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Fragile structured layers on surfaces detected by dynamic atomic force microscopy in aqueous electrolyte solutions
Advanced Powder Technol., Vol. 16, No. 3, pp. 213– 229 (2005) © VSP and Society of Powder Technology, Japan 2005. Also available online - www.vsppub.com
Original paper Fragile structured layers on surfaces detected by dynamic atomic force microscopy in aqueous electrolyte solutions YING LI, YOICHI KANDA, HIROYUKI SHINTO and KO HIGASHITANI ∗ Department of Chemical Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Received 23 July 2004; accepted 13 August 2004 Abstract—In the present study, the detailed characteristics of the layers of water molecules, ions and hydrated ions adsorbed on surfaces in NaCl solutions were investigated by introducing the dynamic method in the use of atomic force microscopy (AFM). We found the following. (1) There exist two kinds of structured layers on solid surfaces in electrolyte solutions — one is a thin, but firm primary layer in which water molecules, cations and hydrated cations are adsorbed directly on the solid surface, and the other is a thick, but fragile secondary layer outside the primary layer. (2) The thickness of the primary layer varies from 0.35 to 1.0 nm with the concentration of NaCl solution. (3) The secondary structure is detectable by introducing the dynamic method in the use of AFM, and the maximum gap at which the fragile structure starts to interact between surfaces was found to be around 4.0 nm at the maximum sensitivity of the present study. Keywords: Fragile structured layers; atomic force microscopy; adhesive force.
1. INTRODUCTION
Various phenomena of colloidal particles in electrolyte solutions are determined by the interaction forces between particle surfaces. It is well known that long-range interactions, e.g. at separations between surfaces h > 10 nm, are well expressed by the classical DLVO theory, but short-range interaction forces, e.g. h < 2 nm, cannot be predicted by the theory [1, 2]. To clarify this discrepancy, extensive studies on the molecular scale have been performed using atomic force microscopy (AFM) and the surface force apparatus (SFA). Israelachvili and Adam [3] carried out the pioneer work to prove that an additional force acts between mica surfaces in aqueous electrolyte solutions at a short separation distance. Later, Israelachvili and Pashley [4 –9] measured the short-range forces between mica surfaces in a series of ∗ To
whom correspondence should be addressed. E-mail: [email protected]
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electrolyte solutions, and concluded that they are attributable to the energy required to dehydrate the adsorbed ions. This short-range force is named the hydration force or structured force [10, 11]. Similar features of short-range forces were also reported in cases of silica surfaces in electrolyte solutions [12 –14]. Furthermore, Pashley pointed out that the strength and range of the hydration force increase with the hydration number. Moreover, the surface gel layer [15], the special adsorption of ions [16] and the surface asperity [17] were considered to give rise to short-range repulsion. Consequently, although advances have been made in understanding the interaction forces, the features at a small separation between two surfaces are still not understood completely. It is also important to note that these microstructures on the particle surface also influence the macroscopic behavior of particles, such as their stability and adhesive force [11, 18, 19]. Recently, we reported on the possibility that there exist two kinds of structured layers on the particle surface in solutions, measuring the interaction forces on the molecular scale with an AFM — one is a thin, but firm primary layer, which is composed of solvent molecules, ions and solvated ions adsorbed directly on the solid surface, and the other is a thick, but fragile secondary layer outside the primary layer [20, 21]. The existence of this secondary layer was also pointed out by Lyklema et al. [22], and very recently by Kim et al. [23] and Feibelman [24]. However, the general and detailed features of the fragile structure have not yet been clarified. In the previous study [21], we employed dimethylsulfoxide (DMSO) as the medium, because the high polarization of DMSO was expected to construct the structured layer on the surface more clearly than water. However, water is the most popular medium, so we must check whether similar phenomena can be observed or not. Hence, in the present study, a series of molecular-scale measurements of interactions between surfaces are carried out in aqueous NaCl solutions under various conditions and much more detailed characteristics of the fragile layer mentioned above are investigated, by introducing a dynamic method in the use of AFM.
2. EXPERIMENTAL
2.1. Materials A muscovite mica plate, which was freshly cleaved before use, was employed as a flat surface with a smoothness on the molecular scale. Either a Si3 N4 cantilever tip coated by SiO2 or the probe of a 5.5 μm colloidal silica was used as the spherical surface. The radius of curvature of the probe tip was found to be 24 ± 5 nm by scanning electron microscopy. The cantilever probe was used to achieve direct contact between the surfaces of the probe tip and the mica plate, by breaking the adsorbed layers with the sharp tip, while the colloid probe was used to investigate interactions between relatively large surfaces.
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The water used here was purified first by an ion-exchange and distillation apparatus, and then by a Millipore apparatus. NaCl of analytical reagent grade was used without further purification as an electrolyte. 2.2. Force measurements All the force measurements were performed using AFM with a liquid cell (Nanoscope IIIa; Digital Instruments). The fluid cell was composed of an upper glass plate with a cantilever, a lower sample plate attached to the piezo system and a Teflon O-ring as the spacer. The spring constant of the cantilever used here was 0.32 ± 0.01 N m−1 , which was determined following the procedure reported elsewhere [25]. The colloid probe was prepared by gluing a silica particle on the apex of the cantilever with an extremely small quantity of epoxy resin. Force measurements were carried out between a mica plate and either the cantilever tip or the colloidal probe in solutions. A detailed description of AFM manipulation for force measurements is provided elsewhere [18], but a brief description was given below. After the fluid cell was filled with a given solution, the lower plate was moved upward to the cantilever with a given scan rate by controlling the piezo system. When the two surfaces start to interact with each other, the cantilever bends, which is detected by the shift of the position on the photodiode of the laser beam reflected on the upper surface of the cantilever. The separation distance was calculated by following the method of Ducker et al. [26]. A typical force curve for surfaces with the same charge in a solution is shown schematically in Fig. 1. When two surfaces approach, they repel each other electrostatically at a relatively long separation distance, but jump into contact at a short separation because of the van der Waals (vdW) attraction. When water molecules, ions and hydrated ions adsorb on the surface and form an adsorbed layer, a very strong repulsive force may appear at a very small separation distance. In this case, the surfaces must be pushed closer to break the adsorbed layers and achieve direct contact. The repulsive force curve at this small separation originates from the breaking energy of the adsorbed layer. After the surfaces were pushed together as much as possible, they were retracted until they detached from each other by jumping out of the cantilever. The adhesive force Fad is defined as the sum of the absolute values of the pull-off force Foff and Frep , where Frep is the force given by extrapolating the electrostatic force curve to happ = 0, as shown in Fig. 1. A detailed explanation of this definition of Fad is given elsewhere [18]. Values of Fad in this study were obtained by averaging the values measured at least 13 different points, using three different mica substrates and three different cantilevers. As for the maximum pushing force Fm , a constant value Fm = 13.5 ± 1.0 nN was taken throughout the present experiment, unless specified. When the medium was changed, the fluid cell was thoroughly flushed by the new solution and left standing for about 40 min to stabilize the system. All the measurements were performed at room temperature (25 ± 2 ◦ C).
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Figure 1. A typical interaction force curve between two surfaces with the same charge in an aqueous electrolyte solution.
It must be emphasized here that the separation between surfaces is regarded as zero in AFM measurements if the two surfaces move with the same speed, but this does not guarantee that two surfaces contact directly with each other. However, this is the only way to define the separation in AFM measurements. Hence, the separation measured in the present study is essentially the apparent separation happ .
3. RESULTS AND DISCUSSION
3.1. Force curves and adhesion at the slowest scan rate First of all, the interaction and adhesion between surfaces in solutions of various NaCl concentrations, Ce , were examined at the lowest scan rate, v = 2 nm/s, to minimize the hydrodynamic effects on force measurements. We consider that interactions between surfaces measured at this very slow scan rate are nearly under the equilibrium condition. 3.1.1. Interactions between the probe tip and mica plate. Figure 2a shows the approaching and separating force curves in NaCl solutions of various concentrations between a probe tip and a mica plate, and Fig. 2b shows the corresponding adhesive forces for various maximum pushing forces Fm . We can slightly detect a long-range
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(a)
(b) Figure 2. Force curves and the corresponding adhesive forces for tip–mica systems given by the slowest scan rate v = 2 nm/s at various NaCl concentrations. (a) Approaching and separating force curves. (b) Adhesive forces under various experimental conditions.
electrostatic repulsive force at Ce = 10−4 M. However, as the concentration of NaCl increases, say Ce = 1.0 M, the long-range force disappears and a small attractive force appears at a small separation distance, which is regarded as the vdW attractive force. It is important to note that an additional short-range repulsive force appears at h < 2 nm, especially at Ce = 10−2 and 1.0 M, and the onset separation for the repulsion force increases with increasing NaCl concentration. As far as the longrange repulsion and the vdW attraction are concerned, the features are consistent
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with the classical DLVO theory. However, the strong repulsion at h < 2 nm is not expressed by the DLVO theory, so that we consider these force curves originate from the breaking energy of the layer composed of water molecules and ions adsorbed on the surface. Figure 2b shows that the value of Fad is independent of not only the electrolyte concentration Ce , but also the maximum pushing force Fm . This constancy of Fad value implies that the sharp probe tip penetrates into the adsorbed layer, and the tip and mica surfaces contact directly.
(a)
(b) Figure 3. Force curves and the corresponding adhesive forces for silica–mica systems given by the slowest scan rate v = 2 nm/s at various NaCl concentrations. (a) Approaching and separating force curves. (b) Adhesive forces under various experimental conditions.
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3.1.2. Interactions between the colloid probe and mica plate. Figure 3a shows the approaching and separating force curves for the same solutions as used in the above experiments, but a colloid probe was employed instead of the cantilever tip. When the NaCl concentration is low, both the electrostatic force and the vdW force appear clearly. These features of F versus Ce are, more or less, the same as those in Fig. 2a, except that the strong repulsive force at h < 2 nm disappears here, even at Ce = 10−2 and 1.0 M. We consider that this disappearance of the repulsion is attributable to the fact that there exist strongly structured layers on surfaces so that they are unable to be broken by the large surface of the colloid probe. Figure 3b shows the corresponding adhesive forces for various values of Fm and Ce . It is clear that the independency of Fad on Fm is the same as shown in Fig. 2b, but the value of Fad does depend on Ce , i.e. Fad value decreases with increasing Ce . The independency of Fad on Fm implies that the structured layer on the surfaces is strong enough not to be broken by the pressure given by this range of pushing force. The dependence of Fad on Ce indicates that the thickness of the adsorbed layer increases with increasing Ce . It is well known that the force normalized by the surface radius, F /R, represents the corresponding free energy between two parallel plates per unit area [8]. This indicates that the values of Fad /R in Figs 2b and 3b are able to be compared quantitatively, even though their radii of surfaces are different. It is clear that the values of Fad /R in Fig. 3b are much smaller than that those in Fig. 2b. These results imply that the surfaces do not contact each other directly in the case of Fig. 3b, i.e. there must be some structured layer sandwiched formed by two surfaces, and the thickness increases with the ionic concentration of the bulk solution. 3.1.3. Gap between surfaces at their adhesion. Assuming that the adhesion is due to the vdW attraction, it is possible to estimate the gap between the surfaces at their adhesion, had . However, this estimation must be very careful, because the estimated gap is very sensitive to the value of the Hamaker constant A, whose true value is not easy to estimate. Nevertheless, it is important to estimate the values of had to know the physical meanings of the data in Fig. 2b and 3b. According to the Lifshitz theory [11], the values of A for the tip–water–mica and silica–water– mica systems are calculated to be 4.314 × 10−20 and 1.074 × 10−20 J, respectively. These values are very similar to the those reported elsewhere; 3.40–3.55 × 10−20 J for the tip–water–mica system and 1.19–1.29 × 10−20 J for the silica–water–mica system [27]. Then the values of had can be estimated using the vdW equation and the measured values of Fad : Fad /R = A/6h2 .
(1)
Figure 4 shows the dependence of had on Ce for the tip–mica and silica–mica systems. It is clear that the gap for the tip–mica system is constant at about 0.35 nm independently of Ce . It is often assumed that the minimum gap between directly contacting surfaces is around 0.4 nm [11]. Not only because of this coincidence
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Figure 4. Comparison between the gaps calculated by (1) for the tip–mica and silica–mica systems at various NaCl concentrations, where v = 2 nm/s.
between these gap values, but also the constancy of Fad given in Fig. 2b, we consider that the probe tip contacts with the mica surface directly. As for the silica–mica system, the gap increases gradually from around 0.70 to around 1.35 nm, so that the difference in gaps between the tip–mica and silica– mica systems varies from around 0.35 to 1.0 nm. We consider that this difference represents the thickness of the firmly adsorbed layer of medium molecules which cannot be destroyed by the given pushing force of the present colloid probe. It is clear that the gap mentioned above is of the same order of the size of a water molecule and hydrated Na+ , i.e. 0.28 and 0.72 nm, respectively, [11]. Hence, it is plausible to assume that the gap is composed of the firmly adsorbed layer of Na+ , hydrated Na+ and water molecules, and the increase of the gap width with Ce is due to the increasing adsorption of these ions. We call this firmly adsorbed layer the ‘primary adsorbed layer’. 3.2. Dependence of force curve and adhesion on scan rate We usually assume that the scan rate does not affect the characteristics of force curves, because the movement of medium molecules is much faster than that of surfaces in the AFM liquid cell, so that an equilibrium between the surface and bulk solution is attained instantaneously at every moment, except for the case in which some structured layers are formed on the surfaces [28]. In this subsection, the effects of the scan rate are examined under various conditions in order to investigate the more detailed characteristics of the structure on the surface. 3.2.1. Dependence of interactions between the probe tip and mica plate on the scan rate. Figure 5a and b shows the dependence of approaching force curves of the tip–mica system on the scan rate v at Ce = 10−4 and 1.0 M, respectively, where the data at v = 2 nm/s are the same as those given in Fig. 2. It is clear that the scan rate hardly affects the force curves, independently of NaCl concentrations, and that the adsorbed layer is negligibly thin in the case of Ce = 10−4 M, but rather thick in
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the case of Ce = 1 M. It is important to note that the thickness of the adsorbed layer can be estimated to be around 1.0 nm from the force curves in Fig. 5b, which agrees well with the thickness of the adsorbed layer at Ce = 1.0 M estimated by Fig. 4. Figure 6 presents the corresponding dependence of Fad on v in NaCl solutions of various concentrations. It is found that values of Fad are independent of not only v, but also Ce , even though the approaching force curves are largely different as shown in Fig. 5a and b. This confirms the conclusion obtained from Fig. 2b, i.e. the probe tip contacts with the mica surface directly under the present experimental conditions.
(a)
(b) Figure 5. Approaching force curves at various scan rates for tip–mica systems. (a) Ce = 10−4 M and (b) Ce = 1 M.
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Figure 6. The adhesive forces calculated using all the data in Fig. 5.
3.2.2. Dependence of interactions between the silica particle and mica plate on the scan rate. Figure 7a and b shows the dependence of approaching force curves of the silica–mica system on v at Ce = 10−4 and 1 M, respectively, where the data at v = 2 nm/s are the same as those given in Fig. 3. It is clear that force curves for Ce = 10−4 M are independent of v, while those for Ce = 1.0 M depend strongly on the magnitude of v. These results indicate the following: (i) Structural adsorbed layers are formed on the surfaces in a concentrated electrolyte solution, which contribute as the repulsive resistance when two surfaces approach. (ii) The layer is mainly composed of cations and solvated cations, because surfaces are negatively charged and the layer thickness increases with increasing Ce . (iii) The structured layer must be very fragile, because it appears only when a large surface and a high approach velocity are employed. (iv) The strength of the structured layer decays with the separation distance, because the apparent thickness of the layer increases with increasing v. This fragile structured layer outside the primary layer is named the ‘secondary structured layer’. There is a possibility that these force curves may be generated by the hydrodynamic drag force caused by the squeezing flow between two approaching surfaces. However, it is found by the theoretical calculation that this hydrodynamic effect is negligibly small [29]. This is also confirmed by the fact that force curves at Ce = 10−4 M do not depend on v, as shown in Fig. 7a. Figure 8a and b shows the dependence of Fad of the silica–mica system on v for various values of Ce , and the corresponding minimum gap had calculated by (1), where the data for the tip–mica system are also shown for the sake of comparison. It is clear that the value of Fad decreases with increasing values of Ce and v. The reason why the value of Fad decreases with Ce at a given value of v is that the thickness of the adsorbed layer becomes thicker with increasing Ce and the gap between surfaces becomes larger at the same maximum pushing force, as shown
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(a)
(b) Figure 7. Approaching force curves at various scan rates for silica–mica systems. (a) Ce = 10−4 M and (b) Ce = 1 M.
in Fig. 8b. On the other hand, the reason why the value of Fad decreases with v at a given value of Ce is that the resistance by the structured layer increases with v, so that the penetration of two surfaces into their adsorbed layers is shortened as v increases, i.e. the minimum gap between surfaces becomes wider, as shown in Fig. 8b. 3.2.3. Primary and secondary layers. We concluded above that the difference of had values between the silica–mica and tip–mica systems at v = 2 nm/s represents
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(a)
(b) Figure 8. Adhesive forces and the corresponding minimum gap between surfaces for silica–mica systems at various scan rates and electrolyte concentrations. (a) Adhesive forces under various conditions. (b) The minimum gaps between surfaces under various conditions. (The data for the tip–mica system are also shown for the sake of comparison.)
the thickness of the primary adsorbed layer. However, this difference increases continuously with v, as shown in Fig. 8b. Hence, it looks that the definite difference between the primary and secondary layers is unable to be defined. To investigate whether we can classify the primary and secondary layers, the dependence of Fad on the maximum pushing force Fm was examined in the cases of v = 2 and 2000 nm/s, as shown in Fig. 9. It is found that the value of Fad for v = 2 nm/s is constant, independently of Fm . We consider that the reason for this constancy of Fad is that
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Figure 9. Dependence of the adhesive force on the maximum pushing force for silica–mica systems at v = 2 and 2000 nm/s.
the fragile structure does not provide resistance for the movement of the probe if the velocity is very slow, so that the probe surface contacts directly the outside of the primary layer and the primary layer is unable to be broken in the range of this pushing force. On the other hand, the value of Fad for v = 2000 nm/s varies with Fm and it approaches asymptotically to the value for v = 2 nm/s. Finally, both values coincide with each other at a strong pushing force, Fm = 35.8 mN/m. This confirms that the thickness given by the data at v = 2 nm/s represents the thickness of the primary adsorbed layer. Hence, we can define the secondary layer, such that the structure formed outside the primary layer is the secondary layer. The secondary layer is so fragile that the apparent thickness varies with v and Fm . Figure 10a shows the approaching force curves for the maximum scan rate we can produce in this study, v = 2000 nm/s, at various values of Fm . It is plausible to assume that the sensitivity to detect the fragile structure increases with the scan rate, but not with the maximum pushing force. Hence, all the force curves in Fig. 10a must have the same sensitivity to detect the fragile structure, so that they must be essentially the same force curve. Because the surfaces contact at the outside surface of the primary layer at Fm /R = 35.8 mN/m, as shown in Fig. 9, the force curve at Fm /R = 35.8 mN/m must cover the whole range of the secondary layer. When the value of Fm becomes smaller, the minimum gap becomes wider. Because this gap can be estimated as in Fig. 8b, the force curves for Fm /R = 5.2, 8.6 and 17.0 mN/m are shifted by their corresponding gaps as shown in Fig. 10b. It is clear that all the force curves coincide well with that of Fm /R = 35.8 mN/m, which we call the master curve here. This indicates that the data presented here are self-consistent and reliable, and the thickness of the fragile structure is about the half of 4.0 nm in the case of the silica–mica system in a 1 M NaCl solution, if the surface property of silica and mica surfaces is nearly the same.
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(a)
(b) Figure 10. Force curves of silica–mica systems at Ce = 1 M for the various maximum pushing forces. (a) The original force curves. (b) The curves shifted by the gap calculated by (1).
3.2.4. Apparent viscosity of solution within the gap. The fragile structure discussed above indicates that the apparent viscosity in the gap between two surfaces must vary with the separation distance. It is well known that, when a sphere approaches a flat plate, the drag force on the sphere Fdrag increases because of the squeezing flow within a small gap, as shown below: Fdrag = 6π μRvβ,
(2)
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Figure 11. Dependence of the apparent relative viscosity calculated from the data at v = 2000 nm/s on the separation.
where μ is the fluid viscosity and β is the correction factor approximated by the following equation: β=
6h¯ 2 + 13h¯ + 2 , 6h¯ 2 + 4h¯
h h¯ = . R
(3)
This equation is not applicable when the viscosity varies with position and the fluid is a structured fluid whose formation is dependent on time, as in the present case. Hence, it is not appropriate to calculate an apparent viscosity using this equation in the present system. Nevertheless, we used this equation to roughly estimate the order of magnitude of the viscosity increase near the surface due to the fragile structure. As mentioned above, the fragile structure depends on the magnitude of Fm and v, so that the apparent viscosity μv = 2000 evaluated using the master curve of v = 2000 nm/s is valid only in the case of v = 2000 nm/s in a 1 M NaCl solution. Substituting the values evaluated by AFM, i.e. values of Fdrag in Fig. 10b and the corresponding approaching velocity of two surfaces, into (2) and (3), the change of the apparent viscosity normalized by the bulk viscosity μ0 (=1.12 × 10−3 Pa s for 1 M NaCl solution), μv = 2000/μ0 , with the separation happ is estimated as shown in Fig. 11, where happ = 0 indicates the outside of the primary layer. It is clear that the apparent viscosity increases rapidly with decreasing happ at happ < 4.0 nm, where the fragile structure starts to interact between the two surfaces. It is important to note that the apparent viscosity near the surface is 3 orders of magnitude greater than that of the bulk solution. Of course this evaluation is not a quantitative one, but we can imagine the steep increase of viscosity near the solid–liquid interfaces. The similar increase of viscosity was also reported elsewhere, in which the viscosity is 6 orders of magnitude greater than that of bulk water [23]. The discussion on the more detailed characteristics of the molecular-scale structure of the secondary layer is out of the range of the present study. To clarify the
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mechanism of the formation of the secondary layer, further sophisticated experiments and the help of the computer simulation will be needed.
4. CONCLUSION
In the present study, detailed characteristics of the layers of water molecules, ions and hydrated ions adsorbed on surfaces in NaCl solutions were investigated by introducing the dynamic method in the use of AFM, and the following conclusions were drawn: (i) There exist two kinds of structured layers on the solid surface in electrolyte solutions — one is a thin, but firm primary layer in which water molecules, cations and hydrated cations are adsorbed directly on the solid surface, and the other is a thick, but fragile secondary layer outside the primary layer. (ii) As for the tip–mica systems, two surfaces contact directly each other with the minimum gap of around 0.35 nm, in spite of the various experimental conditions of the present study. (iii) When the slowest scan rate v = 2 nm/s is employed, the thickness of the primary layer is able to be determined. It is found that thickness of the primary layer varies from 0.35 to 1.0 nm depending on the concentration of NaCl solution. (iv) By introducing the dynamic method in the use of AFM, the fragile secondary structure is able to be detected. The sensitivity to detect the fragile structure increases with increasing scan rate and the maximum gap at which the fragile structure starts to interact between surfaces is around 4.0 nm at the maximum sensitivity of the present study. (v) The apparent viscosity near the solid–liquid interface at the molecular scale increases greatly because of the fragile structure.
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Original paper Fragile structured layers on surfaces detected by dynamic atomic force microscopy in aqueous electrolyte solutions YING LI, YOICHI KANDA, HIROYUKI SHINTO and KO HIGASHITANI ∗ Department of Chemical Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Received 23 July 2004; accepted 13 August 2004 Abstract—In the present study, the detailed characteristics of the layers of water molecules, ions and hydrated ions adsorbed on surfaces in NaCl solutions were investigated by introducing the dynamic method in the use of atomic force microscopy (AFM). We found the following. (1) There exist two kinds of structured layers on solid surfaces in electrolyte solutions — one is a thin, but firm primary layer in which water molecules, cations and hydrated cations are adsorbed directly on the solid surface, and the other is a thick, but fragile secondary layer outside the primary layer. (2) The thickness of the primary layer varies from 0.35 to 1.0 nm with the concentration of NaCl solution. (3) The secondary structure is detectable by introducing the dynamic method in the use of AFM, and the maximum gap at which the fragile structure starts to interact between surfaces was found to be around 4.0 nm at the maximum sensitivity of the present study. Keywords: Fragile structured layers; atomic force microscopy; adhesive force.
1. INTRODUCTION
Various phenomena of colloidal particles in electrolyte solutions are determined by the interaction forces between particle surfaces. It is well known that long-range interactions, e.g. at separations between surfaces h > 10 nm, are well expressed by the classical DLVO theory, but short-range interaction forces, e.g. h < 2 nm, cannot be predicted by the theory [1, 2]. To clarify this discrepancy, extensive studies on the molecular scale have been performed using atomic force microscopy (AFM) and the surface force apparatus (SFA). Israelachvili and Adam [3] carried out the pioneer work to prove that an additional force acts between mica surfaces in aqueous electrolyte solutions at a short separation distance. Later, Israelachvili and Pashley [4 –9] measured the short-range forces between mica surfaces in a series of ∗ To
whom correspondence should be addressed. E-mail: [email protected]
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electrolyte solutions, and concluded that they are attributable to the energy required to dehydrate the adsorbed ions. This short-range force is named the hydration force or structured force [10, 11]. Similar features of short-range forces were also reported in cases of silica surfaces in electrolyte solutions [12 –14]. Furthermore, Pashley pointed out that the strength and range of the hydration force increase with the hydration number. Moreover, the surface gel layer [15], the special adsorption of ions [16] and the surface asperity [17] were considered to give rise to short-range repulsion. Consequently, although advances have been made in understanding the interaction forces, the features at a small separation between two surfaces are still not understood completely. It is also important to note that these microstructures on the particle surface also influence the macroscopic behavior of particles, such as their stability and adhesive force [11, 18, 19]. Recently, we reported on the possibility that there exist two kinds of structured layers on the particle surface in solutions, measuring the interaction forces on the molecular scale with an AFM — one is a thin, but firm primary layer, which is composed of solvent molecules, ions and solvated ions adsorbed directly on the solid surface, and the other is a thick, but fragile secondary layer outside the primary layer [20, 21]. The existence of this secondary layer was also pointed out by Lyklema et al. [22], and very recently by Kim et al. [23] and Feibelman [24]. However, the general and detailed features of the fragile structure have not yet been clarified. In the previous study [21], we employed dimethylsulfoxide (DMSO) as the medium, because the high polarization of DMSO was expected to construct the structured layer on the surface more clearly than water. However, water is the most popular medium, so we must check whether similar phenomena can be observed or not. Hence, in the present study, a series of molecular-scale measurements of interactions between surfaces are carried out in aqueous NaCl solutions under various conditions and much more detailed characteristics of the fragile layer mentioned above are investigated, by introducing a dynamic method in the use of AFM.
2. EXPERIMENTAL
2.1. Materials A muscovite mica plate, which was freshly cleaved before use, was employed as a flat surface with a smoothness on the molecular scale. Either a Si3 N4 cantilever tip coated by SiO2 or the probe of a 5.5 μm colloidal silica was used as the spherical surface. The radius of curvature of the probe tip was found to be 24 ± 5 nm by scanning electron microscopy. The cantilever probe was used to achieve direct contact between the surfaces of the probe tip and the mica plate, by breaking the adsorbed layers with the sharp tip, while the colloid probe was used to investigate interactions between relatively large surfaces.
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The water used here was purified first by an ion-exchange and distillation apparatus, and then by a Millipore apparatus. NaCl of analytical reagent grade was used without further purification as an electrolyte. 2.2. Force measurements All the force measurements were performed using AFM with a liquid cell (Nanoscope IIIa; Digital Instruments). The fluid cell was composed of an upper glass plate with a cantilever, a lower sample plate attached to the piezo system and a Teflon O-ring as the spacer. The spring constant of the cantilever used here was 0.32 ± 0.01 N m−1 , which was determined following the procedure reported elsewhere [25]. The colloid probe was prepared by gluing a silica particle on the apex of the cantilever with an extremely small quantity of epoxy resin. Force measurements were carried out between a mica plate and either the cantilever tip or the colloidal probe in solutions. A detailed description of AFM manipulation for force measurements is provided elsewhere [18], but a brief description was given below. After the fluid cell was filled with a given solution, the lower plate was moved upward to the cantilever with a given scan rate by controlling the piezo system. When the two surfaces start to interact with each other, the cantilever bends, which is detected by the shift of the position on the photodiode of the laser beam reflected on the upper surface of the cantilever. The separation distance was calculated by following the method of Ducker et al. [26]. A typical force curve for surfaces with the same charge in a solution is shown schematically in Fig. 1. When two surfaces approach, they repel each other electrostatically at a relatively long separation distance, but jump into contact at a short separation because of the van der Waals (vdW) attraction. When water molecules, ions and hydrated ions adsorb on the surface and form an adsorbed layer, a very strong repulsive force may appear at a very small separation distance. In this case, the surfaces must be pushed closer to break the adsorbed layers and achieve direct contact. The repulsive force curve at this small separation originates from the breaking energy of the adsorbed layer. After the surfaces were pushed together as much as possible, they were retracted until they detached from each other by jumping out of the cantilever. The adhesive force Fad is defined as the sum of the absolute values of the pull-off force Foff and Frep , where Frep is the force given by extrapolating the electrostatic force curve to happ = 0, as shown in Fig. 1. A detailed explanation of this definition of Fad is given elsewhere [18]. Values of Fad in this study were obtained by averaging the values measured at least 13 different points, using three different mica substrates and three different cantilevers. As for the maximum pushing force Fm , a constant value Fm = 13.5 ± 1.0 nN was taken throughout the present experiment, unless specified. When the medium was changed, the fluid cell was thoroughly flushed by the new solution and left standing for about 40 min to stabilize the system. All the measurements were performed at room temperature (25 ± 2 ◦ C).
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Figure 1. A typical interaction force curve between two surfaces with the same charge in an aqueous electrolyte solution.
It must be emphasized here that the separation between surfaces is regarded as zero in AFM measurements if the two surfaces move with the same speed, but this does not guarantee that two surfaces contact directly with each other. However, this is the only way to define the separation in AFM measurements. Hence, the separation measured in the present study is essentially the apparent separation happ .
3. RESULTS AND DISCUSSION
3.1. Force curves and adhesion at the slowest scan rate First of all, the interaction and adhesion between surfaces in solutions of various NaCl concentrations, Ce , were examined at the lowest scan rate, v = 2 nm/s, to minimize the hydrodynamic effects on force measurements. We consider that interactions between surfaces measured at this very slow scan rate are nearly under the equilibrium condition. 3.1.1. Interactions between the probe tip and mica plate. Figure 2a shows the approaching and separating force curves in NaCl solutions of various concentrations between a probe tip and a mica plate, and Fig. 2b shows the corresponding adhesive forces for various maximum pushing forces Fm . We can slightly detect a long-range
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(a)
(b) Figure 2. Force curves and the corresponding adhesive forces for tip–mica systems given by the slowest scan rate v = 2 nm/s at various NaCl concentrations. (a) Approaching and separating force curves. (b) Adhesive forces under various experimental conditions.
electrostatic repulsive force at Ce = 10−4 M. However, as the concentration of NaCl increases, say Ce = 1.0 M, the long-range force disappears and a small attractive force appears at a small separation distance, which is regarded as the vdW attractive force. It is important to note that an additional short-range repulsive force appears at h < 2 nm, especially at Ce = 10−2 and 1.0 M, and the onset separation for the repulsion force increases with increasing NaCl concentration. As far as the longrange repulsion and the vdW attraction are concerned, the features are consistent
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with the classical DLVO theory. However, the strong repulsion at h < 2 nm is not expressed by the DLVO theory, so that we consider these force curves originate from the breaking energy of the layer composed of water molecules and ions adsorbed on the surface. Figure 2b shows that the value of Fad is independent of not only the electrolyte concentration Ce , but also the maximum pushing force Fm . This constancy of Fad value implies that the sharp probe tip penetrates into the adsorbed layer, and the tip and mica surfaces contact directly.
(a)
(b) Figure 3. Force curves and the corresponding adhesive forces for silica–mica systems given by the slowest scan rate v = 2 nm/s at various NaCl concentrations. (a) Approaching and separating force curves. (b) Adhesive forces under various experimental conditions.
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3.1.2. Interactions between the colloid probe and mica plate. Figure 3a shows the approaching and separating force curves for the same solutions as used in the above experiments, but a colloid probe was employed instead of the cantilever tip. When the NaCl concentration is low, both the electrostatic force and the vdW force appear clearly. These features of F versus Ce are, more or less, the same as those in Fig. 2a, except that the strong repulsive force at h < 2 nm disappears here, even at Ce = 10−2 and 1.0 M. We consider that this disappearance of the repulsion is attributable to the fact that there exist strongly structured layers on surfaces so that they are unable to be broken by the large surface of the colloid probe. Figure 3b shows the corresponding adhesive forces for various values of Fm and Ce . It is clear that the independency of Fad on Fm is the same as shown in Fig. 2b, but the value of Fad does depend on Ce , i.e. Fad value decreases with increasing Ce . The independency of Fad on Fm implies that the structured layer on the surfaces is strong enough not to be broken by the pressure given by this range of pushing force. The dependence of Fad on Ce indicates that the thickness of the adsorbed layer increases with increasing Ce . It is well known that the force normalized by the surface radius, F /R, represents the corresponding free energy between two parallel plates per unit area [8]. This indicates that the values of Fad /R in Figs 2b and 3b are able to be compared quantitatively, even though their radii of surfaces are different. It is clear that the values of Fad /R in Fig. 3b are much smaller than that those in Fig. 2b. These results imply that the surfaces do not contact each other directly in the case of Fig. 3b, i.e. there must be some structured layer sandwiched formed by two surfaces, and the thickness increases with the ionic concentration of the bulk solution. 3.1.3. Gap between surfaces at their adhesion. Assuming that the adhesion is due to the vdW attraction, it is possible to estimate the gap between the surfaces at their adhesion, had . However, this estimation must be very careful, because the estimated gap is very sensitive to the value of the Hamaker constant A, whose true value is not easy to estimate. Nevertheless, it is important to estimate the values of had to know the physical meanings of the data in Fig. 2b and 3b. According to the Lifshitz theory [11], the values of A for the tip–water–mica and silica–water– mica systems are calculated to be 4.314 × 10−20 and 1.074 × 10−20 J, respectively. These values are very similar to the those reported elsewhere; 3.40–3.55 × 10−20 J for the tip–water–mica system and 1.19–1.29 × 10−20 J for the silica–water–mica system [27]. Then the values of had can be estimated using the vdW equation and the measured values of Fad : Fad /R = A/6h2 .
(1)
Figure 4 shows the dependence of had on Ce for the tip–mica and silica–mica systems. It is clear that the gap for the tip–mica system is constant at about 0.35 nm independently of Ce . It is often assumed that the minimum gap between directly contacting surfaces is around 0.4 nm [11]. Not only because of this coincidence
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Figure 4. Comparison between the gaps calculated by (1) for the tip–mica and silica–mica systems at various NaCl concentrations, where v = 2 nm/s.
between these gap values, but also the constancy of Fad given in Fig. 2b, we consider that the probe tip contacts with the mica surface directly. As for the silica–mica system, the gap increases gradually from around 0.70 to around 1.35 nm, so that the difference in gaps between the tip–mica and silica– mica systems varies from around 0.35 to 1.0 nm. We consider that this difference represents the thickness of the firmly adsorbed layer of medium molecules which cannot be destroyed by the given pushing force of the present colloid probe. It is clear that the gap mentioned above is of the same order of the size of a water molecule and hydrated Na+ , i.e. 0.28 and 0.72 nm, respectively, [11]. Hence, it is plausible to assume that the gap is composed of the firmly adsorbed layer of Na+ , hydrated Na+ and water molecules, and the increase of the gap width with Ce is due to the increasing adsorption of these ions. We call this firmly adsorbed layer the ‘primary adsorbed layer’. 3.2. Dependence of force curve and adhesion on scan rate We usually assume that the scan rate does not affect the characteristics of force curves, because the movement of medium molecules is much faster than that of surfaces in the AFM liquid cell, so that an equilibrium between the surface and bulk solution is attained instantaneously at every moment, except for the case in which some structured layers are formed on the surfaces [28]. In this subsection, the effects of the scan rate are examined under various conditions in order to investigate the more detailed characteristics of the structure on the surface. 3.2.1. Dependence of interactions between the probe tip and mica plate on the scan rate. Figure 5a and b shows the dependence of approaching force curves of the tip–mica system on the scan rate v at Ce = 10−4 and 1.0 M, respectively, where the data at v = 2 nm/s are the same as those given in Fig. 2. It is clear that the scan rate hardly affects the force curves, independently of NaCl concentrations, and that the adsorbed layer is negligibly thin in the case of Ce = 10−4 M, but rather thick in
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the case of Ce = 1 M. It is important to note that the thickness of the adsorbed layer can be estimated to be around 1.0 nm from the force curves in Fig. 5b, which agrees well with the thickness of the adsorbed layer at Ce = 1.0 M estimated by Fig. 4. Figure 6 presents the corresponding dependence of Fad on v in NaCl solutions of various concentrations. It is found that values of Fad are independent of not only v, but also Ce , even though the approaching force curves are largely different as shown in Fig. 5a and b. This confirms the conclusion obtained from Fig. 2b, i.e. the probe tip contacts with the mica surface directly under the present experimental conditions.
(a)
(b) Figure 5. Approaching force curves at various scan rates for tip–mica systems. (a) Ce = 10−4 M and (b) Ce = 1 M.
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Figure 6. The adhesive forces calculated using all the data in Fig. 5.
3.2.2. Dependence of interactions between the silica particle and mica plate on the scan rate. Figure 7a and b shows the dependence of approaching force curves of the silica–mica system on v at Ce = 10−4 and 1 M, respectively, where the data at v = 2 nm/s are the same as those given in Fig. 3. It is clear that force curves for Ce = 10−4 M are independent of v, while those for Ce = 1.0 M depend strongly on the magnitude of v. These results indicate the following: (i) Structural adsorbed layers are formed on the surfaces in a concentrated electrolyte solution, which contribute as the repulsive resistance when two surfaces approach. (ii) The layer is mainly composed of cations and solvated cations, because surfaces are negatively charged and the layer thickness increases with increasing Ce . (iii) The structured layer must be very fragile, because it appears only when a large surface and a high approach velocity are employed. (iv) The strength of the structured layer decays with the separation distance, because the apparent thickness of the layer increases with increasing v. This fragile structured layer outside the primary layer is named the ‘secondary structured layer’. There is a possibility that these force curves may be generated by the hydrodynamic drag force caused by the squeezing flow between two approaching surfaces. However, it is found by the theoretical calculation that this hydrodynamic effect is negligibly small [29]. This is also confirmed by the fact that force curves at Ce = 10−4 M do not depend on v, as shown in Fig. 7a. Figure 8a and b shows the dependence of Fad of the silica–mica system on v for various values of Ce , and the corresponding minimum gap had calculated by (1), where the data for the tip–mica system are also shown for the sake of comparison. It is clear that the value of Fad decreases with increasing values of Ce and v. The reason why the value of Fad decreases with Ce at a given value of v is that the thickness of the adsorbed layer becomes thicker with increasing Ce and the gap between surfaces becomes larger at the same maximum pushing force, as shown
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(a)
(b) Figure 7. Approaching force curves at various scan rates for silica–mica systems. (a) Ce = 10−4 M and (b) Ce = 1 M.
in Fig. 8b. On the other hand, the reason why the value of Fad decreases with v at a given value of Ce is that the resistance by the structured layer increases with v, so that the penetration of two surfaces into their adsorbed layers is shortened as v increases, i.e. the minimum gap between surfaces becomes wider, as shown in Fig. 8b. 3.2.3. Primary and secondary layers. We concluded above that the difference of had values between the silica–mica and tip–mica systems at v = 2 nm/s represents
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(a)
(b) Figure 8. Adhesive forces and the corresponding minimum gap between surfaces for silica–mica systems at various scan rates and electrolyte concentrations. (a) Adhesive forces under various conditions. (b) The minimum gaps between surfaces under various conditions. (The data for the tip–mica system are also shown for the sake of comparison.)
the thickness of the primary adsorbed layer. However, this difference increases continuously with v, as shown in Fig. 8b. Hence, it looks that the definite difference between the primary and secondary layers is unable to be defined. To investigate whether we can classify the primary and secondary layers, the dependence of Fad on the maximum pushing force Fm was examined in the cases of v = 2 and 2000 nm/s, as shown in Fig. 9. It is found that the value of Fad for v = 2 nm/s is constant, independently of Fm . We consider that the reason for this constancy of Fad is that
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Figure 9. Dependence of the adhesive force on the maximum pushing force for silica–mica systems at v = 2 and 2000 nm/s.
the fragile structure does not provide resistance for the movement of the probe if the velocity is very slow, so that the probe surface contacts directly the outside of the primary layer and the primary layer is unable to be broken in the range of this pushing force. On the other hand, the value of Fad for v = 2000 nm/s varies with Fm and it approaches asymptotically to the value for v = 2 nm/s. Finally, both values coincide with each other at a strong pushing force, Fm = 35.8 mN/m. This confirms that the thickness given by the data at v = 2 nm/s represents the thickness of the primary adsorbed layer. Hence, we can define the secondary layer, such that the structure formed outside the primary layer is the secondary layer. The secondary layer is so fragile that the apparent thickness varies with v and Fm . Figure 10a shows the approaching force curves for the maximum scan rate we can produce in this study, v = 2000 nm/s, at various values of Fm . It is plausible to assume that the sensitivity to detect the fragile structure increases with the scan rate, but not with the maximum pushing force. Hence, all the force curves in Fig. 10a must have the same sensitivity to detect the fragile structure, so that they must be essentially the same force curve. Because the surfaces contact at the outside surface of the primary layer at Fm /R = 35.8 mN/m, as shown in Fig. 9, the force curve at Fm /R = 35.8 mN/m must cover the whole range of the secondary layer. When the value of Fm becomes smaller, the minimum gap becomes wider. Because this gap can be estimated as in Fig. 8b, the force curves for Fm /R = 5.2, 8.6 and 17.0 mN/m are shifted by their corresponding gaps as shown in Fig. 10b. It is clear that all the force curves coincide well with that of Fm /R = 35.8 mN/m, which we call the master curve here. This indicates that the data presented here are self-consistent and reliable, and the thickness of the fragile structure is about the half of 4.0 nm in the case of the silica–mica system in a 1 M NaCl solution, if the surface property of silica and mica surfaces is nearly the same.
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(a)
(b) Figure 10. Force curves of silica–mica systems at Ce = 1 M for the various maximum pushing forces. (a) The original force curves. (b) The curves shifted by the gap calculated by (1).
3.2.4. Apparent viscosity of solution within the gap. The fragile structure discussed above indicates that the apparent viscosity in the gap between two surfaces must vary with the separation distance. It is well known that, when a sphere approaches a flat plate, the drag force on the sphere Fdrag increases because of the squeezing flow within a small gap, as shown below: Fdrag = 6π μRvβ,
(2)
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Figure 11. Dependence of the apparent relative viscosity calculated from the data at v = 2000 nm/s on the separation.
where μ is the fluid viscosity and β is the correction factor approximated by the following equation: β=
6h¯ 2 + 13h¯ + 2 , 6h¯ 2 + 4h¯
h h¯ = . R
(3)
This equation is not applicable when the viscosity varies with position and the fluid is a structured fluid whose formation is dependent on time, as in the present case. Hence, it is not appropriate to calculate an apparent viscosity using this equation in the present system. Nevertheless, we used this equation to roughly estimate the order of magnitude of the viscosity increase near the surface due to the fragile structure. As mentioned above, the fragile structure depends on the magnitude of Fm and v, so that the apparent viscosity μv = 2000 evaluated using the master curve of v = 2000 nm/s is valid only in the case of v = 2000 nm/s in a 1 M NaCl solution. Substituting the values evaluated by AFM, i.e. values of Fdrag in Fig. 10b and the corresponding approaching velocity of two surfaces, into (2) and (3), the change of the apparent viscosity normalized by the bulk viscosity μ0 (=1.12 × 10−3 Pa s for 1 M NaCl solution), μv = 2000/μ0 , with the separation happ is estimated as shown in Fig. 11, where happ = 0 indicates the outside of the primary layer. It is clear that the apparent viscosity increases rapidly with decreasing happ at happ < 4.0 nm, where the fragile structure starts to interact between the two surfaces. It is important to note that the apparent viscosity near the surface is 3 orders of magnitude greater than that of the bulk solution. Of course this evaluation is not a quantitative one, but we can imagine the steep increase of viscosity near the solid–liquid interfaces. The similar increase of viscosity was also reported elsewhere, in which the viscosity is 6 orders of magnitude greater than that of bulk water [23]. The discussion on the more detailed characteristics of the molecular-scale structure of the secondary layer is out of the range of the present study. To clarify the
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mechanism of the formation of the secondary layer, further sophisticated experiments and the help of the computer simulation will be needed.
4. CONCLUSION
In the present study, detailed characteristics of the layers of water molecules, ions and hydrated ions adsorbed on surfaces in NaCl solutions were investigated by introducing the dynamic method in the use of AFM, and the following conclusions were drawn: (i) There exist two kinds of structured layers on the solid surface in electrolyte solutions — one is a thin, but firm primary layer in which water molecules, cations and hydrated cations are adsorbed directly on the solid surface, and the other is a thick, but fragile secondary layer outside the primary layer. (ii) As for the tip–mica systems, two surfaces contact directly each other with the minimum gap of around 0.35 nm, in spite of the various experimental conditions of the present study. (iii) When the slowest scan rate v = 2 nm/s is employed, the thickness of the primary layer is able to be determined. It is found that thickness of the primary layer varies from 0.35 to 1.0 nm depending on the concentration of NaCl solution. (iv) By introducing the dynamic method in the use of AFM, the fragile secondary structure is able to be detected. The sensitivity to detect the fragile structure increases with increasing scan rate and the maximum gap at which the fragile structure starts to interact between surfaces is around 4.0 nm at the maximum sensitivity of the present study. (v) The apparent viscosity near the solid–liquid interface at the molecular scale increases greatly because of the fragile structure.
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