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Surface chemistry is concerned with events that occur at the surfaces of materials. The interface or surface is symbolised by a / or a – separating the bulk phases. Solid-gas or solid/gas, for example, can be used to illustrate the interface between a solid and a gas. There is no interface between the gases due to complete miscibility. In surface chemistry, the bulk phases we encounter could be pure solutions. The interface’s area is determined by the size of the bulk phase particles. At interfaces, several significant phenomena occur, including corrosion, electrode processes, heterogeneous catalysis etc. Surface chemistry has a wide range of applications in industry, evaluation, and everyday life.
The characteristics of substances differ at the surface and in the bulk because molecules in the bulk are symmetrically drawn in all directions with zero net force, but molecules near the surface are familiarised with attraction unequally. In this unit, we’ll look at a phenomenon related to the surface.
It is the analysis of chemical reactions that take place at the interface of two surfaces, which can be solid-liquid, solid-gas, solid-vacuum, liquid-gas, and so on. Surface engineering refers to some of the applications of surface chemistry. Adsorption, heterogeneous catalysis, corrosion, and crystallisation are some of the phenomena that occur on the surface of substances.
Adsorption Isotherms
The adsorption isotherm is a curve that depicts the variation in the amount of gas adsorbed by the adsorbent as a function of pressure at constant temperature.
Freundlich adsorption isotherm:
Freundlich established a relationship between the amount of gas adsorbed per unit mass of solid adsorbent and pressure at a given temperature in 1909. This equation can be used to express the relationship:
(n > 1) x/ m = k.p 1/ n
where x is the mass of the gas adsorbed on the adsorbent’s mass m at pressure P, and k and n are constants that depend on the nature of the adsorbent and the gas at a certain temperature. The connection is usually depicted as a curve, with the mass of the gas adsorbed per gram of adsorbent plotted against pressure. Physical adsorption decreases with increasing temperature at a fixed pressure, as seen by these charts. At high pressure, these curves always appear to be approaching saturation. Taking the logarithm of log x /m = log k + 1/ n log p… Plotting log x m on the y-axis (ordinate) and log p on the x-axis confirms the validity of the Freundlich isotherm (abscissa). The Freundlich isotherm is valid if the result is a straight line; otherwise, it is invalid. 1/ n is the value of the slope of a straight line. The value of log k is given by the intercept on the y-axis. In a rough sense, the Freundlich isotherm explains adsorption behaviour. The factor 1/ n might have a value of 0 or 1. (probable range 0.1 to 0.5).
When 1/ n = 0, x /m = constant, adsorption is pressure independent. When 1 /n = 1, x/ m = k p, i.e. x/ m p, adsorption is proportional to pressure. Experiment results back up both of these scenarios. At high pressure, the experimental isotherms always appear to approach saturation. Freundlich isotherm cannot account for this. As a result, it fails under high pressure.
Adsorption from Solution Phase
Solids can also absorb solutes from liquids. When an acetic acid solution in water is agitated with charcoal, a portion of the acid is absorbed by the charcoal, and the acid concentration in the solution falls. When the litmus solution is shaken with charcoal, it also turns colourless. When Mg(OH)2 is precipitated in the presence of magneson reagent, the precipitate turns blue. The colour is due to magneson adsorption. In the case of adsorption from the solution phase, the following observations have been made: I As the temperature rises, the amount of adsorption reduces. (ii) As the surface area of the adsorbent grows, so does the extent of adsorption. (iii) The extent of adsorption is determined by the solute concentration in solution. (iv) The adsorption extent is determined by the type of the adsorbent and adsorbate. The exact mechanism of solution adsorption is unknown. Freundlich’s equation approximates the behaviour of adsorption from solution, with the exception that instead of pressure, the concentration of the solution is considered, i.e., x/ m = k C(1/n) (C is the equilibrium concentration). We get log x/ m = logk + 1/ n logC by taking the logarithm of the previous equation.
Langmuir Adsorption Isotherms
Following the Freundlich adsorption isotherm are the Langmuir adsorption isotherms and the BET theory. At low adsorption densities, the Langmuir adsorption isotherms predict linear adsorption and maximal surface coverage at larger solute metal concentrations.
The Langmuir adsorption isotherm is of the following shape:
θ=Kp/(1+Kp)
Where
1.θ is the fraction of the surface that the adsorbed molecule covers.
- The adsorption coefficient, or K, is an equilibrium constant.
- K= ka/kd = adsorption rate constant/ desorption rate constant
- The pressure is denoted by the letter p.
When no interaction between adsorbed species occurs, the Langmuir adsorption is appropriate for monolayer adsorption onto a homogenous surface.
Zeta potential
The zeta potential of any particle in suspension, macromolecule, or material surface is a physical property. It may be used to improve suspension, emulsion, and protein solution formulations, anticipate interactions with surfaces, and improve film and coating development. Knowing the zeta potential can reduce the amount of time it takes to make trial formulations.
Conclusion
Surface chemistry is the discipline of chemistry that studies the different types of surfaces and the species that live on them.
Surface chemistry is concerned with events that occur at the surfaces or interfaces of materials. The interface or surface is symbolised by a hyphen or a slash separating the bulk phases. It is the study of chemical reactions that take place at the interface of two surfaces, which can be solid-liquid, solid-gas, solid-vacuum, liquid-gas, and so on. Surface engineering refers to some of the applications of surface chemistry.
Freundlich adsorption isotherm:
(n > 1) x/ m = k.p 1/ n
where x is the mass of the gas adsorbed on the adsorbent’s mass m at pressure P, and k and n are constants that depend on the nature of the adsorbent and the gas at a certain temperature. The connection is usually depicted as a curve, with the mass of the gas adsorbed per gram of adsorbent plotted against pressure. At high pressure, these curves always appear to be approaching saturation. Using the logarithm,
log k + 1/ n log p = log x /m
Langmuir adsorption isotherms
At low adsorption densities, it predicts linear adsorption and maximum surface coverage at larger solute metal concentrations.
The Langmuir adsorption isotherm is of the following shape:
θ=Kp/(1+Kp).
