how suspension r unstable.
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Physical stability can refer to any number of attributes ranging from appearance to color to viscosity. Many stability parameters are at least partially, if not completely, a function of suspension stability – the ability of the suspension to remain in its original state. This is critical for pharmaceutical formulations since any change could negatively impact its performance. Suspension stability is the result of various physical and electrochemical forces over which formulators often feel that they have little control. However, an understanding of the fundamental forces involved will, in fact, empower the formulator to create more effective and stable formulations.
The preparation of oil-in-water (O/W) and water-in-oil (W/O) emulsions requires emulsifiers and other tensides (1). Similarly, the preparation of suspensions of powders in liquids usually necessitates the use of chemical dispersing aids, including surfactants (2). These materials, dispersants and emulsifiers, while they can also provide some degree of stability to the final suspension or emulsion are not true stabilizing aids. It is, therefore, often advantageous to consider stabilization as a separate issue. This is particularly so when the systems are to be challenged by extreme process conditions such as freeze/thaw cycling and high temperature/longterm storage. Thus, when stability is an issue it is important to consider more closely the mechanics of the stabilization process.
A suspension comprises, in the simplest case, a mixture of two phases. The phases are described by the terms "dispersed phase" (for the material forming any particles) and the "dispersion medium or vehicle" (for the material in which said particles are distributed). The dispersed (internal) phase can be solid particles, as in nail polish, liquid droplets, such as in an emulsion or air/vapor/gas droplets in a foam. For this discussion, the term “particle” will be used for the dispersed phase be it solid, liquid or gaseous. The (external) liquid phase can be aqueous or non-aqueous; examples are all around us from car polish to salad dressing. The overall properties of the suspension are influenced by the chemical and physical characteristics of both the dispersed phase and the dispersion medium/vehicle and the interactions between these two individual materials when they are mixed together.
The physical condition of the dispersed phase can vary from round, such as oil droplets in emulsions to needle-or plate-like crystals of clays and oxides. A very important physical property that influences the performance of a suspension is the particle size distribution (PSD) (see PSI Technical Brief 2009 Vol 2). The PSD directly affects important characteristics such as the bioavailability of APIs and the tone and hue of colored pigments, such as the iron oxides used in daily-wear skincare formulations. In addition, the PSD determines settling properties and affects rheological behavior (flow and deformation) as can be clearly demonstrated by varying the droplet size in a given emulsion.
The preparation of oil-in-water (O/W) and water-in-oil (W/O) emulsions requires emulsifiers and other tensides (1). Similarly, the preparation of suspensions of powders in liquids usually necessitates the use of chemical dispersing aids, including surfactants (2). These materials, dispersants and emulsifiers, while they can also provide some degree of stability to the final suspension or emulsion are not true stabilizing aids. It is, therefore, often advantageous to consider stabilization as a separate issue. This is particularly so when the systems are to be challenged by extreme process conditions such as freeze/thaw cycling and high temperature/longterm storage. Thus, when stability is an issue it is important to consider more closely the mechanics of the stabilization process.
A suspension comprises, in the simplest case, a mixture of two phases. The phases are described by the terms "dispersed phase" (for the material forming any particles) and the "dispersion medium or vehicle" (for the material in which said particles are distributed). The dispersed (internal) phase can be solid particles, as in nail polish, liquid droplets, such as in an emulsion or air/vapor/gas droplets in a foam. For this discussion, the term “particle” will be used for the dispersed phase be it solid, liquid or gaseous. The (external) liquid phase can be aqueous or non-aqueous; examples are all around us from car polish to salad dressing. The overall properties of the suspension are influenced by the chemical and physical characteristics of both the dispersed phase and the dispersion medium/vehicle and the interactions between these two individual materials when they are mixed together.
The physical condition of the dispersed phase can vary from round, such as oil droplets in emulsions to needle-or plate-like crystals of clays and oxides. A very important physical property that influences the performance of a suspension is the particle size distribution (PSD) (see PSI Technical Brief 2009 Vol 2). The PSD directly affects important characteristics such as the bioavailability of APIs and the tone and hue of colored pigments, such as the iron oxides used in daily-wear skincare formulations. In addition, the PSD determines settling properties and affects rheological behavior (flow and deformation) as can be clearly demonstrated by varying the droplet size in a given emulsion.
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