Interfacial Properties

1

• Interfacial Properties

PHAR 526 Physical Chemistry R. Gary
Hollenbeck Professor, Department of
Pharmaceutical Sciences
2

• Interfacial Properties

• Liquid - Gas
• Liquid - Liquid
• Liquid - Solid

3

• Molecular Forces in the Interface

Vapor
Liquid
4

• Molecular Forces in the Interface

Vapor
Surface
Liquid
5

• Molecular Forces in the Interface

Vapor
Interfacial Region
Liquid
6

• Molecular Forces in the Interface

Molecules in bulk are exposed to a symmetrical
force field.
7

• Molecular Forces in the Interface

Molecules in the interface are pulled into bulk,
minimizing the interfacial area.
8

• Expansion of the Liquid Interface

9

• Expansion of the Liquid Interface

10

• Expansion of the Liquid Interface

11

• Expansion of the Liquid Interface

W work done in expanding the surface area ?A
change in area
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• Expansion of the Liquid Interface

W ?G ? ?A ?G free enery change ? ?
surface or interfacial tension
13

• Dimensions of Surface Tension

? ??G/?A erg/cm2 dyne/cm?
14

• Liquid-Liquid Systems

• Oil and Water

15

• Liquid-Liquid Systems

• Oil and Water

Agitation distributes the oil throughout the
water.
16

• Liquid-Liquid Systems

• When agitation is stopped, the drive to reduce
  interfacial free energy results in

1. Oil droplets will form spheres
17

• Liquid-Liquid Systems

• When agitation is stopped, the drive to reduce
  interfacial free energy results in

1. Oil droplets will form spheres 2.
Spherical oil droplets will coalesce.
18

• Liquid-Liquid Systems

• Oil and Water

To stabilize an emulsion, we reduce free energy
by reducing interfacial tension.
19

• Solid-Liquid Systems

liquid
?
liquid
Work of Adhesion
?
L
LS
?
S
solid
solid
?
?
?
WA -
L
S
LS
20

• Work of Cohesion

liquid
Work of Cohesion
?
liquid
L
?
L
liquid
?
?
?
WC 2
L
L
L
21

• Spreading of a Liquid on a Solid

• Spreading of a liquid on a solid will occur when
  the work of adhesion exceeds the work of cohesion.

WA gt WC S Spreading Coefficient S WA -
WC When S is positive, the liquid will spread on
the solid.
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• Spreading Coefficient

S WA - WC
?
?
?
?
S - - 2
L
L
S
LS
?
?
?
S - -
S
L
LS
23

• Questions

• Why do droplets form a spherical shape?
• Why is the surface tension of water greater than
  the surface tension of oil?
• Why is interfacial tension less than surface
  tension?
• Why does a surfactant lower surface tension?
• Why would addition of a surfactant to a capsule
  dosage form improve the oral bioavailability of a
  drug with low aqueous solubility?

24

• Questions

• Why do droplets form a spherical shape?The
  drive to reduce the systems free energy results
  in a configuration that will have the minimum
  surface area. A sphere is the geometrical shape
  with minimum surface area per a given volume.

25

• Questions

• Why is the surface tension of water greater than
  the surface tension of oil?Surface tension
  derives from the cohesive interactions of
  molecules in a phase. Water is polar, and the
  intermolecular interactions between polar
  molecules are stronger than the interactions
  between nonpolar molecules.

26

• Questions

• Why is interfacial tension less than surface
  tension?When two condensed phases are in
  contact, there is more of an interaction "across"
  the interface than that which exists between a
  condensed phase and a gas.

27

• Molecular Forces in the
• Liquid/Vapor Interface

Dispersion force interactions across the
interface are possible. However, the low density
of the gas phase means few molecules are close
enough to interact.
28

• Molecular Forces in the
• Liquid/Liquid Interface

Dispersion force interactions across the
interface are possible. The higher density of the
neighboring liquid phase means molecules are
close enough to interact. The result is a
diminished pull of interfacial molecules into the
bulk of each phase.
29

• Questions

• Why does a surfactant lower surface
  tension?Surfactant molecules are amphiphilic

Polar region
Nonpolar region
30

• Molecular Forces in the
• Liquid/Vapor Interface

Amphiphilic molecules locate preferentially at
the interface, decreasing the difference between
the two phases.
31

• Molecular Forces in the
• Liquid/Liquid Interface

Amphiphilic molecules locate preferentially at
the interface, decreasing the difference between
the two phases.
32

• Questions

• Why would addition of a surfactant to a capsule
  dosage form improve the oral bioavailability of a
  drug with low aqueous solubility?Drugs must
  dissolve to be absorbed. Usually drugs with low
  aqueous solubility are "low energy" solids (e.g.,
  wax) and do not wet well. Surfactants reduce
  interfacial tension and facilitate the wetting
  and spreading of water on low energy solids.