Solvation and Association Physical Pharmaceutics

Solvation and Association Physical Pharmaceutics


The process of solvation is sometimes called dissolution. Solvation is a kinetic process and is quantified by its rate. It is the attraction and association of molecules of a solvent with molecules or ions of a solute.  When  a  solute  is  soluble  in  a  certain  solvent,  the  solute’s molecules  or  ions  spreads  out  and  became  surrounded  by  solvent  molecules.  

A complex formed of molecule or ion of solute in a solvent is known as a solvation complex. 

Solvation  is  the  process  of  rearranging  solvent  and  solute  molecules  into  solvation complexes  to  distribute  solute  molecules  evenly  within  the  solvent.  Solvation  process  is  affected  by  hydrogen  bonding  and  van  der  Waals  forces  (which  consist  of  dipole-dipole, 

dipole- induced  dipole,  and  induced  dipole-induced  dipole  interactions).  Which  of  these  forces  are  at  play  depends  on  the  molecular  structure  and  properties  of  the  solvent  and solute.  Insoluble  solute  molecules  interact  with  other  solute  molecules  rather  than  break apart and become solvated by the solvent, for example, solvation of functional groups on a surface of ion-exchange resin. In fact solvation is an interaction of a solute with the solvent, which leads to stabilization of the solute species in the solution. Solvation of a solute by water is called hydration. 

Solvation  is,  in  concept,  distinct  from  solubility.  Solubility quantifies the dynamic equilibrium state achieved when the rate of dissolution equals the rate of precipitation. The consideration of the units makes the distinction clearer. The typical unit for dissolution rate is mol/sec.  The  units  for  solubility  express  a  concentration  as  mass  per  volume  (mg/mL), molarity (mol/L) etc. The similarity between solvent and solute determines how well a solute can be solvated by a solvent.



Association or ion association is a chemical reaction wherein ions of opposite electrical charge  come  together  in  solution  to  form  a  distinct  chemical  entity.  Ion associates are classified, according to the number of ions that associate with each other, as ion pairs, ion triplets etc. Ion pairs are also classified according to the nature of the interaction as contact, solvent-shared or solvent-separated. The most important factor that determines the extent of ion  association  is  the  dielectric  constant  of  the  solvent.  Ion associates have been characterized by means of vibrational spectroscopy.  

 Ion pairs are formed when a cation and anion come together:

There are three distinct types of ion pairs depending on the extent of solvation of the two ions:

In the above schematic representation,  the circles represent spheres.  The sizes are arbitrary and not necessarily similar as shown in Fig. 1.3, the cation is colored dark and the anion is colored grey. The area surrounding ions represents solvent molecules in a primary solvation shell;  secondary solvation is ignored.  When both ions have a complete primary solvation sphere, the ion pair may be termed fully solvated. When there is about one solvent molecule between cation and anion, the ion pair may be termed solvent-shared. Lastly, when the ions are in contact with each other, the ion pair is termed a contact ion pair. In contact ion pair the ions retain most of their solvation shell and the nature of this solvation shell is generally not known.  In aqueous solution and in other donor solvents,  metal cations are surrounded by between 4 and 9 solvent molecules in the primary solvation shell, but the nature of solvation of anions is mostly unknown. 

Another name for a solvent-shared ion pair is an outer-sphere complex. Usage of outer-sphere complex is common in coordination chemistry and denotes a complex between asolvated metal cation and an anion. Similarly, a contact ion pair may be termed an inner-sphere complex. The major difference between these three types is the closeness with which the ions approach each other:  The order of closeness is prevented as  Fully solvated  > Solvent-shared > Contact. 

With fully solvated and solvent-shared ion pairs the interaction is primarily electrostatic, but in a contact ion pair, some covalent character in the bond between cation and anion is also present.  An ion triplet may be formed from one cation and two anions or from one anion and two cations. Higher aggregates, such as a tetramer (AB)4, may be formed. Ternary ion associates involve the association of three species.  Another type, named intrusion ion pair, has also been characterized.

Coulomb’s law:

A law stating that as charges repel and opposite charges attract, with a force proportional to the product of the charges and inversely proportional to the square of the distance between them. 

F = Force of attraction

q1q2  –  Magnitude of electric charge

Є – Dielectric constant of the medium

r – Distance between ion

Notes credit:

Mr. Sumeet Manohar Kharat
PSPS’s Indira Institute of Pharmacy, Sadavali
Physical Pharmaceutics I

Video lecture credit:

Dr. Govind Kailash Lohiya
Gurunanak College of Pharmacy, Nagpur
Physical Pharmaceutics I
Unit 1 Solubility of drugs Solvation and Association Physical Pharmaceutics