Interfacial Phenomena & Surfactants 3/4 Flashcards
Types of Interfaces
Gas-Liquid
Gas-Solid
Immicible phases in pharmaceutical systems
Gas-liquid
-solutions exposed to air = Liquid Aerosols
Gas-solid
-solid dosage forms exposed to air = Solid Aerosols
Liquid Liquid
Interfaces
Immicible phases in pharmaceutical systems
Emulsions
Liquid-Solid
Interafaces
Immicible phases in pharmaceutical systems
Suspensions
Solid-Solid
Interfaces
Immicible phases in pharmaceutical systems
Dry powder particles in
tablets/capsule/other dosage forms
Surface Tension
When one phase is a liquid and the other is a
GAS
the net force is called Surface Tension
Cohesive (like molecules) forces
PARALLEL force per unit to measure
Interfacial Tension
*Analagous to surface tension @ gas-liquid or gas-solid interafaces
If the two faces are:
both immiscible liquids
both solids
liquid/solid
the force is called interfacial tension
Phases combine (mix together)
WHEN?
When
ADHESION > COHESION
dissimilar forces > like forces
then the two phases will combine
Surface Tension
determinants
More Polar = Greater Surface Tension
ex. Water / Glycerine
Determined by the degree to which molecueles of the liquid attract to one another (cohere)
Electrostatic interactions
Interfacial Tension vs WATER
determinants
MERCURY IS THE HIGHEST
the less the molecule wants to interact with water = greater interfacial tension
Increasing Temperature
effect on
Surface Tension
Increasing Temperature
v
decreasing surface tension
Surface Free Energy
Potential energy of moving a molecule from the interior of a phase to the surface
work is done against the force of surface tension
Proportional to total surface area
Spreading
When you drop a Liquid on a horizontal flat surface
(solid or immiscible liquid)
Extent of spreading depends on the balance between
Adhesive forces & Cohesive forces
Wetting
= Spreading but only if the flat surface is
SOLID
if adhesive forces > cohesive forces within the droplet
droplet will spread(wet) the solid surface
Spreading Coefficient = S
= Difference between
Work of Adhesion - Work of Cohesion
if POSITIVE (S>0), droplet will spread over the surface
if negative, droplet will remain intact as a lens-shaped globule
Spreading Coefficient
determinants
High spreading coefficients (+) on water
Completely/patially miscible w/ water
ex. ethyl alcohol / ethyl ether
* Substances with low or NEGATIVE coefficients*
* are poorly soluble in water*
Contact Angle
What Degrees are more spreading vs less spreading
Angle between a tagent to the droplet and the surface @ the point of contact.
Contact Angle > 90* = More Spreading
Contact Angle < 90* = Less spreading
Scrupulously Clean Glass
= Very High surface free energy
in respect to water
is high enough to overcome internal cohesive forces in a water droplet
–>Water will completely wet clean glass surface
Adsorption
refers to migration towards or away from a interface (surface)
ALWAYS a surface phenomenon
~sticking to surfaces
tend to spontaneously toward/away from the interface
Absorption
penetraion of 1 substance into the interior of the bulk phase
through physical pores or dissolution in the phase
Positive Adsorbtion
Molecules that move TOWARD the interface
Reduce interfacial tension
++++++
surfactants
Negative Adsorption
Molecules that move AWAY from the interface
INCREASE the interfacial tension
——-
Surfactants
Surface Active Agents
Chemical substances, due to molecular properties,
that tend to migrate interfaces between liquids/surrounding other phases = Positively Adsorbed to surface
Ampiphiles / small polar group attached to a long non-polar group
there are four categories of surfactants
Soaps
first form of surfactant
Animal Fats / Vegetable oils + Hot Water
(in presensce of Alkali metal hydroxides / carbonite salts)
Produce–> SOAPS (free glycerol)
through hydrolysis = saponification
ex. Potassium oleate (from olive oil)
Sodium stearate (from beef tallow)
Anionic Surfactants
-
Highly water soluble surfactants that promote solubilization/suspension of oily/greasy materials in water
Soaps made from natural fats & Oils
Detergents (synthetic, not soaps)
Have NEGATIVE charges (sulfate-tail)
Ex. SDS, sodium alkyl benzensulfonic acid
Cationic Surfactants
++++
Used primarily as antiseptic / antimicrobial preservative
has POSITVE CHARGE (nitrogen w/ ch3’s in tail)
ex. CTAB, cetylphyridinium chloride
Zwitterionic Surfactants
Have both + and -
ex. Alyl bentaines / Lecithin
Nonionic Surfactants
most commonly used in pharmaceutical surfactants
ex. alcohol ethyoxylate, poloxyethylene
Tween / Span
Hydrophilic-Lipophilic Balance
HLB
Scale that compares surface-active properties of various surfactants as they relate to use
Greater (18) = detergents/solubizing agents
Middle (6-12) = wetting/spredding agents
Low (0-3) = antiFoaming agents
Critical Micelle Concentration
Adding surfactant to a aquious solution results in an accumulation of suractant –> Reduces Surface tension
Once surface is Completely Saturated & surface tension can no longer be reduced.
Micelles are formed @ this concenctration
Determining the size of a surfactant molecule
- Use Gibbs adsorption equation
- calculate area occupied by a single molecule
- –> calculate molecular surface area
-
Generate a mololayer film (layer 1 molecule thick) on the surface
- done by ben Franklin, 1 tsp of fatty acid over a pond
Solid Adsorption
Both gases & liquids can adsorb to solids.
Physical Adsorption is reversible
- by changing external conc/pressure
Gas Adsorption
dependedent on Gas pressure & Temperature
5 Types of adsorption isotherms
depend on:
conditions used & gas/solid studied
Type 1 Isotherm
Observed when only a monolayer of gas can be adsorbed onto the solid surface
FREUNDLICH ISOTHERM
LANGMUIR ISOTHERM
(regular curve, y limit)
Type 2 / 3 Isotherm
observed when gases adsorb to the surface of
nonporous solids
but condense to form multiple layers after the initial monolayer has saturated the surface
BET MODEL
(s-looking curve and regular curve w/ x-limit)
Type 4/5 Isotherms
Arise when gas adsorbs onto a
Porous solid
where condensation inside the pores can occur
