529—Chemical reactions and oscillating potential difference variations at an oil-water interface

: B@eL+~hemist~~

and Bi&nerge&s. 9 .( 1?82)‘583-59Q

583

.-‘.

A &ctiod pf J.~Ekkwta~ Cheti, and’coostituting-i.-141 (1982) ’ : Else&r Sequoi5:&AA.; ~u&uuie - Printed in Th& Netherlands -.

__-.;-._,.,_ ..

-._ _-, .’

. ‘. :

-_-;.

:

.\ ,...-

.~

-.

I :.

.-

:.

. ,-

bI&liii&k~.V~W~~~:~~ ~. : ~ . .. -.

&j

March Und

IN’@RFACbE ..

Lubo&&t? de Chimi& P&&& LA. Pti@s k’edex bS (Frye). .__..

&&nuscript re+ved

@a-iVAmR.

176 de W&ersitk

Pierre et Marie Curie. 4, place Jussieu -

7.5231

’

1982)

SUMMARY Oscillatory potential difterence Ad interfacial tension variation &I be observed at an oil-water interface’tintaining charged species when the conditions are such that hydrodynamic instabilities can occur. We pr&se a mechanism based on an experimental study accountable for the relaxation-type oscillations observed_ It involves the coupling of a chemical reaction o&wring in the bulk in the vicinity of the interface with an interfacial transfer by diffusion and adsorption-desorption processes.

INTRODUtiION

.‘Jle. hydrodynamical instabilities related to oscillatory potential difference studied here appear in a two non-miscible liquidjhases system, in a state far from the equilibrium [I]. .~ They can _be observed, for instance, when an aqueous solution of an alkyltrimethylammonium halogenide (&X)-which is a.hydrophobic compound-is brought into contact ‘with _a .solution in nitrobenzene (or nitrdethane) of a hydrophylic compound,. e.g. pick .acid (HPi) [2]_. When superposing the two phases each compound is dissolved in the phase where it is the less soluble and the system is in-a state.,far from equilibrium If the concentrations of both compounds are convenient, spontaneous movements .,of the-interface are observed while the system is returning to e@ibritim. They occur, -for example, as a wave which appears along a glass wall

tid disturbs the-whole inteifrice. They can also- appeG‘.as contractions and expansions m the interface p&me-which’can be.observed owing to a sfiontaneous emulsion visible‘a-moment after the’contact of the phases;. We verified that this emulsion. is not. necessary .for. the oc&rrence[of -the instabihtiks. These two hinds of movements arerelated to local iriterfaci+ tensio&v&iations 131; therefore- they are Marangoni effect& Moreover, :& the timpounds dissolved generate ‘ions, a charged species transfer occurs durirrg the.-movements. This induces a..potent@L difference’between :the two jjh&s; which .oscillates_w$r the vz$iatjon~.of .the. species concentration in botbphasghunRg:the-rno~~rn~~ts. ._’ : ‘.. .-. .. -_ 1 .j ,:.,

:

o302;i598/82/ooaa-~~S~~~75. ;. .

-_.

:.

9 1982 Else+

&&$a

g_&

.: : :__

..

584

.

:

.-..

These instabilities cannot be attr&ted to a density effect re!&ed: to a ‘s&;k transfer from one phase to the other [4] nor to the surface-active compound p&s&g through the interface [5]. They no longer follow the Stemling,and Striven c_+iti [6];~ In another publication [7] we showed that a chemical reaction is n$ce+Ary..to s&t the movements. We now propose a reaction scheme, ‘suppotied-by an ex~e~~ekal method, which can account for the observed phenomenon: EXPERIMENTAL METHODS The movements in the idterface plane do not involve a wall effect so they are easier to approach. For this reason we chose to study them first. We correlated the expansions and contractions in the interface plane with variations ok the jnterfacial tension y. Figure 1 shows the variation of this parameter measured by a stirrup method [S]. The potential difference U between the phases was measured by locating two platinum probes in either .side of the interface millivoltmeter. We observed that the variations

and connecting them to a suitable of U were coupled with those of .y

(Fig. 2). STUDY

OF THE TRANSFER

THROUGH

THE OIL-WATER

INTERFACE

This depends mainly on two processes: diffusion and adsorption-desorption. The question is to point out the influence of each of them in the phenomenon observed. In order to determine this we referred to the behaviour of similar inteifaces submitted_ to an electric field. For instance, an interface made from RPi dissolved in nitrobenzene and KX dissolved in water shows a variation of y coupled with a U variation when an electric constraint is applied. This phenomenon, pointed out by Guastalla [9] is called “electroadsorption”. Gavach [lo] proposed to explain this by assuming that: (1) The potential difference is a concentration polarization one, due to an-

Fig. 1. Variation of the interfacial tension with time. Interface HR: 125X 107’ M.(nit&tikne)/i=,~& 5X 10m3M (water).

i-

.

:

.. .,:

_-

.-

Fig. 21 Variation of the interfacial tension and the potential 3.5 X 10m4 M (nitrobenzene)/C,,CI 4X 10z4 M (water).

difference

585

with time. Interface HR:

accumulation of matter. transferred by migration and diffusion like that at a metal-solution interface. (2) For a field such that Rf and X- ions move towards the interface, an exchange reaction:occurs at the interface between RX and RF% As the interfacial activities pf these two types of compounds are. not the same, a y variation occurs. Then the -y and U variations would. both come-from a transfer of matter induced by the electric Strain; which _ex@ius the% coupling. As the‘theory proposed by Gavach did not fit the major part pf our-experimental curves we proposed a modified treatment taking into account not only the migration and diffusion but also the convection [ 111. Therefore, we ap$ied to our-experiments the classic. treatmemproposed by Rosebrugh &rd Miller [13] for a-metal-solution .inietface under these condit@s~For .a field applied in such a wiy that -R+- and Xmove towards theinterface; a calculation‘similar to,that of Gavach [9] shows that the X-_ ion_is. hi q,&&i ~.&r$erthrough the, interface. In- these conditions. the concentiat@n _variati,on of the X;. ion in n&oben&ne at- the. steady state is given by :- ‘p _ y___~ .:. _~_.~ -_:I .:, ..m: __ _-_./ .... 1 ~.my =x- 3co”+gDk_._; .

.:.

: -,-

: -_..._-...

586

j 2.llP

16’

Fig. 3. Variationof ([exp(C,/Rr)]-1) zene)/‘KBr lo-’ M (water).

where

cx-

is the bulk

coefficient of X-

3.laj

with the currentdensity.InterfaceC16Pi: 5X IO-’ M (nitroben-

concentration

in nitrobenzene,j

the thickness of the diffusion This

(A/m’)

concectration

of X-

in nitrobenzene,

D,-

the current density, $ the Faraday

is diffusion contant and 6

layer in nitrobenzene.

variation

induces

a potential

difference

which

can be ex-

pressed by

Figure3 shows that the [exp(TU/RT] - 1 oersus j cur&, pldtted from the experimental measurements is a straight line in the ~polarization range of this interface. Then we can conclude that the species transfer is brought about by a migration-diffusion-convection process. This implies that adsorption-desorptioni.k very -fast compared

with the other processes.

.. .:

RELEVANCE TO THE INSTABILITY PHENOMENA

fis we observed a coupling between y and U variations during the movements, we assumed that this phenomenon was the reverse of electrdadsorption. .Indeed, in electroadsorption, Variation librium

a potential

difference

while, in the $t$abilities a species concentration

ference. This led to suppose

-applied

phenomenon; variation

indtices .a Sp&ies wheti the-,sj+ni.

occ&

that the same prkesses

&ich

ge&+zs

concetitititi&i

returns :to’ &II& -a pott+j:d.-dif-

were involved’kboth

...

cases,

-.

:: .:.

-.

0

2

4

6

. 6

Fig. 4. Variation of the interfacial tension with the square root of time. Interface HPi: (nitroethane)/C,,Br 5 X 10m3M (water)_

1.25X 10m3 M

Indeed, if this is correct, at the beginning.of the phenomenon, when convection is not yet important, diffusion might be the main process. As a result the variation of y might be a- function _of..the square root of tike. Figure 4 shows the y = f(fi) curve plotted from the experimental y-r curve (Fig. 1). As it is a straight line for the major part .of the rising poi-tion of the curve, we can conclude that diffusion is the main process and thk adsorption-desorption is very fast, which induces -the idea that-an equilibrium abvays .exists between the interface and the sublayers; Then the oscillations~ of y, . a@ : consequently those. of. 0 are= ruled by a modification of the coricentmtiorr .of r-i&i -.surf&e-active ions in ‘the sublayers which- depends on a diffu&+convection~@cess. Moreover, the surfaceiactive- species “concerned are RX,:.a compound$resent in the solution. at the beginning of the .experiments, and

-bulk concentration, scheme should

,,

-: ..-.. ... , :...‘..i_ ,._

:._/:.’ ‘_ ;

,~

_.-. .._-

..:

.-,

SCHEME

OF THE WHOLE

PROCESS

Let us consider the experimental curve y =,f(t) (Fig. 1). At. the beginning of .&I oscillation the interfacial tension is that of a system which would invoIve RX-h&e C,,Br-distributed between the solvents. The y begins to increase which implies that X- is replaced by Pi- at the interface. This probably results from the difflsion of HPi from the organic to the aqueous phase as the systems return .to equilibrium. This compound being entirely dissociated in water, this induces RPi formation and justifies the increase of y. In the second part of an oscillation the decrease of y might be a result of Pi- ions leaving the interface. Indeed, the R+ ions tend to pass into the organic phase as the system returns to equilibrium. Their counter ions are not the X- ions but the Piions which are more soluble in nitrobenzene because of their structure. Now the Piions present in nitrobenzene may react with H+ ions, again giving HPi according to the reaction H&, + Pi,, = HP&, the corresponding equilibrium constant being very important (K= 106). Hence the Pi- concentration would decrease-as would y-and the initial compound HPi would be formed again. This would act as a feedback process which creates a loop and maintains the instability. The reaction scheme would be as follows: Scheme

I

interface

EXPERIMENTAL

EVIDENCE

FOR THIS SCHEME

Supposing this mechanism is correct, if the feedback process giving HPi was suppressed the instabilities would not start. Indeed, if HPi is replaced by a similar compound, KPi for instance, with a formation constant lo3 times smaller, no instability is observed. In fact K+ and Pi- do not combine practicably in nitrobenzene to give KPi again and y rapidly reaches a steady value. Nevertheless, this mechanism does not explain why the decrease of y is so abrupt.. In fact, the formation of HPi by the feedback reaction needs H+ idns in tl& organic phase. These can only come from the aqueous phase, essentially as HX. Thus, this compound is responsible for the decrease of y. We can deduce that the. transfer. of HX is abrupt which signifies that the determining process. of this step- is _ adsorption-desorption and not diffusion. We compared this situation with that observed by. Dupeyrat et al. [12] when

h t;8

. 90

95

8

I

100

105

Fig. 5. HX injection effect in the aqukous phase. Interface HA: 3.5X 10w4 M (nitrobenzene)/C,,Cl (water).

4X 10m4M

studying the penetration of a short chain alcohol iri a water-decane system containing surface-active molecules which occupy the interface. From the conclusions of this work we assumed that adsorption-desorption of H + ions, which are weakly adsorbed at the interface occupied by surface-active species, occurs only when their aqueous concentration has reached a certainthreshold. If this hypothesis is correct, an increase in the aqueous concentration of HX would allow the system to reach the threshold more quickly and may increase the frequency of the oscillations. Figure5 shows that the period of instability decreases after an acid has been injected near the interface. CONCLUSION

Finally, the competition between a transfer process (involving convection, diffusion, adsorption-desorption) and a chemical reaction induces variations of the different species concentrations in both media on either side of the interface. The variation of the surface-active species promotes y variation, while that of the other ionic species induces a variation of the distribution of these species, which generates an oscillation of the distribution potential difference CL In fact, the reaction scheme seems to be. essentially explained without a coupling between the electrical and chemical constraints as required in Bish’s [14] approach. M&over, it- can be : ,j@i+& that the phenomenon studied is an interesting experimental example of --direct conversion of chemical energy into mechanical energy: the.lo&l variation of y, generated.by the local variatiorrof the surface-active Speci& c&entratioxi mdu&s the &iovement. ., NeGe&ieless, the presenti of surface-active species is not necessary to obtain an instabiliiy;.-one can imagine systems where only electricaland-chemical oscillations, re<ing from. the competition bet&en transfer pro&sses and chemical reactions, : :.

:.

.- -:

590

would be observed. This could be the case, for instance, for -similar oscillations observed at the biological membrane level. REFERENCES 1 M. Dupeyrat and J. Michel. XX&me Reunion C.I.T.C.E. Strasbourg, 1969. 2 M. Dupeyrat and E Nakache. CR. Acad. Sci. (Paris), 277 (1973) 599; Film: Mouvements Spontanb B I’Interface de 2 Phases Liquides Non Miscibles, SERDDAV. CN.RS., Paris, 1976. 3 M. Dupeyrat and J. Michel, Biological Aspects of Electrochemistry, Exp. Suppl., 18 i1971) 269. 4 J.C. Berg and CR MO& Chem. E%I~.,24 (1969) 937. 5 H. Linde and P. Schwartz. Leawe Notes in Physics, T.S. Sorensen (Editor), Springer-Verlag. Berlin, 1979. p_ 105. 6 C-V. Stemling and EK. Striven, A.I.Ch.EJ., 5 (1959) 514. 7 M. Dupeynt and E. Nakache, Physicochemical Hydrodynamics, D.B. Spalding (Editor), Advance Publications. London. 1977, p_ 591. 8 J. Guastalla. J. Chim. Phys. 68 (1971) 822; T. Proctor Hall, Philos. Ma&, 36 (1893) 385. 9 J. Guastalla, Proc. 2nd Intern. Congress Surface Activity, Butteworths. London, 1957, pp. 14j and 112: J. Cbim. Phys., 53 (1956) 470. 10 C. Gavach and B. d’Epenoux, J. Elezctroanal. Chem.. 55 (1974) 59. 11 M. Dupeyrat and E. Nakache, J. Colloid Interface Sci., 73 (1980) 332. 12 F. Billoudet and M. Dupeyrat. J. Chim. Phys., 78 (1981) 635. 13 T-R. Rosebrugh and W.L. Miller, J. Phys. Chem., 14 (1910) 816. 14 P. Bisch, Th&se, Bruxelles, 1980.