Fergus Gessner

Effect of sonication on the particle size of montmorillonite clays

This paper reports on the effect of sonication on SAz-1 and SWy-1 montmorillonite suspensions. Changes in the size of the particles of these materials and modifications of their properties have been investigated. The variation of the particle size has been analyzed by DLS (dynamic light scattering). In all cases the clay particles show a bimodal distribution. Sonication resulted in a decrease of the larger modal diameter, as well as a reduction of its volume percentage. Simultaneously, the proportion of the smallest particles increases. After 60 min of sonication, SAz-1 presented a very broad particle size distribution with a modal diameter of 283 nm. On the other hand, the SWy-1 sonicated for 60 min presents a bimodal distribution of particles at 140 and 454 nm. Changes in the properties of the clay suspensions due to sonication were evaluated spectroscopically from dye-clay interactions, using Methylene Blue. The acidic sites present in the interlamellar region, which are responsible for dye protonation, disappeared after sonication of the clay. The changes in the size of the scattering particles and the lack of acidic sites after sonication suggest that sonication induces delamination of the clay particles.

Interações entre corantes e argilas em suspensão aquosa

Adsorption of cationic dyes on clays can be used as a model for the interactions between organic compounds and these minerals. Cationic dyes like methylene blue are used to study these interactions because of the spectroscopic changes observed when their molecules are adsorbed on clay surfaces. Depending on the structure and layer charge of the clay particles several processes may occur, like adsorption of dye monomers and aggregates on the external and internal surfaces of the clay tactoids, migration towards internal surfaces, protonation, etc. Under certain conditions the deaggregation-aggregation of the clay particles are accelerated trapping dye species during these processes. A general scheme is proposed for the processes occurring between clays and dyes in aqueous suspensions, which can be used to explain the behaviour of specific systems.

Photoreduction of safranine by substituted anilinomethanesulphonates

Abstract The reactions between the triplet state of safranine and various anilines and substituted anilinomethanesulphonates (RAMS) have been studied by monitoring the formation of the semireduced form of the dye using flash photolysis. The reaction with RAMS at low concentrations and anilines corresponds to the formation of an encounter complex which allows electron transfer or deactivation. The Hammett reaction constant for poor electron-donating substituents is −2.7. At high RAMS concentrations, the semi-reduced dye yield decreases for increasing quencher concentrations. This effect is ascribed to the formation of a ground state ion-pair complex ( K ≈ 140) with an unfavourable configuration for electron transfer. When excited, this complex is rapidly deactivated by static quenching.

The excimer emission of aromatic hydrocarbons on clays

Abstract The emission of some aromatic hydrocarbons on the surface of hectorite (Laponite and SHCa-1) and montmorillonite (SWy-1, Volclay, STx-1 and Syn-1) clays was studied. p -Terphenyl, biphenyl and naphthalene, when adsorbed on clays showed excimer emissions, as well as the well-known molecular fluorescence bands, but on SWy-1 and Volclay the emissions were relatively weak. As the loading was increased, the excimer emission tends to dominate the emission spectra, particularly for the montmorillonites STx-1 and Syn-1. Emission due to microcrystals on the clay surface was also detected at larger loadings. No excimer emissions were detected for m -terphenyl and 1,1′-binaphthyl even at the larger loadings used.

Effect of ground state association on the photoreduction of basic dyes by anionic N-phenylglycines and its neutral esters

Abstract The reactions between triplet Safranine and substituted anionic N -phenylglycines and its neutral esters were used to determine the character of the association complex between the reactants in the ground state. The equilibrium constants, obtained from the deviations of (a) the double reciprocal plots of semireduced Safranine and (b) Stern—Volmer plots of the Safranine fluorescence quenching, were similar when using both the charged and uncharged reductants. Also, no solvent effect on these constants was detected when changing from methanol to water. From these results, it seems that the ground state complex does not have ion-pair character, and that the static quenching of its excited form is from a singlet state.

Adsorção do corante azul de metileno em partículas de argilominerais: análise dos tamanhos das partículas

Interactions of cationic dye methylene blue (MB) with clay particles in aqueous suspension have been extensively studied. As already known, the number of natural negative charges on the clay modifies significantly the particle sizes dispersed in water and therefore the nature of the interaction with the dye. This work evaluated with UV-Vis spectroscopy method how the clay particle sizes weighted on the adsorption and rearrangement of the dye molecules in aqueous system. The results obtained from light-scattering measurements confirmed that larger particles are found in suspensions containing the high-charged clays as the visible absorption band related to the MB aggregates (570 nm) on these suspensions prevailed.

The adsorption of basic dyes by poly(styrene sulphonate)

Abstract The adsorption of basic dyes (Safranine, Neutral Red, Acridine Orange and Proflavine) on poly(styrene sulphonate) has been studied by measuring the amount of free dye in solution using a photochemical method. Adsorption free energies are in the range 7.1–5.3 kcal mol . Comparing the values of these energies as a function of the structure of the dyes, three different types of interactions can be postulated to contribute to the binding of the dyes to the polymer, viz., electrostatic (c. 4 kcal mol ), hydrophobic and charge-transfer.

Formation of mixed dimers in solutions of basic dyes

The formation of mixed dimers of basic dyes has been studied in aqueous solution at room temperature. These mixed dimers have absorption bands in the visible region at wavelengths shorter and longer than those shown by the self dimers of the dyes having higher and lower excited states, respectively. Both transitions are allowed according to exciton theory. The angle between the dye molecules in the sandwich-like dimers was calculated from the molar absorption coefficients and is in the range 140°–160°. The non-alignment of the molecules may be due to steric hindrance caused by the bulky side groups.