The caffeine structure

Organic Molecular Complexes

An organic coordination compound, or molecular complex, is composed of constituents held together by weak interactions such as donor–acceptor forces or hydrogen bonds. Molecular complexes can be classified according to: The bonding or interaction between substrate and ligand (e.g., electrostatic interaction, charge-transfer, hydrogen bonding and hydrophobic interaction). Type of substrate and ligand forming the complex […]


Complexation is a chemical process in which molecules or molecular assemblies are formed by the combination of substrates (S) and ligands (L). Most often complex (SmLn) formation is a reversible process: $$ m.S+n.L⇄S_mL_n $$ where m substrate molecules, associate with n ligand molecules to form a complex of m:n stoichiometry. In this context, complex formation, […]

Rheogram of a thixotropic material showing a bulge in the hysteresis loop.

Bulges and Spurs in Thixotropy

Bulges and spurs represent complex hysteresis loops observed in pharmaceutical dispersions when sheared in a viscometer. In this process, the shear rate, rather than the shear stress, is increased to a certain point and then decreased, with shear stress measured at each shear rate to create the appropriate rheograms. Figures 4 and 5 illustrate two […]

Thixotropy in plastic and pseudoplastic flow systems.


When the rate of shear is progressively increased and plotted against the resulting shear stress, various types of behaviors can be observed. One might assume that if the rate of shear were reduced after reaching the desired maximum, the downcurve would match the upcurve. In other words, the downcurve would be identical to and superimposable […]

Rheogram or curve of Plastic flow (Bingham bodies)

Non-Newtonian Fluids

Most fluid pharmaceutical products are not simple liquids and do not follow Newton’s law of flow. These fluids are known as non-Newtonian fluids. Non-Newtonian behavior is typically seen in liquid and solid heterogeneous dispersions, such as colloidal solutions, emulsions, liquid suspensions, and ointments. When non-Newtonian materials are analyzed using a rotational viscometer and the results […]

Rheograms or flow curves of Newtonian liquids.

Newtonian Fluids

Newtonian fluids are simple liquids that follow Newton’s law of direct proportionality between shear stress and shear rate. In these fluids, viscosity remains independent of both shear stress and shear rate. The viscosity of simple liquids (which include pure liquids with small molecules and solutions where both solute and solvent are small molecules) depends solely […]

Passive Diffusion

A membrane serves as a selective barrier between different phases, such as inside and outside a cell or between two compartments within a cell. Material can cross this membrane through various mechanisms: 1. Passive Transport: This involves the movement of substances across the membrane without the input of energy. Diffusion, osmosis, and facilitated diffusion are […]


Diffusion is the process by which individual molecules of a substance move from an area of high concentration to an area of low concentration due to random molecular motion, driven by a concentration gradient. Diffusion holds significant importance in pharmaceutical operations. Additionally, it serves as a crucial component in a wide range of applications. These […]