The effects of relative humidity on lactose particle adhesion

Kyung Min Lee, Purdue University

Abstract

Adhesion between protein and solid surface or between pharmaceutical particle and solid surface is one of the major concerns in many research areas and in industrial processes. Adhesion at an interface is the result of several interaction forces, including van der Waals interactions, electrostatic interactions, steric interactions, and chemical bonding. In this study, dextran is oxidized using standard periodate methods to form a desired dialdehyde, allowing for side-on covalent attachment of dextran to amine groups on surfaces of interest. Oxidized dextran maintains an equilibrium between the dialdehyde and a hemiacetal structure. Regardless of initial pH, the oxidation proceeds until all dextran is converted. In the same range, the dialehyde was dominant over the hemiacetal structure, showing only small variations with changes in pH. Colloidal probe method was used to measure adhesion forces of lactose particle mounted on AFM cantilever against stainless steel and Perspex surfaces. The adhesion forces increased with relative humidity in the range of 15% and 60%. The role of increasing relative humidity was investigated and validated using models. At low humidity (RH 15%), the vdW force is dominant over all other forces and the difference in the vdW force between stainless steel and Perspex is explained by the composition of the two substrates and the differences in their roughness Perspex surfaces are significantly smoother than the steel. In our prior work, it was shown that smoother surfaces often produce more interfacial contact and a larger amount of interacting volume within 15 nm of the contact point than when the surfaces are rough. As a result, larger adhesion forces are obtained if the particle is smooth. At 60% RH, the adhesion forces against stainless steel are 4 to 5 times larger than the values at 15% RH. However, the adhesion forces against Perspex increased by only a factor of 1.4 over this range. It is explained by capillary forces. When the particle contacts the substrate, the droplets form liquid bridges between asperities on the particle and the substrate. Each droplet exerts a capillary force on the particle to hold it against the substrate. The rougher surfaces will possess nanodroplets of water with larger radii of curvature, and these will exert a correspondingly larger force on the particles than those with smaller radii of curvature. At the same time, as the system RH increases, the size of the nanodroplets will increase, raising the radius of curvature of the bridges and leading to a larger capillary force. The steel, with the much deeper surface features, can accommodate larger droplets than the Perspex, and thus the increase in adhesion is larger for the steel than the Perspex. Measured adhesion forces were validated by van der Waals force model and capillary force model. At dry conditions (RH=15%), the observed adhesion forces can be explained by vdW forces alone. Comparison between predicted and measured forces shows that the model has good agreement with experimental values. At humid conditions (RH>30%), the increased adhesion forces can be explained by capillary forces. Finally, the volume of water drop between particle and substrate was calculated.

Degree

Ph.D.

Advisors

Beaudoin, Purdue University.

Subject Area

Chemical engineering

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