J. Phys. Chem. 1996, 100, 4205-4212
4205
Water-Solid Interactions under Steady Magnetic Fields: Magnetic-Field-Induced Adsorption and Desorption of Water Sumio Ozeki,* Junichi Miyamoto, Shinji Ono, Chihiro Wakai, and Tomotaka Watanabe Department of Chemistry, Faculty of Science, Chiba UniVersity, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan ReceiVed: September 28, 1995; In Final Form: December 1, 1995X
The equilibrium pressure (p0) of water vapor/solid systems changed upon applying relatively small magnetic fields (H < 10 kG) at 303 K, which refers to the magnetic-field-induced adsorption and desorption (MAD). Only weakly interacting water with solid surfaces, such as water in a multilayer, condensed water in pores, and clusterlike water around functional groups on hydrophobic surfaces, was apt to respond to magnetic fields. Water in the first layer on hydrophilic surfaces did not respond, and micropore-filling water responded only weakly, depending on the micropore size or cluster size of water. There were three types of magneticfield dependence of MAD: magnetoadorption, magnetodesorption, and the magnetodesorption-to-magnetoadsorption transition. The apparent magnetization of water adsorbed on silica and titania was less negative than that of bulk water. Thus, the dependences of MAD on p0, temperature, and H are discussed thermodynamically based on the magnetization changes of the adsorption systems during adsorption.
Introduction Solid surfaces generally having geometrical, and thus energetic heterogeneities which arise from defects, pores, and variously indexed surfaces exert adsorption potentials on the adsorptives. Thus, an adsorbed phase on such solid surfaces may be different from its bulk phase. An example is changes in the magnetic property of adsorptives in adsorbing. Paramagnetic nitrous oxide may form a diamagnetic condensed phase1 on solids at a much higher temperature than the boiling and critical points, which has been ascribed to the NO dimer.2 Also, para/ortho hydrogen conversion is catalyzed by various solid surfaces, especially those having paramagnetic centers.3 It has been reported that the adsorption of paramagnetic NO is affected by steady magnetic fields.4,5 NO adsorption was enhanced on metal oxides by magnetic field, besides FeOOH polymorphs, from which NO was desorbed.4 On microporous materials, such as zeolites and activated carbon fibers, magnetic fields promoted NO micropore filling.5 Some causes for magnetic-field-induced adsorption and desorption (MAD), such as magnetism, adsorption sites, and porosity of solids, have been examined, and some correlation was recognized between them, though the mechanism of the magnetic effect on gas adsorption is not understood at all. There have been many reports concerning the properties of water on solid surfaces and around solutes in aqueous solutions. Water bound to molecules and solids plays very important roles in the activities of solids and the conformation of molecules; also, its own activity is modified by the water-substance interfaces. It is well-known that hydrated or adsorbed water is different from bulk water in its structure and static and dynamic properties. If the magnetic properties of water around substances change via interactions with the surfaces, some phenomena via the changes may be expected. MAD seems to be one of the phenomena based on such magnetic behavior. We have published a preliminary paper showing the experimental possibility of the magnetic effect on water adsorption.6 In this paper we try to confirm the magnetic effect on water adsorption, and discuss a possible mechanism for this phenomenon. * Address correspondence to this author. Fax: 81-43-290-2783. Email:
[email protected]. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4205$12.00/0
TABLE 1: Specific Surface Area and Magnetism of Samples for H2O Adsorption sample
as, m2/g
magnetisma
γ-FeOOH δ-FeOOH R-Fe2O3 γ-Fe2O3 Fe3O4 SiO2 zeolite (MS5A) chrysotile asbestos montmorillonite TiO2 carbon black (NPC) activated carbon fiber (A10) DNA
97 75 28 95 90 320 400 65 10 22 69 1230
antiferrob ferri parasticb ferri ferri dia dia dia dia para dia dia dia
a dia, diamagnetism; para, paramagnetism; ferri, ferrimagnetism; antiferro, antiferromagnetism; parastic, parastic ferromagnetism. b γFeOOH and R-Fe2O3 are paramagnetic at 303 K.
Experimental Section Materials. γ- and δ-FeOOH were prepared by NaNO2 oxidation of an aqueous FeCl2 solution and by rapid oxidation with H2O2 of Fe(OH)2, respectively.4b R- and γ-Fe2O3 were prepared by heating γ-FeOOH in air at 673 K for 2 h and in a vacuum at 473 K for 3 h.4b Magnetite (Fe3O4) was prepared in an N2 atmosphere by coprecipitation from a mixed solution of FeSO4 and FeCl3.4b Chrysotile asbestos (Mg6Si4O10(OH)8) was synthesized from a stoichiometrically mixed solution (pH 13) of aerosil and MgCl2 at 573 K and 150 atm under an N2 atmosphere for 8 h.7 SiO2(JRC4), montmorillonite, TiO2 (P25; anatase), carbon black, and activated carbon fiber A10 were kindly supplied from the Japanese Society of Catalysts, Kunimine Kogyo Co. Ltd., and Nippon Aerosil Co. Ltd., Mitsubishi Kasei Co., and Osaka Gas Co., Ltd., respectively. Molecular sieve 5A (MS5A) was purchased from Gasukuro Kogyo, Co. Ltd. Calf thymus DNA (Sigma) was used without further purification. Gd3+-supported SiO2 was prepared by drying SiO2 (JRC4) dispersed in aqueous GdCl3 solutions (0.01-1 mol/dm3) for 5 h. Table 1 summarizes these samples for water adsorption. All samples, except for carbons and DNA, were examined by means of an X-ray diffractometer (Rigaku Denki Geigerflex 2028). The specific surface area (as) of the samples was © 1996 American Chemical Society
4206 J. Phys. Chem., Vol. 100, No. 10, 1996 estimated from N2 adsorption isotherms at 77 K after DNA, MS5A, and others were degassed at room temperature and 1 mPa for 5 h, at 573 K and 1 mPa for 5 h, and at 383 K and 1 mPa for 5 h, respectively. The surface coverage (θ) of water is defined as the ratio of the amount of water adsorbed to the monolayer capacity of water estimated from the BET method. Water Adsorption under Steady Magnetic Fields. The amount of water adsorbed on solids was determined from the pressure decrease, which was monitored with a Baratron 222C sensor.4 The total volume of the adsorption system was 40 mL, and the whole adsorption system was kept at 303 ( 0.1 K. The sensitivity of the adsorption measurement was about 1 µg/g of the adsorbent, which corresponds to a water-pressure change of 0.01 Torr. The pressure was recorded by a personal computer at 2 s intervals. The adsorption cell, made of quartz, was a rectangle of 4 × 18 × 30 mm3 and an inner width of 2 mm. The water-pressure change was measured upon applying steady magnetic fields of 1.0, 3.8, 7.6, and 9.6 kG to the water vaporadsorbent systems. Permanent magnets on a pair of guide rails were moved using a stick by hand in a few seconds when the quasi-adsorption equilibrium was attained at each water pressure, more than 24 h after introducing water vapor. The homogeneity of the magnetic field at the postion of the adsorption cell was within 2% for 3.8, 7.6, and 9.6 kG and within 5% for 1.0 kG. The samples were pretreated using similar methods as those for N2 adsorption. In some cases, dummy magnets made of aluminum, which were in contact with the magnets, were used to check for any pressure change (e.g., due to a temperature change) associated with insertion of the magnet. However, no pressure change was detected by this procedure. Doubly distilled and deionized water and D2O (>99.5%, Wako Junyaku Co., Ltd.) were used after repeated degasification in a vacuum (1 mPa). Magnetization and ESR Measurements. The magnetization of SiO2 and TiO2, which were enclosed in 5 mm diameter ESR tubes after a pretreatment at 383 K and 1 mPa for 5 h and subsequent water vapor introduction at 303 K, was measured by means of an MPMS2 SQUID magnetometer (Quantum Design, Co.) at 173-303 K, whose precision was 10-7 emu; also, the measured magnetization was on the order of 10-510-4 emu. The apparent magnetic susceptibility of adsorbed water (χapp) was estimated from MH/mH, where the apparent magnetization (MH) of water adsorbed (adsorbed amount, m) under H was estimated from the difference in the magnetization between solids and the solids adsorbing water. The ESR spectra were measured with a JEOL JES-RE2X both before and after water introduction at room temperature. Samples in 5 mm diameter ESR tubes with stopcocks and a water reservor were pretreated under the same conditions as those for the magnetization measurement. Results Magnetic-Field-Induced Adsorption and Desorption (MAD). Figure 1 shows examples of the time course of the amount of water adsorbed with the application of steady magnetic fields. The equilibrium water pressure (p0) changed just after the application of a static magnetic field to the water vapor-solid systems which were in equilibrium, and reached a constant value (p0 + ∆p) within a few minutes. The pressure of the water vapor remained constant up to 50 h under the magnetic field. Upon removing the magnetic field, the pressure recovered reversibly: the MAD phenomenon of water is mainly the magnetic effects on physically interacting water with surfaces. Figure 2 shows the ∆p-p0 relations under 1-9.6 kG. In a carbon-black system, ∆p is proportional to p0, irrespective of
Ozeki et al.
Figure 1. Examples of time course of water pressure with the application of steady magnetic fields to solid/water adsorption systems at 303 K: upper, 1.0 kG; lower, 9.6 kG. Arrows: ON, application of magnetic field; OFF, removal of magnetic field. Sample: activated carbon fiber (A10).
H. The ∆p-p0 plot for A10 seems to comprise two or more linear regions. On the other hand, ∆p in the systems of chrysotile asbestos (MS5A) and probably SiO2 change linearly with p0 above a certain threshold pressure of around 3-4 Torr. The magnetic response of vapor pressure (|∆p|) on chrysotile asbestos was abruptly depressed beyond p0 ∼ 21 Torr. In general, the results may be denoted by
∆p ) ki(p0 - pi)
for pi e p0 < pi+1 (i ) 1, 2, 3, ...) (1)
where ki is the magnetic-response coefficient, which represents the sensitivity for the magnetic response of pressure depending on H, and pi and pi+1 are the lower and upper threshold pressures, respectively, which give the border of the pressure region (or the adsorption phase) having a different magnetic response. For example, in the case of 9.6 kG k1 ) 0 at 0 < p0 < 4.15 Torr, k2 ) 0.020 at 4.14 < p0 < 21, and k3 ∼ 0 at 21 < p0 for chrysotile asbestos and k1 ) 0.018 at 0 < p0 < 9 Torr, k2 ) -0.0095 at 9 < p0 < 15, and (k3 ∼ -0.005 at at 15 < p0) for A10. The equilibrium amount of MAD, the difference between the initial amount of adsorbed water and the final saturation value, is denoted by ∆V, which was calculated from the pressure change (∆p). The largest response was 5% of the total amount (V0) of water adsorbed. The ∆p-p0 relations (Figure 2) are redrawn as changes in ∆V as a function of V0 at 303 K in Figure 3. Weakly bound water on hydrophobic surfaces, such as a (nonporous) carbon black (NPC) and activated carbon fiber (A10), responded to a magnetic field. In other systems, there seemed to be a threshold amount of water adsorbed for each system to respond to a magnetic field, which agrees approximately with each monolayer capacity (14 mg/g for chrysotile asbestos, 26 mg/g for γ-FeOOH, 35 mg/g for silica, and 110 mg/g for MS5A): Water in the first layer on hydrophilic oxide surfaces, such as γ-FeOOH, silica, and chrysotile asbestos, and in ultramicropores of zeolite did not respond to a magnetic field, as can be seen in the ∆V -θ relation (Figure 4). In the case of silica, water in the first layer also responded slightly because the surfaces are partially hydrophobic. 100∆V/V0 for NPC changed with V0 through a maximum or minimum, depending on H, suggesting that water in a multilayer on nonporous NPC seems to be less sensitive to magnetic fields. 100∆V/V0 for A10 decreased stepwise along with increase in V0, as shown in Figure 5. A10 has discrete, slitlike micropores of about 0.7 and 1.0 nm width, whose micropore capacity, determined by the preadsorption method of dye,8 corresponds to 80 and 190 mg of H2O/g, respectively. Thus, the steps on
Magnetic Effect on Water-Solid Interactions
J. Phys. Chem., Vol. 100, No. 10, 1996 4207
Figure 3. Change in the adsorption amount of water (∆V) due to steady magnetic fields as a function of the equilibrium adsorption amount (V0) on various solids at 303 K. Samples: (4) SiO2; (0) molecular sieves (MS5A); (O) chrysotile asbestos; (]) γ-FeOOH. H/kG: solid, 9.6; open, 1.0.
Figure 4. Change in the adsorption amount of water (∆V) as a function of the surface coverage (θ) at 303 K. Samples: (4) SiO2; (0) molecular sieves (MS5A); (O) chrysotile asbestos; (]) γ-FeOOH. H/kG: solid, 9.6; open, 1.0.
Figure 2. 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. Samples: (A) carbon black (NPC) at 1.0 kG (O), 3.8 kG (y), and 9.6 kG (b); (B) SiO2 (b), molecular sieves (MS5A) (O), and γ-FeOOH (y) at 9.6 kG; (C) chrysotile asbestos at 1.0 kG (O), 3.8 kG (y), and 9.6 kG (b); (D) activated carbon fiber (A10) at 1.0 kG (O), 3.8 kG (y), and 9.6 kG (b).
the 100∆V/V0-V0 plot for A10 seem to reflect the difference in the response of micropore filling water in micropores having a discrete size. Also, the steep drop in the ∆p-p0 plot in Figure 2 is considered to arise from capillary condensed water in the
Figure 5. 100∆V/V0 for activated carbon fiber (A10) as a function of V0 at 303 K. H/kG: (O) 1.0; (y) 3.8; (b) 9.6.
cylindrical mesopores of 7.0 nm in diameter of the chrysotile asbestos, which should occur at around a p0 of 22 Torr.9 These results suggest that magnetic fields may affect the water adsorption when the magnetic energy gained by external magnetic fields is sufficiently large to overcome any watersurface interactions and hydrogen bonding in the water network (cluster).
4208 J. Phys. Chem., Vol. 100, No. 10, 1996
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Figure 7. Time course of the pressure of water vapor in equilibrium with chrysotile asbestos with the application of a steady magnetic field (9.6 kG) at various temperatures. Arrows: ON, application of magnetic field; OFF, removal of magnetic field.
Figure 6. Magnetic-field-induced adsorption and desorption of H2O onto various solids at 303 K as a function of the static magnetic-field intensity: (A) O, calf thymus DNA; 0, R-Fe2O3; 9, γ-Fe2O3; b, TiO2; ], Fe3O4; (B) O, SiO2; [, molecular sieves (MS5A); K, chrysotile asbestos; 4, carbon black (NPC); 2, activated carbon fiber (A10); 3, γ-FeOOH; 1, δ-FeOOH.
Figure 6 shows the MAD of water on various solids at 1520 Torr and 303 K as a function of the static magnetic-field intensity. There are three types of magnetic-field dependences of MAD: the magnetodesorption (I; MD) and magnetoadsorption (II; MA) at all magnetic field intensities employed and the magnetodesorption-to-magnetoadsorption transition with the magnetic field (III; MAD). Whereas a calf-thymus DNA and R- and γ-Fe2O3 belong to type I, Fe3O4 and TiO2 belong to type II. Type III includes SiO2, chrysotile asbestos, montmorillonite, MS5A, carbon black, A10, and γ- and δ-FeOOH. The apparent magnetic field of the MD/MA transition, which gives ∆V ) 0, depended on the kind of solid: around 5 kG for γand δ-FeOOH and 8-9 kG for others. The magnetic response in water adsorption was appreciable in SiO2, chrysotile asbestos, and A10, which are all diamagnetic. The FeOOH and Fe2O3 polymorphs each showed the same type I and III, irrespective of their magnetism (Table 1). This suggests that MAD should not be subject to the bulk magnetism of solids, but, rather, to surface-active sites, such as surface defects or adsorption states. This tendency was also observed in the MAD of paramagnetic NO.4 Effect of the Temperature, Isotope, and Paramagnetic Ions on MAD. Figure 7 shows an example of the temperature dependence of ∆p on chrysotile asbestos at 9.6 kG, where p0 at each temperature was different, because the temperature of the closed system was changed. This method was adopted because it was too difficult to measure ∆p at the same equilibrium pressure (p0) when the temperature of the open system was changed. The apparent enhancement in magnetoadsorption due to a temperature increment may arise from the pressure (p0) increment. If this pressure effect is corrected usig eq 1 as p1 ) 4.15 Torr (see the text below eq 1), the magnetic response of
Figure 8. Apparent magnetic susceptibility χapp of water adsorbed on SiO2 (b) and TiO2 (O) at 303 K. Broken line is the χb value of bulk water.
adsorption (k2) seems to decrease along with increasing temperature: k2 ) -0.021 at 299 K, -0.020 at 303 K, and -0.019 at 307 K. The MAD of D2O did not differ significantly from that of H2O around V0 ∼ 7 mg/g: 100∆V/V0 (%) for D2O and H2O on NPC were, respectively, 1.0 and 0.8 ((0.1) at 9.6 kG, -0.9 and -0.75 ((0.1) at 3.8 kG, and -1.1 and -0.75 ((0.15) at 1.0 kG. Although the MAD of H2O and Gd3+-supported silicas seemed to be slightly (less than 10%) depressed by Gd3+, the depression of MAD was insensitive to the amount of Gd3+ (checked by the ESR signal intensities). This suggests that a trace of paramagnetic species should give rise to MAD, that only a certain paramagnetic center on surfaces should cause MAD, or that the paramagnetic centers should play only a small role in MAD. Magnetization Changes during Water Adsorption. Figure 8 shows the apparent magnetic susceptibility (χapp) of water adsorbed on SiO2 and TiO2 at 303 K as a function of the magnetic field intensity. The extrapolated χapp value of water adsorbed on silica to zero magnetic field, -(0.63 ( 0.1) × 10-6 cm3/g, is comparable to that (χb ) -0.72 × 10-6 cm3/g) of bulk water, considering that the magnetic measurements were difficult because of the small amounts of adsorbed water, the small χ value, and, especially, the relatively large magnetization of the sample tube used (ESR tubes). Along with an increase in the H, the χapp value of adsorbed water increased monotonously to become positive (paramagnetic) near an H of 4.5 kG, and reached +0.86 × 10-6 cm3/g at 10 kG. In the case of TiO2, the χapp value at 1 kG was about half that of bulk water, thus suggesting fewer diamagnetic components in the adsorbed phase, even under low H. The χapp of the water phase adsorbed on SiO2 at 10 kG changed from -0.87 × 10-6 cm3/g at 173 K to +0.86 × 10-6 cm3/g at 303 K, through the maximum near to 225 K. The χapp
Magnetic Effect on Water-Solid Interactions
Figure 9. Relationship between the apparent magnetic susceptibility (χapp) of water adsorbed on SiO2 at 10 kG and the reciprocal of temperature.
J. Phys. Chem., Vol. 100, No. 10, 1996 4209 2.0493) almost disappeared and a shoulder (g ) 2.0653) became manifest. TiO2 showed a broad ESR signal (g ) 2.255) and a sharp signal (g ) 2.016) with fine structures, which may be assigned to Ti3+.11 The introduction of water vapor to the TiO2 system diminished the broad signal and enhanced the sharp signal, whose fine structures became more manifest. Although the ESR spectra were apt to be different from run to run, the main features due to water adsorption were common: the signals around g ) 2.00 (free electrons) were enhanced and the others reduced. The characteristics were also observed in other adsorption systems, such as ZnO and MS5A. Obviously, this behavior resulted from the interactions of water molecules with the paramagnetic centers; e.g., the coordination of water molecules to Ti3+, which may lead to more symmetric species, and electron transfer from the paramagnetic centers to the water phase. Discussion
Figure 10. Electron paramagnetic resonance spectra of SiO2 (A) and TiO2 (B) at 303 K: upper, in vacuum; lower, after introduction of water vapor.
above 225 K is proportional to T-1, as shown in Figure 9. Thus, this temperature dependence of positive χapp suggests that some portion of the water phase adsorbed on, at least the silica, should be paramagnetic. ESR Spectra of Silica and Titania Adsorbing Water. Paramagnetic species on SiO2 and TiO2 were observed by the ESR spectra, as shown in Figure 10. The ESR signals of SiO2 appeared at g ) 2.0493 with a shoulder near g ) 2.0653 and around g ) 2.317. Though silica may usually have radicals, such as Si• and SiO•,10 the signals observed here seemed not to agree with them. Upon water adsorption, the main peak (g )
Water-Solid Interactions and MAD. Water in the first layer on hydrophilic surfaces and in the thick adsorption phase (bulklike water) was insensitive to steady external magnetic fields. Water interacting with surfaces, such as clusterlike water around functional groups on the hydrophobic surfaces and water in the multilayer, typically in bilayer and trilayer, seemed to respond to a magnetic field. The confined water in the micropores and the capillary condensed water in the mesopores also responded to a magnetic field, though more weakly than the multilayered water. These facts suggest that some structured water, such as clusters with hydrogen-bonded networks, and water moderately interacting with surfaces may lead to MAD. Thus, magnetic fields seem to affect the adsorption when the magnetic energy gained by an external magnetic field is sufficiently large to overcome water-surface interactions and hydrogen bonding in the water networks. However, the magnetic energy of water seems to be too small, less than 1 cal/mol even under 10 kG, to bring about such a large MAD, considering that the energy for the physical adsorption of water is around 10 kcal/mol. In addition, magnetoadsorption is a remarkable phenomenon, because diamagnetic water tends to be repelled from a magnetic field. From these view points, the magnetic properties of water (phase) could change through an interaction with surfaces. In fact, the trends of the χapp-H relations of water adsorbed on SiO2 and TiO2 are similar to their MAD profiles (Figures 6 and 8); MAD thus seems to relate closely to the magnetization of the adsorbed water. Thermodynamic Characteristics of MAD. It is possible to affect gas adsorption if the magnetization of an adsorptive and/or adsorbent changes during adsorption. When the equilibrium pressure of a water/solid adsorption system (p0) changes to p0 + ∆p due to the application of a magnetic field (H) at T K, the change in the free energy of the gas phase (per mol H2O) is δGg ) RT{ln(p0 + ∆p) - ln p0}. On the other hand, the magnetization change (∆M) of the system due to the adsorption of 1 mol of water under H causes a change in the magnetic free energy of the system, δGm ) ∆MH. In an adsorption equilibrium under a steady magnetic field, δGg + δGm ) 0, and, thus, the relation between ∆p and H may be given by12
RT{ln(p0 + ∆p) - ln p0} ) -∆MH
(2)
Under the experimental condition p0 . ∆p, eq 2 becomes
∆p/p0 ) -∆MH/RT
(3)
When the adsorbed phase comprises several magnetic states,
4210 J. Phys. Chem., Vol. 100, No. 10, 1996
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∆p is given by
∆p/p0 ) ∑mi(-∆MiH/RT)
(4)
i
where ∆Mi and mi are ∆M and the molar fraction of the i state, which is, for example, the j-th layer in an adsorbed multilayer, the capillary condensed phase, the micropore filling phase, etc. Also, each phase may include different magnetic states, such as diamagnetism and paramagnetism, as discussed below. Equation 3 or 4 predicts a linear relation between ∆p and p0 under constant H and T. We have already seen the proportionality between ∆p and p0 in eq 1, where ki would correspond to -mi∆MiH/RT. Also, the equation requires that the temperature rise should reduce MAD according to ∆p ∝ T-2 as long as ∆M or χ is proportional to T-1, as shown in Figure 9. The k2 ()∆p/ (p0 - p2)) value estimated there gave a linear relation against T-2, although the temperature dependence of MAD was delicate, as discussed in Figure 7. Equation 3 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. In fact, as can be seen in Figure 6A, magnetodesorption (type I) and magnetoadsorption (type II) appeared at all H employed. An example for type I is the magnetic-field effect on the dissociation of hydrogen from ferromagnetic hydrides, such as LaCo5Hx,12 although the ∆MH values were 105 times larger than in our case (H > 50 kG). The positive χapp of water adsorbed on SiO2 beyond 4.5 kG and the less diamagnetic water phase on TiO2 than bulk water (Figure 8) demonstrate that ∆p < 0 may be possible at any H, because ∆M > 0 or ∆χ > 0 when ∆M ) ∆χH (∆χ ) χapp χb). On the other hand, the magnetodesorption (∆p > 0) should be ascribed to more diamagnetic water than bulk water from the view point of eq 3. Such water might be referred to as a superdiamagnetic domain having χsd (
