Surface Tension & Surfactants

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Questions and Answers

What unit is used to measure surface tension?

kg m-1 s-2

N m-1 (correct)

J m-2

What effect do soaps and detergents have on surface tension?

They have no effect on surface tension.

They reduce surface tension. (correct)

They eliminate surface tension completely.

They increase surface tension significantly.

How does the surface tension of water change with temperature?

It remains constant.

It varies randomly.

It increases significantly.

It decreases significantly. (correct)

Which of the following has the highest surface tension at 25°C?

Mercury Signup and view all the answers

What is the typical lifetime of a molecule at the surface?

1 x 10-6 s Signup and view all the answers

How do impurities affect surface tension?

They reduce surface tension. Signup and view all the answers

What does surface free energy represent?

The work required to increase surface area by 1 m2. Signup and view all the answers

At what temperature is water's surface tension listed as 72 mN m-1?

25°C Signup and view all the answers

Which of the following is true about the relationship between surface tension and surface free energy?

They are numerically equivalent. Signup and view all the answers

Which characteristic describes hydrophobic substances at the liquid interface?

They float on the surface of the liquid. Signup and view all the answers

What role do surfactants play in the formation of stable emulsions?

They prevent phase separation by reducing interfacial free energy. Signup and view all the answers

Which of the following surfactants is water soluble?

Sodium dodecyl sulphate Signup and view all the answers

What happens when a temporary emulsion is left undisturbed?

It separates back into distinct oil and water phases. Signup and view all the answers

What is the effect of reducing interfacial tension in an emulsion?

It facilitates the creation of a stable monolayer. Signup and view all the answers

Why is the formation of a mixed monolayer important in emulsions?

It creates a charged environment that prevents coalescence. Signup and view all the answers

What term is used to describe the transition region where two immiscible phases come into contact?

Interface Signup and view all the answers

What happens to surface molecules compared to those in the bulk of a liquid?

They have higher energies and reactivities Signup and view all the answers

What is the phenomenon called when one phase is liquid and the other is gaseous?

Surface tension Signup and view all the answers

Which of the following accurately describes what interfacial tension refers to?

The interaction between two liquid phases Signup and view all the answers

Which of the following pairs represents a solid-gas interface?

Solid surface in air Signup and view all the answers

What is meant by the term 'surface free energy'?

Excess energy at the surface compared to the bulk Signup and view all the answers

Which characteristic is true regarding the surface molecules of a liquid?

They are more influenced by attraction from neighboring molecules Signup and view all the answers

In a surface tension scenario, what phase is typically interacting with a liquid phase?

Gas Signup and view all the answers

What occurs to reduce the energy in water droplets?

They become spherical to minimize surface area. Signup and view all the answers

What is a consequence of increasing the surface area of a solid?

It creates more fine particles that are harder to handle. Signup and view all the answers

What is the primary reason for the spherical shape of water droplets?

To minimize total free energy. Signup and view all the answers

What happens to surface molecules in relation to their bulk counterparts?

They are subjected to a net downward pull. Signup and view all the answers

In terms of dynamic equilibrium, what occurs when surface molecules leave the surface and enter the bulk?

They achieve a balance of forces. Signup and view all the answers

Why must energy be supplied when increasing the surface area of a solid or liquid?

To overcome the cohesive forces holding molecules together. Signup and view all the answers

What best describes surface tension in relation to the surface molecules of water?

They endure a higher inward force that creates contraction. Signup and view all the answers

Which of the following statements about surface free energy is true?

Surface free energy is associated with stable states. Signup and view all the answers

What role do surfactants play in preventing coalescence?

They act as a barrier through steric hindrance or electrostatic repulsion. Signup and view all the answers

How does ionization state affect the partition coefficient of a drug?

Unionized drugs exhibit higher partition coefficients than ionized ones. Signup and view all the answers

Which of the following statements about surfactants is NOT true?

Surfactants only contain hydrophilic regions. Signup and view all the answers

Which factor is crucial for a drug's ability to permeate the gastrointestinal tract?

The drug's lipophilicity/hydrophilicity. Signup and view all the answers

What is the main difference between drug solubility and dissolution?

Solubility is about the maximum concentration, whereas dissolution is the process of a drug dissolving. Signup and view all the answers

Which of the following examples represents a surface-active drug?

Chlorpromazine Signup and view all the answers

What is an essential characteristic of excipients in pharmaceutical preparations?

Excipients must be pharmaceutically compatible with active ingredients. Signup and view all the answers

Which of the following is a common natural source of surfactants?

Lecithin from egg Signup and view all the answers

Study Notes

MPharm Programme - Surface Tension & Surfactants

Interface: A transition region where two immiscible phases meet. Examples include liquid-vapor (liquid surface in air), liquid-liquid (emulsions), solid-liquid (suspensions), solid-vapor (solid surface in air), and solid-solid (solid mixtures).
Surface Tension: The force acting parallel to the surface, at right angles to a 1m line anywhere on the surface. Measured in N m⁻¹. Water's surface tension at 25°C is 72 mN m⁻¹.
Surface Tension Decreases with Temperature: Hotter water is a better cleaning agent because its lower surface tension allows it to penetrate pores more effectively.
Surface Tension and Temperature Correlation: The relationship is approximately linear.
Surface Free Energy: Excess energy at the surface compared to the bulk. The work needed to increase surface area isothermally and reversibly by 1 m². Numerically equivalent to surface tension.
Surface Molecules: Have higher energy and experience an inward pull, leaving the surface and entering the bulk in dynamic equilibrium. Typical lifetime is 1 microsecond.
Wetting Phenomena: A solid's interaction with a liquid (e.g., float on surface or sink). Hydrophobic interactions (no affinity for water) cause floating; hydrophilic (affinity for water) causes sinking.
Contact Angle: The angle between the tangent to the liquid surface and the solid surface at the point of contact. Measures the wettability. Zero degrees is absolute wetting; 90 degrees is partial wetting; above 90 degrees is no wetting.
Measurement of Surface Tension: Methods include Wilhelmy plate and du Nouy tensiometer.
Wilhelmy Plate: Thin rectangular plate (glass or mica) attached to a torsion balance; dipped into the liquid under investigation. Force is calculated to detach the plate; this is used for measurements.
du Nouy Tensiometer (Ring Method): Measuring the force needed to detach a platinum ring from the surface/interface. Detachment force = surface tension * perimeter. Ring radii (inner and outer) are needed for calculations.
Surfactants: Substances with both water-loving (hydrophilic) and oil-loving (hydrophobic) structural components in a single molecule. Amphipathic. Hydrophobic groups (typically hydrocarbon chains) have no affinity for water. Hydrophilic group(s) have affinity for water.
Surfactant Action: Orients at the surface/interface, with the hydrophobic portion away from water. This reduces surface tension and interfacial tension. They can act as barriers to coalescence (for example, droplets in liquid emulsions). Examples of surfactants include sodium dodecyl sulfate and cetostearyl alcohol.
Emulsions: Colloidal systems of two or more immiscible liquids. Examples include oil-in-water (o/w) and water-in-oil (w/o) emulsions. Surfactants are crucial for stabilizing emulsions.
Traube's Rule: For dilute solutions of a homologous series of aliphatic alcohols, the ratio of the concentration at the surface layer to that in the bulk increases by a factor of approximately 3 for each additional -CH2 group.

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Description

This quiz explores the concepts of surface tension and surfactants as part of the MPharm Programme. It covers key topics such as the definition of interfaces, the effects of temperature on surface tension, and the importance of surface free energy. Test your knowledge on these essential aspects of pharmaceutical sciences.