Langmuir 1988,4, 469-474 been suggested. In anhydrous salts, such as K,Fe(CN),, the reduction occurs above 200 "C and produces K4Fe(CN),, KCN, and C2Nzgas.29 When water is present in the solid, as in the case of the lithium salt of ferricyanide, the reduction occurs in the 100-160 "C range and has the pentacyanoaquo ferrate(II1) ion as an intermediate. The increased reduction seen on alumina a t elevated temperature, therefore, may also be due to catalyzed reduction of the type seen in pure solids and not requiring hydroxyl ion.
Conclusions The adsorption of potassium ferricyanide on three commercial alumina samples having surface areas of the order of 100 m2/g and pore volumes of about 0.3 mL/g was studied. We find that a significant fraction of the oxidesupported material originates from solution held in the pores after filtration. Thus, spectral and analytical studies
469
performed on the dried oxides are quite misleading. In the case of all the alumina types studied, a significant amount of reduction occurs during the drying process, with very little release of ferrocyanide into solution during the adsorption process. Several possible mechanisms for the reduction of ferricyanide were presented. While reduction by hydroxyl ion probably does occur, other reduction processes similar to those occurring in crystalline salts of ferricyanide also may be significant. The thermally induced reduction of alumina-supported ferricyanide begins near 40 "C while the reduction of alkali metal salts of ferricyanide occurs above 100 OC.
Acknowledgment. We gratefully acknowledge the National Science Foundation and the Division of Materials Research for their support in the form of grants DMR8414566 and DMR-8320556.
Surface Charge Development at the Goethite/Aqueous Solution Interface: Effects of C 0 2 Adsorption Walter A. Zeltner and Marc A. Anderson**+ Water Chemistry Program, University of Wisconsin, Madison, Wisconsin 53706 Received July 16, 1987. I n Final Form: October 14,1987 The effect of C02adsorption on the zero point of charge (ZPC) of aqueous goethite (a-FeOOH) suspensions was studied for both unpurged and N2-purgedsuspensions. Unpurged goethite had a ZPC of 8.1 f 0.1, while goethite purged for 2 months displayed a change in ZPC to 9.0 f 0.3. Electrophoretic mobility measurements performed on both suspensions gave identical isoelectric points of 9.7 f 0.2. These results were explained as being due to C02adsorption in the groove on the goethite (100)face, a conclusion supported by cylindrical internal reflection-Fourier transform infrared spectroscopic studies. Also, the measured goethite ZPC would depend on the relative contribution made to the total goethite surface area by the (100)face. Because goethite grows in the [001] direction, the (100) face makes a smaller contribution to the overall surface area in goethite samples with a higher surface area if the other dimensions remain constant. Thus, one would expect to observe higher ZPC values for higher surface area samples. This conclusion is qualitatively consistent with results of other studies.
Introduction Experimental observations of physicochemical phenomena occurring at solid/aqueous interfaces are more readily interpreted if the solid surfaces are "pristine" in the sense that all adsorbed species are known to the investigator. For studies of aqueous/metal oxide interfaces, it has been suggested that a sensitive test for the absence of adsorbed species (other than the potential-determining H+ and OHions) is to obtain agreement between the zero point of charge (ZPC) measured by potentiometric titration and the isoelectric point (IEP) measured by electrophoretic m~bility.l-~The application of this agreement between ZPC and IEP as a purity test for metal oxide suspensions is gaining wider a~ceptance."'~ Because of its importance in soil fertility, in corrosion control, and as a magnetic tape precursor, goethite (aFeOOH) has often been used as a model colloid in electrical double layer and adsorption s t ~ d i e s . ~ ~ ' ~Goethite J " ~ ~ is readily synthesized in crystalline and has very low s ~ l u b i l i t y .In ~ ~many of these studies, the different goethite samples were characterized by their ZPC t Current address: Water Chemistry Program, University of Wisconsin-Madison, 660 N. Park St., Madison, WI 53706.
values-values which ranged from 7.020 to 8.6.19 When both ZPC and IEP values were given, reasonable agree(1)Parfitt, G.D. Progress in Surface and Membrane Science; Cadenhead, D. A., Danielli, J. F., Eds.; Academic: New York, 1976; Vol. ll, p 181. (2)Hunter, R.J. Zeta Potential in Colloid Science;Academic: New York, 1981;p 229. Parks, G. A. Surf. Colloid Sci. 1982,12, 119. (3)James, R. 0.; (4)Lyklema, J. Adsorption from Solution at the SolidlLiquid Interface; Parftt, G. D., Rochester, C. H., Eds.;Academic: New York, 1983; p 223. (5)Kavanagh, B. V.; Posner, A. M.; Quirk, J. P. Chem. SOC.Faraday Discuss. 1975,59,242. (6)Kavanagh, B. V.; Posner, A. M.; Quirk, J. P. J.Colloid Interface Sci. 1977,61, 545. Stiglich, P. J.; Healy, T. W. Adsorption from (7)James, R. 0.; Aqueous Sohtions; Tewari, P. H., Ed.; Plenum: New York, 1981;p 19. (8)Sprycha, R. J. Colloid Interface Sci. 1984,102, 173. (9)Barringer, E. A.; Bowen, H. K. Langmuir 1985,1, 420. (10)Hsi, C.-K. D.;Langmuir,D. Geochim. Cosmochim. Acta 1985,49, 1931. (11)Sprycha, R. J. Colloid Interface Sci. 1986,110, 278. (12)Kallay, N.; Babic, D.; Matijevic, E. Colloids Surf. 1986, 19,375. (13)Jang, H. M.;Fuerstenau, D. W. Colloids Surf. 1986,21, 235. (14)Penners, N. H. G.; Koopal, L. K.; Lyklema, J. Colloids Surf. 1986, 21, 457. (15)Atkinson, R.J.; Posner, A. M.; Quirk, J. P. J.Phys. Chen. 1967, 71, 550. (16)Hingston, F. J.; Posner, A. M.; Quirk, J. P. J.Soil Sci. 1972,23, 177.
0743-746318812404-0469$01.50/0 0 1988 American Chemical Society
Zeltner and Anderson
470 Langmuir, Vol. 4, No. 2, 1988
ment was observed e.g., Kavanagh et a1.6 reported a ZPC of 8.2 f 0.3 with an IEP of 8.0 f 0.3 while Hsi and Langmuir’O observed a ZPC of 8.5 f 0.3 with an IEP of 8.9 f 0.2. When we measured ZPC and IEP values for a goethite sample prepared in our laboratory, the initial measurements indicated that the IEP was more than 1 pH unit higher than the ZPC. Although this disparity has usually been interpreted as an indication that specific cation adsorption has occurred,4’14Evans et al.19 had observed that nitrogen purging of stock goethite suspensions for 2 months raised the ZPC at least half a pH unit. They attributed this increased ZPC to desorption of C02from their goethite suspensions during the nitrogen purging. However, Evans et al. did not report electrophoretic mobility measurements. Since our goethite systems had been prepared with careful attention to cleanliness, we did not expect significant levels of cation contamination. As a result, it seemed appropriate to conduct a further study of the effects of nitrogen purging on possible COz adsorption in our goethite suspensions. This was especially important since our stock suspensions of goethite in water displayed pH values near 8.7.
n
(17)Yates, D. E.Ph.D. Thesis, University of Melbourne, 1975. (18)Balistrieri, L. S.;Murray, J. W. ACS Symp. Ser. 1979,93,275. (19)Evans, T.D.;Leal, J. R.; Arnold, P. W. J. Electroanal. Chem. 1979,105,161. (20)Sigg, L.; Stumm, W. Colloids Surf. 1980,2, 101. (21)Hansmann, D. D.; Anderson, M. A. Enuiron. Sci. Technol. 1985, 19,544. (22)Anderson, M. A.; Tejedor-Tejedor, M. I.; Stanforth, R. R. Enuiron. Sci. Technol. 1985,19,632. (23)Machesky, M. L.;Anderson, M. A. Langmuir 1986,2,582. (24)Zeltner, W.A.;Yost, E. C.; Machesky, M. L.; Tejedor-Tejedor, M. I.; Anderson M. A. ACS Symp. Ser. 1986,323,142. (25)Atkinson, R. J.;Posner, A. M.; Quirk, J. P. J.Inorg. Nucl. Chem. 1968,30,2371. (26)Fey, M. V.;Dixon, J. B. Clays Clay Miner. 1981,29,91. (27)Goodman, B. A.; Lewis, D. G. J. Soil Sci. 1981,32,351. (28)Van der Woude, J. H. A.; de Bruyn,P. L. Colloids Surf. 1984,12, 179. (29)Ainsworth, C.C.;Sumner, M. E.; Hurst, V. J. Soil Sci. SOC.Am. J. 1985,49,1142. (30)Hsu, P. H.; Marion, G. Soil Sci. 1985,140,344. (31)Chernoberezhskii, Yu. M.Surf. Colloid Sci. 1982,12, 359.
UNPURGED GOETHITE IONIC STRENGTH,
2
3
.001 M
75
*
.01 M .l M
z 50 0 ;1 25 P
8
0
v)
2 -25 t
I
-50 -75
5
6
E
7
9
10
PH
Figure 1. Goethite titrations in NaC104with no nitrogen purging. s125
inn
k\
NINE WEE K PURGE IONIC STRENGTH,
--I \ ‘ 1
.001 M .01 M
a
8 0 vi 2 -25
Materials and Methods Goethite was prepared in plastic vessels from ACS grade ferric nitrate (Fe/OH = 2). This solution was aged 2 days at room temperature and then alkalized to pH 12.5 with NaOH. The resulting precipitate was aged for 6 more days at 60 0C.2s This suspension was freeze-dried after repeated washings with ionexchanged, 0.2-wm-filtered Milli-& (Millipore Corp.) water gave a constant conductivity rinse water. Conventional transmission electron micrographs showed acicular particles with average dimensions about 50 X 15 nm. Taking the third dimension as 10 nm, good agreement is obtained with the measured nitrogen BET surface area of 80 m2 g-l. X-ray diffraction confirmed that crystalline goethite was present and that no noticeable impurities were detectable. Stock suspensions (3&35 g dm-3) were prepared as needed by resuspending the freeze-dried material in Milli-Q water, using sonication to ensure good dispersal, and then storing the suspensions in plastic containers. These suspensionswere then placed in a glovebox and purged with prepurified, NaOH-washed, humidified nitrogen while being stirred for various time periods. One stock suspension was kept in a closed container outside the glovebox for purposes of comparison. Potentiometric titrations were also performed in the glovebox, by using a Mettler DL40 MemoTitrator system with a DG112 combination glass electrode (A0.2-mV sensitivity). Suspension concentrations were kept low to minimize junction potential Titrations were performed under a humidified nitrogen purge in individual polypropylene cups holding 60 mL of
125
>m 100
‘ I-75 -50
‘
1 5
6
8
7
9
10
PH
Figure 2. Goethite titrations in NaC104 after nitrogen purging for 9 weeks. suspension (suspensionconcentration during titration was between 3.0 and 3.5 g/dm3), using NaN03 or NaC104 as supporting electrolyte. Small amounts of the appropriate acid or base (0.15 mL or less of the 0.02 mol dms titrants) were added to the suspensions, which were then permitted to equilibrate overnight before titration. Possible effects of slow surface reactions were minimized as the suspension was always equilibrated within 0.4 pH units of the eventual ZPC, and so-called “fast” titrations were performed with titrant additions made no more than 5 min All titrations were performed by initially adding NaOH until pH 10 and then adding either HN03 or HCIOl until pH -6. Every titration was completed in 2-3 h. Even over this time frame, noticeable titration hysteresis was observable, so that only the acid titrations were analyzed by theoretically subtracting possible supernatant reactions to obtain titration curves.33 Electrophoretic mobility was measured by using a PenKem System 3OOO electrokineticsanalyzer, an instrument which require accurate cell positioning to avoid additional contributions to the measured particle mobility from solvent electroosmosis. The accuracy of the cell position was checked by performing cell calibrations with both the goethite sample and monodisperse latex particles. These calibrations verified that the velocity profile across the cell was parabolic and that particle mobilities were identical at the front and back stationary layers in the cylindrical cell? Changes in the position of the stationary layer due to buildup of particles on the interior cell wall were minimized by thorough cleaning of the cell with mineral acid and base washes after each experiment. For the mobility titration experiments, goethite suspensions were titrated in a nitrogen-purged plastic titration vessel. A Radiometer GK2321C combination electrode was utilized for pH measurement. Dilute suspensions (50-100 mg dm4) were prepared from either purged or unpurged stock suspensions. Additions of base were made by using C02-free NaOH taken from the glovebox and quickly added to a purged supporting electrolyte solution to minimize C02 uptake in the base.3a Systems were preequilibrated overnight near pH 9. Titrant additions were
-
(32)Onoda, G.Y., Jr.; de Bruyn, P. L. Surf. Sci. 1966,4,48. (33)Zeltner, W. A. Ph.D. Thesis, University of Wisconsin-Madison, 1986. (34)Hansmann, D. D.Ph.D. Thesis, University of Wisconsin-Madison, 1985.
GoethitelAqueous Solution Interface
8 -1.0 -2. 0
T I T R A T I O N S FROM pH 9 I N NaClO4
Figure 3. Goethite mobility titrations in NaC104 on purged samples preequilibrated overnight near pH 9.
spaced 30 min to 1 h apart. All measurements were performed at 25 & 1 “C.
Results ZPC Determined by Potentiometric Titration. Figure 1shows the potentiometric titration behavior of an unpurged goethite sample stored outside the glovebox, with NaC104 used as the supporting electrolyte. A common intersection point is observed, which would appear to indicate that the system ZPC is at pH 8.1 f 0.1. (The ordinate has not been converted to “surface charge” because a single, well-defined intersection point was not obtained for all the potentiometric titrations. Because a well-defined intersection is needed to determine the zero surface charge reference condition, all titration curves are reported as “micromoles H+ adsorbed”.) When a similar goethite sample is purged with humidified nitrogen and stirred, the behavior of the sample changes over time. Figure 2 shows the potentiometric titration of goethite after 9 weeks of purging. During this period, the intersection points of the titration curves are observed to move to higher pH values for longer purging times. However, purging for longer than 9 weeks has little further effect on the intersection points. These results agree with the observations of Evans et al.19 Moreover, these intersection points separate from one another at different ionic strengths, and the amount of separation appears to increase as the purging time increases. Similar effects are observed when NaN03 is employed as the supporting electrolyte, although the proton adsorption density at a given pH is slightly higher in NaN03. IEP Determined by Electrophoretic Mobility. Figure 3 shows the electrophoretic mobility behavior of a goethite sample which had been purged for 2 months. The IEP is at pH 9.7 f 0.2. Note that even though goethite mobility values are well separated with ionic strength through the neutral pH region, these values approach each other once the system is acidified to below pH 5.5. Similar behavior is again observed with NaN03 as the supporting electrolyte. Figure 3 shows a small shift in the IEP to a higher pH value as the ionic strength of NaC10, increases, but this behavior is not observed in NaN03 and is not reproducible in NaC104. For these reasons, variations in IEP at different supporting electrolyte concentrations have been attributed to instrumental error. In contrast to the potentiometric titration behavior described above, nitrogen purging does not affect the IEP in these systems. For the unpurged goethite sample stored outside the glovebox, the measured IEP is 9.7 f 0.1 in 0.01 mol dm-3 NaN03. Attempted Verification of C 0 2 Adsorption. The previous results would appear to indicate that C02 contamination affects the behavior of unpurged goethite suspensions. In order to confirm this hypothesis, further experiments were undertaken which were designed to
Langmuir, Vol. 4, No. 2, 1988 471 eliminate the possibility of other sources of cation contamination. (R) After many weeks of purging in a nitrogen atmospLGre, the goethite suspension will be highly anoxic. This condition might cause reduction of Fe(II1) to Fe(I1). This possibility was studied by purging a stock goethite suspension with a C02-freemixture of 80% nitrogen and 20% oxygen (“Medical Breathing Air”). Titration curves obtained from samples purged for 1 or 2 months in this oxidizing atmosphere displayed pH values at the intersection points that were almost identical with those shown earlier for titrations performed in the reducing atmosphere of a nitrogen purge. Thus, if reduction of Fe(II1) to Fe(I1) occurs, it does not significantly affect the potentiometric titration behavior of goethite. (b) Because trace levels of ammonia are present in the environment, NH4+adsorption on the original goethite is possible. Nitrogen purging might remove adsorbed NH4+ cations, which would be expeded to increase the pH of the titration intersection points: as observed experimentally. This possibility was studied by exposing a purged goethite suspension to an atmosphere of pure C02 for 5 days. If the intersection points showed no further change, then NH4+adsorption (or adsorption of some volatile cation) would-be indicated. A shift in the intersection points back near pH 8.1 would suggest C02 adsorption. One complication arises, however, since a goethite suspension at pH -8.7, when exposed to C02at about atmospheric pressure, would be expected to have high levels of carbonate and bicarbonate ions present in the supernate. Subsequent adsorption of these anions on the goethite could affect the ZPC. This possibility was minimized by immediately centrifuging the suspension after exposure to COz, discarding the remaining supernate, and replacing the discarded material with Milli-Q water (purged with nitrogen for 4 days). Titrations were performed after two such replacements of the supernatant liquid. A common intersection point was obtained at pH 8.2 0.1. This result strongly supports the hypothesis that C02 adsorption significantly affects the goethite ZPC. (c) Cation contamination from the purging system itself might occur because at one stage the humidified nitrogen stream is passed through copper tubing. In order to check for copper contamination, an aliquot of a suspension which had been purged for 2 months was acidified and filtered. The filtrate was analyzed by atomic absorption spectroscopy and found to have no detectable copper present. Other contaminants might also be carried over from the purging system. This possibility was studied by using the same supernatant replacement technique described above on another suspension that had been purged for 2 months. When this system was titrated, results similar to those for the 9-week-purged system were obtained. This would imply that there is minimal carryover of contaminants from the nitrogen purge system. (d) A simple comparison of purged and unpurged goethite suspensions was performed with a cylindrical internal reflectance-Fourier transform infrared spectroscopy system.35 The results are shown in Figure 4. Unpurged goethite exhibits two peaks at 1502 and 1340 cm-l (4-cm-’ resolution) which are not present in purged goethite. Although a detailed peak analysis was not performed, these peaks correspond closely with the peaks assigned by Russell et al.36 to adsorbed COz. However, the large peak near 1300 cm-l and the small peak at 1055 cm-’, which
*
(35) Tejedor-Tejedor, M.I.; Anderson, M. A. Langmuir 1986,2, 203. (36)Russell, J. D.; Paterson, E.; Fraser, A. R.; Farmer, V. C. J. Chem. SOC.,Faraday Trans. 1 1975,71,1623.
Zeltner and Anderson
472 Langmuir, Vol. 4, No. 2, 1988
n
GOETHITE
Figure 4. CIR-FTIR spectra of purged and unpurged aqueous goethite suspensions. 35-40 g dm-3suspension concentration;pH 8; no supporting electrolyte used.
A
[I 001
B
C
A
B
C
-
D18
Figure 5. Structure of goethite (100)face.
Russell et al. also reported, are not visible in Figure 4.
Discussion Introduction. If nitrogen purging causes desorption of some form of CO, from goethite, two questions arise. First, why does this desorption raise the apparent ZPC of goethite? Desorption of carbonate or bicarbonate ions should lower the ZPC?20 while desorption of neutral C02 molecules should not affect the ZPC. Second, why is the IEP of goethite unaffected by this desorption? In order to answer these questions, one needs to consider the surface structure of crystalline goethite. Figure 5, based on the diagram given in Russell et shows the (100) face in plan and in section, with both oxide and hydroxide ions indicated by large circles. Fe(II1) ions are shown as small circles. The plan view looks down on the (100)face and shows a trough lying between the C-type surface hydroxyls, which form bidentate bonds to bulk Fe(II1) ions, and the A-type surface hydroxyls, which form monodentate bonds to Fe(II1). The section view looks directly at the (001) face, giving a side view of the trough on the (100) face. Russell et al.= studied COz adsorption on thin goethite films using transmission IR spectroscopy. They proposed that CO, adsorbed by forming a coordinate bond between the oxide ion labeled C', located in the trough of the (100) face in Figure 5, and the carbon atom of the COz molecule, which acts as a Lewis acid site, to produce a monodentate carbonate ion in the trough. Other studies have also indicated that COz adsorbs on iron oxides primarily as monodentate carbonate ions,38,39 but these studies do not (37) Russell, J. D.;Parfitt, R. L.; Fraser, A. R.; Farmer, V. C. Nature (London) 1974, 248, 220.
specify an adsorption site. In order to produce a highly charged carbonate ion in the trough, a large amount of negative charge must be donated to the C02molecule from the goethite crystal. This process would require electron withdrawal from nearest-neighbor oxygen atoms in the crystal, leaving a partial positive charge on the goethite surface hydroxyl groups. This proposed mechanism can be used to answer the questions raised earlier. Variable ZPC. Why does COz adsorption lower the apparent ZPC? The charge redistribution which occurs when COz adsorbs in the trough will directly affect the ability to protonate the surface, thus affecting the ZPC, so that a change in ZPC might be expected. However, this change in ZPC can be discussed in two ways, either in terms of chemical bonding effects or in terms of charge development at or near the surface. If chemical bonding is considered, then the electron withdrawal from nearest neighbors which must occur when COz adsorbs would weaken the surface hydroxyl bonds, making these groups more acidic. However, the effect on the adsorbed carbonate is more difficult to determine because this group undergoes considerable hydrogen bonding with surface hydroxyl groups. Russell et al.36 proposed that the adsorbed carbonate forms hydrogen bonds with A-type surface hydroxyl groups, although hydrogen bonding to C-type groups might also occur. The effects of this hydrogen bonding are apparent in Figure 4. Purged goethite has a broad peak around 3400 cm-' which is attributed to surface hydroxyl groups.35 This peak is separated from the large peak near 3000 cm-', attributed to a combination of interfacial water and bulk 0-H stretching vibration^.^^ However, the hydrogen bonding which occurs when COz adsorbs on goethite broadens the 3400-cm-' peak so much that it is no longer visible. In essence, this hydrogen bonding, possibly combined with other effects we have not considered, acts to prevent titration of the adsorbed carbonate, so that the apparent ZPC shifts to a lower pH when C02 is adsorbed. Charge development at the surface can also be used to explain these results. Again, the electron donation which occurs when C02 adsorbs would cause an excess positive charge to develop at the goethite surface. This increased positive charge at the surface should decrease the ZPC, as is observed for cation a d s ~ r p t i o n . ~ J One ~ ~ *might ~~~~ argue, however, that the identical partial negative charge developed on CO, molecules adsorbed in the trough should also be included in the surface charge, compensating for any induced positive charge at the surface. In such a case, the ZPC should not change. Since the ZPC actually rose by 1 pH unit after the goethite was purged, it appears that the C02adsorption site in the trough cannot be considered to lie on the goethite surface. Instead, the negative charge which develops at this site might better be cdnsidered as an intrinsic charge, similar to that developed in many clay mineral^.^^-^^ In order to balance this negative intrinsic charge at the ZPC, the actual surface must become more positive, so that the ZPC occurs at a lower pH. The redistribution of charge which occurs when C 0 2 adsorbs in the goethite trough seems to produce a state in which the (38) Rochester, C.H.; Topham, S. A. J. Chem. Soc., Faraday Trans. 1 1979, 75,872.
(39) Harrison, J. B.; Berkheiser, V. E. Clays Clay Miner. 1982,30,97. (40) Breeuwsma, k;Lyklema, J. J. Colloid Interface Sci. 1973,43,437. (41)Ottaviani, M.F.; Ceresa, E. M.; Visca, M. J. Colloid Interface Sci. 1985, 108, 114. (42) Van Olphen, H. An Introduction to Clay Colloid Chemistry, 2nd ed.; Wiley: New York, 1977; p 18. (43) Sposito, G.Soil Sci. SOC.Am. J . 1981, 45, 292. (44) Sposito, G.The Surface Chemistry o f Soils; Oxford: New York, 1984; p 36.
GoethitelAqueous Solution Interface
partial negative charge in the trough and any positive charge appearing at the goethite surface both act to lower the ZPC. An alternative explanation for these effects is that C02 adsorption in the trough blocks a site involved in surface charge development on goethite. At this time, however, not enough data are available to determine if the observed decreases in proton adsorption density with purging time are due to some process related to C02 removal from the surface or simply due to aging caused by hydration of the goethite surface, as reported by B a l i ~ t r i e r i . ~ Conse~ quently, it is still an open question whether or not C02 adsorption blocks a reactive surface site. Constant IEP. Why does purging the stock goethite suspension not affect the measured IEP? Since C02 adsorption in the trough simply redistributes charge within the crystal rather than placing cations or anions near the slipping plane (where mobility measurements are made), the net potential at the slipping plane would not be expected to change with adsorption or desorption of a neutral COz molecule. No variations in IEP greater than the 0.2 pH unit uncertainty associated with the mobility determinations were obtained. Implications. One point to note about the results reported above is that the ZPC and IEP of our goethite samples do not agree, even after 2 months of nitrogen purging. This disagreement is most likely due to adsorption of a small amount of a contaminating cation on the g ~ e t h i t e .Such ~ cation contamination might be removed by washing the goethite with a weak acid solution before use, although this has not been tried as yet. Penners et al.14discuss changes in the point of zero charge which were observed when a well-aged, aqueous hematite suspension was washed with mild acid solutions. Although the original sol had an apparent ZPC at pH 8.4,this value shifted to 9.5 after acid washing. No mobility measurements were reported for this sample, but a separate hematite sample had an IEP about pH 9.3. These investigators attributed these changes to cation contamination, although the weak acid wash might also remove adsorbed C02 from their hematite. However, strong COPadsorption on hematite is less likely than on goethite because hematite is not likely to have a trough which would serve as a site for C02 ads ~ r p t i o n . ' Our ~ ~ ~explanation for our observed ZPC shifts requires COz adsorption in such a trough. In any event, further investigations of the preparation of metal oxide surfaces for electrical double layer studies appear warranted, especially for oxides with basic ZPC values. A second point is that current double layer models provide little explanation for the wide variations in reported ZPC and IEP values for different samples of a given metal oxide. However, carbonate adsorption in the trough of the goethite (100) face does seem to have a significant effect on the ZPC of an individual goethite sample. One might expect then that goethite samples which are synthesized separately should have different ZPC values depending on the relative contribution made to the total goethite surface area by the (100) face. Since goethite crystals grow in the direction of the (001) longer crystals will have a smaller surface area by virtue of being larger. However, longer crystals will also have a relatively larger contribution from the (100) face for a constant (45) Balistrieri, L. S. M.S. Thesis, University of Washington, 1977. (46) Parfitt, R. L.; Atkinson, R. J.; Smart,R. St. C. Soil Sci. SOC.Am. h o c . 1975, 39, 837. (47) Cornell, R. M.; M a n , S.;Skamulis, A. J. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2679. (48) Van der Woude, J. H. A.: de Bruvn, P. L.: Pieters. J. Colloids Surf. 1984, 9, 173.
Langmuir, Vol. 4, No. 2, 1988 473
N-
E
80.0
0.0
I ,I -
1 6.~0
7.20
7.60
a.00
a a
1
am
ZPC
Figure 6. Literature correlation between goethite surface area apd ZPC.
crystal width and height. Since a larger (100)face implies more carbonate adsorption, more charge redistribution, and a lower ZPC, it may be possible to show that goethite particles with a smaller surface area have a lower ZPC. Although it would be interesting to test this experimentally, a t this stage all that can be done is to perform a literature search to compare reported goethite surface areas with reported goethite ZPC values obtained by potentiometric titration. Table I lists the results for unpurged samples. These results are also shown graphically in Figure 6. The first six values from five different sources indicate a good correlation between lower surface area and lower ZPC. This correlation is also shown by one of the remaining values. Surprisingly, a second correlation is also obtained for four other values. The final two values are scattered. I t should be expected that not all reported values would correlate well because an ideal correlation would require that all experiments be performed in the same supporting electrolyte and that all goethite crystals have identical widths and heights. However, given these limitations, it may be valid to expect goethite samples with lower surface area to have a lower ZPC. In fact, if a similar correlation were obtained for carbonate-free goethite samples, it would imply that different goethite faces have somewhat different surface chemistries.
Summary We have established that changes in the ZPC for this goethite system are largely caused by C02 adsorption and desorption, although some residual cation contamination also appears to be present. However, such factors as possible NH4+ adsorption, possible reduction of Fe(II1) from the goethite, and possible carryover of material from the purge washing system do not affect the measured goethite ZPC. Nitrogen purging of this goethite has no measurable effect on the IEP. These effects can be explained in terms of a bonding mechanism proposed by Russell et al.36 They suggested that the formation of a coordinate bond between an adsorbing COz molecule and an oxide ion in the groove of the goethite (100)face would produce a carbonate ion which is hydrogen bonded to surface hydroxyl groups. This bonding mechanism causes
474
Langmuir 1988, 4 , 474-481
the surface groups to become more acidic while the reactivity of the carbonate group appears to be minimized in some way.
Acknowledgment. We are grateful to Dr. M. Isabel Tejedor-Tejedor for performing the CIR-FTIR analysis and to Dr. Charles Hill for suggesting several useful
manuscript revisions. This work was funded under contract no. DE-AC02-80EV10467 from the Ecological Research Division, Office of Health and Environmental Research, US.Department of Energy. Registry No. COO,124-38-9; FeOOH, 20344-49-4; goethite, 1310-14-1.
Determination of the Heat of Micelle Formation in Binary Surfactant Mixtures by Isoperibol Calorimetry James F. Rathmant and John F. Scamehorn" School of Chemical Engineering and Materials Science, Institute for Applied Surfactant Research, Uniuersity of Oklahoma, Norman, Oklahoma 73019 Received June 1, 1987. I n Final Form: October 15, 1987 The heat of mixing single-component micelles to form mixed micelles was measured by isoperibol calorimetry. Results are presented for 10 binary surfactant mixtures: 2 nonionic/nonionic systems, 1 cationic/cationicsystem, 5 cationic/nonionic systems, and 2 anionic/nonionic systems. The heat of mixing was exothermic for these systems, with the exception of mixtures of cationic surfactant with a nonionic alkylphosphine oxide, for which endothermic heats were observed. The heab of mixing for anionic/nonionic systems were much more exothermic than for the other mixtures studied. These results indicate that the heat of mixing is strongly dependent on electrostatic interactions and the structure of the surfactantsinvolved. Comparison of the heat of mixing data with the excess Gibbs free energy of mixing, obtained from mixture cmc measurements, suggests that the relative contribution of enthalpic and entropic effects to the nonideal behavior of mixed micelle formation may be quite different for different types of systems.
Introduction Nearly all studies of micelle formation in mixed surfactant systems have focused on how the critical micelle concentration varies with composition; while the mixture cmc is indeed important, it is only one of several mixture properties which must be studied in order to gain a more complete understanding of the process of mixed micelle formation. Any thermodynamically valid model for describing the formation of mixed micelles should accurately describe not only the cmc as a function of composition but also the distribution of each surfactant between the monomeric and micellar forms and the enthalpy and entropy effects upon mixing. Nearly without exception, the models presented in the literature have been judged only on their ability to describe cmc data. Several models for monomer-micelle equilibrium,'-15 based on quite different assumptions, have been proposed and shown to accurately describe mixture cmc data, suggesting that such data do not provide a very rigorous test of the thermodynamic validity of these models. Further model development is of little value until other types of data have been collected which permit a more accurate evaluation of any given model. The emphasis on mixture cmc data is in part understandable since no universal method of measuring both micelle and monomer compositions simultaneously has been developed; however, calorimetry provides an established technique for determining heats of mixing and can therefore be a valuable aid in the study of surfactant thermodynamics. The most common use of calorimetry in the study of surfactants has been to measure enthalpies 'Present address: Pioneering Research, Clorox Technical Center, P.O. Box 493, Pleasanton, CA 94566. *Author t o whom correspondence should be addressed.
of micellization for single surfactant systems. Both ionic16m and nonionicwa surfactants have been studied. The addition of a nonsurfactant component to a single surfactant solution has also been studied enthalpic data have been reported for the solubilization of waterlsurfactant solutions into nonpolar solvents25and the solubilization (1) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. I, p 337. (2) Kamrath, R. F.; Franses, E. I. J. Phys. Chem. 1984, 88, 1642. (3) Rathman, J. F.; Scamehorn, J. F. Langmuir 1986, 2, 354. (4) Nguyen, C. M.; Rathman, J. F.; Scamehorn, J. F. J . Colloid Interface Sci. 1986, 112, 438. (5) Holland, P. M.; Rubingh, D. N. J . Phys. Chem. 1983, 87, 1984. (6) Rosen, M. J.; Hua, X. Y. J. Am. Oil Chem. SOC. 1982, 59, 582. (7) Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1982, 90, 212. (8) Rosen, M. J.; Zhu, B. Y. J. Colloid Interface Sci. 1984, 99, 427. (9) Kamrath, R. F.; Franses, E. I. Ind. Eng. Chem. Fundam. 1983,22, 230. (10) Osbome-Lee, I. W.; Schechter, R. S. J. Colloid Intqrface Sci. 1985, 108, 60. (11) Zhu, B. Y.; Rosen, M. J. J. Colloid Interface Sci. 1984, 99, 435. (12) Nagarajan, R. Langmuir 1985, 1, 331. (13) Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985, 1, 347. (14) Moroi, Y.; Nishikido, N.; Saito, M.; Matuura, R. J . Colloid Interface Sci. 1976,52, 356. (15) Motomura, K.; Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984,262, 948. (16) Birdi, K. S. Colloid Polym. Sci. 1983, 261, 45. (17) Mazer, N. A.; Olofason, G. J. Phys. Chem. 1982,86, 4584. (18)Eatough, D. J.; Rehfeld, S. J. Thermochim. Acta 1971, 2, 443. (19) Kresheck, G. C.; Hargraves, W. A. J. Colloid Interface Sci.1974, 48, 481. (20) Benjamin, L. J . Phys. Chem. 1964, 68, 3575. (21) Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans. Faraday Soc. 1964, 60, 202. (22) Corkill, J. M.; Goodman, J. F.; Harrold, S. P. Trans. Faraday SOC. 1964, 60, 996. (23) Teo, H. H.; Yeates, S. G.; Price, C.; Booth, C. J . Chem. SOC., Faraday Trans. 1 1984,80, 1787. (24) Olofsson, G. J . Phys. Chem. 1985, 89, 1473.
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