Time Dependence of Silica Surfaces on Their Interactions in Water

May 13, 2015 - the forces between surfaces at short separation distances interact in ... (immersion times of 10−60 min) by using an AFM to measure t...
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Time Dependence of Silica Surfaces on Their Interactions in Water and Alkaline Solutions Cathy E. McNamee† and Ko Higashitani*,‡ †

Shinshu University, Tokida 3-15-1, Ueda-shi, Nagano-ken 386-8567, Japan Department of Chemical Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

‡

ABSTRACT: Spherical silica particles are widely used to study the stability of colloids. This is because the stability of silica particles is important in industrial processes and also because monodispersed particles are available in various sizes, allowing fundamental investigations on colloidal stability to be performed. However, it is often assumed that the surface properties of silica do not change during the measurements, e.g. for 60 min. In the present study, we used the atomic force microscope to determine the validity of this assumption by studying the change in the surface microstructure of silica with its immersion time in water and alkaline solutions. The microstructure of silica surfaces was found to change with the immersion time, even in water. This change was especially large for solutions of high pH. These changes are explained in terms of the hydrogen bonds between the silanol groups protruding from the silica surfaces, whose length depends on the solution pH and the immersion time. silica have been extensively investigated.17−20 In spite of these extensive investigations on the properties of silica and their interactions in solutions, the origin of the structural force between silica surfaces in solutions is still not fully understood. Silica rapidly dissolves in solutions of high pH, causing gellike layers to form. Adsorbed layers of water molecules and ions may also form on the silica surfaces. Under these circumstances, the forces between surfaces at short separation distances interact in complicated ways.8,16 In addition, the degree of hydrophilicity, the nanoscale roughness of silica surfaces,21 the pretreatment of silica surfaces,22,23 and the measuring procedures further complicate the mechanism of the shortrange interaction. In these experiments, the surface properties of silica are usually assumed to be unchanged during the measuring period, e.g. 60 min, although this assumption has not yet been verified. Vigil et al.8 and Yaminsky et al.10 reported that immersing silica in pure water or allowing it to contact humid air for long periods of time changes the “repulsive” retract (decompression) force between silica surfaces to the “adhesive” retract force. There is, however, little systematic information on how a silica surface changes under specific conditions, such as the solution pH and the immersion time. We previously reported that the microstructures of highly hydrophilic silica and copper surfaces change even in pure water within 60 min. The interaction force was also seen to change with time.24

1. INTRODUCTION The stability of colloidal particles in aqueous solutions depends on the interaction forces between colliding particles. The classical DLVO theory has been used to estimate the interaction forces between colloidal particles in solutions and has been very successful to explain various macroscopic phenomena of colloids.1 However, the magnitude of mediumsized molecules, such as water molecules, ions, and hydrated ions, is not taken into account in this theory. Therefore, this theory is not applicable to the DLVO interaction forces at short separations, where the layers of adsorbed molecules on both surfaces start to interact with each other. The interaction forces due to these adsorbed layers are known as the hydration force or structural force, and act as repulsive forces in most cases.2,3 Extensive studies have been carried out by using the surface force apparatus (SFA) on the structural force between two mica plates with a molecularly smooth surface.4,5 A repulsive structural force appeared when the electrolyte concentration was high enough, e.g., higher than 10−3 M. This structural force is considered to be due to the layers of polarized water molecules, ions, and hydrated ions that are adsorbed on the mica surfaces, which are negatively charged in aqueous solutions. Silica surfaces have also been used to investigate the structural force between surfaces by using the SFA6−8or the atomic force microscope (AFM).9−16 This is probably because silica is one of the industrially important oxide compounds. Additionally, the existence of silica substrates with surfaces of nanolevel smoothness and the availability of spherical silica particles in a wide variety of sizes allow silica to be easily used in SFA and AFM measurements. Thus, the physical properties of © XXXX American Chemical Society

Received: March 15, 2015 Revised: May 8, 2015

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indicated that the corrosion of the cantilever by the measuring solutions was negligible. The force measurements were performed by moving the colloid probe into and out of contact with the substrate by changing the voltage applied to the piezo. During this time, a split photodiode was used to measure the cantilever deflection change as a function of the piezo displacement (Δx). The Δx was converted from volts to nanometers by using the constant compliance region, which was obtained from the force curve measured with a high load between the probe and the substrate water, when the time of contact of the substrate with water was less than 10 min. The constant compliance was defined as the linear repulsion region in the force curve and occurred after the probe was in contact with the silica substrate. This constant compliance was used for the other force curves measured using the same cantilever and substrate, i.e., the same measuring set. The position of zero distance in the force−distance curves was defined as the onset of the linear repulsion region, which occurred after the probe was in contact with the substrate. The force between the probe and the substrate was obtained by using Hooke’s law

In this study, we systematically investigated how the microstructure of highly hydrophilic silica surfaces change with their time immersed in water and in alkaline solutions (immersion times of 10−60 min) by using an AFM to measure the normal force between a silica probe and a silica substrate and the friction force between a Si3N4 AFM cantilever tip and a silica substrate. The results of this study warn us how carefully the experiments must be carried out with respect to the time after the silica particles or plates (substrates) are immersed in aqueous solutions, especially in solutions of high pH.

2. EXPERIMENTAL SECTION 2.1. Materials. The pure water used in this study was distilled and deionized to give a conductance of 18.2 MΩ cm−1 and a total organic content less than 5 ppm. A 10−4 M NaCl (Wako, Japan) solution of pH 5.8 was used as the reference, in order to avoid the disturbance due to the dissolution of CO2 from the ambient air.25 The 10−4 M NaCl solution of pH 5.8 is therefore referred to as “water” in the following of this article. Solutions with pH > 5.8 were achieved by adding NaOH (Wako, Japan) to the water. Silicon wafers with a 100 nm top layer of silica (Silicon Quest INT., USA) were used after cleaning with ethanol (Wako, Japan), water, and plasma treatment in air for 2 min using the plasma cleaner (10.5 W, PDC-32G, Harrick Plasma, USA). The contact angles of water on the cleaned wafers were less than 5°, indicating that the silica surfaces were highly hydrophilic. 2.2. Methods. 2.2.1. Atomic Force Microscopy: Normal Force Measurements. An atomic force microscope with a contact-mode fluid cell (Digital Instruments NanoScope III Multimode, USA) was used to measure the normal interaction forces between a silica colloid probe and a silica substrate in solutions of various pH. The silica colloid probe was prepared by attaching a silica particle (nominal diameter 6.84 μm, Bangs Laboratory, USA) to the tip of a cantilever (NP-S, Veeco Nano Probe Tips, nominal spring constant k = 0.12 N m−1) by using an XYZ micromanipulator and an epoxy resin glue (Araldite Rapid, Nichiban, Japan). The probes were cleaned prior to use by plasma treatment. The force measurements were performed by first bringing the probe in contact with the substrate, after which it was separated a small distance. The fluid cell was then filled with water, and the force curve for the constant compliance was measured. The fluid in the liquid cell was then changed to the aqueous solution to be studied. Two minutes before the desired time to measure the forces, the solution in the cell was flushed out and replaced with a new solution, and the measuring position on the silica surface changed. The forces were then measured, after 1 min was given for the system to settle. This procedure ensured the removal of any material that may have dissolved into the solution within the measurement period and ensured a fresh surface was used each time. The immersion time indicated how much time had elapsed since the silica surfaces first contacted a solution, i.e., the time since the water used for the constant compliance measurement was injected into the cell. Each set of measurements were done in the identical way in order to ensure the same history. This allowed us to compare the normal forces data under various experimental conditions. The reproducibility of each probe and substrate and the ability to compare data from one probe and substrate to data from other probes and substrates was also checked by first measuring the force curves in the pH 5.8 system at t < 2 min. If the same force curve magnitude and shape was obtained, then the probe and substrate were used. If the same force curve was not obtained, then the probe and substrate were changed. The color of the backside of the cantilever did not change from its original gold color after 1 h immersion in any of the pH solutions. Additionally, the value of signal sum display (“SUM”) on the AFM, which indicates the total amount of light collected by the photodetector in volts, did not decrease after 1 h immersion in any of the solutions. This result indicates that the reflectivity of the cantilever gold coating did not decrease within this time. The absence of a cantilever color change or a decrease in the reflectivity of the cantilever

FN = k Δx

(1)

Here, FN is the normal force and k is the nominal spring constant of the cantilever. The zero force was defined at large cantilever−substrate separations, where no force was acting on the probe. The maximum loading forces (FNmax) were defined as the maximum force measured in the force curves at the smallest separation distance. The force curves that were measured by bringing the probe and the substrate into only slight contact did not show a linear regime at zero or near zero separations. They were therefore classified as being measured with a low FNmax. The force curves that were measured by bringing the probe and the substrate into significant contact showed a linear regime at zero or near zero separations. These force curves were classified as being measured with a high FNmax. The adhesion force was defined as the absolute value of the difference between the approach force extrapolated at h = 0 and the detaching retraction force. The adhesive force values (Fad/R) for each immersion time and solution pH was calculated by averaging the adhesion force values measured in each force curves measured within one measuring time (number measured = 20). The standard deviation in these results was used for the error bars. 2.2.2. Atomic Force Microscopy: Friction Force Measurements. The friction force measurements were measured between a Si3N4 cantilever tip (CSC38/Si3N4/noAl, MikroMasch, USA, tip height = 20 μm, tip radius = 20 nm, cantilever thickness = 1.0 ± 0.3 μm, width = 35 ± 3 μm, length = 250 ± 5 μm, and nominal spring constant k = 0.08 N m−1) and a silica substrate in various solutions, using the AFM in the friction mode and a fluid cell. This cantilever tip was chemically inert in the solutions used in this study. The cantilevers were cleaned by plasma treatment in air prior to use. The friction measurements were performed by first bringing the probe in contact with the substrate, after which it was separated a small distance. The fluid cell was then filled with water, after which the force curve for the constant compliance was measured. The fluid in the liquid cell was then changed to the aqueous solution to be studied. Two minutes before the desired time to measure the friction, the solution in the cell was flushed out and replaced with a new solution, the measuring position on the silica surface changed, and the system allowed to settle for 1 min. The friction force was subsequently measured by applying a normal load to the substrate, while the substrate was slid horizontally underneath the cantilever tip at a speed of 3.60 μm/s. The loading force (Fload) was calculated using

Fload = LvkCF

(2)

Here, Lv is the applied load in volts, and CF is the constant compliance value that allowed volts to be converted to meters. The lateral frictional force (FL) was calculated using FL = B

VLSLKL 2H

(3) DOI: 10.1021/acs.langmuir.5b00932 Langmuir XXXX, XXX, XXX−XXX

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FvdW = A /6h2

Here, VL, SL, KL, and H refer to the difference in the lateral force detector in one complete scan, the lateral detector sensitivity, the lateral spring constant, and the height of the cantilever tip plus half the thickness of the cantilever, respectively. SL was determined from the method of Meurk and others26 to be 3.1 × 10−4 rad/V. The value of KL was calculated using KL =

2kl 2 3(1 + υ)

(5)

where A is the Hamaker constant. The magnitude of FvdW for silica surfaces in water was calculated using A = 0.85 × 10−20 J for silica surfaces in water28 and by assuming that the contribution of the adsorbed layers on the surfaces was negligible. The solid line in the inset of Figure 1 shows the calculated FvdW. Comparison of the van der Waals forces and the measured forces allows us to conclude that there is some kind of adsorbed layer with a thickness greater than 3 nm between the two silica surfaces. This was the case, even though we used a pH 5.8 solution, i.e., water. Figure 2 shows the dependence of the adhesion force between silica surfaces in water on the maximum loading force

(4)

where l is the length of the cantilever. Poisson’s ratio ν is given by the torsional ratio of the cantilever, where a representative value of 0.27 was used.27 The values of FL for each system and time were averaged from the values measured at different positions on the substrate in a 1 min time interval (number = 7). The standard deviation in these results was used for the error bars.

3. RESULTS 3.1. Interaction Forces in Water (pH 5.8 Solutions). The normal forces between the silica probe and silica substrate in water were investigated under various experimental conditions. The approach and retract force curves were measured for immersion times (timm) between 10 and 60 min on a 10 min interval. The immersion times indicated how long had passed since the silica surfaces had first come into contact with the liquids inserted into the AFM liquid cell. Figure 1

Figure 2. Dependence of the reduced adhesive force (Fad/R) between a silica probe and a silica substrate on the reduced maximum loading force (FNmax/R) at various immersion times (timm) in water.

at various immersion times. The adhesion between highly hydrophilic surfaces in water is seen to be negligible or very small for timm ≤ 60 min, even for high loads. This result implies that the structural layers on the silica surfaces were thin and/or strong enough not to be broken by the normal loading force under the present conditions. These data show that highly hydrophilic silica surfaces that are immersed in water recover after contact. Thus, these data are consistent with the results in Figure 1, where all the force curves were seen to coincide with each other for timm ≤ 60 min and FNmax/R < 15 mN/m. In a previous paper, we concluded that a thin gel-like layer formed on a silica surface that was immersed in water for timm = 60 min, although the apparent roughness change of silica surface was not seen until 30 min.24 Our present results are slightly different. In order to know whether or not the present silica surfaces really did not change within 60 min, the lateral (friction) force was measured between a Si3N4 cantilever tip and a silica plate. This was done because the lateral force is known to be much more sensitive to changes in the microstructure of a surface than the normal interaction force.23 As shown in Figure 3, the lateral force (FL) linearly increased with the applied load (Fload). This is in accordance with Amonton’s law. However, the friction coefficient gradually decreased with the immersion time. This reduction is explained by the thickening of the thin gel layer, which was composed of silica complexes that formed on the substrate with time. The gel layer contained water molecules, which acted as a lubricant to reduce the friction force.29 These data indicate that the silica surface structure gradually changed with the immersion time in water. This slight structural change, however, could not be detected by the normal force measurements.

Figure 1. Normal force (FN/R)−separation distance (h) curves between the silica probe and silica plate for immersion times of 10 and 60 min in water of pH 5.8 at low and high maximum loads. The inset shows the approach force curves in the region of small separations.

shows the normal force (FN) versus the separation distance (h) for the timm of 10 and 60 min. The data between 20 and 50 min were omitted from the figure, so as avoid the overlap of many data points. The effects caused by the strength of the maximum loading force was also determined from the force curves by measuring with the low and high maximum loading forces (FNmax/R) of ∼5 and 10 mN/m, respectively, where R is the particle radius of the colloid probe. All the data are seen to nearly coincide, irrespective of the experimental conditions. In addition, an attractive force due to the van der Waals was not observed. This disagreement of the data with the prediction by the DLVO theory is well-known for highly hydrophilic surfaces3 and has been explained by the existence of a steric repulsive force, which prevails over the van der Waals attractive force. This steric force is often regarded to result from the layers of water molecules and ions on the negatively charged surface and/or the thin gel layer on the hydrophilic silica surfaces.8 The van der Waals attractive force (FvdW) between a sphere and a plate can be estimated from C

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therefore defined as FNmax/R < 4 mN/m and FNmax/R > 7 mN/ m, respectively. This steplike increase in the adhesive force with the reduced maximum loading force appeared at nearly the same maximum load for different immersion times. This result implies that the gel layers were crushed at the same loading force and indicates that the structural strengths of the layers were almost the same. This type of steplike change in the adhesive force between surfaces with the applied force has also been reported elsewhere.30−32 The magnitude of the adhesive force at the high loads of FNmax/R > 7 mN/m is also seen in Figure 5 to increase with timm. The adhesive force value was also seen to be nearly independent of the magnitude of FNmax/R. We consider this independency of Fad/R on FNmax/R to be due to the fact that the fragile part of the gel layers suddenly broke to the extent of the crushable depth, which was determined by timm, when the loading force exceeded a critical strength. This crushing caused by the contact of the gel layers resulted in the mutual interpenetration of their gel layers, which enhanced the binding forces between the surfaces. The scattering of the value of Fad/ R, especially at timm ≥ 40 min, is explained by the fact that the microstructure of silica surface changes with time and from place to place. Figure 6A shows the normal interaction forces at low loads of FNmax/R < 4 mN/m for a pH 10 solution. The approach and

Figure 3. Dependence of the friction force (FL) between a Si3N4 cantilever tip and a silica substrate on the loading force (Fload) at various immersion times (timm) in water.

3.2. Interaction Forces in pH 10 Solutions. Silica is highly soluble in the solution of high pH.17−19 Thus, it is plausible to assume that a gel layer (hairy layer) will form on the silica surface in a pH 10 aqueous solution. Hence, we expected the normal force interactions to depend on the strength of contact and interpenetration of the gel layers on the two surfaces. Interpenetration would give a strong adhesion between the two surfaces. After measuring normal forces in pH 10 solutions, we found that there was a critical load above which the adhesive force Fad/R appeared between the silica particle and substrate (see Figures 4 and 5). The surfaces were

Figure 4. Normal force (FN/R)−separation distance (h) curves between the silica probe and silica plate for immersion times of 10 and 60 min in water of pH 10 at low and high maximum loads. The inset shows the approach force curves in the region of small separations.

Figure 6. Normal force (FN/R)−separation distance (h) curves between a silica probe and a silica substrate in a pH 10 solution for immersion times (timm) of 10 and 60 min, when measured at (A) low maximum loads and (B) high maximum loads. The insets show the approach force curves in the region of small separations.

retract force curves coincide, and the silica surfaces are seen to be nonadhesive. These results are explained by the facts that the gel layers on the surfaces were not broken under this low load and that the gel layers acted as steric repulsive layers between the two surfaces for both timm = 10 and 60 min. The repulsive force was seen to be slightly lower when timm increased from 10 to 60 min. This reduction is explained by

Figure 5. Correlation between the reduced adhesive force (Fad/R) between a silica probe and a silica substrate and the reduced maximum loading force (FNmax/R) in water for various immersion times (timm).

not adhesive for FNmax/R < 4 mN/m but became adhesive for FNmax/R > 7 mN/m. The low load and high load regions were D

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Langmuir the partial neutralization of the silica surfaces, caused by the diffusion of Na+ ions from the solution into the negatively charged gel layers on the silica surfaces with time.33 Figure 6B shows typical approach and retract force curves measured with a high load of FNmax/R > 7 mN/m in a pH 10 solution. The features of the force curves are very different than those seen in Figure 6A. Each approach curve showed a steplike region at very small separations (see the inset of Figure 6B). The length of this step became longer as the immersion time increased. A repulsive force was measured for h > 3 nm. The strength of this repulsion slightly decreased with timm. This reduction is explained by the same mechanism that caused the reduction of the repulsive force with timm in Figure 6A. The retract force curves showed an adhesive force between the silica surfaces, which increased greatly as the immersion time increased. This increased adhesion indicates that the thickness of the gel layers on the surfaces increased with timm and that their interpenetration increased. The scan rate used was 1.0 Hz. Thus, the contact time of two surfaces was very short, i.e., much less than 1 s. In spite of this short contact time, a strong adhesive force was seen, especially at timm = 60 min. Further information on the time dependence of silica surface in pH 10 solutions was obtained by measuring the friction between a Si3N4 cantilever tip and a silica plate (see Figure 7).

Figure 8. Normal force (FN/R)−separation distance (h) curves measured at high maximum loads between a silica probe and a silica substrate in solutions of different pH for immersion times (timm) of 60 min. The inset shows the approach force curves in the region of small separations.

Figure 7. Dependence of the friction force (FL) between a Si3N4 cantilever tip and a silica substrate on the loading force (Fload) in pH 10 solutions for various immersion times (timm).

Figure 9. Effects of the immersion time (timm) in various pH solutions on (A) the reduced adhesion (Fad/R) between a silica probe and a silica substrate and (B) the friction (FL) between a Si3N4 cantilever tip and a silica substrate.

The friction force was linearly dependent on the loading force of the cantilever tip to the silica substrate, as expected. However, the friction coefficient greatly decreased as the immersion time increased, causing the friction to become negligibly small at timm = 60 min. This decrease indicates that the gel layer on the silica substrate thickened with the immersion time and that the increasing amount of water within the gel layer effectively acted as a lubricant.34 This reduction of FL with timm is consistent with the increase in the adhesive force with timm that was seen in Figure 6B. Both phenomena are considered to originate from the thickening of the gel layers that occurred with an increased immersion time. 3.3. Interaction Forces of Solutions pH between 5.8 and 10. The interaction forces between the silica surfaces in solutions with pH between 5.8 and 10 were expected to gradually change as the pH of the solution was increased. Figure 8 shows the normal force curves measured with high maximum loads at timm = 60 min in solutions with pH between 5.8 and 10. An obvious change was observed at pH ≥ 8, where a steplike approach force curve and a strong adhesive force was measured. Figure 9A shows the dependence of the adhesive

force on the immersion time for solutions of different pH. The adhesive force increased almost linearly with the immersion time, although some scattering was observed. The change in the friction force with time was also measured (see Figure 9B). The friction force was seen to almost monotonically decrease with the immersion time for all pH. The data in Figures 9A and 9B indicate that the characteristics of the silica surfaces in the solutions with pH between 5.8 and10 gradually changed with the solution pH. The phenomena observed here were the same as those seen for the pH 5.8 or 10 cases; i.e., no particularly different phenomena occurred for the solutions with pH between 5.8 and 10.

4. DISCUSSION The surface of silica immersed in a solution is considered to be more or less covered with a gel layer that can swell.8,16 This layer may thicken as the pH of the solution is increased above the normal pH (pH of pure water) due to the increasing E

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Langmuir dissolution rate of silica with a pH increase.17−20 It is therefore plausible to assume that the steplike changes observed in the approach force curves of Figures 6B and 8 (see insets) demonstrated the crushing of the gel layers on the silica surfaces. In that case, the length of the steps would indicate the depth to which the gel layer could be crushed. This thickness would be thinner than the thickness of the gel layer. If this is correct, then the low and high maximum load approach force curves that were measured for the same immersion time must coincide for the regions, where the silica surfaces were still separated enough that the gel layers on each surface did not contact each other. In order to check this hypothesis, the high maximum load approach force curves that were measured at timm = 10 and 60 min (inset of Figure 6B) were shifted horizontally to the left by the crushable depth. The forces in the long-range separation region of the low and high maximum load force curves were then compared in Figure 10. A good

layers could be crushed at this loading force. On the other hand, an adhesive force was measured for FNmax/R > 7 mN/m (see Figure 5). At present, we have no clear explanation as to why the load at which the layer crushes is different from the load at which surfaces become adhesive. Possible reasons are that a minimum time may be required for the two surfaces to attach after the gel layer crushes and that it may be necessary for the contact to be strong enough to make the adhesion strong. The features of the adhesive forces between silica surfaces found in the present study can be summarized as follows. The adhesive force increased as the pH of the solution was increased and as the time of immersion in the solution increased. The magnitude and range of the adhesive force also depended on the maximum loading force. An interesting feature is that a strong adhesion appeared especially in the pH 10 solution, even though the contact time between the two surfaces was very short, i.e., much less than 1 s. It is out of the scope of this present study to clarify the origin of this adhesive force in detail. However, it is interesting to speculate on the origin of this adhesive force. There are several possible sources of adhesion: the van der Waals attraction, the hydrophobic adhesion, the adhesion due to the cross-linking or bridging of the polymeric chains in the hairy layer, hydrogen bonding by the water molecules, and sintering between the protruding silanol and silicic acidic groups on the silica surfaces. As discussed above, the adhesion in this present study cannot be explained by the van der Waals force. The friction data in Figure 9 indicated that the surfaces were very hydrophilic. Thus, the present adhesion cannot be attributable to the hydrophobic force. As explained in the Experimental Section, each normal force measurement was done after the solution in the AFM liquid cell was flushed out and replaced with a new solution. Therefore, the microstructure on the silica surface may have resembled a hairy layer more than a network-type gel layer. In the case of hairy layers, some of the protruding hairy chains on the silica surfaces may have entangled after the two surfaces came into contact. The retract force curve is known to be nonsmooth and to show a long detachment distance, if the adhesion is due to entanglements between interpenetrating protruding chains on the surfaces.24,38 These features were not seen in the retract force curves shown in Figure 6. Thus, we consider the contribution of the entanglements of the hairy layers to be negligible. A possible mechanism for the adhesion between the silica surfaces is the sintering of silica. Si−O−Si linkages can form by the condensation of ≡SiOH and ≡SiO− groups on silica surfaces that are in contact. This is the main mechanism to form the gel layer and also to form a network of chains of small silica sols in solutions. However, sintering in aqueous solutions at the room temperature is reported to be a slow process.8,16 According to the sintering hypothesis proposed by Vigil et al.,8 the adhesion strength between silica surfaces in water after 1 s contact is estimated to be a few orders smaller than the values shown in Figure 5. Hence, it seems that sintering is not the main mechanism of the adhesion in our present study. Hydrogen bonding is known to be much stronger than the van der Waals attraction. In addition, hydrogen bonds form instantaneously, which explain the presence of a strong adhesion between silica surfaces after only a short contact time. Fuji et al.39 concluded from the measurements of the adhesion between silica surfaces wetted by water that hydrogen bonding is the dominant component of the adhesion force

Figure 10. Comparison of the approach force curves of high maximum load shifted by the crushable depth with those of the low maximum load for pH 10 solutions for immersion times (timm) of 10 and 60 min.

coincidence was observed between the low and high maximum load force curves in the long-range separation region, i.e., the h > 0 nm region shown in Figure 10, confirming that the steplike features seen in the approach force curves arose from the crushing of the fragile gel layers after their contact. This result is consistent with reports of similar steplike features in other approach force curves, which appeared when there was a fragile structured layer on the surface.35,36 The steps were explained by the crushing of the structures in the layer on the surface35 or by the expulsion of adsorbents from the gap between the two approaching surfaces.36 The steplike feature in our force curves look similar to the socalled “jump-in” that is commonly observed in approach force curves. The jump-in is attributable to the van der Waals attraction. However, we consider that the steplike feature seen in our study is not a jump-in. This is because in the case of the van der Waals attraction between surfaces with gel layers the attractive force will be short-ranged, even for gels with increasing layer thicknesses.37 The data in Figure 6B show the completely opposite trend, where the attractive force range increased as the gels became thicker. The absence of a jump-in region in the force curves measured at a low maximum load in Figure 6A and the coincidence of the low and high maximum load force curves at timm =10 and 60 min in Figure 10 also confirm that the present steplike feature did not originate from the van der Waals attraction. It is important to note that the steplike features in the approach force curves occur at FN/R = 2.2−3.0 mN/m (see the insets of Figures 6B and 8). This result implies that the gel F

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Langmuir

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between silica surfaces with a high silanol density. There are numerous reports of the hydrogen bond being strong enough to bind two surfaces.40−43 Therefore, we believe that the adhesion in our present study is mainly attributed to the hydrogen bonds between the silanols on the silica surfaces and also through the water molecules between the two surfaces.

5. CONCLUSIONS We systematically investigated how the interactions between highly hydrophilic silica surfaces change with the immersion time in water and alkaline solutions for the usual AFM measuring period of 60 min. The following results were found. 1. In the case of water (pH 5.8 solution), the approach and retract force curves were repulsive and coincided, independent of the immersion time and the strength of the loading force. However, the friction force measurements showed that the friction force slightly decreased with time. These results indicated that a thin gel layer grew on the silica surface as the immersion time in water increased. The normal force measurements could not detect this change. 2. As the solution pH was increased above pH 5.8, the normal force curve could be classified into two types. One type was for the force curves measured when the loading force was lower than a critical load. In this case, the approach and retract force curves coincided and the surfaces were not adhesive. The change in the normal force curves with time was very small for timm ≤ 60 min. The other type was for the force curves measured when the loading force was greater than the critical load. In this case, the gel layers greatly affected the interactions between the silica surfaces. The thickness of the crushable gel layer and the adhesive force between the surfaces increased with the solution pH and the length of the immersion time. 3. The origin of the increasing adhesion with the solution pH was postulated to be due to the hydrogen bonds between the silanol groups protruding from the silica surfaces.

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AUTHOR INFORMATION

Corresponding Author

*(K.H.) E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partially performed through the Program for Dissemination of Tenure-Track System funded by the Ministry of Education and Science, Japan.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b00932 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b00932 Langmuir XXXX, XXX, XXX−XXX