Langmuir 1996, 12, 2115-2117
A Thermodynamic Aspect of the Magnetic Effect on Water Adsorption Sumio Ozeki,* Junichi Miyamoto, and Tomotaka Watanabe Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan Received September 12, 1994. In Final Form: January 19, 1996
It has been reported that the adsorption of NO is affected by a static magnetic field.1 Some causes for magneticfield-induced adsorption and desorption (MAD) of NO, such as magnetism, adsorption sites, and the porosity of solids, have been examined, and some correlation has been recognized between them. However, the mechanism of the magnetic effect on NO adsorption is not understood at all. In this paper, we consider the characteristics of MAD of water to be a function of the vapor pressure, temperature, magnetic-field intensity, and porosity of solids, and whether or not the MAD of water has any thermodynamic characteristics. Although weakly bound water on hydrophobic surfaces, such as carbon black, activated carbon, and silica, responded to a magnetic field, water in the first layer on hydrophilic oxide surfaces, such as FeOOH and zeolite, did not.2 Multilayered water was less sensitive to a magnetic field, and water in pores responded to a magnetic field depending on the pore size. These results suggest that a magnetic field may affect the adsorption when the magnetic energy gained by an external magnetic field is sufficiently large to overcome any water-surface interactions and hydrogen bonding in the water network (cluster). However, the magnetic energy of water under 10 kG (∼0.1 cal/mol) seems to be too small to bring about such a large MAD, considering that the adsorption energy of water is around a few kilocalories per mole. In addition, magnetoadsorption is an unprecedented phenomenon, because diamagnetic water tends to be repelled from a magnetic field. From these viewpoints, the magnetic properties of water or, more plausibly, the adsorbed water phase could change via interactions with surfaces. It is possible for magnetic fields to affect the gas adsorption if the magnetization of an adsorptive and/or an adsorbent changes during adsorption: When the equilibrium pressure of the adsorbed water phase (p0) changes into p0 + ∆p due to a magnetic field (H) at T K, the relation between ∆p and H may be given by
RT{ln(p0 + ∆p) - ln p0} ) -∆MH
(1)
where ∆M is the change in the magnetization of the system when 1 mol of an adsorptive is adsorbed on the surfaces. Under the experimental condition p0 . ∆p, eq 1 becomes
∆p/p0 ) -∆MH/RT
(2)
This equation predicts that ∆p changes linearly with p0 under constant H and depends on T according to the * All correspondence should be addressed to Sumio Ozeki, Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan. FAX: 81-43-290-2783. E-mail:
[email protected]. (1) Ozeki, S.; Uchiyama, H. J. Phys. Chem. 1988, 92, 7805. Uchiyama, H.; Ozeki, S.; Kaneko, K. Chem. Phys. Lett. 1990, 166, 531. Ozeki, S.; Uchiyama, H.; Kaneko, K. J. Phys. Chem. 1991, 95, 7805. Uchiyama, H.; Kaneko, K.; Ozeki, S. Langmuir 1992, 8, 624. Ozeki, S.; Uchiyama, H.; Kaneko, K. J. Colloid Interface Sci. 1992, 154, 303. (2) Ozeki, S.; Wakai, C.; Ono, S. J. Phys. Chem. 1991, 95, 10558.
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temperature dependence of ∆M. These suggestions were experimentally confirmed: We found the proportionality between ∆p and p0 in all of the systems examined (Figure 1), ∆p ) ki(p0 - pi) for pi < p0 < pi+1 (ki is a constant), and ∆p ∝ T-2 in the systems of asbestos and silica in which ∆M ∝ T-1 (see below). Equation 2 also suggests that there can be a variety of H dependences of ∆p, depending on ∆M; e.g., ∆p > 0 if the magnetization of the system decreases with adsorption and ∆p < 0 when the adsorbed phase is less diamagnetic than bulk water or is paramagnetic. Figure 2 shows examples of the MAD of water on various solids at 303 K as a function of H. Here, ∆v is the change in the amount of water adsorbed, which was estimated from ∆p. In fact, as can be seen in Figure 2A, magnetodesorption (type I) and magnetoadsorption (type II) apepared at all H employed. Equation 2 gives ∆p/p0 ) ((10-4∼10-2) for an assumed χapp ∼ (10-2 cm3/g in the 1∼10 kG range, which were comparable with the observed |∆p/p0| of (0.3∼5) × 10-3. An example for type I is the magnetic-field effect on the dissociation of hydrogen from ferromagnetic hydrides, such as LaCo5Hx,3 although the obtained ∆MH values were more than 105 times larger than in our case. Figure 2B shows another type of H dependence of MAD, the magnetodesorption-to-magnetoadsorption transition (type III). When ∆M ) ∆χH (∆χ ) χapp - χb, where χapp is the apparent magnetic susceptibility of the adsorbed phase and χb the magnetic susceptibility of bulk water), eq 2 gives ∆p ∝ ∆χH2 under constant p0 and T. However, as Figure 2 demonstrates, ∆p does not depend on H2, even in type III. Consequently, ∆χ or χapp must be a function of H. Figure 3 demonstrates such an H dependence of χapp of water adsorbed on silica and TiO2 at 303 K. These trends are similar to their MAD profiles: MAD may be closely related to the magnetization of the adsorbed water. Thus, the dependences of MAD on magnetic-field intensity as well as on the equilibrium pressure and temperature seem to be consistent with the thermodynamical characteristics from eq 2. Since the magnetic-field-induced change (∆v(H)) in amount of water adsorbed is proportional to ∆p, ∆v(H) is used instead of ∆p in the following. The facts that χapp of water on silica was positive over 4.5 kG (Figure 3) and obeyed the Curie law under 10 kG over 225 K and also that the ESR signals of silica changed significantly due to water adsorption at 303 K suggest that at least a portion of the adsorbed water on silica should be in a paramagnetic environment under a static magnetic field. Therefore, we assume that the adsorbed water phase comprises two states, diamagnetic (d) and paramagnetic (p), which have magnetic susceptibilities, χd and χp, a MAD of ∆vd(H) and ∆vp(H), and mass fractions md(H) and mp(H) (md(H) + mp(H) ) 1). The p state does not mean paramagnetic water (such as a hydrated electron) but, rather, water states interacting with paramagnetic centers, such as hydrated radicals on the surfaces and hydrated paramagnetic ions (such as contaminants released by water adsorption). Then, the observed χ and ∆v are denoted by
χ(H) ) md(H)χd + mp(H)χp ) χd + mp(H)(χp - χd) (3) and
∆v(H) ) md(H) ∆vd(H) + mp(H) ∆vp(H) ) ∆vd(H) + mp(H)(∆vp(H) - ∆vd(H)) (4) (3) Yamamoto, I.; Yamaguchi, M.; Goto, T.; Sakakibara, T. Z. Phys. Chem. N. F. 1989, 163, 671.
© 1996 American Chemical Society
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Figure 1. Pressure change (∆p) of solid/water adsorption systems due to steady magnetic fields as a function of the equilibrium pressure (p0) of water at 303 K. Arrows show the ordinate for each plot to refer to. Samples: carbon black (NPC) at 1.0 kG (O), 3.8 kG (y), and 9.6 kG (b); molecular sieves (MS5A) (4) and chrysotile asbestos (2) at 9.6 kG.
Notes
Figure 4. Comparison of estimated ∆v by eq 6 with the experimental ∆v (in Figure 2). Arrows show the ordinate for each plot to refer to. SiO2: (s) estimated; (b) experimental. TiO2: (- - -) estimated; (O) experimental.
∆v(H) ) vd(H) + (apH2 - ∆vd(H))(χ(H) - χd)/(χp - χd) (5) Eliminating ap using eq 5 for H1 ()10 kG),
∆v(H) ) (χp - χ(H))∆vd(H)/(χp - χd) + {H2(χ(H) - χd)/H12}{(∆v(H1) ∆vd(H1))/(χ(H1) - χd) - ∆vd(H1)/(χp - χd)} (6)
Figure 2. Magnetic-field induced adsorption and desorption of H2O onto various solids at 303 K as a function of static magnetic field intensity.2 A (4) calf thymus DNA; (2) R-Fe2O3; (1) TiO2; (3) Fe3O4. B: (b) SiO2; (O) zeolite 5A; (i) carbon black; (Q) activated carbon fiber; (y) γ-FeOOH.
Figure 3. Magnetic susceptibility χ of water adsorbed on SiO2 (b) and TiO2 (O) at 303 K. Broken line is the χb value for bulk water.
where f(H) means that f is a function of H. Equation 3 indicates that χ(H) would change with mp(H), e.g., linearly with H, if mp(H) is in proportion to H, as shown in the χ-H plot for SiO2 (Figure 3), where the χapp values at low H were comparable with that of diamagnetic bulk water (χb ) -0.72 × 10-6 cm3/g). The consistency between the assumed χp and the observed χapp can be maintained, e.g., if mp(H) ∼10-4 for χp ∼ 10-2 cm3/g. Equation 4 is obtained from eq 2, and ∆vd(H) and ∆vp(H) are also each given by eq 2. However, since we have no data about χd and χp, we cannot directly predict ∆vd(H) and ∆vp(H), and thus ∆v(H), by solving eqs 3 and 4. Consequently, we focus on estimating ∆v(H) using the observed χ and observed ∆v at a certain H. Equation 4 is transformed using mp(H) from eq 3 and eq 2 (∆v(H) ∝ ∆χH2) for the p state, i.e., ∆vp(H) ) apH2:
If the H dependence of ∆vd(H) is small over the 1 to 10 kG range, we may assume that all parameters in eq 6 having subscript d should be approximated by those at 1 kG. Then, eq 6 gives ∆v(H) values for the observed χapp values, as shown in Figure 4. The plot approximately fits the experimental values for silica (type III), suggesting that the model and assumptions might be correct. The fit for TiO2 is very poor, because some of the above assumptions are not satisfied, e.g., as can be seen in the positive ∆vd(H) and the less negative χ at 1 kG (inapplicability of eq 3 over 1 kG), which are obviously affected by the paramagnetic phases. In general, eq 6 can reproduce type III, as in silica, but not the saturation behavior (or the very weak H dependence) of ∆v, such as types I and II, which are probably a part of the ∆v profile of type III (e.g., γ-FeOOH) in the low- and high-H regions, respectively. To understand the large or strongly H-dependent magnetodesorption which appeared in the low-H region at less than 1 kG, more diamagnetic water than bulk water must be assumed to exist in the adsorbed phase, which might refer to a superdiamagnetic domain having χsd (
