title: Molecular-dynamics simulations of urea nucleation from aqueous solution.
abstract: Despite its ubiquitous character and relevance in many branches of science and engineering, nucleation from solution remains elusive. In this framework, molecular simulations represent a powerful tool to provide insight into nucleation at the molecular scale. In this work, we combine theory and molecular simulations to describe urea nucleation from aqueous solution. Taking advantage of well-tempered metadynamics, we compute the free-energy change associated to the phase transition. We find that such a free-energy profile is characterized by significant finite-size effects that can, however, be accounted for. The description of the nucleation process emerging from our analysis differs from classical nucleation theory. Nucleation of crystal-like clusters is in fact preceded by large concentration fluctuations, indicating a predominant two-step process, whereby embryonic crystal nuclei emerge from dense, disordered urea clusters. Furthermore, in the early stages of nucleation, two different polymorphs are seen to compete.
authors.affiliation: Institute of Process Engineering, Eidgenössische Technische Hochschule Zurich, CH-8092 Zurich, Switzerland; Facoltà di informatica, Istituto di Scienze Computazionali, Università della Svizzera Italiana, CH-6900 Lugano, Switzerland; and.
authors.forename: Matteo
authors.initials: M
authors.lastname: Salvalaglio
authors.affiliation: Facoltà di informatica, Istituto di Scienze Computazionali, Università della Svizzera Italiana, CH-6900 Lugano, Switzerland; and Department of Chemistry and Applied Biosciences, Eidgenössiche Technische Hochschule Zurich, CH-8092 Zurich, Switzerland.
authors.forename: Claudio
authors.initials: C
authors.lastname: Perego
authors.affiliation: Facoltà di informatica, Istituto di Scienze Computazionali, Università della Svizzera Italiana, CH-6900 Lugano, Switzerland; and Department of Chemistry and Applied Biosciences, Eidgenössiche Technische Hochschule Zurich, CH-8092 Zurich, Switzerland.
authors.forename: Federico
authors.initials: F
authors.lastname: Giberti
authors.affiliation: Institute of Process Engineering, Eidgenössische Technische Hochschule Zurich, CH-8092 Zurich, Switzerland;
authors.forename: Marco
authors.initials: M
authors.lastname: Mazzotti
authors.affiliation: Facoltà di informatica, Istituto di Scienze Computazionali, Università della Svizzera Italiana, CH-6900 Lugano, Switzerland; and Department of Chemistry and Applied Biosciences, Eidgenössiche Technische Hochschule Zurich, CH-8092 Zurich, Switzerland parrinello@phys.chem.ethz.ch.
authors.forename: Michele
authors.initials: M
authors.lastname: Parrinello
mesh_terms: D003460:Crystallization; D056004:Molecular Dynamics Simulation; D012996:Solutions; D013816:Thermodynamics; D014508:Urea / Q000737:chemistry*
chemical_list: D012996:Solutions; D014508:Urea
keywords: enhanced sampling; molecular dynamics; nucleation; solution; well-tempered metadynamics