Direct observation of single particle electrostatic charging by atomic force microscopy.

Purpose: To show that atomic force microscopy (AFM) can be used to directly study the electrostatic charging and dissipation of single pharmaceutical particles.

Materials And Methods: Particles of lactose attached to AFM cantilevers were charged on a glass surface at a relative humidity (RH) of 0.1%. By recording force-distance curves, we use a measurement of the long range electrostatic interaction to compare the generation of charge by contact charging and tribocharging and to study the effect of RH on charge dissipation.

Results: As expected, tribocharging by scanning the particle across the glass surface generates considerably more charge than repeated local contacts. Increasing the RH from 0.1 to 5% over a period of 37 min dissipates the tribo-generated electrostatic charge.

Conclusions: Using a combination of the abilities of AFM to scan in contact mode and record force-distance curves, we have shown a novel method to study electrostatic charging of particles. By measuring the length of the long range electrostatic interaction, we are able to compare different mechanisms of generating charge and to study the effect of RH on charge dissipation.