Salt-Acid-Surfactant Lyotropic Liquid Crystalline Mesophases: Synthesis of Highly Transparent Mesoporous Calcium Hydroxyapatite Thin Films

Even though calcium hydroxyapatite [Ca10(PO4)6(OH)2, HAp] is one of the most investigated materials in the literature, the synthesis of mesoporous transparent thin film of HAp has not yet been reported. We show herein that mixtures of phosphoric acid (H3PO4·H2O, PA), calcium nitrate tetrahydrate [Ca(NO3)2·4H2O, CaN] and non-ionic surfactant [C12H25(OCH2CH2)10OH, C12E10] can self-assemble into stable lyotropic liquid crystalline (LLC) mesophases. The clear aqueous solutions of the mixtures can be spin-coated over any substrate and then calcined to form highly transparent mesoporous HAp (mHAp) thin films. From among the compositions studied, three molar ratios of CaN/PA/C12E10 [3.3:2:1 (low), 5.8:3.5:1 (intermediate) and 8.4:5:1 (high)] were chosen for large-scale preparation to investigate their structural and thermal properties. The mHAp films form at around 300 °C and fully crystalize at 500 °C, retaining their transparency, uniformity and porosity in all compositions with few differences. The surface area and pore volume decrease, and the pore size and pore size distribution increase with increasing annealing temperature for all compositions. Phosphoric acid and calcium nitrate self-assemble into lyotropic liquid crystalline mesophases that can be calcined into sponge-like transparent mesoporous thin films of calcium hydroxyapatite. Copyright © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim