Dielectric-relaxation Spectroscopy of Kaolinite, Montmorillonite, Allophane, and Imogolite under Moist Conditions

-The dielectric behavior of kaolinite, montmorillonite, allophane, and imogolite samples adjusted to a water potential of 33 kPa was examined using a time-domain reflectometry method over a wide frequency range of 103-10 ~~ Hz. A dielectric relaxation peak owing to bound H20 was observed. The observation of this peak required the precise determination of the contributions of dc conductivity. The peak is located at 10 MHz, indicating that the relaxation time of the bound H20 is approximately ten times longer than the relaxation time of bound H20 with organic polymers, such as an aqueous globular-protein solution. The structure of bound H20 differs between phyllosilicates and amorphous phases, based on differences in relaxation strength and the pattern of distribution of the relaxation times. The dielectric process involving rotation of bulk H20 molecules was also observed at 20 GHz. The relaxation strength of bulk H20 increased with an increase in the water content. The interfacial polarization in the diffuse double layer occurred only in montmorillonite and kaolinite, indicating that mechanisms involving the Maxwell-Wagner and surface-polarization effects cannot be extended to include allophane and imogolite. Although these results suggest that additional work is required, a tentative conclusion is that a tangential migration of counter-ions along clay surfaces may be important. Key Words--Allophane, Bound Water, Complex Permittivity, Dielectric-Relaxation Spectroscopy, Imogolite, Interfacial Polarization, Kaolinite, Montmorillonite, Time-Domain Reflectometry. I N T R O D U C T I O N Dielectr ic-relaxat ion spectroscopy probes a molecular envi ronment on the broad t ime domain be tween 10-12-10 -2 s, compared with other methods, such as nuc lear magne t i c resonance ( N M R ) spec t roscopy whose t ime scale is be tween 10-1~ -3 s. Therefore, dielectr ic-relaxation spectroscopy provides data concerning many molecular arrangements of H20 depending on the t ime scale involved (Sposito and Prost, 1982). In c lay-mineral studies, dielectric measurements obtain information about the bound H20 on clay surfaces (e.g., Fripiat etal . , 1965; Mamy, 1968; Weiler and Chaussidon, 1968; Hoekstra and Doyle , 1971; Calvet, 1975; Hall and Rose, 1978) and about the interfacial region between clay particles (e.g., Lockhart , 1980a, 1980b; Raythatha and Sen, 1986). M a m y (1968) and Calvet (1975) per fo rmed dielectric measurements on one-layer hydrated montmori l lonite by changing the temperature at a fixed frequency be tween 300 Hz and 10 kHz. They conc luded that the rotational motions of bound H20 on Naand Krich montmori l loni te are on the same scale as those in free H20. On the other hand, the rotational t imes on the montmori l loni te saturated with bivalent cations are comparable to ice (Sposito and Prost, 1982). The conclusion implies that the relaxation t imes of bound H : O are separated by -105 orders of magnitude, depending on the solvation complexes formed with the exchangeable cations. For an aqueous electrolyte solution, N M R data indicate that the rotational correlat ion t imes for Copyright 9 2000, The Clay Minerals Society H20 molecules in the first hydrat ion sphere around the cat ion might be different by no more than one order in magni tude be tween monova len t and bivalent cations (Hertz, 1973). In general, for isotropic motion, the N M R correlat ion t ime is more closely related to the dielectr ic-relaxat ion t ime because the latter is three t imes the former (e.g., Carrington and McLachlan , 1967). Furthermore, the relat ion is applicable to the correlat ion t ime and the relaxat ion t ime for bound H : O in b iopo lymer solutions (Fukuzaki et al., 1992, 1995). The large difference in the dielectr ic-relaxat ion t ime probably cannot be expla ined by only the small difference owing to the solvat ion of exchangeable cations. Hall and Rose (1978) measured dielectr ic-absorption curves for kaolini te at ve ry low-water contents over the f requency range of 100 to 10 MHz. One absorption peak was found. The mechan i sm responsible for absorption was conc luded to be caused by H : O molecules bound to kaolinite. Judging f rom the values of the relaxat ion time, H20 molecules in a onelayer hydrate have strong similari t ies to an " icel ike structu re" , the exchangeable cat ion is monovalent . These results are not consistent with those for montmor i l lonite (see above). Note, however , that there is ev idence o f another absorption found at near 10 MHz. Lockhart (1980a, 1980b) obtained dielectric-dispersion curves for montmor i l lon i te and kaolini te suspensions over the f requency range similar to that used by Hall and Rose. The suspensions var ied be tween highly