MPharm Programme: Surface Tension & Surfactants
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Questions and Answers

What effect do soaps and detergents have on surface tension?

  • They lower surface tension. (correct)
  • They increase surface tension.
  • They only affect surface tension at high temperatures.
  • They have no effect on surface tension.

Which substance listed has the highest surface tension at 25°C?

  • Ethanol
  • Glycerol
  • Water
  • Mercury (correct)

How does the presence of impurities affect surface tension?

  • It decreases surface tension. (correct)
  • It increases surface tension.
  • It only affects surface tension in liquids with high viscosity.
  • It has no effect on surface tension.

What characteristic do hydrophobic substances display in a liquid?

<p>They float on the surface. (D)</p> Signup and view all the answers

At 25°C, which substance has a surface tension closest to that of octanol?

<p>Liquid paraffin (C)</p> Signup and view all the answers

What is the primary reason surface molecules experience a net inward pull?

<p>They experience stronger cohesive forces. (D)</p> Signup and view all the answers

What does surface tension define regarding force on a surface?

<p>The force acting perpendicular to a drawn line on the surface. (A), The force acting parallel to the surface. (C)</p> Signup and view all the answers

What is the typical lifetime of a molecule at the surface before it enters the bulk?

<p>1 microsecond (D)</p> Signup and view all the answers

What happens to the surface tension of water as temperature increases?

<p>It decreases significantly. (D)</p> Signup and view all the answers

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

<p>It is the work to increase surface area by 1 m². (A)</p> Signup and view all the answers

At what temperature is the surface tension of water approximately 72 mN m-1?

<p>25°C (B)</p> Signup and view all the answers

What effect does higher temperature have on water's cleaning capability?

<p>It makes it a better wetting agent. (D)</p> Signup and view all the answers

What is the relationship between surface tension and surface free energy?

<p>They are numerically equivalent. (D)</p> Signup and view all the answers

What is defined as the transition region where two immiscible phases contact each other?

<p>Interface (C)</p> Signup and view all the answers

What type of interface exists between two liquids?

<p>Interfacial tension (C)</p> Signup and view all the answers

What occurs to reduce the energy at the surface of a liquid?

<p>Formation of droplets (B)</p> Signup and view all the answers

How can the surface free energy of a solid be increased?

<p>Decreasing its particle size (D)</p> Signup and view all the answers

What is a consequence of having higher surface area in solids?

<p>Increased reactivity (D)</p> Signup and view all the answers

What happens to surface molecules compared to those in the bulk?

<p>They have higher energies (D)</p> Signup and view all the answers

Which of the following describes the energy needed to increase the surface area of a solid or liquid?

<p>Energy must be supplied (B)</p> Signup and view all the answers

Surface tension occurs in which type of interface?

<p>Liquid-gas interface (A)</p> Signup and view all the answers

What does the du Nouy tensiometer primarily measure?

<p>The force to detach a ring from a surface (B)</p> Signup and view all the answers

What is the formula for calculating the detachment force using the du Nouy tensiometer?

<p>F = 2π (R1 + R2) γ (D)</p> Signup and view all the answers

What structural characteristic makes surfactants amphipathic?

<p>Having both hydrophobic and hydrophilic components (D)</p> Signup and view all the answers

What is the significance of Traube’s rule in the context of surfactants?

<p>It states that longer chain alcohols are more effective in surfactant solutions (C)</p> Signup and view all the answers

In the structure of surfactants, what does the hydrophobic group typically consist of?

<p>A carbon chain with no affinity for water (D)</p> Signup and view all the answers

What must be true regarding the contact angle when using the du Nouy tensiometer for accurate measurements?

<p>It should be zero (B)</p> Signup and view all the answers

What happens to surfactants in solution in relation to their structural components?

<p>They orient at surfaces with the hydrophobic group away from water (B)</p> Signup and view all the answers

What effect does a longer hydrocarbon chain have on surfactant behavior?

<p>Enhances energetic favorability to adsorb at interfaces (D)</p> Signup and view all the answers

What happens to surface tension when surfactants replace water molecules?

<p>Surface tension decreases due to reduced contractile nature. (C)</p> Signup and view all the answers

Which of the following is a characteristic of emulsions?

<p>They involve immiscible liquids and can be water-in-oil or oil-in-water. (A)</p> Signup and view all the answers

How does the addition of surfactants affect the stability of an oil-in-water emulsion?

<p>Surfactants help form a mixed monolayer, stabilizing the emulsion. (C)</p> Signup and view all the answers

What contributes to the spontaneous separation of oil and water in a liquid paraffin-water emulsion?

<p>The density difference between oil and water. (C)</p> Signup and view all the answers

What is the result of shaking an oil-in-water emulsion?

<p>It temporarily disperses oil droplets in water, increasing interfacial area. (D)</p> Signup and view all the answers

Which statement about surfactants is correct?

<p>They can function as emulsifying agents and foaming agents. (A)</p> Signup and view all the answers

What is the effect of adding sodium dodecyl sulphate to a water-in-oil emulsion?

<p>It stabilizes the emulsion through a mixed monolayer. (B)</p> Signup and view all the answers

Why is the water-water attractive force significant in emulsion stability?

<p>It is stronger than hydrocarbon-hydrocarbon forces. (A)</p> Signup and view all the answers

What is the relationship between contact angle (θ) and wetting of a surface?

<p>θ &lt; 90o indicates good wetting. (D)</p> Signup and view all the answers

What is the purpose of using a Wilhelmy Plate in surface tension measurement?

<p>To determine the force required to detach the plate from the liquid. (C)</p> Signup and view all the answers

In calculating surface tension using the Wilhelmy Plate method, what must be ensured?

<p>The contact angle is zero. (B)</p> Signup and view all the answers

What does the equation γl/v = F/P represent in the context of the Wilhelmy Plate?

<p>The force exerted by the liquid on the plate relative to its perimeter. (C)</p> Signup and view all the answers

How is the perimeter (P) of the Wilhelmy Plate relevant in calculations?

<p>It is used to calculate the surface tension of the liquid. (B)</p> Signup and view all the answers

In the calculation example provided, what is the surface tension of liquid A at 200C?

<p>61.22 mN m-1 (B)</p> Signup and view all the answers

What is a condition for the Wilhelmy Plate to function accurately in static mode?

<p>The force is kept constant to measure changes in surface tension. (A)</p> Signup and view all the answers

Why is the Wilhelmy Plate method referred to as an absolute method?

<p>It involves no corrections or factors in calculations. (B)</p> Signup and view all the answers

Flashcards

Interface

The boundary region where two immiscible phases come into contact.

Surface Tension

The attractive force between molecules at the surface of a liquid, causing the liquid to minimize its surface area.

Interfacial Tension

The attractive force between molecules at the boundary of two immiscible liquids, causing them to minimize their interfacial area.

Surface Free Energy

The excess energy possessed by molecules at the surface compared to those in the bulk.

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Minimizing Surface Area

Reducing the surface area of a liquid reduces its free energy, making it more stable.

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Increasing Surface Area

The process of increasing the surface area of a solid or liquid requires energy input.

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Milling of Solids

Reducing the particle size of a solid increases its surface area, creating more surface free energy.

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Handling Fine Particles

Fine particles have high surface energy, making them difficult to handle and prone to instability.

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Surfactants

Substances that lower the surface tension of a liquid. They do this by concentrating at the liquid's surface, reducing the forces between the molecules.

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Hydrophobic

A material that repels water. It has a low affinity for water and tends to resist being wetted.

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Hydrophilic

A material that attracts water. It easily interacts with water and gets wetted.

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Wetting

The ability of a liquid to spread on a solid surface. It's influenced by the relative attraction of the liquid to the solid and itself. Liquids with lower surface tension and higher attraction to the solid, like water to a hydrophilic surface, wet more easily.

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Liquid/Vapor Surface Tension

The surface tension of a liquid/vapor interface, denoted by γl/v, where γ represents surface tension.

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Interfacial Free Energy (Oil/Water)

The interfacial free energy of an oil/water system, denoted by γo/w, representing the energy needed to increase the interface area.

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Surface Tension of Water

The surface tension of water is 72 mN m-1 at 25°C, meaning it requires 72 millinewtons of force to stretch a 1-meter line on the water surface.

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Surface Tension and Temperature

Surface tension decreases with increasing temperature. This explains why hot water is a better cleaning agent as it can penetrate pores and fissures more effectively due to reduced surface tension.

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Dynamic Equilibrium at the Surface

The process where surface molecules leave the surface and enter the bulk, with an equal number of bulk molecules moving to the surface, leading to a dynamic equilibrium.

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Lifetime of a Surface Molecule

The average time a molecule spends at the surface before moving to the bulk, typically 1 microsecond (1 x 10-6 seconds).

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du Nouy tensiometer

A device that measures the force needed to detach a platinum ring from the surface or interface of a liquid.

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du Nouy tensiometer equation

The force needed to detach a platinum ring from a surface or interface is proportional to the surface tension and the perimeter of the ring.

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Hydrophobic group

The hydrophobic part of a surfactant molecule, usually a hydrocarbon chain, that repels water.

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Hydrophilic group

The hydrophilic part of a surfactant molecule, which attracts water, often containing a charged functional group.

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Traube's rule

The tendency of a surfactant to adsorb at the surface or interface is greater for longer hydrocarbon chains.

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Surfactant orientation

The orientation of surfactant molecules at the surface or interface, with their hydrophobic tails pointing away from the aqueous phase.

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Amphipathic

Surfactant molecules have a dual nature with a hydrophobic tail and a hydrophilic head.

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Surface tension: Energy

The energy required to create a new surface area, which is why liquids tend to minimize their surface area.

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Emulsion

A system that involves two or more immiscible liquids, like oil and water.

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Water-soluble surfactant

A surfactant that is soluble in water, used to create oil-in-water emulsions. Example: Sodium dodecyl sulphate.

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Oil-soluble surfactant

A surfactant that is soluble in oil, used to create water-in-oil emulsions. Example: Cetostearyl alcohol.

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Contact Angle

The angle between a liquid surface and a solid surface at the point of contact. It measures the wettability of a solid by a liquid.

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Absolute Wetting

When a liquid spreads completely over a solid surface. The contact angle is 0 degrees.

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No Wetting

When a liquid does not spread on a solid surface. The contact angle is 180 degrees.

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Wilhelmy Plate Method

A method to measure surface tension using a thin rectangular plate attached to a torsion balance, dipped into the liquid. The plate must be completely wetted (zero contact angle) for accurate measurement.

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Force in Wilhelmy Plate Method

The force required to detach the Wilhelmy Plate from the liquid depends on the surface tension and perimeter of the plate.

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Absolute Wilhelmy Plate Method

The Wilhelmy Plate method is an absolute method, meaning it doesn't require correction factors.

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Static Mode of Wilhelmy Plate Method

Measuring changes in surface tension over time or other variables by keeping the Wilhelmy Plate at a constant depth.

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Surface Tension Calculation

Calculation of surface tension using force (F) and perimeter (P) of the Wilhelmy Plate: γl/v = F/P

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Study Notes

MPharm Programme: Surface Tension & Surfactants

  • Surface tension is a property of liquids at their interface with a gas, where the liquid-gas interface acts like a skin. A similar concept, interfacial tension, applies where both phases are liquids.
  • Surface tension arises from cohesive forces between liquid molecules, pulling them inwards. Surface molecules experience a net downward pull, which leads to a tendency for the surface to contract.
  • Surface molecules have more energy than molecules in the bulk of the liquid, due to their differing interactions with neighbouring molecules.
  • The energy needed to increase liquid surface area is called surface free energy.
  • Water droplets assume a spherical shape due to the minimizing of surface area.
  • Larger surface areas in solids (e.g., fine powders) create more free energy, making them more reactive.
  • Increasing surface area in solids or liquids demands energy input.
  • Surface tension is the force acting parallel to the surface, perpendicular to a one-meter line on the surface. It's numerically equivalent to surface free energy.
  • Surface tension of water (25°C) is 72 mN m⁻¹ and declines with increasing temperatures. This impacts pore/fissure penetration in cleaning agents (hot water is more effective than cold).
  • Surfactants are amphipathic molecules, combining hydrophobic (water-repelling) and hydrophilic (water-attracting) regions.
  • Surfactants lower surface/interfacial tension by adsorbing at interfaces. Hydrophobic tails protrude away from the aqueous phase, while hydrophilic heads engage with the aqueous phase.
  • The longer the surfactant chain, the more energetically favorable adsorption at interfaces and the greater the tendency for the molecule to leave the aqueous phase.
  • Traube's rule describes how the ratio of surfactant concentration at the surface, to the bulk concentration, increases with the number of methylene groups in the alkyl chain. This occurs due to the increase in the tendency to escape from the aqueous phase.
  • Emulsions (colloidal mixtures of two immiscible liquids) are stabilized by surfactants, forming a boundary film between the phases.
  • Surfactants are used in various pharmaceutical preparations (e.g., emulsifying agents, detergents, solubilizing agents, wetting agents, foaming & antifoaming agents, flocculating agents).
  • Several methods exist for measuring surface tension (e.g., Wilhelmy plate method, du Nouy tensiometer).

Measurement Methods

  • Wilhelmy Plate: A thin plate dipped into the liquid; the force needed to remove it is measured to calculate surface tension. Zero contact angle is important for accurate readings. Can be used in static mode to track changes in surface tension over time.
  • du Nouy Tensiometer: A platinum ring is dipped into the liquid and the force to detach it from the surface is measured to establish surface tension. Requires zero contact angle and a correction factor for accurate readings.

Calculation Examples

  • Calculation of surface tension from a Wilhelmy plate force measurement involves dividing the measured detachment force by the perimeter of the plate. Ensuring all measurements are in consistent unit systems (e.g., meters).

Surfactant Action/Examples

  • Liquid paraffin and water emulsions: Surfactants reduce interfacial tension between immiscible liquids, allowing them to be mixed and creating a stable emulsion.

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Surface Tension and Surfactants 2024 (PDF)

Description

Explore the intricate properties of surface tension and surfactants in liquids. This quiz delves into the physics of liquid interfaces, the concept of interfacial tension, and the energetic dynamics involved in surface area changes. Perfect for MPharm students looking to deepen their understanding of these essential topics.

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