Colloid

Beer foam is a colloidal system.

In physics and chemistry, a colloid, colloidal system, colloidal suspension or colloidal dispersion is a system made up of by two or more phases, normally one fluid (liquid or gas) and the other dispersed in the form of generally very fine solid particles, with a diameter between 10^-9 and 10^-5 m. The dispersed phase is the one found in the smallest proportion. Normally the continuous phase is liquid, but colloids can be found whose components are in other states of aggregation of matter.

The name colloid comes from the Greek root "kolas" which means "stickable." This name refers to one of the main properties of colloids: their spontaneous tendency to aggregate or form clots. This is also where the word "glue" comes from, the pasty fluid that is used to glue. Colloids also affect the boiling point of water and are contaminants. Colloids differ from chemical suspensions mainly in the size of the dispersed phase particles. Particles in colloids are not directly visible, they are visible at the microscopic level (between 1 nm and 1 µm), and in chemical suspensions they are visible at the macroscopic level (greater than 1 µm). Also, on standing, the phases of a chemical suspension separate, while those of a colloid do not. The chemical suspension is filterable, while the colloid is not filterable.

Colloidal systems are non-homogeneous systems in which the constituent particles of one or several of its components (dispersed or dispersoid phase) have sizes between 10 and 2000 Å, while the remaining components are made up of smaller particles. at about 10 Å (dispersing phase or scattering medium).

Colloid particles have intermediate properties between chemical solutions and suspensions; they are dispersed without being attached to the solvent molecules and do not settle on standing.

In some cases the particles are large molecules, such as proteins. In the aqueous phase, a molecule folds in such a way that its hydrophilic part is on the outside, that is, the part that can form interactions with water molecules through ion-dipole forces or hydrogen bond forces moves to the left. outer part of the molecule. Colloids can have a certain viscosity (viscosity is the internal resistance that a fluid presents: liquid or gas, to the relative movement of its molecules).

Types of colloids

Colloids are classified according to the magnitude of the attraction between the dispersed phase and the continuous or dispersing phase. If the latter is liquid, colloidal systems are classified as “sols” and are subdivided into “lyophobic” (little attraction between the dispersed phase and the dispersing medium) and “lyophilic” (great attraction between the dispersed phase and the dispersing medium). In lyophilic colloids, the dispersed phase and the dispersing medium are related, therefore they form true solutions and are thermodynamically stable; while lyophobic colloids are those where the dispersed phase and the dispersing medium are not related, can form two phases and are kinetically stable. A fundamental characteristic of lyophobic colloids is that they are not thermodynamically stable, as mentioned above, although they have a kinetic stability that allows them to remain in a dispersed state for long periods of time. Colloidal particles are small enough that their behavior is controlled by Brownian motion and not by macroscopic effects, such as gravitational forces. By adding a certain amount of electrolyte they can coagulate, the amount depends on the valence and nature of the electrolyte. Regarding the classification of colloids, it should also be noted that, if the dispersing medium is water, they are called "hydrophobic" (repulsion to water) and "hydrophilic" (attraction to water).

The following table lists the different types of colloids according to the state of their continuous and dispersed phases:

In principle, there is no fixed rule that establishes the state of aggregation in which both the dispersed phase and the dispersing medium must be found. Therefore, all conceivable combinations are possible, as shown in the table above.

Currently, and due to its industrial and biomedical applications, the study of colloids has gained great importance within physicochemistry and applied physics. Thus, numerous research groups around the world are dedicated to the study of the optical, acoustic, and stability properties and their behavior in the face of external fields. In particular, the electrokinetic behavior (mainly electrophoretic mobility measurements) or the electrical conductivity of the whole suspension.

In general, the study of colloids is experimental, although great efforts are also made in theoretical studies, as well as in the development of computer simulations of their behavior. In most colloidal phenomena, such as conductivity and electrophoretic mobility, these theories only reproduce reality in a qualitative way, but the quantitative agreement is still not completely satisfactory.

Preparation

There are two main ways to prepare colloids:

• Dispersion of large particles or droplets to the colloidal dimensions by grinding, spraying or clutching application (e.g. agitation, mixing or mixing of high cutting tension).
• Condensation of small molecules dissolved in larger colloid particles by precipitation, condensation or redox reactions. These processes are used in the preparation of colloidal silica or colloidal gold.

Stabilization

The stability of a colloidal system is defined by the particles that remain suspended in solution and depends on the forces of interaction between the particles. These include electrostatic interactions and van der Waals forces, because both contribute to the overall free energy of the system.

A colloid is stable if the energy of interaction due to the forces of attraction between colloidal particles is less than kT, where k is Boltzmann's constant and T is the absolute temperature. If this is the case, the colloidal particles will repel or only weakly attract each other, and the substance will remain in suspension.

If the interaction energy is greater than kT, attractive forces will prevail and colloidal particles will begin to clump together. This process is generally called aggregation but is also called flocculation, coagulation, or precipitation. While these terms are often used interchangeably, for some definitions they have slightly different meanings. For example, coagulation can be used to describe irreversible permanent aggregation in which the forces holding the particles together are stronger than any external forces caused by agitation or mixing. Flocculation can be used to describe reversible aggregation involving weaker attractive forces, and the aggregate is usually called unfloc. The term precipitation is normally reserved to describe a phase change from a colloidal dispersion to a solid (precipitate) when subjected to a disturbance. Aggregation causes sedimentation or creaming, therefore the colloid is unstable: if it occurs either of these processes, the colloid will no longer be a suspension.

Examples of stable and unstable colloidal dispersal.

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

• Electrostatic stabilization is based on the mutual repulsion of similar electrical loads. The load of the colloidal particles is structured in a double electrical layer where the particles are charged on the surface, but then attracts counterions (opposed loading sessions) that surround the particle. The electrostatic repulsion between suspended colloidal particles is more easily quantified in terms of the zeta potential. The combined effect of van der Waals attraction and electrostatic repulsion on aggregation is described quantitatively by DLVO theory. A common method for stabilizing a colloid (converting it into a precipitate) is peptization, a process in which it is agitated with an electrolyte.
• The aesthetic stabilization consists of absorbing a layer of polymer or tensioactive on particles to prevent them from approaching the range of attractive forces. The polymer consists of chains that are attached to the particle surface and the part of the chain that extends out is soluble in the suspension medium. This technique is used to stabilize colloid particles in all types of solvents, including organic solvents.

A combination of the two mechanisms (electrosteric stabilization) is also possible.

Stabilization of aesthetic and gel networks.

A method called gel network stabilization represents the primary way to produce colloids that are stable to both aggregation and sedimentation. The method consists of adding a polymer capable of forming a gel network to the colloidal suspension. Particle settlement is hampered by the rigidity of the polymer matrix where the particles are trapped, and long polymer chains can provide steric or electrosteric stabilization to dispersed particles. Examples of such substances are xanthan and guar gum.

Destabilization

Destabilization can be achieved by different methods:

• Removing the electrostatic barrier that prevents the aggregation of particles. This can be achieved by adding salt to a suspension to reduce the length of the Debye sieve (the width of the double electrical layer) of the particles. It is also achieved by changing the pH from a suspension to effectively neutralize the surface load of suspended particles. This eliminates the repulsive forces that keep the colloidal particles apart and allows the aggregation due to van der Waals forces. Minor changes in pH can manifest in a significant alteration of the zeta potential. When the magnitude of the zeta potential is below a certain threshold, typically around ± 5 mV, a rapid coagulation or aggregation tends to occur.
• Addition of a loaded polymer float. Polymer floaters can unite individual colloidal particles by attractive electrostatic interactions. For example, negatively charged clay particles or colloidal silica can be loosened by adding a positively charged polymer.
• Addition of non-adsorbed polymers called depletings that cause aggregation due to entropic effects.

Low-volume fraction unstable colloidal suspensions form clumped liquid suspensions, in which individual clumps of particles sediment if more dense than the suspending medium, or cream if less dense. However, colloidal suspensions of larger volume fraction form colloidal gels with viscoelastic properties. Viscoelastic colloidal gels, such as bentonite and toothpaste, flow like liquids under the shear, but hold their shape when the shear is removed. It is for this reason that toothpaste can be squeezed out of a toothpaste tube, but remains on the toothbrush after it is applied.

Monitoring stability

Measuring principle of multiple light dispersion along with vertical scan

The most widely used technique to monitor the state of dispersion of a product and to identify and quantify destabilization phenomena is multiple light scattering together with vertical scanning. This method, known as turbidimetry, is based in measuring the fraction of light that, after being sent through the sample, is backscattered by the colloidal particles. The backscattering intensity is directly proportional to the mean particle size and the volume fraction of the dispersed phase. Thus, local changes in concentration caused by sedimentation or creaming, and particle accumulation caused by aggregation are detected and monitored. These phenomena are associated with unstable colloids.

Dynamic light scattering can be used to detect the size of a colloidal particle by measuring how fast they diffuse. This method involves directing laser light at a colloid. The scattered light will form an interference pattern, and the fluctuation in light intensity in this pattern is caused by the Brownian motion of the particles. If the apparent size of the particles increases because they are clumped together through aggregation, slower Brownian motion will occur. This technique can confirm that aggregation has occurred if the apparent particle size is determined to be beyond the typical size range of colloidal particles.

Acceleration of methods for predicting service life

The kinetic process of destabilization can be quite long (up to several months or even years for some products) and it is often necessary for the formulator to use additional acceleration methods to achieve a reasonable development time for a new product design. Thermal methods are the most widely used and consist of increasing the temperature to accelerate destabilization (below critical temperatures for phase inversion or chemical degradation). Temperature affects not only viscosity, but also interfacial tension in the case of nonionic surfactants, or more generally the forces of interaction within the system. Storing a dispersion at high temperatures makes it possible to simulate the real-life conditions of a product (for example, a tube of sunscreen cream in a car in summer), but also to speed up destabilization processes up to 200 times. Sometimes centrifugation and shaking are used. They subject the product to different forces that push the particles / droplets against each other, thus helping in the drainage of the film. However, some emulsions would never coalesce in normal gravity, while they do in artificial gravity. Additionally, the segregation of different populations of particles has been noted when centrifugation and vibration are used.

Colloidal systems

• Emulsions: It is called emulsion to a colloidal suspension of a liquid in another inmiscible with it, and can be prepared stirring a mixture of the two liquids or, passing the sample by a colloidal mill called a homogenizer. An emulsion is a system where the scattered phase and the continuous phase are liquid.
• Soles: Liophobic suns are relatively unstable (or stable target); often a small amount of electrolyte or an elevation of temperature is enough to produce coagulation and precipitation of scattered particles.
• Aerosols: Aerosols are defined as colloidal systems with liquid or solid particles very finally subdivided, scattered in a gas. Today the term aerosol, in general language, is synonymous with a metallic container with pressurized content, although it is spoken of atmospheric aerosols.
• Gel: The formation of gels is called gelation. In general, the transition from sun to gel is a gradual process. Of course, gelation is accompanied by increased viscosity, which is not sudden but gradual.
• Espuma: The scattering phase can be liquid or solid and the phase disperses a gas.

Properties of colloids

Adsortment

Due to their size, colloidal particles have an extremely large area/mass ratio, which is why they are excellent adsorbent materials. On the surface of the particles there are forces called Van der Waals and even inter-atomic bonds that, when dissatisfied, can attract and retain atoms, ions or molecules of foreign substances. This adherence of foreign substances to the surface of a particle is called adsorption. Adsorbed substances are held firmly together in layers that are usually no more than one or two molecules (or ions) thick. Although adsorption is a general phenomenon in solids, it is especially efficient in colloidal dispersions, due to the enormous amount of surface area.

Tyndall Effect

It consists of a light beam becoming visible when it passes through a colloidal system. This phenomenon is due to the fact that colloidal particles scatter light in all directions, making it visible. Light rays can be seen passing through a forest, for example, as a result of light scattering by colloidal particles suspended in the forest air. Although all gases and liquids scatter light, the scattering by a pure substance or by a solution is very small, which is generally not detectable.

Brownian movement

Examples of this phenomenon are the movements observed in dust particles that move randomly free in a sunbeam entering through a window (or an open curtain), or dust particles and smoke moving in a beam of light coming from the projection room of a movie theater. The disordered movement of said colloidal particles is due to the bombardment or collision with the molecules of the dispersing medium, and in the cited examples it would be by the molecules present in the air (N², O², Ar, Kr, etc.). The movement is known as Brownian movement in memory of the English botanist Robert Brown, who first observed this irregular movement of particles in 1827, while studying the behavior of pollen grains suspended in water under a microscope. Brownian motion prevents colloidal particles from settling or forming air sediments .

Electrophoresis

It consists of the migration of charged colloidal particles within an electric field. Colloidal particles absorb ions on their surface, charging themselves positively or negatively. Although the entire colloidal system is electrically neutral, these particles travel to the electrodes (cathode and anode) through attractive electrical forces.

Dialysis

It is defined as the movement of ions and small molecules through a porous membrane, called the dialytic or dialysate membrane, but not of large molecules or colloidal particles. Dialysis is not an exclusive property of colloids, since certain solutions can also be dialyzed, for example, in biochemistry dialysis is frequently used to separate protein molecules from aqueous ions. In colloids, dialysis allows purifying the colloidal system, since ions and other small molecules considered impurities are eliminated. Cellophane and membranes of animal origin are used as dialysis membranes.