Ultrasound-assisted/biosurfactant-templated size-tunable synthesis of nano-calcium sulfate with controllable crystal morphology

• We report a biosurfactant-templated sonochemical synthesis route of nano-CaSO4.
• The influences of synthesis strategy on the nano-CaSO4 crystals were researched.
• With increase in biosurfactants/H2O ratios, crystal length and aspect ratio decreased.
• Long rod, hexagonal plate, and plate-like appearance with increasing surfactin ratio.
• The possible mechanism of crystal polymorph variation was suggested.

Nano-sized crystals of alpha calcium sulfate hemihydrate (α-HH) with considerable morphology-dependent properties find promising applications in the clinical fields as a cementitious material. Towards this end, ultrasound-assisted rhamnolipid and surfactin biosurfactant-template route is explored to control the morphology and aspect ratio of nano-CaSO4 by adjusting the mass ratio of rhamnolipid/H2O, surfactin/H2O and rhamnolipid/surfactin. The change in the molar ratio of [SO42-]:[Ca2+] results in modification in variable morphology and size of nano-CaSO4 including long, short rods and nanoplates. With increase in the rhamnolipid/H2O ratio from 1.3 to 4.5, the crystal length decreases from 3 μm to 600 nm with the corresponding aspect ratio reduced sharply from 10 to 3. Similarly, the crystal morphology gradually changes from submicrometer-sized long rod to hexagonal plate, and then plate-like appearance with increase in surfactin concentration. The preferential adsorption of rhamnolipid on the side facets and surfactin on the top facets contributes to the morphology control. The process using 50% amplitude with a power input of 45.5 W was found to be the most ideal as observed from the high yields and lower average l/w aspect ratio, leading to more than 94% energy savings as compared to that utilized by the conventional process. As a morphology and crystal habit modifier, effects of Mg2+ and K+ ions on α-HH growth were investigated to find an optimal composition of solution for α-HH preparation. Mg2+ ions apparently show an accelerating effect on the α-HH growth; however, the nucleation of α-HH is probably retarded by K+ ions. Thus, the present work is a simple, versatile, highly efficient approach to controlling the morphology of α-HH and thereby, offers more opportunities for α-HH multiple applications.