Langmuir 1994,10, 330-333
330
Surface Thermodynamic Properties of Synthetic Hydrotalcite Compounds Z. Li,*rt R. F. Giese,t and C. J. van O s s S ~ ~ Department of Geology, State University of New York at Buffalo, 415 Fronczak Hall, Buffalo, New York 14260, Department of Microbiology, State University of New York at Buffalo, 207 Sherman Hall, Buffalo, New York 14214, and Department of Chemical Engineering, State University of New York at Buffalo, 306 Furnas Hall, Buffalo,New York 14260 Received May 3,1993. In Final Form: September 27,1993" The contact angles of test liquids on the surface of several synthetic hydrotalcite compounds were determined by thin layer wicking. These contact angles were used to solve the Young equation for polar materials to yield the Lifshitz-van der Waals apolar surface tension component, yLW, and the polar Lewis acid/base parameters, ye and ye, of the samples. These values indicate that the synthetic hydrotalcite compounds of this study are monopolar (or nearly so) materials with a relatively modest y L W (21-27 mJ/m2), and they are Lewis bases (ye = 17-32 mJ/m2) with a small or no Lewis acid character (ye = 0). The observed variation of the surface tension components as a function of the chemical composition of the samples is determined by the type of interlayer anion (C032-vs SO2-) and the magnitude of the layer charge (determined by the value of the MgZ+/(MgZ++ AP+) ratio. The fact that the hydrotalcite layer has a net positive charge, in contrast to the negative charge of 2:l phyllosilicates, appears to play little or no role in determining the acidlbase character of these layer structures.
Introduction Synthetic hydrotalcite compounds have the general formula [ M ~ I - ~ A ~ ~ ( O H ) Z Inn-*mH201X-, ~+[A, where 0 < x < 0.33 and An- is an excKangea le anion having a valence of n.1 These compounds were first synthesized as mixed magnesium-aluminum hydroxides [Mg4A12(OH)12I2+[C0~3H2012-with different Mgz+/A13+ratios by Gastuche et a1.2 Brown and Gastuche3 described the structure as being composed of positively charged brucite-like layers (Mg4A12)(OH@+and negatively charged interlayer sheets C032--3Hz0with a rhombohedral unit cell a = 3.065 A and c = 23.015 A. In a series of papers, Miyata1VPGstudied the structures and physicochemicalproperties of a wide variety of synthetic hydrotalcite compounds by using different cation-anion systems such as Mg2+-A13+-S04Z- andMg2+AP+-Cl-. The relatively large anion exchange properties of these materials has led to their use as antacids, antipeptins, and adsorbent substrates.6 The properties of adsorption and desorption are interfacial phenomena whose function is determined by the surface thermodynamic properties of the condensed phases in question. Because these phenomena are of such great importance both scientifically and technologically, it is not surprising that a number of studies have appeared recently concerning the surface thermodynamic properties of clay minerals and related high-surface-area materials. Recent studies of 2:l phyllosilicates have shown them to have relatively large Lifshitz-van der Waalssurface tension values (35 < yLw< 45 mJ/m2),a wide range of values for the Lewis base surface tension parameter (3 < 70 < 45
6
t Department of Geology. t Department of Microbiology. 1 Department of Chemical Engineering. @Abstractpublished in Advance ACS Abstracts, November 1, 1993. (1)Miyata, S. Clays Clay Miner. 1983,31, 305. ( 2 ) Gastuche, M. C.; Brown, G.; Mortland, M. M. Clay Miner. 1967, 7, 177. (3)Brown, G.; Gastuche, M. C. Clay Miner. 1967, 7, 193. (4) Miyata, S. Clays Clay Miner. 1975,23, 369. ( 5 ) Miyata, S.;Okada, A. Clays Clay Miner. 1977,25, 14. ( 6 ) Miyata, S. Clays Clay Miner. 1980,28, 50.
mJ/m2), and a small or zero Lewis acid surface tension parameter (0 < ye < 3L7t8 The interpretation was that the Lewis basicity is due to the external oxygen atoms of each 2:l layer whose lone pair electrons were available for donating, principally by forming hydrogen bonds with surface adsorbed water molecules or hydroxyl groups. In theory, a deficiencyof positive charge in the layer resulting from e.g., substitution of A13+ for Si4+would make these electrons more easily available to act as Lewis bases.
Theory of Surface Thermodynamics The surface tension of a solid or a liquid .is due to two fundamentally different types of interaction. The first arises from apolar Lifshitz-van der Waals (LW) interactions of dipole-dipole, dipole-indued dipole, and fluctuating dipole-induce dipole types. More recently, Fowkess and van Oss et al.1° recognized the importance of polar interactions which derive from the ability of polar materials to accept and/or to denote electrons as described by Lewis acidlbase (AB) theory. The total interfacial free energy of cohesion of a surface of material i is the sum of the AB and LW components ~ ~ ~ i c o h e s= i o An G ~ F W
+A
G
~
~ (1) ~
The LW component of the free energy of material i is simply related to its surface tension by
while the polar (AB) part of the free energy is more complicated because there are two independent parameters, one for the electron donor character, Tie, and the other for the electron acceptor (Lewis acid) character, yie. These are not additive (7) Giese, R. F.; Costanzo, P. M.; van Oss, C. J. Phys. Chem. Miner. 1991. 17. 611.
(8) Norris, J.; Giese, R. F.; van Oss, C. J.; Costanzo, P. M. Clays Clay Miner., in press. (9) Fowkes, D. M. J. Phys. Chem. 1963,67, 2538. (10)van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Reu. 1988, 88, 927.
0743-746319412410-0330$04.50/0 0 1994 American Chemical Society
Properties of Synthetic Hydrotalcite Compounds
Langmuir, Vol. 10, No. 1,1994 331 Once R is determined, the contact angles can be calculated from eq 5 for a series of high-energy liquids needed to determine the surface tension components of the solid. The surface tension components can be derived by solving aset of simultaneous equations (4). The number of testing liquids must be no less than three and among them two must be polar. The thin layer wicking method for measuring the contact angles of powdered materials was successfully applied to several organic and inorganic powdered materials, such as talc and pyrophyllite,' some illites,14 latex beads,12 ad octadecylamine treated talcs.'3
7-
2t / I '0
10
20
40
SO
50
60
70
Experiments
YL
Figure 1. A plot of 27hz/t for sample H-4. The data points marked with a triangle are hydro-carbon liquids which spread on the surface of the material. Also shown is the least-squares line passing through the origin and the hydrocarbon data points, yielding the average pore size (R)as described in the text. The nonspreading liquids, used to solve the Young equation to give the surface tension values for the solid, are shown as squares.
(3) The surface tension components of a solid, yiLW,yie, and Tie, are determined by measuring the contact angles of drops of liquid, whose surface tension values are known, on a smooth surface of the solid. The contact angles and the surface tension components of both the liquid and solid are related by the Young equation for polar materials. YL(i
+ COS e) =
2 ((7sLWYLLW 1112 + (yseYLe)1/2+ (YseYLe)'/2) (4) where B is the contact angle between a liquid L and a solid S. For many materials, it is not possible to obtain a smooth, nonporous surface upon which to measure the contact angles. In this case it is usually possible to indirectly obtain the contact angles by measuring the rate at which the liquid moves through a uniformly dense powder of the material in question. For fine-grained materials, which are not monodispersed, a very convenient way to perform the experiment is to deposit a thin layer of the power onto a glass microscope slide. The technique is termed thin layer wicking. The contact angle of a liquid on the solid for the capillary rise measurement is given by the Washburn equation"
where h is the rise of the liquid in a time t through the powdered substrate, q is the viscosity of the liquid, R is the effective pore size of the powder being measured, and y~ is the total surface tensionof the liquid.12J3 In eq 5, there are two unknowns, R and cos 8. Since the number of unknown quantities is greater than the number of equations, R must be determined independently. Van Oss et d . l 2 showed that R can be found by using several low energy, apolar liquids, typically alkanes ranging from the decane to hexadecane. These liquids spread on the surface of the solid, so that cos B = 1and a plot of 2qh2/t versus y~ yields a straight line with a zero intercept; the slope of this line is the value of R for the experiment (Figure 1). (11) Washburn,E. W. Phys. Reu. 1921,17, 273. (12)van Ow, C. J.; Giese, R. F.; Li, Z.; Murphy, K.; Norris, J.; Chaudhury, M. K.; Good, R. J. J. Adhesion Sci. Technol. 1992,6,413. (13) Li, Z.; Giese, R. F.; van Oss, C. J.; Yvon, J.; Cases, J. J. Colloid Interface Sci. 1993, 156, 279.
Synthesis of Samples. Following Gastuche et al.? solutions were prepared with a 0.25 M concentration in totalcation content for MgClrGHpO and AlCls-GH20. The solutions were mixed to give ratios of AP+/Mgz+equal to 20/80 (sample H-3) and to 30/70 (sample H-4). Subsequently, precipitation was initiated by adding NaOH with a flow rate of 40 mL/min while the solution was vigorously stirred to yield a final pH of 10. Sample H-5 was prepared by using aqueous solutions of MgClrGHz0 and Alp(S0&18H20 (Mg2+= 0.75 mol/L and Ala+ = 0.25 mol/L), which were mixed with NaOH (3.5 mol/L) under the same conditions as for H-3 and H-4.e The resulting precipitates were washed with distilled water until they were deflocculated. The precipitates were then transferred to cellulose dialysis bags immersed in plastic jars containing 1 L of distilled water at room temperature. The water in the jars was changed and the conductivity was checked daily. Finally, the precipitates were freeze-dried and stored until needed. For comparison, a commercialsample of aluminum hydroxide was includedin the thin-layer wicking experiments. This material came in the form of a fine powder and was usedwithout treatment. Thin-Layer Wicking. Previous workl2JS showed that depositing an aqueous powdered suspension on clean glass slides produced better wicking samples than packing the powders into thin capillary tubes. The best results are achievedwhen the thin powder layer slides are smooth and uniform in density. For this study, different ratios of solid to water were tried and 0.4% (volume) was found to be optimum for slide preparation. A greater solids content in the suspension caused the powdered substrates to crack during drying and less solid resulted in an uneven powder layer on the slides. The synthetic hydrotalcite samples (800 mg in each) were suspended in distilled water (200 mL). The suspensions were stirred for at least 24 h to break down the particle aggregatesand achieve maximum dispersion. A 5-mL quantity of the suspension was carefully pipetted onto each glass slide and allowed to evaporate overnight. About 40 slides were made for each sample. The slides were then placed in an oven at 110 OC overnight to remove any moisture adsorbed on the surface of the samples. After heating, the samples were stored in a desiccator until the wicking experiments were performed. In order to verify that preexisting moisture in the small pores of the powder films did not interfere with the thin layer wicking experiments, a second set of samples was prepared and stored in a vacuum desiccator until needed. Just prior to measurement, the slides were allowed to equilibrate with the laboratory atmosphere. Wicking experiments were carried out in glass weighing bottles (Fisher 03-4223) containing 5-10 mL each of the test liquids (Table I). Prior to the initiation of the experiment, a series of small scatches was made in the surface of each slide every millimeter or so. For the hydrocarbon liquids, the slides were suspended above the liquid and the wicking bottle was sealed for 30 min in order to saturate the surfaces of the powder with the hydrocarbon vapor.12 At the moment the slides were placed into the liquid, the time was noted; as the liquid moved up through the powder, the time was also noted as the liquid front reached each of the fiducial scratches in the sample. For each liquid at (14) Li, Z.;Giese, R. F.; van Oss, C. J.; Eberl, D. (Abstract) in 29th Clay Mineral Society Annual Meeting, Houston, 1991.
Li et al.
332 Langmuir, Vol. 10, No. I, 1994 Table I. Surface Tension Components, Parameters, and Viscosities of the Test Liquids Used for Thin-Layer Wicking. at 20 OC ~~
y ~ ,
liquid
mJ/m2 23.83 decane dodecane 25.35 25.99 tridecane tetradecane 26.56 27.07 pentadecane 27.47 hexadecane a-bromonaphthalene 44.4 50.8 diiodomethane 72.8 water 48.0 ethylene glycol 58.0 formamide
yLW,
ye,
ye,
mJ/m* mJ/m2 mJ/m2 23.83 0.0 0.0 25.35 0.0 0.0 0.0 0.0 25.99 0.0 0.0 26.56 0.0 0.0 27.07 27.47 0.0 0.0 0.0 44.4 0.0 50.8 0.0 0.0 21.8 25.5 25.5 1.92 47.0 29.0 2.28 39.6 39.0
viscosity q,P 0.0092 0.0149 0.0188 0.0232 0.0284 0.0345 0.0489 0.028 0.010 0.199 0.0455
Table XI. Chemical Composition and Calculated Anion Exchange Capacity (AEC) Based on the Hydrotalcite Formula.
Table 111. X-ray Analysis of the Synthetic Hydrotalcite Compounds of This Study. hkl H-3 H-4 H-5 JCPDS 22-700 003 8.09 7.74 7.86 7.84 006 4.04 3.88 3.91 3.90 009,012 2.615 2.586 2.596 2.60 2.356 2.318 2.332 2.33 015 vbr 018 ubr 2.024 1.971 2.021 1.990 8(003) 0.55 0.94 1.22 B~ooe) 0.66 1.27 1.38 WOO 200 145 85 The d-spacingsare given in A units, the 8 values are in degrees 28, and e is in A units. Table IV. Values of the Surface Tension Components and Parameters for the Synthetic Hydrotalcite Samples of This Study Along with the Average Pore Size (R)Determined from the Wicking Experiments, the BET Surface Area, and the {-Potential yLW,
ye,
ye,
R , W surface {, cm area.m2/a mV 1.4 3.72 -44.8 0.9 2.1 3.88 -51.0 0.8 3.7 1.34 -19.5 1.7 44.0
initial products AEC mequiv/g samples Mgz+ A13+ Mg2+ Al3+ calculated 0.73 0.27 3.2 0.2 H-3 0.8 0.39 4.6 0.3 0.61 H-4 0.7 H-5 0.75 0.25 0.68 0.32 3.7 0.78 0.22 0.83 0.17 Bb 0.73 0.27 0.77 0.23 Db 0.65 0.35 0.7 0.3 Gb a The cation values listed in the second and third columns are the starting values for the synthesis which produced samples with the composition shown in the fourth and fifth columns. b From Gastuche
samde H-3 H-3' H-4 H-4' H-5 H-5' Al(OH)s muscoviteb SWy-le talcd
et al.2
a These are the second set of samples which were stored under vacuum. Unpublished data, this laboratory (contact angle). e Wyoming montmorillonitela (contact angle). Giese et aZ.1 (wicking).
least three slides were measured. The temperature for wicking experiments was controlled a t 20 1 OC. Surface Area. The surface are was determined by the BET method using the adsorption isotherm of liquid nitrogen a t -196 OC with Monosorb MS-5 made by Quantachrome Corp. The samples were dried at 110 O C in an oven overnight, then were put in the BET cells for out gassing a t 250 O C for 2 h before they were measured. At least three measurements were made for each duplication and two duplications were measured for each sample. {-Potential. The {-potential for the samples of this study were 'measured by the capillary tube method of van Oss e t al.lb A small quantity of each sample was suspended in a phosphate buffer (ionicstrengthof0.015) atapHof7.5. Theelectrophoretic mobilities for each sample were measured in a glass capillary tube which had been precoated with a thin layer of agarose to reduce electroosmotic backflow. The ends of the capillary were sealed with agarose plugs, and the ends of the capillary were placed in troughs containing a saline solution. The electric potential across the capillary was generated by a constant-voltage dc source. The observed mobilities were corrected for electroosmotic backflow (0.28 pm s-l V-l cm) and for the true electrical length of the capillary tube. X-ray and Chemical Analyses. X-ray analyses were performed by Siemens 500 diffractometer with Cu K a radiation, 40 kV/30 mA, and a scanning step of 0.02O with a 2-8 count time per ste~,withthescantakenfrom2~ to52O 20. Thechemicalanalyses were done by ICP (ion coupled plasma) for the major components without the elements for anions. Duplicates were measured for each sample. Since water and the anions were not included in the chemical analysis, it was assumed that the products obeyed the hydrotalcite formula. Under this assumption the amounts for COS" or SO'", and thus the anion exchange capacities of the sample, were calculated and are listed in Table 11.
*
Results It was noticed that all three samples had a lower Mg2+/ (Ala++ Mg2+)ratio than the starting ratio of the solutions. The same phenomenon was observed by Gastuche et aL2 (15) van Oss, C. J.; Fike, R.; Good, R. J.; Reinig, J. M. Anal. Biochem. 1974, 60, 242.
mJ/m2 mJ/m* mJ/m2 21.1 0.0 26.8 22.2 0.1 31.7 24.6 0.0 31.1 26.6 0.2 29.5 27.0 0.0 18.1 24.0 0.2 16.8 40.8 29.6 0.2 38.2 0.5 47.3 41.2 1.5 33.3 31.5 2.4 2.7
~
~~
(see Table I1 for comparison). The X-ray analyses indicated that the samples were pure hydrotalcite compounds without any detectable crystalline impurities. The d-spacings and indices of these samples and the hydrotalcite diffraction data (JCPDS22-700) are listed in Table 111. The crystallite sizes in the (001) direction were calculated from the equation of Williamson and Hall16
where @ is the true width at half height. The value for B was obtained from the apparent width at half height minus the instrument breadth determined by using the (101) peak of quartz at 28 = 26.62', A is the wavelength of the X-ray radiation, t is the crystalline size, and 5 (=Ad/& is the strain in the same direction. The results are listed in Table I11 and are in reasonable agreement with the results of Miyata6 for nonhydrothermally treated samples. The surface area data (Table IV) indicated that the samples which have C03" as the anion have a larger surface area than do the species which have 504% as an anion. However the surface area is much smaller than that reported by Miyata.6 Also, the {-potential for the cosz containing sample is much higher than for the SO42 sample. The surface tension components of these compounds are also listed in Table IV. All three compounds are Lewis base monopoles (ye > 0, ye = 0). For two samples, H-3 and H-4, yLW and 70 increased with decreasing Mg2+/ (Mg2+ + AP+) (increasing layer charge). However for sample H-5, yLW is comparable to the values found for samples H-3 and H-4, but the ye of sample H-5 is considerably smaller than those found for H-3 and H-4. From Table IV it can also be seen that there is little (16)Williamson, G. K.;Hall, W . H. Acta Metall. 1953, I , 22.
Properties of Synthetic Hydrotalcite Compounds difference between the measured surface tension values for two sets of samples, one stored under vacuum and the other not so treated. Thus, preexisting pore moisture does not appear to be a problem, and one can conclude that the reproducibility of the measurements is reasonable. The pore sizes for the samples range from 140 to 350 8, for the samples not stored under vacuum and 80 to 170 8, for the second set of samples. The pore sizes have the same magnitude as the crystalline sizes along the [OOlI direction. This coincidence may indicate that the platy particles are deposited parallel to the glass surface. In this orientation, the pore size may reflect the particle thickness in the direction perpendicular to the flat surface of the glass slide. The large pore size for sample H-5 may be caused by some flocculation of the suspension.
Discussion The values of the surface tension components and parameters for the three hydrotalcite samples are shown in Table IV along with values for related materials. The substitution of magnesium for aluminum in Al(OH)3 results in a dramatic change in surface properties. Aluminum hydroxide is a strong Lewis base (ye = 40.8 mJ/ m2) and a weak Lewis acid (ye = 0.2 mJ/m2). The principal result of the change from aluminum hydroxide to a hydrotalcite is a decrease in Lewis basicity, along with a less dramatic decrease in the LW surface tension component, yLw. The three hydrotalcite samples in this study are all hydrophobic materials although not as hydrophobic as talc.7 Doubtless, the hydrophobicity contributes to the utility of these materials.6 Measurements of the surface tension components and parameters of primarily oxide minerals show that yLw values typically lie in the range of 30-45 mJ/m2,with the value for a freshly cleaved muscovite surface approaching 50 mJ/m2. When exposed to ambient conditions, high energy surfaces such as muscovite quickly become hydrated with one or more layers of highly oriented water molecules. Such a situation is also known to occur with human serum albumin (HSA).17 When dry, HSA has yLw = 41 mJ/m2,ye = 0.13 mJ/m2, and ye = 17.2 mJ/m2. When hydrated (one layer of water of hydration) these values change to yLw = 26.6 mJ/m2, ye = 0.6 mJ/m2, and ye = 75.9 mJ/m2. In this state, approximately 73.5% of the water molecules adsorbed onto the surface of the HSA (17)van Oss, C. J.; Good, R. J. J.Protein Chem. 1988, 7,179. (18)Giese, R. F.;Norris, J.; Costanzo, P.M.;van Oss, C.J. Proc. Znt. Clay Conf. 1989,2, 33.
Langmuir, Vol. 10, No. 1, 1994 333 molecules are oriented with the hydrogen atoms of the water molecules directed toward the HSA surface in order to form hydrogen bonds. The decrease in yLW results from the addition of a layer of water molecules to the HSA surface and the increase in ye reflects the high degree of orientation of these adsorbed water molecules. The values of y L W found for the synthetic hydrotalcites of this study are distinctly smaller than the value for muscovite, which has a negative layer charge, and also smaller than talc, which has no layer charge. It is possible that the change from a negative or zero layer charge to a positive layer charge decreases the value of yLw. By analogy with the hydrated HSA, the small yLW values for the hydrotalcite samples are a reflection of a layer of surface adsorbed water. The fact that the ye values for the hydrotalcites are smaller than those reported for hydrated HSA indicates that the degree of orientation of these surface adsorbed water molecules is not great. The effect of the degree of substitution of Mg2+for A13+on the surface tension values is not clearly seen in these samples, probably because the range of substitution is small and only three samples were examined. The observed differences (Table IV) may simply reflect the treatment each of the samples experienced and the difference in the nature of the interlayer anion. For example, the X-ray diffraction line broadening measurements are consistent with samples H-5 being the most disordered of the three and also having the largest values for yLW and the smallest values for ye. A high degree of internal structural disorder would be associated with an irregular surface structure which, in turn, would make it more difficult for adsorbed water molecules to acquire a high degree of orientation. The {-potential values are negative for all the hydrotalcite samples (measured a t pH = 7.5) with sample H-5 having a smaller value than the other two samples. This difference is likely to be due to the sulfate anion present in H-5 compared to the carbonate of H-3 and H-4. As pointed out earlier, the three samples were measured after different pretreatments. In one case, the samples were stored in a desiccator and in the second, the desiccator was evacuated. The surface tension values for the two sets of measurements are comparable, but the pore sizes for the samples stored under vacuum are distinctly smaller, by roughly a factor of 2, than the same sample stored over samples drred by desiccant in the absence of a vacuum. This may reflect a compaction of the powder layer as residual water is removed by the vacuum prior to the thinlayer wicking experiment.
