The role of the lateral analyte-analyte interactions in the process of TLCband formation

An attempt has been made to investigate intermolecular hydrogen-bonding int eractions among adsorbed analyte molecules (i.e. the so-called lateral inte ractions) and their real impact on the retention process. To this effect, t wo different models Models 1 and 2) were elaborated taking into the account the aforementioned interactions. Model 1 is based on the Langmuir adsorpti on isotherm, implemented with a very simple assumption about the tendency o f 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 compou nd. Model 2 is considerably more sophisticated and makes a clear distinctio n between the formation of a monolayer of adsorbed analyte and the accumula tion 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 accompan ying 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 stat ionary and mobile phases. The results obtained fully confirmed the practica l usefulness of the two approaches. The traditional definition of the R-F coefficient was also reexamined and i ts irrelevance for analytes participating in lateral interactions was demon strated, as was the use of densitograms (rather than flat overall pictures of TLC chromatograms). We have proposed two novel land optional) definition s of the R-F coefficient, which make use of the concentration profiles of a nalytes and have compared the practical performance of each approach.