Dispersion (chemistry)

Last updated
IUPAC definition
Material comprising more than one phase where at least one of the phases consists of finely divided phase domains, often in the colloidal size range, dispersed throughout a continuous phase. [1] Note 1: Modification of definition in ref. [2]

A dispersion is a system in which distributed particles of one material are dispersed in a continuous phase of another material. The two phases may be in the same or different states of matter.

Contents

Dispersions are classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not precipitation occurs, and the presence of Brownian motion. In general, dispersions of particles sufficiently large for sedimentation are called suspensions, while those of smaller particles are called colloids and solutions.

Structure and properties

Dispersions do not display any structure; i.e., the particles (or in case of emulsions: droplets) dispersed in the liquid or solid matrix (the "dispersion medium") are assumed to be statistically distributed. Therefore, for dispersions, usually percolation theory is assumed to appropriately describe their properties.

However, percolation theory can be applied only if the system it should describe is in or close to thermodynamic equilibrium. There are only very few studies about the structure of dispersions (emulsions), although they are plentiful in type and in use all over the world in innumerable applications (see below).

In the following, only such dispersions with a dispersed phase diameter of less than 1 µm will be discussed. To understand the formation and properties of such dispersions (incl emulsions), it must be considered that the dispersed phase exhibits a "surface", which is covered ("wet") by a different "surface" that, hence, are forming an interface (chemistry). Both surfaces have to be created (which requires a huge amount of energy), and the interfacial tension (difference of surface tension) is not compensating the energy input, if at all.

Experimental evidence suggests dispersions have a structure very much different from any kind of statistical distribution (which would be characteristics for a system in thermodynamic equilibrium), but in contrast display structures similar to self-organisation, which can be described by non-equilibrium thermodynamics. [3] This is the reason why some liquid dispersions turn to become gels or even solid at a concentration of a dispersed phase above a critical concentration (which is dependent on particle size and interfacial tension). Also, the sudden appearance of conductivity in a system of a dispersed conductive phase in an insulating matrix has been explained.

Process of dispersion

Dispersion is a process by which (in the case of solid dispersing in a liquid) agglomerated particles are separated from each other, and a new interface between the inner surface of the liquid dispersion medium and the surface of the dispersed particles is generated. This process is facilitated by molecular diffusion and convection. [4]

With respect to molecular diffusion, dispersion occurs as a result of an unequal concentration of the introduced material throughout the bulk medium. When the dispersed material is first introduced into the bulk medium, the region at which it is introduced then has a higher concentration of that material than any other point in the bulk. This unequal distribution results in a concentration gradient that drives the dispersion of particles in the medium so that the concentration is constant across the entire bulk. With respect to convection, variations in velocity between flow paths in the bulk facilitate the distribution of the dispersed material into the medium.

Although both transport phenomena contribute to the dispersion of a material into the bulk, the mechanism of dispersion is primarily driven by convection in cases where there is significant turbulent flow in the bulk. [5] Diffusion is the dominant mechanism in the process of dispersion in cases of little to no turbulence in the bulk, where molecular diffusion is able to facilitate dispersion over a long period of time. [4] These phenomena are reflected in common real-world events. The molecules in a drop of food coloring added to water will eventually disperse throughout the entire medium, where the effects of molecular diffusion are more evident. However, stirring the mixture with a spoon will create turbulent flows in the water that accelerate the process of dispersion through convection-dominated dispersion.

Degree of dispersion

The term dispersion also refers to the physical property of the degree to which particles clump together into agglomerates or aggregates. While the two terms are often used interchangeably, according to ISO nanotechnology definitions, an agglomerate is a reversible collection of particles weakly bound, for example by van der Waals forces or physical entanglement, whereas an aggregate is composed of irreversibly bonded or fused particles, for example through covalent bonds. [6] A full quantification of dispersion would involve the size, shape, and number of particles in each agglomerate or aggregate, the strength of the interparticle forces, their overall structure, and their distribution within the system. However, the complexity is usually reduced by comparing the measured size distribution of "primary" particles to that of the agglomerates or aggregates. [7] . When discussing suspensions of solid particles in liquid media, the zeta potential is most often used to quantify the degree of dispersion, with suspensions possessing a high absolute value of zeta potential being considered as well-dispersed.

Types of dispersions

A solution describes a homogeneous mixture of one material dispersed into another. The dispersed particles will not settle if the solution is left undisturbed for a prolonged period of time.

A colloid is a heterogeneous mixture of one phase in another, where the dispersed particles are usually. Like solutions, dispersed particles will not settle if the solution is left undisturbed for a prolonged period of time.

A suspension is a heterogeneous dispersion of larger particles in a medium. Unlike solutions and colloids, if left undisturbed for a prolonged period of time, the suspended particles will settle out of the mixture.

Although suspensions are relatively simple to distinguish from solutions and colloids, it may be difficult to distinguish solutions from colloids since the particles dispersed in the medium may be too small to distinguish by the human eye. Instead, the Tyndall effect is used to distinguish solutions and colloids. Due to the various reported definitions of solutions, colloids, and suspensions provided in the literature, it is difficult to label each classification with a specific particle size range.

In addition to the classification by particle size, dispersions can also be labeled by the combination of the dispersed phase and the medium phase that the particles are suspended in. Aerosols are liquids dispersed in a gas, sols are solids in liquids, emulsions are liquids dispersed in liquids (more specifically a dispersion of two immiscible liquids), and gels are liquids dispersed in solids.

Components phasesHomogeneous mixtureHeterogeneous mixture
Dispersed
material
Continuous
medium
Solution:
Rayleigh scattering effect on visible light
Colloid (smaller particles):
Tyndall effect on visible light near the surface
Suspension (larger particles):
no significant effect on visible light
Gas GasGas mixture: air (oxygen and other gases in nitrogen)
Liquid Aerosol: fog, mist, vapor, hair sprays, moisted airAerosol: rain (also produces rainbows by refraction on water droplets)
Solid Solid aerosol: smoke, cloud, air particulates Solid aerosol: dust, sand storm, ice fog, pyroclastic flow
GasLiquidOxygen in water Foam: whipped cream, shaving cream Bubbling foam, boiling water, sodas and sparkling beverages
Liquid Alcoholic beverages (cocktails), sirups Emulsion: miniemulsion, microemulsion, milk, mayonnaise, hand cream, hydrated soap unstable emulsion of a soap bubble (at ambient temperature, with iridescent effect on light caused by evaporation of water; the suspension of liquids is still maintained by surfacic tension with the gas inside and outside the bubble and surfactants effects decreasing with evaporation; finally the bubble will pop when there's no more emulsion and the shearing effect of micelles will outweight the surface tension lost by evaporation of water out of them)
Solid Sugar in water Sol: pigmented ink, blood Mud (soil, clay or silt particles suspended in water, lahar, quicksand), wet plaster/cement/concrete, chalk powder suspended in water, lava flow (mix of melted and solid rock), melting ice creams
GasSolid Hydrogen in metals Solid foam: aerogel, styrofoam, pumice
Liquid Amalgam (mercury in gold), hexane in paraffin wax Gel: agar, gelatin, silicagel, opal; frozen ice creams
Solid Alloys, plasticizers in plastics Solid sol: cranberry glass natural rocks, dried plaster/cement/concrete, frozen soap bubble

Examples of dispersions

Milk is a commonly cited example of an emulsion, a specific type of dispersion of one liquid into another liquid where to two liquids are immiscible. The fat molecules suspended in milk provide a mode of delivery of important fat-soluble vitamins and nutrients from the mother to newborn. [8] The mechanical, thermal, or enzymatic treatment of milk manipulates the integrity of these fat globules and results in a wide variety of dairy products. [9]

Oxide dispersion-strengthened alloy (ODS) is an example of oxide particle dispersion into a metal medium, which improves the high temperature tolerance of the material. Therefore these alloys have several applications in the nuclear energy industry, where materials must withstand extremely high temperatures to maintain operation. [10]

The degradation of coastal aquifers is a direct result of seawater intrusion into the and dispersion into the aquifer following excessive use of the aquifer. When an aquifer is depleted for human use, it is naturally replenished by groundwater moving in from other areas. In the case of coastal aquifers, the water supply is replenished both from the land boundary on one side and the sea boundary on the other side. After excessive discharge, saline water from the sea boundary will enter the aquifer and disperse in the freshwater medium, threatening the viability of the aquifer for human use. [11] Several different solutions to seawater intrusion in coastal aquifers have been proposed, including engineering methods of artificial recharge and implementing physical barriers at the sea boundary. [12]

Chemical dispersants are used in oil spills to mitigate the effects of the spill and promote the degradation of oil particles. The dispersants effectively isolate pools on oil sitting on the surface of the water into smaller droplets that disperse into the water, which lowers the overall concentration of oil in the water to prevent any further contamination or impact on marine biology and coastal wildlife. [13]

Related Research Articles

Colloid A mixture of an insoluble or soluble substance microscopically dispersed throughout another substance

In chemistry, a colloid is a mixture in which one substance of microscopically dispersed insoluble or soluble particles is suspended throughout another substance. Sometimes the dispersed substance alone is called the colloid; the term colloidal suspension refers unambiguously to the overall mixture. Unlike a solution, whose solute and solvent constitute only one phase, a colloid has a dispersed phase and a continuous phase that arise by phase separation. To qualify as a colloid, the mixture must be one that does not settle or would take a very long time to settle appreciably.

An emulsion is a mixture of two or more liquids that are normally immiscible. Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion should be used when both phases, dispersed and continuous, are liquids. In an emulsion, one liquid is dispersed in the other. Examples of emulsions include vinaigrettes, homogenized milk, and some cutting fluids for metal working.

Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer are emulsified in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act as emulsifiers/stabilizers. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex/colloid particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and are made of many individual polymer chains. The particles are prevented from coagulating with each other because each particle is surrounded by the surfactant ('soap'); the charge on the surfactant repels other particles electrostatically. When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a 'hairy layer' around a particle that repels other particles, because pushing particles together would involve compressing these chains.

In chemistry, a suspension is a heterogeneous mixture that contains solid particles sufficiently large for sedimentation. The particles may be visible to the naked eye, usually must be larger than one micrometer, and will eventually settle, although the mixture is only classified as a suspension when and while the particles have not settled out.

Mixing (process engineering)

In industrial process engineering, mixing is a unit operation that involves manipulation of a heterogeneous physical system with the intent to make it more homogeneous. Familiar examples include pumping of the water in a swimming pool to homogenize the water temperature, and the stirring of pancake batter to eliminate lumps (deagglomeration).

Dilatant type of fluids

A dilatant material is one in which viscosity increases with the rate of shear strain. Such a shear thickening fluid, also known by the initialism STF, is an example of a non-Newtonian fluid. This behaviour is usually not observed in pure materials, but can occur in suspensions.

Zeta potential Electrokinetic potential in colloidal dispersions

Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface.

The DLVO theory explains the aggregation of aqueous dispersions quantitatively and describes the force between charged surfaces interacting through a liquid medium. It combines the effects of the van der Waals attraction and the electrostatic repulsion due to the so-called double layer of counterions. The electrostatic part of the DLVO interaction is computed in the mean field approximation in the limit of low surface potentials - that is when the potential energy of an elementary charge on the surface is much smaller than the thermal energy scale, . For two spheres of radius each having a charge separated by a center-to-center distance in a fluid of dielectric constant containing a concentration of monovalent ions, the electrostatic potential takes the form of a screened-Coulomb or Yukawa potential,

Flocculation, in the field of chemistry, is a process in which colloids come out of suspension in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution. In the flocculated system, there is no formation of a cake, since all the flocs are in the suspension.

Particle aggregation

Particle agglomeration refers to formation of assemblages in a suspension and represents a mechanism leading to the functional destabilization of colloidal systems. During this process, particles dispersed in the liquid phase stick to each other, and spontaneously form irregular particle assemblates, flocs, or agglomerates. This phenomenon is also referred to as coagulation or flocculation and such a suspension is also called unstable. Particle agglomeration can be induced by adding salts or other chemicals referred to as coagulant or flocculant..

Electroacoustic phenomena arise when ultrasound propagates through a fluid containing ions. The associated particle motion generates electric signals because ions have electric charge. This coupling between ultrasound and electric field is called electroacoustic phenomena. The fluid might be a simple Newtonian liquid, or complex heterogeneous dispersion, emulsion or even a porous body. There are several different electroacoustic effects depending on the nature of the fluid.,

Double layer (surface science) a structure that appears on the surface of an object when it is exposed to a fluid

A double layer is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge, consists of ions adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the "diffuse layer".

Sedimentation potential occurs when dispersed particles move under the influence of either gravity or centrifugation in a medium. This motion disrupts the equilibrium symmetry of the particle's double layer. While the particle moves, the ions in the electric double layer lag behind due to the liquid flow. This causes a slight displacement between the surface charge and the electric charge of the diffuse layer. As a result, the moving particle creates a dipole moment. The sum of all of the dipoles generates an electric field which is called sedimentation potential. It can be measured with an open electrical circuit, which is also called sedimentation current.

A dispersant or a dispersing agent is a substance, typically a surfactant, that is added to a suspension of solid or liquid particles in a liquid to improve the separation of the particles and to prevent their settling or clumping.

A Pickering emulsion is an emulsion that is stabilized by solid particles which adsorb onto the interface between the two phases. This type of emulsion was named after S.U. Pickering, who described the phenomenon in 1907, although the effect was first recognized by Walter Ramsden in 1903.

Emulsion dispersion

An emulsion dispersion is thermoplastics or elastomers suspended in a waterphase with help of emulsifiers.

Colloidal crystal an ordered array of colloid particles, analogous to a standard crystal whose repeating subunits are atoms or molecules

A colloidal crystal is an ordered array of colloid particles and fine grained materials analogous to a standard crystal whose repeating subunits are atoms or molecules. A natural example of this phenomenon can be found in the gem opal, where spheres of silica assume a close-packed locally periodic structure under moderate compression. Bulk properties of a colloidal crystal depend on composition, particle size, packing arrangement, and degree of regularity. Applications include photonics, materials processing, and the study of self-assembly and phase transitions.

Macroemulsions are homogenous transparent thermodynamically unstable systems with particle sizes ranging from 5-140 nm, which form spontaneously when mixed in the correct ratio. Macroemulsions scatter light effectively and therefore appear milky, because their droplets are greater than a wavelength of light. They are part of a larger family of emulsions along with microemulsions. As with all emulsions, one phase serves as the dispersing agent. It is often called the continuous or outer phase. The remaining phase(s) are disperse or inner phase(s), because the liquid droplets are finely distributed amongst the larger continuous phase droplets. This type of emulsion is thermodynamically unstable, but can be stabilized for a period of time with applications of kinetic energy. Surfactants (emulsifiers) are used to reduce the interfacial tension between the two layers, and induce macroemulsion stability for a useful amount of time.

Dispersion Technology Inc is a scientific instrument manufacturer located in Bedford Hills, New York. It was founded in 1996 by Philip Goetz and Dr. Andrei Dukhin. The company develops and sells analytical instruments intended for characterizing concentrated dispersions and emulsions, complying with the International Standards for acoustic particle sizing ISO 20998 and Electroacoustic zeta potential measurement ISO 13099.

In polymer science, dispersion polymerization is a heterogeneous polymerization process carried out in the presence of a polymeric stabilizer in the reaction medium. Dispersion polymerization is a type of precipitation polymerization, meaning the solvent selected as the reaction medium is a good solvent for the monomer and the initiator, but is a non-solvent for the polymer. As the polymerization reaction proceeds, particles of polymer form, creating a non-homogeneous solution. In dispersion polymerization these particles are the locus of polymerization, with monomer being added to the particle throughout the reaction. In this sense, the mechanism for polymer formation and growth has features similar to that of emulsion polymerization. With typical precipitation polymerization, the continuous phase is the main locus of polymerization, which is the main difference between precipitation and dispersion.

References

  1. Slomkowski, Stanislaw; Alemán, José V.; Gilbert, Robert G.; Hess, Michael; Horie, Kazuyuki; Jones, Richard G.; Kubisa, Przemyslaw; Meisel, Ingrid; Mormann, Werner; Penczek, Stanisław; Stepto, Robert F. T. (2011). "Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)" (PDF). Pure and Applied Chemistry . 83 (12): 2229–2259. doi:10.1351/PAC-REC-10-06-03.
  2. Richard G. Jones; Edward S. Wilks; W. Val Metanomski; Jaroslav Kahovec; Michael Hess; Robert Stepto; Tatsuki Kitayama, eds. (2009). Compendium of Polymer Terminology and Nomenclature (IUPAC Recommendations 2008) (2nd ed.). RSC Publ. p. 464. ISBN   978-0-85404-491-7.
  3. NALWA, H (2000), "Index for Volume 3", Handbook of Nanostructured Materials and Nanotechnology, Elsevier, pp. 585–591, doi:10.1016/b978-012513760-7/50068-x, ISBN   9780125137607
  4. 1 2 Jacob., Bear (2013). Dynamics of Fluids in Porous Media. Dover Publications. ISBN   978-1306340533. OCLC   868271872.
  5. Mauri, Roberto (May 1991). "Dispersion, convection, and reaction in porous media". Physics of Fluids A: Fluid Dynamics. 3 (5): 743–756. Bibcode:1991PhFlA...3..743M. doi:10.1063/1.858007. ISSN   0899-8213.
  6. Stefaniak, Aleksandr B. (2017). "Principal Metrics and Instrumentation for Characterization of Engineered Nanomaterials". In Mansfield, Elisabeth; Kaiser, Debra L.; Fujita, Daisuke; Van de Voorde, Marcel (eds.). Metrology and Standardization of Nanotechnology. Wiley-VCH Verlag. pp. 151–174. doi:10.1002/9783527800308.ch8. ISBN   9783527800308.
  7. Powers, Kevin W.; Palazuelos, Maria; Moudgil, Brij M.; Roberts, Stephen M. (2007-01-01). "Characterization of the size, shape, and state of dispersion of nanoparticles for toxicological studies". Nanotoxicology. 1 (1): 42–51. doi:10.1080/17435390701314902. ISSN   1743-5390.
  8. Singh, Harjinder; Gallier, Sophie (July 2017). "Nature's complex emulsion: The fat globules of milk". Food Hydrocolloids. 68: 81–89. doi:10.1016/j.foodhyd.2016.10.011. ISSN   0268-005X.
  9. Lopez, Christelle (2005-07-01). "Focus on the supramolecular structure of milk fat in dairy products" (PDF). Reproduction, Nutrition, Development. 45 (4): 497–511. doi:10.1051/rnd:2005034. ISSN   0926-5287. PMID   16045897.
  10. Oak Ridge National Laboratory; United States; Department of Energy; United States; Department of Energy; Office of Scientific and Technical Information (1998). Development of oxide dispersion strengthened ferritic steels for fusion. Washington, D.C.: United States. Dept. of Energy. doi:10.2172/335389. OCLC   925467978. OSTI   335389.
  11. Frind, Emil O. (June 1982). "Seawater intrusion in continuous coastal aquifer-aquitard systems". Advances in Water Resources. 5 (2): 89–97. Bibcode:1982AdWR....5...89F. doi:10.1016/0309-1708(82)90050-1. ISSN   0309-1708.
  12. Luyun, Roger; Momii, Kazuro; Nakagawa, Kei (2011). "Effects of Recharge Wells and Flow Barriers on Seawater Intrusion". Groundwater. 49 (2): 239–249. doi:10.1111/j.1745-6584.2010.00719.x. ISSN   1745-6584. PMID   20533955.
  13. Lessard, R.R; DeMarco, G (Feb 2000). "The Significance of Oil Spill Dispersants". Spill Science & Technology Bulletin. 6 (1): 59–68. doi:10.1016/S1353-2561(99)00061-4.