Article pubs.acs.org/Langmuir
Electric-Field-Assisted Formation of Nonspherical Microcapsules Rahul B. Karyappa and Rochish M. Thaokar* Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India ABSTRACT: A new method for studying the effect of pH on the polysiloxane network formation using electric fields is presented. The kinetic data obtained using these experiments indicates that the two-step interfacial polycondensation of silanes is strongly dependent on the pH, and the mechanism is essentially different at low and neutral to high values of pH. Very rapid hydrolysis followed by moderate rates of condensation are observed at neutral and high pH. The rate of hydrolysis is drastically reduced, while that of condensation is slightly lowered at low pH as compared to that at high values of pH. The slow hydrolysis reaction at low pH is then exploited to synthesize nonspherical microcapsules. Nonspherical polysiloxane microcapsules with varying aspect ratios from 1.05to 1.97 are synthesized by controlling the applied electric field.
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INTRODUCTION
Microcapsules are Newtonian liquid-filled globules and enclosed by a membrane which not only confines the internal liquid but also protects it from the outside ambient environment. Artificial microcapsules have prominent applications in pharmaceuticals, cosmetics, and food industries for controlling the release of active substances, aromas, or flavors.1 Capsules with ultrathin crosslinked films with gel-like properties are also found in the biological systems. Examples amongst others, include red blood cells (RBCs).2 Polysiloxane membranes made up of cross-linked self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS), prepared with interfacial polycondensation, have emerged as suitable model systems for biological membranes because of their high degree of stability and viscoelastic rheological properties. In the synthesis of polysiloxane microcapsules, the kinetics plays an important role in determining the micro structure of the capsule, and thereby its rheological properties. The interfacial polycondensation of OTS is a twostep reaction. In the hydrolysis step, the OTS reacts with water to form silanols.21
Barring one study3 for the kinetics of OTS-based polysiloxane networks, there is very little information on the mechanism of network formation in OTS-based systems. However, the kinetics of non-OTS polysiloxane networks has been fairly well studied.4−7 The interfacial nature of the process makes the kinetics study difficult. Below we briefly review different methods used in the study of kinetics of polymerized networks. In the case of alkyltrimethoxy or alkyltriethoxysilane self assembled monolayers (SAMs), the rate of hydrolysis is dependent on a number of factors,4 such as water content, pH, temperature, and inductive and steric effects.5−7 Linden et al.8 studied the hydrolysis and condensation reactions of octadecyltrimethoxysilane (ODTMS) based polysiloxane monolayers in the pH range of 0.5−13.5. Surface potential studies revealed that the ODTMS-based polysiloxane networks were positively charged for pH < 3 and negatively charged for pH > 12. From the in situ epifluorescense microscopy study it was observed that at acidic conditions the network structure was gel like, while at basic conditions it was flocculent in nature. The rate of hydrolysis was found to be slow at pH 7, while the rate of condensation was slow at approximately pH 4. The rate of hydrolysis was accelerated by several times when pH was changed by 1 unit in either acidic or basic direction. Degen et al.3 experimentally studied the effect of electric field on the deformation of pendant polysiloxane capsules. They also studied the effect of additives and rheological properties of
In the condensation step the formed silanol reacts with either another silanol (eq 2) or OTS (eq 3) to form the polysiloxane network.21
Received: April 28, 2014 Revised: July 8, 2014 Published: July 10, 2014 © 2014 American Chemical Society
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used to demonstrate a new method to synthesize nonspherical microcapsules with different aspect ratios by changing the applied electric field strength.
polymer network on capsule deformation and specifically described the effect of HCl and KOH on network formation from organosiloxane monomer (OTS). For low HCl concentration, network formation decelerated as compared to that in the case of water, while at higher concentrations of HCl the gelation kinetics became faster. During the hydrolysis step release of chloride anions (eq 1) hinders hydrolysis. However, HCl catalyzes condensation of silanol to polysiloxane (eqs 2 and 3). Therefore, at lower concentrations of HCl, although hydrolysis is slowed down, at higher concentrations the catalytic activity of HCl accelerates the condensation. Addition of KOH accelerates network formation because it catalyzes both hydrolysis of cholorsilanes as well as condensation of silanols. A strong scatter in the 2D storage modulus with time was observed at higher concentrations of KOH, indicating formation of a different gellike structure of the OTS network (flocculent structures8). Precise control of colloids of nonspherical or anisotropic shapes which are found in nature has attracted a lot of attention in recent decades.9 Nonspherical shapes are highly desired for their properties such as anisotropic response to external fields, large surface areas, and unique structure formation.10 Efforts have been devoted to generate particles with various morphologies.11−18 In a microfluidic device a droplet can be confined into a disk, a plug, or a rod19,20 if the droplet size is larger than the microfluidic channel. Husmann et al.21 synthesized nonspherical polysiloxane microcapsules by making use of the spinning-drop apparatus. When a fluid drop is suspended in an immiscible fluid it acquires a spherical shape under the influence of interfacial tension and in the absence of any other force. Such a neutral drop, when subjected to a uniform external electric field, deforms due to the electric stresses that can be attributed to the contrast in conductivities and/or the dielectric constants of the drop and the medium phase. When a conducting drop is suspended in a dielectric medium, an axisymmetric deformation is seen which is a result of a balance of the electric stress (ϵeϵ0Eo2) and the capillary stress (γ/a). The axis of symmetry is always in the direction of the field,22 and the deformation depends upon the capillary number Ca = ((aϵeϵ0Eo2)/γ) (note that a, ϵe, ϵ0, Eo, and γ are the drop radius, dielectric constant of the medium, permittivity of the free space, magnitude of the uniform electric field, and surface tension, respectively). Taylor23 argued that a shape corresponding to the second Legendre polynomial P2(cos θ) is able to balance the electric stress leading to D = (9/16)Ca, where D = ((L − B)/(L + B)) is the Taylor deformation parameter, defined with an ellipse of major axis of length L and minor axis of length B. At small capillary numbers Taylor’s theory is a convenient way to determine the interfacial tension.24−27 The deformation of a typical capsule is estimated by the following definition for the modified Taylor deformation parameter
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EXPERIMENTAL SECTION
Materials and Experimental Setup. Octadecyltrichlorosilane (OTS) (Sigma-Aldrich) was used as purchased without further purification (≥ 90%). Silicone oil of 380 cSt (Merck) and 1000 cSt (Sigma-Aldrich) as well as pH 1.68 (deionized water/potassium tetroxalate dihydrate) (Oakton, Cole-Palmer), 4 (citric acid monohydrate/hydrochloric acid), 6 (deionized water/citric acid, sodium salt), 7, 9 (potassium chloride/boric acid/sodium hydroxide) (Merck) buffer solutions were also used without further purification. Ultrapure deionized (DI) water was obtained from Mili-Q (σ = 0.02 S/m). All chemical compounds were stored under vacuum. All experiments were performed at room temperature (24 °C). A schematic diagram of the experimental setup for studies in deformation and electrosynthesis of a capsule is shown in Figure 1. The system consisted of a rectangular plastic cuvette with a high-voltage generator connected to two parallel copper plate electrodes of size
Figure 1. Schematic drawing of the experimental setup (1, function generator; 2, high-voltage amplifier; 3, high-speed camera; 4, stereo zoom; 5, cuvette with copper electrodes; 6, fiber optic illuminator; 7, computer control).
where A and B are twice the distances of the pole and the equator from the origin before deformation and l and b are corresponding quantities after deformation. Using this concept of electrodeformation, we propose a novel method of studying the kinetics of polysiloxane network formation. The change in kinetics due to a change in the pH of the drop phase is analyzed to understand the mechanism of membrane formation. Insights obtained from the kinetics are
Figure 2. Evolution of degree of deformation of a pH 4 buffer solution drop suspended in silicone oil (μ = 1000 cSt) containing OTS (concentration = 0.05072 mM) in uniform ac electric field (f = 1 kHz and Erms = 3.54 kV/cm) at different times ((circle with ×) t = 30 s, (circle with plus) t = 2 min, (circle with minus) t = 4 min, (circle with dot) t = 6 min, (square with dot) t = 8 min, (square with ×) t = 10 min, and (square with plus) t = 12 min) after introducing the drop in silicone oil containing OTS. 10271
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Figure 3. Deformation (%) of a capsule suspended in oil containing OTS (COTS = 0.05073 mM; silicone oil with OTS (●) μ = 380 cSt, (○) μ = 1000 cSt; silicone oil without OTS (Δ) μ = 380 cSt) as a function of polymerization time (t) in uniform ac electric field (f = 1 kHz and Erms = 3.54 kV/cm), (a) pH 1.68 buffer solution drop, (b) pH 4 buffer solution drop, (c) pH 6 buffer solution drop, (d) pH 7 buffer solution drop, (e) pH 9 buffer solution drop, (f) DI water drop, and (g) hydrolysis reaction times and condensation reaction rates for different drop phases. Initial drop radii ranged between 400 and 600 μm.
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10 mm (L) × 3 mm (B) × 45 mm (H). The distance between the two electrodes was kept at 4 mm. The electric field was generated with a function generator (33220A from Agilent Technologies Pvt. Ltd., USA). This device limits the voltage to between 0 and 20 V of any desired waveform. Applied voltages were amplified by connecting the function generator to a high-voltage amplifier (TREK INC., USA) with a fixed gain of 1000 V/V. The densities of the suspended phase and the surrounding medium phase were almost similar, and therefore, buoyancy effects were discounted. The drop had an incredibly low terminal settling velocity in the high-viscosity medium, and negligible displacement in the gravity direction was observed during the course of the experiment. The drop was placed at the center of the space between the two copper electrodes, and an electric field was applied. Deformation of the capsule was recorded with a high-speed camera (Phantom V 12, Vision Research, USA) and observations made using a stereo zoom microscope (SMZ1000, Nikon Instruments Inc.). The large parallel electrodes (as compared to the capsule and separation) ensured that the electric field was almost uniform. The deformed shape of the drop was axisymmetric with the axis of symmetry perpendicular to the applied electric field, and deformation was calculated from the captured images. The degree of capsule deformation was defined using eq 4.
RESULTS AND DISCUSSION Experimental Study of Kinetics of Interfacial Polymerization Using Electrodeformation. Figure 2 shows the evolution of the drop deformation at different times for drop phase with pH 4. Initially (t = 0) a drop is added into the oil containing OTS, and the time t after addition of the drop to the oil is monitored. The drop is subjected to an electric field at different values of t for a brief period of t* ≅ 3.25 s, and deformation is obtained from the videos/images. Here t* is the time after the field is switched on at any value of t. The drop deformation dynamics is fast at any time t, indicated by the plateauing of the D vs t* plot in Figure 2 after around t* = 2 s. Therefore a total time of t* = 3.25 s for observing steady deformation at any value of t is justified. At small values of t, a large value of the degree of deformation is observed (plateau value of D in the D vs t* plots for, say, t = 30 s and 2 min). After around t = 2 min, the degree of drop deformation reduces as OTS adsorbs at the interface and acts as a surfactant. The results of the 10272
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networks then physically adsorb on the polymer network (shown schematically in Figure 4 D). These can also resist deformation, leading to a gradual decrease in the degree of deformation. The data obtained in Figure 3 are used to understand the kinetics of the OTS network formation. At pH 4 the conditions inside the aqueous drop phase are acidic and the possible reactions are given as follows.3,6,32
above study on the kinetics of deformation were used to design experiments to study the effect of different parameters such as concentration of OTS, electric field, etc., on capsule deformation.28 The steady state (plateau value) of the degree of deformation of the drop/capsule (Figure 2) at any value of t is considered to study the kinetics of the OTS-based polysiloxane network formation (Figure 3). In order to understand the kinetics of surface gelation, we studied the evolution of the steady state degree of deformation of a drop (as discussed in Figure 2) as a function of time. In a series of experiments (Figure 3), we changed the pH of the drop phase keeping OTS concentration in the continuous oil phase constant (OTS concentration 0.05073 mM). Experiments were also performed on a drop of DI water where pH was not controlled at 7. The pH in uncontrolled DI drop is expected to decrease on account of release of HCl in the hydrolysis step. In these experiments, the frequency f and the electric field strength E were kept constant ( f = 1 kHz, Erms = 3.54 kV/cm). The field was switched on for around 4 s, ensuring steady state with respect to capsule deformation (Figure 2), and then switched off for 30 s, ensuring complete restoration and the process was repeated. It should be noted that the response time to deformation of an unreacting drop or a capsule is a few seconds (≈ 2 s, see Figure 2). Typical steady state deformation (Figure 2) curves as obtained by the above procedure are plotted in Figure 3 to study the kinetics of hydrolysis and condensation (network formation). Once a water drop is suspended in an oil that contains OTS at different concentrations (shown schematically in Figure 4 A), diffusion of OTS molecules from the oil to the oil−water interface takes place, followed by hydrolysis of silanes at the oil−water interface (Figure 4 B). Figure 3a shows that at low pH the hydrolysis process is quite slow. The high degree of deformation at this initial point is similar to that of a water drop in oil. The deformation curve shows an initial increase in deformation for about 5−6 min (Figure 3a) due to the surface active behavior of the adsorbed OTS molecules. This represents the first step of the reaction, i.e., hydrolysis, at the oil−water interface. The effect of continuous phase viscosity (Figure 3a−d) on the polysiloxane network kinetics is remarkably insignificant. This suggests that the hydrolysis process is not diffusion but reaction controlled. As the reaction time progresses, cross-linking starts and more and more junction points are formed until a dense network structure appears. This can be observed by a systematic decrease in the degree of deformation with respect to time t (Figures 3a and 4 C). The interfacial polymerization proceeds rapidly with a time constant of only a few minutes. These time scales are in agreement with that reported in the literature.21,30,31 The crosslinking that follows results in a decrease in deformation, indicating initiation of cross-linking, i.e., condensation step where the Si−O−Si bonds form. Several polymer chains, spontaneously formed, thus create a network with dense cross-linking. Thus, the resistance to deformation changes from surface tension forces to elastic forces as polymerization progresses. The large scatter seen during the condensation step (Figure 3a) can be attributed to its stochastic nature. Moreover, it should be noted that the size of the capsule in the above experiments varied between 400− 600 μm. The plateauing out of the degree of deformation over a time period of a few minutes signifies completion of the polymerization reaction at the interface (Figure 3a). Once the entire oil−water interface is covered with the polymer network, the unreacted silanol molecules or patches of the polysiloxane
Acid-catalyzed hydrolysis reaction of OTS initiates with attack of O from H2O on Si, forming a bond wherein the lone pair of O is shared with Si. This results in removal of HCl (eq 6). Acid-catalyzed OTS condensation reaction (eq 7) involves protonation of the −OH group of silanol, thus polarizing the Si−O bond. The oxygen lone pair from another silanol molecule then attacks the Si atom, forming an Si−O−Si bond with elimination of H2O (eq 8).
At neutral and higher pH (Figure 3b (pH 7, neutral) and 3c (pH 9)), hydrolysis is catalyzed by the presence of highly reactive nucleophilic hydroxyl (OH−) anion.3,6,32 The paucity of H+ at neutral and high pH means that protonation of silanes is suppressed and hydrolysis takes place by a simple SN2 mechanism wherein the strong OH− nucleophile instantaneously replaces the Cl− ions from OTS. The hydrolysis reaction is so fast at higher pH that it is not possible to resolve its kinetics in our experiments. The base-catalyzed hydrolysis reaction (eq 9) of OTS is found to be fast as compared to the acid-catalyzed hydrolysis reaction, 10273
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Figure 4. Reaction mechanism for the OTS polymerization at the oil−water interface. (A) Drop suspended in oil containing OTS, (B) OTS molecules hydrolyze to form silanols at the water oil−interface and also in the bulk, (C) silanols then condense to form Si−O−Si (siloxane) network at the oil−water interface, and (D) oil−water interface is then covered with polysiloxane network (motivated by Wang et al.29).
since OH− is a strong nucleophile (as compared to Cl−) which directly attacks the Si atom to displace the Cl−. The condensation rates also appear to be catalyzed by the presence of hydroxyl (OH−) ions (see table (g) in Figure 3). The base-catalyzed OTS condensation reaction (eq 10) involves formation of negatively charged siloxide groups by extraction of the proton from the −OH group of neutral silanol. The negatively charged O− in the siloxide then directly attacks the Si of neutral silanol species, forming an Si−O−Si bond (eq 11). Thus, the condensation reaction in the case of pH 7 and 9 is faster as compared to the condensation rate in the low pH system (table in Figure 3 that involves intermediate formation. It should be noted that the condensation rate (D/min) in the table in Figure 3 are the slopes of the D vs t curves in Figure 3a-e obtained at the beginning of the condensation step. When DI water is used as a drop phase, the time required for the hydrolysis step is about the same as compared to pH 4 buffer solution drop (see Figure 3a−d) probably because of a decrease in the pH of DI water locally due to release of HCl. The decrease in pH is facilitated by complete dissociation or ionization of HCl, which is a monoprotic acid, in water to form a hydronium (H3O+) ion. Therefore, the response of DI water drop is simlilar to that of pH 4 buffer drop. Exact estimates of the decrease in pH in the DI case because of release of HCl could not be made. Figure 5 shows the variation of the interplay of interfacial tension (γ) and Young’s modulus (Es) with respect to time in the different regimes. The interfacial tension in the first (I, where Es = 0) regime can be calculated by Taylor’s theory23 using the deformation D of the drop as D = (9/16)((aϵeϵ0E02)/γ). In the third (III, where γ = 0) regime where only elasticity of the polysiloxane network is present, the Young’s modulus can be estimated28 using the deformation of the capsule as D = (45/16)((aϵeϵ0E02)/Es). The estimated value of γ is found to be 7−10 mN/m, while that of Young’s modulus is 0.3−0.7 N/m. In the second (II, where γ/Es ≠ 0) regime, with known values of γ and Es, the fraction of free surface x (surface where OTS/ silanols are not condensed into polysiloxane network and act as surfactants) can be estimated by a simple model as
D=
2 2 9 aϵeϵ0E0 45 aϵeϵ0E0 x+ (1 − x) 16 γ 16 Es
(12)
Thus, x = 1 indicates that the deformation of a drop is dependent only on the surface activity of the surfactants (regime I), while x = 0 indicates that the deformation of the capsule is dependent only on the elasticity of the polymerized elastic network (regime III). The interfacial tension γ between water and silicone oil devoid of surfactants is 25 mN/m.27 Initially for a period of about 3−4 min, in the case of acidic pH buffer as a drop phase (regime I in Figure 5 for pH 1.68 and 4), the drop deformation increases due to a decrease in interfacial tension corresponding to a decrease in interfacial tension γ from 25 to 7−10 mN/m. The available fraction of free surface (x) then reduces gradually from 1 to 0 until about t = 14 min (regime II in Figure 5 for pH 1.68 and 4). After t = 14 min, the deformation becomes constant (regime III in Figure 5 for pH 1.68 and 4), indicating nonavailability of the free surface or completion of the condensation reaction. As the pH of buffer was increased, the hydrolysis reaction became faster and regime I was not observed (Figure 5 for pH 6, 7, and 9) with the present method. In these cases, γ = 25 mN/m (for a water drop suspended in silicone oil) was considered and the free surface available was estimated. The hydrolysis as well as condensation reactions were so fast that the free surface x reduced to zero very fast. As pH was increased from 6 to 9, the time required for the fraction of free surface to became zero reduced drastically. For pH 9, x changed from 1 to 0 in 1 min. This suggests, more basic the pH, the reactions became faster and faster. Thus the data in Figure 5 regime II can be considered to correspond to the kinetics of polycondensation process. Synthesis of Nonspherical Microcapsules Using Electric Field. A drop with an aqueous phase at pH 4, suspended in silicone oil containing OTS, showed slow hydrolysis reaction rate (hydrolysis time ∼ O (5 min)). This aspect can be exploited to synthesize nonspherical microcapsules. By controlling the applied electric field strength, nonspherical microcapsules with aspect ratios ranging from 1.05 to 2 can be synthesized by continuous application of electric field. The evolution of the drop shape with respect to time for different applied electric field strengths is shown in Figure 8a. When electric field was switched 10274
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Figure 5. Estimation of interfacial tension γ, Young’s modulus Es, and fraction of free surface x of an aqueous drop suspended in silicone oil (μ = 1000 cSt, cOTS = 0.05072 mM) in a uniform electric field. The regime in which the electric forces are balanced by the interfacial tension (γ) alone is shown by regime I. In regime III, the polymerized network elastic forces (Es) alone balance the electric forces. In the middle or regime II, the drop deformation is dependent on a combination of interfacial tension (γ) as well as the elastic forces (Es) (due to partial condensation of the silanols).
off; after 20 min of continuous application of electric field (as shown in Figure 6a), no noticeable shrinking of the capsules was observed, indicating completion of polycondensation reaction at the oil−water interface. A small shrinking though is expected since the capsule deformation balances the electric stress before the field is switched off. Specifically, the deformation of a
spherical capsule is of the order of D = 0.01, which is negligible compared to the deformation D ≈ 0.35 of the nonspherical capsules. This probably is the reason for shrinkage not being observed on switching off the field. When a drop, suspended in silicone oil containing OTS, is subjected to a continuous application of electric field, adsorption 10275
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Figure 6. (a) Evolution of pH 4 buffer solution drop shape suspended in silicone oil (μ = 1000 cSt) containing OTS (concentration = 0.05072 mM) ((circle) Erms = 1.885 kV/cm, (circle with dot) Erms = 2.199 kV/cm, (circle with plus) Erms = 2.514 kV/cm, (circle with ×) Erms = 2.828 kV/cm, (circle with plus) Erms = 3.142 kV/cm, (square) Erms = 3.456 kV/cm, (square with dot) Erms = 3.771 kV/cm, (square with plus) Erms = 4.085 kV/vm, (square with ×) Erms = 5.027 kV/cm, (square with a minus) Erms = 5.656 kV/cm, (triangle) Erms = 6.284 kV/cm). (b) Schematics of three different regimes where drop deformation is dependent on the interfacial tension (γ) or elastic forces (Es) or combination of both (γ + Es). (c) D versus aE02 for a pH 4 buffer solution drop suspended in silicone oil (μ = 1000 cSt) containing OTS (cOTS = 0.05072 mM) (symbols, final shapes obtained experimentally after t = 20 min). (d) Nonspherical microcapsules with different aspect ratios (AR = (1 + D)/(1 − D)).
after switching off the electric field (after t = 20 min) is plotted versus aE02. Here a is the initial drop radius. The curve can be used to synthesize nonspherical microcapsules of deformation D. Thus, the interfacial polycondensation of the OTS to form elastic networks opens opportunities to control the shape of the fluid drops and stabilize nonspherical shapes in uniform electric field as elastic capsules. Figure 6d shows the synthesized nonspherical microcapsules with aspect ratios 1.05 (Erms = 1.885 kV/cm) and 1.97 (Erms = 6.284 kV/cm) at a frequency of f = 1 kHz. Good shape control is observed with variation of electric field, and axisymmetry seem to be preserved, although asymmetry at the poles is observed at high aE02 (a point discussed later in some detail). We now discuss the mechanism of nonspherical capsule formation using schematics (Figure 7) and the evolution of a pH 4 buffer solution drop suspended in silicone oil of 1000 cSt
of OTS at the oil−water interface leads to an increase in the electrodeformation due to a decrease in the interfacial tension (γ). A first regime (until t = 4−5 min, which can be confirmed from Figure 3a) can be considered (as shown in Figure 6a and also shown schematically in Figure 6b) in which the electric forces are balanced by the interfacial tension alone. In the second regime the drop deformation is dependent on a combination of interfacial tension (γ) as well as the elastic forces (Es) (due to partial condensation of the silanols, as shown in Figure 6a, where the D versus t curve starts plateauing, and also shown schematically in Figure 6b). Constant deformation of drop D with respect to t (the third regime, as shown in Figure 6a and also schematically in Figure 6b, suggests that the condensation reaction is complete and the polymerized network elastic forces (Es) alone balance the electric force. In Figure 6c, the D obtained 10276
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Figure 7. Reaction mechanism for the OTS polymerization at the oil−aqueous buffer drop (pH 4) drop interface. (A) Drop suspended in oil containing OTS. (B) Drop deformation and adsorption of OTS molecules at the oil−water interface. (C) Decrease in interfacial area due to silanol molecules acting as surfactants because of which drop deformation increases. (D) Adsorption of more OTS molecules at the interface due to increased surface area. (E) Decrease in interfacial area due to an increase in silanol concentration because of which drop deformation increases. (F) Condensation of silanols to polysiloxane.
containing OTS (cOTS 0.05072 mM) in Figure 8. When a drop is suspended in a dielectric oil containing OTS (Figure 8a and also shown schematically in Figure 7 A) it deforms into a prolate shape because of electric stresses acting on it where the deformation is described by Taylor’s theory.23 These electric stresses are balanced by the capillary stresses. The OTS in the bulk oil phase then adsorbs at the oil−water interface (Figure 7 B) and hydrolyzes to silanol. The formed silanols and also the adsorbed OTS at the oil−water interface act as surfactants. Surfactants are known to lower the interfacial tension. This causes an increase in the deformation of the drop for an applied electric field strength (Figure 8a at t = 4 min and also shown schematically in Figure 7 C). An increase in deformation causes an increase in the surface area of the drop with the volume of the drop remaining constant. Therefore, more OTS molecules can adsorb at the interface because of increased surface area (Figure 7 C). Surfactant concentration increases at the interface, which causes further lowering of the interfacial tension and an increase in the drop deformation. This is further proved in Figure 9, where application of strong fields leads to breakup by tip streaming (Figure 9b) instead of bulbous ends (Figure 9a), which is known to happen only in surfactant-laden drops.33 The process described in Figure 7A−C keeps on repeating until the condensation starts and forms a polysiloxane network at the oil−water interface. The slow rate of hydrolysis observed at low pH is therefore critical to reach a large deformation before polycondensation can start. The renewal of surface area due to stretching leads to continuous migration of OTS to the surface followed by hydrolysis and condensation, which leads to a continuous increase in size. An equilibrium is reached when elastic stress exactly balances the electric stress. After about 17 min the drop deformation attains a constant value which indicates completion of the condensation reaction (Figure 8a at t = 18 min and also shown schematically in Figure 7 F). Some of the capsules synthesized with this method were not axisymmetric (Figure 6d, capsule with AR = 1.17 or 1.97). When a drop is introduced through a syringe in the oil it cannot
be placed exactly at the center of the experimental cell. The OTS then starts adsorbing at the oil−water interface, and due to its surface activity, interfacial tension at the oil−water interface decreases which causes an increase in the drop deformation. With time, the poles of the deformed drop experience a high electric stress and start drifting toward the nearest electrode. This drifting of the deformed drop toward the nearest electrode causes an asymmetry between the two poles, resulting in a sharper shape at the pole nearer to the electrode (note that since AC fields are used, the polarity is not important). In Figure 8 (t = 20 min) the black shade on the right side of the image is actually the electrode. The drifting of the drop can be controlled using even more viscous silicone oil. However, our attempts to dissolve OTS in silicone oil of μ = 10000 cSt were not successful. Another reason for the left−right asymmetry of the capsule can be attributed to the interplay of surface tension and elasticity in the partially condensed capsules at intermediate times (t = 8, 10, 12, and 14 in Figure 6a). The inherent difference in the mechanics of elastic and surface tension forces can lead to asymmetric interfacial forces than can deform the capsule as discussed for Figure 6. After 20 min when the electric field is turned off, it is found that the formed polysiloxane network at the interface arrests the drop shape forming a nonspherical capsule. The change in the surface area of the drop with respect to time is shown in Table 1. The volume remains constant within experimental errors. It is also found that the surface area remained nearly constant after the field is switched off. It should be noted that use of pH 6−9 would make the process difficult to control due to rapid hydrolysis. Figure 8b shows the evolution of a pH 9 buffer solution drop suspended in silicone oil of 1000 cSt containing OTS (cOTS 0.05072 mM). As high pH strongly catalyzes the hydrolysis reaction, the drop deformation is arrested and a very low aspect ratio (AR = 1.03) microcapsule is obtained even for a very high applied electric field (Erms = 6.284 kV/cm). 10277
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Figure 8. Nonspherical polysiloxane microcapsule synthesis with a drop suspended in silicone oil (μ = 1000 cSt) containing OTS (cOTS = 0.05072 mM) under uniform ac electric field ( f = 1 kHz): (a) with pH 4 buffer solution drop (Erms = 6.284 kV/cm, AR = 1.97) and (b) with pH 9 buffer solution drop (Erms = 7.855 kV/cm, AR = 1.03).
Table 1. Estimates of Change in Surface Area of the pH 4 Buffer Solution Drop Suspended in Silicone Oil (μ = 1000 cSt) with OTS (concentration = 0.05072 mM) under Uniform ac Electric Field (f = 1 kHz, Erms = 6.284 kV/cm) Figure
V (m3)
0
1.768 × 10
2.21 × 10−10
b
1
1.890 × 10−6
2.21 × 10−10
c
4
2.025 × 10−6
2.18 × 10−10
d
6
2.170 × 10−6
2.15 × 10−10
−6
2.17 × 10−10
−6
2.22 × 10−10
−6
2.22 × 10−10
−6
2.07 × 10−10
−6
2.13 × 10−10
−6
2.13 × 10−10
−6
2.14 × 10−10
−6
2.14 × 10−10
f g h I j k l 10278
S (m2)
a
e
Figure 9. Breakup of aqueous drop suspended in dielectric oil under uniform electric field: (a) lobes formation breakup mode without OTS in the oil, and (b) tip-streaming breakup mode with OTS (concentration = 0.05072 mM) in the oil.
t (min)
−6
2.350 × 10
8
2.550 × 10
10
2.730 × 10
12
2.800 × 10
14
2.880 × 10
16
2.960 × 10
18
2.990 × 10
20 field off
2.990 × 10
dx.doi.org/10.1021/la501617t | Langmuir 2014, 30, 10270−10279
Langmuir
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Article
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CONCLUSIONS A new method based on electroformation is suggested for understanding the kinetics of interfacial polycondensation processes. The degree of deformation of a spherical globule when subjected to an electric field is used to estimate the progress of reaction and different steps of the reaction. The kinetic data is then exploited to synthesize nonspherical capsules. The method opens up several avenues for understanding interfacial processes. Rigorous modeling relating kinetics to the structure of the interface and thereby to deformation can further elucidate the mechanism of interfacial kinetics. A challenge in the method is to understand faster dynamics, and at present it seems that the method is probably suitable for interfacial processes with slow kinetics. The method of formation of ellipsoidal capsules suggested is attractive because of the absence of residual stresses in them. Large-scale production of these capsules for controlled aspect ratio remains a challenge in addition to their polydispersity. These stabilized nonspherical microcapsules can provide easy paths for microencapsulation, drug delivery carriers, and reaction platforms with increased surface area for a given volume.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +91 (22) 2576 7241. Fax: +91 (22) 2572 6895. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Asfiya Contractor, Prof. V. A. Juvekar and Prof. V. M. Naik for their useful suggestions. R.T. would like to thank the Department of Science and Technology (DST), India, for funding the work.
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dx.doi.org/10.1021/la501617t | Langmuir 2014, 30, 10270−10279
