Vicente Talanquer

NUCLEATION: Measurements, Theory, and Atmospheric Applications

New experiments have succeeded in measuring actual rates of nucleation and are revealing the shortcomings of classical nucleation theory, which assumes that the molecular-scale regions of the new phase may be treated using bulk thermodynamics and planar surface free energies. In response to these developments, new theories have been developed that incorporate information about molecular interactions in a more realistic fashion. This article reviews recent experimental and theoretical advances in the study of nucleation of liquids from the vapor and of crystals from the melt, with particular emphasis on phenomena that relate to particle formation in the atmosphere.

Crystal nucleation in the presence of a metastable critical point

Density functional theory is applied to the study of crystal nucleation in the presence of a metastable critical point. A phenomenological model for fluids with short range interactions is developed to study the influence of critical density fluctuations on the structure of the critical nucleus and the height of the barrier to nucleation. Our results show dramatic changes in the nature of crystal nucleation near the metastable critical point, with nucleation rates increasing several orders of magnitude; this behavior has important consequences for nucleation of colloids and proteins from solution. A nonmonotonic dependence of the critical cluster size on supersaturation is observed under these conditions.

Macro, submicro, and symbolic: The many faces of the chemistry "triplet"

The idea that chemical knowledge can be represented in three main ways: macro, submicro, and symbolic (chemistry triplet) has become paradigmatic in chemistry and science education. It has served both as the base of theoretical frameworks that guide research in chemical education and as a central idea in various curriculum projects. However, this triplet relationship has been the subject of different adaptations and reinterpretations that sometimes lead to confusion and misunderstanding, which complicates the analysis of the triplet’s nature and scope. Thus, the central goal of this paper is to describe some of the existing views of the triplet relationship in chemistry and science education and critically analyse their underlying assumptions. We also propose a general structure of our chemistry knowledge intended to better situate the chemistry triplet in relationship with the different types, scales, dimensions, and approaches that seem to characterise such knowledge. Our proposed model may be useful in the analysis, evaluation, and reflection of educational research results and teaching practices centred on the triplet relationship.

Dynamical density functional theory of gas–liquid nucleation

We present a consistent dynamical nucleation theory based on density functional theory. By considering the properties of stable droplets in closed volumes, the height and shape of the barrier to nucleation are calculated. Contributions from fluctuations in the center of mass of the nucleating cluster are taken into account. Forward and backward rates for cluster dynamics are obtained, and nucleation rates are then evaluated under steady‐state conditions. We test the quantitative effects of several shortcuts to calculating nucleation rates. The predictions of the full theory presented here show very modest changes from those of the simpler nonclassical theory proposed earlier by Oxtoby and co‐workers.

On Cognitive Constraints and Learning Progressions: The case of “structure of matter”

Based on the analysis of available research on students’ alternative conceptions about the particulate nature of matter, we identified basic implicit assumptions that seem to constrain students’ ideas and reasoning on this topic at various learning stages. Although many of these assumptions are interrelated, some of them seem to change or lose/gain strength independently from one another. Overlapping or competing presuppositions about the structure, properties, and dynamics of matter may be able to coexist at any given level, particularly at intermediate stages of expertise. Our results allowed us to suggest common paths in the transition from naïve through novice to expert along relevant dimensions related to the structure and properties of chemical substances. The identification of these cognitive constraints provides a useful framework that educators can use to better understand and even predict many of their students’ learning difficulties. It can also assist in the design and organisation of learning experiences and assessment tools that recognise and take advantage of the most likely trajectories towards expertise (learning progressions) followed by many students.

Explanations and Teleology in Chemistry Education

It has been commonly assumed that teleological explanations are unnecessary and have no place in the physical sciences. However, there are indications that teleology is fairly common in the instructional explanations of teachers and students in chemistry classrooms. In this study we explore the role and nature of teleological explanations and the conditions that seem to warrant their use in chemistry education. We also analyse the learning implications of developing explanations of chemical phenomena within a teleological stance. Our study is based on the qualitative analysis of the instructional explanations presented in traditional chemistry textbooks used in the United States. Our results indicate that teleological explanations are in fact present in these textbooks and help provide an explanatory reason for the occurrence of chemical transformations. Their use is tightly linked to the existence of a rule, principle, or law that governs the behaviour of a chemical system, and that explicitly or implicitly implies the minimisation or maximisation of some intrinsic property. This law or principle tends to provide a sense of preferred direction in the evolution of a transformation. Although teleological explanations seem to have heuristic pedagogical value in chemistry education, they may also lead students to develop alternative conceptions and unwarranted overgeneralisations.

Nucleation on a solid substrate: A density functional approach

We extend the density functional approach to the statistical mechanics of inhomogeneous fluids to calculate the rate of heterogeneous nucleation of the gas‐to‐liquid transition by a planar solid substrate. Comparison with classical nucleation theory (extended to incorporate the line tension that results from three‐phase contact) reveals the inadequacy of the latter approaches as the spinodal is approached. Wetting and drying transitions have a large effect on the usefulness of classical theory. Free energies of formation for critical heterogeneous nuclei and their shapes and density profiles are calculated from density functional theory.

NUCLEATION OF BUBBLES IN BINARY FLUIDS

We have applied density functional methods to predict the nucleation rates of bubbles in superheated, stretched, or supersaturated binary fluid mixtures. Our model uses Lennard‐Jones mixtures, with mixing rules chosen to allow either ideal or nonideal solution behavior. Deviations from the predictions of classical nucleation theory are in general quite large, with the locus of observable bubble nucleation (the kinetic stability limit) following the spinodal (the thermodynamic stability limit) reasonably closely. Comparisons are made with a variety of experiments, and puzzling earlier results are explained, such as the increase in solubility of some gases with temperature at the kinetic stability limit. Further experiments are needed to explore the variety of behavior predicted by the present calculations.

Nucleation in a slit pore

We have employed density functional theory in statistical mechanics to study the nucleation of the gas–liquid transition inside a slit pore. This is the simplest kind of pore, in which a fluid is confined between two infinite and identical planar surfaces. Equilibrium capillary condensation in such systems has been studied in the past. Here, we explore the kinetics of nucleation of the new phase and show that the critical nucleus can either be attached to one of the planes or can bridge the two planes, depending on the conditions of the experiment. We show that the macroscopic capillarity approximation is not quantitatively accurate, but can provide a useful qualitative picture provided that the line tension is incorporated. Comparisons are made with recent simulations of phase transitions in slit pores.

Density functional theory of nucleation: A semiempirical approach

We present a semiempirical approach to the density functional theory of gas–liquid nucleation, in which the same experimental properties used in classical nucleation theory (equilibrium vapor pressure, liquid density, and surface tension) are used to fit three adjustable parameters in the intermolecular potential. This approach allows direct comparison of nucleation rates with experimental data. Agreement with results on nonane from three different experimental groups is reasonable, although the comparison clearly reveals the scatter in those results and suggests that further experimental measurements by different groups on the same systems would be very valuable. For water and the n‐alcohols, this version of density functional theory gives results quite close to classical nucleation theory, implying that it is not a good approximation to describe polar fluids by effective spherically symmetric potentials.