2.2 Mixtures, Solutions, Colloids, Sols, and Gels
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• cohesive attraction of like substances in solutions,
• repulsion of dissimilar molecules,
• adsorption of molecules to surfaces,
• differential diffusion in bulk material,
• osmotic differential diffusion through membranes,
• gravitational separation due to varying buoyancies,
• filtration and sieving,
• fractional vaporization,
• fractional freezing,
• precipitation,
• sedimentation,
• solute vaporization or sublimation,
• differential fluid drag separation and natural winnowing.
Besides taking advantage of natural processes, life systems have a plethora
of techniques using active processes for separating atoms, molecules, and other
particles. Organisms employ such organizational processes to create, transport and
concentrate complex molecular materials and to maintain the activity of life.
Our species has developed methods to direct, control and enhance the separation
of mixtures into products, for use in food production, industry or for the analysis of
individual components, including medical analysis. We give below a few examples
of devices and technologies used in bioengineering and medicine.
• Gravitational separation takes advantage of the varying densities of components
in a mixture. In a fluid, buoyant forces act against gravity to separate materials of
differing density.
• The centrifuge is a device which can separate various-sized nano- and microscale particles in solutions, using acceleration forces much larger than the
acceleration of gravity, provided the intrinsic particle density is greater or less
than the background fluid density. We will describe the centrifuge in more detail
in Chap. 3.
• Evaporation concentrates low vapor pressure dissolved materials from a volatile
solute. Some dissolved materials, when so concentrated, will form pure crystals.
Condensation of the vapor back into a fluid can be used to capture the purified
fluid.
• Water distillation and desalination, solar evaporation, partial freezing, and
reverse osmotic pressure devices, are all used to concentrate potable water.
• Chemical precipitation, filtration, and bacterial conversion of hazardous materials helps generate potable and palatable water. Filtration to remove toxins from
blood is the basis of kidney dialysis.
• Electrodialysis uses a semi-permeable membrane in an electric field to transport
selected ions across the membrane. A major application of electrodialysis is in
the desalination of brackish water.
• Chromatography is an analysis tool that separates substances by diffusion
according to their affinity for a particular solvent. Often, this is done by putting
the mixture on filter paper and then letting the solvent soak in from one side of
the paper.
23
• cohesive attraction of like substances in solutions,
• repulsion of dissimilar molecules,
• adsorption of molecules to surfaces,
• differential diffusion in bulk material,
• osmotic differential diffusion through membranes,
• gravitational separation due to varying buoyancies,
• filtration and sieving,
• fractional vaporization,
• fractional freezing,
• precipitation,
• sedimentation,
• solute vaporization or sublimation,
• differential fluid drag separation and natural winnowing.
Besides taking advantage of natural processes, life systems have a plethora
of techniques using active processes for separating atoms, molecules, and other
particles. Organisms employ such organizational processes to create, transport and
concentrate complex molecular materials and to maintain the activity of life.
Our species has developed methods to direct, control and enhance the separation
of mixtures into products, for use in food production, industry or for the analysis of
individual components, including medical analysis. We give below a few examples
of devices and technologies used in bioengineering and medicine.
• Gravitational separation takes advantage of the varying densities of components
in a mixture. In a fluid, buoyant forces act against gravity to separate materials of
differing density.
• The centrifuge is a device which can separate various-sized nano- and microscale particles in solutions, using acceleration forces much larger than the
acceleration of gravity, provided the intrinsic particle density is greater or less
than the background fluid density. We will describe the centrifuge in more detail
in Chap. 3.
• Evaporation concentrates low vapor pressure dissolved materials from a volatile
solute. Some dissolved materials, when so concentrated, will form pure crystals.
Condensation of the vapor back into a fluid can be used to capture the purified
fluid.
• Water distillation and desalination, solar evaporation, partial freezing, and
reverse osmotic pressure devices, are all used to concentrate potable water.
• Chemical precipitation, filtration, and bacterial conversion of hazardous materials helps generate potable and palatable water. Filtration to remove toxins from
blood is the basis of kidney dialysis.
• Electrodialysis uses a semi-permeable membrane in an electric field to transport
selected ions across the membrane. A major application of electrodialysis is in
the desalination of brackish water.
• Chromatography is an analysis tool that separates substances by diffusion
according to their affinity for a particular solvent. Often, this is done by putting
the mixture on filter paper and then letting the solvent soak in from one side of
the paper.