Short-long-chain mixture, adsorption

This selectivity is subject to a strong solvent effect, and is probably determined by the polarity and solubihty of the substrates. The least soluble thiol usually adsorbs preferentially. Thus, in a mixture of an alkanethiol and an from ethanol solution. Adsorption from acetonitrile, on the other hand, does not show any preference, and adsorption from isooctane results in the preferential adsorption of a hydroxythiol . The preference of long-chain thiols over the short-chain ones and compounds with bulky substituents also depends on the solvent, being much less pronounced in non-polar solvents. Adsorption of octadecanethiol is preferred over adsorption of r-butylthiol in isooctane solution by a factor of only 40-100 (the corresponding ratio in ethanol is 290-710, vide supra), and by just a factor of 3-4 over adsorption of straight-chain butanethiol (compared with a 20-30 ratio for C22 vs C12 preference in ethanol, vide suprd). ... 

Figure 12.25h also illustrates the TIRF signal for the coumarin-tagged short chains, curve 2, as they adsorb from the mixture. Initially the short chains from the mixture are uninfluenced by the presence of the long chains, as the rise in curve 2 is compared to the rise in the 33,000 g/mol adsorption in Figure 12.25a. After this point the short chains are displaced from the surface by the incoming long chains, which continues until substantially all of the short chains are removed.

DAOUD LECLERC Adsorption from a Mixture of Short and Long Chains 25... 

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