The adsorption of the cationic dye rhodamine 6G by montmorillonite (Wyoming bentonite) and laponite (synthetic hectorite) was studied by visible and fluorescent spectroscopy and by X-ray diffraction methods. Adsorption of the dye takes place by the mechanism of cation exchange. X-ray data indicate that the adsorbed cationic dye is located in the interlayer spaces of both minerals. It was found that adsorption prevents the dimerization of the dye, and gives rise to red shifts of the principal absorption band of the dye. A study of the effect of varying concentration of dye or clay on the location and intensity of this band proved to be useful in determining the adsorption capacity of the clay. The observed spectral changes are linked to surface polarity and to clay flocculation. At concentrated suspensions of montmorillonite, an additional weak absorption shoulder is obtained at 470-490 nm, which may be attributed to aggregated dye molecules, probably trapped within the cavities of book-house structured flocs. Due to steric hindrance rhodamine 6G does not give IT interactions with the oxygen plane of the silicate layer. Therefore the adsorption of the dye by expanding clay minerals does not give rise to metachromasy. Fluorescence of the adsorbed dye is reported: it is red shifted and quenched relative to aqueous solutions. The use of fluorescence spectra to determine the saturation point is described, and aging and ultrasonic effects on spectral features are presented and discussed.ZVl GRAUER, DAVID AVNIR et SHMUEL YARIV. Can. J. Chem. 62, 1889 (1 984).Faisant appel a la diffraction des rayons-X et a la spectroscopie visible et de fluorescence, on a CtudiC l'adsorption du colorant cationique rhodamine 6G par la montmorillonite (bentonite du Wyoming) et par la laponite (hectorite synthCtique). L'adsorption du colorant se fait par un mCcanisme impliquant un Cchange de cations. Les donnCes de rayons-X indiquent que, dans le cas de chacun des deux minkraux, le colorant cationique est adsorb6 dans les espaces interstitiels. On a trouvt que l'adsorption empeche la dimCrisation du colorant. A la suite de l'adsorption, la principale bande d'adsorption du colorant se dCplace vers le rouge. Une ttude de l'effet d'un changement de concentration du colorant ou de l'argile sur la position et I'intensitC de cette bande s'avkre un outil utile pour dkterminer la capacitC d'adsorption de l'argile. Les changements spectraux obsewCs sont lies a la polarit6 de la surface et a la floculation de l'argile. Pour des suspensions concentries de montmorillonite, on observe une adsorption supplCmentaire faible qui apparait comme un Cpaulement a 470-490 nm et que I'on peut attribuer une aggregation de molCcuies de colorant qui sont probablement emprisonnCes dans les cavitCs des flocons. i a rhodamine 6G, a cause de I'encombrement stkrique, n'a pas d'interactions IT avec le plan d'oxygenes de la couche de silicate. Par conskquent, l'adsorption du colorant par la dilatation des minkraux argileux ne provoque pas de mCtachromasie. On rapporte la...

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The adsorption of pyronin Y by montmorillonite and laponite has been studied by visible, infrared and X-ray diffraction spectroscopies. The saturation point is much higher in montmorilionite, being 100 and 41 mmol pyronin per 100 g montmorillonite and laponite, respectively. The adsorbed cationic dye is located in the interlayer space. In montmorillonite adsorption leads to metachromasy of the dye and the appearance of a new band at shorter wavelengths than the original band (480 and 545 nm, respectively) even at very small coverages. In laponite, on the other hand, no metachromasy is observed with small amounts of dye. It is observed only when the degree of saturation approaches the saturation point. In montmorillonite the organic cation is oriented with the plain of the rings parallel to the silicate layer. In this parallel orientation n interactions between the oxygen plane of the aiuminosilicate and the aromatic dye give rise to metachromasy of the dye. In laponite the plain of the aromatic ring is tilted relative to the silicate layer and n interactions between the oxygen plane and the aromatic dye do not occur. Metachromasy is observed when dimers or aggregates of dye cations are formed in the interlayer space or in the interparticle space of flocs of laponite. '.-.I 500 550 600 650

pattern from test to test, and poesibly traces of gas trapped within the pores of the sandpack, are causing reproducibility problems.The enthalpy of displacement can now be expressed per mole of surfactant adsorbed (see last column of Table 11). From these data we estimate the enthalpy of displacement a t plateau adsorption to range between -4.8 and -6.5 kJ/mol. This extremely low heat effect points to weak physisorption of the surfactant molecule on the sandstone surface.Finally, our enthalpy of displacement values are compared with literature data obtained by No11 et al.,3 Berg et al.: and by Rouquerol and Partyka6 in Table IV. Noll et aL3 determined the heat of preferential adsorption of a saturated aqueous solution of sodium decylbenzenesulfonate (or sodium dodecylbenzenesulfonate) onto Cleveland sand in a liquid-flow calorimeter; their values are not directly comparable with our data because the amount adsorbed is unknown. Berg et al.4 measured the enthalpy of adsorption of solutions with increasing sodium dodecylbenzenesulfonate concentrations on silica gel in a flow calorimeter; the heat of dilution in their experiments was negligible, but unfortunately, the amount adsorbed was not determined. Rouquerol and Partyka6 determined the enthalpy of displacement of sodium dodecylbenzenesulfonate on Fontainebleau sand in a batch calorimeter; these authors corrected their results for the enthalpy of dilution and they also determined the amount adsorbed.It would therefore appear that our results can be compared with those of Rouquerol and Partykas only. Their value of the enthalpy of displacement of -10.3 kJ/mol for sodium dodecylbenzenesulfonate (Fontainebleau sand) compares well with our range of -4.8 to -6.5 kJ/mol for the shorter chain sodium decylbenzenesulfonate (Bentheim sand). Further work will deal with a wider range of surfactants and with improving the reproducibility of the experiment. ConclusionsThe results reported in this paper show that liquid-flow microcalorimetry can be used together with a suitable analytical technique for the simultaneous determination of the enthalpy of displacement of solvent by solute at the solid/liquid interface and the adsorption isotherm. This combination of techniques should represent an additional and potentially valuable tool for the study of adsorption from solution at the solid/liquid interface.The enthalpy of displacement of sodium p(2-decyl)-benzenesulfonate at a water/Bentheim sandstone interface is low and of the order of -5 kJ/mol. This points to very weak physisorption of the surfactant molecule onto the sandstone surface.Regietry No. Sodium p-(2-decyl)benzenesulfonate, 73602-650.The adsorption of R~( b p y )~~+ and Ru(phen)32+ on the clay mineral laponite in aqueous suspensions was investigated by using time-correlated single-photon counting (SPC) and excited-state resonance Raman ('Mi3) spectroscopy. The results show that these metal complexes have a high affinity for binding to the clay surface. The emission decays of both complexes are nonexponential and...

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