Article pubs.acs.org/Langmuir
Interaction Forces between Bubbles in the Presence of Novel Responsive Peptide Surfactants Thakshila S. Balasuriya† and Raymond R. Dagastine*,†,‡ †
The Department of Chemical & Biomolecular Engineering, The Particulate Fluids Processing Centre, The University of Melbourne, Parkville, Victoria 3010, Australia ‡ Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, Victoria 3168, Australia ABSTRACT: Reversibly switchable surfactants are increasingly important for controlling foam stability in many industrial applications because they can be recycled as foaming and antifoaming agents. Novel stimuli responsive peptide surfactants have been previously studied to classify the molecular conformation at the air−water interface before and after switching. In this study, Atomic Force Microscopy (AFM) was used to correlate the peptide conformation to directly measured changes in colloidal interaction forces between immobilized air bubbles before and after switching. The surface tension values extracted from the AFM force measurements were compared to macroscopic pendant drop measurements to help elucidate the importance of desorption in the switching mechanisms for some of the peptides. Results were compared to previous studies of macroscopic foam-column stability using these peptides. Differences in foam stability and the AFM force measurements were apparent, highlighting variations in directly measured equilibrium colloidal interactions to macroscopic bulk behavior. These results help elucidate the connection between the switching mechanism of novel stimuli responsive peptide surfactants and their effect on colloidal scale interactions between bubbles.
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charging or discharging of the amidine group,16 and stimuli responsive polymers with ionic liquid precursors.17 More recently, Middelberg and co-workers have reported on a peptide-based surfactant that is pH sensitive.18,19 These small peptides were successfully used to reversibly form and break foams and emulsions via changing the pH of the solution.20−22 We use AFM to study the stability of the interactions between two bubbles (radius from ∼30 to 200 μm) in the presence of novel pH sensitive, reversibly switchable peptide surfactants18,19 at the air−water interface. The peptides studied include two switchable peptides, AM118 (Ac-MKQLADSLHQLARQVSRLEHA-NH2) and Lac21E23 (Ac-MEELADSLEELARQVEELESA-NH2), with different solution-based switching mechanisms and a control nonswitching peptide Lac21, (AcMKQLADSLMQLARQVSRLESA-NH2). Earlier work in the development of these peptides has correlated the changes in molecular conformations during switching (summarized in Figure 1 for AM1 and Lac21E), via the addition of ions or pH changes to surface tension, interfacial rheology measurements, and foam-column stability studies.22−24 These studies provide highly useful information on the impact of the switching behavior of these peptides, but only an indirect picture of how these peptides affect the interaction forces between bubbles that ultimately govern foam stability. In addition, foam-column
INTRODUCTION Foams are widely used in industrial processes for a variety of purposes such as enhanced oil recovery,1 froth flotation,2 and in the cosmetic and food industries.3,4 In such processes, controlling or switching foam stability is an important aspect typically achieved using foaming or antifoaming agents. Understanding the interfacial interactions that govern bubbles within foams is imperative for stability. The interactions of foams have been an avid area of study since the first thin films studied by Derjaguin and Churaev5 and the advent of the Scheludko cell to study the liquid drainage between air−water interfaces.6 Recently, atomic force microscopy (AFM) has been used to directly measure interactions between drops and bubbles.7,8 Quantitative comparisons between experiments and theory has led to an unprecedented understanding of the interplay of surface forces, deformation, hydrodynamic drainage, and the effects of curvature to control the criteria for bubble stability.9,10 The development of reversibly switchable surfactants is an active area of study because of foam switchability requirements in industrial processes. Reversibly switchable surfactants reduce the amount of material needed for forming or breaking foams because they can be recycled due to their ability to switch between foaming and antifoaming agent states via an external stimulus. Examples of such surfactants that have been developed in the past are polymers switched via pH swings and temperature,11−13 azobenzene surfactants switched via UV irradiation,14,15 long chain alkyl amidines switched via the © 2012 American Chemical Society
Received: April 23, 2012 Revised: November 22, 2012 Published: November 26, 2012 17230
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EXPERIMENTAL METHOD
Materials. Peptide surfactants Lac21, AM1, and Lac21E, based on sequences in the work of Dexter and co-workers,18,19 were customsynthesized by GenScript USA Inc. with a purity of >95%, with end modification acetylation at the N-terminal and amidation at the Cterminal. Stock solutions were prepared in a 25 mM HEPES [4-(2hydroxyethyl)-1-piperazineethanesulfonic acid] (Scharlab S.L., Barcelona, Spain) buffer solution at pH 7.4. Milli-Q water and glassware used to prepare buffer solutions and peptide stock solutions were autoclaved prior to preparation, in order to maximize protein solution longevity. The following concentrations were used for the various solutions used in this study: 120 μM peptide surfactants in the HEPES buffer, 200 μM zinc sulfate heptahydrate (Merck, Victoria, Australia) for AM1 metal binding, and aliquots of a 1.6 N acid (HCl) and a 1.8 N base (NaOH) solutions for pH corrections. Atomic Force Microscopy. The experiment was performed using a MFP-3D-BIO AFM from Asylum Research (Santa Babara, CA). The force interactions between two immobilized air bubbles were measured, where one bubble was positioned on the cantilever and the other bubble on the substrate directly below it, so that their axis of rotation was identical. A custom-made rectangular cantilever8 with a raised circular gold pattern at one end was used for the platform of the top bubble. The cantilever spring constant, Kc, measured using the method of Hutter and Bechhoefer,27 ranged from 0.1 N/m to 0.35 N/ m for the cantilevers used in this study. Prior to use, the cantilevers were hydrophobized by soaking them overnight in decanethiol (Sigma-Aldrich Pty. Ltd., New South Wales, Australia) diluted with ethanol to form self-assembled monolayers of decanethiol on the gold surface. The substrate was a 60 mm diameter glass Petri dish, hydrophobized via boiling in an alcohol such as ethanol at 200 °C for 2 h.28 Micrometer-sized air bubbles with radii ranging from 30 to 200 μm were generated on the glass Petri dish surface in Milli-Q water using a custom-made ultrasonic transducer device, ELAC Nautic with RF generator type LVG 60-10, operated at a frequency of 515 kHz and a power output of approximately 25 W. A concentrated HEPES buffer solution was then added to the Petri dish along with aliquots of acid or base for pH correction. Prior to sonication, Milli-Q water used for bubble generation was saturated with nitrogen gas by bubbling through a constant stream of gas so that any components of atmospheric air affecting peptide switching or the interfacial interaction between the bubbles could be ignored. During AFM experiments, the Petri dish with bubbles was fitted onto the AFM stage and the cantilever was mounted onto the tip holder and lowered toward the Petri dish. Once the cantilever was fully submerged in water and brought near the surface, a chosen air bubble was attached to the cantilever by using the AFM as a micromanipulator. Air bubbles preferentially transferred to the cantilever tip due to the higher hydrophobicity of the cantilever compared to that of the alcohol-treated Petri dish.9 Micromolar quantities of the peptide were added to the Petri dish after an air bubble was attached to the cantilever. Bubbles were given 30 min to equilibrate after the addition of peptide or any change in solution conditions. The two bubbles are aligned with the geometry shown schematically in Figure 2a, and a video microscopy image of the two unaligned bubbles on the cantilever and substrate are shown in Figure 2b. The radius of the bubble, Rc, and the radius of the bubble on the substrate, Rs, were determined using optical video microscopy by comparison with the known cantilever dimensions. The contact angles of the bubbles on the cantilever, θc, and on the substrate, θs, were determined using the geometric relationship given by
Figure 1. The reported conformation of Lac21, Lac21E, and AM1 at the air−water interface at different pH values, in the absence or presence of metal ions. The nonswitching peptide, Lac21, is at the mobile detergent state (MDT) at both pH 3 and 7. Lac21E has a reasonable interfacial coverage at pH 3, and at pH 7 it is completely removed from the interface. AM1 is at the MDT in pH 7 and 3. It is able to cross-link at the interface at pH 7 in the presence of Zn2+ (black spheres), and when the pH is lowered to 3, AM1 returns to the MDT.
stability is a measure of the behavior of a new air−water interface created at each solution state, not the effect of changing the state of the peptide at an existing interface. This is significant, as the latter case may be more relevant for longerlasting bubble systems or emulsions. This study makes use of a well-developed methodology to directly measure the forces between two immobilized bubbles in aqueous solutions, using AFM7,8,25,26 to correlate the changes in peptide conformation to the interaction that exists between two air−water interfaces on the scale of nanometers. This approach has the advantage of being able to use the same interface for both solution conditions of the peptide and being able to decouple dynamic interactions, such as hydrodynamic drainage or interfacial rheology effects from equilibrium surface forces in a systematic fashion. This is difficult to achieve in macroscopic measurements such as foam stability studies. In addition, there is a conceptual gap in attempting to link the molecular conformational changes of the peptides at the air−water interface to bulk measurements without elucidating the connections between the molecular scale and the colloidal interaction forces. Thus, an understanding of the impact of the switchable peptide behavior on interaction forces between bubbles can then be utilized to better understand more complex bulk measurements in general.
⎛r ⎞ θi = 2π − sin−1⎜ i ⎟ ⎝ Ri ⎠
(1)
where r is the radius of contact of the bubble on the surface and i is either s or c. The cantilever was then lowered toward the chosen air bubble on the substrate and repeated force curves between the two bubbles were recorded. Additional aliquots of acid or base were added to the Petri dish to change the solution pH to onset peptide switching. 17231
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significant simplifications to the analysis of the theoretical models, resulting in an analytic formula that relates the measure of force in this region to the relative change in piezo motion, ΔX,31 and is given by
ΔX = hf +
F ⎛ ⎛ FR ⎞ 2πσ ⎞ ⎜⎜ln⎜ ⎟⎟ ⎟ + B(θc) + B(θs) − 1 − 2πσ ⎝ ⎝ 8πσR cR s ⎠ K ⎠
(2) where hf is an offset distance, σ is the interfacial tension, and R is defined by 1/R = 1/2 [1/(Rc) + 1/(Rs)]. The function B(θ) (eq 3) is based on the assumptions that the three-phase contact line is pinned32 and is given by
Figure 2. (a) A schematic diagram of two opposing air bubbles with the same central axis in aqueous buffer solution, and (b) a video microscopy image of two air bubbles in solution, one immobilized on the cantilever and another on a hydrophobized substrate, generated through ultrasound. The bubbles are shown off-axis for a clear view of both, simultaneously.
B(θ) = 1 +
1 ⎛⎜ 1 + cos θ ⎞⎟ ln 2 ⎝ 1 − cos θ ⎠
(3)
Thus, the slope of this region is dependent on the geometric parameters and the surface tension of the bubbles; therefore, the surface tension can be determined by the regression to this region if the bubbles' radii and contact angles are known.
A large number of force curves were recorded for cases where the interaction between bubbles was repulsive. Pendant Drop Tensiometry. Pendant drop tensiometry (PDT) was performed using a Dataphysics OCA20 tensiometer to examine the surface tensions of the interface at different pH conditions. The surface tension was measured in two different configurations (as shown in Figure 3). The first was for an air bubble in a peptide buffer
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RESULTS AND DISCUSSION Interactions between bubbles formed in buffer solutions were measured for each peptide. If repulsive interactions were observed first, then the same bubble pair, hence the same interface, was used to investigate the “switched” state through solution exchange for both pH directions (from 7 to 3 and 3 to 7). Table 1 summarizes the observations for the AFM Table 1. Experimental Observations of the AFM Bubble Force Measurements for the Interactions between Bubble Pairs in Aqueous Solutions of Either 120 μM Lac21, AM1, or Lac21E at Different pH Conditions.a AFM bubbles
Figure 3. The two configurations used for measuring surface tension using PDT. (a) An air bubble immersed in peptide buffer solution in a clear quartz cell, and (b) a peptide buffer solution drop in air.
foam column18,23
solution in a clear quartz cell (Figure 3a), which allowed the interfacial tension to be monitored continuously while the peptide was switched via addition of acid or base to the buffer solution. It must be noted that the solution was agitated to ensure the complete mixing of acid or base. The second configuration was the opposite of that just outlined, where the surface tension of an aqueous drop of peptide buffer solution in air was measured (Figure 3b). In this case, the pH could not be adjusted in situ, so the surface tension of peptide solutions at different pHs were measured separately. Force Curve Analysis. When two bubbles approach each other, their interaction could be repulsive, in which case the bubbles will be stable; or the interaction could be attractive, in which case the bubbles will coalesce and one bubble will peel off the surface and attach to the other. When the interaction is repulsive, the approach force between bubbles can be analyzed to extract the surface tension of the bubbles. Detailed theoretical models to describe the interplay between surface forces, interfacial deformation, and hydrodynamic drainage behavior between bubbles and drops of this size have been developed using a modified Young−Laplace equation and Reynolds lubrication theory to assist in the quantitative analysis of this type of AFM measurement. These models are summarized in the work of Dagastine and coworkers7 and Chan and co-workers,10,29 in addition to Carnie and coworkers.30 At higher forces, in other words, when the force curve exhibits a pseudolinear region as the disjoining pressure in the thin liquid film between the bubbles equals the Laplace pressure of the bubbles, the film approaches a constant thickness. This allows for
pH
Lac21
AM1
Lac21E
7 3 7/Zn2+ 3/Zn2+ 7 3 7/Zn2+ 3/Zn2+
c c c c c c c c
r c r c c c s c
r r − − c s − −
The overall bubble interaction was classified with “c” for coalescence and “r” for repulsion interactions. Earlier foam-column studies (conducted by Middelberg and co-workers18,23) for the same peptides are also summarized. Immediate foam collapse is denoted with a “c”, and a metastable foam is denoted with an “s”. a
experiments between bubble pairs for each peptide surfactant Lac21, AM1, and Lac21E at solution conditions that are expected to switch the conformation of AM1 and Lac21E. Coalescence was observed between bubble pairs in the presence of Lac21 for all conditions (an example is given in Figure 4a). When coalescence occurs, one bubble peels off the surface (usually the bubble on the substrate) and sticks to the other bubble to form one large bubble, as depicted in the inset of the schematic diagram in Figure 4(a). The formation of one bubble appears as a sharp spike in Figure 4(a). The coalescence events were observed at slow piezo drive rates of less than 1 μm/s, where hydrodynamic drainage forces were negligible.7,8,33 Thus, the Lac21 peptides present at the interface are insufficient to create a short-range repulsion to overcome the van der Waals (vdWs) attraction between the bubbles.34 However, at fast approach speeds (>1 μm/s), the bubbles were stabilized by hydrodynamics, where the thin liquid film between 17232
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Figure 5. A typical AFM force vs distance curve, where the force is recorded as a function of ΔX, as defined in the inset for bubble sizes of 15 μm on the cantilever and 15 to 100 μm on the substrate. The blue and red dots denote approach and retraction of the cantilever, respectively. The solid black line is the analytic model fitting to the approach force curve at high forces. The inset schematic diagram visualizes the interaction observed.
The AFM force curves can be analyzed to extract the surface tension of the interface in cases where the bubble interaction is repulsive. A previous analysis of the interactions between deformable interfaces measured using AFM has shown that at sufficiently high forces the slope of the force curve follows an analytic relationship with the geometry (i.e., radii and contact angle shown in the inset of Figure 5) and the surface tension of the bubbles, represented by eq 2.31 The surface tension values obtained from repulsive-force curve fitting are shown in Figure 6 for pH switches in both directions (acidic and basic). Independent pendant drop tensiometry (PDT) measurements were also used to track the changes in surface tension of an air bubble in a 120 μM Lac21E peptide buffer solution switching from pH 7 to 3 and from 3 to 7. These results show the adsorption/desorption of Lac21E peptides regardless of the initial pH (Figure 6). The surface tension obtained from fitting the AFM measurements show that when switched from pH 7 to 3, the peptides adsorb to the interface. However, when switching from pH 3 to 7, the surface tension did not reach the expected value based on the PDT measurements (see Figure 6). This discrepancy was initially a surprising result, as numerous previous studies using AFM in the presence of small synthetic surfactants have shown excellent agreement between surface tension measured using PDT and AFM using both bubbles and drops over a large range of concentrations.7−9,37,38 In addition, previous studies using microWilhelmy plate-type measurements, using a cylindrical37 or a nanoneedle38 attached to the tip of an AFM cantilever to measure the surface or surface tension of drops and bubbles of this size, have shown excellent agreement with PDT measurements for a range of surfactants. Thus, this observation is not expected to be an artifact of the AFM measurement, and a discussion of the details of these two measurements is required. The dynamics of the PDT measurements were on the scale of one to two minutes, where the observation time was much longer. Similarly, the AFM measurements employed equilibration times of up to 30 min after solution switching, thus both measurements appear to no longer demonstrate any time dependence to their surface tensions. The AFM and PDT
Figure 4. (a) A typical AFM force vs distance curve for bubble coalescence, where the force is recorded as a function of ΔX, as defined in the inset, for bubbles sizes ranging from 15 μm on the cantilever (Rc) and 15 to 100 μm on the substrate (Rs). The red and blue dots denote the approach and retraction of the cantilever, respectively. The inset diagram visualizes bubble coalescence. The dimensions [radius (R) and contact angle (θ)] of the bubbles on the cantilever and substrate used in the analytic model are also indicated in the inset diagram. (b) Typical force curves for air bubbles laden with 120 μM Lac21 at fast piezo drive speeds for the approach (top curves) and retract (bottom curves). The force curves display hydrodynamics at various speeds greater than 1 μm/s. The lines connecting the dots at each speed serve as a guide for the eye.
bubbles has insufficient time to adequately thin for a vdWs attraction to drive coalescence.7,8,35,36 An example of this is shown in Figure 4b for 120 μM Lac21-laden air bubbles interacting at various piezo drive speeds. The coalescence behavior between bubbles is consistent with previously published foam-column studies for Lac2118,23 (stability and collapse results for all three peptide surfactants are compiled in Table 1) that also report immediate collapse, consistent with an attractive surface force. For the first switchable system studied (Lac21E), a repulsive interaction, as shown in Figure 5, was observed between bubble pairs in the presence of Lac21E at both pH 7 and 3, for concentrations of 10 μM and 120 μM, respectively (Table 1). In contrast, bubble interactions with 5 μM Lac21E resulted in coalescence at both pHs. This suggests that the concentration of Lac21E must be well above 5 μM for a sufficient amount of surfactant to adsorb to the interface and overcome short-range (on the order of 5−10 nm8) vdWs attraction. 17233
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when using small volumes but because the discrepancy is for a system with desorption, not adsorption, this is unlikely to be relevant. The only notable difference left is that the PDT measurements were agitated. One could speculate that the gentle agitation assists in the desorption process due to the relative size of the peptide compared to traditional lower molecular weight surfactants that have not shown this discrepancy. In a larger context, difficulty with the desorption of surfactants is unlikely to be a surprise to any practitioner in surface science; for example, this is often why surface science laboratories employ very extensive cleaning procedures to clean glassware and surfaces for experimental investigations.26 In the context of this system, it does suggest that the desorption mechanisms of switchable systems may be more complex when the surfactant is a larger molecule than previously studied simple systems such as sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB). The AFM measurements in the presence of Lac21E exhibit repulsive interactions between air bubbles regardless of the bulk solution pH. These interactions are the result of the interplay between repulsive surface forces and bubble deformation.7 If the pressure in the film is comparable to the Laplace pressure of the bubbles, the film ceases to thin and instead grows laterally, resulting in an increasingly repulsive force.29 At pH 3, repulsive interactions are expected as Lac21E lowers the surface tension, decreases the Laplace pressure of the bubbles, and provides some short-range repulsion from either steric barriers or a residual charge on the peptide. At pH 7, the AFM observations are more complicated due to the path dependence of the switching of Lac21E. For bubbles initially formed in pH 3 solutions, the fitted surface tension values suggest only a small desorption of the Lac21E when switched to pH 7, thus the same stability mechanisms as pH 3 are expected. For bubbles initially formed at pH 7 solutions, repulsion is still observed and the surface tension is consistent with the PDT measurements indicating only a small amount of Lac21E adsorbed. Previous studies on force measurements between bubbles and drops have shown that even small amounts of surfactant are often sufficient to result in a repulsive interaction.7,8,29 Furthermore, the air−water interface has an isoelectric point (IEP) between 3 and 4 and is highly negatively charged at pH 7.34 Thus, it is possible that a residual surface charge on the bubble surface at pH 7 contributes to repulsive AFM bubble interactions for Lac21E peptides at pH 7, since the surface tension from AFM force curve fitting indicates a reasonably low level of adsorption. Indeed, Tabor and co-workers conducted an AFM study to investigate gas bubble interactions and have shown that air bubbles only coalesce when the bulk pH is in the region of 3.5− 6.5, and bubbles were always repulsive at and above pH 7.34 Previous foam-column experiments showed that a change to pH 7 resulted in immediate foam collapse.23 Bubble size and native surface charge appear to be more important for equilibrium force measurements at pH 7 in AFM, whereas the dynamic nature of foam and the range of varying bubble sizes may have a more dominant role in the foam-column behavior. AFM bubble experiments were also performed for the peptide AM1. The repulsive interaction shown in Figure 7 was observed between a number of bubble pairs in AM1 solutions at pH 7 and in the absence or presence of Zn2+. Conversely, coalescence between bubbles was always observed at pH 3 regardless of the presence of Zn2+. In addition, the analytic model was fitted to AM1 repulsive force curves, as shown by
Figure 6. The surface tension of the air−water interface of an air bubble in a 120 μM Lac21E peptide buffer solution obtained using PDT. The pH of the solution was changed several times at various intervals starting with (a) pH 7 and (b) pH 3, as indicated by the spanning arrows above the x axis. Surface tension obtained from fitting the AFM force curves for pH change from 7 to 3 (a) and 3 to 7 (b) is also indicated at the top of both graphs.
measurements both involve air bubbles immersed in a peptide buffer solution and employ the same air−water interface during the change of solution pH via the addition of an acid or base. The two measurements differ in the size of the bubbles, where AFM bubble radius ranges from 30 to 200 μm, whereas the apparent PDT radius ranged from 1 to 3 mm. Recent work by Alvarez and co-workers39 has shown that bubble size may impact adsorption dynamics based on kinetic rate-limited versus diffusion-limited transport mechanisms, but as these measurements are at long time scales and exhibit time invariant behavior, this is not expected to be important. The remaining two significant differences in the measurement are the larger solution volume and gentle agitation employed in the PDT measurements. The results with agitation produced readily reproducible measurements compared to those without agitation. In addition, agitation was employed in the PDT measurements in the literature for these systems.23,40 The fluid cell in AFM was not agitated due to space constraints and agitation would dislodge the immobilized bubbles. Solution exchange was always done with care to perturb the system as little as possible. Bulk concentration depletion could be an issue 17234
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Previous work on foam-column stability has shown enhanced foam stability at pH 7 and collapse at pH 3, where the presence of Zn2+ is required for a stable foam (as summarized in 0). This suggests that the switchability of the AM1 peptide is due to the cross linking in the film from the Zn2+ and the increase in the interfacial film strength for a dynamic foam with less dependence on equilibrium surface forces. Conversely, on the basis of the changes in equilibrium surface forces, one could speculate that if AM1 was used in a formulation, pH may be sufficient to switch stability. It is worth noting that the previous AM1 studies also observed an increase in surface tension using PDT at pH 3 with a similar PDT value to the one reported in this work.18,20 The difference in the current and previous studies is that the current study observes a 10 mN/m change in the surface tension when the pH is changed from pH 7 to 3, whereas in the previous study, the surface tension change was 4 mN/m for the same concentrations of AM1.18,20 There are several commercial sources for these peptides and some small variability in these materials is possible, thus some difference in the surface tension may be expected. This is unlikely to affect the cross linking with Zn2+ or force the behavior since in both this study and earlier work, the surface tension increased to approximately the same value at pH 3.
Figure 7. An approach leg of a repulsive force curve for 120 μM AM1 at pH 7 fitted with the analytic model (solid black line) to obtain the surface tension.
the solid line in Figure 7. PDT for a peptide buffer drop in air was also performed at pH 3 and 7. The surface tension results from both methods are summarized in Table 2 for comparison.
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Table 2. Comparison of the Surface Tension Results from Force Curve Fitting and PDT for 120 μM AM1a
CONCLUSIONS This study highlights the importance of making direct links between peptide conformation at the air−water interface and the equilibrium colloidal interaction forces to develop structure−function relationships for switching surfactant systems. These results illuminate the origins of the potential differences in liquid drainage between AFM bubbles and foam columns. The geometry and dynamics of drainage between a bubble pair in the AFM setup is very different to that in a foam column, which consists of a network of polyhedral-shaped gas pockets connected through a thin liquid film with junctions called Plateau borders.41 Differences in the length scales of the bubbles and the time scales of drainage could also lead to changes in the switching behavior of the peptide at the interface. This study also indicates that the effects of pathdependent behavior and interfacial equilibrium states are more pronounced when the air−water interface is maintained during switching of interfacial adsorbents. Adsorption of proteins is a complex process and the investigation of protein foam stability via air−water interfaces adds complexity to bubble−bubble interactions, such as surface forces and thin film drainage. The inability to draw general conclusions from the AFM bubble behavior and the interaction forces of the three different peptide surfactants at the air−water interface highlights the complexity of such biomolecules and the added complexity when deformable interfaces are involved. This study highlights the fact that the equilibrium forces in the thin film are unique for the three peptide surfactants, which differ only slightly in their primary amino acid sequences. This is why it is critical to have an understanding of the fundamental forces such that those in bulk systems could be better understood. Nevertheless, this study highlights that, in general, bubble stability is governed by the three key factors which are the hydrodynamic drainage behavior of the thin film in between the bubbles, the bubble deformation, and the equilibrium surface forces in the thin film, regardless of the presence of various surfactants at the interface.
AM1 surface tension (mN/m) pH
theoretical model
pendant drop tensiometry
7 3
39 ± 5 −
40 ± 2 52 ± 2
a PDT was performed for a peptide buffer solution drop in air, measured at pH 7 and pH 3 separately. The surface tension for AM1 solution at pH 7 and 3 in the presence of Zn2+ showed no difference to those reported in this table.
The surface tension obtained from fitting the AFM force curve data was similar to the PDT measurements for AM1 at pH 7. At pH 3, where coalescence was always observed from the AFM measurements, the PDT measurements showed an increase in surface tension, indicating description of some of the peptides from the interface. The surface tension changes measured from the AFM and PDT measurements are consistent with the observed interaction forces. Previous interfacial rheology studies of AM1 at the air−water interface have correlated the complexation of Zn2+ with AM1 at pH 7 to an increased interfacial stress at large strains.18,20 Interestingly, in the AFM measurements, the bubbles do not require a cohesive interfacial film for bubble stability; rather, the mere presence of interfacial adsorbents is sufficient to produce a repulsive interaction at pH 7, even in the absence of Zn2+. This is most likely reflective of the smaller deformation to the interface and the quasiequilibrium nature of the measurement. The coalescence observed between bubbles at pH 3 can be attributed to the desorption of the peptide surfactant at the interface that renders the air−water interface less deformable. Therefore, air bubbles are able to interact at closer separation distances on approach such that the vdWs attraction force can take over, leading to a coalescence event.7,8,35,36 Thus at pH 3, the interfacially adsorbed peptides were insufficient to stabilize the thin film formed between the two air bubbles. 17235
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(18) Dexter, A. F.; Malcolm, A. S.; Middelberg, A. P. J. Reversible active switching of the mechanical properties of a peptide film at a fluid-fluid interface. Nat. Mater. 2006, 5, 502−506. (19) Dexter, A. F.; Middelberg, A. P. J. Peptides as functional surfactants. Ind. Eng. Chem. Res. 2008, 47, 6391−6398. (20) Malcolm, A. S.; Dexter, A. F.; Middelberg, A. P. J. Foaming properties of a peptide designed to form stimuli-responsive interfacial films. Soft Matter 2006, 2, 1057−1066. (21) Malcolm, A. S.; Dexter, A. F.; Middelberg, A. P. J. Peptide surfactants (Pepfactants) for switchable foams and emulsions. Asia-Pac. J. Chem. Eng. 2007, 2, 362−367. (22) Dexter, A. F.; Middelberg, A. P. J. Switchable peptide surfactants with designed metal binding capacity. J. Phys. Chem. C 2007, 111, 10484−10492. (23) Middelberg, A. P. J.; He, L.; Dexter, A. F.; Shen, H. H.; Holt, S. A.; Thomas, R. K. The interfacial structure and Young’s modulus of peptide films having switchable mechanical properties. J. R. Soc., Interface 2008, 5, 47−54. (24) Jones, D. B.; Middelberg, A. P. J. Micromechanical testing of interfacial protein networks demonstrates ensemble behavior characteristic of a nanostructured biomaterial. Langmuir 2002, 18, 5585− 5591. (25) Vakarelski, I. U.; Lee, J.; Dagastine, R. R.; Chan, D. Y. C.; Stevens, G. W.; Grieser, F. Bubble colloidal afm probes formed from ultrasonically generated bubbles. Langmuir 2008, 24, 603−605. (26) Tabor, R. F.; Grieser, F.; Dagastine, R. R.; Chan, D. Y. C. Measurement and analysis of forces in bubble and droplet systems using AFM. J. Colloid Interface Sci. 2012, 371, 1−14. (27) Hutter, J. L.; Bechhoefer, J. Calibration of atomic-force microscope tips. Rev. Sci. Instrum. 1993, 64, 1868−1873. (28) Biggs, S.; Grieser, F. Atomic force microscopy imaging of thin films formed by hydrophobing reagents. J. Colloid Interface Sci. 1994, 165, 425−430. (29) Chan, D. Y. C.; Dagastine, R. R.; White, L. R. Forces between a rigid probe particle and a liquid interface. i. the repulsive case. J. Colloid Interface Sci. 2001, 236, 141−154. (30) Carnie, S. L.; Chan, D. Y. C.; Lewis, C.; Manica, R.; Dagastine, R. R. Measurement of dynamical forces between deformable drops using the atomic force microscope. I. theory. Langmuir 2005, 21, 2912−2922. (31) Manica, R.; Connor, J. N.; Dagastine, R. R.; Carnie, S. L.; Horn, R. G.; Chan, D. Y. C. Hydrodynamic forces involving deformable interfaces at nanometer separations. Phys. Fluids 2008, 20, 032101/1− 032101/12. (32) Nespolo, S. A.; Chan, D. Y. C.; Grieser, F.; Hartley, P. G.; Stevens, G. W. Forces between a rigid probe particle and a liquid interface: Comparison between experiment and theory. Langmuir 2003, 19, 2124−2133. (33) Webber, G. B.; Edwards, S. A.; Stevens, G. W.; Grieser, F.; Dagastine, R. R.; Chan, D. Y. C. Measurements of dynamic forces between drops with the AFM: Novel considerations in comparisons between experiment and theory. Soft Matter 2008, 4, 1270−1278. (34) Tabor, R. F.; Chan, D. Y. C.; Grieser, F.; Dagastine, R. R. Anomalous Stability of Carbon Dioxide in pH-Controlled Bubble Coalescence. Angew. Chem., Int. Ed. 2011, 50, 3454−3456. (35) Aston, D. E.; Berg, J. C. Thin-film hydrodynamics in fluid interface: Atomic force microscopy. Ind. Eng. Chem. Res. 2002, 41, 389−396. (36) Dagastine, R. R.; Prieve, D. C.; White, L. R. Forces between a rigid probe particle and a liquid interface III. Extraction of the planar half-space interaction energy E(D). J. Colloid Interface Sci. 2004, 269, 84−96. (37) McGuiggan, P. M.; Wallace, J. S. Maximum force technique for the measurement of the surface tension of a small droplet by AFM. The Journal of Adhesion 2006, 82, 997−1011. (38) Uddin, M. H.; Tan, S. Y.; Dagastine, R. R. Novel characterization of microdrops and microbubbles in emulsions and foams using atomic force microscopy. Langmuir 2011, 27, 2536−2544.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +61 (3)8344 4704. Fax: +61 (3) 8344 4153. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank F. Grieser and D. Y. C. Chan for their valuable discussions. We thank the ARC for funding and the PFPC for providing infrastructure support.
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REFERENCES
(1) Lake, L. Enhanced Oil Recovery; Prentice Hall: NJ, 1989. (2) Innovations in Flotation Technology; Mavros, P., Matis, K. A., Eds.; NATO Science Series E; Kluwer Academic Publishers: New York, 1992; Vol. 208. (3) Komtasu, H.; Yamada, H.; Fukushima, S. Rheological properties of soap foam. 1. Apparatus for viscoelastic measurement on foam. J. Soc. Cosmet. Chem. 1978, 29, 237−246. (4) Akers, R. J. Foams: Proceedings of a Symposium by the Society of Chemical Industry, Colloid and Surface Chemistry Group; Academic Press Inc. Ltd: London, 1976. (5) Churaev, N. V.; Starov, V. M.; Derjaguin, B. V. The shape of the transition zone between a thin film and bulk liquid and the line tension. J. Colloid Interface Sci. 1982, 89, 16−24. (6) Ivanov, I. B.; Radoev, B.; Manev, E.; Scheludko, A. Theory of the critical thickness of rupture of thin liquid films. Trans. Faraday Soc. 1970, 66, 1262−1273. (7) Dagastine, R. R.; Manica, R.; Carnie, S. L.; Chan, D. Y. C.; Stevens, G. W.; Grieser, F. Dynamic forces between two deformable oil droplets in water. Science 2006, 313, 210−213. (8) Vakarelski, I. U.; Manica, R.; Tang, X.; O’Shea, S. J.; Stevens, G. W.; Grieser, F.; Dagastine, R. R.; Chan, D. Y. C. Dynamic interactions between microbubbles in water. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 11177−11182. (9) Lockie, H. J.; Manica, R.; Stevens, G. W.; Grieser, F.; Chan, D. Y. C.; Dagastine, R. R. Precision AFM measurements of dynamic interactions between deformable drops in aqueous surfactant and surfactant-free solutions. Langmuir 2011, 27, 2676−2685. (10) Chan, D. Y. C.; Klaseboer, E.; Manica, R. Film drainage and coalescence between deformable drops and bubbles. Soft Matter 2011, 7, 2235−2264. (11) Mathur, A. M.; Drescher, B.; Scranton, A. B.; Klier, J. Polymeric emulsifiers based on reversible formation of hydrophobic units. Nature 1998, 392, 367−370. (12) Mertoglu, M.; Garnier, S.; Laschewsky, A.; Skrabania, K.; Storsberg, J. Stimuli responsive amphiphilic block copolymers for aqueous media synthesised via reversible addition fragmentation chain transfer polymerisation (RAFT). Polymer 2005, 46, 7726−7740. (13) Dumortier, G.; Grossiord, J.; Agnely, F.; Chaumeil, J. A review of poloxamer 407 pharmaceutical and pharmacological characteristics. J. Pharm. Res. 2006, 23, 2709−2728. (14) Shin, J. Y.; Abbott, N. L. Using light to control dynamic surface tensions of aqueous solutions of water soluble surfactants. Langmuir 1999, 15, 4404−4410. (15) Deshmukh, S.; Bromberg, L.; Smith, K. A.; Hatton, T. A. Photoresponsive behavior of amphiphilic copolymers of azobenzene and N,N-Dimethylacrylamide in aqueous solutions. Langmuir 2009, 25, 3459−3466. (16) Liu, Y.; Jessop, P. G.; Cunningham, M.; Eckert, C. A.; Liotta, C. L. Switchable surfactants. Science 2006, 313, 958−960. (17) Ma, X.; Ashaduzzaman, M.; Kunitake, M.; Crombez, R.; Texter, J.; Slater, L.; Mourey, T. Stimuli responsive poly(1-[11-acryloylundecyl]-3-methyl-imidazolium bromide): Dewetting and nanoparticle condensation phenomena. Langmuir 2011, 27, 7148−7157. 17236
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Article
(39) Alvarez, N. J.; Walker, L. M.; Anna, S. L. A microtensiometer to probe the effect of radius of curvature on surfactant transport to a spherical interface. Langmuir 2010, 26, 13310−13319. (40) Svitova, T. F.; Wetherbee, M. J.; Radke, C. J. Dynamics of surfactant sorption at the air/water interface: Continuous-flow tensiometry. J. Colloid Interface Sci. 2003, 261, 170−179. (41) Nguyen, A. V. Liquid drainage in single plateau borders of foam. J. Colloid Interface Sci. 2002, 249, 194−199.
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