Tensammetry with accumulation on the hanging mercury drop electrode

Analytica Chimica Acta, 175 (1985) 55-67 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

TENSAMMETRY WITH ACCUMULATION ON THE HANGING MERCURY DROP ELECTRODE Part 3. The Behaviour of Triton X-100 in Mixtures with PEG-9000

ZENON zUKASZEWSKI*,

HANNA BATYCKA

Institute of General Chemistry,

and WEODZIMIERZ

ZEMBRZUSKI

Technical University of PoznaA, 60-965 Poznaii (Poland)

(Received 16th November 1984)

SUMMARY The behaviour of Triton X-100, which can be present in monomeric or associated form, and its mixtures with PEG-9000, which does not undergo association, is described. The tensammetric curve of Triton X-100 alone shows one or two peaks at negative potentials, depending on the concentration of Triton X-100, i.e., on the presence of associated forms. For < 2 mg l-1, there is one broad peak, related to monomers of Triton X-100. The calibration plot for this peak is sigmoidal but its rising section (0.05-0.20 mg I-‘) is approximately linear. The calibration curve of the second, much narrower, peak related to associated forms of Triton X-100, grows parabolically with increasing concentration of Triton X-100. The behaviour of a mixture of PEG-9000 with a larger amount of Triton X-100 is similar to the behaviour of a model mixture of components with sufficiently different properties (e.g., PEG 1500/PEG 9090). The peak for PEG-9000, the stronger surfactant, is relatively less affected by a large amount of Triton X-100. Even this effect can be decreased by using a suitable preconcentration potential (-1.45 V vs. SCE) so that PEG-9000 can be determined in the presence of a lOOO-fold amount of Triton X-100. Both peaks of Triton X-100 are greatly decreased by the presence of PEG-9000 and the broad peak can be completely suppressed. Triton X-100 cannot be determined accurately in the presence of unknown amounts of PEG-9000.

The accumulation of surfactants on the hanging mercury drop electrode (HMDE) prior to the measurement stage, together with the differentiating action of the preconcentration potential, increases the analytical possibilities of tensammetry in comparison with its classical variant based on the DME. These new possibilities can be important for the analysis of surfactant mixtures, which has been illustrated for the example of a mixture of poly(ethylene glycols) having different molecular weights [l] . However, this example concerns an exceptional case of a mixture of surfactants that does not undergo association, and thus is rather a strictly intermediate model than one having practical significance. Usually, surfactants undergo association, in stages after exceeding a certain concentration. Each variety of a surfactant (i.e., in terms of different degrees of association) behaves as an individual surfactant in adsorption processes. Hence, a surfactant which undergoes association behaves as a surfactant mixture after exceeding a certain concentra-

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0 1985 Elsevier Science Publishers B.V.

56

tion. This complicates the tensammetric curve and so the possibilities for analysis. Of course, the presence of another surfactant in the system, given the possibilities of mutual interferences between surfactants and their competition in the adsorption process, further complicates the interpretation of tensammetric data for such systems. That is why it is necessary to investigate the behaviour of surfactants undergoing association and their mixtures under conditions of tensammetry with accumulation on HMDE. The purpose of this investigation was first to establish the behaviour of Triton X-100 as an example of a surfactant which undergoes association even when present alone in solution, as well as its behaviour in a mixture with a stronger nonionic surfactant which does not undergo association, and then to estimate the possibility of using tensammetry with accumulation on the HMDE to determine the components of such mixtures. Triton X-100 is a polydispersed polyoxyethyleneoctylphenol monoether having an average of 9.5 oxyethylene units. Polyoxyethylenealkylphenol monoethers are very widely used surfactants but their biodegradation is not satisfactory. That is why methods are needed for the determination of trace concentrations of these surfactants. In addition to a study of the behaviour of Triton X-100 present alone in solution, the behaviour of Triton X-100 in a mixture having a stronger surfactant was also examined. A poly(ethylene glycol) having a molecular weight (m.w.) of 9000 was chosen. The behaviour of PEG-9000 was very well known from the earlier papers [l, 21, and did not create any additional complications of the system from its own association. The behaviour of polyoxyethylenealkylphenol monoethers, including Triton X-100, under conditions of classical tensammetry has been described in numerous papers, and was critically reviewed by Jehring in his monograph [ 31. The critical micelle concentration of Triton X-100 is 0.016% [4] or 0.0053% [5], depending on the conditions of its determination. EXPERIMENTAL

An OH-105 polarograph (Radelkis) was used with a voltage scan rate of 400 mV min-‘. The applied amplitude of the alternating voltage was 2 mV. Controlled-temperature Kemula electrode equipment (Radiometer) was used with an additional mercury pool auxiliary electrode. The potential was checked with a digital voltmeter (N-517; Mera-Tronic, Poland). All potentials cited in this paper are against the saturated calomel electrode. Triton X-100 (Merck) and poly(ethylene glycol) of m.w. (nominal mean) 9000 (Fluka) were used without additional purification. The sodium sulphate used for preparation of the supporting electrolyte was purified by double crystallization and ignition at 600°C. All solutions were prepared in water thrice-distilled from quartz. Only freshly distilled water was used. Only glass and quartz vessels were used. The supporting electrolyte in all studies was aqueous 0.5 M sodium sulphate.

57 RESULTS AND DISCUSSION

The behaviour of Triton X-l 00 F’reconcentration of Triton X-100 on the HMDE causes the appearance of one or two negative peaks on the tensammetric curves, depending on the concentration of Triton X-100; this is similar to the case of classical tensammetry. Some examples of these peaks are shown in Fig. 1. The less negative peak at about -1.6 V vs. SCE appears independently of the concentration of Triton X-100. This peak is wide and sometimes bulges on its decreasing section, indicating that it may be composed of two closely adjacent peaks. The more negative of the two peaks of Triton X-100 (at about -1.8 V) is very narrow and appears for concentrations of Triton X-100 exceeding l-2 mg 1-l. It is possible to assume, as with classical tensammetry [3], that a broad peak corresponds to adsorption of the Triton X-100 monomer and the narrow peak to associated forms of Triton X-100. The influence of the preconcentration potential on the heights of both negative tensammetric peaks of Triton X-100 was investigated over a wide range of potentials, and for surfactant concentrations from 0.05 to 50 mg 1-l. The most characteristic dependences, which display the behaviour of both negative peaks of Triton X-100, are shown in Fig. 2. A similar dependence for a longer preconcentration time is shown by curves (a) and (b) in Fig.3. For concentrations of Triton X-100 less than 2 mg 1-l only the one broad peak appears on the tensammetric curves. The dependence of this peak height on

I ) b

c

t”

-I 7

200

I

OIpA

mV

Fig. 1. AC. polarograms of Triton X-100. Concentration of !I’riton X-100: (a) 1.0, (b, c) 10 mg 1-l. Preconcentration potential: (a, b) -1.30 V, (c) -1.70 V vs. SCE. Preconcentration time 5 min.

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50 1

DeposItIon

potential

(V)

Fig. 2. The dependences of the heights of both tensammetric peaks of ‘Priton X-100 on the preconcentration potential: (---) the broad peak; (-) the narrow peak. Concentration of Triton X-100: (a) 0.5, (b) 1.0, (c’) 2.0, (d’) 5.0, (e’) 7.0, (f) and (f’) 10 mg 1-l. Preconcentration time 5 min; 100 mm = 0.40 PA.

Deposition

potential

(V)

0.1

0 3 3

I IO

0.5 5 20

Concentration

0.7 7 30

1.0 b IO c d

(mg l-‘)

Fig. 3. The dependences of the heights of tensammetric peaks on preconcentration potential for 10 mg 1-l Triton X-100: (a, b) alone in solution; (a’, b’, c’) in a mixture of 10 mg 1-l PEG9000. Preconcentration time 10 min; 100 mm = 0.80 PA. See text for further explanation. Fig. 4. The dependences of the heights of the tensammetric peaks of Triton X-100 on its concentration: (-) the broad peak; (---) the narrow peak. Curves: (a) 0.01-0.10 mg 1” ; (b) 0.1-1.0 mg 1-l; (c, c’) l-10 mg 1-l; (d) 10-30 mg 1“. Preconcentration potential -1.00 V vs. SCE; preconcentration time 5 min; 100 mm = 0.40 MA.

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the preconcentration potential varies greatly with the concentration of Triton X-100. For most preconcentration potentials, increase in the Triton X-100 concentration is the primary cause of the changing height of the broad peak (curves a, b, f, Fig. 2). The decrease of the broad peak is accompanied by the appearance of the second, narrow peak. This effect may be caused by the competitive adsorption of associates of Triton X-100 with respect to the monomer. The dependences of the height of the broad peak on the preconcentration potential and on the concentration of Triton X-100 are obviously very complicated (Fig. 2). This is in part due to a shift of the surfactant desorption potential as the surfactant concentration increases; for example, the desorption peak potential changes from -1.584 V at 0.5 mg 1-l to -1.719 V vs. SCE at 10 mg 1-l. The narrow peak of Triton X-100 (Fig. 1, curve b) first appears at a concentration of 2 mg 1-l after 5 min of preconcentration. However, there are indications of this peak even at lower concentrations, particularly for long preconcentration times. The dependences of the height of this narrow peak on the preconcentration potential for different concentrations of Triton X-100 are displayed in Fig. 2 (curves c’, d’, e’ and f’) and, for a lo-min preconcentration time, also in Fig. 3 (curve b). Figure 2 shows a disproportionately sharp increase of this peak height with increasing surfactant concentration. There is also a considerable rise of the corresponding curves in Fig. 2 as the preconcentration potential is shifted towards 0 V. Somewhat different is the corresponding dependence in Fig. 3 (curve b), because of the longer preconcentration time (10 min). Two maxima are visible, separated by a minimum at a preconcentration potential of about -1.0 V vs. SCE. All these results testify to the easier adsorption of associates (which is probably responsible for the appearance of the narrow peak) in the range of preconcentration potentials from 0 to -0.6 V, and also, for higher concentrations of Triton X-100 and longer preconcentration times, in the range from about -1.4 to -1.6 V vs. SCE. Worse conditions for preconcentration of associates in the intervening potential range seem to be connected with preferred adsorption of Triton X-100 monomer (the broad peak) in this range (Fig. 2, curve a). The other curves in Fig. 3 are discussed in detail later in this paper. Given a sufficiently high concentration of Triton X-100, its associates adsorb over a wider potential range than the monomers (cf. curves f and f’ in Fig. 2). Between curves (f) and (f’) there is a narrow potential range in which the preconcentration of only associates of Triton X-100 is possible. The separate narrow peak obtained under these conditions is clear from Fig. l(c). The dependences of the heights of both peaks of Triton X-100 on the concentration were investigated for the range 0.01-30 mg 1-l; a 5-min preconcentration at -1.00 V vs. SCE was used in all cases. The results are shown in Fig. 4. The calibration curve relevant to the broad peak has an approximately linear section between 0.2 and 0.4 mg 1-l Triton X-100 (curve b). However, the start of this curve is almost parabolic; this is shown more

60

clearly by curve (a) which pertains to a lower range of Triton X-100 concentration. At concentrations exceeding 0.4 mg 1-l the peak height decreases with increasing Triton X-100 concentrations (see curve c); in parallel, the narrow peak appears and increases. In the concentration range examined, the height of the narrow peak increases parabolically, beginning from 1 mg 1-l (curves c’ and d’). The results for l-30 mg 1-l show an approximately linear dependence if the square root of the peak height is plotted against the concentration of Triton X-100. Solutions more concentrated than 30 mg 1-l were difficult to test because of strong foaming during the initial deaeration of the solution. This range of concentrations is typical for classical tensammetry. Apart from the preconcentration potential and surfactant concentration, the preconcentration time is also important. The dependence of the height of the broad peak on the preconcentration time was investigated over the range 0.5-21 min; a preconcentration potential of -0.50 V vs. SCE was used with two concentrations (0.05 and 0.20 mg I-‘) of Triton X-100. The results are shown in Fig. 5. When only monomers of Triton X-100 exist in the solution (i.e., for the special case of 0.05 mg l-l), the dependence of the peak height on preconcentration time is similar to those of peak height vs. concentration of Triton X-100 (cf. curve a in Fig. 5 and curve b in Fig. 4). In its rising section this dependence is approximately linear. The formation of the plateau corresponds to attainment of an equilibrium between the elec-

150.

d / i /'X

b' ___ _______-__.,_______c__----------) 5 IO 15 20 -140 Deposltlon

time (mm)

-1.50

-160

-170

-100

DeposItIon potential (mm)

Fig. 5. The dependence of the height of the broad tensammetric peak on preconcentration time at different concentrationsof Triton X-100 (solid lines): (a) 0.05; (b) 0.20mgl-‘. (---) The initial stage of formation of the narrow peak of Triton X-100 (0.2 mgl-I) at increasing preconcentration time. Preconcentration potential -0.50 V vs. SCE; 100 mm = 0.40 PA. Fig. 6. The dependences of the heights of 1-r PEG-9000 and 1.0 mg 1-l Triton X-100 (b) broad peak. (c) The relationship for (---) The dependence for 0.05 mg 1-l 100 mm = 0.40 PA.

both tensammetric peaks of a mixture of 0.05 mg on preconcentration potential: (a) narrow peak; the broad peak of 1.0 mg 1-l Triton X-100 alone. PEG-9000 [l]. Preconcentration time 10 min;

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trode surface and the solution (i.e., accumulation of the surfactant stops). It is interesting to note that after increasing preconcentration times, the narrow peak corresponding to the associates is not significant. With the higher concentration of Triton X-100 (0.2 mg l-l), there is an equilibrium between the electrode surface and the solution for all the preconcentration times tested. The very small concentration of associates in solution means that the narrow peak does not appear for short preconcentration times but later there is partial displacement of the monomers from the mercury surface by associates; the mutual influence of the two varieties of Triton X-100 appears as a reduction in the broad peak (curve b) and an increase in the narrow peak (curve b’). For analytical purposes, both peaks can be used. Based on the height of the broad peak, it is possible to determine Triton X-100 in the range 0.050.20 mg 1-l (Fig. 4, curve b). The linear range of the calibration curve can be shifted by increasing or decreasing the preconcentration time. The appearance of the narrow peak on the tensammetric curve is a signal that the level of concentration of Triton X-100 necessary for formation of associates has been exceeded. It is difficult to judge the degree of association, but undoubtedly it is much smaller than that of micelles, because the narrow peak appears even for Triton X-100 concentrations two orders of magnitude less than the critical micelle concentration (c.m.c.). The narrow peak may also be used to determine Triton X-100 in the range l-30 mg ll’, by using the appropriate calibration curve. The behaviour of a mixture of Triton X-l 00 and PEG-9000 Because Triton X-100 at concentrations <2 mg 1-l is monomeric in solution, but monomeric and associated at higher concentrations, the behaviour of mixtures of Triton X-100 and PEG-9000 greatly depends on the concentration of Triton X-100. A mixture with <2 mg 1-l Triton X-100 should behave like a typical two-component mixture, whereas a mixture with > 2 mg 1-l Triton X-100 should behave as a three-component mixture. The behaviour of two mixtures was investigated: (1) a mixture of PEG9000 and 1 mg I-’ Triton X-100, and (2) a mixture of PEG-9000 and 10 mgl-’ Triton X-100. First, the influence of the preconcentration potential on the behaviour of the tensammetric curves of a mixture of 0.05 mg 1-l PEG-9000 and 1 mg 1-l Triton X-100 was studied; the preconcentration time was 10 min at a potential in the range from 0 to -1.85 V vs. SCE. When the preconcentration potential was more positive than -1.61 V, two peaks appeared on the tensammetric curves; the first peak, at less negative potential, was wide and corresponded to monomeric Triton X-100; the second was narrow, corresponding to PEG-9000. When the preconcentration potential was more negative than -1.61 V, only one narrow peak appeared, corresponding to PEG-9000. The dependence of the heights of both peaks on the preconcentration potential for the range from -1.40 to -1.82 V vs. SCE is shown in Fig. 6 (curves a and b). In this interesting range of preconcentration poten-