PROPERTY-STRUCTURE RELATIONSHIP OF SOLUTE-STATIONARY PHASE COMPLEXES OCCURRING IN A MOLECULAR MECHANISM BY PENETRATION OF ELUITE IN BONDED ALKYL CHAINS IN REVERSED-PHASE LIQUID-CHROMATOGRAPHY

The analysis of the chromatographic behaviour of homologous series wit h a large range of chain lengths, 1 less than or equal to n(c) less th an or equal to 32, allowed a second break in the curves of log k' vs. n(c) to be observed. This occurs independently of the nature of the se ries, the length of bonded chain (6 less than or equal to n(bp)less th an or equal to 22), the analysis temperature (when it is higher than t he temperature of phase transition); also, the qualitative mobile phas e composition does not affect the phenomenon in partially aqueous reve rsed-phase liquid chromatography or in the non-aqueous reversed-phase mode (for mixtures CHCl3-MeOH, CHCl3-MeCN, H2O-MeOH or H2O-MeCN rich i n MeOH or MeCN). The two breaks in the curves mark the limits of three zones which are characterized by two different values of n(c):n(crit. ) and n(sat.). The first describes the phenomenon for values of n(c) l ess than or equal to n(crit.). In this range n(crit.) is independent o f the homologous series but varies with the bonded chain length, n(bp) . The methylene selectivity alpha is different for each series and a c orrelation with the increment of molecular volume of a methylene group , Delta V-CH2, for each of them is established. The second break appea rs for values of n(crit.)less than or equal to n(c) less than or equal to n(sat.). In this case, the value of the methylene selectivity is t he same regardless of the series. The same evolution can be demonstrat ed by examination of the increment of molecular volume of a methylene group. The third zone such as n(c) greater than or equal to n(sat.) sh ows a behaviour identical with that of the second zone but with lower values of alpha and Delta(CH2). The value of n(sat.) is independent of the nature of the homologous series but is related to the bonding den sity of alkyl chains on silica. The results obtained are consistent wi th a molecular mechanism of interaction by insertion of the solute ins ide the bonded phase. The contacts are more or less tight depending on the space available between the chains of the stationary phase. This leads to three geometries of contact whose energetic phenomena are dif ferent.