Physical colloid chemistry is a science that studies the chemical and physical properties of surface phenomena and dispersed systems.
Definitions
Physical colloid chemistry is associated with disperse systems. By them it is customary to understand such states in which one or more substances are in a dispersed (fragmented) state by weight of the second substance. The fragmented phase is called the dispersed phase. A dispersion medium is called a medium in which the dispersed phase is in fragmented form.
Adsorption and surface phenomena
Physicalcolloid chemistry considers surface phenomena that occur at the interface of dispersed systems.
Among them, we note:
- wetting;
- surface tension;
- adsorption.
Physicalcolloid chemistry analyzes important technical processes related to wastewater and air purification, mineral processing, metal welding, painting of various surfaces, lubrication, and surface cleaning.
Surface tension
Organic and physical colloid chemistry explain the phenomena occurring at the interface. Let us analyze the system, which consists of gas and liquid. The molecule that is inside the system is affected by attractive forces from the nearest molecules. The molecule, which is located on the surface, is also affected by forces, but they are not compensated.
The reason is that in the gaseous state the distances between the molecules are quite large, the forces are almost minimal. The internal pressure is trying to pull deep into the liquid molecule, as a result, compression occurs.
To create a new interface, for example, stretching into a film, it is necessary to perform work against internal pressure. There is a direct relationship between the energy expended and the internal pressure. The energy concentrated in molecules located on the surface is considered free surface energy.
Fundamentals of Thermodynamics
The main tasks of physical colloid chemistry include the calculation of thermodynamic equations. Depending on the reaction in question, it is possible to determine the possibility of its spontaneous course.
Due to the instability of thermodynamic systems, processes occur that are associated with the enlargement of particles, accompanied by a decrease in the interface.
The reasons for the change in the thermodynamic state
What factors influence the magnitude of surface tension?
First of all, it is important to highlight the nature of substances. The magnitude of the surface tension is directly related to the features of the condensed phase. With increasing polarity of the bond in the substance, an increase in the tensile force occurs.
The state at the interface is also affected by temperature. In the case of its increase, the forces acting between the individual particles in the substance decrease.
The concentration of substances dissolved in the analyzed fluid also affects the state of the thermodynamic system.
There are two kinds of substances. TID (surface-inactive substances) increase the magnitude of the tension of the solution in comparison with an ideal solvent. Such substances are strong electrolytes. Surfactants (surfactants) reduce the amount of tension at the interface in the resulting solution. With an increase in these substances in solution, their concentration in the surface layer of the solution is observed. Polar organic compounds are acids, alcohols. They incorporate polar groups (amino, carboxyl, hydroxo), as well as a non-polar hydrocarbon chain.
Features of sorption
Physicalcolloid chemistry (STR) includes a section on sorption processes. Adsorption is a process of spontaneous change in the surface layer of the concentration of substances relative to their quantity in the volume of phases.
An adsorbent is a substance on the surface of which deposition occurs. An adsorbent is a substance that can precipitate. Adsorbate is a precipitated substance. Desorption is the reverse of adsorption.
Types of Sorption
The teacher of physical colloid chemistry talks about two types of adsorption. In the case of physical precipitation, a small amount of energy is released, which is comparable to the heat of condensation. This process is reversible. In the case of an increase in temperature, adsorption decreases, and the rate of the reverse process (desorption) increases.
The chemical version of adsorption is irreversible; not an adsorbent, but a surface compound leaves the surface. During chemisorption, heat is high; it is commensurate with the size of the thermal effect of the chemical reaction. With an increase in temperature, chemisorption increases, and the interaction between substances increases.
As an example of chemisorption, we note the adsorption by the surface of a metal of oxygen from the air; it is studied by physical colloid chemistry. Tasks and solutions are often associated with determining the magnitude of the tension arising at the interface between two media.
To quantify the pronounced adsorption, absolute adsorption is used. It characterizes the amount of adsorbate (in mol) per unit area of the adsorbent taken. The plans of physical colloid chemistry include a quantitative determination of this quantity.
Adsorbent Characterization
Physical and colloidal chemistry pays special attention to the analysis of types of adsorbents, their practical application. Depending on the surface size of the adsorbent, a different amount of adsorbed substance is possible. The most effective adsorbents are substances with a developed surface: colloids, powders, porous reagents.
As the main quantitative characteristics of adsorbents, specific surface area and bulk porosity are distinguished. The first value shows the ratio of the surface of the adsorbent to the mass. The second characteristic assumes the features of its structure.
In colloid chemistry, two types of adsorbents are distinguished. Non-porous substances are created by solid particles, forming the porous structure of the “powder diaphragm” with close packing. The pores between them are the gaps between the grains of matter. The structure may have a micro- or macroporous structure. Porous adsorbents are structures that consist of grains having internal porosity.
In physical chemistry, special attention is paid to the characterization of coarsely dispersed systems. They are powder formulations that are formed from powder grains when pressed or densely packed into tubes. The resulting systems have certain thermodynamic characteristics, the study of which is the main task of physical colloid chemistry.
There is a division of the process (taking into account the nature of the adsorbent) into ionic, molecular, colloidal adsorption. The molecular process is associated with solutions of weak electrolytes or dielectrics. Adsorption of dissolved substances on the surface of the solid adsorbent occurs.
Part of the active centers on the surface of the adsorbent are occupied by solvent molecules. During the process of precipitation, the solvent and adsorbent molecules compete.
Conclusion
Physical and colloidal chemistry are important areas of chemistry. They explain the main processes occurring in solutions, allow calculations of the amount of heat released (absorbed) during the formation of new substances. The basic law used in quantitative calculations is the Hess law. It links together several thermodynamic characteristics inherent in substances: enthalpy, entropy, energy. The thermodynamic process of the formation of complex substances from simple (initial) components can be considered from the point of view of the Hess law. The calculations allow us to determine the effectiveness of the process.