IntroductionThe control of crystallization is crucial to many diverse biological and industrial processes. For example, many ocean fish rely on macromolecules to prevent ice crystallizing in their blood,1the production of a particular polymorph is essential in drug formulations,2and impeding crystallization in oil and gas pipes prevents pipe blockage.3Control may be affected by promoting crystallization of a particular polymorph or morphology or conversely by inhibiting crystallization. The former is achieved through promoting nucleation, whereas the latter occurs through inhibiting crystal growth. In both cases, an additive may achieve the desired effect provided that a sufficiently strong interaction exists between part of the additive and the crystalline species. Consequently, a particular functional group can act as either a nucleation promoter or crystal growth inhibitor, depending upon its aggregationstate. If the additive aggregates to form a planar surface, then nucleation is promoted provided that the crystallizing material adsorbs onto this surface. However, if the additive is molecularly dispersed, then it may adsorb onto a growing crystal and inhibit further growth by impeding the attachment of additional crystalline material. The question then arises, at what aggregation state will a species switch from a crystal growth inhibitor to a nucleation promoter in a particular system,i.e.what range of emulsion/microemulsion droplet sizes will promote crystallization? What is the role of surface curvature and how do the dynamics of the system (i.e.the rate at which the micelles/emulsion droplets move and the molecular residence time in the micelles/emulsion droplets) affect the process? Previous studies have shown that size and polymorphic form can be controlled through crystallization in micellar, microemulsion and emulsion systems.4–9However, a systematic study of the role of surface curvature and aggregation state upon promoting nucleation and inhibiting crystal growth as a function of supersaturation has not been conducted to our knowledge. This manuscript details preliminary results obtained in investigating these issues.We have investigated the effect of carboxylic acids and their sodium salts on the crystallization ofl-asparagine monohydrate. This system was chosen because the carboxylic acids and their sodium salts are readily available from standard chemical suppliers in a range of different molecular weights, and with increasing length of hydrocarbon tails, and range from water soluble, through to micellar species, to insoluble species. In addition, different architectures are also available for the polymeric species,vizdendritic species and copolymers. Asparagine was chosen because this material forms a conglomerate, and so the effect of ionic strength can be decoupled from supersaturation by comparing the behaviour of supersaturatedl-asparagine and an equivalent concentration of undersaturateddl-asparagine. In addition, previous studies10,11have shown that the habit can be affected by the addition of tailor-madeadditives at concentrations ofca.2–20 mg ml−1.l-Asparagine crystallizes as the monohydrate with an orthorhombic unit cell ofP212121symmetry and unit cell parameters ofa= 5.593 Å,b= 9.827 Å,c= 11.808 Å,Z= 4.12The crystal is held together by a network of hydrogen bonds. The typical aqueous morphology is prismatic, being slightly elongated along thea-direction and bounded by {012} and {011} faces together with smaller {101}, {111}, {020} and {002} faces.