Spontaneous Recurrence of Deposition and Dissolution of a Solid

We investigated the spontaneous recurrence of deposition and dissolution of camphor layer on the surface of camphor methanol solution. This recurrence...
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Spontaneous Recurrence of Deposition and Dissolution of a Solid Layer on a Solution Surface Tomohiro Sasaki,† Nobuhiko J. Suematsu,‡,§ Tatsunari Sakurai,† and Hiroyuki Kitahata*,† †

Department of Physics, Graduate School of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Graduate School of Advanced Mathematical Sciences and §Meiji Institute of Advanced Study of Mathematical Sciences, Meiji University, 4-21-1 Nakano, Tokyo 164-8525, Japan

‡

S Supporting Information *

ABSTRACT: We investigated the spontaneous recurrence of deposition and dissolution of camphor layer on the surface of camphor methanol solution. This recurrence is a novel rhythmic process concerned with solid−liquid phase transition. To elucidate the underlying mechanism, we measured the solution temperature at different times, and found that the temperature increased and decreased repetitively, correlating with the camphor layer’s deposition and dissolution. These experimental results show that the solution temperature plays an important role in recurrence of deposition and dissolution.

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INTRODUCTION Spatiotemporal patterns often emerge in dynamical systems far from equilibrium.1 Many rhythmic processes and pattern formation phenomena have been observed in living systems, which are regarded as systems far from equilibrium. In abiotic systems, rhythmic processes such as the Belousov−Zhabotinsky (BZ) reaction,1,2 salt−water oscillators,3 and candle flame oscillations,4 as well as pattern formation phenomena such as the BZ reaction, 1,5 Rayleigh−Bénard convection, 6 and Liesegang ring,7 have been widely investigated. Such oscillatory processes and pattern formation phenomena in abiotic systems deal mostly with nonlinear chemical reactions, hydrodynamics, and phase transitions. Regarding phase transitions, there have been several studies on pattern formation phenomena, including crystal growth of ascorbic acid,8,9 chemical gardens,10,11 and breaking of generated layers.12,13 Cyclic growth and dissolution of camphor crystals subject to cyclic temperature variation has been reported recently.14,15 However, the emergence of rhythmic processes induced by phase transitions has never been reported. In the present paper, we report a novel spontaneous recurrence of deposition and dissolution of a camphor layer on the surface of a camphor methanol solution, during the solution’s evaporation. To elucidate the underlying mechanism, we measured the solution temperature and weight at different times. Based on the experimental results, we suggest a mechanism of spontaneous recurrence of deposition and dissolution of the camphor layer on the solution surface. This recurrence can constitute a good experimental model of phasetransition-induced rhythmic processes. © XXXX American Chemical Society

EXPERIMENTAL SETUP Methanol solution of camphor was prepared by adding 1.5 g of (+)-camphor (Wako, first grade) per 1.0 mL of methanol (Wako, special grade). A 5.0 mL sample of the solution was poured into a Petri dish (60 mm in diameter), and was monitored from above by using a charge-coupled device (CCD) camera (STC-MC33USB, SENTECH; 30 frames per second (fps)). A black paper was placed under the Petri dish for capturing clear images, and the acquired images were analyzed by using an image processing software (ImageJ, NIH). We obtained the averaged brightness of the solution surface at different times. To observe the dissolution process of the camphor layer with higher resolution, we added red food coloring to the camphor methanol solution and conducted high-resolution observations of the camphor layer’s dissolution. A 30 μL sample of the solution was poured on a Teflon board. We monitored the camphor layer’s dissolution on the solution surface from a slanted angle by using a high-speed camera (VW6000, KEYENCE), operated at 250 fps. To elucidate the underlying mechanism, we measured solution temperature as a function of time by using a thermocouple (L-TN-SP-K, TOA DENKI). The thermocouple was located near the solution surface to detect the temperature change caused by the phenomenon at the surface. In addition, images were acquired by use of the CCD camera, and we Received: April 9, 2015 Revised: June 17, 2015

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DOI: 10.1021/acs.jpcb.5b03413 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 1. Spontaneous recurrence of deposition and dissolution of the camphor layer on the surface of the camphor methanol solution. (a) Averaged brightness of the camphor methanol solution as a function of time. The time t = 0 s corresponds to pouring the camphor methanol solution into a Petri dish. (b) Magnified view of the averaged brightness of camphor methanol solution in (a). (c) Snapshots at t = (i) 434.00, (ii) 434.40, (iii) 463.73, (iv) 495.77, (v) 496.17, (vi) 537.07, (vii) 580.77, (ix) 617.07, and (x) 653.03 s. (i)−(x) correspond to the time indicated with (i)−(x) in (b). The corresponding movie is available as Supporting Information (jp5b03413_si_002.mpg), together with the movie of the recurrent process observed for a droplet surface on a lab bench (jp5b03413_si_003.mpg).

camphor was dissolved completely was adopted as a saturated temperature. We performed three measurements for each solution.

obtained the solution temperature and the brightness as a function of time. We also simultaneously measured the solution weight and the averaged brightness. The Petri dish was positioned on an electronic balance (GR-200, A&D; time resolution 1 s), and the solution images were acquired by using the CCD camera for obtaining the time series of the weight and the brightness along with the images. All experiments were performed at room temperature (23 ± 2 °C). In order to investigate how the dissolution of camphor to methanol affects the temperature, we measured the dissolution heat of camphor to methanol. A 15 mL sample of methanol was poured into a glass vessel (30 mm in diameter) and was stirred by using a magnetic stirrer for approximately 800 s until a constant solution temperature was obtained. Here we adopt the stirred system to make it easier to distinguish the effect of the dissolution on the temperature change by accelerating the dissolution process. Then, 1.0 g of camphor was added into the methanol and the temperature was measured as a function of time by use of the thermocouple. For the control experiment, we performed the same experiment with water replacing the methanol. We also measured the saturated solubility of camphor to methanol. Five samples were prepared, for which 1.50, 1.75, 2.00, 2.25, and 2.50 g of camphor were dissolved per 1 mL of methanol. The solution and the magnetic stirring bar were bottled up in a glass vessel, and then the vessel was put inside a constant temperature reservoir (low temperature incubator IJ 100W, Yamato). First, the vessel was kept at low temperature, preventing the camphor from dissolving perfectly. Then, we raised the reservoir temperature by 0.2 °C while stirring the solution by using the magnetic stirrer and waited for approximately 1800 s. Then we checked whether the camphor had been dissolved or not by eye. The temperature when the

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EXPERIMENTAL RESULTS Figure 1 demonstrates the recurrence of deposition and dissolution of the camphor layer on the surface of the camphor methanol solution. Figure 1a shows the averaged brightness of the solution surface as a function of time. The brightness was nearly constant up to t ∼ 200 s, when the camphor layer was deposited for the first time. Concurrent with the camphor layer deposition, the brightness began to increase. At t ∼ 300 s, the camphor layer was instantaneously dissolved, which caused a rapid decrease in the solution brightness. After the first dissolution, recurrent deposition and dissolution were observed. Figure 1b is the magnified view of Figure 1a, and Figure 1c shows the snapshots of recurrent deposition and dissolution of the camphor layer on the solution surface. The camphor layer in Figure 1b(i),c(i) was suddenly dissolved and the brightness decreased (Figure 1b(ii),c(ii)). Then, the camphor layer was deposited again and the brightness increased (Figure 1b(iii),c(iii)). Soon after, the camphor layer in Figure 1b(iv),c(iv) was dissolved again (Figure 1b(v),c(v)), and then the cycle of deposition and dissolution of the camphor layer repeated. We also measured the time between two adjacent dissolutions of the camphor layer, where the period was defined as a distance between adjacent local maxima. The calculated period was 52 ± 10 s (see the Supporting Information). By using the high-speed camera, we captured the snapshots of the camphor layer’s dissolution on the solution surface, shown in Figure 2. When the camphor layer was dissolved, the solution oozed from a certain point, and spread all over the B

DOI: 10.1021/acs.jpcb.5b03413 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Figure 3b). The weight loss rate increased when the camphor layer was dissolved. As the camphor layer was deposited again, the weight loss rate decreased again. The weight loss rate’s increase and decrease occurred repetitively. The results for the measurement of the dissolution heat are shown in Figure 4. We found that the solution temperature

Figure 2. Snapshots of the camphor layer’s dissolution on the solution surface, taken from a slanted angle by using a high-speed camera. The frequency of snapshots was 1/50 s.

Figure 4. Solution temperature versus time, for camphor dissolved to methanol (blue curve) or water (red curve). Camphor was added to the solution at t = 800 s.

decreased by approximately 0.9 °C when camphor was dissolved to methanol. The temperature did not change so much when camphor was added to water, though almost all camphor remained in water without dissolution. Thus, we found that the process of the dissolution of camphor into methanol was endothermic. Considering the heat capacity of methanol is 81.6 J mol−1 K−1,16 the dissolution heat can be calculated as −4.1 kJ mol−1. The results on the saturated solubility of camphor into methanol depending on temperature are shown in Figure 5. We

Figure 3. (a) Averaged brightness (red curve, left vertical axis) and temperature (blue curve, right vertical axis) of the camphor methanol solution as a function of time, obtained by simultaneously performed measurements. The data for longer time are shown in the inset. (b) Averaged brightness (red curve, left vertical axis) and weight (blue curve, right vertical axis) of the camphor methanol solution as a function of time, obtained by simultaneously performed measurements. The data for longer time are shown in the inset. In (a) and (b), the camphor methanol solution was poured into a Petri dish at time t = 0 s.

Figure 5. Amount of camphor, cmax, dissolvable to 1 mL of methanol, versus temperature. Error bars denote maxima and minima of three measurements for each solution. The blue line shows the result of linear fitting. The calculated fitting line was cmax = 0.043T + 0.77, where T is the solution temperature in degrees Celsius.

obtained the relation according to which the amount of camphor dissolvable to 1 mL of methanol increased with increasing temperature by about 0.043 g K−1. This result is comparable with the results that the dissolution of camphor to methanol is endothermic.

surface. The spreading speed of the solution was approximately 70 mm s−1. Figure 3a shows the temperature and the averaged brightness of the camphor methanol solution as a function of time, obtained by simultaneously performed measurements. Prior to the camphor layer’s dissolution, the temperature decreased monotonically (Figure 3a, inset). After the camphor layer was deposited at t ∼ 300 s, the temperature increased and decreased repetitively, correlating with the camphor layer’s deposition and dissolution. Figure 3b shows the weight and the averaged brightness of the camphor methanol solution as a function of time. The weight loss rate suddenly decreased when the camphor layer was deposited at t ∼ 200 s (arrow in the inset of

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DISCUSSION As shown in Figure 1, we observed spontaneous recurrence of deposition and dissolution of the camphor layer on the surface of the camphor methanol solution. Below, we discuss the mechanism of this spontaneous recurrence. Before the recurrence started, the solution temperature decreased monotonically and the weight loss rate was large, as shown in the insets of Figure 3. This suggests that methanol C

DOI: 10.1021/acs.jpcb.5b03413 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B mainly evaporated from the surface of the camphor methanol solution, and that evaporation of methanol caused the decrease in temperature owing to the heat of evaporation. At t ranging from 200 to 300 s, the camphor layer was generated and covered the entire solution surface. At this time, the weight loss rate suddenly decreased and the solution temperature stopped decreasing. This suggests that methanol evaporation was inhibited as the camphor layer was generated. Then the recurrent increases and decreases of the temperature and the weight loss rate occurred and were correlated with deposition and dissolution of the camphor layer, as shown in Figure 3. We consider that the temperature increased and approached the ambient temperature because methanol evaporation was inhibited according to the camphor layer’s deposition. Meanwhile, the temperature decreased when the camphor layer was dissolved, and the weight loss rate increased. We consider that the increase in the weight loss rate was owing to the methanol evaporation from the solution surface when the camphor layer was dissolved. Actually, we precisely observed the process of dissolution and found that the dissolved region expanded like a wave on the surface, as shown in Figure 2. In addition, we observed that the solution temperature decreased when the camphor was dissolved to methanol (see Figure 4). To summarize the above, the decrease in the solution temperature was attributed to both the methanol evaporation and the dissolution of camphor to methanol. From Figure 2, we observed that the camphor layer’s dissolution started at a certain point. Since the saturated solubility of camphor to methanol increased with increasing temperature as in Figure 5, the dissolution was caused by the temperature increase that correlated with the camphor layer deposition, as seen in Figure 3a. We hypothesized that the solution spread all over the surface owing to the wetting property of methanol on camphor, and we observed the dynamics of wetting by placing a methanol droplet on a camphor disk. The methanol droplet wetted the surface of the camphor disk completely (see Supporting Information), suggesting that the solution spreads all over the surface due to wetting. Based on the above discussion we suggest a mechanism of spontaneous recurrence of deposition and dissolution of the camphor layer on the solution surface; the proposed mechanism is shown schematically in Figure 6. First, methanol evaporates from the surface of the camphor methanol solution, and the solution temperature decreases owing to the heat of evaporation. Then, the solubility decreases and the camphor concentration increases on the solution surface. As a result, the deposition of the camphor layer occurs on the solution surface, inhibiting evaporation. As evaporation is inhibited, the heat loss by evaporation decreases and heat comes in from the environment, and the solution temperature increases and approaches the ambient temperature. Then, the solubility increases as well and the camphor layer becomes easier to dissolve. After a certain time, the camphor layer exhibits a sudden dissolution because the solubility increases. The solution oozes from the dissolved point utilizing the capillary phenomenon and spreads all over the surface by wetting. With the dissolution of camphor to methanol and evaporation of methanol on the solution surface, the solution temperature decreases, and the deposition of camphor layer occurs again. Taken together, the recurrence can be realized as a switching between fast dissolution of the camphor layer owing to the good wettability, and slow deposition owing to the decreasing

Figure 6. Schematic of the proposed mechanism of spontaneous recurrence of deposition and dissolution of the camphor layer on the surface of the camphor methanol solution. (i)−(ii) The solution oozes from a certain point, spreads all over the surface, and dissolves the camphor layer. (ii)−(iii) Methanol evaporates and the solution temperature decreases. Therefore, deposition of the camphor layer occurs owing to the decrease in temperature. (iii)−(i) The solution temperature increases because the evaporation rate becomes smaller owing to the camphor layer. Thus, the solubility of camphor to methanol increases.

saturated solubility caused by the evaporation. In fact, we confirmed that the spontaneous recurrence did not occur, when the Petri dish with the solution was covered with a glass plate (data not shown). The quantitative measurement of the layer thickness and observation of the microscopic structure of the layer remain as future work.

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CONCLUSION In the present study, we observed spontaneous recurrence of deposition and dissolution of a camphor layer on the surface of a camphor methanol solution. This recurrent phenomenon is a solid−liquid spontaneous phase transition. We suggested the mechanism underlying this recurrence, in which an alternation of the solution temperature is a key factor. Namely, the recurrence was realized as temperature-dependent switching between fast dissolution and slow deposition. Thus, this recurrent phenomenon can constitute a good experimental model of phase-transiton-induced rhythmic processes.

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ASSOCIATED CONTENT

S Supporting Information *

This material includes (i) measurement of periods between adjacent dissolutions of the camphor layer, (ii) observation of wetting dynamics of a methanol droplet placed on a camphor disk, and (iii) the movie (jp5b03413_si_002.mpg, 10 times faster than real time) of the spontaneous recurrence of deposition and dissolution of the camphor layer corresponding to Figure 1 and the movie (jp5b03413_si_003.mpg, in real time) of the spontaneous recurrence observed for a droplet on a lab bench. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcb.5b03413.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.jpcb.5b03413 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported in part by Grants-in-aid for Scientific Research (C) to H.K. (No. 26520205) and for Scientific Research on Innovative Areas “Fluctuation & Structure” to T.S. and H.K. (No. 25103008), Cooperative Research Program of “Network Joint Research Center for Materials and Devices” to H.K., and the Core-to-Core Program “Nonequilibrium dynamics of soft matter and information” to T.S. and H.K. from the Japan Society for the Promotion of Science (JSPS).

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DOI: 10.1021/acs.jpcb.5b03413 J. Phys. Chem. B XXXX, XXX, XXX−XXX