Composite adsorption isotherms from binary liquid mixtures

2 Composite adsorption isotherms from binary liquid mixtures 

A mass mA of insoluble adsorbent is introduced into a binary solution, containing n° moles of component (1) and n moles of component (2), before the adsorbent is immersed into a solution, where n° is the total number of moles of solution before adsorption (n° = n° + rd), and then the solution is allowed to reach equilibrium. After the adsorption takes place, n is the number of moles of component (1) on the surface of unit mass of solid adsorbent, giving a total adsorption of (mAn ) of component (1) whereas, n is the number of moles of component (2) on the surface of unit mass of solid adsorbent, giving a total adsorption of mAn ) of component (2). The number of moles of component (1) that remain in solution at the adsorption equilibrium is denoted by and the number of 

In order to use the mol fraction terms in the above expressions, from (n,/n2 = x,/x2), we can write 

Since (n2x, = n,x2), subtracting Equation (718) from Equation (717) yields 

In practice, the adsorption from solution behavior can often be predicted qualitatively in advance, in terms of the polar/non-polar nature of the solid and of the solution components. A non-polar adsorbent will tend to adsorb non-polar adsorbates strongly and polar adsorbates weakly, and vice versa. In addition, non-polar solutes will tend to be 

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Figure 10.1 Composite adsorption isotherms for adsorption on solids from binary liquid mixtures a. U-shaped isotherm, b. 5-shaped isotherm.

When solid particles are dispersed in liquid medium, solid-liquid interfacial interactions will cause the formation of an adsorption layer, the so-called lyosphere, on their surface. The material content of the adsorption layer is the adsorption capacity of the solid particle, which may be determined in binary mixtures if the adsorption excess isotherm is known [45-50], Due to adsorption, the initial composition of the liquid mixture, x°, changes to the equilibrium concentration Xi. This change, x - Xi = Axi, can be determined by simple analytical methods. The relationship between the reduced adsorption excess amount calculated from... 

From a knowledge of the adsorption, immersion, and wetting properties of solid particles, we have examined the influence of particle-particle and particle-liquid interactions on the stability and structure formation of suspensions of hydrophobic and hydrophilic Aerosil particles in benzene-n-heptane and methanol-benzene mixtures. For the binary mixtures, the Hamaker constants have been determined by optical dispersion measurements over the entire composition range by calculation of the characteristic frequency (Vk) from refractive index measurements [7,29,36,64], The Hamaker constant of an adsorption layer whose composition is different from that of the bulk has been calculated for several mixture compositions on the basis of the above results. Having the excess isotherms available enabled us to determine the adsorption layer thickness as a function of the mixture composition. For interparticle attractive potentials, calculations were done on the basis of the Vincent model [3-5,39]. In the case of hydrophobic particles dispersed in benzene- -heptane and methanol-benzene mixtures, it was established that the change in the attractive potential was in accordance with the interactions obtained from rheological measurements. 

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