Interfacial Phenomena & Surfactants 3/4

Question 1

Types of Interfaces

Gas-Liquid

Gas-Solid

Answer 1

Immicible phases in pharmaceutical systems

Gas-liquid

-solutions exposed to air = Liquid Aerosols

Gas-solid

-solid dosage forms exposed to air = Solid Aerosols

Question 2

Liquid Liquid

Interfaces

Answer 2

Immicible phases in pharmaceutical systems

Emulsions

Question 3

Liquid-Solid

Interafaces

Answer 3

Immicible phases in pharmaceutical systems

Suspensions

Question 4

Solid-Solid

Interfaces

Answer 4

Immicible phases in pharmaceutical systems

Dry powder particles in

tablets/capsule/other dosage forms

Question 5

Surface Tension

Answer 5

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

Question 6

Interfacial Tension

*Analagous to surface tension @ gas-liquid or gas-solid interafaces

Answer 6

If the two faces are:

both immiscible liquids

both solids

liquid/solid

the force is called interfacial tension

Question 7

Phases combine (mix together)

WHEN?

Answer 7

When

ADHESION > COHESION

dissimilar forces > like forces

then the two phases will combine

Question 8

Surface Tension

determinants

Answer 8

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

Question 9

Interfacial Tension vs WATER

determinants

Answer 9

MERCURY IS THE HIGHEST

the less the molecule wants to interact with water = greater interfacial tension

Question 10

Increasing Temperature

effect on

Surface Tension

Answer 10

Increasing Temperature

v

decreasing surface tension

Question 11

Surface Free Energy

Answer 11

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

Question 12

Spreading

Answer 12

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

Question 13

Wetting

Answer 13

= Spreading but only if the flat surface is

SOLID

if adhesive forces > cohesive forces within the droplet

droplet will spread(wet) the solid surface

Question 14

Spreading Coefficient = S

Answer 14

= 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

Question 15

Spreading Coefficient

determinants

Answer 15

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*

Question 16

Contact Angle

What Degrees are more spreading vs less spreading

Answer 16

Angle between a tagent to the droplet and the surface @ the point of contact.

Contact Angle > 90* = More Spreading

Contact Angle < 90* = Less spreading

Question 17

Scrupulously Clean Glass

Answer 17

= 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

Question 18

Adsorption

Answer 18

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

Question 19

Absorption

Answer 19

penetraion of 1 substance into the interior of the bulk phase

through physical pores or dissolution in the phase

Question 20

Positive Adsorbtion

Answer 20

Molecules that move TOWARD the interface

Reduce interfacial tension

++++++

surfactants

Question 21

Negative Adsorption

Answer 21

Molecules that move AWAY from the interface

INCREASE the interfacial tension

——-

Question 22

Surfactants

Surface Active Agents

Answer 22

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

Question 23

Soaps

Answer 23

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)

Question 24

Anionic Surfactants

-

Answer 24

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

Question 25

**Cationic Surfactants** **++++**

Answer 25

Used primarily as **antiseptic / antimicrobial preservative** has POSITVE CHARGE (nitrogen w/ ch3's in tail) ex. **CTAB, cetylphyridinium chloride**

Question 26

**Zwitterionic Surfactants**

Answer 26

Have both + and - ex. **Alyl bentaines / Lecithin**

Question 27

**Nonionic Surfactants**

Answer 27

most commonly used in **pharmaceutical surfactants** **ex. alcohol ethyoxylate, poloxyethylene** **Tween / Span**

Question 28

**Hydrophilic-Lipophilic Balance** **HLB**

Answer 28

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

Question 29

**Critical Micelle Concentration**

Answer 29

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**

Question 30

Determining the size of a surfactant molecule

Answer 30

1. Use **Gibbs adsorption equation** 1. calculate area occupied by a single molecule 2. --\> calculate molecular surface area 2. **Generate a mololayer film** (layer 1 molecule thick) **on the surface** 1. **​**done by ben Franklin, 1 tsp of fatty acid over a pond

Question 31

**Solid Adsorption**

Answer 31

Both gases & liquids can adsorb to solids. **Physical Adsorption is reversible** - by changing external conc/pressure

Question 32

**Gas Adsorption**

Answer 32

dependedent on **Gas pressure & Temperature** **5 Types of adsorption isotherms** depend on: conditions used & gas/solid studied

Question 33

**Type 1 Isotherm**

Answer 33

Observed when only a **monolayer of gas** can be adsorbed onto the solid surface **FREUNDLICH** ISOTHERM **LANGMUIR** ISOTHERM (regular curve, y limit)

Question 34

**Type 2 / 3** **Isotherm**

Answer 34

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)

Question 35

**Type 4/5** **Isotherms**

Answer 35

Arise when gas adsorbs onto a **Porous solid** where **condensation** inside the pores can occur