Colloids and Surfaces A: Physicochemical and Engineering Aspects

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1 Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (213) Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journa l h o me pag e : High stability of the bovine serum albumine foams evidenced in Hele Shaw cell M. Krzan, H. Caps, N. Vandewalle GRASP, Physics Department B5a, University of Liege, B4 Liege, Belgium h i g h l i g h t s g r a p h i c a l a b s t r a c t It is possible to generate foams with only water/protein mixtures. Foam production depends on the protein concentration. Foam production presents a threshold, similar to the CMC for a classical surfactant. Protein foams are heterogeneous and exhibit a relatively slow gravitational drainage. a r t i c l e i n f o Article history: Received 16 November 212 Received in revised form 29 December 212 Accepted 2 January 213 Available online 18 January 213 Keywords: Foams Proteins Bovine serum albumine Hele Shaw flows a b s t r a c t The generation of a reproducible two-dimensional foams is performed by flipping many times a Hele Shaw cell partially filled with a foaming agent/water mixture. This process of foam generation is observed and depends on the chemical composition of the solution. Bovine Serum Albumin (BSA) and Sodium Dodecyl Sulfate (SDS) foams are studied using this system. Protein-based foams exhibit particular characteristics that are emphasized: (i) heterogeneous cellular structures due to bubble size segregation and (ii) relatively high stability of the foam. Some clues are given regarding the formation of heterogeneous structures in such foam systems. 213 Elsevier B.V. All rights reserved. 1. Introduction Aqueous foams are dispersions of gas bubbles in a surfactant solution. They exhibit linear viscoelastic behavior when they are subjected to small shear stresses while they flow like viscous liquids when the applied shear stress is large enough to trigger bubble rearrangements [1]. Foams are used in many technological applications, which include cleaning, water purification, waste treatment, ore flotation, drilling oil, fire fighting as well as production of food or ultra light nanocomposites [2 9]. Corresponding author at: Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Cracow, Poland. Tel.: ; fax: address: nckrzan@cyf-kr.edu.pl (M. Krzan). The generated foams may exist for some period of time. However, from the thermodynamic point of view, the unbalanced surface free energy in the foams has to be decreased. Therefore foams are unstable dispersions by their own nature and must finally disintegrate into individual component phases. Foams irreversible evolve with time (ageing process) througt the following dynamic processes, which are the foam free drainage, coalescence and coarsening [1 3,1,11]. The main physical process taking place in aqueous foams is the free drainage, due to the influence of gravitational acceleration, viscous force and capillary pressure between the adjacent bubbles. The drainage process means that the top of the foam becomes dry while the bottom remains wet. As a result, the dry foam is formed by polyhedral bubbles with thin edges, while the bubbles in the bottom part of the foams are still spherical. Because drainage is linked to foam stability and other rheological properties of foam systems, this process has been intensively studied experimentally and theoretically in the case of foams created /$ see front matter 213 Elsevier B.V. All rights reserved.

2 M. Krzan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (213) from various surfactant solutions and from different geometrical conditions and gravitational conditions [12 18]. When the liquid films between the bubbles are very thin, they may eventually break. The merging of two bubbles as a result of the rupture of the film between them is known as coalescence. Larger bubbles appear in the foam and the number of bubbles decreases. If this continues, the whole foam collapses [1,19] The coarsening occurs due to the gas diffusion. In the result the larger bubbles grow at the expense of smaller bubbles. As the results smaller bubbles may finally disappear [1]. For the last years, protein adsorption phenomena received a special attention on both scientific and technological research areas. Proteins are complex molecules with unique properties, which determine the life in our planet. From their easy biodegradation and natural recycling, raise a motivation for using them in various industrial applications. Proteins adsorption on boundary surfaces is important for a variety of disciplines including material and food engineering, biological and medical sciences, and the pharmaceutical and cosmetic industries [2 28]. Just looking in food engineering we found the large quantities of food foams using in angel-food cakes, meringues, dessert toppings and other products. Therefore the stability and mechanical strength of aqueous foams were studied intensively especially in the field of the food applications, where the presence of aerated structure provides the essential texture of the product. The studies and analysis were performed in three dimensional system. The researchers tried to find the relationship between the adsorption processes in the biosurfactant solutions and its foaming properties [11,22,26 34]. Since proteins own a high molecular mass it is difficult to compare their surface active properties with common water surfactants [22]. The major interactions involved in the protein adsorption processes are hydrophobic, hydrogen-bonding and electrostatic effects [2 25,32,35,36]. Their dual, polymeric and polyelectrolyte nature leads the mixed, steric and electrostatic repulsions during the adsorption processes. Depending on their morphology and surface, proteins may get strongly adsorbed at the air/water interface or not [37]. Foaming properties of proteins are therefore influenced by a large number of parameters including thermal or chemical preprocessing conditions, method of foaming, whipping time and the physical and chemical properties of the proteins as well as the environmental factors like ionic strength or ph [31,37 4]. In general, the proteins during the foam formation diffuse from the aqueous phase and adsorb at the air water interface due to the compatibility of their hydrophobic groups with the hydrophobic character of the interfaces [8,41]. Protein molecules can unfold to a certain degree and reorient at the interface with polar groups exposed towards water phase and the non-polar groups towards the air phase. The protein adsorption is thermodynamically favorable due to the simultaneous dehydration of the hydrophobic portions of the protein [3]. The previously hidden hydrophobic part to the molecules could be exposed to the interface due to the proteins natural flexibility. It potentially is leading to interfacial denaturisation of the molecules. This in turn, leads to the decrease of the interfacial tension and to the formation of more or less stable interfacial protein films. The adsorption rate of proteins, as the most important factor for foam formation, depends on the protein concentration, the molecular weight and the structure of the proteins used. Moreover, the proteins form viscoelastic films via intermolecular interactions. It leads the steric resistance against the rupture of thin liquid films. In the results the foam stability is significantly improved [25,37,42]. However, generally a relatively high protein concentrations are necessary for formation of stable thin liquid films. As clearly seen, up to now the most important process of these has not been clearly identified. Therefore, new experimental works are necessary. In the paper a two-dimensional foams are studied in vertical Hele Shaw cell. It is commonly used experimental system for studies over the foam drainage [43,44]. This way of producing a foam is useful via their use in fundamental research. The experiments, simulations and theories are easier to develop in 2D. For example, in the two-dimensional foams between two parallel plates of glass, each line of thin liquid film has an energy proportional to its length and surface tension, three dimensional effects such as the curvature in the vertical plane appearing only in the value of the proportionality constant [45,46]. In our experiments foams are generated by imposing an intermittent drainage in a Hele Shaw cell. The cell is partially filled with a foaming agent/water mixture. The considered protein is the Bovine Serum Albumin (BSA). For comparison, a common solution based on SDS is used as a reference. The next section details the experimental setup and procedure. Results concerning the foamability, the foam stability and the drainage inside the foam are then presented in Section 3. Those results are discussed in Section 4. Some previous results with SDS were indeed reported in [47]. During the experiments the evolution of foam structure is monitored by image analysis. The accuracy of the experimental setup and procedure has been reported in recent papers [47,48]. 2. Experimental setup The experimental setup consists of a Hele Shaw cell ( cm 3 ), which is fixed to an horizontal axis for controlled rotation [47]. A sketch of the setup is present in Fig. 1. Due to the small foamability of the protein mixture emphasized below, a specific experimental protocol has been used for the cell preparation. The cell glasses are first cleaned with hot tap water and then treated by sulfochromic acid in order to avoid any organic waste traces. Thereafter, the glasses are purified with twice distilled water. The obtained clean and hydrophilic glasses are then conditioned for at least 8 h within a very diluted agent water mixture of the studied protein. This preparation method allows us to obtain some hydrophobic monolayer of the studied compound on the glass wall of the experimental cell. Immediately after being removed out of this bath, the cells are filled with the test solution. This rapid last step avoid the glass surfaces to be contaminated by dust particles. Each solution is prepared a few hours before the experimental run. This time duration (a few minutes) is too short for any decomposition or hydrolysis of the studied agent. During each experiment, the Hele Shaw cell is first partially filled with 6 ml of protein (or SDS)/water mixture. Pure bi-distilled water (ph 5.8) has been used, at a room temperature 22 ± 1 C. The cell is fixed to the horizontal rod and set into rotation with the help of a micro-controller driven motor. Starting from a vertical position of the cell, successive rotations by an angle are performed. Each step is a flip of the Hele Shaw cell, such that the liquid phase Fig. 1. Sketch of the experimental setup used for foam generation and analysis. See text for details.

3 114 M. Krzan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (213) tends to fall towards the bottom of the cell. During the flips, thin liquid films are created between the glass walls of the cell and a foam is thus generated, bubble after bubble. The number of bubbles in the cell is growing after each flip, giving information on the foamability of the aqueous solution. However, the free drainage along the bubble edges takes place, due to the gravitational forces. In order to avoid as much as possible the drainage during the foam generation, the delay between two successive flips is fixed to at most 2 s. A flip sequence is thus consisting of the rotation of the cell (typically 2.5 s) followed by a 2 s delay. This sequence is repeated one hundred times. Thereafter the cell is left in a vertical position for free drainage during 15 min. Using a high definition CCD camera, images of the foam are recorded after each flip and during the drainage sequence as well. Image analysis is then performed in order to extract relevant information such as the foamability, the drainage rate, the foam structure and its stability. It should be noted that due to the specificity of the proteins, the experimental protocol has been deeply changed from the one used in [47,48], resulting in different values for the number of bubbles, even for SDS solutions. Nevertheless, the obtained results remain highly reproducible, with variations of typically less than 5% for the generated numbers of bubbles. Each experiments were repeated at least three times. The results presented in the paper show the experimental results which were in the best accuracy with the averages. In the next section, the experimental data obtained with BSA and SDS will be presented. The BSA concentration ranges from mol/dm 3 to mol/dm 3 while the concentration of SDS was between.1 mol/dm 3 and.1 mol/dm 3. In order to rely the foam properties to the liquid ones, we also performed equilibrium and dynamic surface tension measurements using du Nüoy ring method by Lauda TE1C tensiometer and the Krüss Contact-Angle-Drop-Shape-Analysis System DSA 1M with the DSA3 software. In the pending drop method the drops were formed on the tip of a stainless steel capillary needle, which was surrounded by a cuvette. The cuvette was filled with water saturated atmosphere to avoid droplet evaporation. The metal needle was attached to a volume control system, which was a steppermotor driven syringe. All experiments were performed at the room temperature 22 ± 1 C and for each concentration at least three measurements have been made. During the experiments the video frames of the pendant drop were recorded with the frequency equal 2 Hz. The video frames have been automatically processed by fitting the drop profile with the Laplace equation of capillarity. It should be noted that for SDS, in the case of the tested concentrations (c.5 mol/dm 3 ) the equilibrium state of adsorption was achieved immediately [49,5], while in the case of BSA a period of adsorption longer than 2 minutes was necessary [25,37,51 53]. 3. Experimental results In the following section, we present experimental results concerning various aspects of the foam generated by the method presented above. The measurements we have performed concern the ability of the solution to foam, the foamability; the stability of the foam; the geometrical structure of the foam and the gravitational drainage inside the foam Foamability Along the successive flips, the number N of bubbles in the cell increases before it tends to an asymptotic value N, which depends mainly on the initial liquid content [47] and the concentration of surface active agent (or protein). This second assumption is clearly confirmed in Figs. 2 and 3, where the number of bubbles measured after the 1 flips is reported for various BSA and SDS N ( ) e-8 1e-7 1e-6 C (mol/dm 3 ) Fig. 2. Black dots report the number N of bubbles in the cell after 1 flips as a function of the concentration of BSA. Open circles represent the equilibrium surface tension measurements of BSA-water solutions. Dashed curves are guides for the eyes. The vertical arrow indicates the saturation in surface tension, similarly to the CMC for a surfactant. concentrations. Measurements of the equilibrium surface tension of the various solutions are also reported on the same plots. At first sight, both curves exhibit a growth above a concentration threshold and then saturate to some value. In the followings, this number of bubbles after 1 flips will thus be considered N. In the case of SDS, the threshold for foam generation appears close to the critical micellar concentration (CMC) range M [54,55]. This is confirmed by the equilibrium surface tension measurements (also shown in Figs. 2 and 3). For the BSA case, a similar behavior is also observed. In that case, the threshold could be related to the protein Critical Aggregation Concentration (CAC). In theory protein as amphiphilic molecules could also form a micelle formation. This kind of behavior has been already reported for -casein and -lactoglobulin [56,57]. In the case of BSA the aggregation was also already observed by measuring synchronous light scattering signals using a common spectrofluorometer [55]. The authors found the small micelles in the case of BSA or HSA concentration range about M. When the BSA concentration exceeds those values, a number of micelles and their sizes significantly get enlarged. The values claims by [55] are in really good agreement with the data presented on the Fig. 2, where the concentration range of ca M as the BSA CAC could be assumed. Seems that thanks to the intermittent drainage of the Hele Shaw cell, the dynamics of covering thin liquid films by the N ( ) C (mol/dm 3 ) Fig. 3. Black dots report the number N of bubbles in the cell after 1 flips as a function of the concentration of SDS. Open circles represent the equilibrium surface tension measurements of SDS-water solutions. Continuous curves are guides for the eye. The vertical arrow points to the CMC. CMC γ [mn/m] ( ) γ [mn/m] ( )

4 M. Krzan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (213) N 5 4 N E-7 M 3. E-7 M 4.5 E-7 M 6. E-7 M 9. E-7 M 1.2 E-6 M 1.5 E-6 M t (s) 2 C=.6 M C=.8 M C=.1 M C=.2 M C=.35 M C=.5 M t (s) Fig. 4. Number N of bubbles in the Hele Shaw cell as a function of time during the free drainage of protein-based foams. Different BSA concentrations are illustrated (see legend). Fig. 5. Number N of bubbles in the Hele Shaw cell as a function of time during the free drainage of SDS-based foams. Different SDS concentrations are illustrated (see legend). All data exhibit a relevant decrease of the bubble number with time. proteins, in exchange with the aggregates within the solution, can thus be emphasized. It should also be noted that the foams produced from BSA solutions contain a smaller amount of bubbles than the SDS ones, meaning that the average bubble size is larger for the BSA. At high concentrations (above foam generation thresholds), it takes also more flips to generate a BSA foam than for SDS Foam stability The stability of the generated foam has been monitored by taking images of freely draining foams every 1 s, during the first 15 min following each series of 1 flips. Figs. 4 and 5 present the number of bubbles N as a function of time for both BSA and SDS at different concentration values. The first data point at t = s corresponds to the value N presented in Figs. 2 and 3. Any decrease of N means that either bubble edges break (collapse of the foam [19]) or that the coarsening plays a role (evolution to a broader bubble size distribution). Due to the different nature of both chemicals, this latter ripening of the foam is expected to be strongly different for SDS and BSA, as reported in [37,58]. Since the coarsening is expected to occur over longer timescales, we can associate any variation of N with a collapse of the foam. It should be noted that the image analysis algorithm only accounts for bubbles in contact with the glass surfaces. Local rearrangements of bubbles smaller than the Hele Shaw cell lead to the small fluctuations of the number of bubbles N seen in the figures. Typical images emphasizing the burst of bubble edges is proposed in Fig. 6. To describe the long-term stability of the BSA foam the additional measurements with the solution concentration of M were performed. The foam evolution were monitored during the hours after the formation. It proved that the number of bubbles have not changed till ca. 4 min. During this time almost no rearrangements in foam structure were observed. After this time the rapid coalescence and collapsing of about 25% of foam fraction happened. As seen the major difference between BSA and SDS concerns the foam stability itself: while the SDS foam collapses, the foam based on BSA proteins appears to be very stable. The apparent poor foamability of protein-based solutions is counterbalanced by a larger Fig. 6. Image sequence of a part of an SDS foam illustrating coalescence events. Time evolves from top left to bottom right. Images are separated by 3 s.

5 116 M. Krzan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (213) Fig. 7. Snapshots of a BSA foam taken just after flip N = 1. The successive images correspond to the following concentration values: M, M, M, M, M, M and M. foam stability. Above the CMC, the decay rate (about 5 s) of the SDS foam seems to be independent of the concentration. It should be noted that a small drift of the data is seen at the beginning of the BSA curves. It corresponds to local rearrangements of small bubbles along cell walls Foam structure Figs. 7 and 8 present a few snapshots of the foam just after the 1th reversal, for respectively BSA and SDS cases. An increase in the surface agent concentration separates the successive images. The foam structures appear quite different. The BSA foam is composed of bubbles with various shapes and sizes being quite segregated. The top and the bottom of the Hele Shaw cell is mainly covered by large bubbles while the smaller bubbles are located near the center. When the foam is generated on the basis of the SDS surfactant, the size distribution is narrower and the foam is more homogeneous. Such topological differences should originate from the foam generation mechanisms, as discussed in Section 4. Moreover, structural heterogeneities will modify the physical properties of the foam, as illustrated below with the free drainage measurements. The effect of the surfactant concentration is larger in the case of BSA than in the case of SDS. As the concentration is increased, the number of bubbles is larger. The average bubble size is therefore smaller. As already revealed by Figs. 2 and 3, the evolution of the number of bubbles generated after one hundred flips is more pronounced in the case of the protein Foam drainage Just after the hundredth flip, the foam is left at rest. Gravitational drainage is then thinning the bubble edges by mean of decreasing the local liquid fraction. On the foam pictures, this effect can be monitored by measuring the light intensity transmitted through the bubble edges [47,48]. In so doing, the liquid fraction * that is actually measured on the Hele Shaw walls slightly differs from the bulk liquid fraction. Nevertheless, * gives the relevant information about the liquid flows inside the bubble edges. Fig. 9 presents the vertical distribution of liquid fraction values * for both BSA and SDS cases and after different free drainage durations. The SDS foam exhibits a roughly constant and low value of * above the foam/liquid interface (for h > 2 cm). Drainage is seen since the average liquid fraction reaches <*>» 5% after 3 s while it is <*>» 1% soon after foam generation. Averages <...> are considered for h > 2 cm. Fig. 8. Snapshots of a SDS foam taken just after flip N = 1. The successive images correspond to the following concentration values:.6 M,.8 M,.1 M,.2 M.,.35 M and.5 M. The BSA foam exhibits however larger * values and huge fluctuations of * along the vertical h-axis. Those variations of * with large values (up to 2%) are due to the presence of clusters of small bubbles which are known to store more liquid than the Plateau borders around large polygonal bubbles. Drainage is also seen for BSA foams since average values <*> decrease with time. However, the fraction of liquid remains high (around 1%) meaning that the drainage is slower in BSA foams than in SDS foams. The presence of small bubbles may indeed represent some hindrance for liquid motion along bubble edges. 4. Discussion The results obtained with BSA are puzzling when considering previous results using the same foam generation method in a Hele Shaw cell. Indeed, only homogeneous 2d foam structures were found in previous works. Such structures were described by a model based on the splitting of bubbles at each flip. Since the probability to split a bubble depends on its size, the foam generation process selects a peaked bubble size distribution as the final foam structure. BSA foams cannot be described by such a model. h (cm) SDS φ * BSA φ * Fig. 9. The distribution of the liquid fraction values as a function of the vertical position h above the foam/liquid interface. The liquid fraction * is estimated through image analysis. Free drainage of the foam is considered since the liquid fraction is given at three times after foam generation: 4 s, 15 s, and 3 s. Two cases are illustrated: (left) SDS-water mixture at M and (right) BSA water mixture at M.

6 M. Krzan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (213) Two processes may explain the formation of heterogeneities in BSA 2d foams generated with our setup: i. Asymmetric breaking of bubbles could be inhibited by locally strong gradients of the liquid fraction. ii. Small bubbles, when created, are displaced by the liquid motion along the Plateau borders. After a few flips, the liquid tends to follow paths inside the foam. Small bubbles are transported together along those paths. In the results their are collected in big clusters in the center of the cell. The clusters store liquid and enhance the stability of such foams. This size segregation affects the splittings occurring at different positions inside the foam. The larger content of liquid in the central part of the cell leads to more energy and, therefore, more bubble splits. As a consequence of this heterogeneity, the foam drainage itself experience a slowing down in the middle of the foam. The porosity is actually changing from region to region. Another important point which could be responsible for the relatively slow drainage of the protein foam is related to the surface viscosity. We have seen that a threshold is observed in the effect of the protein concentration on the foam generation, as it is observed for the CMC of a surfactant. Nevertheless, the adsorption process of the protein is quite different from the one of surfactant molecules; the mass of the molecule being simply different. The usually admitted Poiseuille flow inside the Plateau Borders and bubbles edges might thus be altered by the presence of the BSA molecules at the interfaces. The related increase in the surface viscosity surely causes a slowing down of the fluid flow. The high BSA foam stability could be also connected with the formation of the stable black films [37]. It should be pointed that during the time of foam ageing the system start to transform from dynamic to equilibrium state of adsorption conditions. As it was proved by [37] in similar cases the thin liquid films can obtain their equilibrium thickness. 5. Conclusion The foamability of BSA has been investigated experimentally in a nearly 2D configuration. This foam production depends on the protein concentration and presents a threshold, similar to the one observed at the CMC for a classical surfactant. The results clearly show different characteristics of the protein foams in comparison to classical surfactant ones, like SDS-based foams. While the foam is less homogeneous it exhibits a relatively slow gravitational drainage. This effect could be related to the foam structure and the surface viscosity. The heterogeneity of the protein-based foam could be explained in terms of asymmetric breaking of bubbles or by the transport of small bubbles along the Plateau borders. 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