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Time Dependence of the Spectra of Methylene Blue-Clay Mineral Suspensions Katrien Y. Jacobs and Robert A. Schoonheydt* Center for Surface Chemistry and Catalysis, Department of Interphase Chemistry, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium Received January 29, 2001. In Final Form: June 4, 2001 Aqueous dispersions of different smectite-type clay minerals were probed by ion-exchange of methylene blue (Mb). The evolution of the absorption spectra with time demonstrates the importance of the strength of the Mb-clay surface interaction, which is affected by the colloidal nature of the suspension and typical clay characteristics. A strong Mb-surface interaction is observed for clay minerals with tetrahedral substitution and a large basal surface area. Mb species adsorb in a more polar environment and show well-resolved bands. Their mobility is small. When the Mb-surface interaction is weaker, Mb-Mb interactions and interactions among the clay particles dominate. This is the case for octahedrally substituted clays with a large edge surface. Mb species experience the influence of the surrounding water phase and of neighboring Mb molecules. They remain mobile, even for longer aging periods. The absorption spectra evidence a less polar, “undefined” adsorption environment.
1. Introduction Aqueous dispersions of swelling clay minerals are known for their colloidal and rheological properties and have several industrial applications.1,2 Numerous studies have been performed to unravel these systems with regard to their colloidal nature, the structure of the clay-water interface, particle interactions, and the stability of the suspensions. Nonetheless, these topics remain a matter of dispute. Factors of importance are the size, shape, and anisotropy of the clay platelets, the location of the layer charge, and the exchangeable cation. Theoretical studies reveal that a negative charge due to tetrahedral isomorphic substitution resides largely at the basal O-plane. An octahedrally located layer charge causes no localized charges at the surface, giving it a more hydrophobic character.2-6 Such differences will have a strong impact on the structuring of the water molecules and the exchangeable cations in the double layer. Three types of counterions are proposed: inner-sphere complexes, in which the cation desolvates and is captured by a ditrigonal cavity; outer-sphere complexes, in which the cation solvates but is still immobilized at the surface; and the diffuse ion-swarm, in which the cation is attracted to the surface but resides in the double layer.5 The relative importance of these three types of cation complexes depends on the clay structure and the nature of the cation. Both clay and cation will then largely determine the orientation of the water molecules in the interface and have important implications on the colloidal and rheological properties of the clay suspensions. Probing aqueous dispersions of clay minerals is not straightforward. One possibility is the use of a dye * Corresponding author: Prof. Dr. Ir. R. A. Schoonheydt. Tel: 0032-16-321622.Fax: 0032-16-321998.E-mail: Robert.Schoonheydt@ agr.kuleuven.ac.be. (1) Industrial Clays, 2nd ed.; Kendall, T., Ed.; Industrial Minerals Information Ltd., a subsidiary of Metal Bulletin Plc: London, 1996. (2) Konta, J. Appl. Clay Sci. 1995, 10, 275. (3) Bleam, W. F. Clays Clay Miner. 1990, 38, 527. (4) Prost, R.; Koutit, T.; Benchara, A.; Huard, E. Clays Clay Miner. 1998, 46, 117. (5) Sposito, G. Clay-water interphase and its Rheological implications. In CMS Workshop Lectures, Vol. 4; Gu¨ven, N., Pollastro, R. M., Eds.; The Clay Minerals Society: Boulder, CO, 1992. (6) Sposito, G.; Prost, R. Chem. Rev. 1982, 82, 553.
molecule: methylene blue (Mb), rhodamine, and ruthenium-trisbipyridine being the most widely explored.7-14 These studies indicate two adsorption environments, generally ascribed to a site at the external surface of the clay and a site in the interlamellar space. More recent work in our lab sustains the viewpoint of Kuykendall and Thomas.10 They distinguish between a site with strong, direct interaction between the dye molecule and the clay surface and a weaker site, where water is held rigidly to the surface and the dye-surface interaction is indirect.14 With loading, the absorption spectra of large aromatic dyes show a decrease in the intensity of the main monomer band, giving rise to a blue-shifted band, also known as the metachromic effect. The same effect is observed upon increasing the concentration of the aqueous dye solution, and it is correspondingly ascribed to the aggregation of the dye molecules.8,13,16 Alternatively, some researchers claim a π-interaction between the lone pair electrons of the surface oxygens with the dye molecule.12,15 Such an interaction cannot be excluded, but it cannot explain the dependence of the metachromic effect on loading, nor does it accord with the observed time dependence of the spectra. The latter indicates that the adsorption process starts with formation of aggregates, which redistribute over the surface leading to deaggregation.16-18 The resulting dyeclay spectra show numerous bands, depending on the properties of the clay mineral, the loading, and the (7) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399. (8) Lopez Arbeloa, F.; Tapia Estevez, M. J.; Lopez Arbeloa, T.; Lopez Arbeloa, I. Clay Miner. 1997, 32, 97. (9) Tapia Estevez, M. J.; Lopez Arbeloa, F.; Lopez Arbeloa, T.; Lopez Arbeloa, I. J Colloid Interface Sci. 1995, 171, 439. (10) Kuykendall, V. G.; Thomas, J. K. J. Phys. Chem. 1991, 94, 4224. (11) Bergmann, K.; O’Konski, C. T. J. Phys. Chem. 1963, 67, 2169. (12) Yariv, S.; Lurie, D. Isr. J. Chem. 1971, 9, 537. (13) Cenens, J.; Schoonheydt, R. A. Clays Clay Miner. 1988, 36, 214. (14) van Duffel, B.; Jacobs, K. Y.; Schoonheydt, R. A. In Proceedings of the 11th International Clay Conference, Ottawa, 1997; Kodama, H., Ed.; p 475. (15) Dobrogowska, C.; Hepler, L. G.; Gosh, D. K.; Yariv, S. J. Therm. Anal. 1991, 37, 1347. (16) Gessner, F.; Schmitt, C. C.; Neumann, M. G. Langmuir 1994, 10, 3749. (17) Neumann, M. G.; Schmitt, C. C.; Gessner, F. J. Colloid Interface Sci. 1996, 177, 495. (18) Cione, A. P. P.; Neumann, M. G.; Gessner, F. J. Colloid Interface Sci. 1998, 198, 106.
10.1021/la010141u CCC: $20.00 © 2001 American Chemical Society Published on Web 07/27/2001
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Table 1. CEC Values for the Different Samples CEC (µmol/g) hectorite
525
barasym montmorillonite Laponite saponite
464 911 525 738
reference 22Na-
determined by the method (ref 13) id. id. from Laporte (500-550 µmol/g) from ref 20
exchangeable cation. As these also determine the colloidal nature of the clay suspension, information about the latter can be obtained as well. Within the present paper, clay mineral suspensions are probed with Mb. Starting from a time-dependence study, a relation will be established between the absorption spectra of the Mb-clay suspensions and clay characteristics and their implication on the nature of the supension. 2. Materials and Methods 2.1. Materials. Three natural smectites, hectorite (SHCa-1, hec), saponite (SapCa-1, sap), and montmorillonite (Swy-1, swy), together with the synthetic mica-montmorillonite barasym (Syn1, bs) were obtained from the Source Clay Minerals repository of the Clay Minerals Society. They were transformed into the Na+-form by triple exchange with 1 M NaCl solutions. After chloride-free washing, the fraction smaller than 0.05 µm was separated and freeze-dried. AFM (atomic force microscopy) measurements however indicated that the particle size within this fraction can still be considerably larger.19 Laponite RD (Laporte Ltd) was used as received. For all clays, 0.1 wt % suspensions were prepared in deionized water at least 2 days before use. During this time, they were sonicated several times for 1 h and stirred in between. The cation exchange capacity (CEC) values of the different clays are given in Table 1. Methylene blue (Aldrich) was used without further purification. Stock solutions (between 5 × 10-5 and 10-4 M) were prepared and kept in the dark. 2.2. Preparation of the Samples. The short time dependence of the spectra of Mb-clay mineral suspensions was studied by direct addition of a small amount of a Mb solution (∼1.16 × 10-4 M Mb) to a 0.1 wt % clay suspension. The suspensions were vigorously shaken and allowed to equilibrate. The maximum loading was 2.5% of the CEC. Higher loadings would disturb the clay concentration too much. Long time dependence (till 1 year) and higher Mb loadings were studied by dialysis of 0.1 wt % clay suspensions against Mb solutions of different concentrations. After overnight end-overend rotation, the suspensions were separated and allowed to age in the dark. The volume ratio of the suspension toward the Mb solution was 1/4 in all cases, while the Mb concentration was adjusted to obtain the desired loading. The maximum loading was 15% of the CEC, which requires a Mb concentration of about 10-5 M Mb. At loadings higher than 15%, the freshly prepared Mb-clay suspensions start to flocculate. For all samples, the adsorption was quantitative, as indicated by the absence of Mb in the equilibrium solutions. 2.3. Analysis. UV-vis absorption spectra of the Mb-clay suspensions were recorded directly after preparation and as a function of time. The short time dependence covers a time span of minutes up to days; the long term dependence starts after 1 day and lasts for 1 year. These transmission measurements were performed on a Perkin-Elmer Lambda 12 spectrophotometer. During the first hour of the short time dependence study, the spectra were taken within 50 s (240 nm/min). The other spectra were recorded at 60 nm/min. Water was used as the reference, and the spectrum of a pure clay suspension was subtracted. Band assignments and interpretations were discussed previously21 and are given in Table 2. (19) van Duffel, B.; Schoonheydt, R. A.; Grim, C. P. M.; De Schryver, F. C. Langmuir 1999, 15, 7520. (20) Post, J. L. Clays Clay Miner. 1984, 32, 147. (21) Jacobs, K. Y.; Schoonheydt, R. A. J. Colloid Interface Sci. 1999, 220, 103.
Table 2. Characteristic Bands of Adsorbed Mba band position (nm)
species
assignment
760 670 653 600-610 570 720
MbH2+ Mb Mb Mb, (Mb)2 (Mb)3, (Mb)n (Mb)2
protonated methylene blue monomer monomer monomer and H-dimerb trimer and higher aggregatesb L-dimersb
a Reference 21. b H-dimers and -aggregates refer to a face-toface antiparallel stack of molecules. L-dimers refer to either a headto-tail arrangement or a slightly distorted antiparallel stack.
3. Results 3.1. Short Time Dependence of the Spectra of Methylene Blue. In the spectral evolution of the adsorption process of Mb in aqueous Na+-smectite suspensions, two steps can be distinguished. The first one shows an instantaneous adsorption and aggregation of Mb molecules followed by formation of monomers absorbing at 670 nm and is observed for all the clay minerals studied. The second is more clay-specific and depends on the loading. Figures 1 and 2 give the spectra of aqueous suspensions of Mb-clay complexes immediately after mixing the clay mineral suspensions with the Mb solution (Figure 1) and at the end of the first step of the adsorption process (Figure 2). In all cases, the same amount of Mb (100 µL, 1.16 × 10-4 M Mb) was added to 1 mL of 0.1 wt % suspensions. In the absence of clay minerals, such a 10-fold dilution of Mb results immediately in a spectrum containing only Mb monomers absorbing at 664 nm. In the presence of clay minerals, the opposite behavior is observed: the spectra of Mb-clay mineral suspensions are dominated by a broad band at 570-590 nm and one at 670 nm, attributed to H-aggregates and monomers of Mb, respectively (Figure 1). The former is dominant, except for Laponite. These H-aggregates appear as a broad and structureless band in the spectra of Mb-hectorite and Mb-saponite suspensions, while for barasym the band is resolved in two components at 570 and 608 nm. The latter is ascribed to dimers; the former, to trimers and higher aggregates. Figure 2 illustrates that after a few minutes the spectra of all Mb-clay minerals are very much alike. The monomer with a maximum at 670 nm is dominant. The dimer band is much weaker and asymmetric at its short wavelength side, due to residual H-aggregates. The spectra of barasym differ slightly in that a band of protonated Mb (MbH2+) arises at 760 nm. The time needed to reach the end of the first adsorption step falls in the range of 3-15 min, except for saponite, where it extends to 2 h. Figures 3 and 4 compare the second phase of the adsorption process for the different smectites. For the Mbbarasym suspension, there is no second step as no spectral changes are observed after the first adsorption step. For Laponite, one observes a continuous decrease in the intensity of the band of Mb aggregates at ∼570 nm and a concomitant intensity increase of the monomer band at 657 nm (Figure 3a). This indicates therefore the breakdown of aggregates into monomers. The spectra of the Mb-saponite suspension show a decrease of the monomer band at 670 nm and a simultaneous increase of the band intensities of dimers at 605 and 723 nm, the latter being ascribed to L-dimers (Figure 3b). Similar changes can be seen in the Mb-hectorite suspension: the monomer band intensity at 670 nm decreases, the band shifts to 657 nm, and both H-dimers and L-dimers are formed. These changes are however much more pronounced and occur over a time span of 10
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Figure 1. Absorption spectra of Mb-smectite suspensions, immediately after mixing the clay suspension with the Mb solution, and comparison with the spectrum of the aqueous Mb solution (left). Smectites: Laponite (Lap), hectorite (Hec), saponite (Sap), and barasym (Bs).
Figure 2. Absorption spectra of Mb-smectite suspensions at the end of the first step of the adsorption process. Abbreviations of smectites are as in Figure 1; time indications: minutes (min) and hours (h).
Figure 3. Absorption spectra of the second phase of the adsorption process for a Mb-Laponite suspension (a: Lap) and a Mb-saponite suspension (b: Sap). Time indications: minutes (min), hours (h), days and weeks.
min to 3 h after the addition of Mb in the case of hectorite, compared to 2 h to 3 weeks for saponite. After 3 weeks, the spectra of Mb on saponite do not change significantly. For hectorite, a time span exceeding 3 h results in a slight breakdown of higher aggregates as shown by the intensity decrease around 570 nm and a concomitant increase of the dimer and monomer bands (Figure 4). 3.1.1. Influence of the Loading. The previous results are valid for a Mb loading corresponding to 2.2% of the CEC of hectorite. At very small loadings, such as 0.110.5% of the CEC of hectorite (CEChec), the same phenomena are observed. Instantaneously formed aggregates break down with formation of monomers absorbing at 670 nm.
Figure 4. Absorption spectra of the second phase of the adsorption process for a Mb-hectorite suspension. Time indications: minutes (min), hours (h), and days (d).
The 670 nm band intensity subsequently decreases, while the monomer band at 652-656 nm grows. This phase is more pronounced for hectorite than for Laponite and saponite. For barasym, the band at 765 nm of MbH2+ continues to increase with time. 3.2. Long Time Dependence. 3.2.1. Dialysis versus Direct Mixing. The long time dependence was studied on Mb-clay suspensions prepared by dialysis of 0.1 wt % suspensions against a Mb solution. This mode of operation has two distinct advantages over the direct mixing method: (1) the Mb solutions contain only monomers and (2) the Mb molecules have to diffuse through the membrane before the adsorption can take place. In this way, a better distribution of the Mb molecules over the clay
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Figure 5. Comparison of two preparation methods for a 2% loaded Mb-saponite suspension: (a) direct addition method and (b) dialysis method.
Figure 7. Long time dependence of Mb-clay suspensions at higher loading: absorption spectra of a 10% Mb-barasym suspension (Bs) and a 2.5% Mb-saponite suspension (Sap).
Figure 6. Time dependence of Mb-clay suspensions at small loading: absorption spectra recorded after 1 day (d) dialysis up to 1 year (y) after preparation.
Figure 8. Long time dependence of Mb-hectorite suspensions at higher loading.
particles is possible at higher loadings. At smaller loadings and constant clay concentration, the preparation method has no significant effect on the spectra of Mb-clay suspensions. This is shown in Figure 5 for the spectra of Mb-saponite suspensions at loadings of ∼2% Mb. However, the time needed to reach identical spectra can strongly differ, as indicated on the figure. 3.2.2. Time Dependence. Spectra of Mb-clay suspensions were recorded after 1 day of dialysis and up to 1 year of aging. At small loadings (∼1-2% of the CEC), the spectra of Mb-clay suspensions show the same trend for all clay minerals. They indicate an increase in the amount of monomers, as illustrated for saponite and hectorite in Figure 6. In the case of barasym, one observes an additional protonation of Mb at the expense of monomers. The spectra at higher loadings are given in Figures 7-9. Very small time dependence is observed for Mb-Laponite suspensions aged up to 1 year. The same holds for Mbbarasym suspensions and Mb-saponite suspensions (Figure 7). In the first case, one observes a slight protonation. In the second case, the typical bands of dimers (615 and 723 nm) increase slightly with time. This occurs in both cases at the expense of the monomer band intensity. More significant spectral changes are observed for hectorite and Wyoming montmorillonite, as illustrated in Figures 8 and 9, respectively. Thus, the spectra of hectorite indicate a high Mb mobility, even at loadings up to 15% of the CEC. Below 15% loading, the intensity of the bands of higher aggregates (570 nm) and protonated Mb decreases, while the bands of monomers and both types of
Figure 9. Long time dependence of Mb-montmorillonite suspensions at 1.44% Mb and 10% Mb. Absorption spectra were recorded up to 6 months (m) after preparation.
dimers (605 and 723 nm) grow. The spectra of the 15% Mb-hectorite sample indicate a small decrease of higher aggregates, protonated Mb, and monomers and a concomitant increase of the 723 nm band due to L-dimers. In the case of montmorillonite, all suspensions starting from a 1.44% loading up to the 15% Mb loading evolve in the same manner. Right after dialysis, the spectra show an intense, fairly sharp monomer band at 670 nm and a slightly broadened H-aggregate band. Within a time scale of 1-6 months, aggregates (570 nm) are formed out of these 670 nm monomers. The resulting spectrum is dominated by two strongly broadened bands of monomers at ∼660 nm and of higher aggregates at 584 nm. At the same time, an association of Mb-clay particles is notice-
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able. At small Mb loading, these associates remain in suspension, forming a network which expels water. The suspensions are less transparent. With increasing Mb loading, more dense particles are formed, which at the 10-15% Mb loading slowly settle down. Visible changes in the stability of the suspensions are also seen for the other clays. Saponite suspensions remain stable to the eye. The suspensions of hectorite and barasym behave similarly: with increasing loading, they first turn into a gel and then form visible clay associates until at the highest loadings, they flocculate. Finally, the suspensions of Laponite all turn into a gel, irrespective of the Mb loading.
Jacobs and Schoonheydt Scheme 1
4. Discussion 4.1. Influence of Clay Characteristics on the Adsorption Process. The ion exchange of Mb on smectites in dilute aqueous suspension is an instantaneous adsorption process followed by a slow redistribution of the Mb molecules over the available surface area. The instantaneous adsorption is clay independent. It expresses the extremely high selectivity of clay minerals for Mb cations and cationic dyes in general.7-11 The result is a nonspecific adsorption, leading to high concentrations of Mb molecules around the clay particles and aggregation of these Mb molecules. For Na+-clays, the combination of both dimers and higher aggregates gives rise to one strongly broadened band with a maximum at 580-590 nm, encompassing several (Mb)n species (n g 2). Similar results have been observed for neutral red and thionine.16,17 Such an instantaneous adsorption does not give an equilibrated Mb-clay mineral suspension in the thermodynamic sense. Thus, a rearrangement of Mb molecules and clay particles occurs, and this can be followed spectroscopically as a function of time. The first step of this rearrangement consists of the partial breakdown of the (Mb)n aggregates and formation of monomers with a typical absorption band at 670 nm. It is identical for all clay minerals studied, indicating that this process mainly takes place in the hydration sphere of the clay platelets. The relative intensity of the monomer to the aggregate band is proportional with the available surface area. Thus, Laponite, consisting of small, well-dispersed particles, has a large available surface area and the 670 nm band is dominant. For barasym, the effective surface area is small and the interaction between Mb and the surface is stronger. The Mb spectrum shows two distinct aggregate bands, at 570 and 615 nm. The band of higher aggregates at 570 nm predominates. The presence of acid sites on the surface of this clay causes a subsequent protonation of the formed 670 nm monomers. In the second phase of the adsorption process, the distribution of Mb molecules depends on the balance between three types of interactions: Mb-surface interactions, Mb-Mb interactions, and interactions among clay particles. When Mb-surface interactions predominate, (Mb)n aggregates are broken down and Mb molecules can displace water molecules from the surface. The clay particles tend to aggregate into more dense particles. The formed Mb monomers are in direct contact with the surface and give rise to the 670 nm band. Compared to the band of a pure Mb solution, this band is red-shifted, which is proof of a more polar environment. The strong interaction between the Mb species and the surface results in resolved, well-defined bands. This is the case for clay minerals with isomorphic substitution in the tetrahedral layer, saponite and barasym being the examples. In addition, large particles, having an important basal surface area, are expected to improve the Mb-surface interaction as well.
In the case of hectorite and Laponite, the isomorphic substitution is located in the octahedral layer. The Mbsurface interactions are weaker. Islands of Mb monomers are formed. They have a characteristic absorption band at 653 nm, blue-shifted with respect to the solution value. This reflects a less polar or more hydrophobic environment at the surface. With time, the clay particles form loose aggregates with a high water content. This accounts for the remaining mobility of the Mb molecules. The small particle size of these clays furthermore emphasizes the importance of the hydration layer and facilitates the association into loose aggregates. Scheme 1 summarizes the adsorption course. 4.2. Long Time Dependence. The long time dependence of the spectra of Mb adsorbed on clay minerals clearly has two regimes, depending on the Mb loading. At small loadings, the adsorbed Mb molecules do not significantly disturb the surface properties of the clay mineral particles on which they are adsorbed. One observes a slow redistribution of Mb molecules over the available surface. As a consequence, the amount of monomeric molecules increases with time. There is no direct evidence for clay particle aggregation. At high loading, the surface properties of the clay mineral particles are affected by the adsorbed Mb molecules. Particle-particle interactions become important. On this basis, the clay minerals studied can be divided into three groups. The first group contains barasym and saponite. These clays are characterized by tetrahedral substitution. Mbsurface interactions are dominant. The spectrum of adsorbed Mb is characterized by well-defined, relatively sharp bands; the monomeric absorption maximum is typically at 670 nm, and the spectral changes with time are minimal. The second group contains hectorite and montmorillonite (Wyoming bentonite). The latter has predominantly octahedral substitution; the former, exclusively. The electrostatic interaction between Mb+ and the negative lattice charge is weaker than in the case of saponite. The spectral characteristics of these suspensions are the following: the bands are broad, the Mb+ monomer absorbs
Spectra of Methylene Blue-Clay Suspensions
at 653 nm, and the spectra change slowly with time over long time periods. The latter is due to particle-particle interactions, resulting in clay particle aggregates and, as a consequence, aggregation of Mb. At intermediate loadings, the clay aggregates have a loose structure, which does not prevent further deaggregation of Mb. They remain well-suspended, as can be seen by the eye. In the case of hectorite, the L-dimer band becomes prominent. At higher loadings, more dense clay particles are formed and Mb deaggregation can no longer proceed. The onset of particle flocculation is observed. The higher CEC of montmorillonite, its large particle size, and the presence of tetrahedral substitution sites all contribute to the stronger aggregation tendency of these clay particles and the subsequent large amount of higher Mb aggregates formed. Laponite forms a separate case. The very strong dispersability of this clay allows a fast equilibration of the Mb distribution, which does not change with time. The spectrum shows a strongly broadened monomer band with a maximum at 655 nm. 5. Conclusions The adsorption of Mb in aqueous clay mineral suspensions is largely determined by the balance between different types of interaction: Mb-surface interactions, Mb-Mb interactions, and interactions among clay particles. Their relative importance is influenced by the degree of dispersion, the location of the layer charge, the particle morphology, and the Mb loading. For well-dispersed Na+smectites, the adsorption starts in the double layer, but after a few minutes the clay characteristics become visible.
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A strong Mb-surface interaction is expected for tetrahedrally substituted clays and for clays with a large basal surface. The Mb species reside at the clay surface and show well-defined bands with distinct maxima. The main monomer band has its maximum at 670 nm. The red shift with respect to the aqueous solution value evidences the higher polarity of the clay surface. At the surface, the mobility of Mb is small and the time dependence of the spectra is weak. Octahedrally substituted clay minerals have a weaker Mb-clay interaction. Mb-Mb interactions and interactions among clay particles prevail. Smaller clay particles with a large edge surface further emphasize this situation. The Mb molecules remain to some extent in the hydration layer of the particles and are not in direct contact with the surface. At small loading, a blue-shifted monomer band at 653 nm reflects the hydrophobic environment near the surface. At higher loadings, broadened and overlapping bands appear, with ill-defined maxima. With time, loose aggregates are formed. As a consequence, L-dimers and aggregates of Mb are formed. The Mb species stay mobile, which gives rise to continuous spectral changes. With increasing loading and time, more water is expelled from the clay particle aggregates, the bands become sharper, and spectral changes become minor. Acknowledgment. The authors thank the Fund for Scientific ResearchsFlanders (FWOsVlaanderen) for their financial support. K. Y. Jacobs thanks the FWO for a research grant. LA010141U
