Solid-gas interface: Adsorption

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Chapter: Pharmaceutical Drugs and Dosage: Interfacial phenomena

If a solid comes in contact with a gas or a liquid, there is an accumula-tion of gas or liquid molecules at the interface. This phenomenon is known as adsorption.


Solid–gas interface

Adsorption

If a solid comes in contact with a gas or a liquid, there is an accumula-tion of gas or liquid molecules at the interface. This phenomenon is known as adsorption. Adsorption refers to the surface binding of a liquid or gas molecule (adsorbate) onto a solid surface (adsorbent). Examples of adsor-bents are highly porous solids, such as charcoal and silica gel, and finely divided powders, such as talc. Adsorbate could be any molecule, such as a drug compound.

Removal of the adsorbate from the adsorbent is known as desorption. A physically adsorbed gas may be desorbed from a solid by increasing the temperature and reducing the pressure. Adsorption is a surface phenom-enon, distinct from absorption, which implies the penetration through the solid surface into the core of the solid.


Factors affecting adsorption

The degree of adsorption depends on the following:

·           The chemical nature of the adsorbent and the adsorbate. Since adsorp-tion is a result of an adhesive process, whereby two types of molecules interact with one another, the nature of the two types of molecules will determine their attractive interactions.

·           Surface area of the adsorbent. Greater the surface area of the adsor-bent, more the absolute amount of adsorbate that can be adsorbed. In modeling the adsorption phenomenon, the amount of adsorbate per unit adsorbent is usually calculated. In this scenario, the specific surface area (surface area per unit mass) of the adsorbent plays a role in deter-mining the amount of adsorbate per unit mass of the adsorbent. This

·           phenomenon indicates that a finely divided solid (of the same mass as a coarse particulate solid) would adsorb greater amount of adsorbate.

·           Temperature. Temperature increases molecular motion, and its effect on adsorption depends on the relative change in the intermolecular forces of attraction between the molecules of the two phases. Generally, an increase in Brownian motion with increasing temperature reduces adsorption.

·           Partial pressure (gas) or concentration (liquid) of the adsorbate. Generally, greater the solute (adsorbate) partial pressure or concen-tration, greater the rate of adsorption.


Types of adsorption

Adsorption can be physical or chemical in nature. Table 8.1 compares the characteristics of physical and chemical adsorption.

1. Physical adsorption

Physical adsorption is rapid, nonspecific, and relatively weak. It is typically mediated by weak noncovalent forces of attraction, such as van der Waals forces, and is reversible. 

Table 8.1 Characteristics of physical and chemical adsorption


Physical adsorption is an exothermic process, since heat is released with the formation of attractive interactions between mol-ecules of the two phases. Physical adsorption may be associated with three phenomena:

·           Monolayer formation: Adsorption of a solute on a solid surface leads to a monolayer formation, as the solute occupies the available surface in a single layer.

·           Multilayer formation: Surface adsorption may continue into multilayer formation if the adsorption is facilitated by the interactions of solute molecules with other solute molecules (that are already adsorbed on the solid surface). Once the monolayer formation is complete and the conditions (such as solute concentration in the liquid or partial pressure of the gas) are supportive, multimolecular adsorption may take place.

·           Condensation: The adsorbate may condense in the pores or capillar-ies of the adsorbent, leading to changes in the kinetics of the rate and the extent of adsorption.

2. Chemical adsorption (chemisorption)

Chemical adsorption or chemisorption is an irreversible process in which the adsorbent gets covalently linked to the adsorbate by chemical bonds.

Chemisorption is specific and may require activation energy. Therefore, this process is slow, and only a monolayer may be formed.


Adsorption isotherms

An adsorption isotherm is a graph that shows the amount of solute/adsorbate adsorbed per unit mass of a solid/adsorbent as a function of the equilibrium partial pressure (P) of the gaseous solute or the concentration (c) of the sol-ute in the liquid at a constant temperature (thus, the term isotherm).

1. Type of isotherms

The isotherms can generally be classified into five types [UK34](Figure 8.3):

·           Type I isotherms (e.g., ammonia on charcoal at 273 K) show a fairly rapid rise in the amount of solute adsorbed with increasing pressure to a limiting value. This phenomenon is due to the adsorption being restricted to a monolayer.

·           Type II isotherms (e.g., nitrogen on silica gel at 77 K) are frequently encountered and represent multilayer physical adsorption on nonpo-rous solids. They are often referred to as sigmoid isotherms. These isotherms are characterized by rapid solute adsorption to a limiting value, which sustains for certain increase in the partial pressure of the solute. Thereafter, multilayer adsorption initiates at an exponentially increasing rate.

·           Type III adsorption isotherm (e.g., bromide at 760°C or iodine at 790°C on silica gel) shows large deviation from Langmuir model, no flattish portion in the curve, and the formation of multilayer films.

·           Isotherm IV is typical of adsorption onto porous solids and involves the formation of a monolayer, which is followed by multilayer forma-tion. An asymptote toward a limiting value is observed after each additional layer formation.

·           Type V isotherm is similar to a type III isotherm in terms of the initial rate of solute adsorption increasing exponentially with solute


Figure 8.3 Types of adsorption isotherms.

concentration or partial pressure. This behavior is seen in relatively few instances in which the heat of adsorption of the solute in the first layer is less than the latent heat of condensation of successive layers. This promotes more rapid deposition of subsequent layers of adsorbed solute over the previous layer. Type III isotherm does not involve an eventual asymptote toward a limiting value, while type V isotherm does.

2. Modeling isothermal adsorption

Adsorption of a solute on a solid substrate at constant temperature (i.e., isothermal conditions) is a kinetic and a thermodynamic equilibrium phe-nomenon that can be described with the help of empirical or semiempirical equations. Modeling adsorption helps us understand a system and builds predictive ability to interpret the implications of changing system variables on the amount of free versus adsorbed solute. For example, in the case of drug adsorption on activated charcoal for preventing drug absorption into the systemic circulation after an oral overdose, the modeling of adsorp-tion isotherm enables simulation of absorption and pharmacokinetics of the drug in the presence and absence of charcoal and the effect of different quantities of drug and charcoal. This can help determine the required dose of charcoal for a given drug overdose. In addition, modeling the adsorp-tion data can be used to generate information about the system that would otherwise be unavailable. For example, gas adsorption on a solid substrate is used to quantify the specific surface area of a solid.

Isothermal adsorption can be modeled by using Freundlich, Langmuir, or BET equations.

2.1 Freundlich adsorption isotherm

Some cases of isothermal adsorption of a gas on a solid can be explained by the empirical Freundlich equation (Figure 8.4a).


where:

Y is the mass ratio of the adsorbent on the adsorbate, given by the ratio of the mass of gas (x) adsorbed per unit mass (m) of adsorbent at the partial pressure of gas (p).

The k and n are constants for a particular system at a constant temperature.


Figure 8.4 Plots showing (a) Freundlich, (b) Langmuir, and (c) BET isotherms.

The Freundlich isotherm, thus, states dependence of the mass of gas adsorbed on the partial pressure of gas with nonlinear kinetics, which depends on the specific combination of the adsorbent, the adsorbate, and the environment. Thus, the constants k and n depend not only on the sub-strate (adsorbate) and the gas (adsorbent) but also on the system (environ-ment, such as other constituents).

The above equation can be written logarithmically as:


A plot of log (x/m) against log p yields a straight line, with slope 1/n and intercept log k. This allows the experimental determination of the constants for a given system.

Freundlich isotherm models multilayer adsorption and mostly represents physical adsorption that does not reach saturation.

2.2 Langmuir adsorption isotherm

Langmuir developed an equation based on the theory that the molecules or atoms of gas are adsorbed on active sites of the solid to form a layer one-molecule thick (monolayer) (Figure 8.4b). Langmuir adsorption isotherm predicts not only a dependence on the partial pressure of gas (p) but also saturable kinetics of the overall rate of adsorption (K), which is defined as the ratio of the forward (adsorption) reaction rate constant (ka) to the reverse (desorption) reaction rate constant (kd). Thus, for the adsorption reaction,


Langmuir adsorption isotherm predicts that:


where, y is the number of available surface adsorption or binding sites occupied by the adsorbent, also expressed as the mass of gas adsorbed per unit mass of adsorbent, and ymax is the total number of surface adsorp-tion or binding sites on the adsorbate, also expressed as the maximum mass of gas that a unit mass of adsorbent can absorb when monolayer is complete.

Therefore, the Langmuir adsorption isotherm predicts that y never exceeds ymax, even as the rates of forward, adsorption, reaction reach but never exceed 1.

The simplified equation of Langmuir isotherm is:


A plot of p/y against p yields a straight line, with 1/ymax as the slope and 1/Kymax as the intercept. This allows the experimental estimation of the values of ymax and K.

Langmuir adsorption isotherm is often indicative of chemisorption and has the following characteristics:

·           Adsorption is localized to the active regions on the surface, and only monolayer adsorption takes place.

·           Heat of adsorption is independent of surface coverage, indicating that all molecules being adsorbed experience the same attractive force, independent of the neighboring adsorbed molecules.

2.3 BET adsorption isotherm

The BET adsorption isotherm models multilayer gas adsorption and assumes that the forces involved in physical adsorption are the same as those responsible for the condensation of the adsorbate.

The BET equation relates the partial pressure of gas (P) with the relative proportion of the adsorbed molecules (Y/Ym) by the equation:


where:

p is the partial pressure of adsorbate

y is the mass of adsorbate per unit mass of adsorbent

P0 is the vapor pressure of adsorbate when the adsorbent is saturated with adsorbate molecules

Ym is the maximum quantity of adsorbate adsorbed per unit mass of the adsorbent

b is the constant proportional to the difference between the heat of adsorption of the gas in the first layer and the latent heat of con-densation in the successive layers

The BET isotherms occur when gases undergo physical adsorption onto nonporous solids to form a monolayer, followed multilayer formation. The BET isotherms have a sigmoidal shape (Figure 8.4c) and represent type II isotherms.

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