Influence of liquid properties of colloidal crystals to thermal stability of colloidal crystals on different substrate surfaces through heterogeneous nucleation

The effects of liquid properties, such as pH and conductivity, on the thermal stability of electrostatically interacting colloidal silica crystals generated from heterogeneous nuclei were investigated mainly by measuring relative reflection spectra. The melting behavior of the colloidal crystals was also investigated. The melting points of both negatively and positively charged 3-aminopropyltriethoxysilane (APS)-treated substrates with different surface charges increased or decreased with increasing or decreasing pH. For both substrates, pH 5.5 was the modulation point of the crystal melting, and the melting point increased with increasing pH. For the negatively charged substrate, the melting point increased slowly as the pH decreased from pH 5.5. Meanwhile, the melting point of the APS-treated substrate increased more with decreasing pH from 5.5 than that of the negatively charged substrate. The cationisation of the substrate increased with decreasing pH, resulting in stronger electrostatic interactions with the negatively charged colloidal crystals and enhanced thermal stability. Next, we investigated the size of colloidal crystal grains in sample bottles for different liquid properties, and the results show that at the higher pH, the smaller crystal grains, indicating that pH affects both the substrates and crystals. Moreover, the binding effect of the APS-treated substrate on the crystals became stronger at lower pH, thereby enhancing the thermal stability of the crystals.