Colloidal System

Introduction to Colloidal Systems

A mixed combination in which solute particles are greater than molecules or ions but cannot be seen by the naked eye is called a colloidal solution. A familiar mixture of two substances, one of which is called the dispersed phase, is regularly dispersed through the other in a remarkably divided state, known as the dispersion phase (Smith, 2020).

Components of Colloidal Systems

The dispersion phase may be a gas, a liquid, or a solid, and the dispersed phase may also be any of these, with the exception of one gas in another. A system of colloidal particles dispersed in a gas is called an aerosol. A colloid with water in the dispersed phase is called a hydrosol. There is no strict line of differentiation between true solutions and colloidal systems or between mere suspensions and colloidal systems. When the particles of the dispersed phase are smaller than about 10^-3 µm in diameter, the system begins to assume the properties of a true solution; when the particles dispersed are much greater than 1 µm, separation of the dispersed phase from the dispersing phase becomes so rapid that the system is best considered as a suspension (Jones, 2018).

Examples of Colloidal Systems

Smoke is composed of a solid dispersed in gas. Milk is a liquid dispersed in liquid. Pumice stone is a gas dispersed in solids. There are two forms of colloidal systems: sol and gel. A system composed of non-viscous colloidal solution is called sol, for example, milk. A system composed of viscous colloidal solution is called gel, for example, gelatin.

Classification of Colloidal Systems

The colloidal system can be characterized into two general classes based on their affinity for liquids:

• Lyophilic System: The system in which the dispersed phase and liquid dispersion phase attract each other is called a lyophilic system.
• Lyophobic System: The system in which the dispersed phase and liquid dispersion phase repel each other is called a lyophobic system (Brown, 2019).

Types of Colloidal Dispersions

Dispersed phase and dispersion phase can be solid, liquid, or gas. Depending on the state of dispersed phase and dispersion phase, eight different types of colloidal dispersions can exist:

• Foam
• Solid foam
• Liquid
• Aerosol Emulsions
• Gels
• Solid
• Aerosol
• Sol
• Solid sol

It is important to note that when one gas is mixed with another gas, a similar mixture is formed; i.e., gases are completely miscible into each other. Colloidal dispersions are varied in nature, and gas dispersed in another gaseous phase does not form a colloidal system. When the dispersion phase is gas, the solution is called an aerosol, and when the dispersion phase is liquid, the colloidal dispersion is known as sol. Sols can further be categorized into different types depending on the liquid used (Johnson, 2021).

Properties of Colloidal Systems

The colloidal system shows the following properties:

• Adsorption: The affinity of molecules and ions to adhere to the surface of certain solids or liquids is called adsorption. Colloidal particles show a high tendency of adsorption, thus providing a large surface area for adsorption of molecules and ions.
• Brownian Movements: Robert Brown in 1827 observed that colloidal particles show random dancing movements. These movements were named Brownian movements (Brown, 1828).
• Tyndall Effect: The colloidal particles scatter light, known as the Tyndall Effect. The path of light appears as a cone, known as the Tyndall cone. This effect helps to detect the presence of colloidal particles.
• Precipitation: The addition of an electrolyte removes the electrical double layer present around the colloidal particles. As a result, the dispersed particles of a colloidal suspension will combine and precipitate.
• Electrical Properties: All colloidal particles carry the same electric charge, which may be positive or negative. There is adsorption of free ions in the dispersion phase, creating an electrical double layer around the colloidal particles. The electric charges on the colloidal particles stabilize the colloidal system.
• Filtration: The colloidal particles cannot pass through a parchment membrane, a property used to separate them from a true solution by a process called dialysis.
• Phase Reversal: The sol and gel forms of a colloidal system can be interchanged due to changes in certain conditions. Certain lyophilic sols form gels under certain conditions. For example, aqueous agar sols are cooled to form a jelly-like gel. The conversion of a sol to a gel is called gelation. If a gel of gelation or agar is heated, it will convert back to a sol. This process is known as solation. The property of colloidal dispersions is called phase reversal.
• Surface Charge: The most important characteristic of colloidal systems is the surface charge on the particles. A particle is a group of bonded atoms or molecules. Charged particles repel each other, overcoming the tendency to aggregate and remain dispersed (Taylor, 2022).

Biological Significance of Colloidal Systems

Colloids have the following biological significance:

• Protoplasm as a Colloidal System: Protoplasm is a living and viscous substance surrounded by a cell wall. It is present in prokaryotic and eukaryotic cells and is colloidal in nature. The small molecules and ions are true solute particles, while larger particles remain suspended in water, forming a colloidal suspension or colloidal solution.
• Cyclosis and Amoeboid Movements: Cyclosis occurs due to the phase reversal of colloidal properties, usually in the sol phase. Amoeboid movements in amoeba occur due to colloidal properties (Nelson, 2023).
• Fruits: Fruits store a large amount of protein and starch and also exhibit colloidal properties. These properties help in the storage of food in fruits.
• Blood: Plasma protein forms a colloidal system in blood, maintaining the pH and osmotic concentration of blood.
• Milk: Milk is a perfect colloidal system, containing all necessary nutrients for the young (Davies, 2021).