1698
KARLDOBLHOFER AND DAVID111. MOHILNER
Electrosorption of 5-Chloro-1-pentanolat the Mercury-Solution Interface' by Karl Doblhofer and David M. Mohilner* Department of Chemistry, Colorado State University, Fort Collins, Colorado
80621
Publication costs assisted by the A i r Force Ofice o f Scientific Research
The electrosorption of 5-chloro-1-pentanol on mercury in the absence of specific ionic adsorption has been studied by means of electrocapillaryand differential capacitance measurements. There is a very well defined potential and corresponding charge density of maximum adsorption. The Esin and Markov plot for the charge density of maximum adsorption is precisely linear with zero slope. However, the Esin and Markov plots for all other charge densities are curves and have an approximate S shape. The electrosorptionisotherm is congruent neither with respect to electrode potential nor with respect to charge density. h proof is given that the well known necessary conditions for congruence of electrosorption with respect to electrode potential or charge density are also sufficientconditions.
The study of the electrosorption of neutral organic molecules at the mercury-solution interface can provide helpful information for further development of a detailed theory of the inner part of the electrical double layer. In addition t o information on the behavior of the organic sorbates themselves, such studies may help illuminate the role of solvent molecules in the inner layer. especially when the experiments are conducted under conditions in which there is no specific ionic adsorption. The present study concerns the adsorption of 5-chloro-1-pentanol on mercury from saturated aqueous solutions of KaF. To our knowledge, this is the first investigation of the electrosorption of this class of substituted aliphatic alcohols to be reported. Both interfacial tension and differential capacitance measurements were made.
I. Experimental Section A . Solutions. Solutions were prepared using water redistilled from alkaline permanganate in a still equipped with a heated fractionating column packed with glass helices. The saturated N a F base electrolyte was prepared from lllallinckrodt AR sodium fluoride which had been heated at 500" for 1 hr in a platinum dish to destroy any traces of organic matter and then recrystallized. The saturated N a F stock solution (0.916 M ) was stored in a 1/2-gallon Teflon bottle in order to avoid contamination due to attack on the walls of a glass container by the concentrated fluoride solution. The Teflon bottle was first tested for possible leaching out of surface active materials by ascertaining that no detectable change in the differential capacitance of a salt solution could be observed after the solution had been stored in the bottle for a period of more than 2 weeks. The 5-chloro-1-pentanol was obtained from Pfaltx and Bauer, Research Chemicals, Flushing, N.Y. It was purified by fractional distillation a t 6 mm of Hg pressure in a microdistillation apparatus, and the fraction boiling at 79" was taken. Mass spectrometric and infrared tests confirmed the molecular The Journal of Phusical Chemistry! VoZ. 76, N o . 11, 1971
structure. Solutions were prepared by weighing 5chloro-1-pentanol and diluting to volume with saturated K a F stock solution. B. Electrodes. Mercury for electrodes was repurified by vacuum distillation. The mercury was used as a meniscus electrode in the capillary electrometer, as a dropping electrode in differential capacitance measurements, and as the auxiliary electrode in the dc potentiostat circuits for both kinds of measurements. A platinized platinum gauze electrode served as a fourth, ac auxiliary electrode in the differential capacitance measurements. I n place of an ordinary reference electrode an Orion fluoride reversible electrode was employed as indicator electmle in both the electrocapillary and differential capacitance measurements. 2 , C. Interfacial Tension Measurements. Interfacial tension was measured with a modified Lippmann capillary electrometer. The fine bore (7-,u radius) gently tapered capillary was viewed at 120 X magnification with a Bausch and Lomb stereozoom microscope. The pressure required to bring the meniscus electrode to the reference position in the capillary was adjusted by means of a closed gas pressure system employing two brass bellovls, one for coarse, the other for fine adjustment. The gas pressure was measured on a (1) Taken in part from the thesis of K. Doblhofer submitted to the Department of Chemistry and the Faculty of the Graduate School of Colorado State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, March 1970. ( 2 ) D. M. Mohilner, Electroanal. Chem., 1, 241 (1966). (3) Indicator electrode, in contrast to an ordinary constant potential reference electrode, denotes an electrode dipping into the same solution used for the electrocapillary measurements which is reversible either to a cation or an anion of that solution. This terminology was introduced and its rationale was given by D. M. Mohilner and N. Hackerman, Electrochem. Acta, 11, 1669 (1966). The fluoride reversible electrode was not strictly required for the measurements reported here. However, this electrode was used in order t o facilitate future comparisons of these measurements with electrosorption studies of 5-chloro-1-pentanol in NaF solutions of other concentrations. The potential of the Orion fluoride electrode in 0.916 M NaF 5s. a 0.1 M KC1 calomel electrode was -0.260 V.
1699
ELECTROSORPTION OF 5-CHLORO-1-PENTANOL
wide-bore mercury manometer which was read with a 1-m cathetometer calibrated to 0.01 mm (Precision Instrument and Tool Co. Ltd., Surrey, England). The same cathetometer was used to measure the head of mercury in the capillary reservoir assembly. I n order to permit the use of the fluoride reversible electrode which has a resistance of about 1 megohm, a solid-state operational amplifier potentiostat was used for potential control. The voltage follower in this potentiostat circuit had FET inputs. The meniscus electrode in the capillary was operated at circuit ground potential and the potential of the fluoride reversible indicator electrode with respect to ground was read at the output of the voltage follower on a Fluke Model 32t.0 871A differential voltmeter. 310.0 f The capillary electrometer was calibrated with 0.05 NazS04at 25" using as a standard the value of 426.2 290.0 3 0 0 . 0 .1 .2 .O -.2 -.P -.6 -.B -1.0 -1,Z -1.6 dyne/cm at the electrocapillary maximum determined EMF I N VOLTS by Smolders from sessile drop measurement^.^ This capillary electrometer had been shown p r e v i o u ~ l y ~ ~Figure ~ 1. Electrocapillary curves for the electrosorption of 5-chloro-1-pentanol on mercury from 0.916 M N a F at 25'. to be capable of very high precision measurements Reading from upper curve to lower, the concentration of of interfacial tension. When used with salt solutions 1.134 X 5-chloro-1-pentanol is: 0.0, 4.29 X such as NaCl or NazSOl the interfacial tension could 1.716 x 10-3, 2.325 x 10-3, 3.487 x lows,4.65 X be determined with a reproducibility of about A0.03 5.67 X 8.59 X 10-3, 1.290 X 1.718 X loe2, dyn/cm. However, in the case of saturated solutions 2.590 X 10-2, 3.436 X 10-2, 4.330 x 1OU2M. of NaF the precision of the measurements was worse from the bridge was applied between a platinized by more than one order of magnitude. Solutions platinum gauze electrode and the DME, which was were deaerated with purified nitrogen bubbling and positioned in the center of the gauze. The dc poan atmosphere of nitrogen was maintained over the tential of the D I I E with respect to the fluoride resolutions during measurements. versible indicator electrode in the same solution was Electrocapillary curves were measured at 25.0" for controlled by a solid-state operational amplifier pothe base electrolyte solution (0.916 M NAF) alone tentiostat. A mercury pool served as auxiliary elecand for each of 13 different concentrations of 5-chlorotrode in this potentiostat circuit. In order to prevent 1-pentanol in this electrolyte. The concentrations of the potentiostat from canceling the ac voltage applied the alcohol ranged from 4.29 X to 4.33 X M. by the bridge, a low pass filter was placed between The experimental data are plotted in Figure 1. The the output of the potential controlling amplifier and data were taken at 50-mV intervals over a voltage the mercury pool. This filter was effectively isolated range from $0.30 to - 1.40 V vs. the fluoride reversible from the bridge circuit by a 100-kilohm resistor. The electrode in each solution. The entire set of data DIVE was connected to the common ground of the was smoothed and then differentiated with respect bridge and potentiostat circuits. The potential of to both electrode potential and logarithm of alcohol concentration by digital computer using a modification7 the indicator electrode with respect to ground was read at the output of the voltage follower with a Fluke of the previously published moving fit technique.8 D. Diflerential Capacitance Measu~ements. Differ- Model 871A differential voltmeter. The method used for determining the bridge balance was a modification ential capacitance at a dropping mercury electrode of that originated by Grahame.lo The output of the (DME) was measured with a General Radio Type 1615-A transformer ratio-arm bridge using a Type (4) C. A. Smolders and E. M. Duyvis, Red. Trav. Chim. Pays-Bas, 1311-A audio oscillator, and a Type 1232-A tuned 80, 635 (1961). amplifier and null detector. The dropping electrode (5) K. Doblhofer, J . Electrochem. SOC.,116, 77C (1969). was drawn from 0.5-mm bore borosilicate glass capillary (6) For detailed design features c j . K. Doblhofer, Ph.D. Thesis, March 1970, Colorado State University. Available from University tubing. The inner wall of the capillary was dewetted Microfilms, Inc., Ann Arbor, Mich. with the vapor of dichlorodimethylsilane~gand then (7) P. R. Mohilner and D. M .Mohilner in "Applications of Comthe capillary tip was recut. puters to Analytical Chemistry," H. B. Mark, Ed., Marcel Dekker, New York, Tu'. Y . , in press. Solutions were deaerated in the same manner as 115, (8) D. M. Mohilner and P. R. Mohilner, J . Electrochem. SOC., in the case of the electrocapillary measurements. A 261 (1968). four-electrode system was used for the capacitance (9) R. Payne, J. Electroanal. Chem., 7, 134 (1964). measurements.6 The ac voltage (5 mV peakto-peak) (10) D. C. Grahame, J.Amer. Chem. SOC.,71, 2975 (1949).
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Figure 2. Photographs of oscilloscopu screm illiisirating method of determining time of bridge balance. (a) Complete picture of null detector output. Horizontal sweep rate = 1 seclmajor division. Balance occurs slightly after 7 sec. (b) Same BS (a) but using delayed sweep. Sweep rate = 0.05 seelmajor division. ( e ) Same as (b) except upper half of screen has been erased and time marks from Tektronix Type 184 time-mark generator recorded. Small time marks occur every 0.01 sec. Largest time mark showing exponential decay which occurs just before bridge balance is the 7.0 see mark (cj. B above).
bridge null detector which was tuned to the frequency of the applied ac voltage was displayed on the screen of a Tektronix Type 564 storage oscilloscope via one channel of a Type 3A1 dual trace preamplifier, and the oscilloscope sweep was controlled by a Type 3B3 dual time base. The delaying sweep of the time base was triggered by a pulse derived by a solid-state operational amplifier differentiator circuit from the very sharp change in output of the null detector which occurred each time a drop fell. The bridge was adjusted to give a minimum output on the null detector a t some time late in the drop life (Figure 2a). I n practice, null detector output was recorded only in the vicinity of the minimum by storing it in the delayed sweep mode using a delayed sweep rate of 50 msec/cm The J o u d
o/ P h y d Chmislry,
Vd. 76,No. 11,1971
DOBLAOFER AND DAVID fit. MOAILNER
(Figure 2b). Then half of the storage screen was erased and a switch was thrown which connected the trigger input of the time base to the trigger output of tl Tektronix Type 184 crystal-controlled time-mark generator. The trigger output of the time-mark generator was set to 1.000 see. Then the first trigger pulse delivered after the switch was thrown triggered the oscilloscope, and time marks were displayed and stored on the screen via the other channel of the Type 3A1 vertical preamplifier. The time corresponding to any time mark could be recognized by amplitude and decay time (cf. Figure 2c). By this method it was very convenient to determine the time of bridge balance to the nearest 0.01 sec. Moreover, there is the advantage of eliminating any uncertainty in the time of bridge balance due to drift in the time base amplifiers of the oscilloscope. I n effect, this method of bridge balance is equivalent to a recalibration of the oscilloscope time base every measurement. The maximum uncertainty in the measurements reported here mas less than 2 parts per thousand. Differential capacitance was measured for the base electrolyte and for 22 different concentrations of 5chlorn-1-pentanol in 0.916 AT NaF. The alcohol cont o 4.33 X lo-* M . centration range was 1.395 X In the case of such measurements, in order to arrive a t valid thermodynamic capacitances, two different kinds of possible nonequilibrium, dc and ac, must be considered. By dc nouequilibrium it is implied that during the growth of the drop, a t fixed de potential, the surface excess of the organic sorbate has not achieved its equilibrium value due to slow diffusion of the sorbate molecules to the electrode surface. A very sensitive indicator of whether or not de equilibrium has been achieved is the measured value of the differential capacitance per unit area at fixed ac frequency but a t different times in the drop life. If dc equilibrium has not been achieved one will observe, in the potential domain of adsorption, a decrease in the measured capacitance C per unit area with increasing time. In the case of the solutions studied here such a decrease was indeed observed early in drop life. However, it was found that even with the most dilute solutions such a change in capacitance was quite negligible (less than 2 parts per thousand) provided the time of bridge balance was delayed by 6 sec, and therefore most of the measurements vere taken with the bridge balanced a t about 7-8 sec in the drop life. The natural drop time of the capillary was approximately 15 sec. The second source of nonequilibrium, which results from the inability of the sorbate molecules to follow the small ac voltage imposed by the bridge, is unavoidable. It was found that in this study a plot of measured capacitance us. square root of frequency always gave a very good straight line with negative slope indicating that this frequency dispersion was diffusion
ELECTROSORPTION O F 5-CHLORO-1-PENTANOL
O 'r
1701 where y is the interfacial tension, qM is the excess charge density on the metal surface, E is the potential of the ideal polarized mercury electrode with respect to the fluoride reversible indicator electrode in the same solution, is the relative surface excess of the 5-chloro-1-pentanol, and pa is the chemical potential of the alcohol in the given solution. Activity coefficients are not available for 5-chloro-1-pentanol in saturated NaF solutions, and therefore it was necessary to use concentrations in place of activities in the data analysis as has been done in nearly all previous studies of electrosorption of organic compound^.'^^'^ This approximation appears justifiable in the case of the present study because most of the important conclusions about the nature of the adsorption isotherm are based on data obtained at quite low concentrations (
