interfacial parameters, such as interfacial tensions, adsorbed amounts, interfacial composition, visco-elastic parameters, at shortest available surface ages and fastest possible interfacial perturbations. The experimental opportunities are presented along with examples for selected systems and theoretical models for a best data analysis. We also report on simulation results and concepts of necessary refinements and developments in this important field of interfacial dynamics. Refereed/Peer-reviewed

Two oppositely charged surfactants in an aqueous solution interact with each other via electrostatic interactions and produce surfactant complexes-catanionics. However, their quantitative influence on the interfacial tension at the solution/air and solution/oil interfaces is still unknown. We have measured the interfacial tension of sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium bromide (DoTAB) aqueous mixtures using the du Nouy ring tensiometer and drop profile analysis tensiometry (PAT-1). For the oil phase, we used hexane. The obtained kinetic and equilibrium experimental results were fitted with the common theoretical Frumkin adsorption model. We found that SDS + DoTAB complexes show much higher surface activity than the single surfactants. At solution/air and solution/oil interfaces, SDS and DoTAB produce ion pairs with low water affinity and relatively low oil solubility. SDS + DoTAB partition coefficient between hexane and water phases is 0.32. Such surfactant complexes with advanced surface characteristics may be beneficial in different industrial branches nowadays. Refereed/Peer-reviewed

Protein adsorption is one of the most complex and intriguing phenomenon in interfacial science. This phenomenon has been addressed in this paper for a short time range using maximum bubble pressure tensiometer. This study presents the issue of negative surface pressure of proteins which has been much of a speculation, with the thermodynamic model. Here beta-lactoglobulin has been studied at different pHs and the experimental results agree with the theoretical model presented. Refereed/Peer-reviewed