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| [[Polyelectrolytes]] are charged [[polymers]] capable of stabilizing (or destabilizing) [[colloid|colloidal emulsions]] through electrostatic interactions. Their effectiveness can be dependent on [[molecular mass|molecular weight]], [[pH]], solvent [[chemical polarity|polarity]], [[ionic strength]], and the [[hydrophilic-lipophilic balance]] (HLB). Stabilized [[emulsions]] are useful in many industrial processes, including [[flocculation|deflocculation]], drug delivery, petroleum waste treatment, and food technology.
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| ==Types of Polyelectrolytes==
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| Polyelectrolytes are made up of positively or negatively charged [[repeat unit]]s. The [[ion|charge]] on a polyelectrolyte depends on the different properties of the solution, such as the degree of dissociation of the monomer units, the solvent properties, salt concentration, [[pH]], and temperature.
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| Polymers become charged through the dissociation of the [[repeat unit|monomer side groups]]. If more monomer side groups are dissociated, the polymer has a higher charge. In turn, the [[ion|charge]] of the polymer classifies the polyelectrolyte, which can be positive (cationic) or negative (anionic).
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| The polymer charge and [[ionic strength]] of the polyelectrolyte in question dictate how thick a polyelectrolyte layer will be. The thickness of a polyelectrolyte then affects its [[adsorption]] ability.<ref name=Saleh>{{cite journal|last=Saleh, N.; Sarbu, T.; Sirk, K.; Lowry, G. V.; Matyjaszewski, K. and Tilton, R. D. | title=Oil-in-Water Emulsions Stabilized by Highly Charged Polyelectrolyte-Grafted Silica Nanoparticles| journal=Langmuir|year=2005|volume=21|pages=9873–9878}}</ref> For more information on polyelectrolyte adsorption, look [[polyelectrolyte adsorption|here]].
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| Some examples of polyelectrolytes can be found in the table below. The properties of the polymers vary with molecular weight and degree of polymerization.<ref>The [[molecular mass|molar masses]] and [[degree of polymerization]] reported are specific examples of [[polyelectrolytes]] synthesized and reported in various studies.</ref>
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| {| class="wikitable" style="text-align:center"
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| ! Polyelectrolyte and Type !! Pk<sub>a</sub> of Monomer Unit (in water)!! Molar Mass (g/mol)<ref name=Kogej>{{cite journal|last=Kogej, K.| title=Association and structure formation in oppositely charged polyelectrolyte-surfactant mixtures| journal=Advances in Colloid and Interface Science|year=2010|volume=158|pages=68–83}}</ref> !! Degree of polymerization<ref name=Kogej />!! Structure
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| | PSS (anionic) || -0.53<ref>{{cite journal|last=Dong, H.; Du, H.; Wickramasinghe, S. R. and Qian, X.| title=The Effects of Chemical Substitution and Polymerization on the pK<sub>a</sub> Values of Sulfonic Acids| journal=J. Phys. Chem.|year=2009|volume=113|pages=14094–14101}}</ref> || 70,000 || 340 || [[File:PSS.png|thumb|75x75px|PSS]]
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| | PAA (anionic)|| 4.35<ref>{{cite journal|last=Dippy, J. F. J.Hughes, S. R. C. and Rozanzki, A.| title=The dissociation constants of some symmetrically disubstituted succinic acids| journal=J. Am. Soc.|year=1959|pages=2492}}</ref> || 10,000|| 140 || [[File:PAA.png|thumb|75x75px|PAA]]
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| | APMA (cationic) || 5.0<ref>{{cite journal|last=Nayak, S. P.| title=Design, Synthesis and Characterization of Multiresponsive Microgels| journal=Thesis, Georgia Institute of Technology|year=2004}}</ref> || 131,000 || 1528 || [[File:APMA.png|75x75px|thumb|APMA]]
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| | PEA (cationic) || 1.2<ref>{{cite journal|last=Unerberg, W. J. M. and Lingeman, H.| title=Determination of pK<sub>a</sub> Values of Some Prototropic Function in Mitomycin and Porfiromycin| journal=J. Pharm. Sciences|year=1983|volume=72|pages=553–556}}</ref> || 3600|| 36 || [[File:PEA.png|thumb|75x75px|PEA]]
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| | Poly-L-arginine (cationic) || 9.0<ref>{{cite book | author=Van Holde, K. E. and Mathews, C. K. | title=Biochemistry| year=1990| publisher=Benjamin-Cummings| isbn=978-0-805-33931-4}}</ref> || 15,000-70,000<ref name=Cha>{{cite journal|last=Cha, J. N.; Birkedal, H.; Euliss, L. E.; Bartl, M. H.; Wong, M. S.; Deming, T. J. and Stucky, G. D.| title=Spontaneous Formation of Nanoparticle Vesicles from Homopolymer Polyelectrolytes| journal=J. Am. Chem. Soc.|year=2003|volume=125|pages=8285–8289}}</ref> || 96-450<ref name=Cha /> || [[File:IPMA.png|thumb|75x75px|Poly-L-arginine]]
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| |}
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| ==Types of Emulsions==
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| The two main types of [[emulsions]] are oil-in-water ([[chemical polarity|nonpolar in polar]]) and water-in-oil ([[chemical polarity|polar in nonpolar]]). The difference depends upon the nature of the [[surfactant]] or [[polyelectrolyte]] in question. The [[hydrophile|hydrophilic]] pieces will attract the polar solvent, creating a water-in-oil emulsion and the [[hydrophobe|hydrophobic]] pieces will attract the nonpolar solvent, creating an oil-in-water emulsion.
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| ==Emulsion Stability==
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| [[File:Emulsion stability.png|thumb|Schematic representation of flocculation mechanism in SDS-stabilized (top) and surfactant-free (bottom) emulsions.]]
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| When there is less [[surface tension|interfacial tension]] between the [[polyelectrolyte|polyelectrolyte particles]] and the [[emulsions]] in question, emulsions are less stable. This is because the polyelectrolyte particles penetrate the [[flocculation|flocs]] in [[suspension (chemistry)|suspension]] less when there is less [[surface tension|interfacial tension]].<ref name=Saleh />
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| [[Polyelectrolytes]] [[adsorption|adsorb]] to the interface the [[emulsion]] and help stabilize it, but may or may not lower the [[surface tension|interfacial tension]]. This means that the oil or water droplets will not [[Coalescence (chemistry)|coalesce]].
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| On their own, [[hydrophobe|hydrophobic]] surfactants cannot stabilize an [[emulsion]]. Although they are attracted to oil, and an oil-in-water emulsion forms, the emulsion will not stay stable for long and will eventually coalesce.<ref>{{cite journal|last=Stamkulov, N. S.; Mussabekov, K. B.; Aidarova, S. B. and Luckham, P. F.|title=Stabilisation of emulsions by using a combination of an oil soluble ionic surfactant and water soluble polyelectrolytes. I: Emulsion stabilisation and Interfacial tension measurements| journal=Elsevier|year=2008|volume=335|pages=103–106}}</ref> With the addition of a polyelectrolyte, electrostatic forces between the oil and water interface are formed and the surfactant begins to act as an “anchor” for the polyelectrolyte, stabilizing the emulsion. In addition to surfactants, nanoparticles can also help stabilize the emulsion by also providing a charged interface for the polyelectrolyte to adsorb on.<ref name=Saleh />
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| ===Molecular Weight Effects===
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| The [[dispersion stability|stability of the emulsion]] can depend on the [[molecular mass|molecular weight]] of the accompanying polyelectrolyte. Polyelectrolytes of a high molecular weight are the most effective at stabilization. This is because they form a substantial steric barrier between oil and water, inhibiting [[particle aggregation|aggregation]]. However, if the polyelectrolyte is too heavy it will not dissolve in the solution. Instead it will form gel lumps and fail to stabilize the emulsion.<ref>{{cite journal|last=Wang, Y.; Kimura, K. and Dubin, P. L.|title=Polyelectrolyte-Micelle Coacervation: Effects of Micelle Surface Charge Density, Polymer Molecular Weight, and Polymer/Surfactant Ratio| journal=Macromolecules|year=2000|volume=3|pages=3324–3331}}</ref>
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| ===pH Effects===
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| The effect of [[pH]] on the [[dispersion stability|stability]] of [[polyelectrolytes]] is based upon the [[functional group]] on the polymer [[backbone chain|backbone]] that is bearing the charge. A protonated [[amine]], for instance, will be much more stable at a lower pH while a [[sulfonate|sulfonate group]] will be more stable at a higher pH.
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| ===Solvent Effects===
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| [[Polyelectrolytes]] will be much more soluble in polar solvents due to the charge on the polymer backbone and will spread out more. In nonpolar solvents, polyelectrolytes will coil becoming more densely packed and, if the backbone is nonpolar, will put the charge on the inside of the packed structure.<ref name=Stokes>{{cite book | author=Stokes, R. J. and Evans, D. F. | title=Fundamentals of Interfacial Engineering| year=1996| publisher=Wiley-VCH| isbn=978-0-471-18647-2}}</ref>
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| ===Ionic Strength===
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| [[File:Effects of Salt Condition on Polyelectrolytes.png|thumb|The effects of salt concentration on polyelectrolytes structure.]]
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| [[Ionic strength]] plays a crucial role in [[dispersion stability|stability]]. In water-in-oil emulsions, as well as many others, the dielectric constant of the solvent is so low that the [[Coulombs Law|electrostatic forces]] between particles are not strong enough to have an effect on emulsion stability. Thus, [[dispersion stability|emulsion stability]] depends greatly on the [[polyelectrolyte|polyelectrolyte film thickness]].<ref name=Steitz>{{cite journal|last=Steitz, R.; Jaeger, W.; and Klitzing, R. V.|title=Influence of Charge Density and Ionic Strength on the Multilayer Formation of Strong Polyelectrolytes| journal=Langmuir|year=2001|volume=17|pages=4471–4474}}</ref>
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| The [[polyelectrolyte|polyelectrolyte film thickness]] is dependent upon its [[ionic strength]].<ref name=Steitz /> charged species on polyelectrolyte chains repel each other, causing the chains to stretch out. As the salt concentration increases, ionic strength increases, and the ions will shield the charges on the polymer chain allowing the polymer chain to form a dense random coil.<ref>{{cite journal|last=Wang, Y.; Kimura, K,; Huang, Q. and Dubin, P. L.|title=Effects of Salt on Polyelectrolyte-Micelle Coacervation| journal=Macromolecules|year=1999|volume=32|pages=7128–7134}}</ref>
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| ==Theory==
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| ===Electrostatic Stabilization===
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| [[Coulombs Law|Electrostatic repulsive forces]] dominate in polyelectrolyte stabilized emulsions.,<ref name=Saleh /><ref name=Fleer>{{cite book | author=Fleer, G. J.; Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T. and Vincent, B. | title=Polymers at Interfaces.| year=1993| publisher=Chapman & Hall| isbn=978-0-412-58160-1}}</ref> Although there are [[steric effects|steric interactions]], they are negligible in comparison. As the [[concentration]] of polyelectrolyte increases, repulsive forces increase. When there are more [[polyelectrolyte]] molecules, the distance between individual particles decreases. As the distance <math>h</math> decreases, the exponential term becomes greater. Consequently, the repulsion energy also increases.
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| [[File:Electrostatic repulsive force vs distance for polyelectrolytes fixed.png|thumb|This graph illustrates eq. 1. It can be seen that as the surface to surface particle distance decreases, the electrostatic repulsive force decreases exponentially.<ref>Adapted from {{cite journal|last=Philip, J.; Mondain-Monval, O.; Calderon, F. L. and Bibette, J.|title=Colloidal Force Measurements in the Presence of a Polyelectrolyte|journal=Journal of Physics D: Applied Physics|year=1997|volume=30|pages=2798–2803}}</ref>]]
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| The general equation for repulsion energy assuming spherical particles (eq. 1):
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| <br /> | |
| <math>V = \frac{64\pi RC k_BT \Gamma e^{-Kh}}{K^2}, </math> | |
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| where <br />
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| :<math>R</math> = particle radius
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| :<math>C</math> = bulk concentration of ions
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| :<math>k_B</math> = Boltzmann constant
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| :<math>\Gamma</math> = reduced surface potential
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| :<math>h</math> = the surface to surface distance of the spherical particles
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| :<math>T</math> = the temperature in Kelvin
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| :<math>K</math> = the Debye length<br />.
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| In addition, [[pH]] and [[ionic strength]] have a great influence on [[Coulombs Law|electrostatic interactions]] because these affect the "magnitude of electrical charge" in solution.<ref name=Guzey>{{cite journal|last=Guzey, D. and McClements, J.|title=Impact of Electrostatic Interactions of Formation and Stability of Emulsions Containing Oil Droplets Coated by β-Lactoglobulin-Pectin-Complexes|journal=Journal of Agricultural and Food Chemistry|year=2007|volume=55|pages=475–485}}</ref> As can be seen from the above equation, the repulsion energy depends on the square of the [[Debye length]]. From the equation for the [[Debye length]], it is demonstrated how ionic strength can ultimately affect the electrostatic interactions in a solution.
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| ===Bjerrum length===
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| Naturally, the question of the distance at which these [[Coulombs Law|electrostatic interactions]] become important arises. This can be discussed using the [[Bjerrum length]]. The Bjerrum length is the distance at which the electrostatic interaction between two charges is comparable to the [[thermal energy]], <math>k_BT</math>. The distance is given by eq. 2:
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| <math>\lambda_B = \frac{e^2}{4\pi \varepsilon_r\varepsilon_0\ k_B T},</math>
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| where<br />
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| :<math>e</math> = elementary charge
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| :<math>\varepsilon_0</math> = vacuum permittivity
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| :<math>\varepsilon_r</math> = relative dielectric constant.
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| ===Surface Charge Density===
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| The factors discussed above can influence the charge on the surface of the polyelectrolyte. The [[surface charge density]] of these surfaces, at low surface potentials, can be modeled using a simplified version of the Grahame equation (eq. 3):
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| <math>\sigma={\varepsilon_r\varepsilon_0\phi_0}{K},</math> | |
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| where<br />
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| :<math>\phi_0</math> = surface potential.
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| Examples of polymers and their surface charge densities can be found in the table below.
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| {| class="wikitable" style="text-align:center"
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| ! Polymer !! Surface Charge Density <math>(\frac{C}{m^2})</math>!! Structure
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| | Latex || -0.06<ref>{{cite journal|last=Gessner, A.; Lieske, A.; Paulke, B. R. and Müller, R. H.| title=Influence of surface charge density on protein adsorption on polymeric nanoparticles: analysis by two-dimensional electrophoresis| journal=European Journal of Pharmaceutics and Biopharmaceutics|year=2002|volume=54|pages=165–170}}</ref> || [[File:Latex.png|thumb|75x75px|Latex]]
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| | Pectin || -0.011<ref name=Guzey /> ||[[File:Pectin.png|thumb|75x75px|Pectin]]
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| | PAA (0.1% dwb in ZrO<sub>2</sub>) || -0.088<ref>{{cite journal|last=Leong, Y. K.; Scales, P. J.; Healy, T. W. and Boger, D. V.| title=Interparticle forces arising from adsorbed polyelectrolytes in colloidal suspensions| journal=Colloids and Surfaces A: Physicochem. Eng. Aspects|year=1995|volume=95|pages=43–52}}</ref> || [[File:PAA.png|thumb|75x75px|PAA]]
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| |}
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| ==Applications==
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| ===Deflocculation===
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| Depending on the situation, [[polyelectrolytes]] can function as either [[flocculation|flocculants or deflocculants]]. In order to [[dispersion stability|stabilize emulsion]], deflocculant polyelectrolytes are required.
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| When repulsive forces between particles overcome the [[intermolecular force]]s in solution and the loose flocculated aggregates separate, deflocculation occurs. As opposed to the loose and easily separated sediments formed in flocculation, sediments formed in deflocculation are tightly packed and difficult to redisperse.
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| The repelling forces in a deflocculation increase the [[zeta potential]], which in turn reduces the [[viscosity]] of the [[suspension (chemistry)|suspension]]. Because of this reduction in viscosity, deflocculants are sometimes referred to as “thinning agents”. These thinning agents are usually [[alkalinity|alkaline]] and raise the [[pH]] of the [[suspension (chemistry)|suspension]], preventing [[flocculation]]. Deflocculants are used as thinning agents in molding plastics, making glassware, and creating clay ceramics.<ref name=Evans>{{cite book | author=Evans, D. F. and Wennerström, H. | title=The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet.| year=1999| publisher=Wiley-VCH| isbn=978-0-471-24247-5}}</ref>
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| ===Petroleum Waste Treatment===
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| [[Polyelectrolytes]] can also act as [[flocculation|flocculants]], separating solids (flakes) and liquids in industrial processes such as [[solubilization]] and oil recovery and they usually have a large cationic [[charge density]].
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| Using [[organic compounds|organic materials]] to refine [[petroleum]] instead of [[iron]] or [[aluminum]] coagulated would greatly decrease that amount of inorganic waste produced.<ref>{{cite journal|last=Luthy, Richard G; Selleck, Robert E; and Galloway, Terry R|title=Surface Properties of Petroleum Refinery Waste Oil Emulsions|journal=Environmental Science and Technology|year=1977|volume=11|issue=13|pages=1211–1217}}</ref> The waste consists of stable oil-in-water emulsions. The addition of various polyelectrolytes to petroleum waste can cause the oil to coagulate, which will make it easier to remove and dispose of, and does not significantly decrease the stability of the solution.
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| ===Drug Delivery===
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| [[File:Ester cleavage.png|thumb|Hydrolysis of an ester bond on poly([[HPMA]]-[[DMAE]]).]]
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| [[Polyelectrolyte]] stabilized emulsions are important in the field of [[nanomedicine]]. In order to function properly, any drug delivery system must be [[biocompatibility|biocompatible]] and [[Biodegradation|biodegradable]]. Polyelectrolytes such as dextran sulfate (DSS), protamine (PRM) or poly-L-arginine all fulfill these requirements and may be used as a capsule with an [[emulsion]] inside.<ref name="Cingolani, R. 2010 419">{{cite journal|last=Cingolani, R.|title=Imatinib-loaded polyelectrolyte microcapsules for sustained targeting of BCR-ABL.sup.+ leukemia stem cells|journal=Nanomedicine|year=2010|volume=5|issue=3|pages=419}}</ref>
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| Oil in water emulsions are currently used as safe [[solvents]] for [[vaccine]]s.<ref>{{cite journal|last=Fox, C.|title=Immunomodulatory and Physical Effects of Oil Composition in Vaccine Adjuvant Emulsions|journal=Vaccine|year= 2011|volume=29|issue=1|pages=9563–9572}}</ref> It is important that these emulsion are [[dispersion stability|stable]] and remain so for long periods of time. Polyelectrolyte stabilized emulsions could be used to increase the shelf life of vaccines. Researchers have been able to develop polyelectrolyte emulsions with more than six month stability.<ref name=Saleh />
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| In addition to being stable for extended periods of time, polyelectrolytes may be useful for vaccines because they can be [[Biodegradation|biodegradable]]. For example, the [[ester|ester bonds]] of the polyelectrolyte poly([[HPMA]]-[[DMAE]]) can undergo [[hydrolysis]] in the human body and [[vero cell|VERO cells]] envelope DSS and use poly-L-arginine to break them down.<ref>{{cite journal|last=De Geest, B. G.; De Koker, S.; Sukhorukov, G. B.; Kreft, O.; Parak, W.; Skkirtach, A.; Demeester, J.; De Smedt, S. and Hennink, W.|title=Polyelectrolyte Microcapsules for Biomedical Applications|journal=Soft Matter|year=2009|volume=5|issue=2|pages=282–291}}</ref> Once the polylelectroyte capsule has been degraded, the emulsion containing drug is released into the body. Researchers have been investigating this drug delivery method to target leukemia cells.<ref name="Cingolani, R. 2010 419"/>
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| ===Food Technology===
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| Because polyelectrolytes may be [[biocompatibility|biocompatible]], it follows that they can be used to [[dispersion stability|stabilize emulsion]] in foods. Several studies have focused on using [[polyelectrolytes]] to induce mixing of [[proteins]] and [[polysaccharides]] in oil-in-water emulsions. DSS has been successfully used to stabilize these types of emulsions.<ref>{{cite journal|last=Antonov, Y.A. and Moldenaers, P.|title=Strong Polyelectrolyte - Induced Mixing in Concentrated Aqueous Emulsions|journal=Food Hydrocolloids|year=2012|volume=28|issue =1|pages=213–223}}</ref> Other studies have focused on stabilizing oil-in-water emsulsions using β-lactoglobulin (β-Lg), a globular protein, and pectin, an anionic polysaccharide. Both β-lactoglobulin and pectin are common ingredients in the food industry. β-lactoglobulin is used in whey protein, which can act as an emulsifier.<ref name=Guzey />
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| ==References==
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| {{Reflist}}
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| [[Category:Chemical processes]]
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