Quantifying the Wettability of ALD-Coated Micro-Particles

Abstract

This research aimed to enhance and quantify the wettability and water affinity of Expancel microparticles which generally demonstrates a lack of affinity with water. To enhance their water affinity, hydrophilic coatings like SiO2 and TiO2 were deposited onto the microparticles using atomic layer deposition (ALD). Various experimental techniques were employed to analyse the affinity, wettability, surface morphology, surface charge, and elemental compositions of the coated particles. TGA was conducted to examine the water affinity by subjecting the particles to direct heat in the temperature range of 25 ̊C to 130 ̊C under inert conditions. Weight loss% w.r.t temperature were measured, with a focus from the initial stage 25 ̊C to 80 ̊C, representing moisture loss from the surface. An increase in weight loss% was observed for SiO2 samples as the number of coating layers increased, indicating enhanced affinity with water. SEM images were captured at different temperatures of 80 ̊C, 100 ̊C, and 130 ̊C to clarify and explain the trend observed in TGA graphs. It was discovered that there is no expansion until 80 ̊C, full expansion at 100 ̊C, and particle ruptures at 130 ̊C due to the fact that the encapsulated gas escapes the particles completely. Experiments to measure WCA were conducted to quantify the wettability of coated particles and compare them to uncoated particles. The data indicated improved wettability for SiO2 and TiO2-coated particles, as evidenced by reduced contact angles. As Expancel microparticles tend to agglomerate in the presence of water, focused beam reflectance measurement (FBRM) analysis was employed to measure real-time agglomerate size. The FBRM measurements demonstrated reduced agglomerate sizes for coated particles compared to uncoated particles. XPS analysis was used to examine the elemental compositions on the particle surface after coating with varying no. of coating layers; higher silicon (Si) concentrations were found as compared to titanium (Ti). ICP-EOS was used to quantify the amount of Si and Ti weight% after varying no. of deposition layer. The data indicated that the amount of Si was measured to be a lot more than the theoretical value, which implied CVD behaviour in our reactor. LEIS analysis revealed that one layer of coating achieved full surface coverage with SiO2, and subsequent coating cycles did not significantly alter the surface composition.