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Objectives, Applications and Factors Affecting Mixing

A mixing process is designed to ensure that the ingredients that make up the overall composition of the mixture are uniformly composed.

Objectives and Applications of Mixing

A mixing process is designed to ensure that the ingredients that make up the overall composition of the mixture are uniformly composed. Mixing is primarily concerned with obtaining a homogeneous product using the least possible energy and time. Other goals of mixing are:

To create single physical mixture

The process of producing this mix may be very simple, such as mixing two or more uniformly sized liquids or solids. As many pharmaceutical formulations contain dilutions of active substances and must be weighed accurately in order to meet dosage uniformity requirements for pharmaceutical dosage forms, the degree of mixing typically requires a high order.

To produce physical change

Physical and chemical changes can be caused by mixing, for instance, creating a soluble solution. An inferior mixing efficiency may be acceptable because mixing is merely accelerating the dissolution and diffusion process that was possible through agitation alone.

To produce dispersion

It is generally recommended to mix for increased stability and effectiveness, particularly when the dispersion of two immiscible liquids or when the dispersion of a solid into a liquid creates a suspension or paste.

To promote chemical reaction

In chemical reactions, mixing can both stimulate and control the reaction. Mixing is important to ensure a uniform product, for example, when pH is being adjusted and the degree of mixing determines whether the reaction will occur.

In addition to combining raw ingredients, mixing is also used to prepare fine emulsions, reduce particle size, perform chemical reactions, manipulate rheology, dissolve components, facilitate heat transfer, among other tasks. To process raw ingredients, handle intermediates, and prepare the finished product, pharmaceutical companies often rely on a variety of mixer styles, even within a single product line.

Factors Affecting Mixing

Several factors can influence mixing efficiency, resulting in uneven distribution of materials, which may lead to incorrect dosage production. Here are some of them:

Material density - Different mixture components with different densities sink through each other, which will have a different effect on the mixer depending on their relative positions. It is recommended to place the denser material at the bottom of the mixer during the mixing process in an attempt to maximize mixing.

Particle size - Material segregation caused by different particle sizes can cause non-uniform distribution of materials. In between larger particles, the smaller particles will settle. A large particle that slips through the void in the open packing is allowed to dilate during the mixing process, resulting in uneven distribution. The influence of gravity on particle size increases the flow properties as particle size increases. Powders that are similar in particle size can be mixed more easily.

Particle shape - The mixing process can be easier when particles possess spherical shapes, but are more challenging when particles have other shapes. The ideal particle is spherical in shape, and when particles depart from this shape, mixing becomes more challenging. If the particles are mixed, they will become interconnected and reduce the risk of segregation.

Particle attraction - The adsorption of liquid films and electrostatic charges on some particles result in attractive forces that cause them to aggregate. Since these properties are surface properties, they cause a decrease in particle size to increase aggregation. Drug-carrier interactions may also be affected by electrostatic and capillary forces, in addition to van der Waals' forces. It is important to realize that adhering forces are greatly affected by many factors, including the size, shape, crystallinity, hardness, and contaminants of the carrier particles, as well as the intensity and duration of the shear forces when mixing.

Proportion of materials - An important factor determining the efficacy of mixing is the proportion of chemicals in a given mixture. A powder mixing process is very dependent on the proportions of materials to be combined. The mixing of powders is easy if the ingredients are of equal strength, but it is difficult to mix a small quantity of powder with a large quantity of another ingredient or diluent. It is important that the substance be mixed in ascending and geometric order of its weight, however, to ensure uniform mixing.

Mixer volume - It is essential that the mixer reserve enough space during mixing to accommodate bed dilation. Keeping this condition in place will allow the powder samples to undergo free mixing to achieve uniform mix with greater efficacy. Filling the container to the top decreases efficiency and may even prevent mixing completely.

Mixing mechanism - For the mixer to ensure efficient mixing of bulk materials, a sufficient shear force and convective movement are essential. According to its mechanism, the mixer selected must apply suitable shear forces so as to achieve more local mixing and ensure convective movement in order for the bulk of the material to pass through this area.

Method of handling - Standard operating procedures must be followed to minimize the risk of segregation after mixing. Separation is common when goods are handled, transported, or packaged. The result is that all bulk powder needs to be thoroughly mixed before being used.

Nature of product - Powder components with rough surfaces may not mix effectively. Generally, very fine substances may enter pores and cavities on the surfaces of larger ingredients due to their size. When such substances are adsorbed on the surface of another powder, the aggregation and segregation of that powder is decreased. By incorporating colloidal silica into zinc oxide, for example, fine dusting powder that is easily mixed can be maintained.

Mixing conditions - Powder mixing theory points out four conditions that need to be followed when mixing powders. They affect the mixing process.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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