Lyotropic liquid crystalline (LLC) phosphoric acid-10-lauryl ether: mesophases, proton conductivity and synthesis of transparent mesoporous hydroxyapatite thin films

Abstract

Many salts, acids, and bases with low deliquescence relative humidity (DRH) can organize non-ionic surfactants into lyotropic liquid crystalline (LLC) mesophases that form a ready platform for the synthesis of mesoporous materials. In this study, we show that phosphoric acid (H3PO4, PA) with low DRH value can also be used as a solvent in assembling non-ionic surfactant (C12H25(OCH2CH2)10OH, C12EO10) into stable LLC mesophases within a broad range of composition (the concentration can be as high as 20 PA/C12EO10 mole ratio). The PA/C12EO10 mesophase is bi-continuous cubic phase (V1) in extremely low concentrations (2 PA/C12EO10 mole ratio), 2D/3D hexagonal phases (H1) at moderate compositions (3 to 5 PA/C12EO10 mole ratio) and micelle cubic (I1) at high, (more than 5) H3PO4/C12EO10 mole ratios, with a typical unit cell parameter of 127, 55, and 116 Å, respectively. The mesophases of the lower concentrated samples (less than 15 mole ratio) have high thermal stability, with melting points greater than 120 oC. However the melting point drops to less than 50 oC for extremely high concentrations (more than 17 PA/C12EO10 mole ratio). The LLC mesophases were also found to exhibit high proton conductivities (~10-3 S/cm) at room temperature. The proton conductivities were even higher (10-2 S/cm) at some elevated temperatures and reduced to (10-4 S/cm) at temperatures less than 0oC. The conductivity in the cubic phase is slightly higher. Both the temperature and composition-dependent conductivity obey the most accepted proton conductivity mechanisms: Grotthuss and Vehicle. We went further to show that the combination of H3PO4 and another low DRH species, such as Ca(NO3)2·4H2O also form stable mesophases; without precipitating salts, under a wide range of concentration, from 5.3/1 to 13.3/1 precursor to surfactant ratio. High acidity stabilizes both the aqueous solution as well as the LLC phases. The clear solutions obtained from the precursor-surfactant mixtures were spin coated on glass substrates (as thin as a few hundred nanometers) and calcined to form transparent nano-size mesoporous hydroxyapatite (HAp) thin films. The formation of semi-crystalline HAp in our synthetic approach is not a straight forward process; it involves the formation of some intermediate products and also requires a calcination temperature of at least 300 oC. The formation, which starts at 300 oC, is preceded by the evaporation of nitric acid and excess water molecules to the surrounding. The crystallization continues at 400 oC and completes at 500 oC, keeping the uniformity, porosity, and transparency of the films. Films of the 5.3/1 ratio, calcined at 300 oC have high surface area of up to 96 m2/g, which dropped down to 20 m2/g at 500 oC. The mesopores start collapsing at around 600 oC. The pore size, pore walls, and the pore volumes were obtained from the N2 sorption measurements and the values are 22.4 nm, 10 nm, and 0.58 cm3/g, respectively. We also investigated the effect of precursor concentration on both the pore sizes, as well as the thicknesses of the pore walls. The results showed a reduction of surface area, and also narrower pore size distribution with increasing concentration. Temperature was also observed to have the same effect on crystallinity in all the compositions studied. All the investigations on these two systems were carried out using XRD (X-ray diffraction), FT-IR (Fourrier Transform Infrared Spectroscopy), Raman spectroscopy, POM (Polarized Light Optical Microscope), N2-sorption measurements, PEIS (Potentiostatic Electrochemical Impedance Spectroscopy), TEM (Transition Electron Microscopy), SEM (Scanning Electron Microscopy) etc.