Glossary

Physicochemical Properties

Physicochemical properties refer to the physical and chemical characteristics of a substance that define its behavior under different conditions. These properties result from the molecular structure and composition of the substance and are fundamental in determining its interactions, stability, and functionality in various applications. In the context of materials science, pharmaceuticals, chemistry, and many other fields, analyzing and understanding physicochemical properties is crucial for the prediction of performance, processing and suitability for the intended use.

• Physical properties can influence a substance’s form and texture, and how it responds to changes in temperature, pressure, and other environmental factors. These include characteristics such as Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting point, boiling point, PolymorphismPolymorphism is the ability of a solid material to form different crystalline structures (synonyms:  forms, modifications).polymorphism, solubility, DensityThe mass density is defined as the ratio between mass and volume. density, viscosity, and particle size.
• Chemical properties are related to a substance’s reactivity with other chemicals. They include properties like pH, chemical stability, OxidationOxidation can describe different processes in the context of thermal analysis.oxidation-reduction potential, and the potential for chemical reactions like hydrolysis, polymerization, or degradation.
• Thermal properties include Specific Heat Capacity (cp)Heat capacity is a material-specific physical quantity, determined by the amount of heat supplied to specimen, divided by the resulting temperature increase. The specific heat capacity is related to a unit mass of the specimen.specific heat capacity, Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity, thermal expansion, and Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature. These properties determine how a substance absorbs and transfers heat, and how it expands or contracts with temperature changes.
• Mechanical properties are relevant in material science and engineering and include hardness, tensile strength, elasticity, and brittleness. These properties define how a material behaves under various forms of mechanical StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress.

Assessment of the physicochemical properties of substances requires a combination of theoretical knowledge and analytical techniques such as Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and Rheology.

Various standards and guidelines, depending on the field and application, regulate the assessment of physicochemical properties of materials. In material science and environmental studies, standards from organizations like ASTM International and ISO provide methodologies for testing and evaluating these properties. In the pharmaceutical industry, guidelines such as those from the International Council for Harmonisation (ICH), particularly ICH Q6A, outline the necessary studies for characterizing the physicochemical properties of drug substances; ICH Q2A (R2), meanwhile, provides guidance on the qualification of impurities in new drug substances. Methodologies and instrumentation to be used for the characterization of pharmaceutical substances are also described in the general chapters of different pharmacopoeias.

Table 1: Thermal analysis applied in physicochemical characterization