Solid-Liquid interface

Solid–Liquid interface

Many pharmaceutical systems deal with the adsorption of solutes from solutions onto solid surfaces. These can be exemplified by the adsorption of drug or hydrophilic polymer on suspended drug particles in a suspension or the adsorption of drug on activated charcoal administered in the case of oral drug overdose.

Modeling solute adsorption

The adsorption of solute molecules from solution may be treated in a man-ner analogous to the adsorption of gas molecules on the solid surface. Isothermal adsorption can be expressed by Langmuir equation in the following form:

where, c is the equilibrium concentration of the solute in the solution and replaces p, the partial pressure of the gas. A plot of c/y against c yields a straight line, and y_max and K can be obtained from the slope and intercept of this plot.

The Langmuir binding isotherm was utilized in determining the affinity and extent of interaction of drug and excipients in the dosage form and the impact of this interaction on oral bioavailability of drugs.

Factors affecting adsorption from solution

Adsorption from solution depends on the following factors:

1. Solubility of adsorbate/solute: The rate of adsorption of a solute is inversely proportional to its solubility in the solvent from which adsorption occurs. For adsorption to occur, solute–solvent bonds must first be broken. The greater the solubility, the stronger are these bonds and, hence, the lower the rate of adsorption. Conversely, the lower the solubility of the solute in the solvent, the higher its rate of adsorption onto the solid adsorbent.

2. Solute concentration: An increase in the solute concentration increases the rate of adsorption that occurs at equilibrium until a limiting value is reached.

3. Temperature: Adsorption is an exothermic process, that is, heat is released when stronger adsorbate–adsorbent bonds are formed. Thus, increase in temperature reduces adsorption. This can also be under-stood as increased Brownian motion of the solute molecules at higher temperature.

4. pH: The pK_a value(s) of the solute determines the relative proportion of ionized and unionized species of the solute and solute solubility in solution as a function of pH. The pH of the solution may also influence surface polarity of the solid substrate by changing the ion-ization, ion adsorption, or selective dissolution, as discussed earlier. The effect of pH on adsorption depends on the nature of intermolec-ular forces between solute and solute, solute and solvent, and solute and solid substrate as a function of the ionization status of an ioniz-able solute. The pH of the solution would also affect the solubility of the solute.

Adsorption generally increases as the ionization of the drug is sup-pressed; that is, the extent of adsorption reaches a maximum when the drug is completely unionized. This is related to higher aqueous solubility of the ionized form. For amphoteric compounds, adsorp-tion is at a maximum at the isoelectric point.

5. Nature of adsorbent/solid substrate: The physicochemical nature of the adsorbent can affect the rate and extent of adsorption by changes in the molecular forces of attraction between the adsorbate and the adsorbent. In addition, the extent of adsorption is proportional to the surface area of the adsorbent. Thus, an increased surface area, achieved by a reduction in particle size or the use of a finely divide or porous adsorbing material, increases the extent of adsorption.

Wettability and wetting agents

Adsorption of the solvent, water, onto a solid substrate is termed wetting. The wettability of a material can be ascertained by observing the contact angle that water makes with the surface. Contact angle is the angle between a liquid droplet and the surface of the solid over which it spreads. As shown in Figure 8.5, the lower the contact angle (θ), the higher the wetting. Contact angle can range from 0° to 180°. For example, mercury does not wet most solid surfaces and its contact angle is well above 120° for most surfaces.

The balance of intermolecular forces involved in determining the adsorp-tion of solute on a solid surface is the same for the adsorption/wetting of solvent/water on a solid surface. Powders, such as sulfur, charcoal, and magnesium stearate, that are not easily wetted by water are called hydro-phobic. Powders, such as zinc oxide, talc, and magnesium carbonate, that are readily wetted by water are called hydrophilic.

A wetting agent lowers the contact angle and aids in displacing an air phase at the surface and replacing it with a liquid phase. Wetting agents could be of the following types:

1. Surfactants: Surfactants with hydrophile-lipophile balance (HLB) values between 7 and 9 are used as wetting agents, generally in the concen-tration of about 0.1% w/v. Surfactants reduce the interfacial tension between solid particles and a vehicle. As a result of the lowered inter-facial tension, air is displaced from the surface of particles, and wetting and deflocculation of dispersed solid particles are promoted. Examples of surfactants used as wetting agents include polysorbates (Tweens) and sorbitan esters (Spans), as well as sodium lauryl sulfate.

2. Hydrophilic colloids: Acacia, bentonite, tragacanth, alginates, and cel-lulose derivatives act as protective colloids by coating hydrophobic particles with a multimolecular layer. This changes the surface proper-ties of the solid, making it more hydrophilic, and promotes wetting.

Figure 8.5 Contact angles from 0° to 180°.

3. Solvents: Water-miscible solvents, such as alcohol, glycerol, and glycols, can act as wetting agents by getting adsorbed on the solid surface, which makes the surface more hydrophilic, and reducing the dielectric constant of water, which can alter the balance of solute solu-bility in the bulk of the solvent versus surface adsorption.