A new method to determine dispersive surface energy site distributions by inverse gas chromatography

Relating Crystal Structure to Surface Properties: A Study on Quercetin Solid Forms

The surface energy and surface chemistry of a crystal are of great importance when designing particles for a specific application, as these will impact both downstream manufacturing processes as well as final product quality. In this work, the surface properties of two different quercetin solvates (quercetin dihydrate and quercetin DMSO solvate) were studied using molecular (synthonic) modeling and experimental techniques, including inverse gas chromatography (IGC) and contact angle measurements, to establish a relationship between crystal structure and surface properties. The attachment energy model was used to predict morphologies and calculate surface properties through the study of their growth synthons. The modeling results confirmed the surface chemistry anisotropy for the two forms. For quercetin dihydrate, the {010} facets were found to grow mainly by nonpolar offset quercetin-quercetin stacking interactions, thus being hydrophobic, while the {100} facets were expected to be hydrophilic, growing by a polar quercetin-water hydrogen bond. For QDMSO, the dominant facet {002} grows by a strong polar quercetin-quercetin hydrogen bonding interaction, while the second most dominant facet {011} grows by nonpolar π-π stacking interactions. Water contact angle measurements and IGC confirmed a greater overall surface hydrophilicity for QDMSO compared to QDH and demonstrated surface energy heterogeneity for both structures. This work shows how synthonic modeling can help in the prediction of the surface nature of crystalline particles and guide the choice of parameters that will determine the optimal crystal form and final morphology for targeted surface properties, for example, the choice of crystallization conditions, choice of solvent, or presence of additives or impurities, which can direct the crystallization of a specific crystal form or crystal shape.

Τhe role of surface energy in the apparent solubility of two different calcite crystal habits

The interplay between polymorphism and facet-specific surface energy on the dissolution of crystals is examined in this work. It is shown that, using cationic additives, it is possible to produce star-shaped calcite crystals at very high supersaturations. In crystallization processes following the Ostwald rule of stages these star-shaped crystals appear to have higher solubility than both their rhombohedral counterparts and needle-shaped aragonite crystals. The vapour pressures of vaterite, aragonite, star-shaped calcite and rhombohedral calcite crystals are measured using thermogravimetric analysis and the corresponding enthalpies of melting are obtained. Using inverse gas chromatography, the surface energy of the aforementioned crystals is measured as well and the surface energy of the main crystal facets is calculated. Combining the effect of facet-specific surface energies and the enthalpies of melting on a modified version of the classical solubility equation for regular solutions, it is proved that the star-shaped calcite crystals can indeed have higher apparent solubility than aragonitecrystals.

Surface Energy Mapping of Modified Silica Using IGC Technique at Finite Dilution

The reinforcing silica filler, which can be more than 40% of an elastomer composite, plays a key role to achieve the desired mechanical properties in elastomer vulcanizates. However, the highly hydrophilic nature of silica surface causes silica particle aggregation. It remained a challenge for many tire manufacturers when using silica-filled elastomer compounds. Here, the silica surface energy changes when the surface is modified with coupling or noncoupling silanes; coupling silanes can covalently bond the silica to the elastomers. The surface energy of silica was determined using inverse gas chromatography (IGC) at finite dilution (FD-IGC) and found to be reduced by up to 50% when the silica surface was silanized. The spatial distribution of silica aggregates within the tire matrix is determined by transmission electron microscopy (TEM) and a direct correlation between aggregate size (silica microdispersion) and work of cohesion from IGC is reported, highlighting surface energy and work of cohesion being excellent indicators of the degree of dispersion of silica aggregates.

Influences of Crystal Anisotropy in Pharmaceutical Process Development

Crystalline materials are of crucial importance to the pharmaceutical industry, as a large number of APIs are formulated in crystalline form, occasionally in the presence of crystalline excipients. Owing to their multifaceted character, crystals were found to have strongly anisotropic properties. In fact, anisotropic properties were found to be quite important for a number of processes including milling, granulation and tableting. An understanding of crystal anisotropy and an ability to control and predict crystal anisotropy are mostly subjects of interest for researchers. A number of studies dealing with the aforementioned phenomena are grounded on over-simplistic assumptions, neglecting key attributes of crystalline materials, most importantly the anisotropic nature of a number of their properties. Moreover, concepts such as the influence of interfacial phenomena in the behaviour of crystalline materials during their growth and in vivo, are still poorly understood. The review aims to address concepts from a molecular perspective, focusing on crystal growth and dissolution. It begins with a brief outline of fundamental concepts of intermolecular and interfacial phenomena. The second part discusses their relevance to the field of pharmaceutical crystal growth and dissolution. Particular emphasis is given to works dealing with mechanistic understandings of the influence of solvents and additives on crystal habit. Furthermore, comments and perspectives, highlighting future directions for the implementation of fundamental concepts of interfacial phenomena in the rational understanding of crystal growth and dissolution processes, have been provided.

A non-invasive method to directly quantify surface heterogeneity of porous materials

It is extremely challenging to measure the variation of pore surface properties in complex porous systems even though many porous materials have widely differing pore surface properties at microscopic levels. The surface heterogeneity results in different adsorption/desorption behaviors and storage capacity of guest molecules in pores. Built upon the conventional Porod's law scattering theory applicable mainly to porous materials with relatively homogeneous matrices, here we develop a generalized Porod's scattering law method (GPSLM) to study heterogeneous porous materials and directly obtain the variation of scattering length density (SLD) of pore surfaces. As SLD is a function of the chemical formula and density of the matrix, the non-invasive GPSLM provides a way to probe surface compositional heterogeneity, and can be applied to a wide range of heterogeneous materials especially, but not limited to, porous media and colloids, using either neutron or X-ray scattering techniques.

Influence of sample preparation on IGC measurements: the cases of silanised glass wool and packing structure

A high amount of silanised wool can influence IGC measurements, especially for low surface energy/area materials.

Comparison of the cohesion-adhesion balance approach to colloidal probe atomic force microscopy and the measurement of Hansen partial solubility parameters by inverse gas chromatography for the prediction of dry powder inhalation performance

The abilities of the cohesive-adhesive balance approach to atomic force microscopy (AFM) and the measurement of Hansen partial solubility parameters by inverse gas chromatography (IGC) to predict the performance of carrier-based dry powder inhaler (DPI) formulations were compared. Five model drugs (beclometasone dipropionate, budesonide, salbutamol sulphate, terbutaline sulphate and triamcinolone acetonide) and three model carriers (erythritol, α-lactose monohydrate and d-mannitol) were chosen, giving fifteen drug-carrier combinations. Comparison of the AFM and IGC interparticulate adhesion data suggested that they did not produce equivalent results. Comparison of the AFM data with the in vitro fine particle delivery of appropriate DPI formulations normalised to account for particle size differences revealed a previously observed pattern for the AFM measurements, with a slightly cohesive AFM CAB ratio being associated with the highest fine particle fraction. However, no consistent relationship between formulation performance and the IGC data was observed. The results as a whole highlight the complexity of the many interacting variables that can affect the behaviour of DPIs and suggest that the prediction of their performance from a single measurement is unlikely to be successful in every case.

Effect of temperature on the surface free energy and acid–base properties of Gabapentin and Pregabalin drugs − a comparative study

Temperature dependent surface energy variations observed in Gabapentin and Pregabalin drugs.