QIU Shixuan，YAN Shen，ZHANG Shengyu，WU Duo

Engineering Research Centre of Advanced Powder Technology, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China

Abstract

Objective Spray drying technology offers the advantage of one-step formation, enabling the efficient preparation of functional microparticles. It is widely used in the food, chemical, and pharmaceutical industries. However, during the drying process，solvent evaporation within droplets causes solute migration and rearrangement, resulting in dried microparticles with complex surface structures and surface compositions that are difficult to quantify. Furthermore, the morphology, size, and crystal forms of the microparticles can all influence their performance. In this study, inverse gas chromatography (IGC) was employed to characterize the surface properties of the microparticles, exploring the relationship between surface composition and surface energetics, and further analyzing the flowability of the microparticles. The study focuses on elucidating the structure-property relationship between surface composition and particle performance, providing guidance for the design of functional microparticles and enabling precise control of their flowability.

Methods Mannitol and sodium stearate were used as model substances. The feed liquid for spray drying was prepared using high-shear stirring, ultrasonic treatment, and microfluidic homogenization. A series of mannitol‒sodium stearate microparticles was then prepared via spray drying at an inlet temperature of 120 °C. The particle morphology was characterized using scanning electron microscopy (SEM), particle size was measured by laser diffraction, crystal form was analyzed using X-ray diffraction (XRD), and the chemical state of carbon on particle surfaces was examined using X-ray photoelectron spectroscopy (XPS). The surface properties of the microparticles were characterized using IGC at infinite dilution conditions: n-heptane, n-octane，and n-nonane were used to measure the dispersive surface energy; iso-octane was used to determine the surface roughness at the molecular level; and ethyl acetate and isopropanol were used to measure the surface reaction energy. In addition, the flowability of the samples was evaluated by measuring bulk density (Carr’s index).

Results and Discussion After spray drying, the resulting microparticles all exhibited a spherical morphology, and their dispersibility improved slightly with increasing sodium stearate content. However, the sodium stearate content had no significant effect on particle morphology or size. XRD results revealed that the addition of sodium stearate disrupted the formation of mannitol’s ordered crystalline structure, resulting in the precipitation of metastable δ-polymorph. XPS results showed that as the sodium stearate content increased, its proportion on the particle surface also increased. This was mainly due to the surface activity and larger molecular size of sodium stearate, causing it to aggregate on the particle surface during drying. IGC results showed that adding sodium stearate increased the dispersive surface energy of the microparticles, but decreased their surface polarity. When the sodium stearate content was 10% w/w, the dispersive surface energy of the particles reached 65. 1 mJ/m², and the surface reaction energy measured using isopropanol as a probe was 11. 42 kJ/mol. Moreover, the addition of sodium stearate disrupted the orderly arrangement of mannitol molecules on the particle surface, resulting in a more complex molecular arrangement. Accordingly, the surface roughness index measured with iso-octane as a probe decreased with increasing sodium stearate content. The flowability of the samples improved significantly with higher sodium stearate content: the Carr’s index of the spray-dried pure mannitol sample was as high as 48. 56%, but decreased to 29. 7% when the sodium stearate content was 10% w/w, which was due to the hydrophobic shell formed by sodium stearate enrichment on the particle surface.

Conclusion The IGC technology provides new insights into the surface properties of spray-dried microparticles. The addition of sodium stearate promotes the formation of a hydrophobic surface on the spray-dried microparticles, enhances the dispersive surface forces, and reduces the surface reaction energy, making the sample less prone to absorbing water during collection, transfer, and storage, and ultimately improving their flowability.

Keywords：mannitol; sodium stearate; spray drying; inverse gas chromatography; surface energetics