LECTURE 04.

LECTURE 04. GENERAL CHARACTERISTICS OF DISPERSED nSYSTEMS

A dispersion is a system in which particles nare dispersed in a continuous phase of a different composition (or state). A ndispersion is classified in a number of different ways, including how large the nparticles are in relation to the particles of the continuous phase, whether or nnot precipitation occurs, and the presence of Browniamotion.

There are three main types of dispersions: Coarse dispersion (Suspension), Colloid and nsollution

Types of ndispersions

n

Dissolved or dispersed phase

Continuous medium

Solution: Homogeneous mixture: Dissolved phase < 1 nanometer

Colloid: Dispersed phase between 1 nanometer and 1 micrometer

Coarse dispersion (Suspension): Heterogeneous mixture: Dispersed phase > 1 micrometer

Structure and Properties of nDispersions

It is still common belief, that dispersions nbasically do not display any structure, i.e., the particles (or in case of nemulsions: droplets) dispersed in the liquid or solid matrix (the n”dispersion medium”) are assumed to be statistically distributed. nTherefore, for dispersions usually percolation theory is assumed to appropriately describe their properties.

In chemistry, a suspension is a heterogeneous mixture ncontaining solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The ninternal phase (solid) is dispersed throughout the external phase (fluid) nthrough mechanical agitation, with nthe use of certain excipients or suspending agents. Unlike colloids, suspensions will eventually settle. An example of a nsuspension would be sand in water. The suspended particles are visible under a nmicroscope and will settle over time if left undisturbed. This distinguishes a nsuspension from a colloid, iwhich the suspended particles are smaller and do not settle. Colloids and nsuspensions are different from solutions, in which the dissolved substance (solute) does not nexist as a solid, and solvent and solute are homogeneously mixed.

A suspension of liquid droplets or fine solid nparticles in a gas is called an aerosol or particulate. In the atmosphere these nconsist of fine dust and soot particles, sea salt, biogenic and volcanogenic sulfates, nitrates, and cloud droplets.

Suspensions are classified on the basis of the dispersed nphase and the dispersiomedium, where the former is essentially nsolid while the latter may either be a solid, a liquid, or a gas. In moderchemical process industries, high shear nmixing technology has been used to create nmany novel suspensions.

COLLOID

A colloid nis a substance microscopically dispersed throughout another substance.

A ncolloidal system consists of two separate phases: a dispersed phase (or internal nphase) and a continuous phase (or dispersion medium) in which nthe colloid is dispersed. A colloidal system may be solid, liquid, or gas.

The ndispersed-phase particles have a diameter of between approximately 1 and 1000 nanometers. Such particles are normally ninvisible in an optical microscope, though their presence cabe confirmed with the use of an ultramicroscope or an electron microscope. Homogeneous nmixtures with a dispersed phase in this size range may be called colloidal naerosols, colloidal emulsions, colloidal foams, colloidal ndispersions, or hydrosols. The dispersed-phase particles or droplets nare affected largely by the surface chemistry present in the colloid.

Some ncolloids are translucent because of the Tyndall effect, which is the scattering of nlight by particles in the colloid. Other colloids may be opaque or have a nslight color.

Colloidal nsolutions (also called colloidal suspensions) are the subject of interface and colloid science. This field of study was nintroduced in 1861 by Scottish scientist Thomas Graham.

Classification

Because nthe size of the dispersed phase may be difficult to measure, and because ncolloids have the appearance of solutions, colloids are sometimes nidentified and characterized by their physico-chemical and transport nproperties. For example, if a colloid consists of a solid phase dispersed in a nliquid, the solid particles will not diffuse through a membrane, whereas with a true solution the ndissolved ions or molecules will diffuse through a membrane. Because of the nsize exclusion, the colloidal particles are unable to pass through the pores of nan ultrafiltration membrane with a size smaller than their own dimension. The nsmaller the size of the pore of the ultrafiltration membrane, the lower the nconcentration of the dispersed colloidal particules remaining in the nultrafiltered liquid. The exact value of the concentration of a truly dissolved nspecies will thus depend on the experimental conditions applied to separate it nfrom the colloidal particles also dispersed in the liquid. This is, a.o., nparticularly important for solubility studies of readily hydrolysed species such as Al, Eu, Am, nCm, … or organic matter complexing these species. Colloids cabe classified as follows:

Hydrocolloids

A hydrocolloid nis defined as a colloid system wherein the colloid particles are hydrophilic npolymers dispersed in water. A hydrocolloid has colloid nparticles spread throughout water, and depending on the quantity of water navailable that can take place in different states, e.g., gel nor sol (liquid). Hydrocolloids can be either irreversible (single-state) or reversible. For example, agar, a reversible hydrocolloid of seaweed extract, can exist in a gel nand solid state, and alternate between states with the addition or eliminatioof heat.

Many nhydrocolloids are derived from natural sources. For example, agar-agar and carrageenan are extracted from seaweed, gelatin is produced by hydrolysis of nproteins of bovine and fish origins, and pectin is extracted from citrus peel and apple pomace.

Gelatin desserts like jelly or Jell-O are made from gelatipowder, another effective hydrocolloid. Hydrocolloids are employed in food nmainly to influence texture or viscosity (e.g., a sauce). nHydrocolloid-based medical dressings are used for skin and wound treatment.

Other nmain hydrocolloids are xanthan gum, gum arabic, guar gum, locust bean gum, cellulose derivatives as carboxymethyl cellulose, alginate and starch.

Interaction between particles

The nfollowing forces play an important role in the interaction of colloid nparticles:

·           nExcluded volume repulsion: This refers nto the impossibility of any overlap between hard particles.

·           nElectrostatic interaction: Colloidal nparticles often carry an electrical charge and therefore attract or repel each nother. The charge of both the continuous and the dispersed nphase, as well as the mobility of the phases are factors affecting this ninteraction.

·           nvan der Waals forces: nThis is due to interaction between two dipoles that are either permanent or ninduced. Even if the particles do not have a permanent dipole, fluctuations of nthe electron density gives rise to a temporary dipole in a particle. This ntemporary dipole induces a dipole in particles nearby. The temporary dipole and nthe induced dipoles are then attracted to each other. This is known as van der nWaals force, and is always present (unless the refractive indexes of the ndispersed and continuous phases are matched), is short-range, and is nattractive.

·           nEntropic forces: According nto the second law of thermodynamics, a system progresses to a state in which nentropy is maximized. This can result in effective forces even between hard nspheres.

·           nSteric forces betweepolymer-covered surfaces or in solutions containing non-adsorbing polymer camodulate interparticle forces, producing an additional steric repulsive force n(which is predominantly entropic in origin) or an attractive depletion force nbetween them. Such an effect is specifically searched for with tailor-made superplasticizers developed to nincrease the workability of concrete and to reduce its water content.

Preparation

There nare two principal ways of preparation of colloids:

·           nDispersion of large particles or ndroplets to the colloidal dimensions by milling, spraying, or application of shear n(e.g., shaking, mixing, or high shear mixing).

·           nCondensation of small dissolved molecules ninto larger colloidal particles by precipitation, condensation, or redox nreactions. Such processes are used in the preparation of colloidal silica or gold.

Stabilization (peptization)

The nstability of a colloidal system is the capability of the system to remain as it nis.

Stability nis hindered by aggregation and by sedimentation phenomena, that determine phase nseparation.

Examples of a stable and of an unstable colloidal ndispersion.

Aggregatiois due to the sum of the interaction forces between particles. If attractive nforces (such as van der Waals forces) prevail over the repulsive ones (such as nthe electrostatic ones) particles aggregate in clusters.

Electrostatic nstabilization and steric stabilization are the two main mechanisms for nstabilization against aggregation.

·                       nElectrostatic stabilization nis based on the mutual repulsion of like electrical charges. In general, ndifferent phases have different charge affinities, so that an electrical double nlayer forms at any interface. Small particle sizes lead to enormous surface nareas, and this effect is greatly amplified in colloids. In a stable colloid, nmass of a dispersed phase is so low that its buoyancy or kinetic energy is too nweak to overcome the electrostatic repulsion between charged layers of the ndispersing phase.

·                       nSteric stabilization nconsists in covering the particles in polymers which prevents the particle to nget close in the range of attractive forces.

A ncombination of the two mechanisms is also possible (electrosteric nstabilization). All the above mentioned mechanisms for minimizing particle aggregation rely on the enhancement of the nrepulsive interaction forces.

Electrostatic nand steric stabilization do not directly address the sedimentation/floating nproblem.

Particle nsedimentation (and also floating, although this phenomenon is less common) narises from a difference in the density of the dispersed and of the continuous nphase. The higher the difference in densities, the faster the particle nsettling.

·           nThe gel network stabilization nrepresents the principal way to produce colloids stable to both aggregation and nsedimentation.

The nmethod consists in adding to the colloidal suspension a green biopolymer able nto form a gel network and characterized by shear thinning properties. Examples nof such substances are xanthan and guar gum.

Steric nand Gel network stabilization.

Particle nsettling is hindered by the stiffness of the polymeric matrix where particles nare trapped. In addition, the long polymeric chains can provide a steric or nelectrosteric stabilization to dispersed particles.

The nrheological shear thinning properties find beneficial in the preparation of the nsuspensions and in their use, as the reduced viscosity at high shear rates nfacilitates deagglomeration, mixing and in general the flow of the suspensions.

Destabilization (flocculation)

Unstable ncolloidal dispersions form flocs as the particles aggregate ndue to interparticle attractions. In this way photonic glasses can be grown. nThis can be accomplished by a number of different methods:

·           nRemoval of the electrostatic barrier nthat prevents aggregation of the particles. This can be accomplished by the naddition of salt to a suspension or changing the pH of a suspension to neffectively neutralize or “screen” the surface charge of the nparticles in suspension. This removes the repulsive forces that keep colloidal nparticles separate and allows for coagulation due to van der Waals forces.

·           nAddition of a charged polymer nflocculant. Polymer flocculants can bridge individual colloidal particles by nattractive electrostatic interactions. For example, negatively-charged ncolloidal silica or clay particles can be flocculated by the addition of a npositively-charged polymer.

·           nAddition of non-adsorbed polymers ncalled depletants that cause aggregation due to entropic effects.

·           nPhysical deformation of the particle n(e.g., stretching) may increase the van der Waals forces more thastabilization forces (such as electrostatic), resulting coagulation of colloids nat certain orientations.

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

Monitoring nstability

Measurement principle of multiple light scattering coupled with vertical nscanning

Numerous experimental techniques have beedeveloped to study particle aggregation. Most frequently used are time-resolved noptical techniques that are based on transmittance or scattering of light.

Light Transmission. The variation of transmitted light through an aggregating suspension cabe studied with a regular spectrophotometer in the visible region. As aggregation proceeds, the nmedium becomes more turbid, and its absorbance increases. The increase of the absorbance can be nrelated to the aggregation rate constant k and the stability ratio cabe estimated from such measurements. The advantage of this techniques is its nsimplicity, but its disadvantage is that it can be only reliably used for nlarger particles or that detailed corrections due to the presence of larger nclusters must be considered. Small particles aggregate rapidly, and in such nsystems it is normally difficult to extract the stability ratio from the ntransmittance quantitatively.

Light scattering. These techniques are based on probing the scattered light from aaggregating suspension in a time-resolved fashion. Static light scattering yields the change nin the scattering intensity, while dynamic light scattering the variation in the apparent hydrodynamic radius. At nearly-stages of aggregation, the variation of each of these quantities is ndirectly proportional to the aggregation rate constant k. At later nstages, one can obtain information on the clusters formed (e.g., fractal ndimension). Light scattering works well for a wide range of particle sizes. nMultiple scattering effects may have to be considered, since scattering becomes nincreasingly important for larger particles or larger aggregates. Such effects ncan be neglected in weakly turbid suspensions. Aggregation processes istrongly scattering systems have been studied with backscattering ntechniques or diffusing-wave spectroscopy.

Probing aggregation of a setting colloidal nsuspension with light scattering coupled with vertical scanning

Single particle counting. This technique offers excellent resolution, whereby clusters made out of ntenths of particles can be resolved individually. The aggregating suspension is nforced through a narrow capillary particle ncounter and the size of each aggregate nis being analyzed by light scattering. From the scattering intensity, one cadeduce the size of each aggregate, and construct a detailed aggregate size ndistribution. If the suspensions contain high amounts of salt, one could nequally use a Coulter counter. As time proceeds, the size distribution shifts towards larger aggregates, nand from this variation aggregation and breakup rates involving different nclusters can be deduced. The disadvantage of the technique is that the naggregates are forced through a narrow capillary under high shear, and the naggregates may disrupt under these conditions.

Indirect Techniques. As many properties of colloidal suspensions depend on the state of naggregation of the suspended particles, various indirect techniques have beeused to monitor particle aggregation too. While it can be difficult to obtaiquantitative information on aggregation rates or cluster properties from such nexperiments, they can be most valuable for practical applications. Among these ntechniques settling tests nare most relevant. When one inspects a series of test tubes with suspensions nprepared at different concentration of the flocculant, stable suspensions ofteremain dispersed, while the unstable ones settle. Automated instruments based non light scattering to monitor suspension settling have been developed, and nthey can be used to probe particle aggregation. The scheme of such instrument nis shown in the animated figure on the right. One must realize, however, that nthese techniques may not always reflect the actual aggregation state of a nsuspension correctly. For example, larger primary particles may settle even ithe absence of aggregation, or aggregates that have formed a colloidal gel will nremain in suspension. Other indirect techniques capable to monitor the state of naggregation include, for example, filtration, rheology, absorption of ultrasonic nwaves, or dielectric properties.

Relevance

Particle aggregation is a widespread phenomenon, nwhich spontaneously occurs iature but is also widely explored in manufacturing. nSome examples include.

Papermaking. Retention aids nare added to the pulp to accelerate paper formation. These aids are coagulating naids, which accelerate the aggregation between the cellulose fibers and filler nparticles. Frequently, cationic polyelectrolytes are being used for that npurpose.

Water Treatment. Treatment of municipal waste nwater normally includes a phase where fine solid particles are removed. This nseparation is achieved by addition of a flocculating or coagulating agent, nwhich induce the aggregation of the suspended solids. The aggregates are nnormally separated by sedimentation, leading to sewage sludge. Commonly used nflocculating agents in water treatment include multivalent metal ions (e.g., Fe³⁺ nor Al³⁺), polyelectrolytes, or both.

Cheese nMaking. The key step in cheese production is the separation of the milk into solid ncurds and liquid whey. This separation is achieved by inducing the aggregatioprocesses between casein micelles by acidifying the milk or adding rennet. The nacidificatioeutralizes the carboxylate groups on the micelles and induces nthe aggregation.

In fluid mechanics, the REYNOLDS NUMBER (RE) nis a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces and consequently nquantifies the relative importance of these two types of forces for given flow nconditions.

The nconcept was introduced by George Gabriel Stokes in 1851, but the Reynolds number is nnamed after Osborne Reynolds (1842–1912), who popularized nits use in 1883.

Reynolds nnumbers frequently arise when performing dimensional analysis of fluid dynamics problems, and as nsuch can be used to determine dynamic similitude between different experimental ncases.

They nare also used to characterize different flow regimes, such as laminar or turbulent flow: laminar flow occurs at low nReynolds numbers, where viscous forces are dominant, and is characterized by nsmooth, constant fluid motion; turbulent flow occurs at high Reynolds numbers nand is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow ninstabilities.

Colloid nsolutions used in intravenous therapy belong to a major group of volume expanders, and can be used for intravenous fluid replacement. Colloids preserve a high colloid osmotic pressure in the blood, and therefore, they nshould theoretically preferentially increase the intravascular volume, whereas other types of volume nexpanders called crystalloids also increases the interstitial volume and intracellular volume. However, there is still controversy nto the actual difference in efficacy by this difference. Another difference is nthat crystalloids generally are much cheaper than colloids. Recently, however, nit has been determined that the use of colloids was bolstered by faked research nstudies.

References:

 1.The abstract of the lecture.

2. intranet.tdmu.edu.ua/auth.php

3. Atkins P.W. Physical chemistry. – New York. – 1994. – P.299-307.

4. en.wikipedia.org/wiki

5.Girolami, G. S.; nRauchfuss, T. B. and Angelici, R. J., Synthesis and Technique in Inorganic Chemistry, University nScience Books: Mill Valley, CA, 1999

6.John nB.Russell. General chemistry. New nYork.1992. – P. 550-599

7. Lawrence D. Didona. Analytical chemistry. – n1992: New York. – P. 218 – 224.

8. www.youtube.com/watch?v=iEJSOhFaHos

9. www.youtube.com/watch?v=BsClg6z_PSw

10. www.youtube.com/watch?v=O3_hpvYlavA

11. www.youtube.com/watch?v=E9rHSLUr3PU

Optional:

·  D.M. nRuthven Principles of Adsorption & Adsorption Processes John Wiley & nSons, New York, 1984.

·  Yiacoumi, nS. and Tien, C. Kinetics of Metal Ion Adsorption from Aqueous Solutions: nModels, Algorithms, and Applications.  nKluwer Academic Publishers, Norwell, 1995.

·  Cohen, nY. and Peters, R.W. Novel Adsorbents and Their Environmental Applications
nAIChE, New York, 1995. 

·  Adsorptioand its applications in industry and environmental protection”, Vol. nI-Applications in Industry,1061 pages.

·  Adsorptioby powders and porous solids F. Rouquerol, J. Rouquerol and K Sing Academic nPress, 1999.

·  D.J. nShaw: Introduction to Colloid and Surface Chemistry, 3^rd Ed., nButterworths, London 1980, ISBN 0-408-71049-7

·  F. nMacRitchie: Chemistry at Interfaces, Academic Press, San Diego, 1990, ISBN n0-12-464785-5

·  nJ.N. Israelachvili: nIntermolecular and Surface Forces, 2^nd Ed., Academic Press, New nYork, 1991, ISBN 0-1237-5181-0

·  C.J. nvan Oss: Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994, nISBN 0-8274-9168-1

·  S. nHartland, R.W. Hartley: Axissymmetric Fluid-Liquid Interfaces, Elsevier, nAmsterdam, 1976, ISBN 0-4444-1396-0

·  K.L. nMittal (Ed): Adsorption at Interfaces, 5^th Ed., Wiley Interscience, nNew York, 1990, ISBN 0-8412-0249-4

·  D.K. nChattoraj, K.S. Birdi: Adsorption and the Gibbs Surface Excess, Plenum Press, nNew York, 1984, ISBN 0-306-41334-5

·  M.J. nRosen: Surfactants and Interfacial Phenomena, 2^nd Ed., John Wiley, nNew York, 1989, ISBN 0-471-83651-6

·  M.J. nRosen (Ed.): Structure performance relationships in surfactants, ACS, nWashington, DC, 1984, ISBN 0-8412-0839-5

·  S.A. nSafran: Statistical Thermodynamics of Surfaces, Interfaces, and Membranes, nAddison-Wesley, Reading MA, 1994, ISBN 0-201-62633-0

·  L.E. nSchramm: Dictionary of Colloid and Interface Science, 2^nd Ed., nWiley-Interscience, New York, 2001, ISBN 0-471-39406-8

·  Journal nof Colloid and Interface Science, Academic Press, New York, ISSN 0021-9797

·  Colloids nand Surfaces A – Physicochemical and engineering aspects, Elsevier, Amsterdam, nISSN 0927-7757

Prepared by nPhD Falfushynska H.