Mrs. Rubenicia RPh Powerpoint about Interfacial Phenomena - PPharm Lecture

1. Interfacial
Phenomena
Ana Marie L. Rubenicia,RPh

2. Interfacial Phenomena
 When phases exist together, the
boundary between two of them is
termed an interface.
 The properties of the molecules
forming the interface are often
sufficiently from those in the bulk
of each phase that they are
referred to as forming an
interfacial phase.

3. Interfacial Phenomena
SURFACE TENSION INTERFACIAL TENSION

4. Interfacial Phenomena
Several types of interface can exist, depending
on whether the two adjacent phases are in the
solid, liquid or gaseous state.
For convenience, we shall divide these various
combinations into two groups, namely liquid
interfaces and solid interfaces.

5. Interfacial Phenomena
Importance I Pharmacy
b. Adsorption of drugs onto adjuncts in
dosage forms.
c. Penetration of molecules through
biological membranes.
d. Emulsion formation and stability.
e. Dispersion of insoluble particles in liquid
media to for suspensions.

6. Interfacial Phenomena
Classification of Interfaces
Phase Interfacial Types & Examples of Interface
Tension
Gas - gas
- No interface possible
Gas - liquid Liquid surface, body of water
уLV exposed to atmosphere
Gas - solid Solid surface, table top
ySV
Liquid - liquid Liquid-liquid interface, emulsion
yLL
Liquid - solid Liquid-solid interface, suspension
yLS
Solid - solid Solid-solid interface, powder
ySS
particles in contact.

7. Liquid Interfaces
Surface and Interfacial Tension
Surface
 The term surface is customarily used when
referring to either a gas-solid or a gas-liquid
interface.
 “Every surface is an interface.”

8. Liquid Interfaces
 Surface
tension- a force
pulling the
molecules of the
interface together
resulting in a
contracted
surface.
- Force per unit area
applied parallel to
the surface.Unit in
dynes/cm or N/m

9. Liquid Interfaces
Interfacial
tension
 Is the force per
unit length
existing at the
interface
between two
immiscible liquid
phases and like
surface tension,
has the units of
dyne/cm..

10. Liquid Interfaces
 Surface Free
energy – increase
in energy of the
liquid and the
surface of the
liquid increase.
-work must be done
to increase liquid
surface.
γ – surface tension
or surface free
energy per unit
surface.

11. Liquid Interface
 Surface Free energy
W=γ ∆A
where W is work done or surface free energy increase
expess in ergs(dynecm); γ is surface tension in dynes/cm
and ∆ A is increase in are in cm sq.
What in the work required to increase area of a
liquid droplet by 10 cm sq if the surface tension is
49 dynes/cm?
W = 49 dynes/cm x 10 cm sq = 490 ergs

12. Liquid Interfaces
Measuring Surface and Interfacial
Tension
e Du Nouy Ring Method
This method also is called the detachable ring method
and is used to measure both the l surface tension
and interfacial tension. It employs a tensiometer
that consists of a hanging platinum-indium ring of
defined geometry connected with a microbalance.
to the surface tension y of the liquid.
2. Capillary Rise Method
If a capillary tube is placed in a liquid that wets
the surface of the capillary, the liquid will rise
inside the capillary tube and its surface will be
concave.

13. Liquid Interfaces
 When oleic acid is
placed on the
surface of a water ,
a film will be formed
if the force of
adhesion b/n oleic
accid molecules and
water molecules is
greater than the
cohesive forces b/n
the oleic acid
molecules
themselves.

14. Liquid Interfaces
 Work of adhesion(Wa), which is the energy
required to break the attraction between the
unlike molecules.(water to oil)
 Work of cohesion(Wc), required to separate
the molecules of the spreading liquid so that it
can flow over the sublayer.(oil to oil and water
to water)
Spreading of oil to water occurs if the work of
adhesion is greater than the work of cohesion.
Spreading coefficient(S) – difference
between Wa and Wc.
Positive S – if oil spreads over a water surface.

15. Liquid Interfaces
Surface and Interfacial Tension
 When a drop of oil is added on the surface of
water, three things may happen:
1. The drop may spread as a thin film on the
surface of water.(positve S)
2. It may form a liquid lens if the oil cannot
spread on the surface of water.(negative S)
3. The drop may spread as a monolayer film
with areas that are identified as lenses.

16. Liquid Interfaces
 Organic liquids on water are unstable
 Effects og Molecular Structure on Spread
Coefficient(S)
a. Polar groups such as COOH or OH such as
propionic acid and ethanol have high values of S.
b. Increase in carbon chains of acids will lead to
decrease of polar-nonpolar char ratio thus
decrease in S on water. Ex are nonpolar liq
petrolatum fail to spread on water.
 Benzene spreads in water because of its weak
cohesive forces.

17. Liquid Interfaces
 For lotions with
mineral oil base
to spread freely
and evenly on the
skin , its polarity
and spreading
coefficient
should be
increase by the
addition of
surfactants.

18. Liquid Interfaces
Initial Spreading Coefficients, S, at 20◦ C
Substance S (dynes/cm)
Ethyl alcohol 50.4
Propionic acid 45.8
Ethyl ether 45.5
Acetic acid 45.2
Acetone 42.4
Undecyclenic acid 32 (250)
Oleic acid
24.6
Chloroform
13
Benzene
8.9
Hexane
3.4
Octane
0.22
Ethylene dibromide
Liquid petrolatum -3.19
-13.4

19. Interfacial Phenomena

20. Interfacial Phenomena
Application of Surface Active
Agents
 In addition to the use of surfactants as
emulsifying agents, detergents, wetting
agents and solubilizing agents, they find
application as antibacterial and other
protective agents and as aids to the
absorption of drugs in the body.
 A surfactant may affect the activity of a drug
or may itself exert drug action.

21. Interfacial Phenomena
Application of Surface
Active Agents
 Foams and Antifoaming agents
© Any solutions containing surface-active materials
produce stable foams when mixed intimately with air. A
foam is relatively stable structure consisting of air pockets
enclosed within thin films of liquid, gas-in-liquid
dispersion stabilized by a foaming agent. The foam
dissipates as the liquid drains away from the area
surrounding the air globules, and the film finally
collapses.

22. Interfacial Phenomena
Application of Surface
Active Agents
 Agents such as alcohol, ether, castor oil, and
some surfactants may be used to break the
foam and are know as antifoaming agents.
 Foams are sometimes useful in Pharmacy but
are usually nuisance and are prevented or
destroyed when possible. The undesirable
foaming of solubilized liquid preparations
poses a problem in formulation.

23. Interfacial Phenomena

24. Interfacial Phenomena
Electric Properties of
Interfaces
 The Electric Double Layer
 Consider a solid surface in contact with a polar
solution containing ions, for example, an aqueous
solution of electrolyte.

25. Interfacial Phenomena
Electric Properties of
Interfaces
 Nernst and Zeta Potentials
- The potential at the solid surface aa’, due to
the potential determining ion, is the
electrothermodynamic (Nernst) potential, E,
and is defined as the difference between the
actual surface and the electroneutral region of
the solution.

26. Interfacial Phenomena
Electric Properties of
Interfaces
 The potential located at the sheer plane bb’ is
known as the electrokinetic, or zeta potential.
The zeta potential is defined as the difference
in potential between the surface of the tightly
bound layer (shear plane) and the
electroneutral region of the solution.

27. Interfacial Phenomena
Electric Properties of
Interfaces
 Zeta potential has practical application in the
stability of systems containing dispersed
particles, since this potential, rather than the
Nernst potential governs degree of repulsion
between adjacent, similarly charged,
dispersed particles.

28. Adsorption at Liquid
Interfaces of liquids
 Occurs at the surface or interfaces
Surface Active Agents/ Surfactants/ Amphifiles
•Substances with part of their molecule
lipophilic and part hydrophilic move on
their own to the surfaces or interfaces of
the liquid, where they lower the surface
or interfacial tension
•The dual character of their
molecule[hydrophilic & lipophilic]
•Often represented in a graphic form as a
circle with a tail
Circle- Hydrophilic part or the polar
part
Tail- lipophilic or the non-polar part

29. Micelles
 Are formed when the active molecules saturate
the surface of the water
 Often spherical; but may also come in different
forms
Cationic-cations
Anionic-anion
Amphoteric-amphoteric ions
Nonionic-nonions

31. Hydrophile- Lipophile Balance
[HLB]
 The number that describes and makes possible to
organize info about the hydrophilic-lipophilic nature
of the surface active molecule
 An arbitrary scale which was developed by GRIFFIN
in 1949
Davis and Rideal23 formula:
HLB= Σ [Hydrophilic group #] – Σ [lipophilic group #]+7

32. HLB Value Use
0-3 Antifoaming agents
4-6 W/O emulsifying agents
7-9 Wetting agents
8-18 O/W emulsifying agents
13-15 Detergents
10-18 Solubilizing agents
Trial and error
 The combination of the surface active agents has a new HLB value equal
to the algebraic mean of both HLB values:
HLBmixture= ƒHLB1 +(1-ƒ)HLB2
Where ƒ is the fraction of surfactant 1 and the fraction of surfactant 2 is (1-ƒ)

33. Monolayers at the Surfaces
• Substances that reduces the surface tension
of a liquid
Gibbs Adsorption Equation:
Where: Γ =Surface concentration in moles per unit area of surface
C=concentration of the substance
γ = surface tension
R= gas constant
T= temperature
/ C= change in the surface tension

34. Example
The concentration of a
surfactant in water is 0.01 mole/L,
and dγ /dC is –5.87 dyne liter mole-1
cm-1. What is the surface
concentration of the surfactant at
20ºC?
Solution:
Γ = (0.01 mole/L)

35. Critical Micelle Concentration [CMC]
 Shows that the surface tension decreases with increasing
concentrations of the surface active agent then after a certain
concentration of the surface active agent, the surface tension stops
decreasing and reaches a plateau.
 The surface is saturated with surface active molecules and any inc in
their conc will cause them to form micelles in the bulk to protect their
hydrophobic groups from the aqueous envt.
 In aqueous media, decreases as the # of carbons in the hydrophobic
grp of the surface active agent increases
 Decreases with electrolytes in soln

36. Critical Micelle Concentration [CMC]
 Molecules can form aggregates in which the hydrophobic portions
are oriented within the cluster and the hydrophilic portions are
exposed to the solvent. Such aggregates can show a variety of
conformations. The shapes of the aggregates depends largely of the
properties of the amphiphilic molecules.
 The proportion of molecules present at the surface or as aggregates
in the bulk of the liquid depends on the concentration of the
amphiphile. At low concentrations amphiphiles will favor
arrangement on the surface. As the surface becomes crowded with
amphiphiles more molecules will arrange into aggregates. At some
concentration the surface becomes completely loaded with
amphiphile and any further additions leads to arrangement into
aggregates. This concentration is called the Critical Micelle
Concentration(CMC). A graph of surface tension vs log of
concentration may be used to determine the CMC point.

38. Tilted-drop Measurement
• The tilted-drop measurement (Fig. 2e) is another angle measurement. In
this technique, a droplet is added to the surface and the advancing and
retreating contact angle are measured as the surface is tilted up until the
droplet reaches a point where it almost moves. This technique is useful to
measure both the receding and advancing contact angles at the same time.
• In general, contact angle measurements serve as a good initial technique
to characterize a surface. However, contact angle measurements need to
be analyzed with care as a number of factors including operator error,
surface roughness, surface heterogeneity, contaminated fluids, and sample
geometry can influence the overall result.

39.  Figure 1 Figures 1A and 1B demonstrate a
difference in wettability. Figure 1A shows how a
water droplet might appear on a hydrophobic
surface such as wax. Figure 1B shows how a water
droplet might appear on a hydrophilic surface such
as a contact lens

40. Figure 2. Five ways that the contact angle (q) can be
measured. (A.) Sessile or Static drop. (B.) Wilhelmy plate
method. (C.) Captive air bubble method. (D.) Capillary rise
method. (E.) Tilting substrate method. Figure adapted from
Ratner, et. al.
Figure 3. A
generalized contact angle plot showing the
advancing (qAdv) and receding (qRec) contact angles.

41. The concentration of the
surface active agent affects:
a. Interfacial tension
b. Osmotic pressure
c. Detergency[ability to remove soil]
d. Light scattering
e. Solubility

42. Interfacial Tension
 Follows a path parallel to that of the surface
tension
 Decreases with increasing concentration of
the surface active agent until the CMC is
reached, then becomes constant

43. Osmotic Pressure
 Increases as the surface active agent increases
 But at CMC it reaches a plateau

44. Detergency, Solubility, Light
Scattering Ability
 Increases sharply when the concentration of
the surface active agent increases beyond
the CMC concentration.

45. MicelLes
 Are aggregates of surface active agents
 Size varies, but is more than 0.1μm
 # of molecules is approximately 50-100
 Are always in equilibrium with monomers of
surface active agents in soln

46. Surface Active Agents
 Hydrophilic and lipophilic
 Reside at interfaces and lower the interfacial
tension
 Can be synthetic or natural
 Anionic, cationic,nonionic and zwitterionic

47. Anionic Surface Active Agents
 H as a negative char ge
 Widely used in the pharmaceutical and cosmetic industries
 H ave an unpleasant taste
 H ave skin irritation potential
 Not compatible with cationic surface active agents
 Compatible with nonionics and zwitterionic surface active
agents

48. Types of Anionic Surface
Active Agents
 SOAPS-fatty acid chain ranges between 12-18
 Sulfates-most popular
 Toothpaste,shampoos and other cosmetic products as well as in fabric detergents
 Sulfonates-sulfur atom connected to the carbon atom
 Molecule is less liable to hydrolysis than are sulfates
 N-Acyl taurines- good skin compatibility
 Exhibit a good stability over wide ranges of pH
 Compatible with hard water since their Mg and ca salts are soluble
 Monoalkyl phosphate-low skin irritation potential
 Used in face and body liquid cleansers
 Acyl isethionate
 Used in soaps and shampoos for their mildness and foaming properties
 N-Acyl sarcocinate-produce a rich foam and have excellent skin compatibilities

49. Cationic Surface Active Agents
 Has a positive charge
 Can be used as bactericidal agents
 Absorb onto negatively charged surfaces
 Are used as hair conditioners and fabric softeners
 Are electrolytes and are incompatible with anionic
surface active agents
 Compatible with nonionics and zwitterionics
 Quaternary ammonium cmpds are among the most
extensively used cationic surface active agents

50. Types of Cationic Surface Active
Agents
 Alkylbenzyldimethyl Ammonium
Salt- germicide
 Alkyl trimethyl Ammonium Salt-
emulsifiers
-are also very effective
germicides

51. Nonionic Surface Active Agents
 Not electrolytes
 Has no charge
 Are not affected as much by the
presence of salts or charges in pH
 Hydrophilic group may contain
hydroxyl groups, polyoxyethylene
groups, or saccharides

52. Types of Nonionic Surface Active
Agents
 P olyoxyethylene A lkyl Ether - ar e widely used in the pharmaceutical and cosmetic
industries
 The longer the polyoxyethylene chain, the mor e hydr ophilic the molecule and
the higher the H L B value
 Fatty acid A lkanolamides- ar e used extensively in shampo os as foam stabilizers and
viscosity enhancers
 Sorbitan Fatty A cid Esters- ar e oil-soluble and form w/o emulsions
 A r e widely used in the combination with poloxyethylene sorbital fatty acid
esters
 P olyoxyethylene Sorbitan fatty A cid Esters [TWEE N]-hydr ophilic and form o/w
emulsions
 Used extensively in the pharmaceutical, cosmetic, and fo od industries
 A lkyl P olyglucoside-used in dishwashing deter gents and shampo os

53. Zwitterion Surface Active Agents
 Compatible with all types of surface active agents
 Can be anionic, cationic or zwitterionic depending on the pH
of the medium they are in
 Main use is as cosurfactants to boost the foaming properties
of other surfactants
N-alkylbetaines-lead to minimal skin irritation
-hard waters does not affect their foaming properties

54. Insoluble Monolayers at Liquid
Surfaces
 Molecules which are not soluble in the bulk of
liquids
 A.k.a. Langmuir films
If the number of molecules on the surface of the
water is low, the molecules will be far away
from each other, trying to cover the whole
surface

55. Langmuir Film Balance
 An instrument that can control the area of water surface
available for the floating fatty acid molecules
 movable barrier that moves tangiential to the water surface
 Data are presented as plots of the surface pressure π as a
function of the area A per molecule
Surface pressure-the horizontal force between the pure
substrate, γ 0, and the surface tension of the substate
with the film on it.

56. Langmuir Film Balance
• A Langmuir film balance facilitates the
controlled preparation of model
membranes at the air/water interface

57. Walking on water
 Small insects such as the water strider can walk on water
because their weight is not enough to penetrate the
surface.
Floating a needle
 If carefully placed on the surface, a small needle can be
made to float on the surface of water even though it is
several times as dense as water. If the surface is agitated
to break up the surface tension, then needle will quickly
sink.
Don't touch the tent!
 Common tent materials are somewhat rainproof in that
the surface tension of water will bridge the pores in the
finely woven material. But if you touch the tent material
with your finger, you break the surface tension and the
rain will drip through.

58. Soaps and detergents
 help the cleaning of clothes by lowering the surface tension of the water
so that it more readily soaks into pores and soiled areas.
Clinical test for jaundice
 Normal urine has a surface tension of about 66 dynes/cm but if bile is
present (a test for jaundice), it drops to about 55. In the Hay test,
powdered sulfur is sprinkled on the urine surface. It will float on normal
urine, but sink if the S.T. is lowered by the bile.
Washing with cold water
 The major reason for using hot water for washing is that its surface
tension is lower and it is a better wetting agent. But if the detergent
lowers the surface tension, the heating may be unneccessary.
Surface tension disinfectants
 Disinfectants are usually solutions of low surface tension. This allow them
to spread out on the cell walls of bacteria and disrupt them. One such
disinfectant, S.T.37, has a name which points to its low surface tension
compared to the 72 dynes/cm for water.