Acetonitrile excess adsorption isotherm

Figure 4-52. Acetonitrile excess adsorption isotherm from water on Zorbax Eclipse XDB-C8 adsorbent (left) normalized filling of adsorbed layer (right). (Reprinted from reference 165, with permission.)...

Figure 2-8. Typical excess adsorption isotherm of acetonitrile from water on the surface of reversed-phase silica.

The analysis of experimental excess adsorption isotherms using equation (2-50) had shown unusual results [22]. The adsorbed layer thickness of acetonitrile adsorbed from water on different types of reversed-phase adsorbents calculated as the ratio of adsorbed layer volume and adsorbent surface area appears to be on average equal to 14 A, which is equivalent to approximately five monolayers of acetonitrile molecules adsorbed on the hydrophobic surface. At the same time, the adsorbed layer thickness of methanol adsorbed from water on the same adsorbents is equal to only 2.5 A, which is equivalent to the monolayer-type adsorption. 

The preferential sorption of the organic component of the solvent is a well known effect in RPLC. Based on their interpretation of adsorption isotherm measurements with water/acetOTiitrile mixtures, Kazakevich and coworkers have inferred that the excess adsorption occurs purely at the surface of the alkyl chains and that acetOTiitrile forms up to five molecular layers atop the alkyl chains. Clearly, from the snapshots and the density profiles in Fig. 3, this layering does not appear. The simulations indicate that the excess adsorption occurs both within the chain region and at the chain surface. Furthermore, the adsorbed solvent at the surface is not pure acetonitrile it is simply a local enhancement of the acetonitrile concentration. 

For experimental verification of these models, Foplewska et al. [33] used binary mixtures of methanol-water and acetonitrile-water as the mobile phases and measured the adsorption equilibrium isotherms of cyclopentanone on two similar adsorbents having different degrees of sruface heterogeneity, a Cis non-endcapped and a Cig endcapped silica. Ehie to its structure, cyclopentanone exhibits affinity for adsorption on the bonded alkyl chains and for the polar, im-covered silica sruface of the adsorbent. Overloaded elution bands of cyclopentanone in piue water were recorded (Figrue 15.3) and the isotherms were derived using an inverse method (see Chapter 3). Five independent parameters (the excess coefficients and the eqiulibrirun constants for partition-adsorption and for... 

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