Pharmaceutical Crystallization

Abstract

Two types of radially symmetric three-dimensional nonlinear photonic crystals with the cylindrical structure and the egglike structure are proposed, from which the conical and the spherical quadratic harmonic waves can be produced, respectively, by three-dimensional quasi-phase matching. First, the cylindrical structures with periodic and aperiodic modulations of the nonlinear coefficient are both studied, showing their significant advantages compared to the corresponding two-dimensional structures. The dependencies of the transverse and the longitudinal phase-matching periods on harmonic propagating directions are also calculated and analyzed. Then, the egglike structure is designed by programming and the distribution of reciprocal vectors is presented, indicating its ability to generate the spherical harmonic as a point light source. The investigation of the intensity distribution on the spherical wavefront is also performed, showing its strong dependence on the harmonic polarization and the quadratic nonlinear coefficients.

Supramolecular gel phase crystallization: orthogonal self-assembly under non-equilibrium conditions

“…Metastable structures are essentially static and kinetically trapped. They exist because the available thermal 10 energy is not sufficient to overcome the activation barrier required to transform them into the most thermodynamically stable state. Dissipative structures are dynamic and require the active and continuous input of energy in order to push the system out of equilibrium.…”

“…Classical nucleation theory is an oversimplification of the mechanism of crystallization and homogeneous, primary nucleation is rarely dominant in the crystallization of molecular species. There is growing evidence for two-step nucleation 45 involving local concentration variation 10 and crystallization outcome is often determined by nucleating high energy surfaces. The recent book by Beckmann is recommended as an introduction into the field.…”

“…In this review we focus upon the combination of small molecule crystallization and gelation; two closely related self-5 assembly processes that occur under non-equilibrium conditions, driven by a supersaturation gradient. A range of possible outcomes of a two-component, orthogonal self-assembly process (such as the one between a gelator and crystallization substrate) as a function of the degree of coupling between small molecule 10 components are summarised in Figure 1. Where there is essentially no interaction between the two components then the outcome of the process is the same as if the self-assembly of each component occurred separately.…”

“…The two orthogonal materials may be physically entangled in longer length scales. The nucleation of each component may be primary, secondary or follow a non-classical two-step mechanism, 10 and crucially the presence of more than 20 one component offers the opportunity for each of the two orthogonal assembly processes to influence the outcome of the other one. However, solubility constraints and kinetic factors mean that each process may not occur at the same rate and hence there is the possibility of temporal as well as spatial separation of 25 the individual assemblies.…”

“…5 Stoichiometric supramolecular co-gels have recently become topical, as in the work of Dastidar on dipyridylurea-carboxylic acid derivatives 22 and the work of Smith on diamine-linked dendrons. 23 There is also a significant field based on metallogelators where the components are metals and gelating 10 ligands. 24 These examples build on earlier work on aminopyrimidine/dialkylbarbituric acid mixtures.…”