(37f) Impact of Silica Nanoparticles on Microstructure and Rheology of Wax Network Suspensions

Suspensions with jammed networks are found in different products such as chocolate, peanut butter, or margarine in the food industries, crude oil-carrying pipelines in the petroleum industries, and creams and ointments in the pharma and consumer goods industries. Organic molecules present in these suspensions serve as crucial structuring agents. The network formed during the cooling plays a pivotal role in determining the texture and rheology of the end products. Common additives like polymers and surfactants are frequently employed to refine the crystal morphology and enhance product properties. Recently, additives containing nanoparticles with different surface characteristics are being explored to improve rheology by modifying the crystal properties.

In the present work, we employ a networked waxy oil system to study the effect of hydrophobic silica nanoparticles (SNPs) on the crystallization behavior of waxes. SNPs have been synthesized using the Stöber method and hydrophobized via attachment of long alkyl chains. We study the influence of alkyl chain length and grafting density on wax crystal morphology and suspension rheology. With the introduction of SNPs in the crystallizing medium, the crystal appearance temperature is reduced, signifying delayed nucleation. Moreover, the morphology of wax crystals changes from plate-like to branched structures. The change in crystal morphology was found to reduce the network-forming tendency of the system with more than one order decrease in viscoelastic moduli and yield stresses. The changes in rheological properties are further correlated to microscopic properties, such as crystal number density, average crystal length, and total free pore volume. SNPs are expected to interfere with the formation of stable nuclei and further adsorb on the growing interfaces of the crystals to inhibit growth in that direction. These results have been confirmed by SEM micrographs obtained for the crystals before and after the incorporation of SNPs. Adsorption on the crystal surface is directly related to the diffusivity and mobility of nanoparticles. Our study also explores the effect of various parameters such as SNP concentration, surface grafting density, and particle size to optimize the efficiency. By shedding light on the mechanisms underlying nano-additive action on wax crystallization, our findings offer valuable insights for the development of tailored solutions to enhance the product performance of similar systems.