The zeta potential of muscovite mica: Counterion complexation by a macrocyclic ligand

The Zeta Potential of Muscovite Mica: Counterion Complexation by a Macrocyclic Ligand PETER J. SCALES,* THOMAS W. HEALY, *'1 AND D. FENNELL EVANSt *Department of Physical Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia; and tDepartment of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received May 22, 1987; accepted August 18, 1987 Addition of the macrocyclic potassium complexing agent cryptand [2.2.2] to an aqueous solution in a fiat plate muscovite mica streaming potential apparatus causes marked changes in the measured zeta potential of the surface. The changes were found to be dependent on the solution pH and the concentration of the cryptand. A dramatic change in the zeta potential, from negative to positive, was observed with an increase in the cryptand concentration at low pH whereas a far smaller change was observed at high pH under similar concentration conditions. The charge reversal at low pH is attributed to the strong adsorption of the diprotonated form of the cryptand. At high pH the dominant solution species is a K ÷ cryptate inclusion complex which adsorbs as strongly as the aqueous K ÷ species but does not significantly alter the potential, presumably because of the size of the complexed cation. © 1988AcademicPress,Inc. INTRODUCTION

Muscovite mica provides a molecularly smooth, planar surface on which to perform streaming potential experiments. It has perhaps not been as extensively investigated as many pure oxide systems but it has been studied in detail as a base for "force balance" measurements (1). Both types of experiments give an insight into the potential at or near the mica-solution interface. The potential is known to be largely dependent upon the degree of hydration, hence size, and valency of diffuse layer counterions derived from solution (2, 3). One would expect the addition of a macrocyclic complexing agent capable of "encapsulating" a counterion to have a significant effect on the calculated zeta potential through, if nothing else, the change in size of the counterion species. Addition of a macrocyclic complexing ligand such as a cryptand to a solution of an anionic dialkyl compound (4) produces a dramatic change in the size and type of self-asTo whom correspondence should be addressed.

sembly species in solution. The change is associated with a decrease in the aggregation number of the self-assembly species with the formation of unilamellar vesicles and micelles from larger aggregates. The explanation put forward is in terms of increased head group repulsion due to the ability of the cryptand to complex a monovalent counterion and displace it from the plane of the surfactant headgroup, so limiting its head group repulsion screening ability. Parallels to such phenomena have been drawn with cationic dialkyl surfactants where the size and type of counterion have subtle effects on aggregate size and stability (5). Our purpose in the present study was to compare the effect of the addition of a cryprand to an ionic solution in the presence of a solid substrate (muscovite mica) with previous studies on solution self-assembly species (4). A cryptand was chosen that is known to form large, stable, inclusion complexes with the potassium ion and other cations (6). A cryptand with a high stability constant for the potassium ion was chosen in order that a large percentage of the solution potassium ions would exist in 391

Journal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

0021-9797/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

392

SCALES, HEALY, AND EVANS 40

inclusion complexes at relatively low molar addition ratios of the cryptand. ~,

EXPERIMENTAL

0

SECTION

Materials -40

4,7,13,16,21,24-Hexaoxa- 1,10-diazabicyclo-[8.8.8]hexacosane (cryptand C222) was obtained from Aldrich (>98% pure) and was used as received. Solutions of C222 were protected from light. A structural representation ofC222 is shown in Fig. 1. All other reagents were analytical grade and used as received. Solutions were prepared using "Milli-Q" grade reagent water. Muscovite mica was from Bihar, India and was generously provided by Dr. R. Pashley, ANU, Canberra, Australia.

-80 2

4

6

8

10

pH

l~G. 2. Zeta potential verses pH for 0.001 m o l . d m -3 KCI on brown muscovite mica.

terrupt laminar flow or enhance capillary entrance effects (9, 10). An equilibration time of 30 min was allowed between measurements.

Streaming Potential Measurements RESULTS AND

Two mica sheets (approx 30 #m thick) were glued individually to standard microscope slides (75 X 25 X 1 mm) using an inert wax and then trimmed to size. A thin layer was then cleaved from the surface of each sheet to expose fresh surface and the plates placed in a streaming potential apparatus to form a rectangular capillary. The apparatus, both design and function, will be described fully in a later publication (7). A background electrolyte concentration of 0.001 mol. dm -3 potassium chloride was used throughout. Liquid was forced through the rectangular capillary using nitrogen gas in both directions and the zeta potential calculated from the slope of the pressure versus potential curve using the Smoluchowski relationship (8). A maximum pressure of 5.0 kPa was used so as not to in-

DISCUSSION

The zeta potential of muscovite mica as a function of pH at a background electrolyte concentration of KC1 of 0.001 mol. dm -3 is shown in Fig. 2. The data are similar in form to streaming potential results for a silica capillary system (3, 11) and show reasonable agreement at pH 5.6 + 0.2 with potential values calculated using a force balance technique (2). The zeta potential of mica at constant ionic strength in solutions with pH values in the range 6 to 10 is seen to be quite constant. Cryptand C222 is known from a previous study (6) to show a number of ionic forms as a function ofpH. The molecule has two amine functional groups with pKa's of 7.28 and 9.60. It forms an inclusion complex with cationic metal ions at high pH (i.e., KC222 +) and has a stability constant (PKstab) of 5.4 with the potassium ion. At low pH (pH's ~< 5), C222 exists /'--N predominately as a divalent "free" ligand (H2C2222+). At pH values between 7 and 9, the concentration of the monovalent free lis~ gand (HC222 +) reaches a maximum but this species is never dominant. There is no evi\ / dence to suggest that inclusion complexes ocFIG. 1. Structural representation of cryptand C222 as cur with the monovalent and divalent free deduced from molecularmodelsof the longchain analog forms of the ligand. Speciation equilibria for the cryptand in solution are as follows: as cited in Ref,(17).

I

Journal of Colloid and InterfaceScience, Vol. 124, No. 2, August 1988

ZETA POTENTIAL OF MICA -2

K÷ '

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,,

,,

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,,

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14

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M o l a r Ratio of C 2 2 2 / K +

FIG. 3. Log~o(concentration cryptatespeciesin solution) as a function of the molar ratio of C222/K÷ for a KC1 concentration of 0.001 tool. dm-3 at pH 5, 7, and 10.2. H2C2222+ *--*HC222 + + H +

(pKal = 7.28)

HC222 + *--*C222 + H +

(pKa2 = 9.60)

KC222 + *--' HC222 + K +

(pKstab =

5.4).

Calculated distribution curves for each species at a background potassium ion concentration of 0.001 tool. d m -3 as a function of the molar ratio of C222/K ÷ at p H 5, 7, and 10.2 are shown in Fig. 3. The curves show H2C2222÷ and K + to be the predominant counterions at p H 5 and KC222 ÷ and K + the predominant counterions at p H 10.2. Complexation of the K ÷ ion at p H 10.2 is clearly observed with the KC222 ÷ species becoming dominant at molar ratios of C222/K ÷ > 0.5. Relative concentrations of the divalent ligand H2C2222÷ are greatly reduced above p H 7. The zeta potential of mica on addition of C222 at both high (>~10.2) and low ( ~ 5 ) p H values are shown in Fig. 4. At high p H the zeta potential is observed to increase slightly (i.e., become less negative) as the molar ratio of

393

C222/K + increases while a dramatic increase is observed at p H 5. The p H 5 curve is indicative that the presence of very small amounts of divalent ligand (i.e., <2 X 10 -5 M ) increases the zeta potential dramatically. This latter trend is in line with observations from force balance measurements (12, 13) and previous streaming potential studies of the addition of di- and trivalent metal ions to mica (3). The zeta potential as a function of p H for the addition of C222 to mica at p H 5 and at variable p H (i.e., at pH's intermediate between 5 and 10), is shown in Fig. 5. Arrows show the direction of the experiment and the p H of each data point is also included. At variable p H (lower curve), the addition of C222 results in an increase in solution p H from 7.0 to 10.1. The zeta potential is observed to increase quite dramatically but then decrease at higher p H and higher molar ratios of C222/K ÷. The relative concentrations of counterion species change markedly in this p H regime. For example, the concentration of the H2C2222÷ ion shows complex behavior. Initially, the concentration increases (pH 7.0-7.9), then decreases (pH 7.9-8.7), increases (pH 8.7-9.2), and finally decreases (pH 9.2-10.1). Addition of acid to the system at this point (pH 10.1) to lower the p H to 5.0 produces a surface of similar potential to that observed under fixed p H conditions. This behavior shows the pres-

40

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,

,

,

,

,

,

,

0

2 -4o pH_> 10.2 -80

.4

.8

1.2

1.6

M o l a r Ratio of C222/K+

FIG. 4. Zeta potential verses the molar ratio C222/K+ for a KC1 concentration of 0.001 mol. d m -3 o n brown muscovite mica at pH 5.0 ___0.2 (O) and at pH >/ 10.2 (O). Journal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

394

SCALES, HEALY, AND EVANS 40 o

5.0

pH = 5 . 0 8 . 2 7 " 3

0

~. -40 ~7.9

.80 ¸

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.8

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Molar Ratio of C222/K*

FIG. 5. Zeta potential verses molar ratio of C222/K ÷ for a KC1 concentration of 0.001 m o l . d m -3 on brown muscovite mica at pH's as indicated. Arrows show the direction of the experiment.

ence of the divalent ion H2C2222+ to be the most important controlling factor on the zeta potential of mica as a function ofpH and concentration. A plot of zeta potential versus -log10 (concentration of H2C2222+) is shown in Fig. 6. The effect of the H + cation under constant ionic strength conditions is shown as a dashed line on the same curve. This allows comparison of the effect of the H2C2222+ ion to the potential determining H + ion on mica. The direction of each experiment is indicated as well as the pH where applicable. Under fixed pH conditions where an increase in the molar ratio of C222/K + produces an increase in H2C2222+ concentration, the divalent ligand is observed to show similar potential determining characteristics as the H + ion. At variable pH, where the H2C2222+ ion concentration both increases and then decreases with C222 addition, deviations from the fixed pH curve are significant. Only data where the concentration of H2C2222+ is increasing are shown in Fig. 6. The data suggest the H2C2222+ ion to be specifically adsorbed. Force balance measurements on mica using the same solution conditions as in this work (14) show that at pH's >i 10.2, the addition of C222 produces an increase in the thickness of the "Stem" layer from 5 to 12 A. This coincides with the adsorption of a single layer of molecules of C222 to the mica surface and Journal of Colloid and Interface Science, Vol. 124, No. 2, August 1988

compares well with the expected diameter as deduced from Fig. 1. Such an increase in size would be expected to move the "plane of shear" further from the mica surface and hence decrease the value of the zeta potential. Similarly, an increase in metal ion size has been associated with an increase in the observed potential at constant ionic strength for both streaming potential and force balance experiments (2, 3). Similar work by Claesson et al. (15) using large monovalent cations on mica has shown that the adsorbed size of the cation would need to be greater than that of the tetrapentylammonium ion to induce a significant increase in the surface potential. At the risk of assuming that C222 adsorbs quite strongly to the mica surface and that results may be compared under slightly different ex•perimental conditions, force balance results from Miller et al. (14) give a potential of -95 mV (pH 10.2) for C222 while results from Claesson et al. give a value o f - 1 0 5 mV (pH 5.7) for tetrapentylammonium ions on mica. Using the argument of the latter authors, one could conclude that the C222 molecule occupies a site area of similar magnitude (i.e., 106 ~2). Physical constraints preclude a much smaller number and surface conductance effects in streaming potential measurements at lower ionic strengths limit a more detailed analysis. Simplistic modeling shows that the 40 • Variable pH o pH =5

•

\\

~

• pH-> 10.2

-40

-80 2

4

6

8

10

-Log lo (concentration of H2C222 ++)

FIG. 6. Zeta potential verses -loglo (concentration of the divalent ligand H2C2222+)for fixed and variable pH conditions. Arrows show the direction of the experiment. Dashed lines show the effect of pH (from Fig. 2).

ZETA POTENTIAL OF MICA zeta potential can be expected to show little or no change. The observed small increase in potential on cryptate addition m a y be due therefore to a cation size effect or due to very small concentrations o f the specifically adsorbed H2C2222÷ ion. Variations o f a few millivolts between experiments in the potential in the absence o f C222 was quite c o m m o n and is in line with observations o f previous studies using muscovite mica (3). Comparisons o f the addition o f C222 to mica with addition to an anionic dialkyl c o m p o u n d (4) at molar ratios o f C 2 2 2 / K ÷ in excess of 0.6 can now be made in light o f the observed behavior in the mica system. As m e n t i o n e d previously, anionic dialkyl c o m p o u n d s appear to show a dramatic decrease in aggregate size with the addition o f C222. This is evidenced by the formation o f unilamellar vesicles and micelles f r o m larger self-assembly structures. Such a decrease in aggregate size is usually associated with a substantial increase in head group repulsion and hence "surface potential" in the surfactant head group plane. Results for mica in the presence o f C 2 2 2 show little or no evidence o f such p h e n o m e n a . A n o t h e r explanation m a y be in terms o f the work of Quintela et al. (16) who considered the effect o f C222 on the micellization behavior o f anionic surfactants. T h e y f o u n d that the average location o f the C222 molecule, as a counterion at the surface, was below rather than in the plane o f the surfactant head groups. The proposed m o d e o f action was that o f increasing micelle curvature and micelle stabilization by decreasing repulsion between head groups. O u r data, while not fully supporting either mechanism, confirms the latter to be m o r e reasonable. CONCLUSION The results obtained for the addition o f C222 to m i c a in the presence o f the m o n o valent metal ion potassium m a y be explained in terms o f the ability o f C 2 2 2 to f o r m m o n o valent and divalent free ligands and m o n o valent metal ion inclusion complexes in so-

395

lution as a function o f pH. The potentials measured are consistent with those calculated from force balance measurements if the effect o f ion size and shear plane displacement are considered. T h e divalent f o r m o f the C222 ligand is observed to adsorb quite strongly to the muscovite mica surface. ACKNOWLEDGMENTS The authors acknowledgethe support of the Australian Research Grants Commission. P. J. S. acknowledgesreceipt of a Commonwealth Post Graduate Research Award from the Australian Government. D. F. E. acknowledgesreceipt of a Willsmore Fellowship from the University of Melbourne. REFERENCES 1. Israelachvili, I. N., and Adams, G. E., J. Chem. Soc. Faraday Trans. 1 74, 975 (1978). 2. Pashley, R. M., ,L Colloid Interface Sci. 83(2), 531 (1981). 3. Lyons, J. S., Furlong, D. N., and Healy, T. W., Aust. J. Chem. 34, 1177 (1981). 4. Miller, D. D., Evans, D. F., Warr, G. G., Bellare, J. R., and Ninham, B. W. J. Colloid Interface Sci. 116(2), 598 (1987). 5. Brady, J. E., Evans, D. F., Warr, G. G., Grieser F., and Ninham, B. W., J. Phys. Chem. 90(% 1853. (1986). 6. Lehn, J. M., and Sauvage, J. P., J. Amer. Chem. Soc. 97, 6700 (1975). 7. Scales P. J., White, L. R., and Healy, T. W., to be published. 8. Overbeek, J. Th., in "Colloid Science" (H. R. Kruyt, Ed.), Vol. 1. Elsevier, Amsterdam/London, 1952. 9. Van Wagenen, R. A., and Andrade, J. D., J. Colloid Interface Sei. 76(2), 305 (1980). 10. Bowen, B. D., J. Colloid Interface Sci. 106(2), 367 (1985). 11. Wiese,G. R., James, R. O. and Healy, T. W., Discuss. Faraday Soc. 52, 302 (1971). 12. Pashley, R. M., J. Colloid Interface Sci. 102(1), 23 (1984). 13. Pashley, R. M. and Israelachvili, J. M., J. Collidlnterface Sci. 97(2), 446 (1984). 14. Miller, D. D., Evans, D. F., Warr, G. G., Bellare, J. R., and Ninham, B. W., to be published. 15. Claesson, P., Horn, R. G., and Pashley, R. M., J. Colloid Interface Sci. 100(1), 250 (1984). 16. Quintela, P. A., Reno, R. C. S., and Kaifer, A. E., J. Phys. Chem. 91, 3582 (1987). 17. Castaing, M., Morel, F., and Lehn, J.-M., J. Membr. Biol. 89, 251 (1986). Journal of Colloid and lnterface Science, Vol. 124, No. 2, August 1988