Article pubs.acs.org/Langmuir
Ionization of a Nonpolar Liquid with an Alcohol Antonio J. F. Bombard*,† and Andrei Dukhin‡ †
Universidade Federal de Itajubá/IFQ, Av. BPS 1303, Itajubá − MG, 37500-903, Brazil Dispersion Technology Inc., 364 Adams Street, Bedford Hills, New York 10507, United States
‡
ABSTRACT: Nonpolar liquids whose dielectric permittivities are close to 2 have very low conductivities, usually below 10 × 10−10 S/m. Their ionization is suppressed by the lack of solvation resulting from the negligible dipole moment of such liquids’ molecules. Ionization could be enhanced by the addition of other substances that could serve as solvating agents, creating inverse micelles around ions and preventing them from reassociating into ion pairs and neutral molecules. Surfactants are normally used for this purpose, but we show here that alcohols could perform a similar function. However, the mechanism of ionization by alcohols turns out to be quite different compared to the mechanism of ionization by surfactant. For instance, the conductivity of poly-α-olefin oil (PAO) depends on the concentration of added octanol (alcohol) as an exponential function above 10% of the octanol content. At concentrations below approximately 10%, octanol does not affect the conductivity at all. This phenomenon has never been observed for surfactant solutions. Apparently, octanol is completely dissolved at concentrations below 10% and forms micelles only above this concentration, which is the cmc for octanol−PAO mixtures. Below the cmc, octanol molecules do not dissociate, despite being able to dissociate in pure octanol, which has a conductivity of about 10 × 10−7 S/m. This again stresses the importance of the solvating factor in the ionization of liquids. Above 10% concentration, octanol molecules form micelles, which become charged by the disproportionation mechanism when they collide. To explain the exponential dependence of conductivity on octanol content, we assume that charged micelles grow in volume with increasing octanol content faster than neutral ones. Ion−dipole interactions are responsible for the preferential adsorption of octanol molecules onto charged micelles. Additional ionization occurs in such larger micelles, which then break down into smaller ones carrying individual electric charges.
1. INTRODUCTION
can be seen in a recent review on the nonaqueous electrochemistry of nonpolar liquids.9 There are other types of amphiphilic substances that in principle could perform similarly to surfactants in nonpolar liquids. The most obvious ones are alcohols: organic compounds in which the hydroxyl functional group (−OH) is bound to a carbon atom. An important class of alcohols includes simple acyclic alcohols, the general formula of which is CnH2n+1OH. Of these alcohols, ethanol (C2H5OH) is the most well known example. The hydroxyl groups (−OH) found in alcohols are polar and therefore hydrophilic, but their carbon chain portion is nonpolar, which makes them hydrophobic. Is it possible that, being amphiphilic in nature just like surfactants, alcohols could enhance the ionization of nonpolar liquids by strengthening ion solvation? If so, then we could learn more about nonpolar liquid ionization by studying their mixtures with alcohols and perhaps even find answers to some still-open questions. Alcohols are also somewhat conducting, containing concentrations of ions that are lower than in water but much higher than in nonpolar liquids. According to classical electrochemistry by
The ionization of nonpolar liquids makes them capable, at least to some degree, of conducting electric current, which is important in many industrial applications such as fuels, battery electrolytes, and electronic paper. It is known that such effects could be achieved with surfactants, both ionic1−7 and nonionic.8−11 These substances create a protective steric layer around the ions, similar to the ions’ solvating layer in polar liquids. Electric charges become encapsulated within these socalled inverse micelles. This steric stabilization prevents ion reassociation and ion-pair formation, which would otherwise dominate the electrochemistry of nonpolar liquids12,13 according to Bjerrum−Onsager−Fuoss theory.14−16 Surfactants are amphiphilic substances having a hydrophilic part (polar head) and a lipophilic part (hydrophobic tail). They dissolve in nonpolar liquids thanks to their lipophilic tail. However, it is the interaction of their polar heads that leads to the formation of the pools with higher polarity inside inverse micelles. For these pools to form, the concentration of surfactant must exceed a certain critical value known as the critical micelle concentration (cmc). Dissociation can occur inside these pools, with further charge separation between two colliding micelles. There are still many open questions regarding this disproportionation model1 of nonpolar liquid ionization with surfactants, as © 2014 American Chemical Society
Received: January 16, 2014 Revised: April 2, 2014 Published: April 2, 2014 4517
dx.doi.org/10.1021/la500221n | Langmuir 2014, 30, 4517−4521
Langmuir
Article
Bockris and Reddy,17 alcohols gain ions because of a so-called “auto-dissociation reaction”, which is one of the proton-transfer reaction types.17 For instance, for ethanol, this reaction is given by the following equation: C2H5OH + C2H5OH ↔ C2H5OH 2+ + C2H5O−
(C20H42) and small amounts of tricosane (C30H62) and its possible isomers. The same PAO liquid was used in both laboratories. The 1-octanol for the Brazilian laboratory (C8H17OH) was purchased from Vetec (a Brazilian branch of Sigma-Aldrich). Experiments in the New York laboratory were conducted with 1octanol anhydrous >99% purchased from Sigma-Aldrich in the US. There was no special treatment applied to adjust the humidity for either PAO or 1-octanol. Mixtures of octanol in PAO were prepared in two ways. Mixtures with octanol content from 0.1 to 50% were prepared by adding octanol to PAO. Mixtures with octanol content from 50 to 100% were prepared by adding PAO to octanol. All preparations were made by weight.
(1)
It would be interesting to learn what happens to the alcohol ions when alcohol is mixed with nonpolar liquid. Do they survive in the nonpolar liquid, or are they neutralized? These are the main goals of this article. We selected poly-α-olefin oil (CH3(CH2)nCH3) as an example of a nonpolar liquid. This is a specific type of olefin that has a poly-α-olefin (sometimes abbreviated as PAO), a polymer made by polymerizing an α-olefin. An α-olefin is an alkene in which the carbon−carbon double bond starts at the α-carbon atom (i.e., the double bond is between carbons 1 and 2 in the molecule). PAO is widely used in a variety of industrial applications. Although the name poly-α-olefin is a commercial name and commonly employed, chemically this liquid is an alkane. After alkene polymerization, there is another reaction: hydrogenating the unreacted double bonds of oligomers or polymers. This has a dielectric permittivity of 2.1 at 25 °C, and the viscosity of this substance can vary. In particular, the PAO liquid that we used has a viscosity of 6.2 mPa·s at 25 °C. PAO is known as practically nonconducting with respect to electric current.18,19 There are several requirements that determined our selection of the alcohol. First, alcohol molecules become increasingly nonpolar and therefore less miscible with polar water as their carbon chains become longer. The elimination of water from the test is extremely important because of its possible effect on conductivity. Having both an alcohol and a nonpolar liquid that are immiscible with water ensures that at least this factor could be eliminated from the data interpretation. The second condition restricts the range of viscosity. It is known that viscosity affects conductivity. If both liquids (alcohol and nonpolar) were to have the same or at least similar viscosity, then this factor would be eliminated from the data treatment as well. We selected 1-octanol (C8H17OH) as an alcohol that meets both conditions. It has a dielectric permittivity (ε) of 10.3 at 20 °C and a viscosity of 6.4 mPa s at 30 °C. In addition, octanol is known for being very close in chemical properties to the lipids of cell membranes. Because of this, it is commonly used for modeling lipids in biosciences. This introduces additional motivation for studying this substance. We prepared octanol−PAO mixtures with different octanol content varying from 0% (pure PAO) to 100% (pure octanol). We then measured the conductivity of these mixtures. Experiments were conducted independently in two laboratories, one in Brazil and the other in New York. We used two different instruments to conduct these experiments. In this way we ensured the reproducibility of the collected data. The conductivity dependence on the octanol content turns out to be very unusual and very informative. We were able to derive a theoretical model describing this dependence and the mechanism of PAO ionization by octanol. It reveals some interesting general insights into the process of nonpolar liquid ionization.
3. INSTRUMENTS To measure the conductivity in nonpolar liquids, we used the DT-1202 by Dispersion Technology Inc. 20 in Brazil and DT-700 by the same company in the New York laboratory. Both instruments have a nonaqueous conductivity probe of the same design. This probe is capable of measuring a wide range of conductivities from 10 × 10−11 to 10 × 10−4 S/m, where the precision of a single measurement is ±(1% + 10 × 10−11 S/m) over the complete range. The precision can be statistically improved over multiple measurements. The probe consists of inner and outer coaxial cylindrical electrodes as well as a guard electrode to eliminate leakage paths between the two. The outer electrode can be unscrewed from the body of the probe to facilitate cleaning of messy samples. Measurement principles are the same for both instruments. During a measurement, the instrument applies a sinusoidal voltage to the outer electrode and measures the current that flows through the sample to the inner electrode. The frequency of this applied voltage is changed depending on the measured conductivity. The lower-conductivity samples are measured at 1 Hz, and the higher-conductivity samples are measured at 10 Hz. The required sine wave voltage values are computed versus time and converted into an actual voltage by a digital-to-analog converter. This voltage, after suitable filtering, is then connected to the outer electrode, simultaneously measured with an analogto-digital converter, and optionally displayed in the application window on the display. The current is measured with a log amplifier, so no range selection is required by the user as was common in older designs. At the same time that the voltage waveform is being applied to the outer electrode, the output signal from the log amp is captured by a second channel of the analog-to-digital converter. The desired current waveform is calculated from the captured log amp values by performing an inverse log calculation. This log/ inverse-log technique allows measurements to be made over many decades of current without the need for any range selection switches. The amplitude and phase of both the voltage and current waveforms are then computed from the first-order Fourier coefficients of the respective waveforms. The complex conductance of the cell contents is then computed from the current/voltage ratio. Finally, the specific conductivity of the sample material is determined by computing the real part of the complex conductance and then multiplying by a cell constant. This cell constant is determined with a calibration procedure that is based on the known dielectric permittivities of air and
2. MATERIALS Poly-α-olefin oil (CH3(CH2)nCH3) was purchased from Chevron Philips. It is a blend of oligomers of 1-decene with isomers (head−head, head−tail, or tail−tail) of mainly eicosane 4518
dx.doi.org/10.1021/la500221n | Langmuir 2014, 30, 4517−4521
Langmuir
Article
The observed conductivity dependence for octanol in PAO differs dramatically from the typical conductivity−concentration dependence observed for surfactants in nonpolar liquids. Ionic surfactants demonstrate a linear dependence, as shown in Figure 3 for AOT in heptane. This figure is reproduced from ref 9.
toluene. This calibration procedure is the same for both instruments. There are some differences in electronics between both instruments and software. These differences should not affect the results of the measurements.
4. EXPERIMENTAL RESULTS The conductivity dependence of the octanol content in PAO is shown in Figure 1, which presents data collected in both the Brazil and New York
Figure 3. Conductivity of heptane with the addition of ionic surfactant AOT. Nonionic surfactants exhibit linear dependence at low concentration, and then the rate of conductivity growth decays with increasing surfactant concentration.8 This dependence is presented in Figure 4 for
Figure 1. Conductivity of the octanol−PAO mixture as a function of the octanol content measured in two different laboratories, plotted on log− log axes.
laboratories. It is seen that the curves are quite similar and have the same trends. The difference could be assigned to the difference in octanol. Apparently, the octanol used in New York was somewhat more conducting than that used in Brazil. The conductivity is plotted on log−log axes. This presentation emphasizes the main features of the peculiar conductivity dependence trend in the low-octanol-content range and in the high range as well. In addition, we plotted just a single conductivity curve, the one that was measured in Brazil, on log−linear axes (Figure 2). This presentation is important in understanding the conductivity dependence in the high octanol range, as will be discussed below.
Figure 4. Conductivity of toluene with the addition of nonionic surfactant Span 80. SPAN 80 in toluene. This figure is reproduced from ref 9. The parabolic dependence at high surfactant concentration reflects the appearance of ion pairs above the critical concentration of ion-pair formation.9 The conductivity dependence that we observe in this study has no resemblance to the behavior observed previously in nonpolar liquids using surfactants. First, the conductivity is practically unchanged up to about 10% octanol content. Then it becomes an exponential function of octanol content up to a concentration of almost 100%, as shown in Figure 2. This unusual behavior requires explanation. We suggest one hypothesis in the next section to explain it.
5. DISCUSSION AND THEORETICAL MODEL The first peculiar point of the conductivity versus octanol concentration curve is that at concentrations below 10 wt % there
Figure 2. Conductivity of the octanol−PAO mixture as a function of the octanol content measured in the Brazil laboratory, plotted on log−linear axes. 4519
dx.doi.org/10.1021/la500221n | Langmuir 2014, 30, 4517−4521
Langmuir
Article
additives. Here we present one hypothesis that might be the first step in explaining this empirical dependence. Let us assume that we have an octanol−PAO mixture with N charged micelles. Then we add a small amount (dW) of octanol. This would change the number of charged micelles by dN. The conductivity is proportional to the number of charged micelles. For conductivity to be an exponential function of the octanol weight fraction (w), there must be a proportionality between dN and N. This leads to the following equation:
is practically no effect. The conductivity remains constant and identical to the conductivity measured for pure PAO. Actually, it is on the low limit of the conductivity probe specifications. This means that octanol contributes no ions to PAO if the octanol concentration is below 10%. This is very surprising because pure octanol is quite conductive compared to PAO: at least 5 orders of magnitude higher as shown in Figure 1. Intuitively, one would expect some proportionality between the conductivity and octanol content in the mixture. This means that one would expect 1/10th of the pure octanol conductivity for an octanol−PAO mixture with 10% octanol. This expected number would be around 10−7 S/m because the conductivity of pure octanol is about 10 −6 S/m (Figure 1). Our experimental results dramatically contradict this intuitive expectation. The measured conductivity for the 10% octanol− PAO mixture is astoundingly 4 orders of magnitude less. We think that this experiment indicates that octanol completely dissolves at concentrations below 10%, dissolving down to single molecules. Some of these molecules are charged in pure octanol before it is added to PAO according to the autodissociation reaction shown in eq 1. Apparently, they somehow lose their charges when dissolved in PAO. There are two ways for charged octanol molecules to become uncharged. They either exchange back a proton when a cation collides with anion or they can build a neutral ion pair. In any case, this experiment demonstrates the importance of the solvation function. PAO cannot solvate charged octanol molecules, and as a result, they become neutral. Carriers of electric charge disappear, and the conductivity of the PAO− octanol mixture remains unchanged. This situation apparently changes at about 10% octanol content. It looks like this is the point at which micelle formation begins: the cmc.21 The interior space of the micelle can be considered to be a small pool of octanol with higher polarity. There is the opportunity for proton exchange between the octanol polar heads inside of such pools, similar to the situation for pure octanol. As a result, octanol molecules can gain charge inside the micelles whereas the micelle itself remains neutral. However, the collision of two such micelles might lead to an exchange of charged octanol molecules. It is possible that the positively charged molecule would remain within one micelle whereas the negatively charged one would jump to the other micelle. As a result, there would be two charged micelles that can serve as carriers of electric current. This mechanism is identical to the disproportionation model that has been widely used to describe the ionization of a nonpolar liquid with surfactants. However, there is a large discrepancy between the disproportionation model and experiment. The model predicts a linear dependence of the conductivity on the concentration of the solvating additive.1 Experiment, however, demonstrates much more rapid conductivity growth: exponential, as follows from Figure 2. This figure shows that the logarithm of conductivity, log(K), is a linear function of the octanol content. This can be present in the following empirical equation K = K 0e Aw
dN = NA dW
(3)
Integrating this equation would lead to the exponential dependence in eq 2. We can therefore conclude from experiment that the number of new charges appearing in the mixture after the addition of octanol is proportional (for some reason) to the number of already-existing charged micelles in the solution. This is the fundamental conclusion that links conductivity measurements with the mechanism of charging. We can suggest one scheme that explains such a relationship; it is illustrated in Figure 5. First, it stresses the point that added
Figure 5. Cartoon illustrating the preferential adsorption of added octanol molecules to existing charged octanol micelles, which then leads to additional proton exchange in such swallowed micelle. This micelle then breaks up into individual ions.
octanol molecules would most likely be attached to the existing charged micelles instead of neutral ones. Interaction between the octanol polar group dipole moment and the micelle electric charge is responsible for such selective attachment. There would be an equilibrium distribution between the concentration of octanol molecules in the charged and neutral micelles, with the energy of the ion−dipole interaction determining the value of the Bolzmann exponential function. In the case when this energy is much larger than kT (where k is the Bolzmann constant and T is the absolute temperature), most of the added octanol molecules would be attached to charged micelles. Therefore, charged micelles would grow in size after the new portion of octanol has been added. This increase would open up the possibility for more proton exchanges inside of such swallowed micelles. We show in the diagram in Figure 5 that this would lead to the appearance of two more charges inside: one positive and one negative. Then, such a large micelle would break into three parts. Internal stresses between charges of the same sign could promote
(2)
where w is the octanol weight fraction content, K0 is the conductivity of pure PAO, and A is some unknown constant. As far as we know, neither of the existing electrochemical theories predicts and explains such a strong conductivity dependence on the concentration of electrolyte or other 4520
dx.doi.org/10.1021/la500221n | Langmuir 2014, 30, 4517−4521
Langmuir
■
such a breakup. As a result, three smaller charged micelles would appear. Basically, we point out the preferential adsorption of the new added octanol molecules in the existing solution of charged octanol micelles, driven by ion (charged micelle)−dipole (octanol molecule polar head) interactions. This leads to a proportionality between the number of new ions and existing ions, which then results in an exponential dependence of conductivity (number of ions) on the octanol weight fraction. At this stage, this model is just a hypothesis. It is possible that there are other models that can also explain the exponential dependence of the conductivity on the concentration of the additive. We do not discuss here the higher range of octanol concentration. At some sufficiently high concentration, octanol becomes the continuous medium and PAO becomes the dissolved one. It is possible that the described conductivity measurement was not sensitive to this phase transition because octanol and PAO had the same viscosities. Additional experiments are required to resolve this phase-transition point.
REFERENCES
(1) Morrison, I. D. Electrical charges in non-aqueous media. Colloids Surf., A 1993, 71, 1−37. (2) Strubbe, F.; Verschueren, A. R. M.; Schlangen, L. J. M.; Beunis, F.; Neyts, K. Generation current of charged micelles in nonaqueous liquids: measurements and simulations. J. Colloid Interface Sci. 2006, 300, 396− 403. (3) Parent, M. E.; Yang, J.; Jeon, Y.; Toney, M. F.; Zhou, Z. L.; Henze, D. Influence of surfactant structure on inverse micelle size and charge for non-polar electrophoretic inks. Langmuir 2011, 27, 11845−11851. (4) Poovarodom, S.; Berg, J. C. Effect of particle and surfactant acidbase properties on charging of colloids in apolar media. J. Colloid Interface Sci. 2010, 346, 370−377. (5) Hsu, M. F.; Dufresne, E. R.; Weitz, D. A. Charge stabilization in nonpolar solvents. Langmuir 2005, 21, 4881−4887. (6) Kim, J.; Anderson, J. L.; Garoff, S.; Schlangen, L. J. M. Ionic conduction and electrode polarization in a dopped nonpolar liquid. Langmuir 2005, 21, 8620−8629. (7) Prieve, D. C.; Haggard, J. D.; Fu, R.; Sides, P. J.; Bethea, R. Two independent measurement of Debye legth in dopped non-polar liquids. Langmuir 2008, 24, 1120−1132. (8) Dukhin, A. S.; Goetz, P. J. How non-ionic “electrically neutral” surfactants enhance electrical conductivity and ion stability in non-polar liquids. J. Electroanal. Chem. 2006, 588, 44−50. (9) Dukhin, A. S.; Parlia, S. Ions, ion-pairs and inverse micelles in nonpolar liquids. Curr. Opin. Colloid Interface Sci. 2013, 18, 93−116. (10) Espinosa, C. E.; Guo, Q.; Singh, V.; Behrens, S. H. Particle charging and charge screening in nonpolar dispersions with non-ionic surfactants. Langmuir 2010, 26, 16941−16948. (11) Guo, Q.; Singh, V.; Behrens, S. H. Electric charging in non-polar dispersions due to non-ionazable surfactants. Langmuir 2010, 26, 3203− 3207. (12) Izutsu, K. Electrochemistry in Nonaqueous Solutions; Wiley-VCH: Weinheim, Germany, 2009. (13) Aurbach, D. Nonaqueous Electrochemistry; Marcel Dekker: New York, 1999. (14) Onsager, L. Report on revision of the conductivity theory. Trans. Faraday Soc 1927, 23, 341−349. (15) Fuoss, R. M.; Kraus, C. A. Properties of electrolytic solutions. IV. The conductance minimum and the formation of triple ions due to the action of columbic forces. J. Am. Chem. Soc. 1933, 55 (), 2837−2399. (16) Bjerrum, N. A New Form for the Electrolyte Dissociation Theory. Proceedings of the 7th International Congress of Applied Chemistry; London, 1929; Section X, pp 55−60. (17) Bockris, J.; Reddy, A. K. Modern Electrochemistry; Plenum Press: New York, 1977; Vol. 1. (18) Tuma, P.; Hesselroth, D.; Brodbeck, T. Next-generation dielectric heat transfer fluids for cooling military electronics. Military Embedded Systems 2009, July 9. (19) Tuma, P. E. Hydrofluoroethers as low-temperature heat-transfer liquids in the pharmaceutical industry. Pharm. Technol. 2000, March 1, 104−116. (20) Dukhin, A. S.; Goetz, P. J. Characterization of Liquids, Nano- and Microparticulates, and Porous Bodies Using Ultrasound, 2nd ed.; Elsevier: Amsterdam, 2010 (21) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: London, 1995−2000; Vols. 1−3.
6. CONCLUSIONS We have established that alcohol added to a nonpolar liquid does not transfer ions existing in the alcohol directly into the nonpolar liquid. Instead, these ions become neutralized by either a buildup of ion pairs or the exchanging of protons back from cation to anion. This neutralization functions below the cmc of alcohol micelle formation in the nonpolar liquid. Above the cmc, the conductivity becomes an exponential function of the alcohol content. There is no existing theory that would explain such a conductivity dependence. Alcohol micelles that form above the cmc can be considered to be pools with a higher liquid polarity, where dissociation and solvation could take place. Alcohol molecule polar heads could exchange protons there and become charged. Then, two colliding micelles could exchange such charged alcohol molecules such that one becomes cationic and the other becomes anionic. We suggest that the observed exponential conductivity dependence is related to the preferential adsorption of newly added alcohol molecules on those already existing in solution as charged alcohol micelles. This adsorption is driven by ion (charged micelle) −dipole (alcohol molecule polar head) interactions. This ensures the proportionality between a number of new ions and already existing ions after the addition of the new portion of the alcohol to the solution. This proportionality is responsible for the exponential dependence of both the ionic strength and conductivity of the alcohol−nonpolar liquid mixture on the alcohol content.
■
Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS AJFB gratefully acknowledges FAPEMIG (Minas Gerais state research agency) for the grants TEC-RDP-00164/2010, APQ00463/2011, and ETC-00044/2013. 4521
dx.doi.org/10.1021/la500221n | Langmuir 2014, 30, 4517−4521
