André Watillon
Université libre de Bruxelles
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Featured researches published by André Watillon.
Journal of Colloid and Interface Science | 1970
Jacques Depasse; André Watillon
Abstract The stability of amorphous silica hydrosols has been studied in the presence of alkali metal cations at various pHs. Results show that silica is very stable within appropriate pH ranges, notwithstanding the presence of very high electrolyte concentrations (sometimes reaching saturation). Moreover, coagulation is often reversible. Although all the alkaline cations are able to coagulate silica between pHs about 7 and 11, they can be classified into two groups according to their behavior in more alkaline media: above pH 11, Li + and Na + cations bring coagulation whereas K + , Rb + , and Cs + remain ineffective. It is suggested that the attraction forces responsible for coagulation are related to acid-base interparticle bonding of two types (salt bonding by acidic cations and silanol-dissociated silanol bonds) rather than to London-van der Waals forces.
Journal of Colloid and Interface Science | 1975
Jeannine Stone-Masui; André Watillon
Abstract To get information about the surface charge, potentiometric and conductometric titrations have been carried out on monodisperse latices. Using these methods, strong acidic groups were detected for latices prepared in the presence of strong acid soaps. Similarly, for latices prepared with weak acid soaps, weak acidic groups were observed even after several purifications with mixed-bed ion exchange resins used in a batch procedure. The influence of the soap chain length on the surface charge has been investigated. An increase of the hydrophobic part of the soap leads to higher surface charges. Furthermore, surface charge-ionic strength relationships were established, which allowed us to calculate the surface dissociation constant of the weak acidic groups. The problem of the origin of surface charge has also been considered by comparison between the aforementioned data and information from molecular weight distribution of the polystyrene chains obtained by gel permeation chromatography. Other latices prepared in the same experimental conditions as for emulsion polymerization but without any soap were also studied.
Journal of Colloid and Interface Science | 1968
Jeannine Stone-Masui; André Watillon
Abstract The contribution of electroviscous effects to the viscosity of monodisperse polystyrene latices has been studied. The sols were prepared by emulsion polymerization using as emulsifier a weak acid soap and a strong acid soap, respectively. Particle sizes were determined by electron microscopy and turbidimetry. Excellent agreement between the two methods was obtained. Zeta potentials and charges of the electrokinetic unit were deduced from electrophoretic mobility measurements at different ionic strengths. The validity of the Einstein equation was demonstrated for perfectly stable latices when the primary electroviscous effect can be considered as negligible. Influences of ζ potential, particle charge, ionic strength, particle size, and rate of shear on the first- and second-order electroviscous effects have been investigated. For the primary electroviscous effect, a good agreement with the Booth Streets equation yielded too low viscosities, values predicted by the recent treatment of Chan, Blachford, and Goring were too high. However, this last theory represents a more realistic approach than the Street one.
Journal of Colloid and Interface Science | 1990
F. Dumont; Jean Warlus; André Watillon
Abstract Several TiO2 hydrosols with point of zero charge (pzc) ranging from 2.8 to 6 were prepared depending on the method of synthesis. The ionic adsorption sequences at the different interfaces were deduced from the critical coagulation concentrations of the corresponding hydrosols. The general behavior can be summarized as follows: on positively charged surfaces, the structure-maker anions are always more adsorbed than the structure-breakers; on negatively charged surfaces, for “high pzc” hydrosols, the cationic sequence is the same as that observed for the anions, whereas, for “low pzc” hydrosols, the reverse sequence is found. The model based on the generalization to the ion-surface interactions of the Gurney concept of the ion-ion interactions, as proposed by Gierst, is used to explain the observed properties of the different hydrosols.
Journal of Electroanalytical Chemistry | 1972
André Watillon; Jeannine Stone-Masui
Summary Conductivities of well characterized monodisperse polystyrene latices have been measured. Experimental values of the surface conductance have been obtained from the Street treatment. Surface conductances have been calculated from the Bikerman equation introducing experimental values of the electrok netic potentials. These are compared with the experimental results. As the Bikerman equation was derived for large ka, however, an extension of this relation for any ka value is proposed. The effects of ζ potential, particle charge and ionic strength on surface conductance have been investigated.
Discussions of The Faraday Society | 1971
F. Dumont; André Watillon
Ferric oxide hydrosols are composed of a haematite core surrounded by an hydrated oxide shell. Interactions have been studied, on both sides of the p.z.c., between ferric oxide surface and a series of anions and cations having characteristics ranging from structure promoters to structure breakers. Although ferric oxide has a moderate crystalline field, the observed adsorption sequences show that the interface acts as a structure promoter for water molecules. The large spreading of the measured adsorption sequences, especially at extreme pH values, proved the coagulation technique to be a good tool for the study of ion-surface interactions. A new type of behaviour has been observed; at low surface potentials the ions behave as indifferent but at higher surface potentials they are specifically adsorbed.
Journal of Colloid and Interface Science | 1976
F. Dumont; Dang Van Tan; André Watillon
Abstract Coagulation values, electrokinetic potentials and peptization properties of ferric oxide hydrosols were measured in the pH range 2–12 in the presence of various monovalent ions. The observed ionic adsorption sequences are explained in terms of the Gurney concept of the ion-ion interactions extended to the ion surface interactions. From stability and peptization measurements, it is shown that the water structure near the interface changes through the p.z.c. The conclusions deduced from the haematite surface study are extended to other interfaces.
Journal of Colloid and Interface Science | 1968
André Watillon; Josette Dauchot
Abstract Monodisperse selenium sols with particle diameters ranging from approximately 40 to 500 mμ were prepared and their particle size distribution was analyzed by comparing the experimental spectra with theoretical curves computed by means of the Mie equations, assuming a normal distribution function. Good agreement was found with the results obtained by means of an ultramicroscopic counting technique and by electron microscopy.
Journal of Colloid and Interface Science | 1967
Josette Dauchot; André Watillon
Abstract With the introduction of recent values of the refractive index of selenium into the Mie equations, the extinction coefficients were computed as a function of wavelength (240 to 1100 mμ) for particle diameters ranging from 0 (20) to 500 mμ. The maxima appearing in the extinction curves were related to the contributions of various electric and magnetic partial waves. The shift of these maxima to the infrared with increasing particle diameters is also analyzed. Finally, the influence of the particle size distribution is discussed.
Journal of Colloid and Interface Science | 1976
R De Backer; André Watillon
Abstract A theoretical approach is proposed to explain the relaxation against frequency of the electrical properties of an heterogeneous system, previously observed and reported. The dispersion of the dielectric constant and of the conductivity of a Pyrex plug embedded in an electrolyte solution is interpreted, using the Maxwell equations and a triphasic model, where the double layer surrounding the particles is considered as the third phase. The computed curves are in good agreement with the experimental data. Different parameters governing the relaxation process (electrolyte concentration, magnitude of the surface conductance, particle radius, and relative volume fractions) are also considered.