Influence of Alkyl Trimethyl Ammonium Bromides on Hydrothermal Formation of α-CaSO4·0.5H2O Whiskers with High Aspect Ratios

In this paper, the influence of alkyl trimethyl ammonium bromides (CnH2n+1(CH3)3NBr, n = 10, 12, 14, 16, 18, abbreviated as ATAB) on the formation of alpha calcium sulfate hemihydrate (α-CaSO4·0.5H2O) whiskers under a hydrothermal condition (135 °C, 3.0 h) was analyzed. Specifically, it focuses on cetyl trimethyl ammonium bromide (C16H33(CH3)3NBr, abbreviated as CTAB). The rising CTAB concentration from 0 to 9.2 × 10−4 mol·L−1 led to the increase of the average aspect ratio of α-CaSO4·0.5H2O whiskers from 80 to 430, since the selective adsorption of CTAB on the negatively-charged side facets of the whiskers inhibited the growth of the whiskers along the direction normal to the lateral facets. The further increase of CTAB concentration above the critical micelle concentration (abbreviated as CMC) showed little effect on the morphology of α-CaSO4·0.5H2O whiskers, considering that CTAB tended to form micelles instead of being adsorbed on the whisker surfaces. Similar phenomena were observed in other ATABs (n = 10, 12, 14, 18).