Molecular dynamics of electrosprayed water nanodroplets containing sodium bis(2-ethylhexyl)sulfosuccinate Giovanna Longhi, a,b Alberto Ceselli, c Sandro L. Fornili, c Sergio Abbate, a,b Leopoldo Ceraulo d,e and Vincenzo Turco Liveri d * The behavior of aqueous solutions of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) subject to electrospray ionization (ESI) has been investigated by molecular dynamics (MD) simulations at three temperatures (350, 500 and 800 K). We consider several types of water nanodroplets containing AOTNa molecules and composed of a fixed number of water molecules (1000), N 0 AOT AOT À anions (N 0 AOT = 0, 5, 10) and N 0 Na sodium ions (N 0 Na = 0, 5, 10, 15, 20): in a short time scale (less than 1 ns), the AOTNa molecules, initially forming direct micelles in the interior of the water nanodroplets, are observed in all cases to diffuse nearby the nanodroplet surface, so that the hydrophilic heads and sodium ions become surrounded by water molecules, whereas the alkyl chains lay at the droplet surface. Meanwhile, evaporation of water molecules and of solvated sodium ions occurs, leading to a decrease of the droplet size and charge. At 350 K, no ejection of neutral or charged surfactant molecules is observed, whereas at 500 K, some fragmentation occurs, and at 800 K, this event becomes more frequent. The interplay of all these processes, which depend on the values of temperature, N 0 AOT and N 0 Na eventually leads to anhydrous charged surfactant aggregates with prevalence of monocharged ones, in agreement with experimental results of ESI mass spectrometry. The quantitative analysis of the MD trajectories allows to evidence molecular details potentially useful in designing future ESI experimental conditions. Copyright © 2013 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article. Keywords: AOTNa; electrospray ionization; aqueous nanodroplets; charged reverse micelle-like aggregates; molecular dynamics simulation Introduction Electrospray ionization (ESI) is a soft technique widely employed in mass spectrometry allowing to generate multiply charged high-molecular weight molecular and/or supramolecular species with minimal chemical decomposition of the molecules under study. In typical ESI-MS experiments, highly charged micrometer- sized droplets are ejected from the tip of the Taylor cone in the surrounding gas phase and are driven by the external electric field towards the counter-electrode. [1–4] During this flight, some out-of-equilibrium processes take place. In particular, the droplet may undergo collisions with surrounding gas molecules and other droplets, and thus evaporation of neutral volatile components (generally, solvent molecules) and/or Rayleigh instability may occur, leading to the emission of charged nanodroplets and ejection of solvated ions. [5–7] These interconnected phenomena are accompanied by changes in the droplet size and charge state as well as in the radial distribution of ionic species within the droplet. It is gener- ally believed that while solvent evaporation and ion spatial rearrangement begin when the droplet is formed, ejection of solvated ions takes place when the charge density reaches a critical value. [8] Solvent molecules and solvated ions are preferen- tially released from the tips of transient protrusions originated by local instabilities of the droplet surface. [9] Thus, the species observed by mass spectrometry are produced through evapo- ration of volatile components and ejection of charged ions. These processes are strongly impacted by experimental conditions (i.e. nature and composition of the electrosprayed solution, temperature of the tip and surrounding gas phase and external electric field). Even though each one of these processes is amenable to a qualitative description, an overall picture is hard to come about, especially at the quantitative level. Together with experimental findings for droplets in the micrometer-size regime, [10] a great deal of detailed and sometimes unex- pected information on such processes has been gained by molecular dynamics (MD) simulations especially for nano-sized droplets. [9,11,12] * Correspondence to: Vincenzo Turco Liveri, Dipartimento STEBICEF, Università di Palermo, Viale delle Scienze Parco d’Orleans II, 90128 Palermo, Italy. E-mail: vincenzo.turcoliveri@unipa.it a Dipartimento di Scienze Biomediche e Biotecnologie, Università di Brescia, Viale Europa 11, 25123 Brescia, Italy b CNISM, Consorzio Interuniversitario Scienze Fisiche della Materia, Via della Vasca Navale 84, 00146 Roma, Italy c Polo Didattico e di Ricerca di Crema, Universita’ di Milano, Via Bramante 65, 26013 Crema CR, Italy d Dipartimento STEBICEF, Università di Palermo, Via Archirafi 32, 90123 Palermo, Italy e Centro Grandi Apparecchiature, UniNetLAb, Via F. Marini 14, 90128 Palermo, Italy J. Mass Spectrom. 2013, 48, 478–486 Copyright © 2013 John Wiley & Sons, Ltd. Research article Received: 2 October 2012 Revised: 22 November 2012 Accepted: 1 February 2013 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/jms.3179 478