THE ROLE OF THE LATERAL ANALYTEANALYTE INTERACTIONS IN THE PROCESS OF TLC BAND FORMATION

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Abstract
An attempt has been made to investigate intermolecular hydrogen-bonding interactions among adsorbed analyte molecules (i.e. the so-called lateral interactions) and their real impact on the retention process. To this effect, two different models (Models 1 and 2) were elaborated, taking into the account the aforementioned interactions. Model 1 is based on the Langmuir adsorption isotherm, implemented with a very simple assumption about the tendency of an analyte to form linear associative n-mers. This kind of intermolecular interaction is most characteristic of aliphatic alcohols, although it can also be observed for a wide variety of the other classes of chemical compound. Model 2 is considerably more sophisticated and makes a clear distinction between the formation of a monolayer of adsorbed analyte and the accumulation of further layers of adsorbed analyte on the original monolayer. Model 2 makes use of the Langmuir and Langmuir–Freundlich isotherms and is well suited to the description of fairly subtle molecular-level effects accompanying the adsorption of carboxylic acids. The validity of Models 1 and 2 was checked experimentally with three higher fatty acids (dodecanoic, tetradecanoic, and hexadecanoic) as test solutes. TLC was performed with cellulose powder and decalin, respectively, as stationary and mobile phases. The results obtained fully confirmed the practical usefulness of the two approaches. The traditional definition of the R F coefficient was also reexamined and its irrelevance for analytes participating in lateral interactions was demonstrated, as was the use of densitograms (rather than flat overall pictures of TLC chromatograms). We have proposed two novel (and optional) definitions of the R F coefficient, which make use of the concentration profiles of analytes and have compared the practical performance of each approach.