Browsing by Author "Segall, Robert L."

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Ionic oxides: distinction between mechanisms and surface roughening effects in the dissolution of magnesium oxide
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (09-12), 1978) Segall, Robert L.; Smart, Roger St. C.; Turner, Peter S.
The mechanism of the rate determining step (r.d.s.) for dissolution of well characterised, very perfect MgO “smoke” crystals, has been re-evaluated by studies of the dependence of log (rate) on pH. Surface potential barrier modification is the most likely r.d.s. when solution diffusion is not limiting. Electron microscopic studies of changes in surface structure of partly dissolved crystals have been related to rate changes during dissolution. There is a doubling of surface area per unit mass in the first 10% of dissolution, due to initial attack at defect sites, after which the increase in surface area per unit mass (to more than ten times the initial area) is due to both decreasing particle size and surface roughening.
Semiconducting Oxides: Effects of Electronic and Surface Structure on Dissolution Kinetics of Nickel Oxide
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (07), 1978) Jones, Colin F.; Segall, Robert L.; Smart, Roger St. C.; Turner, Peter S.
The effect of decreasing dissolution rate of nickel oxide per unit surface area in acid solution with increasing prior annealing temperature (700–1450°C) has been shown to occur over a wide range of pH. The linear dependence of log (rate) on pH may be explained on a model of non-oxidative dissolution in which the pH variation changes the overpotential at the surface. Electron microscopy shows a different mode of attack at pH < 0. The presence in solution of a strong oxidizing ion such as cobaltic causes a very large increase in dissolution rate (> 200 fold) for all prior annealing temperatures but the oxide annealed at 1450°C is still the slowest to dissolve in the presence of cobaltic ions. This is believed to be because it has the lowest kink site density, the role of the Co3+ being hole injection into the p-type semiconductor at kink sites. The general conclusion of the work is that the supply of the majority carriers (the holes) may be rate limiting in the dissolution process.
Semiconducting oxides: infrared and rate studies of the effects of surface blocking by surfactants in dissolution kinetics
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (07), 1978) Jones, Colin F.; Segall, Robert L.; Smart, Roger St. C.; Turner, Peter S.
For nickel oxide annealed at different temperatures (after preparation from the hydroxide), surface blocking by adsorbed anionic surfactant reduces dissolution rates per unit surface area in acid solution. Cationic and non-ionic surfactants have little effect. Infrared studies indicate coulombic interaction of the anionic group with the positive surface charge, with little molecular structural modification. For NiO annealed at 700°C, there is a systematic decrease in rate with anionic surfactant concentration to a minimum rate at ≈ 20% of the rate without surfactants. The minimum rate is found close to monolayer coverage. For 1450°C annealed material, weaker attraction of adsorbed surfactant, due to lower surface charge on the less imperfect surfaces, produces less reduction of the rate and the minimum rate (≈ 35% of the rate without surfactant) occurs at concentrations corresponding to about six times monolayer coverage. Surface blocking by the anionic surfactant causes a reduction (to 10%) of the enhanced dissolution rate produced by Co3+ in solution; a cationic surfactant again has no significant blocking effect and oxidation by Co3+ gives a more than one hundred fold increase in rate. Rate changes are discussed in relation to changes in the oxide structure caused by pretreatment and in relation to the structure of the adsorbed surfactant layer.