Effect of Tetraalkylammonium Cations on Gas Coalescence at a

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Effect of Tetraalkylammonium Cations on Gas Coalescence at a Hydrogen-Evolving Microelectrode Lorena M. A. Monzon,* Alice J. Gillen, Matthias E. Mobius, and J. M. D. Coey School of Physics, SNIAMS building, Trinity College Dublin, Dublin 2, Ireland ABSTRACT: Hydrogen gas evolution at the surface of a microelectrode may result in periodic release of single bubbles larger than the electrode diameter. Bubbles often grow by incorporating smaller bubbles that coalesce with them. To explore the coalescence, we investigate how a series of six tetralkylammonium cations (TXA+), where the number of carbons on the alkyl chain varies from 1 to 6, affects the oscillatory behavior of the gas-evolving microcathode. Different concentrations of TXA+ bromide salts ranging from a few micromolar up to 1 M were added in the acid electrolyte. The frequency of bubble release and the transition from periodic to aperiodic release are related to the inhibition of bubble coalescence and gas streaming. The concentration range where this transition occurs depends strongly on the cation hydrophobicity and it ranges from very small values for the hydrophobic cations to over 1 M for the most hydrophilic one. For some of the TXA+ cations, the transition shows a smooth increase in release frequency before switching completely to bubblestream behavior, while for others the transition is abrupt. A smooth increase in the gas oscillator frequency with concentration indicates that the adsorption of TXA+ cations on the bubble surface is mass transport-limited. The inhibition of bubble coalescence by the smallest cations is electrochemically driven, facilitated by specific interactions established between the ions and the electrode surface. of the interface between bubbles.10 Therefore, bubble coalescence is still expected to take place in the presence of αα and ββ salts, but at a much slower rate.10 On the other hand, it has been recently demonstrated that ions of the same charge adsorbed at the surface of submicrometer-size bubbles tend to stabilize them, because the Coulomb repulsion opposes the surface tension of the bubble11 and charge separation caused by differences in the ions’ surface affinity is not determined so much by ion-hydration forces but by the ions’ influence on the dielectric properties of interfacial water.12,13 Experiments carried out using in situ environmental transmission electron microscopy (TEM)14 or on transparent Pt electrodes15 indicated that the nucleation of hydrogen takes place both in the solution and at surface irregularities and it occurs after supersaturation has been reached,14,16,17 with hydrogen pressures varying inversely with electrode size.16,17 For example, nanobbules are found to grow on nanoelectrodes only if the solution is supersaturated by a factor of over 300 while in microelectrodes this value can be up to 20.16,17 Once solubility limit is exceeded, gas which is initially dissolved in the electrolyte nucleates near but not necessarily on the electrode surface; bubbles then precipitate wetting the electrode.14 For this reason, species both at the electrode surface as well as in

1. INTRODUCTION The ability of some ion-pairs to inhibit the coalescence of gas bubbles formed by passing gas through an orifice into aqueous solution seems to depend on their relative affinity for the gas/ liquid interface.1−5 The inhibition of bubble coalescence is strongly ion-specific, exhibiting a well-defined transition concentration that is characteristic of each salt. Ions have been classified by Craig2 into two groups α and β. It was later proposed that α cations and β anions could be located away from the gas/liquid interface and α anions and β cations preferentially adsorbed at it.6 This classification was based on 0.1 M electrolytes and inhibition of bubble coalescence observed with αα and ββ salts, i.e. aqueous solutions containing ions pairs where one of the ions has an affinity for the interface and the other does not.6 Since then, this classification has been challenged because some electrolytes of ion-pairs that were not supposed to influence coalescence actually do when their concentration is increased over 0.1 M.7 In fact, a more nuanced description considers that concentration gradients of ions are established at the air−liquid interface whose maxima are at different depths,8 with ion pairing interactions and bulk electrolyte concentrations affecting the ionic interfacial distributions.9 For many years there were debates on whether or not hydration forces played a role on bubble coalescence.1−5,10 However, given this process is driven by collisions in the electrolytes, ions most likely affect the dynamics of film thinning and rupture by slowing down the re-equilibration rate © 2015 American Chemical Society

Received: March 18, 2015 Revised: May 7, 2015 Published: May 13, 2015 5738

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Table 1. Structure Parameters of All the TXA+ Ions Calculated for the Structures Shown in Figure 6, Taking the Bond Angles of 104.5° and Bond Lengths of C−N, C−C, and C−H to be 1.43 Å, 1.54, and 1.09 Å, Respectively and Considering the Ions from TEtA+ Onwards Enclosed in a Circle Corresponding to the Total Projected Area, With the Effective Area Being Equivalent to the Surface Occupied by Each Cationa

a

The theoretical surface coverage is calculated taking the effective area of each ion and considering a 100% surface excess. Second column is the number of carbons on the alkylchains of each TXA+ cation. Third column contains some of the molecular structures. Average Vfoam and tfoam values obtained from shaking experiments (Figure 4). potential used is much lower than that needed to reduce the tetralkylammonium cations, ensuring that the salts do not undergo chemical decomposition during electrolysis.21 Even at the most negative overpotentials, the production of hydrogen is driven by the reduction of H+ and not by water dissociation that would occur when the H+ concentration in the diffuse layer drops below 10−7 M.22 A three-electrode configuration was used, including a Pt wire counter electrode and a Ag/AgCl/Cl reference electrode. The aqueous electrolyte was a 1.2 M H2SO4 solution, containing different concentrations of tetraalkylammonium bromides salts (purity >99%, Sigma-Aldrich) that were used as received. The salts were tetramethylammonium (TMeA+), tetraethylammonium (TEtA+), tetrapropylammonium (TPrA+), tetrabutylammonium (TBuA+), tetrapenthylammonium (TPeA + ), and tetrahexylammonium (THexA+), with the alkyl chain ranging from 1 to 6 carbons long. The frequency of bubble release was determined from power spectra derived from the overpotential signals, using Origin Pro 8.6 software. The electrochemical experiments were repeated a number of times with different sets of solutions to test reproducibility. The transition concentration range where bubble release switches from periodic to bubble stream is characteristic of each ion, with slight variations that may be due to dilution errors. Increased roughness of the Pt microelectrode gives rise to oscillatory electrochemical signals which have several different fundamental frequencies. The corresponding videos show bubbles of different sizes being released from the electrode at different frequencies. Prior to each measurement, the microelectrode was mechanically polished with diamond lapping pads of 6, 3, 1, and 0.05 μm, and washed with copious amounts of DI water between each step. After rinsing, the electrode was placed in the cell, and electrochemically cleaned with successive pulses at 3.0 and −3.0 V in 1.2 M H2SO4 solution. Oscillatory potential curves were recorded during 240 s, with a 0.004 s sampling rate. Static surface tension was measured with for all electrolytes with the du Nouy ring technique using a Krus K12 tensiometer with a platinum ring. The reported average values were calculated from 4−5 measurements of each solution. Relative viscosity values for TMeABr solutions were determined from the average of five measurements of the flow times using an Ubbelohde viscometer immersed in a thermostatic bath at 25 °C. Time was allowed for the solution to adjust to the water bath temperature before any measurement was carried out. The values were calculated by taking the ratio between the flow times of each solution with respect that of a 1.2 M H2SO4 reference.

the nearby solution can influence the nucleation and growth of bubbles. Experiments performed with a bubbling tube immersed in 0.1 M electrolytes have shown that Tetramethylammonium bromide (TMABr) does not affect the coalescence of bubbles and was classified as a βα pair,2 with the cation presumably having some affinity for the gas/liquid interface. TMeA+ belongs to a family of tetraalkylammonium cations (TXA+) where the alkylchain length determines the hydrophobicity of the salt, thereby giving rise to a wide range of physicochemical properties. TXA+ salts are extensively used as supporting electrolytes in organic media because of their chemical stability,18 as well as in the fabrication of low-density selfassembled monolayers where the size of the cation controls the molecular spacing in the film.19 They can promote or impede foam stability of common surfactants and serve as flotation collectors.20 All the cations in this family belong to the β group yet, their influence on bubble coalescence should be expected to depend on how they modify the structure of the gas/liquid interface. Here, we report on the influence of six members of the TXA+ family on the coalescence of electrochemically generated hydrogen gas with X = Me, Et, Pr, Bu, Pe, and Hex, where the number of carbons on the alkyl chain increases from 1 to 6, in that order (see full names below, Table 1 and Figure 6 for more details). The nucleation and growth of hydrogen gas on the surface of a microelectrode offers a versatile way to study bubble coalescence as one single bubble grows at a time, and its frequency of release providing a direct measure of the bubble growth dynamics. These results are compared with simple foamability tests to check the surfactant-like behavior of these salts and their effect on bubble coalescence.

2. EXPERIMENTAL SECTION Hydrogen gas was produced at the upward-facing surface of a commercial Pt working microelectrode (CHI PT100, CHI Instruments, Bee Cave, TX) 100 μm in diameter in order to allow free detachment of the bubbles. Electrochemical experiments were performed at 0.5 mA with a CHI 660A potentiostat operating in the galvanostatic mode, at a current density of ∼6.4 A cm−2. The range of 5739

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Langmuir Structure parameters of all the TXA+ ions were calculated taking the bond lengths of C−N, C−C, and C−H to be 1.43, 1.54, and 1.09 Å, respectively, with bond angles of 104.5°. Model molecular structures were built using the Orbit molecular building systems. Optimized structures can be found in ref.23 Foam formation was evaluated by shaking centrifuge sample vials containing 10 mL of 1.2 M H2SO4 solution and each TXABr salt.24 Bubble growth was observed and recorded simultaneously with the electrochemical response. Image sequences were taken using a PCO 1200HS high-speed monochrome camera at a frame rate of 600 s. Magnifications between 3.5 and 5.2 were achieved using a Sigma 180 mm 1:3.5 Alp Macro DG lens with a 140 mm macro bellows. The hydrogen bubbles were illuminated from behind using either a PHLOX planar backlight or Fiber Light DC- 950 light source with a diffusion plate.

3. RESULTS The electrolytic production of hydrogen at the surface of a microelectrode is known to behave as a gas oscillator under certain experimental conditions.25,26 A single bubble grows until it reaches a certain size and then it detaches from the electrode surface. A large bubble occludes almost all the microelectrode surface while somehow leaving parts of it still accessible for electron transfer to the electrolyte.15,27 These large, single bubbles grow by diffusion of dissolved gas to the bubble surface and by coalescence with tiny bubbles which appear spontaneously from the nearby supersaturated solution, as well as with those growing at any exposed electrode surface.28,29 The typical oscillating electrochemical signals (Figure 1a, inset) arise from the resistance associated with the occlusion of the electrode surface by the growing hydrogen bubble. As it grows, the overpotential, E, at constant current passes through a minimum before spiking back to less negative E when the bubble detaches from the surface.15,27 The frequency of release, f, determined from the power spectral density plots (Figure 1b), is influenced by variables such as the surface tension, the nature of the electrolyte, external magnetic fieldm, and the current density employed during hydrogen production.15 In the absence of organic additives f is ∼0.5 Hz at our set current of 0.5 mA. This value may vary with surface roughness and it is particularly influenced by slight vertical misalignment of the electrochemical cell. In Figure 1, a reduction in average bubble size due to the presence of surfactant, from 700 μm to about 250 μm in diameter and an increase in f from 0.8 to ∼18 Hz is associated with a drop in the average potential from ∼ −1.00 V to −0.82 V, which can be understood considering that a bigger bubble occludes much of the electrode surface, thereby drawing more power to maintain the same current. The difference between the highest and lowest potentials diminishes with bubble size (Figure 1a, inset). To determine how much of the hydrogen produced ends up inside the bubbles we calculate the current efficiency by observing the bubbles detaching from the electrode surface. At the slowest frequency f recorded or in other words, passing a current of 0.5 mA for 2 s, 5.2 × 10−9 moles of H2 gas should be formed if the current conversion rate is 100%. Assuming that hydrogen behaves as an ideal gas at room temperature and normal atmospheric pressure, the volume occupied by 5.2 × 10−9 moles of hydrogen gas would be 1.3 × 10−4 cm3, which is equivalent to a bubble of 650 μm diameter, in agreement with observations. The current efficiency for this electrogenerated hydrogen gas is close to 100%.15

Figure 1. Influence of TBuA+Br− concentration on bubble release frequency: electrochemical signals associated with the growth of single hydrogen bubbles at the surface of a microelectrode. (a) Time evolution of the overpotential and (b) noise spectra from where the frequency of bubble release is determined. Inset in (a) shows a magnification of the oscillating overpotential signals, with a significant increase in f caused by a small change in the TBuA+ concentration. Electrolyte: 1.2 M H2SO4 with the addition of 50 μM (black line) and 120 μM TBuABr (red line). Average bubble size obtained in these conditions are 700 and 250 μm diameter, respectively. Pt microelectrode diameter is 100 μm. Current is set at 0.5 mA.

To determine how the concentration of a series of tetralkylammonium cations (TXA+) affects the oscillatory behavior of a gas-evolving microcathode, different concentrations of TXA+ bromide salts were dissolved in the acid electrolyte, going from a few micromolar up to 1 M solutions. In general, a transition from gas oscillator to streaming behavior is observed in a narrow concentration range. Figure 2 shows experiments recorded in the transition concentration range of TBuA+, where the first peak of the power spectrum and its harmonics shift toward higher f values, consistent with periodic release of progressively smaller bubbles at the surface of the microelectrode. The complete disappearance of well-defined peaks in the power spectrum corresponds to random bubblestream behavior where coalescence is inhibited (Figure 2, last 5740

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behavior depicted in Figure 5 indicates that the scavenging growth mechanism by random collision of bubbles is increasingly inhibited when the concentration of TBuA+ approaches to Cst = 0.2 mM. It is known that in the absence of surfactant, large hydrogen bubbles form on Pt electrodes as a result of collisions and coalescence but small ones are formed on Cu and Fe electrodes as they detach before coalescence can take place.28 Detachment of buoyant gas bubbles occurs when the surface adhesive forces cannot longer restrain them,28 therefore the presence of TBuA+ at the electrode surface could also alter the surface interactions and favor an early detachment of the bubbles. Figure 5a and b summarize the experimental values of f and surface tension, γ, corresponding to the transition concentration region for each of the TXA+ cations. Cst for each cation is indicated with vertical dashed lines. More than 4 orders of magnitude variation in Cst seen for these cations appears to be related to a decrease in surface tension, γ, determined for stagnant solutions of the whole series. However, the variation of γ with the ionic concentration of these ions is not sufficient to address their effect on the coalescence of hydrogen at the microelectrode because TPrA+, TEtA+, and TMeA+, change their periodic bubbling behavior at concentrations where they are not spontaneously adsorbed at a gas−liquid interface (Figure 5b). Other interesting features are the fact that THexA+ and TPeA+ exhibit similar electrochemical behavior to each other, almost coinciding with that of TBA+, with coalescence inhibition taking place in the concentration range between 0.2 and 1.0 × 10−4 M. Moreover, there is a clear contrast in the transition behavior exhibited by the ions: THexA+, TPeA+, TBuA+, and TMeA+ show a smooth increase in f before switching to complete bubble-stream whereas the transition is abrupt for TPrA+ and TEtA+. Table 1 also lists some structural parameters which were obtained by considering the TXA+ cations in their most stable geometries (see Figure 6a−g).23 The positive charge of these cations is localized on the central nitrogen atom, which imparts solubility to the corresponding salts in water, with the solubility limit also controlled by the nature of the anion and the acid present in the electrolyte. The coordination around the N atom is tetrahedral. TMeA+ is spherical but the larger members of the family are pseudoplanar, with the chains adopting a cross-shape configuration. A top view of all the structures are shown in Figure 6, with Figure 6d revealing the tetrahedral conformation around the N atom of TBuA+. The wide variety in the physicochemical properties of TXA+ cations arises mainly from their differences in size and their corresponding interaction with water.34,35 These molecules are water structure-makers, in the sense that water arranges itself around them. The smallest cation of this series, TMeA+, can be considered as a hydrophilic species, with its behavior in aqueous electrolytes in between that of the alkali ions and the larger TXA+ cations. Unlike the rest of the series, TMeA+ has no tendency toward self-association in water36 and shows anomalous ion-pairing behavior in voltammetric measurements37 which has been attributed to specific cation-solvent interactions.38−40 A comparison of the electrochemical adsorption of TMeA+, TEtA+, TPrA+, and TBuA+ on gold electrodes revealed that while TMeA+ remains hydrated in solution and does not adsorb at the gold surface, TEtA+ and TPrA+ show specific interactions with the electrode material, and TBuA+ forms a condensed structure at the interface which is metal-independent and dominated by hydrophobic inter-

Figure 2. Crossover from periodic bubble release to aperiodic streaming at the TBuA+ transition concentration range. Top panels are snapshots of bubble released from Pt microelectrode. Bottom panel: analysis of the oscillatory overpotential signals to determine the frequencies of bubble release, marked with corresponding arrows at the top of each peak. The concentration value corresponding to a loss of the periodic signal (spectra in orange) is the stream threshold concentration, Cst.

panel on the top right). We call the concentration at which this streaming sets in, the stream threshold concentration, Cst. The behavior of successive rising bubbles in a fluid has been associated with that of an inverted dripping faucet,30,31 where the time intervals between successive drops at a constant flow rate reflects the dynamics of these systems, their periodicity and the transition to chaotic streaming.32,33 Analysis of a hundred time intervals (tbubble) in the transition concentration range of TBuA+ described in Figure 2 are presented in Figure 3. The panels on the left show that the dynamic behavior obtained at low f is not random but periodic, with a slow time evolution of f. On the other hand, the panels on the right with f values of 20−50 Hz, indicate that the gas production becomes increasingly irregular as streaming sets in, with deviation from the average tbubble that can be as much as 40%. Foam formation via flask shaking was evaluated in the transition concentration range for all TXA+ cation solutions as a test of coalescence in the absence of electrochemical effects. Figure 4 shows pictures taken of the flasks at Cst immediately after shaking, with average foam volumes (Vfoam) and corresponding foam lifetimes (tfoam) summarized in Table 1. tfoam is the time it takes for the foam to disappear. These tests reveal that the TXA+ cations behave like surfactants, stabilizing tiny bubbles and retarding their coalescence. In the case of TBuA+ the coalescence is seen to be inhibited below 0.2 mM. Therefore, if one can assume that single bubbles in this system arise at least in part from the coalescence of tiny bubbles, as it has been shown previously for similar systems,28,29 then the 5741

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Figure 3. Temporal pattern of bubble intervals, tbubble, corresponding to the transition concentration range of TBuA+Br−, obtained at a constant flow gas rate set by a 0.5 mA current. Transition from periodic to chaotic gas oscillator is evidenced when the TBuA+ concentration is increased from 50 to 200 μM (a)−(h). Average f values are (a) 0.75 Hz, (b) 3.2 Hz, (c) 4.2 Hz, (d) 5.8 Hz, (e) 22 Hz, (f) 32 Hz, (g) 34 Hz, and (h) 36 Hz.

actions.41 Other physicochemical properties of TXA+ salts such as their electrical conductivity,42 their partition between aqueous and organic electrolytes,43 and their interaction with water molecules deduced from NMR relaxation, neutron scattering and dielectric spectroscopic studies44 also differentiate TMeA+ from the rest of the series, with TEtA+ and TPrA+ exhibiting semihydrophilic behavior and then hydrophobic behavior from TBuA+ onward. The influence of all six molecules on the inhibition of bubble coalescence also follows

this trend (Figure 5). We now discuss these three classes of cations in more detail.

4. DISCUSSION 4.1. Hydrophobic Cations: TBuA+, TPeA+, and THexA+. Accumulation of the hydrophobic cations at the solid/liquid and liquid/gas interfaces arises when they are expelled from aqueous solutions and it occurs when their bulk concentration is very low. For this reason, they tend to inhibit bubble coalescence in the submillimolar concentration range, between 5742

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Figure 4. Snapshots of sample vials taken right after shaking by hand for 20 s. Sample volume is 10 mL, electrolyte is 1.2 M H2SO4 solution + TXABr at Cst. Average foam volumes (Vfoam) and corresponding lifetimes (tfoam) are summarized in Table 1

Figure 6. Molecular structures of tetraalkylammonium cations in their optimized conformations.23 (a−h) Top view images of the cations in their pseudoplanar configuration. All the structures have a tetrahedral configuration around the central N atom, as shown in (d). (a) TMeA+; (b) TEtA+; (c) TPrA+; (d), (e); and (h) TBuA+; TPeA+ in (f) extended and (i) bent configurations; THexA+ in (g) extended and (j) bent configurations. Panels at the bottom correspond to TBuA+, TPeA+ and THexA+ cations in conformations that occupy a similar area, indicated with in dashed orange circle. The larger cations TPeA+ and THexA+ have more freedom to bend their alkyl chains, without major steric effects.47

transport dynamics toward the interface.45 If mass transport governs the excess of TXA+ ions on the bubble surface, the coverage is low. For example, assuming a 10% surface coverage of a bubble of radius 0.2 mm whose release frequency is 1 Hz, this corresponds to a surface excess of 3.5 × 10−11 mol cm−2 which could be obtained from 9.7 × 10−6 cm3 of a 20 μM TXABr solution. As a result, a diffusion zone surrounding the bubble is created with an estimated thickness of 18 μm. Assuming a diffusion coefficient of ∼10−6 cm2 s−1, a mass transport rate of 1.1 × 10−11 mol cm−2 s−1 is obtained, confirming that at low concentrations the surface excess of TBuA+, TPeA+, and THexA+ is limited by the mass transport of these ions as well as by their hydrophobicity.46 The concentration ranges where TBuA+, TPeA+, and THexA+ switch from periodic to chaotic gas oscillator are similar, with a smooth transition toward a stream of tiny bubbles as the bulk concentration is raised. Even though TXA+ cations are rather weakly surface-active species, reflected by their short tfoam values deduced from foamability tests (Table 1), micellization does occur in aqueous solutions of the hydrophobic members of this group, with TPeA+ and TBuA+ reported to exhibit similar critical micelle concentration (cmc) values in the millimolar range.47 The trend of Vfoam and alkylchain length obtained goes through a maximum at TPeA+ which arises from a balance between lateral cohesiveness and film elasticity.48 Therefore, TBuA+, TPeA+, and THexA+ behave like surfactant molecules, with coalescence retarded by large surface tension gradients and their ability to stabilize cavities.49 Identical Cst values obtained for the

Figure 5. Transition from electrochemical gas oscillator to bubblestream behavior as a function of concentrations for different TXA+ salts. The figures only display values obtained in the transition concentration range, for clarity. Below this range, f values are equivalent to those obtained with pure acid solution and above Cst (indicated by the vertical dashed lines) the E-t plots are not periodic. (a) Frequency and (b) surface tension vs concentration for the six TXA+ salts. Lines connecting data points are added as a guide to the eye. Horizontal dotted black line in (b) corresponds to the surface tension of the electrolyte without addition of TXA+ salts.

0.2 and 2.0 × 10−4 M, with all the cations exhibiting almost identical Cst values of about 2 × 10−4 M (Figure 5). The differences observed below Cst, result from the interplay between the relative hydrophobicity of the ions and their 5743

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inhibited with an abrupt transition from a slow gas oscillator to bubble-stream behavior. Considering the crossover to bubblestream obtained with TPrA+, and given this process is localized at the electrified interface, a surface excess of 5 × 10−10 mol cm−2 which corresponds to almost 100% coverage of the electrode would require 3.9 × 10−8 cm3 of a 10−3 M TPrA+ solution. The thickness of the diffusion zone will be ∼5 μm, in agreement with a linear rather than a spherical diffusion regime. From these parameters, a mass transport rate of 2 × 10−9 mol cm−2 s−1 is calculated, indicating that the transport of TPrA+ cations is not limiting the adsorption process, as it did in the case of the hydrophobic TXA+ cations, thereby explaining why the transition from full coalescence to gas stream with TPrA+ and TEA+ is never smooth but abrupt (Figure 5). 4.3. Hydrophilic Cation: TMeA+. The behavior of TMeA+ is anomalous in our experimental system, as it is the only hydrophilic cation. Above 0.1 M, the frequency of the TMeA+ gas oscillator starts to increase progressively with concentration, unlike TPrA+ and TEtA+ (Figure 5a). The electrochemical response is also peculiar, exhibiting values that are more negative for smaller bubbles, not the other way around (see Figure 8 in comparison to Figure 1). Here, the TMeA+ concentration is of the same order of magnitude as the proton concentration, with protons being consumed at the cathode surface. Therefore, conditions in the almost organic-like chemical environment near the electrode are different from those in the bulk. Considering Ohm’s law, the larger overpotential needed to attain the same current value indicates that the electrochemical system has become more resistive, probably because of changes in viscosity, which is enhanced at the interface. Our relative viscosity measurements obtained with the bulk electrolytes show an increase of up to 10% for 1 M TMeA+ concentration, and it may be locally larger, given the accumulation of these ions at the cathode interface. However, a direct relationship between increased relative viscosity and inhibited coalescence is debateable.1,2,7,50 Instead, more traditional electrochemical concepts can be used to explain the influence of TMeA+ on the inhibition of bubble coalescence. The steady f shift resembles that obtained with hydrophobic TXA+ cations, which was given by the cations’ low bulk concentration and corresponding progressive surface coverage, gradually affecting bubble coalescence. Therefore, a low TMeA+ surface excess should be operative here too. Considering that TMeA+ is in huge excess in the electrolyte and would still remain in solution without interacting directly with the electrode surface,38,41 only at very high concentrations could TMeA+ start to be specifically adsorbed, as evidenced by potentiodynamic scans shown in Figure 7. Hence, if this step is kinetically slow, it should give rise to low surface coverage and therefore to a gradual increase in f for the electrochemically induced bubble inhibition process. Considering that even with 3 M TMeA+, the surface tension of a 1.2 M H2SO4 aqueous solution is only marginally decreased, one can infer that the surface activity of TMeA+ is negligible.

electrogenerated bubble stream arise from the fact that longer alkyl chains are able to bend, with TBuA+, TPeA+ and THexA+ effectively occupying a similar area at gas/liquid interfaces when the surface excess of these cations is rather large.47 Pictures of these structures are shown in the bottom panels of Figure 6. Foam formation is also found with solutions just below Cst, although Vfoam and tfoam are lower than those cited in Table 1, suggesting a less effective inhibition of bubble coalescence is at play and coinciding with the transition to chaos evidenced in the electrochemical gas oscillator in this same concentration range. 4.2. Semihydrophilic Cations: TPrA+ and TEtA+. The transition concentrations of TPrA+ and TEtA+ for the electrochemical gas oscillator are 1 and 3 orders of magnitude higher than those of hydrophobic TXA+ cations, with only TPrA+ exhibiting significant foam height in the flask-shaking experiments (see Figure 4 and 5). These ions do not form micelles in aqueous solutions,47 yet their inhibition of bubble coalescence is electrochemically driven nevertheless, facilitated by the specific interactions they establish with the electrode surface. Figure 7 shows electrochemical scans for the hydrogen

Figure 7. Cathodic potentiodynamic scans recorded at 50 mV/s corresponding to the hydrogen reduction reaction in 1.2 M H2SO4 M electrolytes. Pt microelectrode diameter 100 μm. Tetraalkylammonium bromide salts are added each at their Cst concentration. Shifts on the onset are proportional to TXA+ bulk concentration. Dashed lines are scans obtained when the hydrophobic TXA+ ions are present in the solution, where the surface roughness of the microelectrode has a strong influence the electrochemical signal at low overpotentials.

evolution reaction recorded with H2SO4 electrolytes containing TPrA+, TEtA+, and TMeA+ at their corresponding Cst values. The surface excess of TXA+ is evidenced by the shift of the potentiodynamic curve toward more negative overpotentials and by lower current gradients which are proportional to the TXA+ concentration. The physisorption of TXA+ slows down the electron transfer reaction and this is proportional to the concentration rather than the hydrophilic or semihydrophobic nature of the TXA+ cations. Similar results have been observed in experiments where nucleation of the hydrogen bubbles takes place at the surface of a nanoelectrode, with surfactants also promoting the nucleation at a lower level of supersaturation.17 Once the adsorption of TPrA+ and TEtA+ at the bubble surface has been electrochemically induced, coalescence is

5. CONCLUSIONS By exploring the effect of the series of tetraalkylammonium salts, TXA+Br− on the electrolytic production of hydrogen on a Pt microelectrode, we have gained an understanding of the factors influencing gas coalescence. According to Craig’s2 classification, the entire TXA+Br− family are βα ion pairs and therefore were not expected to interfere with the bubble coalescence process. Nonetheless, we have found a partial or 5744

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Langmuir

complete inhibition of coalescence of electrogenerated hydrogen bubbles as a result of the interactions of the TXA+ cations with gas/liquid and solid/liquid interfaces, which is determined by their relative hydrophobicity and the action of the electric field. The behavior of the ion pairs H+/SO42−, Na+/SO42−, H+/ ClO4−, and Na+/ClO4− at the hydrogen-evolving microelectrode follows the same trend as that found for bubbles released from an orifice. Only Na+/SO42− and H+/ClO4− inhibit coalescence. For the TMeA+Br− ion pair, we observed the same behavior at a microelectrode as Craig observed at an orifice but when the concentration is increased beyond 0.1 M, the electrogeneration of bubbles become progressively chaotic. Other inorganic salts previously thought to not influence coalescence have been reported to switch behavior in a similar fashion, although at a concentration much higher than 1 M.7 We think that while it may be useful to classify ions as α and β, this is too black-and-white. A more nuanced description is that provided by Enami’s8 and Jungwirth’s9 work where they nicely show how ions position themselves at different depths of the interface, depending on the experimental conditions. For bulky members of the TXA+ family, the their interaction with interfaces is governed by their hydrophobicity as well as their bulk concentration: TBuA+, TPeA+, and THexA+ with long alkylchain lengths of 4, 5, and 6 carbons, respectively, behave as surfactant molecules evidenced by the foam created in the flask-shaking tests and the fact that very low concentrations suffice to inhibit coalescence. TPrA+, TEtA+, and TMeA+, with short alkylchain lengths of 3, 2, and 1 carbons are semihydrophobic or hydrophilic in that order. Although their foam formation is poorer than those obtained with the hydrophobic TXA+ species, they do effectively influence bubble coalescence at a gas evolving microelectrode. Bubbles generated electrochemically in the presence of these cations differ from those produced by bubbling gas through tubes or by shaking a flask, as they grow at an electrochemically charged triple phase boundary, by a growth process that involves both direct gas diffusion and coalescence with tiny bubbles in the vicinity.15,16,28 Inhibition of coalescence in the presence of TPrA+, TEtA+, and TMeA+ takes place in solutions where the ions are not yet expelled toward the gas/liquid interface; the surface tension of the solutions is the same as that of the pure electrolyte. However, considering the specific adsorption of TXA+ cations to the surface of the cathode, it is thought that the liquid film surrounding electrogenerated bubbles contains a larger surface excess of TXA+ cations than that present at the air/liquid interface of the corresponding stagnant electrolyte in equilibrium conditions. Common surfactants drive the nucleation of single nanobubbles at lower gas supersaturation levels, in agreement with the classical heterogeneous nucleation theory which predicts that a reduction in surface tension decreases the energy barrier for the nucleation of bubbles.17 Hence, a question that remains to be answered is whether or not TXA+ cations facilitates the initial nucleation of bubbles at lower gas saturation values. In any case it is sure; that the electrochemical growth of bubbles can be finely controlled by the nature and concentration of TXA+ Br−/SO42− salts in the electrolyte.

Figure 8. Electrochemical signals associated with the growth of single hydrogen bubbles at the surface of a microelectrode as a function of TMeA+ concentration. (a) Time evolution of the overpotential and corresponding (b) noise spectra recorded with solutions containing TMeABr, concentrations indicated in the figures legend. (c) f values and average potentials at the end of plateau, as a function of bulk TMeABr concentration. Electrolyte: 1.2 M H2SO4 + 0.1 to 1 M TMeABr. Pt microelectrode diameter is 100 μm. Current is set at 0.5 mA.

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DOI: 10.1021/acs.langmuir.5b01003 Langmuir 2015, 31, 5738−5747

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Science Foundation Ireland, as part of the MNM project, contract 13/ERC/13261. REFERENCES

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DOI: 10.1021/acs.langmuir.5b01003 Langmuir 2015, 31, 5738−5747