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, […]

Cutter mill

Cutter Mill

The cutter mill (Fig. 1) is very similar to the hammer mill in operation, except that sharp blades are used in place of blunt surfaces. Hammer and cutter mills are especially suited for brittle or fibrous materials and can be considered the most commonly encountered mills for roller compaction (in dry granulation). Construction and Working […]

Pin mill

Pin Mill

Pin mill is a method of comminution that acts by producing particle impact and attrition. Particle size reduction occurs by impaction with the pins and by attrition between pins as the particles travel outwards under the influence of centrifugal force. Construction and Working of Pin Mill Pin mills consist of two horizontal steel plates with […]

Fluid Energy Mill

  Fluid energy mill, also known as jet mill or micronizing, is a size reduction method that operates through particle impaction and attrition. Figure 1 illustrates a form of a fluid energy mill. Both circular and oval-path designs are available for this type of milling. Construction and working of Fluid Energy Mill In the fluid-energy […]

Hammer mill

Hammer Mill

A hammer mill carries out size reduction by using impact. Hammer mills consist of a series of four or more hammers hinged on a central shaft which is enclosed within a rigid metal case (fig.1). The material is fed at the top or center, thrown out centrifugally, and ground by the impact of the hammers […]

Ball mill in operation showing correct cascade action.

Ball Mill

A ball mill is an example of a comminution method which produces size reduction by both impact and attrition of particles. The ball mill consists of a horizontally rotating hollow vessel of cylindrical shape with the length slightly greater than its diameter. The mill is partially filled with balls of steel or pebbles, which act […]

Mechanisms of Comminution


Comminution, or size reduction, is the operation carried out for reducing the size of bigger particles into smaller ones of the desired size and shape with the help of external forces. Moreover, the function of size reduction may be to aid efficient processing of solid particles. Additionally, it facilitates powder mixing or the production of […]

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 […]