Interfacial forces between a silica particle and phosphatidylcholine monolayers at the air-water interface

Interfacial forces between a silica or borosilicate particle in water and phospholipids at the air-water interface were studied using the Monolayer Particle Interaction Apparatus. This instrument allowed the forces to be measured as the colloidal probe approached the monolayer from the liquid phase. The proper working principle of this setup was demonstrated by measuring the forces between a particle and a mica plate in 0.1 mM NaCl. The effect of the alkyl chain length on the adhesion between the particle and the monolayer was investigated using four different 1,2-dialkyl-sn-glycero-3-phosphocholines (DMPC, DPPC, DSPC, and DBPC), which had 14, 16, 18, and 22 carbon atoms per alkyl chain, respectively. The adhesion force increased with the square of the particle radius. The lipids in the liquid-expanded (LE) phase showed an attraction to the particle, explained by an electrostatic attraction and/or the formation of a three-phase contact line that lead to a capillary force. All monolayers showed an adhesion in their retract force curve, which decreased with an increased chain length and surface pressure. Interfacial stiffness was generally seen to increase with the phospholipid chain length and to decrease with surface pressure, explained by an increase in the intermolecular van der Waals interaction and a decrease in the interfacial tension, respectively. The adhesion between the particle and monolayer was concluded to be controlled by the contact area between the particle and monolayer, and therefore the monolayer stiffness and the electrostatic interactions.