Heterogeneous Dispersed Systems: A Comprehensive Guide
Scattered Heterogeneous Systems
Introduction
Dispersions are systems that incorporate substances of different natures into a single formula. This versatility, combined with their physical properties, allows for easy handling and application. Most consist of dispersed systems such as emulsions, suspensions, gels, foams, and aerosols.
Scattered Heterogeneous Systems
Dispersed systems are classified based on the particle size of the dispersed phase. Solutions have a very small particle size and are homogeneous systems, unlike the rest, which are heterogeneous or multiphase systems.
Factors Affecting Stability
Dispersed systems are inherently unstable. Factors affecting their stability include:
- Particle size of the dispersed phase: The smaller the size of the dispersed phase, the more stable the system.
- Viscosity: Increased viscosity reduces the tendency for phase separation.
- Electric charge: In many cases, stability depends on the pH. Particles repel each other and remain separate.
- Temperature: Increased temperature increases the energy and motion of particles in the dispersed phase and decreases the viscosity of the dispersing phase.
- Surface-active agents: These lower the interfacial tension and promote contact between the two phases.
Colloidal Dispersions
Colloidal dispersions belong to the category of dispersed systems. In these systems, the dispersed phase is evenly distributed in an external phase or dispersing medium.
Features
- The dispersed phase is not visible to the naked eye.
- Dispersions are not fully transparent.
- Changes in pH can affect stability.
- They are classified according to the affinity between the dispersed and dispersing phases.
Gels
The term “gel” encompasses a group of semi-solid excipients, which may be suspensions of small inorganic particles or large organic molecules interpenetrated by a liquid. The former is a suspension, which, if small enough, could be considered a colloid. The latter is dissolved but may be considered a heterogeneous system, i.e., a lyophilic colloid.
Properties and Applications of Colloidal Systems
There are three key properties:
- Rheological behavior: The viscosity of the gel helps achieve stability of emulsions and suspensions. It is a suitable cosmetic carrier as transparent gels can be obtained.
- Dispersing ability: Colloids can disperse insoluble lipophilic substances in an aqueous medium without adding organic solvents and without modifying the appearance of the aqueous medium.
- Stability: They are able to provide stability.
One drawback of gels is their tendency to exude water. They usually include glycerin or polyol to counteract this.
Suspensions
Suspensions are dispersed heterogeneous systems in which the dispersed phase is solid and insoluble in the dispersing phase. They appear cloudy.
Properties
- They settle slowly.
- They are easily redispersed by stirring.
- They do not cake upon settling.
- They have a viscosity that allows them to be poured.
Emulsions
An emulsion is a heterogeneous dispersed system consisting of two immiscible liquid phases, one of which is divided. One phase is called the internal, dispersed, or discontinuous phase, and the other is the external, dispersing, or continuous phase.
Features
Emulsions have a smooth consistency. In terms of consistency, creams have a higher viscosity.
Types of Emulsion
Emulsions are typically classified according to their type. The type of an emulsion indicates which phase is internal and which is external. This is determined by the emulsifier used. Oil is denoted by O and water by A. The first initial corresponds to the nature of the internal phase, and the one after the slash corresponds to the external phase.
- O/A emulsion (oil-in-water):
- Conducts electric current
- Feels soft and is easily absorbed
- Is stained with water-soluble dyes
- Used in day creams, lotions, and milk bases
- A/O emulsion (water-in-oil):
- Does not conduct electric current
- Viscous and oily
- Is stained with lipophilic dyes
- Used in night creams and treatments
- A/S or silicone emulsion:
- The internal phase is aqueous, and the external phase comprises silicone derivatives (oil-free)
- A/O/A emulsion:
- Aqueous phase dispersed in an oil phase, which in turn is dispersed in an aqueous solution
- O/A/O emulsion:
- Oily phase dispersed in an aqueous phase, which in turn is dispersed in another oil phase
Components of an Emulsion
- Aqueous phase: Water is the major component of this phase. It should be deionized water.
- Oil phase: It consists of lipophilic substances and includes all fatty substances such as hydrocarbons, fatty alcohols, waxes, fatty acids and their esters, lanolin and its derivatives, or silicone.
- Emulsifiers or surfactants: Emulsifiers are substances that facilitate the formation and stabilization of the emulsion by reducing the surface tension between the phases. They are also called surface-active agents or surfactants. They have a common feature: the molecule has two distinct parts, one of which is soluble in water (hydrophilic) and the other is soluble in fat (lipophilic). All these compounds are soluble or partially soluble in both water and fat.