Experimental study of interfaces separating immiscible electrolyte solutions vibrating in a capillary

NOTES Experimental Study of Interfaces Separating Immiscible Electrolyte Solutions Vibrating in a Capillary INTRODUCTION Fundamental properties of a vibrating interface separating an aqueous electrolyte solution and an electrolyte solution in nitrobenzene have been previously presented in papers (1-3). The results of complementary studies of vibrating water-nitrobenzene and water-l,2-dichloroethane interfaces are presented in this paper. The latter has been studied recently: the equilibrium potentials (4) and the potentials of ion transfer have been determined (5).

EXPERIMENTAL M e a s u r e m e n t conditions. The measurements have been carried out using the technique and the vessel described in Ref. (2) replacing the platinum electrodes by smallsize calomel electrodes of resistance below 0.5 kft. The liquid-liquid interface located in a calibrated glass capillary was stimulated to vibrate by applying sinusoidal pressure of 10 Hz frequency; it was generated by the vibrator W coupled with a metal bellows and was transmitted pneumatically (Fig. 1). Stimulation by a single pressure pulse was also studied and the electrical response of the liquid-liquid system was being recorded with a memory oscilloscope of 10 Mft input resistance. The studies of liquid-liquid interface polarized by a constant voltage have been carried out using the system presented in Fig. 1. The interface was polarized by stimulating a direct current flow from the galvanostat G through the platinum electrodes introduced into the aqueous phase. Alternating voltage generated by the vibrating interface was measured with a nanovoltmeter of 100 M~2 input resistance and 54 dB selectivity as in the case of a nonpolarized interface. The potential of the polarized interface with respect to a nonvibrating organic tetrabutylammonium tetraphenylborate (TBATPhB) solution-aqueous tetrabutylammonium chloride (TBAC1) interface was measured with the constant voltage meter of 104 M~ input resistance. The materials and the studied systems were prepared in the way described in Refs. (2, 4, 5).

RESULTS AND DISCUSSION The studies of amplitude characteristics of the opencircuit voltage, U, and of the short-circuit current, I~,

generated by a sinusoidally vibrating water-l,2-dichloroethane (1,2-DCE) interface have demonstrated that the characters of their shapes and of their dependences on capillary diameter, organic phase slug length, and number of interfaces are similar to those observed for the waternitrobenzene interface (2). These experimental findings have confirmed that the electrical equivalent circuit elaborated for the vibrating mercury-electrolyte solution interface (6) is applicable for the water -1,2-DCE interface. The water -1,2-DCE interface exhibits higher mechanoelectric effectivity than the water-PhNO2 interface in the presence of the same electrolytes. It is illustrated by Fig. 2 and by comparison of the curves presented in Fig. 3. It indicates that the water-l,2-DCE system is more polarizable, possibly due to lower ion concentration in the 1,2-DCE phase caused by their self-association (5). The experiments have also been carried out concerning the mechanoelectrical transduction by the system containing TBATPhB in the organic phase and pure solvent, i.e., triply distilled water with no electrolyte before contacting the organic solution, as the aqueous phase. The measurements were done immediately after filling the vessel. Remarkable voltage values have been obtained; it indicates a high polarizability of the systems. The system containing 1,2-DCE yielded higher voltage values in this case, too (Fig. 3). Figure 4 presents the plot of alternating voltage generated by the water-1,2-DCE and water-PhNO2 interfaces as a function of polarization potential E measured with respect to a nonvibrating organic TBATPhB-aqueous TBAC1 interface, i.e., with respect to the organic solventwater interface reversible with respect to the TBA ÷ ion

FIG. 1. The scheme of the experimental set-up. W, Vibrator; M, bellows; G, galvanostat; VAC, selective nanovoltmeter; VDC, constant voltage meter; w, aqueous solution; o, organic solution.

267 Journal of Colloid and Interface Science, Vol. 105,No. 1, May 1985

0021-9797/85 $3.00 Copyright© 1985by AcademicPress,Inc. All rightsof reproductionin any formreserved.

268

NOTES u

U mV

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FIG. 2. Amplitude characteristics of the open-circuit voltage, U: (e) 0.01 M TBATPhB in 1,2-DCE, (©) 0.01 M TBATPhB in PhNO2; 0.01 M NaBr in both cases.

U

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mV 25-

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FIG. 3. Amplitude characteristics of the open-circuit voltage, U: (e) 0.01 M TBATPhB in 1,2-DCE/distilled water, (O) 0.01 M TBATPhB in PhNO:/distilled water.

Journalof ColloidandInterfaceScience,Vol. 105,No. 1, May 1985

-1oo

-50

o

,-5'o

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FIG. 4. Dependences of the open-circuit voltage, U, on the polarization potential, E, 0.01 M solutions. The liquid-liquid systems: (O) TBATPhB in 1,2-DCE/aqueous NaBr, (×) TBATPhB in PhNO2/aqueous NaBr.

(4) in a partition equilibrium. The TBA + ion concentrations in both phases were identical and amounted to 0.01 M. As the plots do not represent currents, the observed minima of the U = f(E) curves can hardly be assigned to the potential of zero charge of the studied liquid-liquid interfaces. This method which has been applied in the work (3) to the case of an interface separating immiscible electrolyte solutions is similar to that used in the case of a mercury-solution interface (7-9). We have determined the polarization of the mechanoelectric effect generated by the interface separating immiscible electrolyte solutions. The voltage response caused by a pressure jump was recorded to this aim. A typical course is illustrated by Fig. 5a and the response peak voltage values are given in Table I for several studied systems. Of course, the peak voltage values depend on the pressure variation rate; those collected in Table I were obtained at the same pressure jump conditions. Microscopic observations indicated no pistonlike displacement of the organic phase slug during the pressure jump; remarkable changes occurred instead in the surfaces of both interface menisci. It is diagramaticaUy illustrated by Figs. 5b and c. The results presented in Table I and in Fig. 5 permit determination of the polarity of the studied effect. At a positive Galvani potential value, °A = ~P° -- ~pw > 0, e.g., in the presence of the w

NOTES

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m~

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0.5s

269

It also results from the data collected in Table I that the voltage generated by the interface separating the TEAPi solutions in partition equilibrium is negligible. It supports the assumptions of the water-nitrobenzene interface potential being practically zero (4, 5, 12) and of the water-1,2-dichloroethane interface potential being small (15 mV) (4, 5) in the presence of TEAPi. ACKNOWLEDGMENTS

, -(:

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1~

u=o

,

b)

We are indebted to Professor Dr. R. Joos, Universitaire Instelling Antwerpen, for his suggestions to carry out some experiments described in the present paper. The authors are grateful to Mrs. A. Kaszubska, M. Sci., for her participation in the experimental work.

c)

REFERENCES

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FIG. 5. (a) The voltage response of the liquid-liquid interface to stimulation by a single pulse; system number 3 of Table I. (b) The diagrammatic explanation of the appearance of electric signal due to deformation of a liquid-liquid interface (c) with respect to the unperturbed interface (b). TBA+ ion (4), the mechanical movement generates a higher potential at the side of the interface whose area increases during the movement. It is in agreement with the observations of mercury drop formation in an electrolyte solution (10) and of a vibrating mercury-electrolyte solution interface (11). TABLE I The Systems Stimulated by a Single Pressure Pulse, 0.01 M Solutions

No.

Organic phase

Aqueous phase

1 2 3 4 5 6

NaTPhB in PhNO2 TBATPhB in PhNO2 TBATPhB in 1,2-DCE TBATPhB in 1,2-DCE TEAPi in PhNO2 TEAPi in 1,2-DCE

NaC1 NaC1 NaBr LiC1 TEAPi TEAPi

Response voltage peak (mV)

-25 -30 -35 -40 +0.05 +0.2

1. Koczorowski, Z., and Kotowski, J., J. Colloid Interface Sci. 66, 584 (1978). 2. Kotowski, J., Kalifiska, A., and Koczorowski, Z., 3". Colloid Interface Sei. 86, 442 (1982). 3. Koczorowski, Z., Kalifiska, A., and Figaszewski, Z., J. ElectroanaL Chem. 139, 303 (1982). 4. Koczorowski, Z., and Geblewicz, G., J. Electroanal. Chem. 152, 55 (1983). 5. Geblewicz, G., and Koczorowski, Z., J. Eleetroanal. Chem. 158, 37 (1983). 6. Figaszewski, Z., and Koezorowski, Z., Roczniki Chem. 46, 481 (1972). 7. Koczorowski, Z., D~bkowski, J., and Minc, S., J. Electroanal. Chem. 13, 189 (1967). 8. Figaszewski, Z., and Koczorowski, Z., Collect. Czech. Chem. Commun. 48, 1 (1983). 9. Imai, H., Inouye, S., and Chaki, S., Bull, Chem. Soc. Japan 31, 767 (1958). 10. Levich, V., "Physicochemical Hydrodynamics," Prentice Hall, Englewood Cliffs, New York, 1962. 11. Koczorowski, Z., Kucharska, E., and Figaszewski, Z., J. Appl. Electrochem. 10, 191 (1980). 12. Koczorowski, Z., J. Electroanal. Chem. 127, 11 (1981). ZBIGNIEW KOCZOROWSKI JAN KOTOWSKI

Department of Chemistry University of Warsaw Pasteura 1 02-093 Warsaw, Poland Received March 15, 1984; accepted September 7, 1984

Journal o f Colloid and Interface Science, VoL 105, No. 1, May 1985