Stability of polyvinyl chloride latex. II. Coagulation by metal chelates

Stability of Polyvinyl Chloride Latex II. Coagulation by Metal Chelates ~'2 R. V. LAUZON 3 AND E. M A T I J E V I C 4 Institute of Colloid and Surface Science and Department oJ Chemistry, Clarkson College of Technology, Potsdam, New York 13676

Received January 21, 1971; accepted April 6, 1971 Coagulation of a polyvinyl chloride latex by two metal chelate counterions, Co(dipy)~+ and Co(phen)~+, as well as by the chelating agents 2,2~-dipyridyl and 1,10-phenanthroline has been studied. It was found that Co(phen)~ + coagulates the latex more efficiently than the corresponding dipy complex. The former can also reverse the charge of the latex particles. This is consistent with the greater adsorptivity of the Co(phen)~+ which was established earlier. The chelating agents coagulate the same latex only if pH is sufficiently low to produce protonated counterions. Again the phenanthroline species coagulates more efficiently than the protonated dipyridyl ion.

INTRODUCTION In a previous study using silver halide sols it was shown t h a t the chelation of metal ions significantly enhanced their coagulation and reversal of charge ability (1). For example, the critical coagulation concentration (cce) of Ni 2+ for a negatively charged silver bromide sol was found to be ~ 1 0 -8 M and for Ni(phen)~ + N 10-7 M. In addition, the unhydrolyzed nickel ion could not reverse the charge of the silver bromide sol whereas the complex ion Ni(phen)~ + rendered the sol particles positively charged at a concentration as low as 5 X 10-~ M. Another advantage of metal chelates is t h a t m a n y of these complexes are stable over a broad p H range and, therefore, coagulation 1 Supported by the U.S. Army Research Office (Durham) Grant No DA-ARO (D)-31-124 G 656. Part I, see Ref. (2). 3 Part of a PhD Thesis by R. V. Lauzon. Present address: Dow Chemical Co., Designed Polymers Research, Midland, Michigan 48640. 4 Published in lieu of the plenary lecture given by the senior author (E.M.) at the Fourth Scandinavian Symposium on Surface Chemistry held at TylSsand, Sweden, August 1970. Journal of Colloid and Interface Science,

and reversal of charge studies could be ex~ended to p H values which would normally cause the hydrolysis of uncomplexed metal ions or even the precipitation of corresponding metal hydroxides. For these reasons it seemed of considerable interest to establish whether such behavior of metal chelates was true for any lyophobic colloid or if it was specific for silver halide sols. Here, a study is reported of the stability of a polyvinyl chloride (PVC) latex in the presence of tris(dipyridyl)cobalt(III) perchlorate, Co(dipy)~(ClO4)a, and tris (phenanthroline)cobalt(III) perchlorate, Co(phen)3(C104)3. In the first communication of this series (2) the adsorption of the same chelates on PVC latex was investigated using the radioactive tracer technique and it was shown that Co(phen)~ + has a greater specific adsorption potential than Co(dipy)~ + with respect to the PVC latex/solution interface. Furthermore, the chelating agents (2,2 ~dipyridy] and 1,10-phenanthroline), when added uncomplexed, also coagulated silver

Vol. 37, No. 2, October1971 296

STABILITY OF POLYVINYL CHLORIDE LATEX halide sols and reversed their charge, doing so more efficiently at higher pH values (3). The same two compounds have been employed in the study with PVC latex to be reported here. The PVC latex used in this work consists of spherical particles uniform in size, stabilized by dodeeyl sulfate ions. The charge density of the latex particles could be changed by varying the concentration of sodium dodeeyl sulfate (SDS). This colloidal system was chosen because a number of studies have shown that various latexes behave as typical lyophobie colloids when coagulated with simple eounterions of different charges (4-13). Although the effects of hydrolyzed metal ions have been investigated (7), no work on latex stability in the presence of metal chelates or chelating agents has been reported heretofore. EXPERIMENTAL A. Materials. The polyvinyl chloride (PVC) latex, sodium dodecyl sulfate (SDS), the chelates [Co(dipy)a(ClO4)a and Co(phen)a(C104)a] and the chelating agents [2,2'-dipyridyl (dipy) and 1,10-phenanthroline (phen)] used in this work have been the same as described in the first paper of the series (2). B. Methods. Depending on the late:~- coneentration, two methods were used for studying the stability of the PVC sols. At high PVC concentrations (3.6 ;4 10-3 gm/ml), a set of test tubes was prepared containing a constant amount of latex and varying concentrations of cobalt chelate or ligand material, in a total of 10 ml. The systems were then stirred on a magnetic stirrer, with the use of Teflon coated stirring bars, for 1 rain and let stand for 2 hr. During this time, which was equal to the equilibration time used for the adsorption experiments (2), the unstable systems flocculated and settled. The amount of settling varied in the critical concentration range. A 1-ml aliquot was taken from each supernatant and diluted to 10 ml. The absorbance of this diluted aliquot was

297

then measured in a Beckman D.U. Spectrophotometer with a slit width of 0.18 mm at a wavelength of 546 m~ using l-era cells. The critical coagulation concentration (eec) was determined by plotting the absorbanee as a function of the log of the chelate concent.ration and extrapolating the line of steepest slope to the high absorbance level (i.e., to the absorbance of stable systems). At a low latex concentration (1.8 X 10-4 gm/ml), a Brice-Phoenix Light Scattering Photometer was used to measure the Rayleigh ratio at a scattering angle of 45 ° using a wavelength of 546 m~. The components of the systems were prepared in two sets of test tubes, each with a final volume of 5 ml. The first set contained the latex at a constant concentration, whereas the second set contained the cobalt chelates or the ligand materials in gradually decreasing concentrations. The contents of each test tube of the second series was mixed with the sol in the test tubes of the first series. This was done by pouring the corresponding solutions back and forth between the test tubes ten times. The test tubes containing the sols were then placed in a the~mostated water bath mainrained at 25°C. After desired periods of time, the tubes were inserted into the photometer cell, and readings were taken. The log Rayleigh ratio was plotted as a function of the log of the electrolyte concentration and the ecc determined by an extrapolation of the line of steepest slope to high Ilayleigh ratio (i.e., that of the stable sol). The ccc varied slightly when measurements were made at 4 and 24 hr after mixing the reacting components. The change in Rayleigh ratio was more pronounced at 24 hr and, therefore, the results obtained at this time are reported in this work. Since the systems did not change at longer times, the data for the two sol concentrations can be compared. In the two procedures used for measuring coagulation concentrations, no increase in turbidity was observed in the coagulation range. Instead, the colloidal material sep-

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LAUZON AND MATIJEVI~

arated in the form of flocs and subsided to the bottom of the tube. The mobilities of the latex particles were determined in a microelectrophoresis cell as described earlier (2, 14, 15). RESULTS

A. Coagulation with Chelates. Figure 1 shows several coagulation curves for PVC latex in the presence of Co(dipy)s(C104)s at different sodium dodecyl sulfate (SDS) concentrations. The open and full symbols represent latex concentrations of 3.6 X 10 -3 gm PVC/ml and 1.8 X 10-4 gm PVC/ml, respectively. The log Rayleigh ratio serves as ordinate for the lower and the absorbanee for the higher latex concentration. The pH at the critical coagulation concentration (eee) ranged from 5.0 to 6A for the higher latex concentration and from 6.2 to 6.5 for the lower. In both cases the eec for the: chelate increases as the total concentration of the stabilizing species (SDS) in the system is increased. The coagulation of PVC latex by Co(phen)s(ClO4)s gave similar type of curves.

S.D,S. : /7"0x10"3 M

In the latter case the effect of pH was also investigated. The ccc at the lower latex concentration (PVC: 1.8 )< 10-4 em/ml SDS: 5.6 × 10.4 M) was reduced when the pH was changed from 6.6 to 2.9 (6.6 X 10-5 M and 1.0 X 10-5 M, Co(phen)~ +, respectively). Figure 2 shows that the latex with no added surface active agent (total SDS 1.9 X 10.6 M) can be restabilized by Co(phen)s(C104)s due to charge reversal. In this case the cee is 2.9 X 10-~ M. However, over the chelate concentration range of 2.6 X 10-51.0 X 10-4 M the latex is stable again. The superimposed mobility curve shows that the charge reversal takes place, although the onset of the second stability region does not coincide with the latex of a positive charge. The isoelectric point is found at the chelate concentration of 3.4 X 10-s M. The ccc, at similar sodium dodecyl sulfate concentrations, are lower foi Co(phen)s(C104)s than for Co(dipy)3(C1Q)s. This is shown in Fig. 3 in which the logarithm of the ccc is plotted as a function of the logarithm of the total concentration of the

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FIG. 1. C o a g u l a t i o n c u r v e s of p o l y v i n y l c h l o r i d e l a t e x ( P V C ) w i t h Co(dipy)3(C104)8 for t w o l a t e x c o n c e n t r a t i o n s a t v a r i o u s s o d i u m d o d e c y l s u l f a t e (SDS) c o n c e n t r a t i o n s : P V C , 3.6 X 10-3 g m / m l ( o p e n s y m b o l s ) ; S D S , 7.2 X 10-5 M ( O ) ; 3.8 X 10-4 M ( A ) ; 7.0 X 10-6 M ([2). P V C , 1.8 X 10 -4 g m / m l (full s y m b o l s ) ; S D S , 1.9 X 10-6 M ( 0 ) ; 5.4 X 10-~ M ( A ) ; 1.9 X 10 -5 M ( R ) . Journal of Colloid and Interface Science, Vol. 37, No. 2, October 1971

STABILITY OF POLYVINYL CHLOI~IDE LATEX

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F'm. 2. Coagulation and restabilization of PVC (1.8 X 10-~ gm/ml) by Co(phen)3(C104)3. The stability data are represented by the triangles, and the corresponding mobility curve is given by the circles. The isoelectrie point is at 3.4 X 10-~ M Co (phen)3 (CIQ)3. -2 PVC = 3.6 o

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as a function of total sodium dodecyl sulfate concentration for a PVC latex of 3.6 X 10-8 gm/ml. Circles and triangles represent systems coagulated with Co (dipy) 3(C104)3 and Co (phen)~ (C1Q) 3, respectively. stabilizing species (SDS) for a latex concentration of 3.6 X 10-3 gm/ml. The circles and triangles represent systems with Co(dipy)a(C104)3 and Co(phen)3(C10,)3, respectively. I n both cases, the log ccc increases linearly

with the increasing concentration of SDS ( > 1 × 10-4 M), although the p h e n a n t h m line chelate is g somewhat more effective coagulant. Figure 4 gives the analogous plot for a lower latex concentration (1.8 × 10 -4 g m / m l ) . The phenanthroline chelate has a much lower ccc t h a n the dipyridyl chelate when no extra SDS is added to the system (i.e., when the total emulsifier concentration is 1.9 X 10- 6 M). Systems containing higher SDS concentrations show similar ecc for both chelates. B. Coagulation with Chelating Agents. I n order to ascertain whether the ligand materials were responsible fol coagulation of the P V C latex, a s t u d y was conducted to determine the coagulation efficiency of the ligands themselves. Investigations at the high latex concentration revealed t h a t 2 , 2 ' dipyridyl could not cause coagulation above p H 3.9, even at the highest concentrations used. Similarly, 1,10-phenanthroline did not coagulate the latex above p H 5. Figure 5 compares the coagulating efficiency of Co(phen)3(ClO4)s and 1,10-phenanthroline for a P V C latex to which sodium dodecyl sulfate was added at p H 3. The total

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LAUZON AND MATIJEVIC

SDS concentration is 3.8 X 10-~ M. The circles and triangles represent coagulation curves in the presence of the ligand and the chelate, respectively. The chelate acts as a 1

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much more efficient coagulant than the corresponding ligand material. Figure 6 shows the effect of SDS upon the coagulation of PVC (1.8 X 10.4 gm/ml) by 1,10-phenanthroline at constant pH (= 3). Again the eee was higher in the presence of larger amounts of SDS. However, in each ease the chelating agent was less efficient as a coagulating species than its corresponding chelate. Under the same conditions the 2,2t-di pyridyl requires a higher concentration to destabilize the latex than 1,10-phenanthroline. For example, for PVC 3.6 X 10-8 gm/ml, SDS 2.8 X 10.3 M and pH 3, the tee values for 2,2~-dipyridyl and 1,10phenanthroline are 4 X 10-3 M and 1 X 10.8 M, respectively. DISCUSSION

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LOG MOLAR CONC. OF SODIUM DODECYL SULFATE FIG. 4. T h e critical coagulation c o n c e n t r a t i o n as a f u n c t i o n of sodium dodeeyl sulfate concentration for a PVC latex c o n c e n t r a t i o n of 1.8 X 10.4 gm/ml. The circles a n d triangles represent Co(dipy)3(C104)3 a n d Co(phen)~(CIO4)8, respectively.

Studies with silver halide sols showed metal chelates in general, and Co(dipy)~ + or Co(phen)] + in particular, to be exceedingly efficient coagulating and reversal of charge agents over a broad pH range (1,3). It was hoped that this behavior would be characteristic of all hydrophobic sols. The results presented here indicate that considerably higher concentrations of the

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Journal of Colloid and Interface Science,

MOLAR

Vol. 37, N'o. 2, October 1971

STABILITY OF POLYVINYL CHLORIDE LATEX & S.D.S. = 1.9 x IO'eM o Sl D. S. = 5.4 x }O'tt_MM S.O.S. =1.9 x IO-*N

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FIG. 6. Coagulation of PVC latex (1.8 × 10.4 gm/ml) with 1,10-phenanthroline at three different sodium dodeeyl sulfate concentrations, at pH = 3. Concentrations of SDS: 1.9 X 10.8 M (A); 5.4 X 10.6 M (O); 1.9 X 10.5 M (,~). same chelates are required to coagulate the PVC latex and that the charge reversal of the latex is not as easily achieved as in the ease of silver halides. Also, the concentration of the stabilizing ion (dodeeyl sulfate) has a preponderant effect upon the latex stability. The eee values of the chelates increase considerably with increasing amount of SDS in the system. As in the ease of silver halides the Co(phen)~ + is a better coagulating agent than the corresponding dipyridyl complex. The latter effect is explained by the higher adsorption potential of the former ehelate, which would therefore show a greater ability to reduce the Stern potential (2). The high efficiency of the chelates in the destabilization of silver halide sols is most, likely due to strong interactions of constituent silver ions with the eounterions, particularly the coordination of Ag+ with chelating ligands. Such interactions are not possible with latex particles. In the latter ease the

301

addition of chelates has in part a stabilizing effect as it enhances the adsorption of the stabilizing dodeeyl sulfate ions on latex partides (2). Thus, the chelates cause two compensating effects: an increase in surface potential owing to stronger emulsifier adsorption and a decrease in Stern potential due to chelate adsorption. Indeed, when the SDS concentration is very low only the second effect is dominant and the coagulation concentration of the Co(phen)~ + is found to be very low (Fig. 4). The coagulation results with chelating agents rather than with the corresponding cobalt chelates offer further evidence for the different type of interactions taking place at silver halide/solution and PVC/solution interface, respectively. It was shown that 2,2'-dipyridyl and 1,10-phenanthroline coagulate and reverse the charge of a silver bromide sol more efficiently at high than at low pH (3). Contrary to this the same ligand materials cannot destabilize the PVC latex at high pH values at all. Only when pH is low enough for the protonation of these compounds to take place, the coagulation of the latex is observed. Thus in the latex system the coagulation is caused by the cationic eounterions formed by the protonation of chelating agents. Over the pH range 2-3 the two ligand materials exist almost entirely in the monoprotonated form (3), giving counterions of 1+ charge. However, the eec values for these species are much lower than expected for the simple monovalent eounterions. For example, the eec of Na+ for the same latex is ~--0.1 M (16). This would indicate that the protonated chelating agents adsorb on latex particles whereas Na + does not. The protonated phenanthroline seems to adsorb more strongly than the protonated dipyridyl since the former coagulates at a lower eee. At higher pH values only neutral species are present which cannot destabilize the latex. On the other hand the same neutral chelating agents can coordinate with silver ions of a silver halide sol and replace the

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LAUZON AND MATIJEVI~

stabilizing halide ions. As a result the silver halide son are readily destabilized at higher p H values and, moreover, their charge can be reversed (3). The coordination M t h silver is apparently stronger with neutral rather than with protonated chelating agents and this is why the silver halide sols are more easily coagulated at higher p H values. This then explains the difference in the behavior of the two tyophobie sols towards the same solute environment. REFERENCES 1. MATIJEVId, E., AND KOLAK, N., ] . Colloid Interface Sci. 24,441 (1967). 2. LAVZON,R. V., ANDMATXJEVId,E., J. Colloid Interface Sci. (submitted). 3. MATIJEVId, E., :KOLAK,N., AND CATONE, D., J. Phys. Chem. 73, 3556 (1969).

6. FORCE, C. G., AND MATIJEVId, E., Kolloid-Z. Z. Polym. 224, 51 (1968). 7. MAwIzEvId, E., AND FORCE, C. G., Kolloid-Z. Z. Polym. 225, 33 (1968). 8. OTTEWILL,R. H., ANn SHAW, J. N., Discuss. Faraday Soc. 42~ 154 (1966). 9. NEIMAN, P~. E., LYASHENKO, O. A., AND KIRDEEVA, A. P., Kolloid. Zh. 28, 110 (1966);

Engl. transl., pp. 92, 93. 10. NEIMAN, R. E., AND LYASHENKO, 0, ~k., Kolloid. Zh. 27,254 (1965). 11. NEI~AN, R. E. et al., Kolloid Zh. 23, 732 (1961), Engl. transl., pp. 618-623. 12. WENNING, H., Kolloid-Z. 154, 154 (1957). 13. VOWTSKV,S. S., AND PANICH, R. M., Colloid. J. (USSR) 18, 645 (1956); 19,273 (1957). 14. MATIJEVId, E. MATm~I, K. G., OTT~WILL, R. H., AND KERKER, M., J. Phys. Chem. 65,

826 (1961).

4. G~,ENE, B. W., AN]) SAUND]~RS, F. L., or. Colloid Interface Sci. 31, 39 (1969).

15. MATIJEVI~,E., AND STRYKER,L. J., J. Colloid Interface Sei. 31, 39 (1969).

5. TEOT, A. S., AND DANIELS, S. L., Environ. Sci. Technol. 3, 825 (1969).

16. BIB~AV,A., Master's Thesis, Clarkson College of Technology, Potsdam, N.Y. (1971).

Journal of Colloid and Interface Science, VoI.37, No. 2, October1971