  While almost any kind of face mask offers some protection against particles
and pathogens of different sizes, the most efficient ones make use of a layered
structure where one or more layers are electrically charged. This electret
layer is essential to efficient filtration of difficult-to-capture small
particles, yet the exact nature of electrostatic capture with respect to both
the charge on the particles and the electret fibres as well as the effect of
immediate environment remains unclear. Here, we explore in detail the
electrostatic interaction between the surface of a single charged electret
fibre and a model of SARS-CoV-2 virus. Using Poisson-Boltzmann electrostatics
coupled to a detailed spike protein charge regulation model, we show how pH and
salt concentration drastically change both the scale and the sign of the
interaction. Furthermore, the configuration of the few spike proteins closest
to the electret fibre turns out to be as important for the strength of the
interaction as their total number on the virus, a direct consequence of spike
protein charge regulation. The results of our work elucidate the details of
virus electrostatics and contribute to the general understanding of efficient
virus filtration mechanisms.
