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ARTICLES IN THE BOOK

  1. Atomic force microscope
  2. Atomic nanoscope
  3. Atom probe
  4. Ballistic conduction
  5. Bingel reaction
  6. Biomimetic
  7. Bio-nano generator
  8. Bionanotechnology
  9. Break junction
  10. Brownian motor
  11. Bulk micromachining
  12. Cantilever
  13. Carbon nanotube
  14. Carbyne
  15. CeNTech
  16. Chemical Compound Microarray
  17. Cluster
  18. Colloid
  19. Comb drive
  20. Computronium
  21. Coulomb blockade
  22. Diamondoids
  23. Dielectrophoresis
  24. Dip Pen Nanolithography
  25. DNA machine
  26. Ecophagy
  27. Electrochemical scanning tunneling microscope
  28. Electron beam lithography
  29. Electrospinning
  30. Engines of Creation
  31. Exponential assembly
  32. Femtotechnology
  33. Fermi point
  34. Fluctuation dissipation theorem
  35. Fluorescence interference contrast microscopy
  36. Fullerene
  37. Fungimol
  38. Gas cluster ion beam
  39. Grey goo
  40. Hacking Matter
  41. History of nanotechnology
  42. Hydrogen microsensor
  43. Inorganic nanotube
  44. Ion-beam sculpting
  45. Kelvin probe force microscope
  46. Lab-on-a-chip
  47. Langmuir-Blodgett film
  48. LifeChips
  49. List of nanoengineering topics
  50. List of nanotechnology applications
  51. List of nanotechnology topics
  52. Lotus effect
  53. Magnetic force microscope
  54. Magnetic resonance force microscopy
  55. Mechanochemistry
  56. Mechanosynthesis
  57. MEMS thermal actuator
  58. Mesotechnology
  59. Micro Contact Printing
  60. Microelectromechanical systems
  61. Microfluidics
  62. Micromachinery
  63. Molecular assembler
  64. Molecular engineering
  65. Molecular logic gate
  66. Molecular manufacturing
  67. Molecular motors
  68. Molecular recognition
  69. Molecule
  70. Nano-abacus
  71. Nanoart
  72. Nanobiotechnology
  73. Nanocar
  74. Nanochemistry
  75. Nanocomputer
  76. Nanocrystal
  77. Nanocrystalline silicon
  78. Nanocrystal solar cell
  79. Nanoelectrochemistry
  80. Nanoelectrode
  81. Nanoelectromechanical systems
  82. Nanoelectronics
  83. Nano-emissive display
  84. Nanoengineering
  85. Nanoethics
  86. Nanofactory
  87. Nanoimprint lithography
  88. Nanoionics
  89. Nanolithography
  90. Nanomanufacturing
  91. Nanomaterial based catalyst
  92. Nanomedicine
  93. Nanomorph
  94. Nanomotor
  95. Nano-optics
  96. Nanoparticle
  97. Nanoparticle tracking analysis
  98. Nanophotonics
  99. Nanopore
  100. Nanoprobe
  101. Nanoring
  102. Nanorobot
  103. Nanorod
  104. Nanoscale
  105. Nano-Science Center
  106. Nanosensor
  107. Nanoshell
  108. Nanosight
  109. Nanosocialism
  110. Nanostructure
  111. Nanotechnology
  112. Nanotechnology education
  113. Nanotechnology in fiction
  114. Nanotoxicity
  115. Nanotube
  116. Nanovid microscopy
  117. Nanowire
  118. National Nanotechnology Initiative
  119. Neowater
  120. Niemeyer-Dolan technique
  121. Ormosil
  122. Photolithography
  123. Picotechnology
  124. Programmable matter
  125. Quantum dot
  126. Quantum heterostructure
  127. Quantum point contact
  128. Quantum solvent
  129. Quantum well
  130. Quantum wire
  131. Richard Feynman
  132. Royal Society's nanotech report
  133. Scanning gate microscopy
  134. Scanning probe lithography
  135. Scanning probe microscopy
  136. Scanning tunneling microscope
  137. Scanning voltage microscopy
  138. Self-assembled monolayer
  139. Self-assembly
  140. Self reconfigurable
  141. Self-Reconfiguring Modular Robotics
  142. Self-replication
  143. Smart dust
  144. Smart material
  145. Soft lithography
  146. Spent nuclear fuel
  147. Spin polarized scanning tunneling microscopy
  148. Stone Wales defect
  149. Supramolecular assembly
  150. Supramolecular chemistry
  151. Supramolecular electronics
  152. Surface micromachining
  153. Surface plasmon resonance
  154. Synthetic molecular motors
  155. Synthetic setae
  156. Tapping AFM
  157. There's Plenty of Room at the Bottom
  158. Transfersome
  159. Utility fog

 



NANOTECHNOLOGY
This article is from:
http://en.wikipedia.org/wiki/Colloid

All text is available under the terms of the GNU Free Documentation License: http://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License 

Colloid

From Wikipedia, the free encyclopedia

 

In general, a colloid or colloidal dispersion is a substance with components of one or two phases, a type of mixture intermediate between a homogeneous mixture (also called a solution) and a heterogeneous mixture with properties also intermediate between the two. Typical membranes restrict the passage of dispersed colloidial particles more than they restrict the passage of dissolved ions or molecules; i.e. ions or molecules may diffuse through a membrane through which dispersed colloidal particles will not. The dispersed phase particles are largely affected by the surface chemistry existent in the colloid.

Many familiar substances, including butter, milk, cream, aerosols (fog, smog, smoke), asphalt, inks, paints, glues, and sea foam are colloids. This field of study was introduced in 1861 by Scottish scientist Thomas Graham.

The size of dispersed phase particles in a colloid range from one nanometer to one micrometer. Dispersions where the particle size is in this range are referred to as colloidal aerosols, colloidal emulsions, colloidal foams, or colloidal suspensions or dispersions. Colloids may be colored or translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid.

Classification of colloids

Colloids can be classified as follows:

Interaction between colloid particles

The following forces play an important role in the interaction of colloid particles:

  • Excluded Volume Repulsion: This refers to the impossibility of any overlap between hard particles.
  • Electrostatic interaction: Colloidal particles often carry an electrical charge and therefore attract or repel each other. The charge of both the continuous and the dispersed phase, as well as the mobility of the phases are factors affecting this interaction.
  • van der Waals forces: This is due to interaction between two dipoles which are either permanent or induced. Even if the particles don't have a permanent dipole, fluctuations of the electron density gives rise to a temporary dipole in a particle. This temporary dipole induces a dipole in particles nearby. The temporary dipole and the induced dipoles are then attracted to each other. This is known as van der Waals force and is always present, is short range and is attractive.
  • Entropic forces: According to the second law of thermodynamics, a system progresses to a state in which entropy is maximized. This can result in effective forces even between hard spheres.
  • Steric forces between polymer-covered surfaces or in solutions containing non-adsorbing polymer can modulate interparticle forces, producing an additional repulsive steric stabilization force or attractive depletion force between them.

Stabilization of colloid suspensions

Stabilization serves to prevent colloids from aggregating. Steric stabilization and electrostatic stabilization are the two main mechanisms for colloid stabilization. Electrostatic stabilization is based on the mutual repulsion of like electrical charges. Different phases generally have different charge affinities, so that a charge double-layer forms at any interface. Small particle sizes lead to enormous surface areas, and this effect is greatly amplified in colloids. In a stable colloid, mass of a dispersed phase is so low that its buoyancy or kinetic energy is too little to overcome the electrostatic repulsion between charged layers of the dispersing phase. The charge on the dispersed particles can be observed by applying an electric field: all particles migrate to the same electrode and therefore must all have the same sign charge.

Destabilizing a colloidal suspension

Unstable colloidal suspensions form flocs as the particles aggregate due to interparticle attractions. This can be accomplished by a number of different methods:

  • Removal of the electrostatic barrier that prevents aggregation of the particles. This can be accomplished by the addition of salt to a suspension or changing the pH of a suspension to effectively neutralize or "screen" the surface charge of the particles in suspension. This removes the repulsive forces that keep colloidal particles separate and allows for coagulation due to van der Waals forces.
  • Addition of a charged polymer flocculant. Polymer flocculants can bridge individual colloidal particles by attractive electrostatic interactions. For example, negatively charged colloidal silica particles can be flocculated by the addition of a positively charged polymer.
  • Addition of nonadsorbed polymers called depletants that cause aggregation due to entropic effects.

Unstable colloidal suspensions of low volume fraction form clustered liquid suspensions wherein individual clusters of particles fall to the bottom of the suspension (or float to the top if the particles are less dense than the suspending medium) once the clusters are of sufficient size for the Brownian forces that work to keep the particles in suspension to be overcome by gravitational forces. However, colloidal suspensions of higher volume fraction form colloidal gels with viscoelastic properties. Viscoelastic colloidal gels such as toothpaste flow like liquids under shear but maintain their shape when shear is removed. It is for this reason that toothpaste can be squeezed from a toothpaste tube, but stays on the toothbrush after it is applied.

Colloids as a model system for atoms

In physics, colloids are an interesting model system for atoms. Colloidal particles are large enough to be observed by optical techniques such as confocal microscopy. Many of the forces that govern the structure and behavior of matter such as excluded volume interactions or electrostatic forces govern the structure and behavior of colloidal suspensions. For example, the same techniques that can be used to model ideal gases can be used to model the behavior of a hard sphere colloidal suspension. Additionally, phase transitions in colloidal suspensions can be studied in real time using optical techniques and are analogous to phase transitions in liquids.

Colloids in biology

In the early-20th century, before enzymology was well understood, colloids were thought to be the key to the operation of enzymes; i.e., the addition of small quantities of an enzyme to a quantity of water would, in some fashion yet to be specified, subtly alter the properties of the water so that it would break down the enzyme's specific substrate, such as a solution of ATPase breaking down ATP. Furthermore, life itself was explainable in terms of the aggregate properties of all the colloidal substances that make up an organism. As more detailed knowledge of biology and biochemistry developed, of course, the colloidal theory was replaced by the macromolecular theory, which explains an enzyme as a collection of identical huge molecules that act as very tiny machines, freely moving about between the water molecules of the solution and individually operating on the substrate, no more mysterious than a factory full of machinery. The properties of the water in the solution are not altered, other than the simple osmotic changes that would be caused by the presence of any solute.


See also

  • Entropic force


 

 
Retrieved from "http://en.wikipedia.org/wiki/Colloid"