Emergence of Complex Behavior in Chemical Cells: The System AlCl3

May 7, 2014 - Understanding and mastering the synthesis of these systems may lead to new technologies where complex structures are grown rather than ...
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Emergence of Complex Behavior in Chemical Cells: The System AlCl3−NaOH Jerzy Maselko,*,† Micah Kiehl,† Jordan Couture,† Agnieszka Dyonizy,§ Vitaliy Kaminker,‡ Piotr Nowak,§ and J. Pantaleone‡ †

Department of Chemistry and ‡Department of Physics and Astronomy, University of Alaska, Anchorage, Alaska 99508, United States Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wroclaw, 3070 Poland

§

S Supporting Information *

ABSTRACT: Chemical cells that spontaneously form in simple inorganic systems are presented. The cells are surrounded by semipermeable membranes that allow water and some ions to diffuse through. These cells exhibit dynamical behaviors that are typically associated with biological entities. These behaviors may be used to perform tasks such as rotation or linear translation in the vertical and horizontal directions. Yet another system builds “curtains”. Behaviors are controlled by a complex network of physical and chemical processes that are organized in space and time. The type of dynamical behavior is determined by the chemical composition of the cell and the environment. By studying these systems we may learn general rules for the growth of living entities, or at least about the spontaneous growth of complex chemical structures. Understanding and mastering the synthesis of these systems may lead to new technologies where complex structures are grown rather than assembled.

1. INTRODUCTION Chemical gardens1−9 were discovered over 300 years ago and were initially of interest because their growth processes and structures superficially resembled biological systems. Semipermeable membranes form around metal salt solutions producing inorganic cells with different colors and shapes. Osmosis into the cell drives the growth of these cells to form structures such as tubes. In traditional chemical gardens the solution surrounding the cell was silicate; however other anions such as phosphates, aluminates, metal oxalates, and even organic anions10−21 have been used together with a large range of cations, including organic cations, to produce similar structures. There are many analogies between biological cells and chemical gardens. In chemical gardens, chemical cells are formed spontaneously and have semipermeable membranes that allow chemicals to diffuse inside where they react and the products can then diffuse out, keeping these cells continuously far from thermodynamic equilibrium. These cells may form complex structures and it may be possible to use them to perform some task. Processes in these cells are controlled by networks of chemical and physical reactions that are precisely organized in space and time and are open-ended, forming hierarchies of complex structures with increasing complexity. However, these cells do not multiply in the sense of biological cells. Besides the possible applications of these systems, or their possible aid in understanding fundamental aspects of biological systems, these chemical cells are a fascinating subject to study. In the experiments discussed here, aluminum chloride is the metal salt and, instead of silicate, hydroxide solution (some© 2014 American Chemical Society

times with the addition of carbonate solution) is used for the exterior solution. For these chemicals the resulting chemical cells have dynamics that are far more diverse and energetic than have been seen previously in traditional chemical gardens. This leads us to use a word such as behavior that is not typical for inorganic chemistry. However, the systems studied here resemble living matter in many ways. Thus we occasionally, and with caution, use words appropriate for living systems. The relation between the problem of the origin of life and chemical gardens has been studied and discussed in many papers.22−26 Understanding the origin of life is one of the most intriguing problems in science. Unfortunately, the synthesis of a biological cell by assembling chemicals is still impossible, as indicated by hundreds of scientific papers. Even the formation of a protocell,27−29 an object that lies in the huge gap between nonliving and living matter, is still beyond our grasp. Protocells are a very important idea in studies of the origin of life because they are considered to be the first step in the transition from nonliving to living matter. The simplest biological cell requires many essential subcomponents: a semipermeable membrane (with complex structures for transmembrane communication and transportation), a system of storing and translating information (DNA, RNA), and metabolic pathways. These three subsystems have to ensure perfect synchronization in coping processes and a high fidelity that is balanced with the possibility of mutations that are the basis of evolution. The cells Received: December 11, 2013 Revised: April 11, 2014 Published: May 7, 2014 5726

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unique feature is that the tube end often spontaneously branches symmetrically and simultaneously into five or six smaller tubes. This is visible in the table of contents graphic. Tube growth ends within a couple of seconds after rupture, and after that the tube slowly dissolves. Visually, the tube producing relaxation oscillations are quite impressive; for a video of the process, see the Supporting Information. To illustrate the dynamics of the ejections, the luminosity around the chemical cell is plotted versus time in Figure 2. The

in chemical gardens may provide a path toward the formation of protocells.

2. RESULTS AND DISCUSSION 2.1. Methods. A precipitation membrane was formed by introducing a pellet made of ground aluminum chloride into a solution of sodium hydroxide with varying amounts of sodium carbonate. The exterior solution was placed into a 65 mm × 60 mm rectangular glass container and filled to a depth of 200 mm. Aluminum chloride hexahydrate crystals were ground, shaped into a pellet using a pellet maker, and then dropped into the solution. The diameter of the pellet was 10 mm, the height was 5 mm, and the mass was 0.5 g. The growth of the structures was monitored with a video camera for later frame-by-frame analysis. 2.2. Periodic Ejections. Pellets made from AlCl3 were submerged in a solution of NaOH. A semipermeable membrane of Al(OH)3 forms around the pellet, creating a cell containing a solution of aluminum chloride. Osmotic flow into this cell increases the interior pressure until the membrane ruptures and the interior solution is ejected, decreasing the pressure. For NaOH concentrations in the range from 3 to 5 M, the fluid ejections occur periodically. While pressure-driven relaxation oscillations are common in chemical gardens,9 the current system is unique because the amount of material ejected is relatively large (sometimes several percent of the total interior solution) and because the quickly ejected fluid forms twisting, branching tubes that soon dissolve (see Figure 1). The

Figure 2. Image brightness around the chemical cell (from a side view) versus time. The intensity increases quickly when a tube is ejected and then decreases slowly as the tube dissolves. The time t = 0 corresponds to when the pellet was dropped into the solution. The NaOH concentration was 3 M. Luminosity was measured with Tracker video analysis and a modeling tool for physics education.

luminosity of a white cell (one without ink) against a black background is an approximate measure of the observed area of the cell plus tubes. The quick rise accompanying tube ejection, followed by the slow decay as the tube dissolves, is readily apparent in this plot. This sawtooth pattern is characteristic of relaxation oscillations. Note that the amplitude in the plotted data is not a good measure of the amount of solution ejected because the angle the tube makes with the camera varies and sometimes the tube is hidden behind the pellet. The period of the relaxation oscillations is observed to depend on the concentration of the exterior solution and also on the age of the chemical cell. Sometimes the eruptions are well separated in time with a relatively regular period. At other times, the eruptions occur in bursts, with multiple tubes being created in a short time interval, followed by a period of quiescence. The physics behind these eruptions is similar to that behind closed tube oscillations in chemical gardens, as discussed in ref 8. The eruptions occur when the pressure in the chemical cell is sufficiently large to cause a rupture in the membrane. Ruptures occur when the stress in the membrane exceeds a critical value. When the chemical cell is approximated as a sphere, the stress in the membrane is given by PR σ= (1) 2t

Figure 1. Photographic overlay showing the ejection of material from the chemical cell. The ejected material forms a slender tube shown at times (1) 0.08 s and (2) 0.5 s after ejection begins (tube (2) is shown in a green pseudocolor, painted using Photoshop). The change in length and position between (1) and (2) illustrates how the tube grows and moves. The cell here has been dyed black with ink to improve visibility. The NaOH concentration was 3 M.

relaxation oscillations continue until the AlCl3 pellet inside the cell is fully dissolved, after which the interior concentration decreases and the osmotic flow stops; this takes about 5 to 10 min. Thereafter the structure decreases in size from dissolution, and soon the entire structure disappears. The ejected material emerges through the membrane rupture with speeds typically of about 3−5 cm/s. The interior solution then reacts with the surrounding solution, forming a new membrane which then channels the emerging solution into a tube. Using the observed speed, the tube diameter, and the kinematic viscosity of water, the Reynolds number for the emerging solution is about 30−100. The direction of growth of the tube end is constantly changing, probably an instability similar to the flutter observed in a freely hanging hose pipe.30 A

where P is the pressure difference across the cell membrane, R is the sphere radius, and t is the membrane thickness. This stress can be related to the strain in the membrane and hence to the change in volume of the chemical cell

ΔV 3 = σ (2) V 2E where V is the volume of the cell, ΔV is the change in volume, E is the elastic modulus of the membrane, and the Poisson ratio 5727

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of the membrane has been taken to be 1/2. When the membrane ruptures, the observed values of ΔV/V vary from around 1 to 5%, and thus this is also approximately the ratio of the critical stress to the elasticity of the membrane. The chemical time scales for membrane creation and dissolution are relatively short, thus it is to be expected that the membrane parameters will be relatively constant over the cell’s life, varying only with the interior concentration of the chemical cell. Concentrations inside the cell may be influenced by arm growth because bursts of arm growth have been observed to coincide with the pellet inside the chemical cell moving and even turning completely over. Variations in the internal concentration from such a movement may lead to membrane nonuniformities and also to changes in the rate of osmosis. These factors will influence the period of the oscillations. The critical value of the pressure when the membrane ruptures may be estimated from the observed initial speed of the emitted tube. The interior pressure pushing the ejecta out of the membrane will be approximately balanced by the dynamic pressure ρv2. Video was analyzed, and the speed of the ejected fluid was measured with the Tracker software. Using the observed initial speed, the dynamic pressure can be estimated to be a few Pascals. Similar values have been measured directly in experiments performed in chemical gardens.8 The complex dynamics of this system means that there may be some aspect of these dynamics that could be exploited to perform a useful function. For example, the system might be useful as a chemical clock to periodically release material into the surrounding solution. Another possibility might be to use the irregular tube growth to create the motion of the chemical cell. Controlling the function of the chemical cell should be possible by adjusting the size or shape of the seed or the composition of either the seed or the environment. The essential processes responsible for the dynamics of this system are outlined in the flowchart in Figure 3. This figure is divided (by dashed lines) into four parts: (1) Initial conditions. By changing the initial conditions, the cell operation and function can be adjusted. (2) Growth. The spontaneous formation of the chemical cell. (3) Operation. The loop describing the competing processes that produce the oscillations. (4) Function. In this case, the tube growth. 2.3. Cell Movement. If the NaOH concentration is decreased, the periodic ejections change to a more continuous ejection of material. Accompanying this change is an increase in the movement of the original cell. Two types of cell movement are shown in Figure 4. Along the top row of Figure 4 is shown an example of the movement that can accompany periodic ejections. The cell membrane and the remains of the pellet inside the cell rotate counterclockwise. This type of rotation is not seen in traditional chemical gardens, probably because the forces in the current case are much larger. The exact mechanism responsible for this rotation is unknown. The rotation could be caused by uneven interior chemical concentrations causing uneven growth in the membrane, or it could be due to internal pressure gradients acting on the pellet. The visual appearance is reminiscent of an organism “turning over”. Along the bottom of Figure 4 is shown an example of behavior at an environment concentration below where periodic ejections occur. Instead a large, bulbous tube is extruded from the bottom of the pellet. While tube growth is

Figure 3. Flowchart showing the chemical and physical processes responsible for the periodic ejections.

common in traditional chemical gardens, this growth is unusual in that the old membrane shrinks while the new membrane grows. The result is a linear translation of the center of mass of the structure along the vessel. The front grows about 0.3 cm/ min. The visual appearance is of an organism crawling along the floor. 2.4. Buoyancy Oscillator. A pellet made from AlCl3 was submerged into a solution of 0.30 M NaOH and 0.20 M Na2CO3. The addition of Na2CO3 to the environment adds new chemical reactions which change the dynamics from the previous cases. As before, an Al(OH)3 membrane forms around the pellet, creating a cell containing a solution of aluminum chloride. However, unlike the previous case, small gas bubbles form inside the chemical cell, causing it to move up to the surface after a short time. There it expands and parts of the cell break off. These secondary cells (about 1 cm in size) fall down through the solution. Subsequently these secondary cells will change direction and move upward. The vertical motion of these small, secondary cells can be oscillatory, as shown in Figure 5. In general, oscillations tend to form when there are two competing processes. For the chemical cell studied here these processes are (1) the formation of the CO2 bubbles inside the cell and (2) the release of the bubbles from the exterior of the cell. The period and amplitude of the oscillations are determined by the rate of bubble growth and the factors influencing bubble release. The vertical motion of the cell through the fluid results from changes in the buoyancy of the cell. The gas bubbles are created by a series of chemical reactions. First the aluminum ions undergo the reaction Al3 + + H 2O = Al(OH)2 + + H+ 5728

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Figure 4. Sequences of photographs showing cell movement for 3 M (top and middle) and 2 M (bottom) NaOH. The video is included in the Supporting Information. Along the top row, the cell can be seen to turn over (follow the position of the “head” on the right side of the cell, indicated by the red arrow) with some periodic arm creation. The pictures correspond to 0, 5, and 7 s. In the middle row, a separate rotation is visible. These pictures correspond to 6, 11, and 13 s. Ink was added to the pellet to make the arms visible. On the bottom, the cell continuously extrudes a large, transparent arm which slides along the bottom (follow the position with respect to the bubble fixed on the bottom). The times are 35, 140, and 252 s after the pellet drop.

bubbles, R is the effective radius of the cell, and g is the acceleration of gravity. The left-hand side of eq 3 represents the drag force on the cell from the surrounding fluid, and the righthand side is the upward buoyant force from the gas bubbles (first term) minus the net weight of the fluid and membrane which make up the cell (second term). Thus, the magnitude and direction of the cell’s motion is determined by the relative size of the buoyant force of the gas bubbles versus the net weight of the cell. The observed parabolic motion of z(t) between bubble releases (Figure 5) is easily explained by an approximately constant rate of bubble growth and/or cell dissolution. Buoyancy oscillations driven by bubble growth have been described previously for objects placed in carbonated beverages (see, e.g., ref 31.). However, the oscillations described here differ in important ways from those. The oscillations discussed here involve a chemical reaction within the cell while the oscillations in soda are driven by nucleation. Consequently our chemical cells typically dissolve after a few buoyancy oscillation cycles. Also, for the oscillation in soda, the top and bottom surfaces of the experimental vessel play a crucial role in limiting the motion while for the oscillations we have observed they are not essential. In principle, this buoyancy oscillator can also be used to perform a function, hence it too can be thought of as a chemical motor. In principle, the period and amplitude of the oscillations can be controlled by changes in the composition of the seed and/or the exterior solution. 2.5. Curtain Formation. This case is initially identical to the previous one; a pellet of AlCl3 is submerged into a solution of 0.30 M NaOH and 0.20 M Na2CO3. In the previous case, we

causing the solution inside the cell to become acidic. Then CO32− in the exterior solution diffuses to the interior. This creates bubbles of carbon dioxide from the reactions CO32 − + 2H+ = H 2CO3 H 2CO3 = H 2O + CO2

The creation of gas bubbles increases the buoyant force on the cell. While the CO2 bubbles are always observed to be created on the inside surface of the cell membrane, their location is not constant. The outer surface of the cell is continuously dissolved by the reaction Al(OH)3 + OH− = Al(OH)4 −

At the same time a new membrane is being created on the inside surface by the continuous reaction of OH− ions diffusing from outside and reacting with the interior Al3+ solution. This causes the CO2 bubbles to effectively migrate through the membrane, reach the outside surface, and be released. Bubble release decreases the buoyancy of the cell which leads to downward movement. The vertical motion can be described approximately by the equation 6πμR

dz = ρVBg − ΔρVg dt

(3)

Here, z is the height of the cell, μ and ρ are the viscosity and density of the exterior solution, Δρ is the density difference between the interior and exterior solutions, VB is the volume of gas bubbles in the cell, V is the volume of the cell without the 5729

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presumably because the membrane is less elastic for these concentrations of the exterior solution. The ejected solution is denser than the surrounding solution so the tubes grow downward and resemble stalactites. CO2 bubbles form on the inside of these stalactites, causing them to rotate upward and lay on the top surface of the solution. In a short time the top surface is completely covered with these buoyant chemical tubes. From this surface layer of tubes a second generation of stalactites can form. These tend to grow downward, forming a curtain around the interior of the vessel. This situation is shown in Figure 6. If the pellet is large enough, these curtains can cover all of the walls.

Figure 6. Complex precipitation structure that grows spontaneously in a very simple chemical system. The chemical cell spontaneously constructs a curtain for the experimental vessel. The numbers along the bottom indicate the time in minutes and seconds. The last picture on the right was made with a larger pellet, and in this case almost the entire upper part of the experimental vessel is covered.

3. CONCLUSIONS The systems described here resemble biological systems in many different ways. One way is the growth process, in which seeds change to a chemical cell. Another way is the structures produced, which involves a chemical cell surrounded by a semipermeable membrane, sometimes with tubular “arms” or “branches”, as for the periodic ejections, or in an amalgamation of cells as for the curtain. Yet another way is the chemistry of the cell, where chemicals diffuse into the cell, react, and this then drives different behaviors. The behaviors of these systems are probably the most important way that they resemble biological systems. The behaviors are quite complex, far beyond what is normally seen in chemical gardens. This trait is important because when the dynamics are complex then it may be possible to exploit the system to produce a desired outcome. The functionality of these systems is, of course, defined by humans, unlike biological systems where it is determined by evolution. But functionality in a system that grows is especially interesting because it holds the promise of a new technology where entities are grown as in biology as opposed to assembly from prefabricated parts as is usually done in human technology. The spontaneous formation of chemical cells that can perform tasks may have a great impact on studies of the origin of life. Research in this area seeks to find a chemical cell that can evolve to form a biological cell. The systems discussed here may also be useful as base units in the developing discipline called systems chemistry.32−34 In systems chemistry, the growth is required to have sequences of chemical and physical processes leading to a more complicated whole. The system is changing continuously, with the previous state determining the sequence of processes that follow. The systems are growing on trajectories that can be controlled. For the systems discussed

Figure 5. (a) Sketch showing the migration of CO2 bubbles through the chemical cell membrane from the continuous dissolution of the exterior surface and formation of new membrane on the interior surface. The typical size of an actual chemical cell is 0.6 cm. (b) Plot of the height of the chemical cell versus time. The red circles indicate when a gas bubble was released. (c) Series of photographs from a typical experiment and the time of the photograph in seconds.

examined the behavior of the small, secondary chemical cells that contained AlCl3 in solution. Here we examine the dynamics of the main chemical cell which contain the undissolved remains of the AlCl3 pellet. When the pellet is dropped into the exterior solution, it falls to the bottom. Membrane forms around the pellet, creating the chemical cell. In a few seconds, this cell floats to the surface because of the CO2 bubbles produced inside of it. The osmolarity of the interior AlCl3 solution is much higher than the osmolarity of the environment, thus the cell grows in size due to osmotic movement of water into the cell from the outside. As for the ejection oscillator, the osmotic inflow causes the interior pressure to increase, which leads to the membrane rupturing. However here the solution ejected through the rupture emerges in a slower and more continuous manner, 5730

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here there are many possible ways to control the growth trajectory. Besides the obvious possible changes in the composition of the seed or the environment, changes are possible by altering the delivery system.15,17,35 For example, one solution can be pumped or dripped into the other with varying rates and locations. Today we are just beginning to understand how to manipulate these processes, but what has already been observed is staggering and exciting.

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ASSOCIATED CONTENT

S Supporting Information *

Video of a moving cell. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; tel: 907-786-4697. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by NSF Grant 1011656. REFERENCES

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