Volume Fraction Dependent Morphological Transition of Silica Particles Derived from Sodium Silicate

The volume fraction dependent morphological transition of droplets during the evaporation of colloidal silica solution was investigated using the spray-drying method. The colloidal solution was prepared from sodium silicate using the sol-gel method. Spray drying with a tubular reactor was used in the experiment, with the volume fraction of the colloidal silica varying from 15 to 2%. It was demonstrated that a morphological transition from a sphere shape to a donut-like shape takes place at a colloidal volume fraction of between 4% and 2%, even when the drying rate remains moderate and is not extremely fast. A spherical silica particle shape was found at a colloidal volume fraction of between 15% and 8%. The morphological transition depends strongly on the volume fraction of the colloids in the droplets. Further, the ?-potential of the particles in the droplet also affects the transition by applying an interparticle electrostatic force. The same high surface charge of sol silica provides a repulsive interaction between the sol particles inside the droplet. The transition is hindered when the colloid volume fraction is increased because of the inherent spatial constraint. The Fourier-transform infrared (FT-IR) spectra of both the spherical and donut-like particles confirm the chemical bonding of the powder silica product. Moreover, X-ray diffractometry (XRD) analysis revealed an amorphous phase of the silica particles produced from spray drying. These preliminary results open up a new path for controlling the formation of a wide variety of silica particles using the spray-drying method. In addition, the different silica particle morphologies enable a variety of particle applications.

Colloidal silica; Particle morphology; Sodium silicate; Spray drying; Volume fraction

The spray-drying method is an established technology for synthesizing a material with controllable morphology. The method enables continuous production of nanostructure powders with high surface area and high purity, and it can be scaled up to ton-order quantities (Xiong et al., 2017). In addition, the particles produced from this method are agglomeration-free and have a monodisperse size, which is highly useful for material processing (Nandiyanto & Okuyama, 2011). The industrial-scale spray-drying method has been long established because it is a simple, cost-effective system (Isobe et al., 2005).

The production of silica particles by spray drying has attracted much interest in recent decades because of the ability of the method to produce several kinds of particles with different morphologies (Tsapis et al., 2005; Vehring et al., 2007). The morphology of silica particles determines their practical applications (Qomariyah et al., 2018a). Spray drying of colloidal precursors containing silica particles has been reported by several researchers (Waldron et al., 2014; Dixit et al., 2016). The reported applications of silica particles from spray drying include their use as catalysts and absorbents, as well as in chromatography and drug delivery systems (Bahadur et al., 2011; Wang & Friess, 2017).

Iskandar et al. (2003) used an aerosol-assisted spray method to produce silica particles, using a commercial colloidal silica suspension as the precursor. However, application on a large scale has been limited because of the high cost of the silica source (Ui et al., 2009). This drawback is an interesting topic for further study of the production of silica particles with controllable morphology using the spray-drying method with a low-cost silica source. In general, silica particles are synthesized using tetraethyl orthosilicate (TEOS) (Cho et al, 2009; Wang et al., 2010) or trimethoxy vinyl silane (TMVS) as the silica source to obtain monodisperse and spherical-shaped particles (Zainal et al., 2013; Wilson & Mahmud, 2015). Particle size can be altered by adding higher concentrations of TEOS and TMOS. Unfortunately, reliable preparation of larger amounts of silica particles and effective control of the reaction mechanism is difficult because of the high cost and rapid hydrolysis and condensation reaction rate of this silica source. Industry would prefer to use an alternative low-cost source of silica such as sodium silicate, which is also known as water glass.

In the drying process of the small droplets containing nanoparticles, evaporation drives the shrinkage of the droplets, and the constituent particles are assembled through particle interaction (Lyonnard et al., 2002). Furthermore, the oscillation of the droplet shape is a basic mechanism for the buckling process of the droplet. It has been observed that the drying process strongly affects the shrinkage of the droplet. When the rate of drying is slow enough, the droplet shrinks in an isotropic manner and the final particle shape remains spherical because of the high internal surface energy. However, if the rate of drying is sufficiently fast, droplet deformation occurs because of its instability. This leads to the formation of non-spherical shapes, such as donut-like particles. The formation of this kind of morphology can be affected by various factors, such as the volume fraction of the precursor. The parameter that determines the formation of particles inside the spray-drying reactor is called the Peclet number (Pe), which is defined as the ratio of the mixing time of the nanoparticles on the droplet to the droplet drying time. A Pe value much greater than 1 is regarded as a fast-drying process, which can produce hollow or donut-like morphologies. However, if Pe << 1, the drying process is slow enough for the particle to produce a spherical morphology because of isotropical shrinking of the droplet. Therefore, in addition to the volume fraction of the colloidal solution, the drying process plays a role in determining particle morphology.

In addition, the morphology of the synthesized particle through spray drying can be tuned by the physical properties of the drying medium. This tuning of the particle-particle interaction inside the droplets affects the final particle morphology (Qomariyah et al., 2018b). Other parameters, such as the particle concentration inside the initial droplet, droplet size, surface tension and hydrodynamic properties, can affect the sphericity of the final particle morphology (Iskandar et al., 2003; Widiyastuti et al., 2016). Several researchers (Iskandar et al., 2003; Pitchumani et al., 2009) have attempted to control the morphology of silica particles produced by the spray-drying method using sodium silicate solution as the silica source. However, to the best of the present authors’ knowledge, none of the studies has considered the morphology change based on the drying process (slow or fast) in the spray-drying reactor. The morphology of particles is an important issue in the application of silica particles. Hollow particles are preferred for inorganic catalyst, adsorption and gas separation applications. On the other hand, spherical and donut-like particles are desirable for pigments and as carrier particles in drug delivery.

This study was conducted to investigate the morphological transition of droplets consisting of sol silica particles with different volume fractions. It was observed that the morphology transition from spherical to donut-like shapes takes place solely by varying the volume fraction of sol silica particles in the initial droplet. Furthermore, the presence of particle interactions inside the droplets also strengthens the buckling process, even at a slow drying rate. A possible mechanism of the morphological transition is also discussed in this paper. An understanding of the morphological transition of silica particle formation will enable wider application of silica. The use of the spray-drying method is also feasible for large-scale production in industry.

The morphological transition of silica particles from spherical to donut-like particles was investigated regarding the dependence on volume fraction, which varied from 15% to 2%. Sodium silicate solution was used as the silica source. Even at a slow drying rate, the buckling process took place by hydrodynamic and particle-particle interactions, which eventually caused the morphological transition when the colloid volume fraction was small. The spherical particles formed at colloidal volume fractions of between 15% and 8%, whereas the donut-like particles formed at lower volume fractions (4% to 2%). However, the higher colloid volume fractions of between 8% and 15% are favorable for spherical-shaped particle formation, because of the inherent constraints on space availability; the morphological transition to the donut-like shape is hindered. Particle interaction in the droplets containing sol silica particles also plays an important role in the formation of donut-like morphology. FTIR analysis revealed that the silica particles were formed by the presence of siloxane bonding in all the samples. XRD analysis also proved the formation of silica particles with an amorphous phase. This preparation method offers an economical approach that exploits an abundant and cheap material, sodium silicate, and the well-understood spray-drying technique. Furthermore, the method is suitable for economical and large-scale production of silica powder.?

The authors are grateful for the financial support provided by the PMDSU research grant 2018 from the Directorate of Research and Public Service, Directorate General of Research Strengthening and Development, Ministry of Research, Technology and Higher Education of the Republic of Indonesia, with contract No. 819/PKS/ITS/2018. We also extend our gratitude to Mr. Fahad Nizar Sasmita and Mr. Hafidz Rifki Novaldi for their assistance with the experiment.  

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