*3.3.2 Dissociation constant (pKa) assessment*

pKa is the dissociation constant of a drug and they are available as weak bases or acids in the solution, hence drugs can exist in the ionised or un-ionised form at particular pH. The aqueous solubility of drugs depends on the ionisation and fraction of ionised to the unionised form of the drug. The un-ionised form of drugs is lipophilic, thus permeating through the bilayer membrane however, the ionised substances are lipid insoluble hence permeation is slow. Three parameters that are crucial for absorption are the ionisation constant, which is the un-ionised form of drug at the gastrointestinal tract site available for absorption to show its efficacy [57].

The degree of ionisation depends on the Henderson-Hasselbalch equation and can be determined by UV spectroscopy, potentiometric titration, and titrimetric methods from intrinsic solubility data. A parameter that also considers a compound's ionisation state is the ionisation constant (pKa). Understanding pKa is crucial for predicting the absorption route of weakly acidic or basic medications [3, 58].

The equations for basic and acidic compounds are mentioned below:

```
For acidic drugs : pH=log pKa+ratio of un ion − ised to ionised drug (2)
```
For basic drugs : pH=log pKa+ratio of ionised to un ionised drug − (3)

%Ionisation 10 pH-pKa /1+10 pH-pKa ,where is t = ( ) ( ) *Ka* he dissociation constant (4)

Weakly acidic drugs with around 4 pKa value are absorbed fast from the stomach because drug molecules available here in an un-ionised state at this pKa value in the stomach. Similarly, basic drugs with an 8 pKa value are available in an unionised form and easily absorbed from the intestine [58]. Various methods used for the determination of the partition coefficient are a shake-flask method, chromatographic determination, microelectrometric titration, counter current and filter probe. pKa determination at the preformulation stage is important because of the following reasons [59].


#### *3.3.3 Partition coefficient (Log P) assessment*

The ratio of unionised drugs dispersed in the aqueous and organic phases is known as the partition coefficient. This is helpful to predict drugs' ability to cross the bilayer. Lipinski's Rule of 5 has been used to predict solubility and permeability.

The log P can be determined with the formula log P= oil/water at equilibrium. ( ) (5)

A compound's lipophilicity is represented by its Log P value, 0 log P value indicates that the drug substance is similarly soluble in both n-octanol and water. While 2 Log P values indicate the hydrophilic nature of the drug and 5 indicates the lipophilic nature. A suitable absorption profile ranges between log P values of 1 and 3, while a Log P value of less than 1 and more than 6 indicates poor permeability. The software tools are nowadays very helpful for Log P value determination for example Molecular Modelling Pro™ 6.27 software [60].

#### *3.3.4 Distribution coefficient (Log D)*

Log D provides an approximation of the lipophilicity of drug molecules in blood plasma at pH 7.4. It can be determined by correlating the drug retention time compared with a similar compound with a known log P value. Log D values consider the possibility of drug molecules in an ionic state [3, 14].

#### *3.3.5 Thermal effect (enthalpy of solution)*

The effect of heating on drug solubility can be measured in the form of heat of solution. The heat released or absorbed during the dissolution of a mole of solute in a large volume of solvent is referred to as the heat of the solution. The ideal temperature range should typically include 5, 25, 37, and 50°C. For the endothermic process, the heat of the solution is considered positive and negative for exothermic. Positive heat of solution with an increase in temperature leads to an increase in the drug solubility hence at the preformulation stage, with the use of the heat of solution formula, the optimum drug solubility can be determined. The heat of solution between 4 and 8 kcal/mol indicates un-ionised forms of weak bases and acidic drugs dissolved in water [36].

#### *3.3.6 Common ion effect (Ksp)*

Pre-formulation evaluation performed for solubility determination must not avoid the common ion effect since the common ions are responsible for salt solubility reduction.

Le Châtelier's principle states that when an equilibrium is out of balance, the reaction will change to put it back in balance. An equilibrium between a weak acid or base and a common ion will shift in favour of the reactants. The common ions suppress the ionisation of a weak acid in the presence of a weak base or acid by producing more comparable product ions [61]. Hence, adding common ions to the solution may shift the reaction toward the reactant to dismiss excess product stress in the form of precipitation leading to a decrease in the solubility. For example, the solubility of weakly basic drugs in acidic (HCl) solution is diminished when they are administered as HCl salts due to Cl common ions. Hydrochloride salt's intrinsic dissolution rate evaluation between water, and water containing 1.2% w/v NaCl, and 0.9% w/v NaCl in 0.05 M HCl medium suggest a common ion interaction pathway. Following this, if the drug's solubility has not decreased, the drug can suitable to administer as a chloride salt; otherwise, it should be discarded. Hence, to get optimum solubility common ions effect must be avoided [14].
