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A colloid is a mixture in which one substance of microscopically dispersed insoluble particles are suspended throughout another substance. However, some definitions specify that the particles must be dispersed in a liquid, and others extend the definition to include substances like aerosols and gels. The term colloidal suspension refers unambiguously to the overall mixture (although a narrower sense of the word ''suspension'' is distinguished from colloids by larger particle size). A colloid has a dispersed phase (the suspended particles) and a continuous phase (the medium of suspension). The dispersed phase particles have a diameter of approximately 1 nanometre to 1 micrometre. Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color. Colloidal suspensions are the subject of interface and colloid science. This field of study was introduced in 1845 by Italian chemist Francesco Selmi and further investigated since 1861 by Scottish scientist Thomas Graham.

Classification

Colloids can be classified as follows: Homogeneous mixtures with a dispersed phase in this size range may be called ''colloidal aerosols'', ''colloidal emulsions'', ''colloidal foams'', ''colloidal dispersions'', or ''hydrosols''.

Colloid compared with solution

A colloid has a dispersed phase and a continuous phase, whereas in a solution, the solute and solvent constitute only one phase. A solute in a solution are individual molecules or ions, whereas colloidal particles are bigger. For example, in a solution of salt in water, the sodium chloride (NaCl) crystal dissolves, and the Na+ and Cl- ions are surrounded by water molecules.  However, in a colloid such as milk, the colloidal particles are globules of fat, rather than individual fat molecules.

Interaction between 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 that are either permanent or induced. Even if the particles do not 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 (unless the refractive indexes of the dispersed and continuous phases are matched), is short-range, and is attractive. *Steric forces between polymer-covered surfaces or in solutions containing non-adsorbing polymer can modulate interparticle forces, producing an additional steric repulsive force (which is predominantly entropic in origin) or an attractive depletion force between them.

Sedimentation velocity

The Earth’s gravitational field acts upon colloidal particles. Therefore, if the colloidal particles are denser than the medium of suspension, they will sediment (fall to the bottom), or if they are less dense, they will cream (float to the top). Larger particles also have a greater tendency to sediment because they have smaller Brownian motion to counteract this movement. The sedimentation or creaming velocity is found by equating the Stokes drag force with the gravitational force: $m_Ag=6\pi \eta rv$ where $m_Ag$ is the Archimedean weight of the colloidal particles, $\eta$ is the viscosity of the suspension medium, $r$ is the radius of the colloidal particle, and $v$ is the sedimentation or creaming velocity. The mass of the colloidal particle is found using: $m_A =V\left(\rho_1 - \rho_2\right)$ where $V$ is the volume of the colloidal particle, calculated using the volume of a sphere $V = \frac\pi r^3$, and $\rho_1-\rho_2$ is the difference in mass density between the colloidal particle and the suspension medium. By rearranging, the sedimentation or creaming velocity is: $v = \frac$ There is an upper size-limit for the diameter of colloidal particles because particles larger than 1 μm tend to sediment, and thus the substance would no longer be considered a colloidal suspension. The colloidal particles are said to be in sedimentation equilibrium if the rate of sedimentation is equal to the rate of movement from Brownian motion.

Preparation

There are two principal ways to prepare colloids: * Dispersion of large particles or droplets to the colloidal dimensions by milling, spraying, or application of shear (e.g., shaking, mixing, or high shear mixing). * Condensation of small dissolved molecules into larger colloidal particles by precipitation, condensation, or redox reactions. Such processes are used in the preparation of colloidal silica or gold.

Stabilization (peptization)

The stability of a colloidal system is defined by particles remaining suspended in solution at equilibrium. Stability is hindered by aggregation and sedimentation phenomena, which are driven by the colloid's tendency to reduce surface energy. Reducing the interfacial tension will stabilize the colloidal system by reducing this driving force. Aggregation is due to the sum of the interaction forces between particles. If attractive forces (such as van der Waals forces) prevail over the repulsive ones (such as the electrostatic ones) particles aggregate in clusters. Electrostatic stabilization and steric stabilization are the two main mechanisms for stabilization against aggregation. * Electrostatic stabilization is based on the mutual repulsion of like electrical charges. In general, different phases have different charge affinities, so that an electrical 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 weak to overcome the electrostatic repulsion between charged layers of the dispersing phase. The electrostatic repulsion between suspended colloidal particles is most readily quantified in terms of the zeta potential, a measurable quantity describing electrical potential at the slipping plane in an electrical double layer. * Steric stabilization consists in covering the particles in polymers which prevents the particle to get close in the range of attractive forces. A combination of the two mechanisms is also possible (electrosteric stabilization). All the above-mentioned mechanisms for minimizing particle aggregation rely on the enhancement of the repulsive interaction forces. Electrostatic and steric stabilization do not directly address the sedimentation/floating problem. Particle sedimentation (and also floating, although this phenomenon is less common) arises from a difference in the density of the dispersed and of the continuous phase. The higher the difference in densities, the faster the particle settling. * The gel network stabilization represents the principal way to produce colloids stable to both aggregation and sedimentation. The method consists in adding to the colloidal suspension a polymer able to form a gel network and characterized by shear thinning properties. Examples of such substances are xanthan and guar gum. Particle settling is hindered by the stiffness of the polymeric matrix where particles are trapped. In addition, the long polymeric chains can provide a steric or electrosteric stabilization to dispersed particles. The rheological shear thinning properties find beneficial in the preparation of the suspensions and in their use, as the reduced viscosity at high shear rates facilitates deagglomeration, mixing and in general the flow of the suspensions.

Destabilisation

Unstable colloidal dispersions can form either ''flocs'' or ''aggregates'' as the particles assemble due to interparticle attractions. Flocs are loose and flexible conglomerates of the particles, whereas aggregates are compact and rigid entities. There are methods that distinguish between flocculation and aggregation, such as acoustic spectroscopy. Destabilization can be accomplished by 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 neutralise 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. Minor changes in pH can manifest in significant alteration to the zeta potential. When the magnitude of the zeta potential lies below a certain threshold, typically around ± 5mV, rapid coagulation or aggregation tends to occur. *Addition of a charged polymer flocculant. Polymer flocculants can bridge individual colloidal particles by attractive electrostatic interactions. For example, negatively charged colloidal silica or clay particles can be flocculated by the addition of a positively charged polymer. *Addition of non-adsorbed polymers called depletants that cause aggregation due to entropic effects. *Physical deformation of the particle (e.g., stretching) may increase the van der Waals forces more than stabilisation forces (such as electrostatic), resulting coagulation of colloids at certain orientations. 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 bentonite and 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.

Monitoring stability

Multiple light scattering coupled with vertical scanning is the most widely used technique to monitor the dispersion state of a product, hence identifying and quantifying destabilisation phenomena. It works on concentrated dispersions without dilution. When light is sent through the sample, it is backscattered by the particles / droplets. The backscattering intensity is directly proportional to the size and volume fraction of the dispersed phase. Therefore, local changes in concentration (''e.g.''Creaming and Sedimentation) and global changes in size (''e.g.'' flocculation, coalescence) are detected and monitored.

Accelerating methods for shelf life prediction

The kinetic process of destabilisation can be rather long (up to several months or even years for some products) and it is often required for the formulator to use further accelerating methods in order to reach reasonable development time for new product design. Thermal methods are the most commonly used and consists in increasing temperature to accelerate destabilisation (below critical temperatures of phase inversion or chemical degradation). Temperature affects not only the viscosity, but also interfacial tension in the case of non-ionic surfactants or more generally interactions forces inside the system. Storing a dispersion at high temperatures enables to simulate real life conditions for a product (e.g. tube of sunscreen cream in a car in the summer), but also to accelerate destabilisation processes up to 200 times. Mechanical acceleration including vibration, centrifugation and agitation are sometimes used. They subject the product to different forces that pushes the particles / droplets against one another, hence helping in the film drainage. However, some emulsions would never coalesce in normal gravity, while they do under artificial gravity. Moreover, segregation of different populations of particles have been highlighted when using centrifugation and vibration.

As a model system for atoms

In physics, colloids are an interesting model system for atoms. Micrometre-scale 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 used to model ideal gases can be applied to model the behavior of a hard sphere colloidal suspension. In addition, phase transitions in colloidal suspensions can be studied in real time using optical techniques, and are analogous to phase transitions in liquids. In many interesting cases optical fluidity is used to control colloid suspensions.

Crystals

A colloidal crystal is a highly ordered array of particles that can be formed over a very long range (typically on the order of a few millimeters to one centimeter) and that appear analogous to their atomic or molecular counterparts. One of the finest natural examples of this ordering phenomenon can be found in precious opal, in which brilliant regions of pure spectral color result from close-packed domains of amorphous colloidal spheres of silicon dioxide (or silica, SiO2). These spherical particles precipitate in highly siliceous pools in Australia and elsewhere, and form these highly ordered arrays after years of sedimentation and compression under hydrostatic and gravitational forces. The periodic arrays of submicrometre spherical particles provide similar arrays of interstitial voids, which act as a natural diffraction grating for visible light waves, particularly when the interstitial spacing is of the same order of magnitude as the incident lightwave. Thus, it has been known for many years that, due to repulsive Coulombic interactions, electrically charged macromolecules in an aqueous environment can exhibit long-range crystal-like correlations with interparticle separation distances, often being considerably greater than the individual particle diameter. In all of these cases in nature, the same brilliant iridescence (or play of colors) can be attributed to the diffraction and constructive interference of visible lightwaves that satisfy Bragg’s law, in a matter analogous to the scattering of X-rays in crystalline solids. The large number of experiments exploring the physics and chemistry of these so-called "colloidal crystals" has emerged as a result of the relatively simple methods that have evolved in the last 20 years for preparing synthetic monodisperse colloids (both polymer and mineral) and, through various mechanisms, implementing and preserving their long-range order formation.

In biology

Colloidal phase separation is an important organising principle for compartmentalisation of both the cytoplasm and nucleus of cells into biomolecular condensates, similar in importance to compartmentalisation via lipid bilayer membranes - a type of liquid crystal. The term biomolecular condensate has been used to refer to clusters of macromolecules that arise via liquid-liquid or liquid-solid phase separation within cells. Macromolecular crowding strongly enhances colloidal phase separation and formation of biomolecular condensates.

In the environment

Colloidal particles can also serve as transport vector of diverse contaminants in the surface water (sea water, lakes, rivers, fresh water bodies) and in underground water circulating in fissured rocks (e.g. limestone, sandstone, granite). Radionuclides and heavy metals easily sorb onto colloids suspended in water. Various types of colloids are recognised: inorganic colloids (e.g. clay particles, silicates, iron oxy-hydroxides), organic colloids (humic and fulvic substances). When heavy metals or radionuclides form their own pure colloids, the term "''eigencolloid''" is used to designate pure phases, i.e., pure Tc(OH)4, U(OH)4, or Am(OH)3. Colloids have been suspected for the long-range transport of plutonium on the Nevada Nuclear Test Site. They have been the subject of detailed studies for many years. However, the mobility of inorganic colloids is very low in compacted bentonites and in deep clay formations because of the process of ultrafiltration occurring in dense clay membrane. The question is less clear for small organic colloids often mixed in porewater with truly dissolved organic molecules. In soil science, the colloidal fraction in soils consists of tiny clay and humus particles that are less than 1μm in diameter and carry either positive and/or negative electrostatic charges that vary depending on the chemical conditions of the soil sample, i.e. soil pH.

Intravenous therapy

Colloid solutions 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 should theoretically preferentially increase the intravascular volume, whereas other types of volume expanders called crystalloids also increase the interstitial volume and intracellular volume. However, there is still controversy to the actual difference in efficacy by this difference, and much of the research related to this use of colloids is based on fraudulent research by Joachim Boldt. Another difference is that crystalloids generally are much cheaper than colloids.

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