Particle Shape, Specific Surface, Methods for Determining Surface Area, Permeability, Adsorption

Particle Shape

A sphere's radius, or more often, diameter, can be used to calculate its size. The length of a cube's side or diagonal can be used to indicate its size. Particles, on the other hand, can take a variety of forms, ranging from needle shape to irregular polygonal, as seen in Figure. Quantitatively measuring and defining the size of these particles may be difficult. However, the use of finely divided powders in pharmaceutical unit activities necessitates a numerical definition of particle size, ideally as a single number, in order to compare different powder kinds and batches of the same substance.

A three-dimensional object can be defined by a single number in terms of the property of interest by using a one-dimensional particle attribute (such as surface area or volume) and describing it in terms of an equivalent sphere. The criterion for particle size equivalence to a sphere is determined by the powder's intended function or application. Surface area-based equivalence is required, for example, when using a powder for surface catalysis or comparing the dissolution rates of various batches.



Two parameters can be used to characterize irregular-shaped particles:

• An analogous sphere's diameter. Milling and granulation are two powder processing methods that aim to change the shape of particles to make them more spherical.
• Aspect ratio is the proportion of a par-longest ticle's to smallest axis. A sphere would have one, while a needle-shaped particle would have the greatest. Aspect ratio is a tool for determining how much a form deviates from a perfect sphere.

Many frequently used particle size measuring methods quantify a particle's size in terms of an analogous sphere's diameter. This description comes with a number of assumptions and/or limitations. When defining particle size in terms of the diameter of an equivalent sphere, for example, the criterion used to determine equivalence must be taken into account. In terms of volume or surface area, two particles can be defined as equal. As a result, the diameter of a sphere with a similar volume or surface area may be used to estimate a particle's size.

Specific Surface

Surface area is a crucial physical attribute that determines how well pharmaceutical powders, components, APIs, and excipients operate. Differences in the surface area and porosity of particles inside a material, even though they are the same physical size, can have a significant impact on crucial performance aspects for pharmaceutical goods. Purification, processing, mixing, tableting, and packaging of pharmaceutical goods all rely heavily on surface area. The diameter of an equivalent sphere is employed in several commonly used particle size measurement procedures. A number of assumptions and/or constraints are made in this explanation. When defining particle size in terms of the diameter of an equivalent sphere, for example, the criterion employed to generate equivalence must be addressed. Two particles can be considered equal in terms of volume or surface area. As a result, particle size may be estimated using the diameter of a sphere with a similar volume or surface area.

This method entails exposing solid materials to gases or vapors under various circumstances and determining the weight absorption or sample volume. Analyzing this data yields physical properties for the solid, such as specific surface area, porosity, total pore volume, and pore size distribution. Using gas sorption to study pores can also reveal details about the pores' physical structure and shape. Gas adsorption can also be used to measure the roughness of a pharmaceutical product's surface. Roughness of the surface can have an impact on the performance properties of products like oral pills and inhalable dry powders. Dissolution, stability, friability, coating adhesion, mix homogeneity, particle to particle adhesion, powder flow, and powder packing are critical performance attributes directly related to the surface area. A rough surface can make small drug particles interact with larger inert carrier particles in dry powder inhalation formulations.

Methods for determining surface area

Pretreatment of solid samples to remove impurities (water and CO2) absorbed from the environment using vacuum, heat, and a gas flow is required for surface area measurement. The solid is then chilled to a cryogenic temperature of -195°C under vacuum before an adsorptive is introduced. After each dosage of adsorptive, the pressure is allowed to achieve equilibrium, and the adsorbed quantity is calculated. Calculating the quantity of gas necessary to form a monolayer on the solid's outer surface at various temperatures and pressures yields an adsorption isotherm. It is possible to calculate the surface area of an object by measuring the area of the monolayer formed by these gas molecules. This approach uses the USP 846> protocols to determine a specified surface area.

Permeability

The Biopharmaceutics Classification System is a classification system for medications based on their solubility and permeability. Using the factors of solubility and intestinal permeability, this system limits the prediction. The solubility categorization is based on a USP aperture (United States Pharmacopoeia). Intestinal permeability is classified using a comparison to an intravenous infusion. All of these criteria are critical since orally delivered medications account for 85 percent of all pharmaceuticals sold in the United States and Europe.

The amount of a pharmacological substance absorbed in humans is determined in part by mass transfer rates across the human intestinal membrane and in part by the amount of a pharmaceutical substance absorbed in humans. Non-human systems capable of forecasting medication absorption in humans can also be utilized as an alternative. According to mass-balance analysis or when compared to an intravenous dose, a substance that is extremely permeable in humans absorbs 90% or more of the given dose.

Adsorption

Heinrich Kayser, a German scientist, initially invented the word adsorption in 1881. Adsorption is a typical term for a surface phenomenon in which particles cling to a substance's top layer. It usually involves the molecules, atoms, or even ions of a dissolved gas, liquid, or solid adhered to the surface. Surface energy is the primary cause of adsorption. A partial exposure of surface particles tends to attract additional particles to the affected area. Adsorption is found in a variety of physical, natural, biological, and chemical systems, and it is used in a variety of industrial applications.

Adsorption is the process of a material accumulating in molecular forms at increasing concentrations on the surface. Hydrogen, Nitrogen, and Oxygen are all gases that adsorb on activated charcoal. Additionally, it must be kept in mind that adsorption does not mean absorption. The mechanisms involved in the two processes are completely different.

The adsorption process requires two components -

• Adsorbate: A material that adheres to another substance's surface. H2, N2, and O2 gases, for example.
• Adsorbent: A substance's surface where adsorbate adsorbs. Charcoal, Silica gel, and Alumina are just a few examples.

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