M. Borkovec et al., AFFINITY DISTRIBUTIONS AND ACID-BASE PROPERTIES OF HOMOGENEOUS AND HETEROGENEOUS SORBENTS - EXACT RESULTS VERSUS EXPERIMENTAL-DATA INVERSION, Colloids and surfaces. A, Physicochemical and engineering aspects, 107, 1996, pp. 285-296
Ionization properties of simple polyprotic acids and bases (oligoproti
c carboxylic acids, polyelectrolytes, latex particles, proteins) can b
e described quantitatively with Ising models. These mechanistic models
incorporate relevant molecular details and allow determination of exa
ct affinity distributions (pK spectral. At present, such a molecularly
based approach is too involved for complex heterogeneous substances,
but the affinity distributions of these materials can be calculated by
direct numerical inversion of the experimental data. Even though this
problem is ill-posed, special numerical techniques can be used to sta
bilize the procedure and td select distributions that are smooth or th
at consist of a small number of isolated discrete peaks, which can be
interpreted as a small number of ionizable sites. The results of these
inversion techniques are discussed for a number of examples of enviro
nmental relevance such as oxides and humic substances, and for the sim
ple model systems. Based on such observations for simple systems, we c
onclude that it is essentially impossible to extract true affinity dis
tributions of complex environmental sorbents.