J . Phys. Chem. 1984,88, 2119-2124
/ P 0
2119
units of cm3 molecule-1 s-l. The value of k(HO+CO) thus cm3 molecule-'s-', derived is (2.35 f 0.13 ( c T ) ) X The present value for k(HO+CO) is significantly smaller than those in Table I reported previously. One potential complication in the present results was examined and was shown to be unimportant. Namely, Kurylo and Laufer have shown qualitatively that isotope exchange reactions such as H180+ C1602 Cl60I8O H160 and H180 C l 6 0 ClSO H160 can occur at low diluent pressures but not in the presence of 760 torr of SF6.I8 Thus, although these reactions were not expected to be operative at 700 torr of air, analogous isotope-scrambling reactions could have affected the present results on the H I 6 0 + 12C'80reaction but not on the H160 I3Cl6Oreaction. Consequently, an attempt was made to observe 12C'60180as a product of the H160 + 12C1802 reaction by irradiating a mixture containing C H 3 0 N 0 (20 ppm), NO (10 ppm), 12C1802 (5 ppm) in 700 torr of air for 15 min. Note that these experimental conditions are virtually identical with those used to measure the relative rate constants (cf. Table I1 and 111). N o 12C160180 could be detected above the present sensitivity limit of approximately 10 ppb. A plausible explanation for the apparent discrepancy between the present value and those listed in Table I cannot be offered at this time. Interestingly, according to D e M ~ r ehis , ~ ongoing work on this reaction indicates that the magnitude of the pressure enhancement in N 2 (or Nz 0,) at 1 atm is about 40-50%, in constrast to earlier reports which have shown a larger enhancement, Le., 100%.l9 Furthermore, recent resonance-absorption experiments by Hofzumahaus and Stuhl at atmospheric pressure of N, resulted in values for the rate constant ranging from 2.1 X to 2.5 X cm3 molecule-' s-'.~O The present results appear to be compatible with these findings. However, it is clear that further extensive studies are required to firmly establish accurate values of k(HO+CO) as a function of N2-O2 pressure and temperature.
+
+
-
+
-
+
+
N
0
2
4
Figure 2. Plots of eq I for l3CI6Oand '*C1*0.Data points indicated by
open circles and triangles are for 13C160and filled points are for lzC1sO obtained by using CH30N0and CzH50N0,respectively. The slope of the line is based on the weighted average value given in Tables I1 and 111.
kinetic method at 299 f 2 K and in the presence of 735 torr of air, Le., (8.48 f 0.20(a)) X cm3 molecule-' s-l.15 As critically reviewed by these authors, other pertinent literature values for k(HO+C2H4) are also consistent within combined uncertainties, Le., 7.7 A 1.6,167.85 f 0.79,5 and 10.0 A 1.7"in (15) Atkinson, R.; Aschmann, S.M.; Winer, A. M.; Pitts, J. N., Jr. Int. J . Chem. Kinet. 1982, 14, 507.
Registry No. HO, 3352-57-6; 13C160, 1641-69-6;12C180, 4906-87-0. (16) Lloyd, A. C.; Darnall, K. R.; Winer, A. M.; Pitts, J. N., Jr. J . Phys. Chem. 1976,80, 789. (17) Overend, R.; Paraskevopoulos, G. J . Chem. Phys. 1977, 67, 674. (18) Kurylo, M. J.; Laufer, A. H. J . Chem. Phys. 1979, 70, 2032. (19) See, for example: "Chemical Kinetic and Photochemical Data for Use in Stratospheric Modeling", US.Government Printing Office: Washington DC, 1981; NASA Evaluation no. 4. (20) Hofzumahaus, A.; Stuhl, F., private communication.
Preparation of Porous Alunite and Its Water Adsorption Masahiro Kurata, Katsumi Kaneko, and Katsuya Inouye* Department of Chemistry, Faculty of Science, Chiba University, Chiba 260, Japan (Received: March 17, 1983; In Final Form: August 23, 1983) A particular alunite (KA13(S04),(OH)6)with high specific surface area, 200-240 m2/g from BET monomolecular Nzand H 2 0 adsorptions, has been synthesized by boiling a K2S04-A12(S04),mixed solution with a 5/1 K/AI ratio for 3 h, followed by washing the precipitates with a large amount of water (4 L/g of alunite). The X-ray diffraction patterns of the synthetic alunite were sharp and identical with those of mineral alunite, but interlamellar reflexions (001) are diminutive. The adsorption isotherms of the alunite were determined for H 2 0 at 30 O C to calculate the apparent pore-size distribution. It was postulated that the pores are slits intervening between very thin lamellae. The internal structure and mechanism of formation of the porous alunite were discussed with reference to the chemical composition, DTA-TG curves, and the dielectric constant variations with the H 2 0 adsorption. The possibility of the alunite as an adsorbent has been suggested.
Introduction A widely deposited aluminum basic sulfate mineral, alunite, has the chemical composition of KA13(S04)2(0H)6and a hexagonal layer structure of the space group R3m. The lamella spreading in the a-b plane is composed of Al-centered octahedra surround by two oxygens and four hydroxyls and S-centered tetrahedra forming SO4,- with four oxygens. Three of the four 0022-3654/84/2088-2119$01.50/0
oxygens connecting with sulfur are bonded with aluminum. Potassium ions are situated in interlamellar spaces to bind each neighboring lamella through coordinating bonds of the cations with OxYgens and hYdroxYls.''2 (1) S. B. Hendricks, Am. Mineral., 22, 773 (1937).
0 1984 American Chemical Society
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The Journal of Physical Chemistry, Vol. 88, No. 10, 1984
We are interested in alunite and its homologue jarosite (KFe3(S04),(OH),) in anticipation of their potentiality as adsorb e n t ~ . During ~ the course of examination, it was realized that the possibility as an adsorbent depends strongly on whether alunite, and jarosite as well, with high specific surface area could be obtained, despite their favorable characteristics in thermal stability, crystallographic structure embracing conceivable adsorptive sites, and very sparing solubility. Alunite crystals are synthesized easily by the procedures proposed by Brophy et al! and then by Parker;5 both authors describe the refluxing method of stoichiometric mixed solutions of aluminum sulfate and potassium sulfate for 2-4 days. The samples thus obtained gave X-ray diffraction patterns of crystalline alunite, but the nitrogen BET specific surface area was not more than 1 m2/g. A particular yet simple method to prepare alunite with high surface area will be described in this paper. In order to confirm the structure of the “porous” alunite produced, the results of H,O adsorption will be discussed with reference to variations in dielectric constant with the adsorption, X-ray diffraction, thermal analyses, and electron microscopic observation. Experimental Section Alunite was synthesized by refluxing the mixed solutions of 0.45-1.13 M A12(S04)3and 0.30-2.25 M KzSO4 for 3 h. The initial pH value was adjusted to 2.60-3.70 by addition of solid KOH. Because of slow dissolution of AlZ(SO&, it was convenient to add the KzSO4 solution, together with KOH when necessary, to the Alz(S04)3solution previously dissolved into hot distilled water. The precipitates were filtered off and washed with distilled water of different amounts in the range of 1-5 L/g of alunite on a dry basis. After the washing, the precipitates were dried at 110 OC for 3 h. The most appropriate conditions for preparing the alunite with high surface area were 0.45 M Alz(S04)3and 2.24 M KzSO4 with a K/Al molar ratio of 5 (pH 3.70) and washing with water (4 L/g of alunite on a dry basis). All reagents used were of analytically pure grade. For chemical analyses, alunite was dissolved in hot 12 N HC1 and diluted with distilled water at suitable concentrations. The AI3+ was analyzed by a titration method with EDTA.6 The K+ was determined gravimetrically by precipitating with sodium tetraphenylborate.’ The SO4,- was determined by the conventional BaS04 gravimetry. The specific surface area was measured by Shibata surface area meter 1000 by nitrogen adsorption at liquid-nitrogen temperature. The specific surface area from the H 2 0adsorption isotherms at 30 OC was calculated by the BET equation. The T G weight change was measured in the temperature range 100-600 OC at the temperature rise of 10 OC/min. The DTA curve was obtained in air with an A1,0, reference at the temperature rise of 10 OC/min up to 600 OC. The X-ray diffraction was done by an automatic diffractometer (Rigakudenki Series 2028) operated at 35 kV and 10 mA with the Fe K a irradiation filtered by Mn foil. A JEOL JSM-25s scanning electron microscope was used for the observations of synthetic alunites coated with a spattered gold. The transmission electron micrographs were taken by a Hitachi HU- 12 for the carbon-coated samples. The H 2 0 adsorption isotherm was obtained at 30 f 1 OC by measuring the weight gain at the adsorption equilibrium after 2 days. The H,O adsorption reaches equilibrium at each vapor pressure within 24 h. The dielectric loss of the alunite was determined at room temperature by a typical Schering bridge circuit* operated at 1 (2) R. Wang, W. F. Bradley, and H. Steinfink, Acta Crystallogr., 18, 249 (1965). ( 3 j K . Inouye, I. Nagumo, K. Kaneko, and T. Ishikawa, Z . Phys. Chem. (Wiesbaden), 131, 199 (1982). (4) G. P. Brophy, E. S . Scott, and R. A. Snellgrove,Am. Mineral., 47, 112 (1 ‘ 962). ($R. L. Parker, Am. Mineral., 47, 127 (1962). ( 6 ) K. Ueno, “Chelate Titration”, Nankodo Publishing Co., Tokyo, 1979, p 245. (7) Japanese Industrial Standards, M-8853, 1976. \ -
Kurata et al.
K l A l ratio
Figure 1. Variation of specific surface area by nitrogen adsorption with the K/A1 mole ratio in the initial solution. Precipitates were washed with water (4 L/g of alunite on a dry basis). A12(S0& concentration: open circles, 0.45 M; filled circles, 0.90 M.
I /
0,
0
I
1
I
2
3
L
amount of washing w a t e r ( L f ’ )
Figure 2. Increase in the specific surface areas with the amount of water for washing the alunite prepared from the sulfate mixed solution with a K/A1 ratio of 5: open circles, surface area by N2 adsorption; filled circles, surface area by H,O adsorption. kHz, connected with an amplifier and oscilloscope to check the balancing point. By use of the capacitance value of the sample-filled cell and the resistances and capacitances in each branch of the bridge, the apparent dielectric constant of the sample was calculated. Alunite filled in the gap between two concentric aluminum tubes was kept at different water-vapor pressures for 200 h at 30 f 1 “ C to reach the adsorption equilibrium.
Results Synthesis of Porous Alunite. As explained in the Introduction, Brophy et ale4and Parker5 synthesized alunite from the mixed sulfate solution of the stoichiometric K/A1 ratio of by boiling the solution at pH 2.60. The examination of this procedure gave alunites of low specific surface area (C1 m*/g) by nitrogen adsorption (&). It was found after various trails that the high surface area alunite can be prepared by boiling the solution containing a large excess of K+ and washing the precipitates with abundant water. Figure 1 shows the variation in S, of the alunite obtained with K/A1 ratios in the range 0.3-6.0 by refluxing for 3 h, filtration, and washing with distilled water (4 L/g of alunite). The surface area increases noticeably with the K/Al ratio above 3, irrespective of the difference in the solution concentration; we hence used conveniently the solution with the K/Al ratio of 5 in most of the following preparations. The refluxing time for 3 h was selected on the basis of the changes in surface area and X-ray diffraction patterns of the products, showing that alunite crystals are sufficiently grown up to the saturation in size within 1 h. The (8) S. Fich and J. L. Potter, “Theory of A-C Circuit”, Prentice-Hall, Englewood Cliffs, NJ, 1958, Chapter 3.
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2121
Preparation of Porous Alunite and Its Water Adsorption
A
I
I
201
I
,
I
I
I
I
I
0.5
0.1
u
0
1 2 3 4 amount o f washing w a t e r ( L g - l )
Figure 3. Changes of analytical ratios of K/Al and S04/A1 in alunite crystals with washing. The initial solution has a K/A1 mole ratio of 5 .
I
I
I
I
I
100
2M)
300
400 temperature(
'C
I
I
500
600
/
)
Figure 5. TG and DTA curves of synthetic alunites. The numbers on DTA curves denote the amounts of washing water in L/g of sample.
003 101
I
303 220
110
IT
I
1
2 8 ( Fe Kd)
Figure 4. X-ray diffraction patterns of synthetic alunite with 200 m2/g
surface area (upper) in comparison with those of mineral alunite (lower, ref 5 ) showing the relative intensity of each diffraction line. yield of alunite increased almost linearly with the K/Al ratio in the range 2-6, though it is still as low as 35% at the maximum. The yield was raised to approximately 75% by elevating the pH to 3.70 by the addition of KOH. Over this pH, Al(OH)3 is predominantly produced instead of alunite. It is noteworthy that the alunite crystals with high specific surface area are obtained by washing the precipitates from a solution with a high K/A1 ratio with large amounts of water as demonstrated in Figure 2, which shows the increase in S, up to 220 m2/g with the amount of washing. Figure 2 also gives the change of the H 2 0 specific surface area (Sw) obtained from H 2 0 adsorption isotherms at 30 OC (cf. Figure 7). It is noted that this sample has the electron microscopic particle size ranging from 1.0 to 3.0 pm. Chemical Composition. Excessive ions occluded in precipitates are washed out during the course of washing. Figure 3 shows the changes of K/AI and S04/Al ratios with the washing, approaching stoichiometric values of 0.33 and 0.67, respectively. An excess decrease in potassium content below the stoichiometric value was observed in the sample washed with water (4L/g of alunite). It is noticeable that the alunites from solutions containing sulfates of K/Al below 2 showed neither a decrease in the K/Al ratio nor an increase in the surface area by the similar washing procedure. CrystallographicStructure. The X-ray diffraction of the typical synthetic alunite with high surface area gave very sharp patterns and low background scattering, which indicates that only a small amount of the amorphous portion exists in the sample (Figure 4). This type of diffraction pattern was observed for all the alunites, irrespective of different K/AI ratios in the initial solution. It is recognized that diffraction results of the synthetic alunites are identical with those of mineral alunites reported by Parker.5 There is, however, a seemingly important difference in that the patterns from interlamellar (001) lattices, Le., (003), (006), and (009), are fairly strong for the mineral, whereas the (003) and (009) lines are unobservable and the (006) line is faint for the synthetic alunite. This difference and the fact that other diffraction patterns of the synthetic alunite are sharp and show no indication of amorphous impurities such as Al(OH), indicate the
(a)
(b)
Figure 6. Electron microscopic images of synthetic porous alunite with S N of 220 m2/g: (a) agglomerated particles by scanning electron microscope; (b) a very thin portion by transmission electron microscope.
The line on each picture corresponds to 1.S ym and 100 nm for (a) and (b), respectively. formation of a particular high-order structure in the synthetic alunite. Thermal Changes. Figure 5 gives the T G curve and typical DTA curves of the synthetic alunites. The TG curve showed no considerable variation with the preparation conditions. The DTA curve, on the other hand, varies with the amount of washing. An endothermic dehydration peak in the 100-200 "C range and an exothermic peak at 310 OC gradually manifests itself with the washing, and in particular a noticeable endothermic peak in the temperature range 520-570 "C tends to move to higher temperature range with the degree of washing the crystals. It was confirmed by the X-ray diffraction examinations that the latter endothermic change corresponds to the decomposition The difference in the K/A1 ratio and concentration of the initial solution showed no influence on the last endothermic peak at 520-570 O C . It will well be, if one assumes the opening of internal structure is caused by washing, that the above decomposition reaction becomes more difficult because of the degrading of the three-dimensional crystalline state which is necessary for the structural transformation. The exothermic peak at 310 OC, becoming sharper with the washing, appears to be pertinent to the second weight decrease in the TG curve. This change will be due to the recrystallization of alunite itself with evolution of some amount of strongly bound water in the internal structure. Particle Morphology. Figure 6 gives (a) SEM and (b) TEM images of the porous alunite. The approximate size of apparent
Kurata et al.
2122 The Journal of Physical Chemistry, Vol. 88, No. 10, 1984
p o r e radlus ( n r n )
Figure 8. Pore-size distribution of synthetic alunites. The amounts of washing water for 1 g of alunite, produced from the solution with a K/A1 ratio of 5 , are given on each curve as 0-4 (L/g of alunite). Square symbols indicate values for the alunite from the solution with a K/AI ratio of 3, after washing with water (4 L/per g of alunite).
0
0.4
0.2
0.6
0.8
1.0
relative pressure(P/P,)
Figure 7. Adsorption isotherms of H 2 0 on alunite at 30 OC,washed with different amounts of water. The number on each curve denotes the amount of washing water in L/g of sample: open symbols, adsorption;
filled symbols, desorption. particles ranges from 1 to 3 Hm. It appears that the sample is a cemented agglomerate of thin, nearly hexagonal layers. There is no evidence of pores in the particles. An interesting spiral growth is observed on the tops of several agglomerates. Low intensity of (001) diffraction patterns (loc. cit.) seems plausible, if one accepts the lamellae incling with each other. Minute crystallites of the size of ca. 10 nm are observed by TEM in some very thin particle fragments, as in the case of jarositeS3 This will merit further examinations. Adsorption of H 2 0 . The adsorption isotherms of H 2 0 on alunite, which had been synthesized from the solution with the K/Al ratio of 5 , changed with the amount of washing water, as illustrated in Figure 7 . It appears that the isotherms change from type I1 of the BET classificationg to type IV with the amount of washing water. The type I1 curve has been generally considered to indicate the presence of micropores in the sample, though the solids showing type I1 isotherms are called “nonporous”, whereas type IV indicates mesopores or macropores together with micropores.1° The isotherms for the samples 0-3 are characterized by concave changes in the low-pressure range, nearly horizontal changes in the PIPo range of 0.4-0.9, and finally a steep rise of adsorption above PIPo of 0.95. As for the alunite washed with an abundant amount of water (curve 41, the isotherm shape is different from the others; it shows linear increase of adsorption with the relative pressure. With the progress of washing with water (>2 L/g of alunite), the adsorption-desorption hysteresis becomes noticeable in the intermediate relative pressure range, and furthermore another hysteresis is observed in the low-pressure range. When the sample is washed with water (4 L/g of alunite), the latter hysteresis is so obvious as to show a nonreversible adsorption. These adsorption results seem to mean that washing the alunite synthesized from high K/A1 ratio solutions conceives the removal (9) S.Brunauer, P. H. Emmett, and E. Teller, J . Am. Chem. Soc., 60,309 (1938). (10) S. J. Gregg and K. S. W. Sing, “Adsorption, Surface Area, and Porosity”, Academic Press, London, 1967, Chapters 1-3.
of occluded excess ions (Figure 3) to form a porous structure with high surface area (Figure 2). Nevertheless, this verifies perhaps the development of intercrystallite spaces, because no changes in the X-ray diffraction patterns have been observed (Figure 4), except the diminishing intensity of the interlamellar (004 patterns. Gonzalez-Calbet et al.” discussed recently the nonreversible adsorption-desorption curves of nitrogen on very thin iron oxide hydroxide crystals, with reference to the work by Paterson and Tait,I2 to conclude that the pores calculated are due to the interstices between crystal particles. The BET surface area for the H 2 0 adsorption (Sw) was already given in Figure 2 with respect to the progress of pore opening by washiqg. S, and Sware almost identical after the formation of pores, but insufficient washing results in a wide difference between SNand Sw. The Swvalue of the alunite without washing amounts to approximately 100 m2/g, suggesting that H 2 0 molecules are well adsorbed in micropores originally present in the precipitates, even though the S, value of the same sample is only 1 m*/g. It is noticeable from a practical standpoint that the porous alunite has a high hygroscopic capacity. The amount of water adsorbed will reach at least 200 mg/g, that potentiates to be comparable with the generally acclaimed hygroscopic capacity of a typical solid desiccant silica gel (ca. 0.5 g of H 2 0 / g of silica gel). These results are supported by the variations of pore-size distribution curves in Figure 8. The calculation of reliable pore-size distribution has still been a controversial subject as was comprehensively discussed by Gregg and Sing,’O yet the classical method proposed by Barret et al.13 was applied to the present adsorption-desorption results. For obtaining the adsorption film thickness ( t value) on the adsorbent surfaces, to be added to the “Kelvin radius”, the Halsey equation14was employed. The original alunite precipitates before washing already have micropores of 0.7-nm radius, which tend to increase its volume with the progress of washing. By washing the sample with water (4 L/per g of alunite), the pore-size distribution maximum shifts to approximately 1.6 nm in radius. It will be responsible for the change in adsorption behavior shown in Figure 7 . Despite many arguments on the plausibility of the micropores less than 2 nm in radius, it could not be denied that these variations of pore-size distribution with the formation of internal surfaces are relevant to other results obtained. Figure 8 also shows that the alunite sample prepared from the initial solution with a K/AI ratio below 3 does not produce the pores of the larger radius, even after washing with water (4 L/per g of alunite). (1 1) J. M. Gonzalez-Calbet, M. A. Alario-Franco, and M. Gayoso-Andrade, J . Inorg. Nucl. Chem., 43, 257 (1981). (12) E. Paterson and J. M. Tait, Clay Miner., 12, 345 (1977). (13) E. P. Barret, L. G. Joyner, and P. P. Halenda, J . Am. Chem. SOC., 73, 373 (1951). (14) G. D. Halsey, J . Chem. Phys., 16, 931 (1948).
Preparation of Porous Alunite and Its Water Adsorption
I
0
P i
non-porous
r”1 1
n
u1
.-C
0
“m 5 0
Vm
100
150
H 2 0 a d s o r b e d ( m g g-’)
Figure 9. The increase in dielectric constant with the adsorption of water on porous and nonporous alunites: open symbols, adsorption; filled symbols, desorption.
Dielectric Constant. In Figure 9, the relationships between the dielectric constant increase (AE)caused by the adsorption of H 2 0 and the amount of H 2 0 are compared for the “nonporous” alunite from the solution of the stoichiometric K/AI ratio (1/3) and for the “porous” alunite from the solution with a K/A1 ratio of 5. The AE is the difference between E of a water-adsorbed sample and that of a dry sample. The variations of A€ with the amount of H 2 0 adsorbed differ distinctly with the difference in internal structure of each alunite: the nonporous alunite shows a steep rise of A€ at the point of water adsorption of approximately 50 mgJg, whereas the porous alunite demonstrates a gradual increase in A€ in the H 2 0 adsorption range below 140 mg/g, following by a steep rise of AE. For both the porous and nonporous alunites, no hysteresis of AE has been observed. Discussion The experimental results have revealed that a procedure using an initial solution with superabundant potassium ions over aluminum gives synthetic alunite crystals with high surface area as high as 200-240 m2/g, if the precipitates produced are washed with a large amount of water (>3 L/g of alunite). It is interesting that this porous alunite still keeps a surprisingly well crystalline structure by X-ray diffraction, but the diffraction patterns corresponding to the interlamellar (001) lattices are diminutive. The X-ray patterns shown by the synthetic alunites were identical irrespective of the K/Al ratio in the initial solution and the amount of water used for washing the precipitates. This fact suggests that alunite crystallites of the same composition are always formed by consumption of relevant ions in necessary quantities. The remains after washing out the excess amount of K+ and S042ions contain almost stoichiometric quantities of each component, as shown in Figure 3, although the amount of K+ appears to become somewhat below the stoichiometric ratio. The large number of studies devoted to the mechanism of hydrolysis of, in particular, iron salt acidic solutions has given us abundant information on the mechanism to produce oxides, oxide hydroxide, and basic sulfate compounds. A detailed review by Spiro et al.I5 describes various experimental techniques to investigate into the products that are not only crystalline materials but polymeric complexes with undefined compositions. In a series of extensive studies on the preparation of monodispersed hydrous oxide sols by Matijevic et al., aluminum salt hydrolysis was exarnined.16+’’ Furthermore, Matijevic has discussed the mechanism (15) T. G. Spiro and P. Saltman, “Structure and Bonding”, Vol. 6, Springer-Verlag, West Berlin, 1969, p 116. (16) R. Brace and E. Matijevic, J . Inorg. Nucl. Chem., 35, 3691 (1973).
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2123 of formation of “alunite mineral group”, where it is suggested that the existence of certain cations, mostly divalent transition-metal ions, produces a considerable portion of “soluble” products to inhibit particle formation.’*J9 We can thus presume that the excessive potassium ions form alunite crystallites which are so minute that they are soluble or dispersable in water during the continuous washing procedure. The yield of alunite increases with the K/A1 ratio in solution, though it is at best 35%, but doubling the A13+ concentration increased the yield to approximately 50%, suggesting that only a portion of pertinent ions are used for crystalline alunite formation. The nitrogen surface area in Figure 1 jumps up between values of 2 and 3 for the K/Al ratio, meaning an abrupt opening of internal surfaces from a certain K/AI ratio. On the other hand, Figure 2 demonstrates the difference in surface areas by N2 and H20;the porous alunite has spaces large enough to embrace either N2 or H20. The opening of the pores of 1.6-nm radius becomes distinctive at the sample washed with water (>4 L/g of alunite) (Figure 8). From ref 20, the van der Waals radii of N2and H 2 0 are 0.41 and 0.28 nm, respectively. This molecular size difference influences indirectly on the difference between SNand Swin Figure 2 as far as the alunite contains only the pores of 0.7-nm radius. It seems likely that the internal surface relevant to the H 2 0 adsorption is the interstices between platelike alunite crystallites. The shape of the hysteresis cruve in Figure 7 of the porous alunite is close to that of the type B hysteresis loop according to the de Boer classification2’ and suggests that the pores are neither parallel-plate type nor bottleneck type, but they must contain slanting slit-shaped interstices between aggregates of lamellae. The diminishing intensity of (001) patterns in the porous alunite and the S E M image (Figure 6) as well will support this view. Consequently, the porous alunite seems to be an aggregate system composed of thin layers, between which intervening spaces of varying pore size exist. The excess amount of K+ in the solution used for the synthesis plays a role, in fact, to cement crystalline lamellae before being washed out. Because K+ ions can enter only into interlamellar spaces crystallographically, a lower K/Al value of the washed alunite than the stoichiometric 1 / 3 value (Figure 3 ) is conceivable. The intentional formation of soluble portion in precipitates, to be washed out afterwards, could be a general procedure to make a porous compound, which otherwise has low internal surface area. As for the reasons why the porous alunite has a high adsorbability for H20, there will be a t least two main factors to make molecules enter into pores: the size relationship between pores and adsorbate and the electronic effect of adsorbent surfaces. So far as the molecular dimension is concerned, H 2 0 molecule (0.28 nm in radius) can easily enter into the 1.6-nm pores shown in Figure 8. As for the electronic factor, we tried to estimate the residual electrostatic valence according to Pauling22 for each oxygen and hydroxyl exposed on lamellar walls (a-b) and cross section (a-c), to indicate that the (a-b) lamellar planes charge positively, while the (a-c) sections charge negatively. The molecular adsorption will occur on the positively charged wide internal surfaces made out of lamellae. A separate experiment23to determine the adsorbability of water-soluble dyes on the porous alunite has shown that the negative dyes such as Acid Red 88 and Orange I1 are adsorbed on the alunite effectively, while the adsorption of the positively dissociating dyes such as Methylene Blue and Crystal Violet does not occur. The presence of positive charges on the crystal planes may be due also to the cationic defects (17) D. L. Catone and E. Matijevic, J . Colloid Interface Sci., 48, 291 ( 1974).
(18) E. Matijevic, R. S. Sapieszko, and J. B. Melville, J . Colloid fnterfuce Sci., 50, 567 (1975).
(19) R. S. Sapieszko, R. C. Patel, and E. Matijevic, J . Phys. Chem., 81, 1061 (1977). (20) R. D. Shelton and A. H. Nielsen, J . Chem. Phys., 21, 2178 (1953). (21) J. H. de Boer, “The Structure and Properties of Porous Solids”, Butterworth Publishing Co., London, 1958, p 68. (22) L. Pauling, J . Am. Chem. SOC.,51, 1010 (1929). (23) K. Inouye et al., to be submitted for publication.
2124
J . Phys. Chem. 1984, 88, 2124-2127
revealed by the low K/A1 ratio in the porous alunite (loc. cit.). It is noted that there has been no signs of swelling along the c axis of the porous alunite even after the water adsorption at PIPo of 1.0 and 30 OC for 48 h, although the X-ray diffraction patterns of (001) lattices are generally faint. The changes of dielectric constant with water adsorption suggest the state of HzO molecules adsorbed on porous and nonporous synthetic alunites. Comparing the E change (Figure 9) with the adsorption isotherms (Figure 7) and with the pore-size distribution (Figure 8), it is suggested that the steep rise of Ac corresponds to the filling up the 1.6-nm radius pores in the porous alunite with water molecules. In other words, water up to 135 mg/g is adsorbed in the pores in a less mobile state. The fact that no hysteresis has been observed in the dielectric measurements suggests that HzO molecules form perhaps a particular monotypic water structure in the slitlike pores. The water molecules up to the point where A€ commences to rise steeply, 45 (point A) and 135 mg/g (point B) for nonporous and porous samples, respectively, are considered to be bonded so strongly as to show lower E increases. It does not necessarily mean, however, that the A€ rise occurs at the point where the BET monolayer adsorption is complete, because the calculated values of monomolecular capacity (0,) for the nonporous and porous samples are 30 and 65 mg/g, respectively. The weakly bound water molecules on the nonporous alunite above point A seem to correspond to the multilayer physisorption, while the molecules adsorbed on the porous alunite in the range between v, and point B correspond to those adsorbed
in multilayered states in the slitlike pores. The water molecules above point B will involve those adsorbed on outer surfaces of porous alunite particles. On the state of water molecules adsorbed on a-Fe,O,, McCafferty and ZettlemoyerZ4concluaed by the dielectric relaxation study that only the monomolecularly adsorbed water is immobile on the oxide surface, but water in succeeding layers is mobile. The discrepancy in dielectric behavior between water molecules on a-Fez03and those on porous alunite may indicate that the bonding of water molecules with alunite internal surfaces is stronger than with outer surfaces of nonporous a-FeZO3. Acknowledgment. This work was ,financially supported by a special grant for environmental science from the Japanese Government, Ministry of Education, Science, and Culture. We are indebted to Dr. T. Ishikawa (Osaka University of Education) for congenial interest and discussion during the experiments. The dielectric measurements were carried out by H. Omata. The SEM and TEM electron micrographs were kindly taken by Dr. S. Kurita and Prof. Y. Watanabe of this University, respectively, to whom our thanks are due. Registry No. KA13(S04)2(OH)6, 12253-07-5; HzO, 7732-18-5; alun-
ite, 1302-91-6. (24) E. McCafferty and A. C. Zettlemoyer,Discuss. Faraday SOC.,52,239 (1971).
Extension of the Falkenhagen Equation to the Conductivity of Concentrated Electrolyte Solutions Mario Della Monica,* Andrea Ceglie, and Angela Agostiano Dipartimento di Chimica, 70126 Bari, Italy (Received: April 6, 1983; In Final Form: September 29, 1983)
The inadequacy of the Falkenhagen-Wishaw-Stokes equation to account for the conductance of some concentrated electrolyte solutions is discussed in this work. Through an analysis of the dependence on the medium viscosity of the single retarding effects on the ionic mobility it has been shown that the introduction of a correction factor in the electrophoretic term of the conductivity equation of the NaSCN in water system accounts for the increasing trend of the Walden product found in the range of concentration 3-9 M. The proposed equation has also been tested with satisfactory results for KBr, KCl, and LiCl in water at 25 OC.
Introduction Although there are many reports in the literature on thermodynamic and transport processes of dilute electrolyte solutions, very few studies deal with higher concentrations. This is probably due to the lack of a suitable model to describe the behavior of the latter. With regard to salt conductivity in water, models and equations having limited applicability in single systems have been proposed. However, these prove to be inadequate when generalized. In analyzing the specific conductivity curve of some systems in water, Postler' proposes the introduction into the Pitts equation2 of a log c term whose coefficient can be determined graphically from the experimental data. The suggested model is the Debye-HUckel one; but the introduction of an adjustable term which varies for each system is a limiting factor in the application of the derived equation to systems of different concentrations. (1) M. Postler, Collect. Czech. Chem. Commun., 35, 535 (1970). (2) E. Pitts, Proc. R. SOC.London, Ser. A , 217, 43 (1953). (3) P. Debye and E. Hiickel, Phys. Z . , 24, 305 (1923).
In fact, the introduction of an arbitrary factor does not allow a distinction between the contribution to conductance variability due to increasing ionic interactions with the concentration and that due, for example, to ion-pair associations which seem to be present in every system at high concentration. Angel14 proposes a different approach to the problem of ion mobility in highly concentrated solutions. His model implies that the energy connected with a mass-transfer process (conductivity, viscosity, etc.) depends on the temperature up to the point where a temperature To,known as the glass-transition temperature, is reached. Below this temperature, the system becomes a glass and any process involving mass transfer is hindered. Following this model, for each solution, both the Totemperature, at which the system becomes a glass, and, alternatively, the concentration No,at which the system behaves like a glass, at a given temperature, can be established. This theory has been successful in some aqueous systems such as Ca(NO&- and ~~
~
~
(4) C. A. Angell, J. Phys. Chem., 70, 3988 (1966); C. A. Angell and E. J. Sare, ibid., 52, 1058 (1970).
0022-365418412088-2124$01.50/0 0 1984 American Chemical Society
