Estimation of Microplastic Sticking Coefficient and Enthalpy of Bonding Using Atomic Force Microscopy

Abstract:

Microplastics are an emerging contaminate with a number of unanswered questions related to their fate and bioavailability. Their propensity for aggregation is a critical characteristic that contributes to the fate of microplastics in the environment. The traditional kinetic model for aggregation relies on the probability of two particles sticking together when they collide, called the sticking coefficient. The sticking coefficient can be estimated from the approach force curves for a particle attached to an atomic force microscope tip. A second model for aggregation, based on statistical thermodynamics, was recently proposed, but the accuracy of its results hinge on the determination of the energy in the contact point between particles, termed the enthalpy of bonding. The enthalpy of bonding has never been theoretically or experimentally determined. We hypothesize that this term can be estimated using the retraction curves of an atomic force microscope. Herein, we present experimentally determined values of the sticking coefficient and the enthalpy of bonding for plastic microspheres on various materials under different pH and ionic strength conditions using atomic force microscopy. Determining these two values will allow for a better understanding of microplastic aggregation and allow for direct comparison between the traditional kinetic and statistical thermodynamic models.