Irregular patterns of polarographic maxima in surfactant dispersions

J. ElectroanaL Chem., 175 (1984) 299-312

299

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

IRREGULAR PATTERNS OF POLAROGRAPHIC MAXIMA IN SURFACTANT DISPERSIONS

T I N K A PLESE and VERA ZUTI(~

Center for Marine Research, "Rudjer Bogkovik" Institute, Zagreb (Yugoslavia) (Received 25th October 1983; in final form 20th March 1984)

ABSTRACT Heterodispersions of fluid surfactants, such as unsaturated fatty acids and their esters, produce irregular oscillations of polarographic maxima of Hg(lI) in seawater owing to random collisions of aggregates of variable size with the mercury electrode/solution interface. By recording the current-time "~urves at the potential of polarographic m a x i m u m in diluted heterodispersions it is possible to characterize single events of coalescence and relaxation of fluid aggregates into the adsorbed layer at the mercury electrode/solution interfaces. The technique developed enables a direct analysis of organic aggregates in natural waters.

INTRODUCTION

Streaming maxima on current vs. potential curves at the dropping mercury electrode have been reported extensively in the polarographic literature ([1-3] and references therein) so that a large number of experimental observations is available today. All the classical theories of polarographic maxima assumed that geometrical non-uniformities of the system are an initial condition, and therefore electrochemical and hydrodynamic equations cannot be applied rigorously. More recently a view was introduced that the maxima could be induced by the instability inherent in electrochemical, hydrodynamic systems--interracial turbulence at liquid-liquid interfaces. Frumkin [4] was the first to draw attention to the close relationship between the phenomena of polarographic maxima and the "Maragoni effect" well known in the chemical engineering literature. Thus, stationary and nonstationary convective streamings are generated by surface tension gradients at the interface and amplified to macroscopic instabilities demonstrated by a measurable increase in current [5,6]. Suppression of streaming maxima by surfactants provides a basis for their determination (a technique first introduced by Heyrovsky as "the adsorption analysis" [7] although no general solution of theoretical aspects of this problem has been accepted. Here, we want to draw attention to the phenomenon of an irregular pattern of polarographic maxima in natural aquatic samples (Fig. 1, curve 3) first observed in

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Fig. 1. Polarographic maxima during reduction of 10 3 M Hg(II) and dissolved oxygen in: (1) artificial seawater in absence of surfactants; (2) a typical Adriatic seawater sample; and (3) a sea surface microlayer sample. p h y t o p l a n k t o n cultures a n d seasurface m i c r o l a y e r s a m p l e s [8,9]. This p h e n o m e n o n is i m p o r t a n t for the s t u d y of the n a t u r e a n d the fate of organic m a t t e r in n a t u r a l waters (e.g. flocculation processes in estuaries [10] a n d should b e of f u n d a m e n t a l electrochemical interest as well. A t this stage we will limit ourselves to a direct p r e s e n t a t i o n of the e x p e r i m e n t a l facts. METHODOLOGY S i m u l t a n e o u s r e d u c t i o n of 1 0 - 3 M H g ( I I ) a n d dissolved oxygen in c o n c e n t r a t e d electrolyte solutions such as seawater ( I = 0.7) p r o d u c e b r o a d p o l a r o g r a p h i c m a x i m a of the first a n d second k i n d (Fig. 1.1).

301

Surfactants present in natural waters give measurable suppression of the maxim u m which serves as a basis for the characterization of adsorbable organic matter [8,9,11-18]. The form of the maxima is, generally, very regular, smooth and highly reproducible (Fig. 1.2). Suppression of the maximum at a constant potential is a measure of the surfactant activity of the solution, while the potential dependence of the suppression serves as a qualitative characterization through comparison with model surfactants and their mixtures. In some samples of natural waters (surface waters with high biological activity, sea surface microlayer or mixing zone in estuaries) irregular, but reproducible patterns of polarographic maxima have been observed (Fig. 1.3). Problems of elucidation of surfactant activity from such irregular signals and identification of organic material responsible for such effects motivated us to study this behaviour in more detail, using a model system. EXPERIMENTAL Measurements were performed in a 100 ml all glass cell open to air (so that measured solutions were equilibrated with atmospheric oxygen) and thermostated at 20 o C. The working electrode was a fast dropping mercury electrode (the capillary used in this work had a flow rate of 5.2 m g / s and a drop time of 2.2 s) and an Ag/AgC1 electrode was used as reference. A Pt counter electrode completed the three-electrode configuration. A 0.5 ml aliquot of 0.2 M HgC12 solution was added to a 100 ml of sample just prior to measurement. If not specifically stated natural samples and test solutions were not filtered. A PAR model 174 Polarographic Analyser in connection with a 7004 B HewlettPackard recorder was used for registration of polarograms. For a.c. measurements a P A R model 170 electrochemical system was used. Registration of i - t curves was performed using a Gould digital storage oscilloscope OS 4000 with an OS 4001 output unit. Prolonged contact (>/15 min) of the solution to be measured with a mercury pool should be avoided. Registration of the polarogram was usually completed within 10 min after insertion of the dropping mercury electrode into the test solution. Reproducibility of independent measurements is + 2% at the concentration level of 1 m g / 1 (of dissolved surfactants). Materials

Artificial seawater, used as the electrolyte, was of the following composition: 0.6 M NaC1 10 -3 M KBr and 5 x 10 -3 M N a H C O 3. The Adriatic seawater had 38%o salinity and p H = 8.2. Stock dispersions of fatty acids and esters were prepared by shaking a given amount (5-100 rag) in the artificial seawater, or 5 x 10 - 3 M N a H C O 3 solution for 4 h. Freshly prepared dispersions were measured to avoid any significant transformation, such as oxidation of double bonds. Preparation of

302

dispersions in the ultrasonic bath did not effect the polarographic response. Axenic phytoplankton cultures of microflagellate Dunaliella tertiolecta were prepared in the Biological Institute, Dubrovnik. Seasurface microlayer samples were taken with a stainless steel net (the Garret sampler). Surfactants of the highest commercial purity were used without any further purification. Special care was taken with respect to purity of water, glassware and the atmosphere in the laboratory. Organic traces from the sodium chloride were eliminated through prolonged heating and the addition of active carbon to the stock solution. All the potentials are reported with respect to the potential where the current of the mercury wave equals zero ( + 60 mV vs. SCE). RESULTS

Choice of the model surfactant Among typical surfactants, relevant to aquatic organic matter [19], only oleic acid was found to cause the irregular pattern of polarographic maxima of Hg(II) in air-saturated seawater, similar to those of natural samples (Fig. 1, curve 3). Soluble surfactants, such as detergents, polysaccarides, polypeptides or sparingly soluble saturated fatty acids (e.g., lauric and stearic) do not cause irregular oscillations; polarographic maxima, more or less supressed, are perfectly regular and smooth. Dispersions of inactive material, such as T-A1203 or clays do not affect the shape or height of maxima in the artificial seawater (Fig. 1., curve 1) up to concentrations of 10 g/1. Figure 2 shows the effect of several unsaturated compounds (9 mg/1) which are all sparingly soluble fluid surfactants. Oleic, linoleic and linolenic acids are negatively charged at the p H of seawater (pH = 8.2, p K = 4.7 [20,21]). There are no solubility data for seawater, but it is known that, in general, solubility increases with the number of double bonds due to an increase in polarity of the aliphatic chain (Tanford [21]). Three types of behaviour are apparent: (1) oleic and linoleic acids have very similar effects: (a) significant suppression of maxima at positive potentials, (b) irregular oscillations over the whole range of potentials within the adsorption range ( - 100 mV > E > - 1250 mV), (c) appearance of a sharp maximum at disorption potentials (-- - 1200 mV) that is higher than the maximum of the second kind in the absence of surfactants (Fig. 1.1), (2) methyl oleate shows significantly lower suppression of the maximum at positive potentials, but an important suppression at potentials corresponding to less positive charges, irregular pattern is present, as well as a maximum at the desorption potential, (3) in contrast to oleic and linoleic acid, linolenic acid suppresses the positive maximum almost completely, the polarogram is regular over the whole potential range, resembling that of a soluble surfactant such as Triton-X-100 by the extent and the form of suppression, except for the negative potentials, where desorption takes place (without appearance of the sharp maximum).

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304

The effect of concentration of the same materials on the forms of polarographic maxima is schematically presented in Fig. 3. For a comparison, Fig. 4 illustrates the effect of a soluble surfactant, Triton-X-100, and a saturated fatty acid. The suppression of positive maxima by oleic acid, linoleic acid, and methyl oleate is accompanied by irregular oscillations even at the lowest concentrations and both effects increase with increasing concentration up to 150 mg/l. Maxima at negative potentials appear at low concentrations as soon as the suppression of positive maxima becomes pronounced. They show typical features of maxima of the third kind [4]. Oleate and linoleate are typical of the surfactants that might be expected to form maxima of the third kind [22]: they are strongly adsorbed at hydrophobic surfaces, sparingly soluble sith strong attractive interactions both in aqueous solutions and at aqueous interfaces, due to the hydrophobic effect [21] and they form liquid crystal structures [23,24]. Alternatively, negative maxima observed might be ascribed to an increase in the capacitive charge towards more negative potentials, as produced by desorption of the surfactant [25]. If suppression of maxima is measured at a constant potential ( - 3 0 0 mV, averaged maximum current is measured), the adsorption isotherm-like curves are obtained (Fig. 5). They reflect both the adsorbability and the rate of convective

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Fig. 3. Polarograms (only envelopes drawn) of 10 -3 M Hg(II) in artificial seawater in the presence of: oleic acid (0.36, 0.89, 1.78, 5.3, 8.9, 17.8, 35.6, and 77 mg/1), methyl oleate (1.76, 8.8, 17.6, 35.2, 87.9 and 152 mg/1), linoleic acid (0.09, 0.36, 0.90, 2.7, 4.5, 6.3, 13.5 and 45 mg/1, and linolenic acid (0.018, 0.04, 0.18, 0.36, 0.54, 0.91, 1.8, 3.6, 5.4 and 12.7) mg/l.

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diffusion of surfactants, since at low c o n c e n t r a t i o n s in solution s u p p r e s s i o n b y s t r o n g l y a d s o r b a b l e surfactants is c o n t r o l l e d b y the rate of mass transport. A l t h o u g h all the three u n s a t u r a t e d acids have similar m o l a r masses ( M = 280 g), linolenic acid has a similar effect to the soluble s u r f a c t a n t T-X-100, while the m e a s u r a b l e r a n g e for oleic a n d linoleic acids indicates a higher aggregation n u m b e r . M e t h y l oleate is clearly less a d s o r b a b l e at the positively c h a r g e d interface.

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Figure 6 shows the effect of filtration and centrifugation on the polarographic maxima i n a dispersion of 89 mg 1- ] oleic acid in artificial seawater. After centrifugation at 8000 rpm, only a few irregularities are apparent on the polarogram, while the height of the maximum is slightly increased. If the same dispersion is filtered (0.45 /~m pore size is arbitrarily chosen by oceanographers to separate dissolved from particulate constituents of natural waters) the irregular oscillations disappear completely and surfactant activity of the sample decreases (the m a x i m u m height increased). This experiment shows clearly that only dispersed surfactants could be responsible for the irregular pattern. The fact that dispersions of saturated fatty acids cause no irregularities indicates that fluidity [21], i.e. mobility of aggregates, is another neccessary condition. i - t curves

To get more insight into the phenomena of irregular oscillations the i - t curves were recorded under potentiostatic conditions. Dispersions of oleic acid were studied. Figure 7 shows i - t curves at different potentials where oleic acid is adsorbed and an irregular pattern is observed. At a constant potential each i - t curve is different, ranging from smooth and featureless ones to highly perturbed ones with one or more irregular oscillations in sequence. Perturbations range from very large ones (ip - 50/~A) to very small ones (the lowest amplitudes detectable are limited by the noise level), but show a general feature: in the first phase a sharp, almost vertical rise of current, sometimes even

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above the value in absence of surfactants, with a slower descending part (50-200 ms). In the second phase, the spike current drops below the value extrapolated according to the form of the signal before perturbation. There is a linear correlation

308

between the "spike" heights and the drop of current below the "normal" i-t curve that follows the spike. The form of the spikes in i-t curves and the corresponding rise-time are reminiscent of sudden capacity changes more than of a Faradaic phenomenon, although it is difficult to explain the very high amplitude in the absence of any external perturbation of the potential. To clear up this point the corresponding changes in capacity current were followed by imposing the a.c. signal under otherwise identical conditions. The effect of perturbation on the capacity current is measurable, but the irregular pattern is much less pronounced. During the perturbation, amplitudes and widths of spikes are significantly lower than in the d.c. response, for an otherwise equal decrease of current below the initial value before the perturbation. The spikes in d.c. curves are predominantly Faradaic. During the second phase of perturbation the capacity current decreases, as in the case of the d.c. response, but after the perturbation the initial current value is reached more readily. Natural heterodispersions, such as a phytoplankton culture (Dunaliella tertiolecta)

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containing phytoplankton cells and fragments, such as fluid vesicules [9], show a similar irregular pattern in i - t curves (Fig. 8), and an analogous dependence on the electrode potential, as oleic aggregates. However, individual perturbations are less pronounced due to competitive adsorption of soluble surfactants and/or difference in fluidity of the aggregates. DISCUSSION

We have presented the evidence that the irregular pattern in polarographic maxima (to our knowledge a phenomenon that has not been described in the polarographic literature) is caused by dispersions of fluid surfactants. The perturbations in the current-time curves (Fig. 7) of variable amplitude and frequency indicate a stochastic process, which can be interpreted as random collisions of surface active aggregates of variable size with the interface of the mercury electrode/aqueous solutions. Thus, each perturbation corresponds to a single event of coalescence.

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This process seems to have some common features with the periodic oscillations observed when the mercury drop electrode is placed at the interface of two liquids with different surface tensions [5,26,27]. However, since the appearance of two liquid phases at the electrode, when immersed in a diluted heterodispersion, should be random, no periodicity would be expected. The characteristic form of perturbation (schematically presented in Fig. 9) is rather complex and could be interpreted by the following sequence of events. (1) Before the arrival of an aggregate, current is controlled by convective diffusion driven by electrochemical and hydrodynamical instability at the interface (typical conditions of a maximum of the first kind). (2) An aggregate arrives at the interface. This has little effect on the current. (3) The aggregate disintegrates, its material spreads over the surface and gives, locally, a patch of lower interfacial tension (7 - A~,); the time scale of spreading is much shorter than the drop life. (4) Owing to the generated gradient Ay/Ax, the patch rapidly spreads towards the regions of higher surface tension (Maragoni effect [5], driving force A~/Ac. AcS/Ax) and this gives rise to a transient turbulent transport of matter from the bulk to the surface, superimposed on the initial flux. This is manifested as the peak current ip. The time scale of the event is of the order 10-3-10 -2 s. The area under the peak is a measure of the size of the aggregate. (5) After this event, the surface concentration of oleic acid is increased (AF), and the Faradaic current is lowered (Ai) with respect to the value before the onset of the perturbation. The decrease in current, is roughly proportional to the peak height ip. Coalescence and disintegration of aggregates at the electrode take place only within the range of potential where surfactant molecules are adsorbed, Collisions at

311 m o r e negative, or m o r e positive, p o t e n t i a l s d o n o t lead to coalescence a n d disintegrat i o n - t h e aggregates b e h a v e as inert particles (e.g. A1203), as there is n o m e a s u r a b l e p e r t u r b a t i o n of the c u r r e n t response. A p o s s i b i l i t y t o elucidate size, d i s t r i b u t i o n a n d reactivity of surface active aggregates in very dilute h e t e r o d i s p e r s i o n s (>~ 100 /~g/1) from the p o l a r o g r a p h i c response is most interesting for the c h a r a c t e r i z a t i o n of a highly reactive a n d i m p o r t a n t c o m p o n e n t of a n a t u r a l aquatic system [10], which is n o t a m e n a b l e to analysis b y the s t a n d a r d methods. A n a t t e m p t will be m a d e to assess the aggregation n u m b e r of an aggregate in a future p u b l i c a t i o n . It is a l r e a d y clear that the smallest aggregates, such as simple micelles c a n n o t b e d i s t i n g u i s h e d from soluble surfactants b e c a u s e of their small a m p l i t u d e of p e r t u r b a t i o n (in c o m p a r i s o n to the noise level) a n d / o r the short time of their r e l a x a t i o n [28]. T h e r e l a x a t i o n time of the r e o r g a n i z a t i o n of an aggregate into the a d s o r b e d layer at the electrode c a n be c h a r a c t e r i z e d m o r e directly f r o m the w i d t h of the c u r r e n t spike ( A t ) . T h e r e l a x a t i o n times observed, 5 0 - 2 0 0 ~ts, c o r r e s p o n d to the r e l a x a t i o n times of oleic acid films at the a i r / s e a w a t e r interface [29]. A l t h o u g h the r a t e of p e r t u r b a t i o n s a n d their a m p l i t u d e s are a function of the size d i s t r i b u t i o n in the aqueous phase, to o b t a i n the actual d i s t r i b u t i o n in the h e t e r o d i s p e r s i o n detailed analyses of the e x p e r i m e n t a l d a t a are n e e d e d [30]. ACKNOWLEDGEMENTS W e wish to t h a n k Prof. H a n s L y k l e m a for his interest a n d c o m m e n t s on the coalescence events. T h e financial s u p p o r t of the S e l f - m a n a g e m e n t C o m m u n i t y for Scientific W o r k of S R C r o a t i a is gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13

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B. (~osovi6, V. Zuti6 and Z. Kozarac, Croat. Chem. Acta, 50 (1977) 229. K.A. Hunter and P. Liss, Water Res., 15 (1981) 203. B. (~osovi6 and V. Zuti6, Thalassia Jugosl., 17 (1981) 197. K.A. Hunter and P. Liss, Limnol. Oceanogr., 27 (1982) 322. K.A. Hunter, Geochim. Cosmochim. Acta, 47 (1983) 197. W.D. Garret, in D.W. Hood (Ed.), Organic Matter in Natural Waters, Inst. Mar. Sci., Univ. of Alaska, 1970, p. 469. P. Somasundaran and K.P. Anathapadmangham, in K.L. Mittal (Ed.), Solution Chemistry of Surfactants, Vol. 2, Plenum Press, New York, 1979, p. 780. C. Tanford, The Hydrophobic Effect, Wiley-Interscience, New York, 1980. E.V. Stenina, A.N. Frnmkin, N.V. Nikolaeva-Fedorovich and I.V. Osipov, J. Electroanal. Chem., 62 (1975) 11. V. Luzzati, in D. Chapman (Ed.), Biological Membranes, Physical Facts and Function, Academic Press, London, 1968, p. 71. J.A. Lucy, in ref. 23, p, 233. Comment of the referee. H. Jehring, N. Viet Huyen and E. Horn, J. Electroanal. Chem., 88 (1978) 265. G. Wessler, P. Schwartz, H. Linde, H. Jehring and C. Hirche, Heyrovsk~' Mem. Congr. on Polarography, Prague, 1980. E. Lessner, M. Taubner and M. Kahlweit, J. Phys. Chem., 86 (1982) 3167; E. Lessner and I. Frahm, ibid., 86 (1982) 3032. D.J. Drag~evi6 and V. Pravdi6, Croat. Chem. Acta, 53 (1980) 1. V. Zuti6, T. Plebe, J. Tomai6 and T. Legovi6, Mol. Cryst. Liq. Cryst., in press.