Suspensions Containing Large Solid Particles

The huge interface associated with colloids is the reason why colloid and surface chemistry are often studied together. Colloidal dimensions imply that there are numerous surface molecules due to the large surfaces present. For example, 1 litre of a latex paint suspension containing 50% solids with a particle size of 0.2 pm has a total particle surface area of 15 000 m. However, to form such huge interfaces, e.g. by dispersing water in the form of droplets in an oil, we need to... 

There are a large number of processes in the chemical industries that handle a variety of suspensions of solid particles in liquids. The application of filtration techniques for the separation of these heterogeneous systems is sometimes very costly. If, however, the discrete phase of the suspension largely contains settleable particles, the separation can be effected by the operation of sedimentation. The process of sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening,... 

The USP has numerous requirements, e.g., ophthalmic solutions [need be] essentially free from foreign particles, suitably compounded and packaged for instillation into the eye, or ophthalmic suspensions [need contain] solid particles dispersed in liquid vehicle intended for application to the eye [1]. Ophthalmic suspensions are required to be made with the insoluble drug in a micronized form to prevent irritation or scratching of the cornea. A finished ophthalmic ointment must be free from large particles and must meet the requirements for leakage and for metal particles under ophthalmic ointments . These and other requirements will be discussed further in subsequent sections. 

The sol-gel method is widely used to obtain oxide layers on the walls of microchannels. This method is advantageous because a large variety of compositions can be produced, and porosity and surface texture can be tailored. The sol-gel method is also used for the preparation of particulate porous catalytic supports [155,201,202], The colloidal metal oxide sols can be prepared by various methods such as reactions of metal salts with water or by hydrolysis and polycondensation of metal alkoxides. The latter is the most versatile procedure and has been investigated extensively. Often the sol contains varying concentrations of solid particles, and the procedure is no longer a sol-gel but rather a hybrid method, with the coating medium being a mixture between a sol and a suspension (Table 3). 

Suspensions may contain not just solid particles and water but also emulsified oil and even dispersed gas bubbles. In the large Canadian oil sands mining and processing operations, bitumen is disengaged from the sand matrix in suspensions created in large tumblers. The bitumen is then separated from the suspension by a flotation process in which the flotation medium is a suspension of fine particles that also contains emulsified oil (bitumen) and dispersed air bubbles see Chapter 13). 

The continued inhalation of air containing large quantities of solid particles in suspension may cause severe pulmonary disorder, by mere mechanical irritation, and apart from any poisonous quality in the substance such is the case with the air of carpeted ball-rooms, and of the workshops of certain trades, furniture-polishers, metal-filers, etc. 

The traditional definition of emulsions (1) as consisting of two liquids, of which one is dispersed in the other in the form of macroscopic droplets, was modified by the lUPAC Commission for Nomenclature (2) to include lyotropic liquid crystals. This change was justified by the fact that a large number of commercial emulsions within the areas of foods, pharmaceutics, and personal care contain such structures. Commercial emulsions frequently also contain solid particles, but such systems are usually not called emulsions, but rather emulsions-suspensions to avoid having the term emulsions covering the majority of dispersed systems. 

A suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation and, unlike colloids, will settle. 

Phenomena at Liquid Interfaces. The area of contact between two phases is called the interface three phases can have only aline of contact, and only a point of mutual contact is possible between four or more phases. Combinations of phases encountered in surfactant systems are L—G, L—L—G, L—S—G, L—S—S—G, L—L, L—L—L, L—S—S, L—L—S—S—G, L—S, L—L—S, and L—L—S—G, where G = gas, L = liquid, and S = solid. An example of an L—L—S—G system is an aqueous surfactant solution containing an emulsified oil, suspended soHd, and entrained air (see Emulsions Foams). This embodies several conditions common to practical surfactant systems. First, because the surface area of a phase iacreases as particle size decreases, the emulsion, suspension, and entrained gas each have large areas of contact with the surfactant solution. Next, because iaterfaces can only exist between two phases, analysis of phenomena ia the L—L—S—G system breaks down iato a series of analyses, ie, surfactant solution to the emulsion, soHd, and gas. It is also apparent that the surfactant must be stabilizing the system by preventing contact between the emulsified oil and dispersed soHd. FiaaHy, the dispersed phases are ia equiUbrium with each other through their common equiUbrium with the surfactant solution. 

An aerosol is generally defined as a suspension of solid or liquid particles in a gas, and the most evident example of an aerosol is the air (Horvath, 2000). However, in atmospheric research, the term aerosol usually denotes the suspended particles that contain a large proportion of condensed matter, whereas clouds are considered as separate phenomena (Poschl, 2005). 

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