Interfacial Phenomena: Surface Active Agents

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Questions and Answers

What is the parameter that reflects the degree of wetting?

Viscosity

Surface tension

Adhesive forces

Contact angle (correct)

What must be decreased to increase wetting?

Viscosity

Surface area

Contact angle (correct)

Cohesive forces

What type of material has a small contact angle with water?

Greasy

Hydrophobic

Non-polar

Hydrophilic (correct)

What type of molecule is necessary to introduce in order to dispense one in the other?

Amphiphile molecule Signup and view all the answers

What effect does increasing the temperature have on surface and interfacial tensions?

Decreases them Signup and view all the answers

What is the key factor for an amphiphile to be concentrated at the interface?

Balancing polar and nonpolar groups Signup and view all the answers

What is the phenomenon observed when a drop of oil is added to water?

The drop spreads as a thin film on the surface of water Signup and view all the answers

What does spreading of lotions, creams, and sunscreens on the skin depend on?

The surface tension and interfacial tension of the liquids involved Signup and view all the answers

How is the displacement of dirt in wound washing achieved?

By using detergents to reduce the interfacial tension Signup and view all the answers

In what form does a drop of oil remain on the surface of water if it cannot spread?

It forms a lens and then disappears Signup and view all the answers

What causes the formation of micelles in the bulk when more surface active agent is added to water?

Saturation of the water surface with surface active molecules Signup and view all the answers

What happens to the solubility of a drug as the concentration of the surface active agent increases beyond the CMC concentration?

Increases sharply Signup and view all the answers

What does the HLB number of a surfactant measure?

The balance between hydrophobic and hydrophilic portions of the surfactant Signup and view all the answers

What type of surfactants have low HLB values and act as water-in-oil emulsifiers?

Oil-soluble surfactants Signup and view all the answers

What is the main reason for micelles to form in the bulk as more surface active agent is added to water?

To protect their hydrophobic groups from the aqueous environment Signup and view all the answers

What effect does an increase in the HLB value of a surfactant have on its properties?

Increases its oil-in-water emulsifying ability Signup and view all the answers

What is the main reason for the majority of particles dispersed in water acquiring a negative charge?

Adsorption of the hydroxyl ion Signup and view all the answers

What is the function of pH and pKa in determining the charge on particles?

To affect the ionization of groups at the particle surface Signup and view all the answers

Which type of compounds are both gram-negative and gram-positive organisms susceptible to?

Cationic quaternary compounds Signup and view all the answers

How do particles dispersed in liquid media acquire a charge mainly?

Through ionization of groups at their surface Signup and view all the answers

What is the primary reason for particles dispersed in water acquiring a negative charge?

Selective adsorption of a particular ionic species Signup and view all the answers

In what way do charges on particles arise from ionization?

By ionization of groups situated at the surface Signup and view all the answers

What may be added to the solution to cause selective adsorption and charge acquisition by dispersed particles?

$\text{OH}^-$ ions Signup and view all the answers

What is the significance of the hydration difference between hydronium and hydroxyl ions in relation to particle charge?

$\text{OH}^-$ ions are more hydrated than $\text{H}^+$ ions, causing preferential adsorption Signup and view all the answers

What is the angle between a liquid droplet and the surface over which it spreads called?

Contact angle Signup and view all the answers

What does a contact angle of Ɵ = 0° indicate?

Complete wetting Signup and view all the answers

Which type of material has a small contact angle with water?

Hydrophilic material Signup and view all the answers

What can be added to decrease the contact angle and increase wetting?

Surfactant Signup and view all the answers

What is the main requirement for the preparation of a suspension?

Dispersion of solid in liquid Signup and view all the answers

Why does mercury rise in the capillary tube with a convex surface?

Due to cohesive forces being more than adhesive forces Signup and view all the answers

What is the initial spreading coefficient when benzene is added to water?

8.9 dyne/cm Signup and view all the answers

What happens to the spreading coefficient after equilibration of benzene and water?

It becomes negative Signup and view all the answers

Why does benzene retract and form a lens on the surface of water after mutual saturation?

Due to a negative final spreading coefficient Signup and view all the answers

What is the importance of considering molecular structures in relation to spreading coefficient?

It affects the degree of wetting Signup and view all the answers

What is the reason for the surface tension of water to stop decreasing and reach a plateau?

The formation of critical micelle concentration (CMC) Signup and view all the answers

Why do surface active agents reduce the surface tension of water?

They interfere with hydrogen bonding between water molecules Signup and view all the answers

What causes the surface tension to decrease with increasing concentrations of the surface active agent?

Interference with hydrogen bonding between water molecules Signup and view all the answers

What is the work of adhesion equal to?

The newly created surface tensions minus the interfacial tension Signup and view all the answers

Which formula expresses the spreading coefficient (S) for a liquid?

$S = \gamma_S - (\gamma_L + \gamma_{LS})$ Signup and view all the answers

Under what conditions will a liquid spread as a film?

When the work of adhesion is greater than the work of cohesion Signup and view all the answers

Study Notes

Wetting and Surface Tension

The degree of wetting is reflected by the contact angle, which is the angle between a liquid droplet and the surface over which it spreads.
A small contact angle is indicative of high wetting, while a large contact angle indicates low wetting.
To increase wetting, the surface tension of the liquid or the interfacial tension between the liquid and the solid surface must be decreased.

Surface Active Agents

Amphiphiles are molecules that are concentrated at the interface between two phases, with a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail.
Increasing the temperature decreases the surface and interfacial tensions, allowing for greater wetting and spreading.

Emulsions and Micelles

When a drop of oil is added to water, it forms a sphere and minimizes its contact area with the water to reduce its energy.
The formation of micelles in the bulk occurs when more surface active agent is added to water, causing the surfactant molecules to aggregate and form a spherical structure.
As the concentration of the surface active agent increases beyond the CMC (critical micelle concentration), the solubility of a drug increases.

Surfactants

The HLB (hydrophilic-lipophilic balance) number of a surfactant measures its balance between hydrophilic and hydrophobic properties.
Surfactants with low HLB values act as water-in-oil emulsifiers, while those with high HLB values act as oil-in-water emulsifiers.
Increasing the HLB value of a surfactant increases its hydrophilicity and its ability to form oil-in-water emulsions.

Particle Charge

The majority of particles dispersed in water acquire a negative charge due to the greater hydration energy of the hydroxyl ion compared to the hydronium ion.
pH and pKa values determine the charge on particles, with particles becoming more negatively charged at high pH and low pKa values.
Particles dispersed in liquid media acquire a charge mainly through ionization, and the hydration difference between hydronium and hydroxyl ions affects the charge.

Particle Dispersion

The primary reason for particles dispersed in water acquiring a negative charge is the greater hydration energy of the hydroxyl ion compared to the hydronium ion.
Charges on particles arise from ionization, and the hydration difference between hydronium and hydroxyl ions affects the charge.
Selective adsorption and charge acquisition by dispersed particles can be achieved by adding ions to the solution.

Spreading Coefficient

The spreading coefficient (S) measures the ability of a liquid to spread on a surface and is equal to the work of adhesion.
The formula for the spreading coefficient is S = γ_s - γ_l - γ_sl, where γ_s is the surface tension of the solid, γ_l is the surface tension of the liquid, and γ_sl is the interfacial tension between the liquid and the solid.
A liquid will spread as a film under the conditions of S > 0, where the surface energy of the solid is greater than the sum of the surface energy of the liquid and the interfacial energy between the liquid and the solid.

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Description

Test your knowledge about surface active agents and their role in decreasing interfacial tension between oil and water. Explore the concepts of interfacial phenomena and the behavior of amphiphile molecules at the interface of oil and water.