Disintegration mechanisms of charged nanodroplets: novel systems for applying methods of activated processes

Abstract

We review recent advances in the understanding of ejection mechanisms of solvated ions and charged macromolecules from highly charged nanodroplets. While the physical basis for the instability leading to droplet fragmentation is relatively well understood, a description of molecular mechanism of the fragmentation in complex systems is still missing. Development of a comprehensive model for the droplet fragmentation is further complicated by chemical modifications of the charged macromolecules (macroions) in a changing droplet environment. We highlight several different molecular simulation techniques used to study fragmentation of charged droplets with different solutes. Ejection of simple ions is analysed using theory of activated processes and transfer reaction coordinate (TRC). The TRC was shown to adequately represent complex rearrangement of solvent molecules in the course of evaporation. The critical value of the square of the charge to volume ratio for spontaneous ejection of simple solvated ions from aqueous droplets is found to be very close to that predicted by Rayleigh's model. On the contrary, the presence of macromolecules adds a level of complexity into the system where the charge-induced instabilities cannot be described by a conventional theory such as Rayleigh or ion-evaporation mechanism. Additional charge -charge interactions between charged sites on a macromolecule dramatically change the macroion ejection mechanism. Molecular dynamics simulations reveal a number of distinct scenarios: contiguous extrusion, dryingout, star-like formation of solvent surrounding a macroion and pearl formation along the macromolecular chain.

Novel ionization processes for use in mass spectrometry: ‘Squeezing’ nonvolatile analyte ions from crystals and droplets

“…Another possibility is that as loss of matrix from the charged particle occurs and the charge density reaches the Rayleigh limit, Coulomb bursting occurs producing smaller particles. There is also the consideration that with nanoparticles of a few nm diameter, the solid state may not exist . The size at which the charged residue model can be expected to produce the observed results is when the particles predominately have only a single nonvolatile analyte molecule.…”

“…There is also the consideration that with nanoparticles of a few nm diameter, the solid state may not exist. [172][173][174] The size at which the charged residue model can be expected to produce the observed results is when the particles predominately have only a single nonvolatile analyte molecule. It is possible that an ion evaporation-like mechanism might occur through an FD mechanism, but ejection of have only aproteins from a solid particle is also difficult to envision.…”

Dynamics and thermodynamics of decay in charged clusters

“…The competition between ejection and fission for n = 309 and the absence of fission at smaller n can be understood from an idealised analysis by Consta and Malevanets of a spherical droplet splitting into two daughter droplets with conservation of total volume and of total charge [2]. Symmetrical decay (fission) is favoured when electrostatic energy dominates over surface energy, since the decrease in electrostatic energy of separating two highly charged subclusters compensates for the large increase in surface area caused by creating two spheres from one with the same total volume.…”

“…Deformation leads to a barrierless decrease in the sum of these energies beyond the limiting charge Q R = π 8ǫ 0 γD 3 , where γ is the surface tension of the droplet interface and ǫ 0 is the permittivity of free space. Rayleigh reached this classic result within the first page of his elegantly short article [1], concealing a somewhat involved derivation [2].…”

Charge-induced instabilities of droplets containing macromolecular complexes

“…12 In this model, the droplet stability is determined by two competing factors: the surface tension force that holds the droplet intact and the Coulomb repulsion between the charges of the same sign that tends to fragment the droplet. Theory has shown 12,13 that the stability of the droplet with respect to small fluctuations is determined by the ratio of the square of the charge to the droplet volume. The critical value of the parameter where the two competing factors compensate each other is known as the Rayleigh limit:…”