Enhancing the volumetric heat storage capacity of Mg(OH)2 by the addition of a cationic surfactant during its synthesis

Abstract

It is here investigated the effect of the addition of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) during the precipitation of Mg(OH)2 on structural, morphological and physical properties of the resulting hydroxide. The thermochemical behaviour as heat storage material of the final samples is investigated. Mg(OH)2 is precipitated from a solution containing Mg(NO3)2 precursor at pH 11.5, using NH4OH as base, at T = 25 °C. CTAB concentration varies in the range 0.5–20.0 mM. For comparison a sample of Mg(OH)2 is prepared in absence of surfactant. The results obtained show that, at the above synthesis conditions, exists an optimum CTAB concentration value (2.0 mM) which (i) promotes the formation of well separated Mg(OH)2 particles, (ii) enhances specific surface area, (iii) lowers the hydroxide mean particle diameter and (iv) increases the bulk density likely due to the peculiar stacked configuration of hydroxide particles. As main result, Mg(OH)2 prepared at the optimum CTAB concentration exhibits the highest volumetric stored/released heat capacity, ∼560 MJ/m3, almost two times higher than that measured over Mg(OH)2 prepared in absence of CTAB. Cyclic experiments evidence an excellent stability of the sample up to 13 dehydration/hydration reactions.