Definition
Mixing is defined as an unit operation in which two or more components are treated in such a way that each particle / molecule of the components lies as nearly as possible in contact with a particle / molecule of the other components.
If this is achieved it produces a theoretical ‘ideal’ situation, called a perfect mix.
Objectives of mixing:
To make simple physical mixture
Physical change
Dispersion
Promotion of reaction.
To make dispersions.
To achieve uniformity in the composition of a product.
Types of Mixtures
Mixtures may be divided into three types that differ in their behavior as,
Positive Mixtures.
Negative Mixtures.
Neutral Mixtures.
Positive mixtures:
Positive mixtures are formed from materials such as gases or miscible liquids, which mix spontaneously and irreversibly by diffusion and tend to approach a perfect mix.
There is no input of energy required.
In general, such materials do not present any problems in mixing.
e.g. Mixing of sodium chloride and sugar in water.
Negative mixtures:
In negative mixtures, after mixing, the components will tend to separate out. If this occurs quickly, then energy must be continuously input to keep the components in dispersed state.
Negative mixtures are more difficult to form and a higher degree of mixing efficiency is required.
e.g. Calamine lotion, Emulsions and suspensions.
Neutral mixtures
Neutral mixtures are static in their behavior, the components having no tendency to mix spontaneously, nor do they segregate when mixed.
e.g. Pastes, ointments and mixed powders.
Applications of Mixing:
In preparation of Emulsions and Suspensions.
In preparation of granules in tablet manufacturing.
Blending of powders in capsule manufacturing.
In low dose drug formulation the potent drug is mixed well with diluents to form a dosage form.
Physical mixtures of certain drugs with cyclodextrin are found to have more bioavailabilities e.g. Nimesulide with B cyclodextrin.
Factors affecting process of Mixing:
Handling of Mixture:
Once mixture is formed the vibrations in handling and transportation may cause change in the state of mixing.
Duration of Mixing:
To achieve homogeneity in the mixture the mixture has to be mixed for sufficient duration of time.
The time required for mixing can be decided by doing preformulation studies.
Type of Mixture used:
Different substances are mixed by different mechanisms, the mixer should be selected appropriately for mixing particular substances.
e.g. Solid powders are preferably mixed by using Double cone blenders while emulsions are made using Silversons homogeniser.
Particle Size:
For uniform mixing particle sizes of all the components of mixture should have the same particle size.
Particle Shape:
Irregular shaped particles do not produce homogeneous mixture while spherical shaped particles produce good quality mixtures.
Charge on Particles:
The particles of certain substances possess static electric charges on them and hence influence mixing.
The particles with opposite charge form a good mixture while similar charged particles do not form a mixture.
Particle Density:
In a mixture the particles with heavier densities tend to settle at the bottom in a mixture while those with lesser densities accumulate on top of the mixture.
To form a good mixture all particles should have identical densities.
Proportion of Mixing Material.
The quantities of the materials to be mixed influence the process of mixing, to form a good mixture,
The materials with equal quantities are mixed first.
Then ' ‘Ascending Order” of materials as per their quantities are mixed.
Difference between Solid and Liquid Mixing:
Commonly Asked Questions.
Define Mixing. Discuss Objectives and Factors affecting the process of Mixing.
Define mixing. Differentiate Liquid Mixing and Solid Mixing,