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Nikolai D. Denkov - One of the best experts on this subject based on the ideXlab platform.
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role of surfactant adsorption and surface properties for the efficiency of pdms silica Antifoams
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2021Co-Authors: Nadya Politovabrinkova, Slavka Tcholakova, Nikolai D. Denkov, M Hristova, V Georgiev, M Grandl, F AchenbachAbstract:Abstract We study how the composition of various surfactant mixtures affects the efficiency of mixed PDMS–silica antifoam in foamed surfactant solutions. First, systematic experiments are performed to characterize the surface and foam film properties of the studied surfactant solutions. The spreading, bridging and entry coefficients are calculated and the spreading ability of the antifoam is characterized by microscopy observations and by surface tension measurements. Next, the initial antifoam activity and the antifoam durability are characterized in foam tests. The obtained results reveal that the antifoam efficiency in solutions of low-molecular mass surfactants with low surface dilatational modulus depends strongly on the density (area-per-molecule) of the respective adsorption layer. The addition of nonionic surfactants, which increase the mean area-per-molecule in the mixed adsorption layer, enhances significantly the antifoam activity and durability. In contrast, the addition of surfactants, which decrease the mean area-per-molecule, suppresses the antifoam activity. Furthermore, we found that surfactant mixtures which form condensed adsorption layers on the solution surface suppress strongly the antifoam activity. As an extreme, the condensed adsorption layer formed from the natural surfactant Quillaja saponin suppresses the antifoam spreading even at highly positive spreading coefficient which results also in very poor AF efficiency. The obtained results rationalize in a coherent way the observed differences in the AF activity and durability in mixed solutions of various ionic, nonionic and zwitterionic surfactants.
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Antifoaming Action of Oils
2015Co-Authors: Nikolai D. Denkov, Krastanka G. MarinovaAbstract:Antifoams are widely used for control of the foam stability in various products (detergents, paints, pharmaceuticals, and many others). A significant progress in the understanding of the foam destruction mechanisms by oil-containing Antifoams has been achieved recently. Experiments with Antifoams comprising silicone oil and hydrophobic silica showed that the antifoam entities (emulsified globules or lenses floating on the solution surface) easily form unstable oil bridges between the two surfaces of the foam film. These bridges rapidly stretch in radial direction, due to uncompensated capillary pressures at the oil-water and oil-air interfaces, and eventually rupture the foam films. As a result, the foam is destroyed within several seconds by the mixed solid-liquid Antifoams. In contrast, drops of silicone oil deprived of silica are unable to enter the foam film surface due to significant entry barriers. In these systems, the oil drops are expelled into the neighbouring Plateau borders (PBs), and the foam collapse is observed at a much later stage of the foam evolution, when the drops are compressed by the walls of the narrowing PBs (defoaming time on the order of minutes and hours). The magnitude of the entry barriers can be quantified by the so called Film Trappin
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mechanisms of foam destruction by oil based Antifoams
Langmuir, 2004Co-Authors: Nikolai D. DenkovAbstract:Oils and mixtures of oils with hydrophobic particles are widely used in various technologies and consumer products to control foaminess and foam stability. The aim of this review is to summarize our current understanding of the mechanisms of foam destruction by such substances, which are usually called Antifoams or defoamers. The experimental results show that two types of antifoam can be distinguished (called for brevity "fast" and "slow") which differ in the modes of their action. Fast Antifoams are able to rupture the foam films at the early stages of film thinning. As a result, fast Antifoams destroy completely the foam in less than a minute, in a typical foam-stability test. Microscopic observations have shown that the fast Antifoams rupture the foam films by the so-called "bridging" mechanisms, which involve the formation of oil bridges between the two surfaces of the foam film. The stability/instability of these oil bridges is explained by using the theory of capillarity. In contrast, the oily globules of the slow Antifoams are unable to enter the surfaces of the foam films and are first expelled into the Plateau borders (PBs). Only after being compressed by the narrowing walls of the PBs (due to water drainage from the foam), are the globules of the slow Antifoams able to enter the solution surface and to destroy the adjacent foam films. Typically, the process of foam destruction by slow Antifoams requires much longer time, minutes or tens of minutes, and a residual foam of well-defined height is observed in the foam tests. The experiments show that there is no direct relation between the magnitudes of the entry, E, spreading, S, and bridging, B, coefficients, on one side, and the antifoam efficiency, on the other side. The only requirement for having active antifoam, with respect to the bridging mechanisms, is that B should be positive. On the other hand, the barrier preventing the emergence of pre-emulsified antifoam globules on the solution surface (so-called "entry barrier") is of crucial importance for the mode of foam destruction and for the antifoam efficiency. Measurements of the entry barrier with recently developed film trapping technique (FTT) showed that Antifoams possessing low entry barriers act as fast Antifoams, whereas high barriers correspond to slow or inactive Antifoams, although E, S, and B coefficients could be strongly positive in the latter case. A good agreement between the magnitude of the entry barrier, measured by FTT, and the height of the residual foam, in the presence of slow Antifoams, was experimentally established and theoretically explained. The importance of various factors, such as the size of antifoam globules, oil spreading, kinetics of surfactant adsorption, hydrophobicity of solid particles in mixed oil-solid Antifoams, and the presence of amphiphilic additives (foam boosters), is discussed from the viewpoint of the mechanisms of antifoaming. The main experimental methods, used for studying the modes of antifoam action, are briefly described.
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Model studies on the mechanism of deactivation (exhaustion) of mixed oil-silica Antifoams
Langmuir, 2003Co-Authors: Krastanka G. Marinova, Slavka Tcholakova, Nikolai D. Denkov, And Stoian Roussev, Martial DeruelleAbstract:Antifoams are important components of many commercial products, such as detergents, paints, pharmaceuticals, and others.1 Antifoams are also used in various technologies, such as pulp and paper production, fermentation, and oil processing. It has been shown2 that mixed liquid-solid Antifoams (e.g., those comprising silicone oil and hydrophobic silica) usually have much higher activity than their individual components, if taken separately. Amajorproblemin thepractical application ofAntifoams is the gradual loss of their activity in the course of foam destruction. This process is termed “antifoam exhaustion” or “deactivation”, and several possible explanations have been proposed in the literature.3-14 Most often, the explanations are as follows: (i) the antifoam globules reduce their size in the course of foam destruction and eventually become too small to rupture efficiently the foam films;6,8,10 (ii) the antifoam, initially deposited on the surface of the foaming solution, is gradually emulsified
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model studies of the effect of silica hydrophobicity on the efficiency of mixed oil silica Antifoams
Langmuir, 2002Co-Authors: Krastanka G. Marinova, Slavka Tcholakova, Nikolai D. Denkov, Martial DeruelleAbstract:Mixtures of poly(dimethylsiloxane) oil and hydrophobized silica are used for foam control and are termed antifoam compounds or mixed oil−silica Antifoams. In a previous study, we found experimentally a well-pronounced maximum in the antifoam efficiency at a certain optimal silica hydrophobicity. The reasons for this peculiar dependence are studied in the present paper by performing two series of experiments. First, compounds of silica and oil are prepared under different conditions (with and without heating and/or stirring) and the antifoam efficiency of these compounds is compared by foam tests. Second, several characteristics of the studied compounds, such as viscosity, entry barrier, and thickness of the layer of spreading oil, are measured, and their contribution to the compound performance is analyzed. The results show that the optimal hydrophobicity of the silica particles is related to a minimum in the entry barrier (measured by the film trapping technique), which determines how easily the compound...
Krastanka G. Marinova - One of the best experts on this subject based on the ideXlab platform.
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Antifoaming Action of Oils
2015Co-Authors: Nikolai D. Denkov, Krastanka G. MarinovaAbstract:Antifoams are widely used for control of the foam stability in various products (detergents, paints, pharmaceuticals, and many others). A significant progress in the understanding of the foam destruction mechanisms by oil-containing Antifoams has been achieved recently. Experiments with Antifoams comprising silicone oil and hydrophobic silica showed that the antifoam entities (emulsified globules or lenses floating on the solution surface) easily form unstable oil bridges between the two surfaces of the foam film. These bridges rapidly stretch in radial direction, due to uncompensated capillary pressures at the oil-water and oil-air interfaces, and eventually rupture the foam films. As a result, the foam is destroyed within several seconds by the mixed solid-liquid Antifoams. In contrast, drops of silicone oil deprived of silica are unable to enter the foam film surface due to significant entry barriers. In these systems, the oil drops are expelled into the neighbouring Plateau borders (PBs), and the foam collapse is observed at a much later stage of the foam evolution, when the drops are compressed by the walls of the narrowing PBs (defoaming time on the order of minutes and hours). The magnitude of the entry barriers can be quantified by the so called Film Trappin
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Model studies on the mechanism of deactivation (exhaustion) of mixed oil-silica Antifoams
Langmuir, 2003Co-Authors: Krastanka G. Marinova, Slavka Tcholakova, Nikolai D. Denkov, And Stoian Roussev, Martial DeruelleAbstract:Antifoams are important components of many commercial products, such as detergents, paints, pharmaceuticals, and others.1 Antifoams are also used in various technologies, such as pulp and paper production, fermentation, and oil processing. It has been shown2 that mixed liquid-solid Antifoams (e.g., those comprising silicone oil and hydrophobic silica) usually have much higher activity than their individual components, if taken separately. Amajorproblemin thepractical application ofAntifoams is the gradual loss of their activity in the course of foam destruction. This process is termed “antifoam exhaustion” or “deactivation”, and several possible explanations have been proposed in the literature.3-14 Most often, the explanations are as follows: (i) the antifoam globules reduce their size in the course of foam destruction and eventually become too small to rupture efficiently the foam films;6,8,10 (ii) the antifoam, initially deposited on the surface of the foaming solution, is gradually emulsified
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model studies of the effect of silica hydrophobicity on the efficiency of mixed oil silica Antifoams
Langmuir, 2002Co-Authors: Krastanka G. Marinova, Slavka Tcholakova, Nikolai D. Denkov, Martial DeruelleAbstract:Mixtures of poly(dimethylsiloxane) oil and hydrophobized silica are used for foam control and are termed antifoam compounds or mixed oil−silica Antifoams. In a previous study, we found experimentally a well-pronounced maximum in the antifoam efficiency at a certain optimal silica hydrophobicity. The reasons for this peculiar dependence are studied in the present paper by performing two series of experiments. First, compounds of silica and oil are prepared under different conditions (with and without heating and/or stirring) and the antifoam efficiency of these compounds is compared by foam tests. Second, several characteristics of the studied compounds, such as viscosity, entry barrier, and thickness of the layer of spreading oil, are measured, and their contribution to the compound performance is analyzed. The results show that the optimal hydrophobicity of the silica particles is related to a minimum in the entry barrier (measured by the film trapping technique), which determines how easily the compound...
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role of oil spreading for the efficiency of mixed oil solid Antifoams
Langmuir, 2002Co-Authors: Nikolai D. Denkov, Krastanka G. Marinova, Slavka Tcholakova, Asen HadjiiskiAbstract:The role of oil spreading in the process of foam destruction by oil-based Antifoams (oils or mixed oil-solid compounds) has been the subject of a long debate in the literature. To clarify some aspects of this problem, we compare the entry barriers of antifoam globules in the presence and in the absence of a prespread layer of oil on the surface of the surfactant solution. The film trapping technique is employed to measure precisely the critical capillary pressure, at which the entry of the antifoam globules on the solution surface occurs. The experimental results show that the prespread oil layer reduces by several times the entry barrier for mixed oil-silica Antifoams, as compared to the barrier in the absence of spread oil. Thus, the oil spreading facilitates the entry of mixed antifoam globules and the subsequent bridging and rupture of the foam films. A simple mechanistic explanation of this effect is given, taking into account the main role of the solid particles in mixed Antifoams, namely, to pierce the asymmetric oil-water-air film, formed when an antifoam globule approaches the solution surface. This explanation is expressed in terms of the threephase contact angles solid-water-oil and solid-water-air, when spherical solid particles are considered.
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optimal hydrophobicity of silica in mixed oil silica Antifoams
Langmuir, 2002Co-Authors: Krastanka G. Marinova, Nikolai D. Denkov, Paul Branlard, Yves Giraud, Martial DeruelleAbstract:Mixtures of a properly chosen oil (poly(dimethylsiloxane) (PDMS) or hydrocarbon) and hydrophobic particles (e.g., hydrophobized silica) have a strong deteriorating effect on foam stability, even when introduced in a very low concentration. These mixtures are widely used for foam control and are commonly termed antifoam compounds. In the present study, we check experimentally how the antifoam efficiency depends on the hydrophobicity of the solid particles. For this purpose, we prepare antifoam compounds by mixing silicone oil (PDMS) and hydrophilic silica at room temperature. The mild stirring of this mixture leads to a gradual PDMS adsorption on the silica surface, making it more hydrophobic. This process is very slow at room temperature and takes weeks before reaching the final, most hydrophobic state of the particles. Thus, we are able to check how the antifoam activity changes along the process of silica hydrophobization. Solutions of three surfactants (one ionic and two nonionic) are used as foaming m...
Martial Deruelle - One of the best experts on this subject based on the ideXlab platform.
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Model studies on the mechanism of deactivation (exhaustion) of mixed oil-silica Antifoams
Langmuir, 2003Co-Authors: Krastanka G. Marinova, Slavka Tcholakova, Nikolai D. Denkov, And Stoian Roussev, Martial DeruelleAbstract:Antifoams are important components of many commercial products, such as detergents, paints, pharmaceuticals, and others.1 Antifoams are also used in various technologies, such as pulp and paper production, fermentation, and oil processing. It has been shown2 that mixed liquid-solid Antifoams (e.g., those comprising silicone oil and hydrophobic silica) usually have much higher activity than their individual components, if taken separately. Amajorproblemin thepractical application ofAntifoams is the gradual loss of their activity in the course of foam destruction. This process is termed “antifoam exhaustion” or “deactivation”, and several possible explanations have been proposed in the literature.3-14 Most often, the explanations are as follows: (i) the antifoam globules reduce their size in the course of foam destruction and eventually become too small to rupture efficiently the foam films;6,8,10 (ii) the antifoam, initially deposited on the surface of the foaming solution, is gradually emulsified
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model studies of the effect of silica hydrophobicity on the efficiency of mixed oil silica Antifoams
Langmuir, 2002Co-Authors: Krastanka G. Marinova, Slavka Tcholakova, Nikolai D. Denkov, Martial DeruelleAbstract:Mixtures of poly(dimethylsiloxane) oil and hydrophobized silica are used for foam control and are termed antifoam compounds or mixed oil−silica Antifoams. In a previous study, we found experimentally a well-pronounced maximum in the antifoam efficiency at a certain optimal silica hydrophobicity. The reasons for this peculiar dependence are studied in the present paper by performing two series of experiments. First, compounds of silica and oil are prepared under different conditions (with and without heating and/or stirring) and the antifoam efficiency of these compounds is compared by foam tests. Second, several characteristics of the studied compounds, such as viscosity, entry barrier, and thickness of the layer of spreading oil, are measured, and their contribution to the compound performance is analyzed. The results show that the optimal hydrophobicity of the silica particles is related to a minimum in the entry barrier (measured by the film trapping technique), which determines how easily the compound...
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optimal hydrophobicity of silica in mixed oil silica Antifoams
Langmuir, 2002Co-Authors: Krastanka G. Marinova, Nikolai D. Denkov, Paul Branlard, Yves Giraud, Martial DeruelleAbstract:Mixtures of a properly chosen oil (poly(dimethylsiloxane) (PDMS) or hydrocarbon) and hydrophobic particles (e.g., hydrophobized silica) have a strong deteriorating effect on foam stability, even when introduced in a very low concentration. These mixtures are widely used for foam control and are commonly termed antifoam compounds. In the present study, we check experimentally how the antifoam efficiency depends on the hydrophobicity of the solid particles. For this purpose, we prepare antifoam compounds by mixing silicone oil (PDMS) and hydrophilic silica at room temperature. The mild stirring of this mixture leads to a gradual PDMS adsorption on the silica surface, making it more hydrophobic. This process is very slow at room temperature and takes weeks before reaching the final, most hydrophobic state of the particles. Thus, we are able to check how the antifoam activity changes along the process of silica hydrophobization. Solutions of three surfactants (one ionic and two nonionic) are used as foaming m...
Sarah J Routledge - One of the best experts on this subject based on the ideXlab platform.
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Antifoams the overlooked additive
Pharmaceutical bioprocessing, 2014Co-Authors: Sarah J Routledge, David R Poyner, Roslyn M. BillAbstract:Present research has found that Antifoams can have a broad range of effects upon bioprocesses, both on the culture environment and upon the cells themselves.
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beyond de foaming the effects of Antifoams on bioprocess productivity
Computational and structural biotechnology journal, 2012Co-Authors: Sarah J RoutledgeAbstract:Antifoams are often added to bioprocesses with little knowledge of their impact on the cells or product. However, it is known that certain Antifoams can affect the growth rates of both prokaryotic and eukaryotic organisms in addition to changing surface properties such as lipid content, resulting in changes to permeability. This in turn can be beneficial to a recombinant protein production system for soluble proteins, as has been demonstrated by increased secretion of α-amylase and GFP, or achievement of greater yields of protein due to increased biomass. However, in some cases, certain concentrations of Antifoams appear to have a detrimental effect upon cells and protein production, and the effects vary depending upon the protein being expressed. These findings emphasise the importance of optimising and understanding antifoam addition to bioprocesses.
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the effect of Antifoams upon recombinant protein production in yeast
2012Co-Authors: Sarah J RoutledgeAbstract:Foaming during fermentation reduces the efficiency of the process leading to increased costs and reduced productivity. Foaming can be overcome by the use of chemical antifoaming agents, however their influence upon the growth of organisms and protein yield is poorly understood. The objective of this work was to evaluate the effects of different Antifoams on recombinant protein production. Antifoam A, Antifoam C, J673A, P2000 and SB2121 were tested at different concentrations for their effect on the growth characteristics of Pichia pastoris producing GFP, EPO and A2aR and the yield of protein in shake flasks over 48 h. All Antifoams tested increased the total GFP in the shake flasks compared to controls, at higher concentrations than would normally be used for defoaming purposes. The highest yield was achieved by adding 1 % P2000 which nearly doubled the total yield followed by 1 % SB2121, 1 % J673A, 0.6 % Antifoam A and lastly 0.8 % Antifoam C. The Antifoams had a detrimental effect upon the production of EPO and A2aR in shake flasks, suggesting that their effects may be protein specific. The mechanisms of action of the Antifoams was investigated and suggested that although the volumetric mass oxygen transfer coefficient (kLa) was influenced by the agents, their effect upon the concentration of dissolved oxygen did not contribute to the changes in growth or recombinant protein yield. Findings in small scale also suggested that Antifoams of different compositions such as silicone polymers and alcoxylated fatty acid esters may influence growth characteristics of host organisms and the ability of the cells to secrete recombinant protein, indirectly affecting the protein yield. Upon scale-up, the concentration effects of the Antifoams upon GFP yield in bioreactors was reversed, with lower concentrations producing a higher yield. These data suggest that antifoam can affect cells in a multifactorial manner and highlights the importance of screening for optimum antifoam types and concentrations for each bioprocesses.
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Antifoam addition to shake flask cultures of recombinant Pichia pastoris increases yield.
Microbial cell factories, 2011Co-Authors: Sarah J Routledge, Christopher J. Hewitt, Nagamani Bora, Roslyn M. BillAbstract:Pichia pastoris is a widely-used host for recombinant protein production. Initial screening for both suitable clones and optimum culture conditions is typically carried out in multi-well plates. This is followed by up-scaling either to shake-flasks or continuously stirred tank bioreactors. A particular problem in these formats is foaming, which is commonly prevented by the addition of chemical antifoaming agents. Intriguingly, Antifoams are often added without prior consideration of their effect on the yeast cells, the protein product or the influence on downstream processes such as protein purification. In this study we characterised, for the first time, the effects of five commonly-used antifoaming agents on the total amount of recombinant green fluorescent protein (GFP) secreted from shake-flask cultures of this industrially-relevant yeast.
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Antifoam addition to shake flask cultures of recombinant Pichia pastoris increases yield
2011Co-Authors: Sarah J Routledge, Nagamani Bora, Christopher Hewitt, Roslyn M. BillAbstract:Pichia pastoris is a widely-used host for recombinant protein production. Initial screening for both suitable clones and optimum culture conditions is typically carried out in multi-well plates. This is followed by up-scaling either to shake-flasks or continuously stirred tank bioreactors. A particular problem in these formats is foaming, which is commonly prevented by the addition of chemical antifoaming agents. Intriguingly, Antifoams are often added without prior consideration of their effect on the yeast cells, the protein product or the influence on downstream processes such as protein purification. In this study we characterised, for the first time, the effects of five commonly-used antifoaming agents on the total amount of recombinant green fluorescent protein (GFP) secreted from shake-flask cultures of this industrially-relevant yeast. Addition of defined concentrations of Antifoam A (Sigma), Antifoam C (Sigma), J673A (Struktol), P2000 (Fluka) or SB2121 (Struktol) to shake-flask cultures of P. pastoris increased the total amount of recombinant GFP in the culture medium (the total yield) and in the case of P2000, SB2121 and J673A almost doubled it. When normalized to the culture density, the GFP specific yield (μg OD595 -1) was only increased for Antifoam A, Antifoam C and J673A. Whilst none of the Antifoams affected the growth rate of the cells, addition of P2000 or SB2121 was found to increase culture density. There was no correlation between total yield, specific yield or specific growth rate and the volumetric oxygen mass transfer coefficient (kLa) in the presence of antifoam. Moreover, the Antifoams did not affect the dissolved oxygen concentration of the cultures. A comparison of the amount of GFP retained in the cell by flow cytometry with that in the culture medium by fluorimetry suggested that addition of Antifoam A, Antifoam C or J673A increased the specific yield of GFP by increasing the proportion secreted into the medium. We show that addition of a range of antifoaming agents to shake flask cultures of P. pastoris increases the total yield of the recombinant protein being produced. This is not only a simple method to increase the amount of protein in the culture, but our study also provides insight into how Antifoams interact with microbial cell factories. Two mechanisms are apparent: one group of Antifoams (Antifoam A, Antifoam C and J673A) increases the specific yield of GFP by increasing the total amount of protein produced and secreted per cell, whilst the second (P2000 or SB2121) increases the total yield by increasing the density of the culture
Slavka Tcholakova - One of the best experts on this subject based on the ideXlab platform.
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role of surfactant adsorption and surface properties for the efficiency of pdms silica Antifoams
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2021Co-Authors: Nadya Politovabrinkova, Slavka Tcholakova, Nikolai D. Denkov, M Hristova, V Georgiev, M Grandl, F AchenbachAbstract:Abstract We study how the composition of various surfactant mixtures affects the efficiency of mixed PDMS–silica antifoam in foamed surfactant solutions. First, systematic experiments are performed to characterize the surface and foam film properties of the studied surfactant solutions. The spreading, bridging and entry coefficients are calculated and the spreading ability of the antifoam is characterized by microscopy observations and by surface tension measurements. Next, the initial antifoam activity and the antifoam durability are characterized in foam tests. The obtained results reveal that the antifoam efficiency in solutions of low-molecular mass surfactants with low surface dilatational modulus depends strongly on the density (area-per-molecule) of the respective adsorption layer. The addition of nonionic surfactants, which increase the mean area-per-molecule in the mixed adsorption layer, enhances significantly the antifoam activity and durability. In contrast, the addition of surfactants, which decrease the mean area-per-molecule, suppresses the antifoam activity. Furthermore, we found that surfactant mixtures which form condensed adsorption layers on the solution surface suppress strongly the antifoam activity. As an extreme, the condensed adsorption layer formed from the natural surfactant Quillaja saponin suppresses the antifoam spreading even at highly positive spreading coefficient which results also in very poor AF efficiency. The obtained results rationalize in a coherent way the observed differences in the AF activity and durability in mixed solutions of various ionic, nonionic and zwitterionic surfactants.
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Model studies on the mechanism of deactivation (exhaustion) of mixed oil-silica Antifoams
Langmuir, 2003Co-Authors: Krastanka G. Marinova, Slavka Tcholakova, Nikolai D. Denkov, And Stoian Roussev, Martial DeruelleAbstract:Antifoams are important components of many commercial products, such as detergents, paints, pharmaceuticals, and others.1 Antifoams are also used in various technologies, such as pulp and paper production, fermentation, and oil processing. It has been shown2 that mixed liquid-solid Antifoams (e.g., those comprising silicone oil and hydrophobic silica) usually have much higher activity than their individual components, if taken separately. Amajorproblemin thepractical application ofAntifoams is the gradual loss of their activity in the course of foam destruction. This process is termed “antifoam exhaustion” or “deactivation”, and several possible explanations have been proposed in the literature.3-14 Most often, the explanations are as follows: (i) the antifoam globules reduce their size in the course of foam destruction and eventually become too small to rupture efficiently the foam films;6,8,10 (ii) the antifoam, initially deposited on the surface of the foaming solution, is gradually emulsified
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model studies of the effect of silica hydrophobicity on the efficiency of mixed oil silica Antifoams
Langmuir, 2002Co-Authors: Krastanka G. Marinova, Slavka Tcholakova, Nikolai D. Denkov, Martial DeruelleAbstract:Mixtures of poly(dimethylsiloxane) oil and hydrophobized silica are used for foam control and are termed antifoam compounds or mixed oil−silica Antifoams. In a previous study, we found experimentally a well-pronounced maximum in the antifoam efficiency at a certain optimal silica hydrophobicity. The reasons for this peculiar dependence are studied in the present paper by performing two series of experiments. First, compounds of silica and oil are prepared under different conditions (with and without heating and/or stirring) and the antifoam efficiency of these compounds is compared by foam tests. Second, several characteristics of the studied compounds, such as viscosity, entry barrier, and thickness of the layer of spreading oil, are measured, and their contribution to the compound performance is analyzed. The results show that the optimal hydrophobicity of the silica particles is related to a minimum in the entry barrier (measured by the film trapping technique), which determines how easily the compound...
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role of oil spreading for the efficiency of mixed oil solid Antifoams
Langmuir, 2002Co-Authors: Nikolai D. Denkov, Krastanka G. Marinova, Slavka Tcholakova, Asen HadjiiskiAbstract:The role of oil spreading in the process of foam destruction by oil-based Antifoams (oils or mixed oil-solid compounds) has been the subject of a long debate in the literature. To clarify some aspects of this problem, we compare the entry barriers of antifoam globules in the presence and in the absence of a prespread layer of oil on the surface of the surfactant solution. The film trapping technique is employed to measure precisely the critical capillary pressure, at which the entry of the antifoam globules on the solution surface occurs. The experimental results show that the prespread oil layer reduces by several times the entry barrier for mixed oil-silica Antifoams, as compared to the barrier in the absence of spread oil. Thus, the oil spreading facilitates the entry of mixed antifoam globules and the subsequent bridging and rupture of the foam films. A simple mechanistic explanation of this effect is given, taking into account the main role of the solid particles in mixed Antifoams, namely, to pierce the asymmetric oil-water-air film, formed when an antifoam globule approaches the solution surface. This explanation is expressed in terms of the threephase contact angles solid-water-oil and solid-water-air, when spherical solid particles are considered.