Journal of colloid and interface science, Jan 24, 2015
Oscillating drop tensiometry was applied to study adsorbed interfacial layers at water/air and wa... more Oscillating drop tensiometry was applied to study adsorbed interfacial layers at water/air and water/hexane interfaces formed from mixed solutions of β-lactoglobulin (BLG, 1μM in 10mM buffer, pH 7 - negative net charge) and the anionic surfactant SDS or the cationic DoTAB. The interfacial pressure Π and the dilational viscoelasticity modulus |E| of the mixed layers were measured for mixtures of varying surfactant concentrations. The double capillary technique was employed which enables exchange of the protein solution in the drop bulk by surfactant solution (sequential adsorption) or by pure buffer (washing out). The first protocol allows probing the influence of the surfactant on a pre-adsorbed protein layer thus studying the protein/surfactant interactions at the interface. The second protocol gives access to the residual values of Π and |E| measured after the washing out procedure thus bringing information about the process of protein desorption. The DoTAB/BLG complexes exhibit h...
Advances in colloid and interface science, Jan 12, 2014
On the surface of bubbles rising in a surfactant solution the adsorption process proceeds and lea... more On the surface of bubbles rising in a surfactant solution the adsorption process proceeds and leads to the formation of a so called Rear Stagnant Cap (RSC). The larger this RSC is the stronger is the retardation of the rising velocity. The theory of a steady RSC and steady retarded rising velocity, which sets in after a transient stage, has been generally accepted. However, a non-steady process of bubble rising starting from the initial zero velocity represents an important portion of the trajectory of rising, characterized by a local velocity profile (LVP). As there is no theory of RSC growth for large Reynolds numbers Re » 1 so far, the interpretation of LVPs measured in this regime was impossible. It turned out, that an analytical theory for a quasi-steady growth of RSC is possible for small Marangoni numbers Ma « 1, i.e. when the RSC is almost completely compressed, which means a uniform surface concentration Γ(θ)=Γ∞ within the RSC. Hence, the RSC angle ψ(t) is obtained as a fun...
ABSTRACT The recently described new experimental tool called Drop and Bubble Micro Manipulator (D... more ABSTRACT 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 beta-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. (c) 2013 Elsevier Ltd. All rights reserved.
ABSTRACT Protein adsorption is one of the most complex and intriguing phenomenon in interfacial s... more ABSTRACT 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. (c) 2013 Elsevier Ltd. All rights reserved.
Current Opinion in Colloid & Interface Science, 2013
ABSTRACT Mixed protein–surfactant adsorption layers at liquid interfaces are described including ... more ABSTRACT Mixed protein–surfactant adsorption layers at liquid interfaces are described including the thermodynamic basis, the adsorption kinetics and the shear and dilational interfacial rheology. It is shown that due to the protrusion of hydrophobic protein parts into the oil phase the adsorption layers at the water–hexane interface are stronger anchored as compared to the water-air surface. Based on the different adsorption protocols, a sequential and a simultaneous scheme, the peculiarities of complexes between proteins and added surfactants are shown when formed in the solution bulk or at a liquid interface. The picture drawn from adsorption studies is supported by the findings of interfacial rheology.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2010
ABSTRACT Oil-in-water emulsion films PEO-PPO-PEO copolymers Steric forces Electrostatic forces Br... more ABSTRACT Oil-in-water emulsion films PEO-PPO-PEO copolymers Steric forces Electrostatic forces Brush-to-brush steric forces Oil/water interface charge a b s t r a c t Oil-in-water emulsion films were used to investigate the origin of electrostatic and steric surface forces between two oil droplets in aqueous solution of nonionic polymeric surfactants. The Thin Liquid Film–Pressure Balance Technique was used. Equivalent thickness of oil-in-water emulsion films stabi-lized by PEO-PPO-PEO triblock copolymers (Pluronic ® F108 and P104) vs. pH was measured at constant capillary pressure and ionic strength. The isoelectric point at the negatively charged film interfaces, cor-responding to zero diffuse double layer potential was found. This demonstrates that the origin of the electrostatic repulsion in the emulsion films studied is associated to preferential adsorption of OH − ions. Disjoining pressure isotherms for the emulsion films obtained at different pH were fitted to de Gennes' scaling theory for steric interaction between polymer 'brush' layers with the 'brush' thickness as a fit-ting parameter. The fit obtained was satisfying and yielded PEO-'brush' thickness of 9.6 nm. These results allowed the conclusion that in absence of diffuse double layer forces the stabilization of oil-in-water emulsion films from PEO-PPO-PEO triblock copolymers was due to 'brush-to-brush' steric repulsion.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2014
ABSTRACT The presented drop and bubble micro manipulator (DBMM) is a new experimental tool for th... more ABSTRACT The presented drop and bubble micro manipulator (DBMM) is a new experimental tool for the quantitative analysis of the interaction between two drops or two bubbles or even between a single drop with a bubble in a liquid medium. For each drop or bubble, a coarse and a very fine dosing system allow controlling their size or changing it in respect to a given protocol. An accurate pressure sensor for each drop/bubble can record changes in the capillary pressure with a selected sampling rate in order to follow critical processes between the two liquid menisci, such as coalescence. The stability of liquid films between drops or bubbles can be studied also under external perturbations, i.e. a harmonic oscillation with a given frequency and amplitude can be generated in order to find critical conditions for the coalescence. The described tool can be used in different experimental protocols. Among them there are options to mimic the situation of multilayers and multiple emulsions. Recording of dynamic processes as video with a standard frame rate is possible as well. Alternatively, a fast video camera can be mounted to monitor fast processes, such as the coalescence of two drops or bubbles.
Journal of colloid and interface science, Jan 24, 2015
Oscillating drop tensiometry was applied to study adsorbed interfacial layers at water/air and wa... more Oscillating drop tensiometry was applied to study adsorbed interfacial layers at water/air and water/hexane interfaces formed from mixed solutions of β-lactoglobulin (BLG, 1μM in 10mM buffer, pH 7 - negative net charge) and the anionic surfactant SDS or the cationic DoTAB. The interfacial pressure Π and the dilational viscoelasticity modulus |E| of the mixed layers were measured for mixtures of varying surfactant concentrations. The double capillary technique was employed which enables exchange of the protein solution in the drop bulk by surfactant solution (sequential adsorption) or by pure buffer (washing out). The first protocol allows probing the influence of the surfactant on a pre-adsorbed protein layer thus studying the protein/surfactant interactions at the interface. The second protocol gives access to the residual values of Π and |E| measured after the washing out procedure thus bringing information about the process of protein desorption. The DoTAB/BLG complexes exhibit h...
Advances in colloid and interface science, Jan 12, 2014
On the surface of bubbles rising in a surfactant solution the adsorption process proceeds and lea... more On the surface of bubbles rising in a surfactant solution the adsorption process proceeds and leads to the formation of a so called Rear Stagnant Cap (RSC). The larger this RSC is the stronger is the retardation of the rising velocity. The theory of a steady RSC and steady retarded rising velocity, which sets in after a transient stage, has been generally accepted. However, a non-steady process of bubble rising starting from the initial zero velocity represents an important portion of the trajectory of rising, characterized by a local velocity profile (LVP). As there is no theory of RSC growth for large Reynolds numbers Re » 1 so far, the interpretation of LVPs measured in this regime was impossible. It turned out, that an analytical theory for a quasi-steady growth of RSC is possible for small Marangoni numbers Ma « 1, i.e. when the RSC is almost completely compressed, which means a uniform surface concentration Γ(θ)=Γ∞ within the RSC. Hence, the RSC angle ψ(t) is obtained as a fun...
ABSTRACT The recently described new experimental tool called Drop and Bubble Micro Manipulator (D... more ABSTRACT 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 beta-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. (c) 2013 Elsevier Ltd. All rights reserved.
ABSTRACT Protein adsorption is one of the most complex and intriguing phenomenon in interfacial s... more ABSTRACT 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. (c) 2013 Elsevier Ltd. All rights reserved.
Current Opinion in Colloid & Interface Science, 2013
ABSTRACT Mixed protein–surfactant adsorption layers at liquid interfaces are described including ... more ABSTRACT Mixed protein–surfactant adsorption layers at liquid interfaces are described including the thermodynamic basis, the adsorption kinetics and the shear and dilational interfacial rheology. It is shown that due to the protrusion of hydrophobic protein parts into the oil phase the adsorption layers at the water–hexane interface are stronger anchored as compared to the water-air surface. Based on the different adsorption protocols, a sequential and a simultaneous scheme, the peculiarities of complexes between proteins and added surfactants are shown when formed in the solution bulk or at a liquid interface. The picture drawn from adsorption studies is supported by the findings of interfacial rheology.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2010
ABSTRACT Oil-in-water emulsion films PEO-PPO-PEO copolymers Steric forces Electrostatic forces Br... more ABSTRACT Oil-in-water emulsion films PEO-PPO-PEO copolymers Steric forces Electrostatic forces Brush-to-brush steric forces Oil/water interface charge a b s t r a c t Oil-in-water emulsion films were used to investigate the origin of electrostatic and steric surface forces between two oil droplets in aqueous solution of nonionic polymeric surfactants. The Thin Liquid Film–Pressure Balance Technique was used. Equivalent thickness of oil-in-water emulsion films stabi-lized by PEO-PPO-PEO triblock copolymers (Pluronic ® F108 and P104) vs. pH was measured at constant capillary pressure and ionic strength. The isoelectric point at the negatively charged film interfaces, cor-responding to zero diffuse double layer potential was found. This demonstrates that the origin of the electrostatic repulsion in the emulsion films studied is associated to preferential adsorption of OH − ions. Disjoining pressure isotherms for the emulsion films obtained at different pH were fitted to de Gennes' scaling theory for steric interaction between polymer 'brush' layers with the 'brush' thickness as a fit-ting parameter. The fit obtained was satisfying and yielded PEO-'brush' thickness of 9.6 nm. These results allowed the conclusion that in absence of diffuse double layer forces the stabilization of oil-in-water emulsion films from PEO-PPO-PEO triblock copolymers was due to 'brush-to-brush' steric repulsion.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2014
ABSTRACT The presented drop and bubble micro manipulator (DBMM) is a new experimental tool for th... more ABSTRACT The presented drop and bubble micro manipulator (DBMM) is a new experimental tool for the quantitative analysis of the interaction between two drops or two bubbles or even between a single drop with a bubble in a liquid medium. For each drop or bubble, a coarse and a very fine dosing system allow controlling their size or changing it in respect to a given protocol. An accurate pressure sensor for each drop/bubble can record changes in the capillary pressure with a selected sampling rate in order to follow critical processes between the two liquid menisci, such as coalescence. The stability of liquid films between drops or bubbles can be studied also under external perturbations, i.e. a harmonic oscillation with a given frequency and amplitude can be generated in order to find critical conditions for the coalescence. The described tool can be used in different experimental protocols. Among them there are options to mimic the situation of multilayers and multiple emulsions. Recording of dynamic processes as video with a standard frame rate is possible as well. Alternatively, a fast video camera can be mounted to monitor fast processes, such as the coalescence of two drops or bubbles.
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Papers by Georgi G Gochev