40 Questions
What is the parameter that reflects the degree of wetting?
Contact angle
What must be decreased to increase wetting?
Contact angle
What type of material has a small contact angle with water?
Hydrophilic
What type of molecule is necessary to introduce in order to dispense one in the other?
Amphiphile molecule
What effect does increasing the temperature have on surface and interfacial tensions?
Decreases them
What is the key factor for an amphiphile to be concentrated at the interface?
Balancing polar and nonpolar groups
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
What does spreading of lotions, creams, and sunscreens on the skin depend on?
The surface tension and interfacial tension of the liquids involved
How is the displacement of dirt in wound washing achieved?
By using detergents to reduce the interfacial tension
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
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
What happens to the solubility of a drug as the concentration of the surface active agent increases beyond the CMC concentration?
Increases sharply
What does the HLB number of a surfactant measure?
The balance between hydrophobic and hydrophilic portions of the surfactant
What type of surfactants have low HLB values and act as water-in-oil emulsifiers?
Oil-soluble surfactants
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
What effect does an increase in the HLB value of a surfactant have on its properties?
Increases its oil-in-water emulsifying ability
What is the main reason for the majority of particles dispersed in water acquiring a negative charge?
Adsorption of the hydroxyl ion
What is the function of pH and pKa in determining the charge on particles?
To affect the ionization of groups at the particle surface
Which type of compounds are both gram-negative and gram-positive organisms susceptible to?
Cationic quaternary compounds
How do particles dispersed in liquid media acquire a charge mainly?
Through ionization of groups at their surface
What is the primary reason for particles dispersed in water acquiring a negative charge?
Selective adsorption of a particular ionic species
In what way do charges on particles arise from ionization?
By ionization of groups situated at the surface
What may be added to the solution to cause selective adsorption and charge acquisition by dispersed particles?
$\text{OH}^-$ ions
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
What is the angle between a liquid droplet and the surface over which it spreads called?
Contact angle
What does a contact angle of Ɵ = 0° indicate?
Complete wetting
Which type of material has a small contact angle with water?
Hydrophilic material
What can be added to decrease the contact angle and increase wetting?
Surfactant
What is the main requirement for the preparation of a suspension?
Dispersion of solid in liquid
Why does mercury rise in the capillary tube with a convex surface?
Due to cohesive forces being more than adhesive forces
What is the initial spreading coefficient when benzene is added to water?
8.9 dyne/cm
What happens to the spreading coefficient after equilibration of benzene and water?
It becomes negative
Why does benzene retract and form a lens on the surface of water after mutual saturation?
Due to a negative final spreading coefficient
What is the importance of considering molecular structures in relation to spreading coefficient?
It affects the degree of wetting
What is the reason for the surface tension of water to stop decreasing and reach a plateau?
The formation of critical micelle concentration (CMC)
Why do surface active agents reduce the surface tension of water?
They interfere with hydrogen bonding between water molecules
What causes the surface tension to decrease with increasing concentrations of the surface active agent?
Interference with hydrogen bonding between water molecules
What is the work of adhesion equal to?
The newly created surface tensions minus the interfacial tension
Which formula expresses the spreading coefficient (S) for a liquid?
$S = \gamma_S - (\gamma_L + \gamma_{LS})$
Under what conditions will a liquid spread as a film?
When the work of adhesion is greater than the work of cohesion
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.
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.
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