Nucleation in non-homogeneous diffusing-reacting systems occurs under a changing supersaturation. Under these conditions, the rate of creation of supersaturation determines the value of the supersaturation threshold at the nucleation time, i.e., the actual maximum metastability level. Measurements of metastable zone-widths and their dependence on the supersaturation rate are here reported for different carbonates and sulphates. By analogy with nucleation behaviour by continuous cooling of saturated solutions, a semiempirical relation appears to govern this dependence. Finally, this relation is ehecked for solutions doped with additives and metastability measurements in U-tube gel systems are proposed to compare inhibitor effectiveness.

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Lithium propanoate and pentanoate were characterized by DSC, single crystal and powder XRD and FTIR and impedance spectroscopies. Lithium propanoate presents a solid-to-solid transition (SII-SI) at T(ss) = (549.1 +/- 0.7) K on first heating that varies on the second and next ones, followed by a fusion at T(f) = (606.1 +/- 0.5) K. For lithium pentanoate, two solid-to-solid transitions (SIII-SII and SII-SI), at T(ss) = (205.5 +/- 0.5) K and T(ss) = (325.2 +/- 0.7) K, respectively, and a melting point at T(f) = (576.5 +/- 0.3) K were found. The crystal structures for both compounds were characterized at 100 and 298 K (and for the lithium propanoate also at 160 K). Single-crystal XRD showed that the SII phase of both compounds has a monoclinic structure with the same symmetry group (P2(1)/c). This is the first time that a single-crystal structure has been reported for any member of the lithium alkanoates series, so far. FTIR and impedance spectroscopies were also carried out to better characterize the solid phases in these compounds.

Lead(II) alkanoates, from hexanoate to dodecanoate, have been analyzed by means of XRD, optical microscopy, DSC, FTIR, and electric spectroscopy. Four different phases have been identified, corresponding to the three thermal transitions measured by DSC:  two of them solid (crystal and “intermediate” phases), and another two fluid (neat phase and isotropic liquid). Powder crystal XRD data indicate that the samples present a bilayered structure. The analysis of the (00l) spacing dependence with temperature in the three ordered phases strongly points to the intermediate phase to be a rotator phase. Optical microscopy and FTIR versus temperature also confirm a structural change from the crystal to the intermediate phase and its solid-state nature. Electrical conductivity maps the thermal transitions of the samples and shows a high ionic conductivity in the intermediate phase, which does not depend much on the carbon chain length. The high conductivity values (3 orders of magnitude higher in comparison with that of the ordered crystal at room temperature) obtained for the intermediate phase gave a further support to the existence of a rotator mesophase in the lead(II) alkanoate series.

Supersaturation is not enough to interpret nuc\eation behavior in crystal growth in gels. The explanation for the first precipitate location needs complementary crystallization criteria. The experimental testing of both mass-transfer and pH evolution is the background to establish rigorous hypotheses. From these data, the actual distribution of free ions, iomc associations and supersaturation throughout the gel column may be found. In the case of nuc\eation of strontiamte and witherite crystals, although the critical supersaturation conditions remain necessary, a second concept, the "equality range" of the reagent concentrations, has to be introduced to account for observations. With these dual conditions, the behaviour of the described gel-diffusion system is correctly reproduced. The significance of pH evolution is also experimentalIy shown. Nuc\eation density and growth mechamsms are finally explained on the base to supersaturation rate and kinetic roughness.