Chemo-Mechanical Coupling in Reactive Droplets - American

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Chemo-Mechanical Coupling in Reactive Droplets Jan Szymanski, Jerzy Gorecki, and Marcus J. B. Hauser J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp402308t • Publication Date (Web): 03 Jun 2013 Downloaded from http://pubs.acs.org on June 5, 2013

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The Journal of Physical Chemistry

Chemo-Mechanical Coupling in Reactive Droplets. Jan Szymanskia , Jerzy Goreckia,b,∗ , Marcus J. B. Hauserc a

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. b

Faculty of Mathematics and Natural Sciences, Cardinal Stefan Wyszynski University Dewajtis 5, Warsaw, Poland c

Abteilung Biophysik

Institut f¨ ur Experimentelle Physik Otto-von-Guericke-Universität Magdeburg Universitätsplatz 2, 39106 Magdeburg, Germany ∗

[email protected] June 3, 2013

Abstract Chemo-mechanical coupling is studied in droplets containing an oscillating Belousov-Zhabotinsky reaction medium surrounded by a

1

ACS Paragon Plus Environment


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solution of lipids in n-decane. We identified experimental conditions at which the oxidation state of the catalyst significantly changes the interfacial tensions of the BZ droplet. During oxidation episodes a fraction of the catalyst incorporates into the membrane, whereas during the slower reduction phase it returns to the aqueous phase. The catalyst is partly replaced by lipids that were originally dissolved in the organic phase, thus increasing the surface of the droplet. As result of this increase in surface area, a droplet placed in a trench on the bottom of the reactor periodically elongates in phase with the chemical oscillations.

Keywords: Droplet, lipids, Belousov-Zhabotinsky reaction, oscillations, interfacial tension, periodic elongation

2


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1

Introduction

A number of interesting results demonstrating mechanical motion generated by a chemical reaction have been published recently. Autonomous motion is one of the attributes of living matter, so all examples of chemo-mechanical coupling sound attractive for scientists interested in, among others, the chemical origins of life. Chemo-mechanical coupling has been observed in very different systems. Self-propelled swimmers that are driven by symmetry breaking catalytic properties1 are one of the simplest phenomena of this kind. Many interesting examples of chemically induced motion are observed in media where elastic constants strongly depend on concentrations of reagents. If concentrations of relevant substances change periodically in time due to an oscillating chemical process then the mechanical motion can be generated by periodic contraction and expansion. Frequently studied examples of such systems are pH-sensitive polymers coupled with different variants of the Belousov-Zhabotinsky (BZ) reaction 2 . Complex processes of chemomechanical coupling are responsible for motility and transport in living organisms, like, for example, the formation of lamellipodia or pseudopodia in migrating cells slime mold

4,5

3,4

or the periodic contraction and expansion of veins in a

.

A large number of observations of chemo-mechanical effects have been reported for systems where forces are generated by changes in interfacial tension resulting from the production of surface active molecules. Probably the first observation of motion driven by interfacial interactions was reported by Lord Rayleigh who observed that a camphor grain floating on water exhibits irregular motion due to the interfacial tension gradient and named this 3



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phenomenon the ”camphor dance” 6 . When a disk of camphor is placed on water, it starts to move in one direction almost at a constant velocity 7 . Its motion is driven by an asymmetrical distribution of the camphor layer on the water surface that surrounds the disk

8

and its speed depends on both

the inhomogeneity of the layer generation and evaporation. Spontaneous motion of a droplet related to changes in the interfacial tension was also found in other systems, for example a droplet of pentanol or of other aliphatic alcohols

11

9,10

on water, a phenanthroline disk on ferrous

sulfate solution 12 and surfactant-encapsulated oil droplets in aqueous media 13−18

. Surfactant-covered BZ droplets in an organic phase were shown to

squirm and move

19

. In all these systems the chemical energy is transduced

into motion via changes in interfacial tension resulting from the presence of reagents of a chemical reaction

20

. The case of 1,10-phenanthroline is worth

mentioning because the 1,10-phenanthroline molecule has both hydrophobic and hydrophilic domains, and it shows a surface activity similar to the camphor. When a disk of phenanthroline is either placed on the surface of water or on a diluted aqueous solution of FeSO4 , uniform motion of the grain is observed. The uniform motion changes to intermittent motion (periodically changing between motion and rest) with increasing concentration of FeSO4 and both its period and resting time increase. The mechanism of this characteristic motion is related to the composition of the surface-active layer formed by 1,10-phenanthroline and tris-(1,10-phenanthroline) iron complex ([Fe(phen)3 ]2+ , ferroin) as the driving force and to the solubility of ferroin in the aqueous phase

12

.

Convective motion of a liquid and associated spontaneous motion of

4


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a floating object can be induced by spatio-temporal pattern formation in a reaction-diffusion system through interfacial tension. In such reactiondiffusion-convection systems the reaction changes the surface tension, leading to surface fluxes due to Marangoni convection, which then couple back onto the reaction 21−23 . In the BZ reaction system the interfacial tension values of the oxidized and reduced states differ

24

. Therefore, convective flows in the

bulk phase induced by repetitive changes in interfacial tension can appear in the BZ medium. Those flows can generate a spontaneous motion of a small droplet of the BZ reaction medium

25,26

.

In this paper we investigate an example of chemo-mechanical coupling through a periodical, chemically driven interfacial tension. In experiments discussed below we used phospholipids (asolectin) to stabilize the droplets, because we found that such lipids allow for excitable communication between droplets necessary for information processing applications

27

. We report pe-

riodic expansion of a droplet of oscillatory BZ solution placed on the bottom of a plexiglass dish and covered with a solution of lipids in n-decane. The droplet volume remains constant, but due to the difference in interfacial tension between the reduced and oxidized states, a large contact surface between the droplet and the organic phase is preferred in the oxidized state. We present a discussion of the observed phenomenon.

2

Materials and methods

Stock solutions of sodium bromate (1.5 M, Fluka), malonic acid (1.0 M, Alfa Aesar), sulphuric acid (3.0 M, Chempur) and potassium bromide (1.0 M, ABCR) were prepared using deionized water (Millipore, ELIX 5). The 5



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0.025 M stock solutions of the calatytic complexes ferroin and bathoferroin hexasulphonate (BPHS) were prepared by mixing iron (II) sulphate (Roth), in molar proportion ratio 1:3, with 1,10-phenanthroline (Roth) or disodium salt of bathophenanthrolinedisulphonic acid (Alfa Aesar), respectively. The aqueous Belousov-Zhabotinsky (BZ) phase was set up by mixing the aforementioned solutions in a small beaker in the order: sulphuric acid, bromate, malonic acid, bromide and adding the catalyst after disappearance of brownish color which was due to bromine production. The organic phase consisted of a predetermined amount of soybean lipid extract asolectin (Sigma) dissolved in n-decane (Sigma), at the mass concentration of 5 mg/ml. Alternatively, pure DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine, Avanti Polar Lipids) at the concentration 2.5 mg/ml was used. The aqueous droplets were placed in shallow, 1 mm deep trenches drilled in the bottom of a PMMA (plexiglass) reactor filled with the organic phase and their temporal evolution was recorded using a digital video camera. Experiments were performed at room temperature (≈ 295 K). The observed phenomenon of droplet elongation seems to be qualitatively identical if the temperature differs by a few kelvins.

3

Results

We observed that a droplet containing the BZ reaction placed in a trench drilled in a PMMA plate and covered with lipid solution in n-decane elongates keeping its volume constant. The droplet expands symmetrically along the main axis of the trench. An unidirectional expansion as shown in Fig. 1 was obtained by placing a droplet close to one end of the trench such that the 6


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droplet expansion in one direction was prevented by the trench wall. We observed that the droplet elongation is not uniform in time, but it is composed of steps representing expansion and contraction. The steps were synchronized with the chemical oscillations of the BZ medium: Each oxidation step was accompanied by expansion, followed by a brief period of contraction (Fig. 2). We found that the presence of lipids in the organic phase is necessary for droplet expansion. When lipids were present the behavior depends only slightly on their concentrations in the organic phase, as evidenced by Fig. 1. For all used concentrations of lipids the observed extension of a droplet during the course of the reaction was between 2 and 3 times of its original length.

7



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(a)

(b)

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(c)

time

(d)

(e)

2 mm

0

time/min (f)

Figure 1: Caption8 on the next page.


50

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Caption of Figure 1

Changes of shape for a single droplet placed in the trench reactor. (a), (b), (c) - top views of the droplet after 0, 25, and 50 minutes from the start of the reaction, (d) - the corresponding space-time plot created by analysis of pixels along the trench length (blue line), (e), (f) - space-time plots for other concentrations of asolectin in n-decane. The horizontal axes covers 50 minutes, the initial droplet diameter is 2 mm. BZ solution composition: 0.6 M sulphuric acid, 0.1875 M NaBrO3 , 0.175 M malonic acid, 0.06 M KBr, 1.7 mM BPHS complex, the surrounding medium is (a,b,c,d) 2 mg/ml, (e) 5 mg/ml, (f) 20 mg/ml solution of asolectin in n-decane.

9


(a)

(b)

2 mm 0 time/min

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time


5

(c)

Figure 2: Caption on the next page.

10


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Caption of Figure 2

The time evolution of a 1 mm × 2 mm BZ droplet resting in a trench on PMMA substrate. BZ solution composition: 0.6 M sulphuric acid, 0.225 M NaBrO3 , 0.175 M malonic acid, 0.06 M KBr, 1.7 mM BPHS complex, the surrounding medium is 2.5 mg/ml solution of DOPC in n-decane. (a) the top view of the droplet. The white lines indicate the positions used for creating space-time plots., (b, c) - space-time plots along the horizontal (b) and the vertical (c) lines showing changes of shape for a droplet surrounded by solution with DOPC concentration 2.5 mg/ml over 5 minutes. The dark vertical lines in Fig 2c mark the position of the gas bubbles located slightly above the intersection of the white lines in Fig 2a.

11



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To verify whether the shape changes were actually related to the oxidation process and to exclude other explanations (e.g. influence of molecular bromine), a series of experiments was performed. In these studies, the droplets contained reaction mixtures of different reactivities. First, we considered droplets in which a reaction medium was exclusively oxidizing. To this purpose, the droplets contained only the catalyst, sulphuric acid, and a small amount of sodium bromate (while malonic acid was absent). This medium oxidized the metal catalyst complex, which remained in the oxidized state. Shortly after starting the reaction, we observed a dramatic shape change of the droplet containing the ferric BPHS complex which is immersed in an n-octane phase that contains lipids (Fig. 3(a)). Second, we tested the importance of the presence of phospholipids in the organic phase. Experiments lacking phospholipids did not support any shape changes of the droplets (Fig. 3(b)). Finally, we investigated the role of the identity of the catalyst. Whereas shape elongations were found in droplets containing the ferric BPHS complex, droplets containing the classical BZ catalyst ferriin always retained their spherical shape (Fig. 3(c)). Further experiments were performed to understand the expansion of the droplets. Droplets containing only the acidified solution of the catalytic BPHS complex were filmed from the side while resting on a flat PMMA surface. Two series of experiments were performed: first with the reduced, then with the oxidized catalyst. The results point to the critical role of lipids in the observed phenomena. In pure n-decane, droplets containing reduced and oxidized catalyst look similar, since they retain their spherical geometry (Fig. 4(a), 4(b)). When lipids are available in the organic phase the oxidized

12


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(a)

(b)

(c)

Figure 3: Space-time plot showing oxidation in a droplet containing different mixtures of reagents: (3(a)) 0.3 M H2 SO4 , 0.029 M NaBrO3 and 4.9 mM BPHS iron(II) complex immersed in asolectin solution, (3(b)) 0.3 M H2 SO4 , 0.015 M NaBrO3 and 4.9 mM ferroin in asolectin solution, (3(c)) 0.3 M H2 SO4 , 0.022 M NaBrO3 and 4.9 mM BPHS iron(II) complex immersed in pure n-decane. At the begin of each experiment, the droplet diameter was 1 mm. The time, flowing downwards, covers 97, 88 and 75 seconds respectively. droplet flattens out almost immediately on contact with the surface, whereas the droplet with reduced catalyst retains its almost spherical shape (Fig. 4(c), 4(d)).

13



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(a)

(b)

(c)

(d)

Figure 4: Snapshots of droplet placed on a flat plexiglass surface. Droplets containing non-reactive solutions of 0.3 M H2 SO4 and 5 mM BPHS iron complex at oxidation number +II (a, c) and 0.3 M H2 SO4 , 0.03 M NaBrO3 and 4.9 mM BPHS iron complex at oxidation number+III (b, d), respectively. The droplets are viewed from the side while resting on a PMMA plate immersed in n-decane (a, b) and 5 mg/ml asolectin solution in n-decane (c, d). The scale bars are 1 mm. The droplet volume was 2 µl for lipid solution and 1.5 µl for pure n-decane.

14


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The snapshots of stationary droplets that are placed on a flat surface illustrated in Fig. 4 allow for the extraction of information about the interfacial tension between the aqueous BZ solution (L), the organic phase (O) and the plexiglass surface (S). From the digitalized images we measured the contact angle, the droplet height z0 and the height corresponding to the maximum width of the droplet zm . Moreover, if the top surface of the droplet was not flat, we measured the radius of curvature at the droplet top r0 . If the droplet is flat r0 = ∞. The Young-Laplace equation

28,29

provides the fundamental relation be-

tween the mean curvature of the surface element expressed by its two principal radii R1 , R2 and the pressure element ∆p across it:

∆p = γLO (

1 1 + ) R1 R2

(1)

where γLO is the interfacial tension between the BZ-solution inside a droplet and the surrounding organic phase. Let us consider a droplet oriented as in Fig. 5. The x-axis is horizontal and parallel to the surface of the plexiglass. The z-axis is the symmetry axis of the droplet. The top of the droplet is located at the origin of the coordinate system (Z,X), thus it corresponds to z = 0. The surface of the trench is located at z = −z0 and the droplet width reaches its maximum at z = −zm . The pressure can be calculated as:

∆p = −g∆ρz + p0

(2)

where g is the gravitational field strength and ∆ρ is the difference between the densities of the BZ-solution and the surrounding organic phase. We assumed that the density of BZ-solution is the same as that of 0.3M sulphuric acid 15



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(1.019 g/cm3 ) and the density of the organic phase equals to the density of n-decane (0.78 g/cm3 ). Thus ∆ρ ≈ 0.24 g/cm3 . In Eq.(2) p0 denotes the additional pressure generated by the curvature at the top of the droplet. If the droplet is flat at its top, like the one in Fig. 4(d), then p0 = 0. For droplets characterized by non-zero curvature at the top (like that in Fig. 4(a-c)) p0 = 2γLO /r0 . This result is a consequence of the Young-Laplace equation and the assumption that both radii of curvature at the droplet top are equal: R1 (z = 0) = R2 (z = 0) = r0 . We can simplify the problem assuming that the principal radius R1 is related to the droplet curvature in the vertical plane (here (X,Z)) whereas the other principal radius R2 is constant and equal to r0 . This approach generalizes the standard assumption

29

that R2 is much larger than R1 and

thus 1/R1 plays the dominant role in Eq.(1). For flat, disk-shaped droplets, like those observed in the oxidized BZ-solution, this assumption seems to be satisfied because R2 >> R1 , and we have 1/r0 = 0 (p0 = 0), so that the commonly used theory is not modified. In the other case (1/r0 6= 0) by comparing pressure generated by the surface curvature (Eq.(1)) with the hydrostatic pressure (Eq.(2)) we get:

−gρz + γLO

1 1 2 = γLO ( + ) r0 R1 r0

(3)

1 1 = γLO r0 R1

(4)

Eq.(3) transforms into: −gρz + γLO

which is argued to be a reduction to the one dimensional problem with curvature calculated on the (X, Z) plane only and modified pressure at the droplet top. 16


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Let us introduce the function x(z) that describes the droplet surface on the (X, Z) plane as illustrated on Fig. 5. The curvature 30

by the derivatives of the function x(z) (see s

µ

2

d x(z)  1 = / R1 dz 2

1+

1 R1

can be expressed

):

dx(z) dz

¶2

3 

(5)

and Eqs.(1, 2, 4, 5) combine to: s

µ

2

d x(z) gρz 1 + ) = (− 2 dz γLO r0

1+

dx(z) dz

¶2

3 

(6)

The solution of this equation has the form αz 2 + 2 rz0 + 2C dx(z) =q dz 4 − (αz 2 + 2 rz0 + 2C)2 where α =

gρ . γLO

The derivative

dx(z) dz

diverges to −∞ when z → 0− therefore

C = −1. The function x(z) has a maximum at z = −zm thus γLO =

(7)

2 gρzm 2(1 − zm /r0 )

dx(z) dz

= 0 and: (8)

We can also relate the interfacial tension γLO with the contact angle Θ between the droplet and the surface of the reactor. Straightforward calculations lead to well known relationship between the interfacial tension and the droplet height z = −z0

28

:

γLO =

gρz02 2(1 − cos(Θ) − z0 /r0 )

(9)

Eqs.(8,9) give different, independent estimations for the value of γLO based on selective features of a droplet measured in an experiment. For the droplets shown in Fig. 4 we obtained the following average values of the interfacial 17



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tensions: ox − oxidized solution, no lipids γLO = 3 mJ/m2 red − reduced solution, no lipids γLO = 6 mJ/m2 ox = 0.2 mJ/m2 − oxidized solution, with lipids γLO red − reduced solution, with lipids γLO = 3.1 mJ/m2

where the superscripts

red

and ox refer to droplets with oxidized and reduced

catalyst. These values are significantly smaller than the surface tensions (the interfacial tension of air/solution interface) of BZ solution reported in the literature. For the ferroin catalyzed reaction (catalyst concentration 3.2 mM) Kusumi et. al.

24

measured 63.4 mJ/m2 in the reduced solution and 65.7

mJ/m2 in the oxidized one. We have measured the surface tension of BPHS system and observed the opposite trend. For the concentrations of reagents as in Fig.4 we found that the surface tension of the reduced solution is 68.1 mJ/m2 and that of the oxidized one 63.2 mJ/m2 . Therefore, the changes in surface tension due to catalyst oxidation show that the oxidized catalyst, i.e. bathoferriin haexasulphonate, is more active than its reduced form, thus displaying the same trend as observed for droplets in the organic phase. The presence of an organic phase can significantly lower the interfacial tensions. The values of interfacial tensions for droplets in n-decane given above are in a good agreement with the interfacial tensions measured for glycerol-1-mono-oleate covered BZ droplets in a continuous oil phase consisting of squalene. Whereas BZ droplets covered with the pristine mono-olein surfactant possess an interfacial tension of γLO = 1.3 mJ/m2 , the interfacial tension was shown to rise to 2.7 mJ/m2 when the surfactant was brominated

18


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19

. Young’s law describing the forces at the contact line implies that γSO −

(γLO cos(Θ) + γLS ) = 0, where γSO and γLS are the interfacial tension values between PMMA and the organic phase and between PMMA and the solution of BZ-reagents respectively. The value of γSO , i.e. the interfacial tension between the organic phase and the PMMA substrate, should not depend on the oxidation level of the catalyst in a droplet. Therefore, subtracting Young’s law for the oxidized and reduced catalyst and the same type of surrounding organic phase (i.e., with or without lipids) we get: ox red red ox γLS − γLS = γLO cos(Θred ) − γLO cos(Θox )

If we substitute the estimated values of interfacial tension, we obtain: red ox − for oil and no lipids γLS − γLS = 2.3 mJ/m2 red ox − for oil with lipids γLS − γLS = 2.5 mJ/m2 .

These data show that the interfacial tension in the oxidized state is always lower than the corresponding tension in the reduced state. Therefore, in the oxidized state it is energetically favorable to increase the surface of the droplet and reduce the contact surface between PMMA and the organic phase. Moreover, the difference between the interfacial tensions for the rered ox duced and the oxidized catalyst (i.e. γLS − γLS ) does not seem to depend on

the presence of lipids. Regardless if lipids are present or absent the differred ox red ox ence γLO − γLO is around 3 mJ/m2 and the difference γLS − γLS is around

2.5 mJ/m2 . Although the differences in the interfacial tensions measured in absolute numbers are almost the same the changes in shape of a droplet resulting form the catalyst oxidation depend on the ratios:

19


ox γLO red γLO

and

ox γLS . red γLS

Here,


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³

ox γLO red γLO

´ without lipids

≈

1 2

³ and

ox γLO red γLO

´ with lipids

≈ 0.06. These numbers indicate

that when the catalyst is in the oxidized state then the surface between the droplet and the organic phase can be enlarged at relatively low energetic cost if compared with the case when the catalyst is in the reduced form. Moreover the relative decrease in γLO resulting from the oxidation of catalyst is more important when lipids are present. As result, we observe a significant difference between the shapes of droplets in the oxidized states surrounded by an organic phase with and without lipids. Comparing the values of interfacial tensions, we can propose the following model for the observed droplet expansion: During the oxidation stage of BZ reaction in the droplet, bathoferriin hexasulphonate is formed autocatalytically. This oxidized form of the catalyst is highly surface active and it incorporates into the interface between the droplet and the surrounding organic phase. The incorporation leads to the decrease in the interfacial tensions. This process seems to be similar for the organic phases containing or lacking lipids. The presence of lipids is important because they also lower the interfacial tension between the BZ solution and the organic phase. For asolectin or DOPC, the combined decrease in the interfacial tensions coming from lipids and the oxidized catalyst is so large that it modifies the droplet shape. During the reduction phase of the BZ-cycle a part of the reduced bathoferroin leaves the interface and returns to the aqueous domain of the droplet. Their positions at the interface are occupied by lipids and the interfacial tensions increase. In this phase a droplet shrinks.

20


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z x

x 0 p 0

x(z) x(-zm) -zm ϑ

-z0

Figure 5: The orientation of a droplet on a flat surface considered in the analysis of interfacial tensions. The x-axis is horizontal and parallel to the surface of the plexiglass. The z-axis is the symmetry axis of the droplet. The top of the droplet is located at the origin of coordinate system (Z,X) thus it corresponds to z = 0. The surface is located at z = −z0 and the droplet width reaches its maximum at z = −zm . The droplet surface is described by the curve x(z).

21



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4

Page 22 of 31

Discussion

The oscillations in reagent concentrations during the course of the BelousovZhabotinsky reaction are accompanied by changes in interfacial tension at air/liquid and liquid/liquid interfaces

20,31

. However, none of the results

described to date involve shape changes as dramatic as those reported in the present communication. We measured the periodic elongation of a droplet by a factor of 3. In order to account for such large expansion three conditions need to be satisfied, namely the presence of suitable substrates, catalysts and lipids. We observed elongation of droplets on plexiglass substrate, but not for droplet placed on glass. The effect can be found when using the modified BZ catalyst (BPHS), but not for the classic ferroin. Our experiments confirmed elongation of droplets when asolectin (Fig.1) was used as the surfactant covering the aqueous BZ droplet.

Asolectin

is a mixture of lipids obtained from soybean extracts that is often used in studies of proteins or enzymes in reconstituted membranes. Asolectin consists of a mixture of lecithin (i.e. phosphatidylcholines), cephalin (i.e. phsophatidylethanolamines) and phosphatidylinisitol. In order to establish which of the components of asolectin supports the shape changes of reacting BZ droplets, we repeated the experiments using either DOPC and DOPS (1,2-dioleoyl-sn-glycero-3-phospho-L-serine) instead of asolectin as surfactant. The fact that DOPC supports the shape changes of reactive BZ droplets (Fig. 2), whereas DOPS does not, indicates that the surfactant should have a positive head group (as in the case of DOPC and of lecithin and cephalin from asolectin), while phospholipids with a negatively charged head group, as DOPS, do not seem suitable to induce shape changes in reaction BZ droplets. 22


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Interestingly, DOPC and DOPS are both unsaturated lipids, as is the largest fraction of the lipids in asolectin. However, the presence of unsaturated aliphatic chains in the phospholipids does not seem to be essential for the emergence of shape oscillations in the BZ droplets, as seen from the fact that DOPC supports these shape changes whereas DOPS does not. The main difference between the classic BZ catalyst ferroin and the modified BPHS complex is the presence of additional sulphonic groups in the latter molecule (Fig. 6). Both, the reduced and the oxidized form of bathoferroin are more surface active than their analogous ferroin/ferriin counterparts. The sulphonic groups can be assumed to enter into interactions with the hydrophilic, polar heads of the lipid molecules forming a monolayer around droplets. It is known that the oxidized forms of these metal catalyst complexes tend to incorporate in micelles 32 , which is consistent with the observation that aromatic organic anions substituted with sulphonate groups exhibit strong affinity toward lipid layers

33

.

(a)

(b)

Figure 6: Structural formulae of ligands constituting ferroin (a) and BPHS (b) catalysts.

23



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The shape adopted by droplets containing only the acidified solution of the oxidized catalytic complex is consistent with the situation in which there is a lowered interfacial tension between two liquid phases (the droplet phase and the bulk phase) 34 . As the BPHS catalytic complex undergoes oxidation during the course of the Belousov-Zhabotinsky reaction, the nature of its interactions with the lipid monolayer changes in a way that leads to a different organization of the membrane, which in turn leads to decrease in interfacial tension. Recently, it has been shown that a reaction front of the BZ reaction in a lamellar phase of a phospholipid reorganizes the lipid bilayer, such that the persistence length of this bilayer increases. In other words, the BZ reaction increases the degree of ordering of the bilayer membrane

35

.

The calculations have shown that in the oxidized phase it is energetically preferable to increase the surfaces of contact between droplet and the organic phase and between the droplet and substrate at the cost of oil-surface contact if compared to the reduced phase. The membrane-catalyst interactions also explain why the shape changes are absent when the anionic lipid DOPS is used instead of the cationic DOPC (or asolectin). As the polar heads in DOPS are negatively charged, their interactions with the BPHS complex are much less probable due to electrostatic repulsion.

5

Acknowledgements

The research was supported by the NEUNEU project sponsored by the European Community within FP7-ICT-2009-4 ICT-4-8.3 - FET Proactive 3: Bio-chemistry-based Information Technology (CHEM-IT) program. The authors are also grateful to dr K. Noworyta for his help with surface tension 24


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measurements for the reduced and oxidized BPHS solutions.

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Figure 7: Figure for TOC. The space-time plot along the yellow line.

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Chemo-Mechanical Coupling in Reactive Droplets - American

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