interfacial phenomena

Interfacial PhenomenaAna Marie L. Rubenicia,RPh

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.


SURFACE TENSION INTERFACIAL TENSION


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.


Importance I Pharmacya.Adsorption of drugs onto adjuncts in

dosage forms.b.Penetration of molecules through

biological membranes.c.Emulsion formation and stability.d.Dispersion of insoluble particles in

liquid media to for suspensions.

Interfacial PhenomenaClassification of

Interfaces

Solid-solid interface, powder particles in contact.ySS

Solid - solid

Liquid-solid interface, suspensionyLS

Liquid - solid

Liquid-liquid interface, emulsionyLL

Liquid - liquid

Solid surface, table topySV

Gas - solid

Liquid surface, body of water exposed to atmosphereуLV

Gas - liquid

No interface possible-Gas - gas

Types & Examples of InterfaceInterfacial Tension

Phase

Liquid InterfacesSurface 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.”

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

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..

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.

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

Liquid InterfacesMeasuring Surface and Interfacial Tension

1. 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.

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.

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.

Liquid InterfacesSurface 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.

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.

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.

Liquid Interfaces

50.445.845.545.242.4

32 (250)24.6138.93.4

0.22-3.19-13.4

Ethyl alcohol

Propionic acid

Ethyl ether

Acetic acid

Acetone

Undecyclenic acid

Oleic acid

Chloroform

Benzene

Hexane

Octane

Ethylene dibromide

Liquid petrolatum

S (dynes/cm)Substance

Initial Spreading Coefficients, S, at 20◦ C

Interfacial PhenomenaApplication 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.


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.


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.

Interfacial PhenomenaElectric 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.


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.


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.


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.

Adsorption at Liquid Interfaces

Occurs at the surface or interfaces of liquids

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

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

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

HLB Value

Use

0-3

4-6

7-9

8-18

13-15

10-18

Antifoaming agents

W/O emulsifying agents

Wetting agents

O/W emulsifying agents

Detergents

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)

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

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)

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

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.

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.

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

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.

The concentration of the surface active agent affects:

a. Interfacial tensionb. Osmotic pressurec. Detergency[ability to remove soil]d. Light scatteringe. Solubility

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

Osmotic Pressure

Increases as the surface active agent increases

But at CMC it reaches a plateau

Detergency, Solubility, Light Scattering Ability

Increases sharply when the concentration of the surface active agent increases beyond the CMC concentration.

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

Surface Active Agents

Hydrophilic and lipophilic Reside at interfaces and lower the

interfacial tension Can be synthetic or natural Anionic, cationic,nonionic and

zwitterionic

Anionic Surface Active Agents Has a negative charge Widely used in the pharmaceutical and

cosmetic industries Have an unpleasant taste Have skin irritation potential Not compatible with cationic surface

active agents Compatible with nonionics and

zwitterionic surface active agents

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

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

Types of Cationic Surface Active Agents Alkylbenzyldimethyl Ammonium

Salt- germicide

Alkyl trimethyl Ammonium Salt-emulsifiers

-are also very effective germicides

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

Types of Nonionic Surface Active Agents Polyoxyethylene Alkyl Ether- are widely used in the

pharmaceutical and cosmetic industries The longer the polyoxyethylene chain, the more

hydrophilic the molecule and the higher the HLB value Fatty acid Alkanolamides- are used extensively in shampoos as

foam stabilizers and viscosity enhancers Sorbitan Fatty Acid Esters- are oil-soluble and form w/o

emulsions Are widely used in the combination with poloxyethylene

sorbital fatty acid esters Polyoxyethylene Sorbitan fatty Acid Esters [TWEEN]-hydrophilic

and form o/w emulsions Used extensively in the pharmaceutical, cosmetic, and

food industries Alkyl Polyglucoside-used in dishwashing detergents and

shampoos

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

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

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, γ0, and the surface tension of the substate with the film on it.

Langmuir Film Balance

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

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.

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.

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