451 — Electret state and hydrated structure of melanin

TSDC spectroscopy of relaxational and interfacial phenomena

“…128) melanins, which can store electrical charge and/or polarisation, at different water content by the TSDC technique (F p = 1 × 10 1 -5 × 10 3 kV/m with or without blocking electrodes with Teflon at a heating rate 0.08-0.1 K/s). They showed that (i) natural melanin in the choroid is a natural electret at physiological temperature; (ii) TSDC on synthetic melanin is due to the presence and/or release of water; (iii) there exist two kinds of water such as ice-like and structured ('easy' according to the authors [384]) water; and (iv) the significance of hydration structure for the properties of melanin.…”

“…Bridelli et al [384] studied natural (choroid from bovine eye) (Fig. 127) and synthetic (L-dopa) (Fig.…”

“…The intensity of the A band (attributed by the authors [384] to dipolar and dc relaxations) is a linear function of F p up to 1 MV/m, and neither the position T m nor the shape of the band are modified by the applied voltage. However, the T m and I m values depend on polarisation temperature.…”

“…The activation energy of relaxation is between 30-64 kJ/mol for powders and 65-80 kJ/mol for pellets, depending on hydration degree. The I band (assigned by the authors [384] to MW polarisation effects) position depends on the hydration degree and thermal history of the sample (T m increases after sample heating). The intensity is a linear function of the F p value.…”

Thermogravimetry and differential scanning calorimetry of natural and synthetic melanins

“…Evidence exists in the literature that melanins contain water in their structure [5][6][7]. It was demonstrated that part of the water can be lost from melanin by heating up to 423 K, whereas another part is retained in the inner structure of the melanin polymer, even at temperatures higher than 423 K. Simultaneous DTA, TGA and MS analysis indicate that at least some of the thermally induced effects in melanin samples of various origins result from various concentrations of water present in the analyzed melanin samples [8].…”