Adsorptive stripping tensammetry of commercially available oxyethylated alcohols: Part 2. Surfactants having C16C18 hydrophobe

EktroanalysS 3 ( 1991) 963-972

Adsorptive Stripping Tensammetry of Commercially Available Oxyethylated Alcohols: Part 2. Surfactants Having C1,-Cl8 Hydrophobe

Andrzej Szymanski and Zenon Lukaszewski’ Institute of Chemisny, Technical University of Poznun, ul. Piotrowo 3, 60-9G5 Poznun, Poland Receiued August 2, 1990.

ABSTRACT The behavior of oxyethylated straight chain alcohols having the CI6-C,, hydrophobe and 10, 18, 25, 30, and 40 oxyethylene subunits was investigated. The dependences of the peak height on the concentration of surfactant, preconcentration time, and preconcentration potential were examined. Roughly, the behavior of the surfactants having 10-30 oxyethylene subunits can be described as the behavior of the system of two peaks: the dispersed one, which shifts toward a negative direction with the increase of concentration, and the narrow one, which has a stable position. The narrow peak is useful for analytical purposes whereas the dispersed one makes the background current worse. The surfactant having 40 oxyethylene subunits forms two narrow closely located peaks, useful for analysis. The peaks of all surfactants increase linearly with the increase of the number of oxyethylene subunits, under selected conditions.

INTRODUCTION

Presently employed trace analaysis of all oxyethylated surfactants, including oxyethylated alcohols, is based on their preconcentration using specific properties of oxyethylene chain (coprecipitation with Dragendorff re- agent, extraction of complex with Co( 1I)thioisocyanate or with picric acid) so that only the total content of surfactant having oxyethylene chain is determined (e.g., the so-called BiAS [ 1, 21). It is impossible to distinguish the content of certain surfactants in such a result. The only exceptions are oxyethylated alkylphenols, which can be determined in such a mixture [3,4].

Tensammetry offers the analytical signal that is, to a certain degree, characteristic of particular surfactants. The substantial improvement of the detection limit can be achieved if the determination is performed after the adsorptive preconcentration of the surfactant on HMDE (AST). Unfortunately, the weak point of tensammetry, both classical and AST, is the complicated behavior of mixtures of surfactants. On the other hand, a gradual increase of knowledge about the behavior of certain surfactants can lead to achieving enough knowledge, to allow qualitative analysis, using the characteristic peaks on the tensammetric curve. Adsorptive stripping tensammetry is a very

‘To whom correspondence should be addressed.

Q 1992 VCH Publishers, Inc.

semiti* tool for quantitative determination of single surfactants.. This method can be successfully used for the investigation of separation processes and is much easier than other methods. Finally, progress in the separation, which wbuld enable the selection of the single surfactants or at least narrow fractions, could be used both in AST and in the other methods of trace analysis of surfactants.

Oxyethylated alcohols, the most frequently used group of commercially available nonionic surfactants, con- sist of three main subgroups. One of these subgroups has the straight-chain C,,-C,, hydrophobe and a different length of oxyethylene cpain, giving the variety of properties of these surfactants. The aim of this article is to characterize this group of oxyethylated alcohols under conditions of AST. Attention was focused primarily on the effect of the length of oxyethylene chain.

The behavior of oxyethylated 0x0-alcohols having the C,,,-CI3 hydrophobe was the subject of Part 1 [ 5 ] . Also straight-chain oxyethylated alcohols having the C,,-C,, hydrophobe were fragmentarily examined (5-71. As for the oxyethylated alcohols having the c16-c,, hydrophobe, workers have published articles investigating the behavior of oxyethylated n-octadekanols having an oxyethylene chain of 6,10, and 14 oxyethylenesubunits [6] as well as the behavior of their mixtures [S]. The behavior of Rokanol RZ-12 (surfactant having c,6-C,, and av. 12 oxyethylene subunits) alone and in the mixtures with Triton X-100 and

963 1040-0397/32/$3.50 + 25

964 Szymanski and Lukaszewski

sodium dodecylbenzenesulphonate has also been re-

Weiss’s work discusses the behavior of oleyl ethoxy- late Prawozell W-OF-100 under conditions of classical tensammetry [ lo , 111. A paper about the behavior of oxyethylated alcohols having a different number of oxyethylene subunits under the condition of dropping mercury electrode also gives a derivative chronopoten- tiometric method [ 121.

This article primarily considers surfactants having a higher oxyethylation degree than those examined in the above papers.

ported 191.

ExpeRLMENTAI;

Apparatus and Reagents The voltammetric equipment was the same as described recently (51.

RGURE 1. Tensammetric curves for different concentrations of Marlipal1618/10 at constant preconcentrati conditions (curves b-J) and for different preconcentratbn times at constant concentration (curves k-0). Curve a corresponds to pure supporting electrolyte. The Wid points denote the zero of current for the corresponding curve. Concentration of surfactant (mg/L): (a) 0, (b) 0.05, (5) 0.1, (d) 0.2, (e) 0.3, (1) 0.4, (a) 0.5, (h) 0.7, (i) 1.0, and (j) 10, (k-o) 0.2. Preconcentration potential: - 1.20 V; preconcentration time(s): (a-j) 300, (k) 60, (I) 120, (m) 180, (n) 300 and (0) 360.

The following oxyethylated alcohols were used without additional purification: Marlipal 161W10, Marlipal 1618/18, Marlipal 161W25, and Marlipal 1618/40 (Hulls), Rokanol 0-30 and Rokanol K7 (Rokita, Poland).

Purified sodium sulfate was used for preparation of the aqueous 0.5 M base electrolyte [5]. All solutions were prepared in water triple-distilled from quartz. Only freshly distilled water was used.

Preconcentration was done in stirred solution at the required potential with a new mercury drop (51. The preconcentration potential was measured from the mo- ment of drop formation. The tensammetric curve was recorded in the negative direction after a 30 s quiescent period [ 5 ] .

RGURE 2. Representative tensammetric curves for different concentrations of Marlipal 1618118 (curves b-e) and Marlipal 1618/25 (curves f-I). Curve a corresponds to pure supporting electrolyte. Concentration of surfactant (mg/L): (a) 0, (b and f) 0.05, (c and g) 0.10, (d and h) 0.20, (a and i) 0.50 and (i) 1.0. Preconcentration potential: - 1.20 V; preconcentration time: 300 s.

’

m m V c---

Q3P4

zw>mV

i

0

Adsorptive Stripping Tensammetry of Commercially Available Oxyethylated Alcohols 965

RESULTS Influence of Surjktants' Concentration on the Shape and Height of Tensammetric Peaks The results of this series are shown in Figures 1-3. Figure 1 (curves b-j) shows the representative tensammetric curves of Marlipal1618/10. Figure 2 shows such curves for Marlipal 1618'18 and for Marlipal 1618'25, and Figure 3 shows the curves for Marlipal1618/40. Figure 4 shows the dependences of the peak height on the concentration for all five surfactants of this series (including Rokanol O-30).

The behavior of Marlipals 1618/10,1618/18, and 1618/ 25 is similar from the point of view of the shape of tensammetric curve. Two highly coinciding peaks appear. The less negatively situated of these peaks is wide but the second one is narrow. The degree of coincidation of peaks is different for particular surfactants, and it increases with the length of oxyethylene chain in the surfactant structure (compare Figures 1-3). The increase in the concentration of surfactant also causes an increase ofcoincidation degree due to the shift of the less negatively situated peak toward negative direction. The other peak keeps its position (approximately - 1.8 V). Finally, due to the gradual shift of the

FIGURE 3. Tensammetric curves for different concentrations of Marlipal 1618/40. Conditions as in Figure 2, except surfactant concentration (mg/L): (a) 0, (b) 0.04, (c) 0.10, (d) 0.20, (e) 0.30, (f) 0.40, (9) 0.50, (h) 0.70, (i) 1.0, and (j) 2.0.

I

less negatively situated peak, one common narrow peak is formed. The behavior of Rokanol O-30 is very similar to that of Marlipal 1618'25. Closer observation of tensam- metriccurves ofMarlipals 1618/10and 1618'18 leads tothe additional conclusion that prior to the formation of the wide peak the series of small refused peaks appear at the lower concentration of the surfactant (see Figure 1 curves b and c, Figure 2, curves b and c). The increase in the concentration of surfactant or preconcentration time causes the formation of the wide refuse peak from these small peaks.

The behavior of Marlipal 1618/40 (Figure 3) is slightly different. Two coinciding peaks also appear here. How- ever, both are narrow. They are located at -1.81 and - 1.83 V VS. SCE.

All dependences shown in Figure 4 have the growing section up to 0.3-0.4 mg/L, and a plateau appears for the higher concentration of surfactant. However, for the cases of Marlipal1618'40 and Rokanol O-30 this second section can be defined as the straight line with small slope rather than the typical plateau. The peak height increases with the increase of the number of oxyethylene subunits in the surfactant molecule.

More detailed investigation of the growing sections of curves in Figure 4 has indicated that the curve for Marlipd 1618/10 is nonlinear and complicated. This is shown in Figure 5 (curve a) together with the tensammetric curves that correspond to the characteristic point of this curve. The distortion of linearity of the calibration curve (see curve a) seems to be connected with the appearance of an additional narrow coinciding peak on the tensammetric curve. Fortunately, the removal of this effect by the addition of ethyl acetate to the solution was indicated in the simultaneously conducted investigation. The calibration curve of Marlipal1618/10 in the presence of 18 gA. of ethyl acetate in the solution and the slightly changed base electrolyte (approx. 0.012 M, i.e., 1 g/L of sodium bicarbonate was also added) is shown in the cume b of Figure 5.

The peak height of investigated surfactants increases with the increase of the number of oxyethylene subunits in the molecule of surfactant, at a constant surfactant concentration (see Figure 4). The dependences of the peak height on the number of oxyethylene subunits in the molecule of surfactant are shown in Figure 6. Curves a and b show the data for surfactants having hydrophobe C,6-C,, for their concentrations 2 m@ (i.e., corresponding to the plateau of calibration curves: curve a) and 0.2 m@ (i.e., corresponding to the growing range of calibration graph: curve b). Curve c is added for comparison and shows the results for BRIJ-35, Oxetal C114, and Rokanol K-7, for surfactants having lauryl hydrophobe and the average number of oxyethylene subunits of 20, 14, and 7, respectively. The concentration of these surfactants corresponds to the plateau of calibration curves. All the curves show that the analytical signal linearly increases with the increase of ethoxylation degree of the surfactant.

Dependence of Height and Shape of Tensammetric Peaks on Preconcentratdon Time The results are shown in Figure 7. Two concentrations were used: 0.2 mg/L. (dashed lines in the figures) and


FIGURE 4. Calibration graphs for Marlipal 161 8/10 (a), Marlipal 1618/18 (b), Marlipal 1618/25 (c), Rokanol 0-30 (d), and Marlipal 1618/40 (e). Preconcentration potential: - l .20 V; preconcentration time: 300 s.

36

1.2

0 0 012 04 06 Q8 LO 12 14 16 18 2.0

c/mg.l-'

1 mg/L (solid lines in the figures). Almost all the curves presented in Figure 7 are typical. These curves show the range of growth of the peak with the increase of preconcentration time. Only the preconcentration times within this range can be used for quantitative analysis. The formation of a plateau for the dependence of the peak height on the preconcentration time means the saturation of the electrode surface and thus the end of accumulation ability. The position of the plateau increases with the growth of the concentration of surfactant due to the formation of equilibrium corresponding to the higher

concentration level (approximately proportionally to loga- rithm of concentration). Some deviation from the typical curve is observed for Marlipal 1618/10 for concentration 1 m a . The tensammetric curves related to Figure 7 supply additional information about the changes in the number and shape of tensammetric peaks of Marlipals 1618/10 and 1618/18 and about the transformations of these peaks. These transformations are highly visible in Figure 1, curves k-o, where three originally almost sepa- rate peaks of Marlipal1618/10 appear and increase gradually, due to the increase of preconcentration time (i.e., the

FIGURE 5. Calibration graphs for Marlipal 1618/10 in the ab- sence (a) and in the presence (b) of ethyl acetate, illustrated with the characteristic tensammetric curves. In the case of curve b the base electrolyte consists of 0.5 M sodium sulfate and approx. 0.012 M sodium bicarbonate (1 g/ L). Concentration of ethyl acetate (g/L): (a) 0, (b) 18. Preconcentration potential: - 1.20 V; preconcentration time: 300 s.

50 100 I50 200 clpgl-'

Adsorptive Stripping Tensammetry of Commercially Avaikabk Oxyethylared Alcohols 967

0 5 10 15 20 25 Number of EO-subunits

amount of surfactant on the electrode surface) and change into the single wide dispersed peak.

The Dependences of the Peak Height on the Preconcentration Potential (h us. Ed These dependences were examined using different concentrations of surfactants corresponding both to the growing range of calibration graph and to the plateau of this graph. The preconcentration time was equal 300 s in all the experiments. Two surfactants from the investigated group having extremal oxyethylation degree (Marlipal

FIGURE 6. The dependences of the height of the tensammetric peak on the average number of oxyethylene subunits of surfactant: (a) and (b) oxyethylated straight chain alcohols having C,,-C,, hydrophobe, (c) oxyethylated straight chain dodecanols. In (a) and (c) the peak height corresponds to the plateau of calibration graph (2 mg/L); (b) is measured at the concentration of 0.2 mglL. Preconcentration potential: - 1.20 V; preconcentration time: 300 s.

1618/10 and Marlipal 1618/40) were examined within the wide range of concentration. All the surfactants of these group were examined for concentration of 0.5 m a ( i e , corresponding to the plateau of the calibration graph) and for concentration of 0.05 mg/L (i.e., corresponding to the growing range ofcalibration graph). The results are shown in Figures 8, 9, 10, and 11, respectively; characteristic tensammetric curves of Marlipal 1618/10 are shown in Figure 12.

The obtained picture is very complicated. The h-E, curves for higher concentrations of the surfactants are typical (Figure lo), but those obtained for the concentra-

t./min

FIGURE 7. The dependences of the peak height on the preconcentration time for Marlipals: 1618/10 (a, a‘), 1618/18 (b, b’), 1618125 (c, c’) and 1618/40 (d, d’). Concentration of surfactant: (a, b, c, and d) (dashed lines) 0.2 mg/L; (a’, b’, c’, and d’) (solid lines) 1 .O mg/L. Preconcentration potential: - 1.20 V.


peak height on the preconcentration potential for Marlipal 1618/10 at different

0 -02 -0.1 -Q6 -0.8 -lO -12 - i d -t6 -18 -2.0 EC/V

tions corresponding to the growing range of the calibration graphs represent a great variety of behavior (Fig- ures 8,9, and 11). The h-E, curves for Marlipal1618/40 are typical if concentration is higher than 0.2 mgk (curves c-e in Figure 9), but the decrease of concentration below this value causes the substantial deviation of h-E, curves-from the ideal shape (curves a and b in Figure 9). Still much stronger deviations from the typical shape indicate the h-E, curves for Marlipal1618/10 (Figure 8). The shape of h-E, curves for extremes among the investigated concentrations (0.01 and 0.5 mg/L, curves a and d, respectively) shows little difference from the typical shape. However, within the intermediate range of concentration (0.05 and 0.1 m@, curves b and c) the observed picture is thor-

oughly untypical. First of all, curve c, which corresponds to the higher concentration (0.1 m a ) , is located below curve b, which corresponds to the lower concentration (0.05 m@) within the wide range of preconcentration potential. The shape of curve b, consisting of two sections located on two different levels, is also untypical. The untypical behavior of h-E, curves seems to be connected with the appearance of the second narrow peak on the tensammetric curves. From now on this peakwill be called “narrow peak (111.’’ Figure 12 illustrates this effect. The tensammetric curve a was recorded from a solution of 0.05 m@ concentration at the preconcentration potential - 0.80 V, i.e., under the conditions still corresponding to the formation of the single well-shaped peak. The change

0 . m b 1 d

1

2.4 - T 3 ’ -s I 1 1 11

1.2-

FIGURE 9. The dependences of the peak height on the preconcentration potential for Marlipal 1618/40 at different concentrations of surfactant (mgiL): (a) 0 0.01, (b) 0.05, (c) 0.20, (d) 0.50, and (e) 0 -02 -04 -06 -0.8 -10 -12 -11 -16 -18 -2.0 1.0. Preconcentration time: 300 s. E. JV


FIGURE 10. The dependences of the peak height on the preconcentration potential for Marlipal 1618/10 (a), Marlipal 1618/18 (b), Marlipal 1618125 (c), Rokanol 0-30 (d) and Marlipal 161 8/40 (0). Concentration of surfactant: 0.50 mg/L. Preconcentration time: 300 5.

-18 -2D

of the preconcentration potential on the value of - 1.40 V, at the same concentration, causes the appearance of a much lower double peak (curve b). The components of this double peak so closely coincide that they are almost indistinguishable. On the other hand, the use of the previ- ous value of preconcentration potential ( - 0.80 V) but a higher concentration of Marlipal 1618/10 (0.1 m e ) also causes the formation of a double peak (see curve c) that is much lower than the peakobtainedat the lower concentration at the same preconcentration conditions (curve a). The single narrow peak (11) was recorded using the preconcentration potential located in the vicinity of the potential of this peak ( - 1.79) (see curve d).

Figure 11 completes the h-E, curves of the investigated surfactants for their concentration of 0.05 m e . Two

Q6-

of these curves (a and e) appeared in the other corresponding figures and were considered there. They are shown in Figure 11 for the purpose of comparison. As for the other curves, that ofMarlipal1618/18 deviates from the typical shape observed for higher concentration of this surfactant (see Figure 11, curve b), but this deviation is not so high as in the cases of Marlipal 1618/10 (curve a) or Marlipal 1618/40 (curve e). The characteristic maximum on the h-E, curve of Marlipal1618/18 is formed. A much smaller maximum is formed on the curves for Marlipal 1618/25 and RokanolO-30 (curves c and d, respectively). The height of this maximum decreases in the sequence: Marlipal 1618/18, Marlipal 1618/25, Rokanol 0130; the shapes of h-E, curves becomes more and more typical in the same sequence.

Ol .O -02 -01 -0.6 -0.8 --;Pv -12 -U -16 -

FIGURE 11. The dependences of the peak 1 height on the preconcentration potential for

Marlipal 1618/10 (a), Marlipal 1618/18 (b), - Marlipal 1618/40 (e). Concentration of sur- 9 -20 factant: 0.05 mg/L. Preconcentratin time:

300 s.

) Marlipal 1618/25 (c), Rokanol 0-30 (d) and


@mV, I I

OlpA I

0

FIGURE 12. Tensammetric curves that illustrate the changes on the number and height of the peaks of Marlipal 161 8/10 with the change of preconcentration potential and concentration of surfactant within the range of the parameters corresponding to the appearance of narrow peak (11). Concentration of surfactant (mg/L): (a) and (b) 0.05, (c) and (d) 0.10. Preconcentration potential: (a) and (c) - 0.80 V, (b) - 1.40 V, (d) - 1.79 V; preconcentration time: 300 s.

DISCUSSION The tensammetric curves of oxyethylated alcohols having the cl6-c1, hydrophobe seems to behave similarly as those having the C,,-C,, hydrophobe, particularly within the range of higher concentrations. Within this range the behavior of considered peakscan be roughly described as the system of two transforming peaks: the wide one, which moves toward negative direction with the increase of concentration of surfactant or preconcentration time, and the narrow one, which has a stable position. However, in reality, the wide peak is different in this case from that for the oxyethylated alcohols having the C,,-C13 hydrophobe. Its nature is complex. The wide peak forms in accordance with the following scheme: Two or three very small peaks, or rather bulges, appear on the tensammetric curves at the low concentration of surfactant. These peaks are visible in Figure 1, curves b-c and k-n, and Figure 2, curves b and c. The less negatively located on these peaks moves toward

negative direction with the increase of concentration or preconcentration time and thus coincide with the following peak. The wide and more negatively located peak (but now better visible) is formed (e.g., Figure 1, curves d-f). This common peak moves toward negative direction in the same way. Finally, one common wide peak is formed, which is located in the closest vicinity of the narrow peak (e.g., Figure 1, curves g-i). Further, these peaks also coincide, and only asingle common narrow very high peak exists (see Figure 1, curve j). This last stage is very similar for the oxyethylated alcohols having the C]~-CIR hydrophobe and those having the C,,-CI3 hydrophobe (see [5]) . In both groups of oxyethylated alcohols the common single peak appears at a similar concentration, i.e., 5-10 m@ for each surfactant of this group, with the exception of Marlipal 1618/40.

The shift of peaks toward negative direction with the increase of concentration of surfactant is theoretically predicted [ 131 and was demonstrated in Part 1 for Oxetal D104 [ 51. With respect to the considered transformations of the wide peak, the increase of preconcentration time causes results that are similar to those caused by the increase of concentration.

Thus the wide peak for oxyethylated alcohols having the C,,-C,, hydrophobe has a complex nature and is formed because of the coincidation of several peak-components. This peak is slightly different for each particular surfactant of the investigated group depending on the ethoxylation degree. The increase of degree also causes shifting of the wide peak toward negative direction, at equal concentration (compare Figure 1, curve d, Figure 2, curves dand h; i.e., curves for 0.20 m@of Marlipals 16181 10, 1618/18, and 1618/25, respectively). Generally, the wide peak of all the investigated oxyethylated alcohols here, with the exception ofMarlipal1618/40, is refused and badly shaped within the range of concentration, which is interesting from the viewpoint of trace analysis. This peak can be considered as the unfavorable component of the base line current rather than as a useful analytical signal. However, the characteristic pattern of the discussed peak can be useful for identification of surfactant, based on its tensammetric curve. The analytical signal here can be the narrow peak; i.e., this case differs from the oxyethylated alcohols having the C,,-C,, hydrophobe, where the wide peak is well shaped and is the analytically useful signal.

The case of Marlipal1618/40 is also different. Here, the high oxyethylation degree shifts the wide peak toward a negative direction and the coincidation “lifts” it in such a degree that, together with the narrow peak, a comparatively well shaped double peak is formed (see Figure 3). This favorable shape of the peak, which appears for Marlipal 1618/40 within the analytically useful range of concentration, appears for the other surfactants at a much higher concentration, outside the most interesting range. The left peak of Marlipal 1618/40 considered here is the main component of this double peak. It is higher and wider than the more negative peak and, because of its shape, cannot be called as the wide peak any longer. Actually, this peak is the analytical signal in the case of Marlipal 1618/40.

As has already been mentioned, only the narrow peak can be used as the analytical signal for Marlipals 1618/10,


1618/18,1618/25, and Rokanol O-30. In all the cases with the exception of Marlipal 1618/10 this peak behaves comparatively typically: it steadily grows with the increase of concentration of the surfactant (see Figure 4) and the preconcentration time (see Figure 7). This dependence can be roughly considered as linear within the certain range of concentration or preconcentration time. It is necessary to emphasize that the narrow peak increases with the increase of the number of the oxyethylene subunits in the molecule of surfactant, which will be discussed later in detail.

Besides the narrow peak considered above (called “narrow peak (I)”) a second narrow peak appears in the case of Marlipal1618/10, which is located more negatively than narrow peak (I) and is called “narrow peak (II).” The appearance of this peak is accompanied by two untypical phenomena. The calibration graph (see Figure 5) indicates the deviation on regular growth: At first the peak grows, then the growth is disturbed, the curve falls, and finally the peak grows again. Just at the concentration corresponding to the first turning point of the calibration graph the narrow peak (11) appears on the tensammetric curves (see Figure 5) . The effect is visible within the whole range of preconcentration potential; however, narrow peak (11) appears quickly, i.e., for a lower concentration for preconcentration potentials more negative than - 1.0 V. The disturbance of regular growth of the calibration graph causes a problem from the viewpoint of the analysis. Fortunately, this effect can be removed by the addition of ethyl acetate to the determined solution. In such a case the calibration graph rebuilds the sigmoidal shape (see Figure 5, curve b). The appearance of narrow peak (11) was also reported for oxyethylated n-octadekanol 18-6 (6 oxyethylene subunits) and for a higher concentration of surfactant 18-10 (10 oxyethylene subunits) [6]. A trace of this peak is visible on the tensammetric curves of surfactant 18-14 (14 oxyethylene subunits) [6].

The great complexity of transformations of tensammetric peaks of surfactants of the considered group seems to be caused by the multistage association of surfactants. These stages of formation of micelles are not precisely described. However, it is obvious that a multistage process is running in the sequence: monomer, dimer, simple associate forms, micelle. Particular species of the same surfactant behave like single surfactants in the adsorption processes, i.e., they can compete or undergo the mutual influences of both attractive or repulsive nature. The investigated surfactants having the C,&, hydrophobe do not indicate the presence of the distinct peak that could correspond to the monomer form. Such is the wide peak of Triton X-100 I141 or Oxetal D104 [5]. In this work the monomer form probably corresponds to the less negatively located bulge or small peak visible on the tensammetric curves at a low concentration of surfactant (see Figure 1, curves b-c and k-m, and Figure 2 , curves b-c). The question arises, what is the reason for the difference between the behavior of Oxetal D104 (C,,) and similar surfactants on the one hand, where the well-shaped wide peak of monomer form appears and it gradually trans- forms into the peak of associate forms, and on the other hand, the behavior of surfactants considered in this work? The reason probably is that the surfactants considered

here, because of having a longer hydrophobe, much more easily undergo the association; this, among other reasons, can be concluded from the lower values of cmc for these surfactants. The easier association causes these processes to occur within the lower concentration range. The range of predominant existence of the monomer form in the solution is probably located below the best sensitivity of this measuring method, and the monomer form is not clearly indicated. On the other hand, the range of concentration corresponding to a much stronger signal already corresponds to the associate forms. The appearance of the narrow peak undoubtedly corresponds to some associate form of surfactant but not to the micelle (the concentration is much below the cmc value [ 151). The presence of narrow peak (11) for Marlipal 1618/10 is probably connected with the existence of some associate form of surfactant (called below, “form B”) having a higher association degree than the form corresponding to narrow peak ( I ) (called below, “form A”). This associate form corresponding to narrow peak (11) can be separately preconcentrated on the electrode surface (see Figure 12, curve d) at a sufficiently negative preconcentration potential selected in such a way that the preconcentration of less associated forms is impossible. This means that form B cannot be formed on the electrode surface from the less associated components and so it must originate from the solution.

The scheme origination of the peaks discussed above seems to be the most probable explanation. However, it is slightly probable that the impurities of the investigated surfactants, which are only commercially available polydispersal products, can also play a certain role in the origin of these peaks.

The complex behavior of h-E, dependences is also of interest. On the one hand, such curves obtained at the comparatively high concentration are typical (see Figure 10). On the other hand, h-E, dependences obtained within the range of concentration typical of the trace analysis are complicated, and the choice of the best E, value is difficult. Within the higher range of concentration the highly associate forms are predominant in the solution, and the species having the lower degree of association or

, impurities can be present in the solution only at a negligi- ble concentration. That is why the typical h-E, dependences for the high concentration of surfactant reflect the behavior of associate forms. Within the range of lower concentration, which is much more interesting for the trace analysis, the different species having various degrees of association are simultaneously present in the solution at the comparatively similar concentration. These species compete in the adsorption processes, and their h-E, dependences are complicated (see Figure 8, Figure 9, curves a and b, and Figure 11) and reflect all the complica- tions that characterize the behavior of mixtures. However, these dependences should be investigated because these data are necessary for the proper selection of preconcentration potential.

The case of Marlipal 1618/10 is an excellent example of the picture of competition of two differently associated species with different ranges of adsorption (see Figures 8 and 12). At the concentration of 0.05 mgiL o f this surfactant, within the range of 0-( - 0.80) V the less associated form A is predominant (see Figure 8, curve b). This form is


represented by narrow peak (I) (see Figure 12, curve a). At the same concentrat ion within the range of ( - 1.20)-( - 1.80), the highly associated form B, which corresponds to narrow peak (II), seems to compete effec- tively with the less associated form. The double peak that appears (see Figure 12, curve b) is lower than on curve a, because of the competition of both forms or the decrease of concentration of form A, which is partially transformed into form B. At twice the concentration of Marlipal1618/10 (0.10 mg/L) form B demonstrates its presence within the whole range of investigated potentials. The double peak that appears (Figure 12, curve c) is lower than the peak of single form A (Figure 12, curve a). As already mentioned, this form can be preconcentrated separately (Figure 12, curved). However, at concentrations lower than 0.05 mg/L only a single well-shaped peak appears, corresponding to form A (see Figure 5 ) .

The discussed effect seriously complicates the choice of the best analytical conditions. On the other hand, it reflects the presence of an intermediate form between monomer and micelle. Thus the effect creates theopportu- nity to investigate different forms in the association process. Such an investigation has not yet been possible.

The analytical problems connected with the appearance of the narrow peak (11) can be removed by the addition of ethyl acetate to the solution. It is obvious that, in the presence of ethyl acetate, the association process operates differently and the intermediately associated forms differ from those in the pure water solution. A similarly successful modification has been the introduction of the methanol addition in respect to the tensammetric determination of Oxetals T105 and C114 [5 ] . The effect of ethyl acetate will be published in another paper.

The maximum visible on the h-E, curve of Marlipal 1618/18 and, in a smaller degree, on the curve of Marlipal 1618/25 (see Figure 11, curves b and c) is typical of the behavior of mixtures. This is because these surfactants are polydispersal mixtures or a mixture of components that differ with the association degree. A similar maximum appears on the h-E, curve for the two-component mixture of PEG 1500 and PEG 9000, although both components separately have typical h-E, curves, without a maximum [ 161. Such a maximum was observed on the h-E, curve of surfactant 18/14, i.e., having the n-octadecyl hydrophobe and an average of 14 oxyethylene subunits [6]. The same types of maximum on the h-E, curves are typical of the mixture of oxyethylated alcohols [7, 91.

The case of Marlipal1618/40 differs from the others. In this case the h-E, dependences reflect the behavior o f another peak. The height of the less negatively located peak is measured here, i.e., the peak corresponding to the wide peak for the other cases (although this peak is narrow rather than wide). That is why this peak behaves, to a certain degree, differently from the narrow peak of the other surfactants.

The effect that ha.. a certain practical importance is the dependence of the signal on the ethoxylation degree (see Figure 6). The linear increase of the peak height with the increase of the number of oxyethylene subunits in the molecule structure has been determined both for the positions of plateau of particular calibration graphs as well

as for certain concentrations located within the growing range of the calibration graph. Similarly, linear dependence has been found for three surfactants with the C,, hydrophobe (see Figure 6, curve c). However, the considered effect is observed only if the preconcentration potential is located within the range of ( - 1.20)-( - 1.30) V (see Figure 11).

The practical profit of the discussed effect is the possibility of the quick and easy determination of the number of oxyethylene subunits in the surfactant if the hydrophobe and concentration of surfactant are knon-n. The number of oxyethylene subunits provides important information for the characterization of oxyethylates. Pres- ently, it is most often determined using gas chromatogra- phy after the linkage of the oxyethylene chain with hydro- bromic acid [ 171. The linearity of the signal on the number of oxyethylene subunits enables workers to predict the height of the analytical signal if the structure of the surfactant is known or to obtain qualitative information when attempting identification of an unknown surfactant.

ACMVO WLEDGMENT This work u)as supported by Kesearch Pmgram CPBP 01.17.

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Keck, E. Kunkel, E. Matthijs, J. Waters, and R. J. Watkinson, Ti?nsiak Lkterg. 23 (1986) 4.

3. M. Ahel and W. Giger, Anal. Chem. 57 (1985) 1577. 4. M. Ahel and W. Giger, Anal. Chem. 57 (1985) 2584. 5. A. Szymanski and 2. Lukaszewski, E l e c ~ o a n a l . 3(1991)17. 6. M. K. Pawlak and 2. Lukaszewski, Anal. a i m . Acta 202

(1987) 85. 7. 2. Lukaszewski and M. K. Pawlak: The Behavior of

0.uyethvlated Alcohols under Conditions of Tensammetry with Adsorptive Accumulation on HMDE, in M. R Smyth and J. G. Vos (Eds): Elech-ocbmktq), Sensors andAm&sb, Else- vier, Anlsrerdam, pp. 119-124 (1986).

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Po’olavoRraphie, Akademie-Verlag, Berlin, 1974. 12. P. Holmqvist, Anal . Chim. Acta 90 (1977) 35. 13. B. B. Damaskin, 0. A. Petrij, and W. W. Batrakov:Adsorpcija

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15. P. Becher: Micelle Formation in Aqileoifi ana‘ Nonaqueous Solutions, in Nonionic Surfatants, M. J. Schick (Ed.): M. Dekker, New Y a k , (1966) pp. 478-575.

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Documents

Adsorptive stripping tensammetry of commercially available oxyethylated alcohols: Part 2. Surfactants having C16C18 hydrophobe