In chemistry, a suspension is a heterogeneous mixture of a fluid that contains solid sufficiently large for sedimentation. The particles may be macroscopic to the naked eye, usually must be larger than one micrometre, and will eventually settling, although the mixture is only classified as a suspension when and while the particles have not settled out.
Properties
A suspension is a heterogeneous mixture in which the
solid particles do not dissolve, but get suspended throughout the bulk of the
solvent, left floating around freely in the medium.
[Chemistry: Matter and Its Changes, 4th Ed. by Brady, Sense, ] The internal phase (solid) is dispersed throughout the external phase ,fluid, through mechanical (action), with the use of certain or suspending agents.
An example of a suspension would be sand in water. The suspended particles are visible under a microscope and will settle over time if left undisturbed. This distinguishes a suspension from a colloid, in which the colloid particles are smaller and do not settle.[The Columbia Electronic Encyclopedia, 6th ed.] Colloids and suspensions are different from solution, in which the dissolved substance (solute) does not exist as a solid, and solvent and solute are homogeneously mixed.
A suspension of liquid droplets or fine solid particles in a gas is called an aerosol. In the Earth's, the suspended particles are called particulates and consist of fine dust and soot particles, sea, biogenic and volcano genic [sulfates, [nitrate, and cloud droplets.
Suspensions are classified on the basis of the dispersed and the dispersion medium, where the former is essentially solid while the latter may either be a solid, a liquid, or a gas.
In modern chemical process industries, high-shear mixer has been used to create many novel suspensions.
Suspensions are unstable from a thermodynamic point of view but can be kinetically stable over a longer period of time, which in turn can determine a suspension's shelf life. This time span needs to be measured in order to provide accurate information to the consumer and ensure the best product quality.
"Dispersion stability refers to the ability of a dispersion to resist change in its properties over time."[ “Food emulsions, principles, practices and techniques” CRC Press 2005.2- M. P. C. Silvestre, E. A. Decker, McClements Food hydrocolloids 13 (1999) 419–424.]
Technique monitoring physical stability
Multiple light scattering coupled with vertical scanning is the most widely used technique to monitor the dispersion state of a product, hence identifying and quantifying
Destabilisation phenomena.
[I. Roland, G. Piel, L. Delattre, B. Evrard International Journal of Pharmaceutics 263 (2003) 85-94][C. Lemarchand, P. Couvreur, M. Besnard, D. Costantini, R. Gref, Pharmaceutical Research, 20-8 (2003) 1284-1292][O. Mengual, G. Meunier, I. Cayre, K. Puech, P. Snabre, Colloids and Surfaces A: Physicochemical and Engineering Aspects 152 (1999) 111–123][P. Bru, L. Brunel, H. Buron, I. Cayré, X. Ducarre, A. Fraux, O. Mengual, G. Meunier, A. de Sainte Marie and P. Snabre Particle sizing and characterization Ed T. Provder and J. Texter (2004)] It works on concentrated dispersions without dilution. When light is sent through the sample, it is back scattered by the particles. The backscattering intensity is directly proportional to the size and volume fraction of the dispersed phase. Therefore, local changes in concentration (
sedimentation) and global changes in size (
flocculation, aggregation) are detected and monitored. Of primary importance in the analysis of stability in particle suspensions is the value of the
zeta potential exhibited by suspended solids. This parameter indicates the magnitude of interparticle electrostatic repulsion and is commonly analyzed to determine how the use of
adsorption and pH modification affect particle repulsion and suspension stabilization or destabilization.
Accelerating methods for shelf life prediction
The kinetic process of destabilisation can be rather long (up to several months or even years for some products) and it is often required for the formulator to use further accelerating methods in order to reach reasonable development time for new product design. Thermal methods are the most commonly used and consists in increasing temperature to accelerate destabilisation (below critical temperatures of phase and degradation). Temperature affects not only the viscosity, but also interfacial tension in the case of non-ionic surfactants or more generally interactions forces inside the system. Storing a dispersion at high temperatures enables simulation of real life conditions for a product (e.g. tube of sunscreen cream in a car in the summer), but also to accelerate destabilisation processes up to 200 times including vibration,
centrifugation and agitation are sometimes used. They subject the product to different forces that pushes the particles / film drainage. However, some emulsions would never coalesce in normal gravity, while they do under artificial gravity.
[ J-L Salager, Pharmaceutical emulsions and suspensions Ed Françoise Nielloud,Gilberte Marti-Mestres (2000)] Moreover, segregation of different populations of particles have been highlighted when using centrifugation and vibration.
[P. Snabre, B. Pouligny Langmuir, 24 (2008) 13338-13347]
Examples
Common examples of suspensions include:
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Mud or muddy water: where soil, clay, or silt particles are suspended in water
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Flour suspended in water
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Chalk suspended in water
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Sand suspended in water
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Oil incorporated in water
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Juice vesicles suspended in lemonade
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Blood suspended in plasma
See also
