Concentration adsorption layer, crystal growth

The increased concentration of the inhibitor in the surface layer relative to the bulk solution is caused by adsorption. The adsorption may be at a kink, step, or terrace site and be effective in influencing the step velocity. Unfortunately, the experimental determination of adsorption isotherms of inhibitors is rare and the approach in the past has been to try and obtain information indirectly from the analysis of crystal growth data. 

In this chapter we discuss the rates of adsorption, paying special attention to those few cases where information on the rate of specific adsorption (reaction of an adsorbate in the adsorption layer) is available. Furthermore, we elaborate on the chemical processes involved in the dissolution of minerals and concentrate on the dissolution of oxides, silicates, and carbonates, which play an enormous rx)le in the chemical weathering and erosion. We try to demonstrate that in most cases the rate-determining step in the dissolution is a chemical reaction at the surface of the mineral. Thus we have here an excellent example of the relationship between surface stracture and reactivity. Surface chemistry plays an equally important role in the formation of the solid phase (precipitation, nucleation, and crystal growth). Nature s selectivity is reflected in the creation of a crystal and its growth. 

To fully assess the properties of suspension concentrates, three main types of measurements are required. Firstly some information is needed on the structure of the solid/solution interface at a molecular level. This requires investigation of the double layer properties (for systems stabilised by ionic surfactants and polyelectrolytes), adsorption of the surfactant or polymer as well as the extension of the layer from the interface (adsorbed layer thickness). Secondly, one needs to obtain information on the state of dispersion on standing, such as its flocculation and crystal growth. This requires measurement of the particle size distribution as a function of time and microscopic investigation of flocculation. The spontaneity of dispersion on dilution, i.e. reversibility of flocculation needs also to be assessed. Finally, information on the bulk properties of the suspension on standing is required, which can be obtained using rheological measurements. The methods that may be applied for suspension concentrates are described briefly below. 

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