Dissolution is the capacity of solute to be dissolved whereas solubility is the rate of solute dissolving in a solution. Dissolution is a kinetic process whereas solubility is a thermodynamic process. In dissolution, solute dissolves in a solvent to form a solution whereas solubility is the outcome of dissolution. Dissolution is a kinetic process that is performed by the dissolution test apparatus USP, IP, BP dissolution apparatus for the oral solid dosage form and is used to find the rate of a solute dissolving in a solution, in which the process the solutes are always soluble in the medium or solvent.
Surface area Physicochemical properties of the drug The temperature of the process The concentration of the solute Viscosity and polarity of the solvent. Solubility is a thermodynamic process in which the chemical property refers to the ability of a particular solute, the substance, to liquefy in the solution or solvent.
Solubility is used for finding out the solute capacity to be dissolved in a particular solvent quantity, in which the process solutes are not always soluble in the medium or solvent it depends on the solute properties. The concentration of the solute The temperature of the system The polarity of the solvent The polarity of the solute The pressure of the system. Commonly asked questions on solubility and dissolution are as follows.
Both solubility and dissolution rate are two different phenomena. The solubility of the sample in a solvent may be poor, but its dissolution rate might be high. A solute, on the other hand, can be very soluble but take a long time to reach its final saturation concentration. Process of dissolution. Dissolution rates of solids in liquids. Factors affecting the rate of dissolution.
Measurement of dissolution rates of drugs from dosage forms. Methods of expressing solubility and concentration. Distribution of solutes between immiscible liquids.
While a chemical with a fast dissolution rate often has a high solubility and vice versa , this is not always the case. The differences are explained in the chapter. Some of these may be manipulated by the formulator. Solutions are encountered frequently in pharmaceutical development, either as a dosage form in their own right or as a clinical trials material.
Additionally, almost all drugs function in solution in the body. This chapter discusses the principles underlying the formation of solutions from solute and solvent and the factors that affect the rate and extent of the dissolution process.
This process will be discussed particularly in the context of a solid dissolving in a liquid as this is the situation most likely to be encountered in the formation of a drug solution, either during manufacturing or during drug delivery. Further properties of solutions are discussed in Chapter 3 and Because of the number of principles and properties that need to be considered, the contents of each of these chapters should only be regarded as introductions to the various topics.
The student is encouraged, therefore, to refer to the bibliography cited at the end of each chapter in order to augment the present contents. It uses a large number of pharmaceutical examples to aid in the understanding of physicochemical principles. This chapter will begin by clarifying some of the key terms relevant to solutions.
A solution may be defined as a mixture of two or more components that form a single phase that is homogeneous down to the molecular level. The component that determines the phase of the solution is termed the solvent ; it usually but not necessarily constitutes the largest proportion of the system.
The other component s are termed solute s and these are dispersed as molecules or ions throughout the solvent, i. The transfer of molecules or ions from a solid state into solution is known as dissolution.
Fundamentally, this process is controlled by the relative affinity between the molecules of the solid substance and those of the solvent.
The extent to which the dissolution proceeds under a given set of experimental conditions is referred to as the solubility of the solute in the solvent. The solubility of a substance is the amount of it that passes into solution when equilibrium is established between the solute in solution and the excess undissolved substance. The solution that is obtained under these conditions is said to be saturated. A solution with a concentration less than that at equilibrium is said to be subsaturated.
Solutions with a concentration greater than equilibrium can be obtained in certain conditions; these are known as supersaturated solutions. Since the above definitions are general ones, they may be applied to all types of solution involving any of the three states of matter gas, liquid, solid dissolved in any of the three states of matter, i.
However, when the two components forming a solution are either both gases or both liquids, then it is more usual to talk in terms of miscibility rather than solubility. Other than the name, all principles are the same.
One point to emphasize at this stage is that the rate of solution dissolution rate and amount which can be dissolved solubility are not the same and are not necessarily related. In practice, high drug solubility is usually associated with a high dissolution rate, but there are exceptions; an example is the commonly used film-coating material hydroxypropyl methylcellulose HPMC which is very water soluble yet takes many hours to hydrate and dissolve.
The majority of drugs and excipients are crystalline solids. Liquid, semi-solid and amorphous solid drugs and excipients do exist but these are in the minority. For now, we will restrict our discussion to dissolution of crystalline solids into liquid solvents.
Also, to simplify the discussion, it will be assumed that the drug is molecular in nature. The same discussion applies to ionic drugs. Again, to avoid undue repetition in the explanations that follow, it can be assumed that most solid crystalline materials, whether drugs or excipients, will dissolve in a similar manner.
The dissolution of a solid in a liquid may be regarded as being composed of two consecutive stages. First is an interfacial reaction that results in the liberation of solute molecules from the solid phase to the liquid phase. This involves a phase change so that molecules of solid become molecules of solute in the solvent in which the crystal is dissolving. After this, the solute molecules must migrate through the boundary layers surrounding the crystal to the bulk of solution. These stages, and the associated solution concentration changes, are illustrated in Figure 2.
Dissolution involves the replacement of crystal molecules by solvent molecules. This is illustrated in Figure 2. The process of the removal of drug molecules from a solid, and their replacement by solvent molecules, is determined by the relative affinity of the various molecules involved.
On leaving the solid surface, the drug molecule must become incorporated in the liquid phase, i. At times, dissolution might occur due to a chemical reaction and not due to the pure solubility of the solute. This should not be confused over solubility. When a solute is purely soluble, one should be able to obtain the solute back again after the evaporation of the solvent. Dissolution is the process where a solute dissolves in a solvent to form a solution.
Therefore, this has a kinetic effect. Dissolution can occur at various rates and sometimes for a solute to completely dissolve in a solvent it might require quite a length of time. During the process of dissolution, the structural integrity of the solute is broken down into individual components, molecules or atoms, and the outcome of dissolution is referred to as solubility. Dissolution too is governed by similar physical principles as for solubility, but dissolution itself is a kinetic process.
The rate of dissolution depends on various factors; mechanical mixing, nature of solvent and solute, mass of dissolved material, temperature etc. Your email address will not be published.
Comments So clear and specific explanation, really helpful!
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