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Some remarks on determinations of surface free energy changes of some solids from zeta potential measurements
NOTES Some Remarks on Determinations of Surface Free Energy Changes of Some Solids from Zeta Potential Measurements In the earlier publications (1-7) we presented the results of zeta potential measurements for some solids (sulfur, Teflon, polystyrene, quartz, polypropylene) in water, the dry surface of which was previously wetted with n-alkane or n-alcohol. The wetting of the surface with n-alkane changes the zeta potential value about 20-30 mV in relation to the value of bare surface, at the same pH as that of water. For sulfur this change reaches almost 100 mV. From our measurements on bare sulfur (1) and quartz (heated to 900"C) (3), it appears that in a relatively wide range of moderate pH values of water (without a background electrolyte) zeta potential value is practically unaffected. A similar behavior may be found in the literature for sulfur (8), quartz (9-1 I), polystyrene (12, 13). As the determination of zeta potential by the streaming potential method was conducted at a constant pH, temperature, and ionic composition, the observed changes of zeta potential in water at a natural pH may be ascribed to the present film. As yet we have not proposed any quantitative mechanism for the observed relationship, based on double-layer theory. Some suggestions were made in the published papers (I-7). Because the studied solids are nonionogenic in water (with the exception of heated quartz to some extent), especially after covering the surface with n-alkane, and the ionic strengths are low, the double electric layer at the interface cannot be included into a reversible model, where charges cross the interface. These systems do not achieve electrochemical equilibrium. Nonspecific adsorption of H ÷ and OH- takes place on the hydrocarbon chains (or rather negative adsorption of hydrogen ions) (12, 14). The structure of water neighboring with hydrophobic surface becomes more fixed than that in bulk (15-19) and the water dipoles may be preferentially oriented in relation to the dipoles in the next layer (17-18). The above effects lead to an adsorbed charge (and potential) and, as a consequence balancing the diffuse charge in the liquid. On the basis of Stem's theory: -Cro = o'1 + o'2,
[1]
where tr0 is the charge density on the solid surface, o'1 is the charge density in the Stern layer, and trz is the diffuse charge density. For nonionogenic surface -or l equals o'2 and 00 equals
08, the potential on the solid surface and in the Stern plane, respectively (12, 14). Assuming that zeta potential g = 08, then 00 = ~. On the other hand, the Galvani potential difference A~, is equal to the sum of 00 and AX potential, resulting from permanent dipole orientation (20). A~ = ~o + AX.
[2]
If the potential from the excess charge 00 = 0, then AX = A~. In some papers (21, 22) zeta potential is considered as the sum of A0 and 5 X potentials. = aO + A×.
[3]
When the free energy of the alkane film changes with the film thickness then it seems reasonable to assume that potential will also change. It is also noteworthy that from the studies (23) it appears that in solutions below 10-4 mole/dm a the compact layer is not formed completely. The above considerations show possible reasons for changes of zeta potential of the solid after covering their surface with n-alkane. In the light of the stated changes of zeta potential value of the solids the question arises about a possible dependence between zeta potential and specific volume of n-alkane film. To test whether such a dependence exists both parameters must be determined. The determinations of the specific film volumes were possible by applying the technique (1-7), in which the needed volumes of alkane were dosed from a Hamilton microsyringe into glass ampoules containing cooled powdered samples of the tested solids. The Hamilton microsyringe assures reproducible dosed volumes. Previously, by an independent adsorption technique (1-7) the BET specific surfaces of the samples tested were determined. The applied wetting technique was verified on sulfur by passing, at a constant flow rate (120 bubbles per rain), the vapor of heptane (in nitrogen) throughout powdered sulfur and then measuring zeta potential in water. Figure 1 show the results for zeta potential changes obtained by both techniques. Curve 1 in this figure shows zeta changes against time of heptane vapor adsorption in minutes, and curve 2 shows the changes determined for samples wetted from the microsyringe (1) relative to the number of statistical monolayers of heptane (1, 2). It is seen
567 0021-9797/82/040567-03502.00/0 Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
568
NOTES
-90
This confirms the validity of Eq. [4] for film pressure determinations in systems with nonionogenic solids in water. Theoretically Eq. [4] may probably be derived from the Lippman equation for a polarizable interface and the Gibbs adsorption equation (26). We suppose that water dipoles' contribution must also be considered here. The solution of this problem is still being studied by us.
2~~1...,,
-80 ~-~0
~.-6o ~-50 -40
REFERENCES
-~0
,'= ,~ e~, ~b ,~ ~ ~ ,,4 ~ :~,;0 FIG. 1. Zeta potential of sulfur covered with nheptane film, in water. Curve 1--relative to time (rain) of heptane vapor adsorption on the dry sulfur samples, Curve 2--relative to statistical monolayers of heptane, the samples wetted from microsyringe in glass ampoules.
that for adsorption up to 24 min both curves coincide. The equilibrium for dynamic conditions (curve 1) is achieved after 120 rain of adsorption and is equivalent to almost four statistical monolayers. A very similar value of heptane adsorption in dynamic conditions was recently determined by a gas chromatography technique for sulfur/n-heptane-vapor system (24). The results in Fig. 1 clearly show that in the tested systems, zeta potential may be used as an "adsorption parameter." This induced us to apply an empirical equation analogous to Gibbs' adsorption equation for determinations of the film pressure rr in the solid/ film/vapor system. The equation used (1-7) was as follows: 7r = VoA
vd In ~,
[41
where R, T are the gas constant and temperature, respectively, v0 is the molar volume of n-alkane, A is the total surface of the sample, ~ is the zeta potential value for bare solid surface, ~ is the zeta potential for the film volume v. The maximum Ir value, depending on the specific film volume, may correspond to spreading wetting or immersional wetting processes (7). The values of zr were determined by graphic integration of Eq. [4] (1-7). Thus determined maximum ~r values were then used for calculations of the dispersion part of the surface free energy of the solid 3~t~,applying the usual method (25). The calculated values for the tested solids agree well with those from contact angle measurements and also with the literature data. Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
1. Chibowski, E., and Waksmundzki, A., J. Colloid Interface Sci. 64, 380 (1978). 2. Chibowski, E., and Waksmundzki, A., J. Colloid Interface Sci. 66, 213 (1978). 3. Chibowski, E., J. Colloid Interface Sci. 69, 326 (1979). 4. Chibowski, E., and Hotysz, L., J. Colloid Interface Sci. 77, 37 (1980). 5. Chibowski, E., Przem. Chem. 59, 557 (1980). 6. Chibowski, E., Przem. Chem. 59, 606 (1980). 7. Chibowski, E., and Holysz, L., J. Colloid Interface Sci. 81, 8 (1981). 8. Healy, T. W., and Maignard, M. S., in "Flotation--A. M. Gandin Memorial" (M. C. Fuerstenanu, Ed.), Vol. 1, Chap. 9, AIME, New York, 1976. 9. Sidirowa, M. P., Siemina, L. A., et al., Kollod. Zh. 38, 722 (1976). 10. Jednacfik, J., Prawdi~, V., and Heller, W., J. Colloid Interface Sci. 49, 16 (1974). I1. Laskowski, J., and Kitchener, J. A., J. Colloid Interface Sci. 29, 670 (1969). 12. Show, D. J. in "Introduction to Colloid and Surface Chemistry," pp. 134, 151. Butterworths, London, 1970. 13. Norde, W., and Lyklema, J., J. Colloid Interface Sci. 66, 257-295 (1978). 14. Parreira, H. C., and Schulman J. H., in "Solid Surface and the Gas-Solid Interface" (R. F. Gould, Ed.), p. 260. Advances in Chemistry Series, No 33, Amer. Chem. Soe., Washington D. D., 1961. 15. Nemethy, G., and Sharaga, H. A., J. Phys. Chem. 66, 1773 (1962). 16. Nemethy, G., and Sheraga, H. A., J. Chem. Phys. 36, 3401 (1962). 17. Herman, R. B., J. Phys. Chem. 75, 363 (1971). 18. Herman, R. B., J. Phys. Chem. 76, 2754 (1972). 19. Drost-Hansen, W., Ind. Eng. Chem. 61, 10 (1969). 20. Sonntag, H., and Pilgrimm, H., Prog. Colloid Polym. Sci. 61, 87 (1976). 21. Kumar, R., J. Colloid Polym. 257, 95 (1979). 22. Rastagi, R. P., and Ram Shabad, J. Phys. Chem. 81, 1953 (1977).
NOTES 23. Kreker, M. (Ed.), "Surface Chemistry and Colloids," Vol. 7, p. 171. Butterworths, London/ Boston, 1975. 24. Chibowski, E., Bilifski, B., Waksmundzki, A., and W6jcik, W., J. Colloid Interface Sci. 86, PN 5255 (1982). 25. Zcttlemoyer, A. C., in "Hydrophobic Surfaces" (F. M. Fowkes Ed.), p. 1. Academic Press, New York/London, 1969.
569
26. Holly, F. L, J. Colloid Interface Sci. 61, 435 (1977). EMIL CHIBOWSKI Department of Physical Chemistry Institute of Chemistry M. Curie-Skt'odowska University 20-031 Lublin, Poland Received May 20, 1981
Journal of Colloid and Interface Science, Vol.86, No. 2, April1982
[1]
where tr0 is the charge density on the solid surface, o'1 is the charge density in the Stern layer, and trz is the diffuse charge density. For nonionogenic surface -or l equals o'2 and 00 equals
08, the potential on the solid surface and in the Stern plane, respectively (12, 14). Assuming that zeta potential g = 08, then 00 = ~. On the other hand, the Galvani potential difference A~, is equal to the sum of 00 and AX potential, resulting from permanent dipole orientation (20). A~ = ~o + AX.
[2]
If the potential from the excess charge 00 = 0, then AX = A~. In some papers (21, 22) zeta potential is considered as the sum of A0 and 5 X potentials. = aO + A×.
[3]
When the free energy of the alkane film changes with the film thickness then it seems reasonable to assume that potential will also change. It is also noteworthy that from the studies (23) it appears that in solutions below 10-4 mole/dm a the compact layer is not formed completely. The above considerations show possible reasons for changes of zeta potential of the solid after covering their surface with n-alkane. In the light of the stated changes of zeta potential value of the solids the question arises about a possible dependence between zeta potential and specific volume of n-alkane film. To test whether such a dependence exists both parameters must be determined. The determinations of the specific film volumes were possible by applying the technique (1-7), in which the needed volumes of alkane were dosed from a Hamilton microsyringe into glass ampoules containing cooled powdered samples of the tested solids. The Hamilton microsyringe assures reproducible dosed volumes. Previously, by an independent adsorption technique (1-7) the BET specific surfaces of the samples tested were determined. The applied wetting technique was verified on sulfur by passing, at a constant flow rate (120 bubbles per rain), the vapor of heptane (in nitrogen) throughout powdered sulfur and then measuring zeta potential in water. Figure 1 show the results for zeta potential changes obtained by both techniques. Curve 1 in this figure shows zeta changes against time of heptane vapor adsorption in minutes, and curve 2 shows the changes determined for samples wetted from the microsyringe (1) relative to the number of statistical monolayers of heptane (1, 2). It is seen
567 0021-9797/82/040567-03502.00/0 Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
568
NOTES
-90
This confirms the validity of Eq. [4] for film pressure determinations in systems with nonionogenic solids in water. Theoretically Eq. [4] may probably be derived from the Lippman equation for a polarizable interface and the Gibbs adsorption equation (26). We suppose that water dipoles' contribution must also be considered here. The solution of this problem is still being studied by us.
2~~1...,,
-80 ~-~0
~.-6o ~-50 -40
REFERENCES
-~0
,'= ,~ e~, ~b ,~ ~ ~ ,,4 ~ :~,;0 FIG. 1. Zeta potential of sulfur covered with nheptane film, in water. Curve 1--relative to time (rain) of heptane vapor adsorption on the dry sulfur samples, Curve 2--relative to statistical monolayers of heptane, the samples wetted from microsyringe in glass ampoules.
that for adsorption up to 24 min both curves coincide. The equilibrium for dynamic conditions (curve 1) is achieved after 120 rain of adsorption and is equivalent to almost four statistical monolayers. A very similar value of heptane adsorption in dynamic conditions was recently determined by a gas chromatography technique for sulfur/n-heptane-vapor system (24). The results in Fig. 1 clearly show that in the tested systems, zeta potential may be used as an "adsorption parameter." This induced us to apply an empirical equation analogous to Gibbs' adsorption equation for determinations of the film pressure rr in the solid/ film/vapor system. The equation used (1-7) was as follows: 7r = VoA
vd In ~,
[41
where R, T are the gas constant and temperature, respectively, v0 is the molar volume of n-alkane, A is the total surface of the sample, ~ is the zeta potential value for bare solid surface, ~ is the zeta potential for the film volume v. The maximum Ir value, depending on the specific film volume, may correspond to spreading wetting or immersional wetting processes (7). The values of zr were determined by graphic integration of Eq. [4] (1-7). Thus determined maximum ~r values were then used for calculations of the dispersion part of the surface free energy of the solid 3~t~,applying the usual method (25). The calculated values for the tested solids agree well with those from contact angle measurements and also with the literature data. Journal of Colloid and Interface Science, Vol. 86, No. 2, April 1982
1. Chibowski, E., and Waksmundzki, A., J. Colloid Interface Sci. 64, 380 (1978). 2. Chibowski, E., and Waksmundzki, A., J. Colloid Interface Sci. 66, 213 (1978). 3. Chibowski, E., J. Colloid Interface Sci. 69, 326 (1979). 4. Chibowski, E., and Hotysz, L., J. Colloid Interface Sci. 77, 37 (1980). 5. Chibowski, E., Przem. Chem. 59, 557 (1980). 6. Chibowski, E., Przem. Chem. 59, 606 (1980). 7. Chibowski, E., and Holysz, L., J. Colloid Interface Sci. 81, 8 (1981). 8. Healy, T. W., and Maignard, M. S., in "Flotation--A. M. Gandin Memorial" (M. C. Fuerstenanu, Ed.), Vol. 1, Chap. 9, AIME, New York, 1976. 9. Sidirowa, M. P., Siemina, L. A., et al., Kollod. Zh. 38, 722 (1976). 10. Jednacfik, J., Prawdi~, V., and Heller, W., J. Colloid Interface Sci. 49, 16 (1974). I1. Laskowski, J., and Kitchener, J. A., J. Colloid Interface Sci. 29, 670 (1969). 12. Show, D. J. in "Introduction to Colloid and Surface Chemistry," pp. 134, 151. Butterworths, London, 1970. 13. Norde, W., and Lyklema, J., J. Colloid Interface Sci. 66, 257-295 (1978). 14. Parreira, H. C., and Schulman J. H., in "Solid Surface and the Gas-Solid Interface" (R. F. Gould, Ed.), p. 260. Advances in Chemistry Series, No 33, Amer. Chem. Soe., Washington D. D., 1961. 15. Nemethy, G., and Sharaga, H. A., J. Phys. Chem. 66, 1773 (1962). 16. Nemethy, G., and Sheraga, H. A., J. Chem. Phys. 36, 3401 (1962). 17. Herman, R. B., J. Phys. Chem. 75, 363 (1971). 18. Herman, R. B., J. Phys. Chem. 76, 2754 (1972). 19. Drost-Hansen, W., Ind. Eng. Chem. 61, 10 (1969). 20. Sonntag, H., and Pilgrimm, H., Prog. Colloid Polym. Sci. 61, 87 (1976). 21. Kumar, R., J. Colloid Polym. 257, 95 (1979). 22. Rastagi, R. P., and Ram Shabad, J. Phys. Chem. 81, 1953 (1977).
NOTES 23. Kreker, M. (Ed.), "Surface Chemistry and Colloids," Vol. 7, p. 171. Butterworths, London/ Boston, 1975. 24. Chibowski, E., Bilifski, B., Waksmundzki, A., and W6jcik, W., J. Colloid Interface Sci. 86, PN 5255 (1982). 25. Zcttlemoyer, A. C., in "Hydrophobic Surfaces" (F. M. Fowkes Ed.), p. 1. Academic Press, New York/London, 1969.
569
26. Holly, F. L, J. Colloid Interface Sci. 61, 435 (1977). EMIL CHIBOWSKI Department of Physical Chemistry Institute of Chemistry M. Curie-Skt'odowska University 20-031 Lublin, Poland Received May 20, 1981
Journal of Colloid and Interface Science, Vol.86, No. 2, April1982