Khalid T MaaroofMSc. Pharmaceutical sciences

School of pharmacy – Pharmaceutics department

1Online access: bit.ly/physicalpharmacy

Phase equilibria

Physical Pharmacy

10/31/2015

What was equilibrium?

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Phase rule Coexistence of phases can only occur over a

limited range. phase rule is a relationship for determining the

least number of intensive variables that can be changed without changing the equilibrium state of the system.

P is the number of phases present F is termed the number of degrees of freedom C in represents the number of components in the

system

What is intensive variable???

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The number of degrees of freedom: is the least number of intensive variables that must be fixed/known to describe the system completely.

Examples of intensive variables are: temperature, pressure, concentration….

Systems Containing One Component

This is the phase diagram for a typical pure substance.

at higher pressure, the melting temperature would be higher because the line between the solid and liquid areas slopes slightly forward.

Increasing pressure at constant temperature changes liquid to solid and vice versa

The same explanation for liquid vapour boundary

The point labelled as C in the phase diagrams is critical temperature.

Note the process of sublimation as shown on the graph; below the point T.

Point T is called triple point.

Where all three lines meet, you must have a unique combination of temperature and pressure where all three phases are in equilibrium together

Normal melting and boiling points

The phase diagram for water In the case of water, the

melting point gets lower at higher pressures. Why?

Imagine lowering the pressure on liquid water along the line in the diagram below.

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Systems Containing more than one Component

CONDENSED SYSTEMS a maximum of three degrees of freedom is possible

in a two-component system. More components more degrees of freedom

In two component one phase system (maximum no of variables) F = 2 – 1 + 2 = 3

E.g. temperature, pressure, and concentration we frequently choose to disregard the vapor phase

and work under normal conditions of 1 atm pressure. Now we can use planar figures instead of three

dimensional (this will be easier) condensed systems: When we only consider solid

and liquid phases

Two-Component Systems Containing Liquid Phases

The maximum temperature at which the two-phase region exists is termed the critical solution or (upper consolute temperature).

Upper and lower consulate temperatures

Two-Component Systems Containing Solid and Liquid Phases: Eutectic Mixtures

two components are completely miscible in the liquid state and completely immiscible as solids,

Eutectic composition “contact melting”: lowest melting

temperature over a composition range. eutectic composition is the composition of

two or more compounds that exhibits a melting temperature lower than that of any other mixture of the compounds

The primary criterion for eutectic formation is the mutual solubility of the components in the liquid or melt phase.

Lidocaine and prilocaine, two local anesthetic agents, form a 1:1 mixture having a eutectic temperature of 18◦C

Phase Equilibria in Three-Component Systems

In systems containing three components but only one phase.

four degrees of freedom are temperature, pressure, and the concentrations of two of the three components.

Only two concentration terms are required because the sum of these subtracted from the total will give the concentration of the third component.

Temperature and pressure are considered to be constant.

Triangular Diagram

Ternary Systems with One Pair of Partially Miscible Liquids

e.g. Water and benzene + alchohol

• Alchohol provide solvation

• binodal curve

Alterations of the binodal curves with changes in temperature.

Ternary Systems with Two Pairs of Partially Miscible Liquids

Ternary Systems with Three Pairs of Partially Miscible Liquids

F=0 Why???

Questions !

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