Electrophoretic Mobility of Hydroxy-Aluminum and Sodium-Hectorite in Aqueous Solutions
- James B. Harsh ,
- H. E. Doner and
- D. W. Fuerstenau
The electrokinetic behavior of colloids titrated with acid or base in the presence of hydrolyzable metal cations is often modeled by assuming the substrate surface is partially or completely coated with the metal hydroxide. In aged OH-Al-clays much of the Al(OH)3 surface precipitate resides in the interlayer, but it is not known if the external surfaces of aged OH-Al clays contain Al(OH)3. We precipitated Al(OH)3 on hectorite and measured electrophoretic mobility (µ), which is a function of the electric potential at the shear plane of the substrate, with varying pH, time, and ionic strength. The behavior was compared to that of Na-hectorite in the presence and absence of AlCl3. We found that (i) the OH-Al-hectorite had a more positive µ than the Na-hectorite between pH 3 and 5.5, but had a more negative µ between pH 5.5 and 7; (ii) µ became more negative with increasing NaCl concentration; and (iii) neither clay equilibrated rapidly with time. Ion exchange experiments showed that Na displaced exchangeable Al and Mg that originated from dissolution of the hectorite and Al(OH)3. There was no evidence for significant Al(OH)3 coating external surfaces, because this would have resulted in a more positive µ for the clay at all pH values below the point of zero charge of Al(OH)3 (pH = 9). Exchangeable cations on external surfaces primarily determined the shear plane potential in this system.
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