WEB Optimisation of hydroxyapatite texture using CTAB template
Hydroxyapatite (HA), Ca5(PO4)3(OH), is a natural ceramic with extensive interest in areas such as biomaterials, adsorbents and catalysis. In the last decades, considerable efforts have been devoted to control the nano-texture of HA, in particular for the sake to increase their total area surface and finely tune their porosity at a nanoscale level. To aim that, several approaches have been proposed in the literature, specially using surfactants as nuclear template during the synthesis. However, no systematic studies have been published in order to correlate and control the texture properties of HA and the surfactant behavior in the experimental conditions of HA synthesis.
The work aims to analyse cetrimonium bromide (CTAB) behavior within conditions simulating HA synthesis in order to better understand and control its aggregation level before considering HA synthesis. To achieve that, different CTAB concentrations (0.4 to 100 mmol*L-1) were evaluated by Dynamic light scattering (DLS) in HA reaction conditions (pH = 10.5 and presence of phosphate ions) at 25 and 50 ⁰C. Interestingly enough at low concentration (i.e. < 10 mmol*L-1), CTAB tends to form aggregates with a mean size over 100 nm while exhibiting bimodal population of surfactant aggregates. Above 10 mmol*L-1 only unimodal size micelles have been distinguished in the autocorrelation curves derived from DLS analysis. No significant difference in CTAB behavior has been noticed at the two temperatures investigated.
In a second step several batches of HA have been synthetized adopting CTAB concentration adjusted in function of their aggregation regime. After purification and drying at 100 °C, HA has been analyzed by FTIR, XRD, TGA, BET and TEM techniques. It was clearly observed that the specific surface area of HA can be increased significantly (up to 150 m2/g) in a narrowed range of CTAB concentration. Moreover, size and shape of HA nanoparticels are remarkably affected by surfactant concentration.
From the behavior of CTAB micelles in function of time, temperature and concentrations, the most appropriate medium to perform HA synthesis assisted by CTAB surfactant and its effect in HA texture properties was identified.