Why do electrolytes increase the surface tension of a liquid? Explain, with the help of Gibbs adsorption isotherm.
Electrolytes increase the surface tension of a liquid (mainly water) because their ions are preferentially repelled from the liquid-air interface and prefer to stay in the bulk of the solution, leading to a phenomenon called negative adsorption. This can be explained using the Gibbs adsorption isotherm, which relates surface tension to the concentration of solutes at the interface.
We know that the Gibbs adsorption isotherm provides a quantitative relationship between the change in surface tension (γ) of a solution and the concentration (C) of a solute at constant temperature (T) and pressure (P):
where:
Γ is Surface excess concentration
γ is Surface tension
c is Concentration of the solute (electrolyte) in the bulk solution.
R is Gas constant
T is Absolute temperature
dγ/dc is Rate of change of surface tension with solute concentration.
For electrolytes, dγ/dc > 0, means surface tension increases with increase in the concentration of electrolyte because, electrolytes exhibit negative adsorption (γ < 0), as ions are repelled from the surface. The negative 'Γ' indicates that the surface has fewer ions than the bulk makes it harder to pull the electrolyte away from the bulk and onto the new surface. This increased resistance to surface formation leads to higher surface tension.
Surface tension (γ) arises due to the cohesive forces between molecules at the surface of liquid. In pure water, these forces are due to hydrogen bonding.
When electrolytes (e.g., NaCl) are dissolved, their ions (Na+ and Cl−) interact with water molecules due to ion-dipole interactions. These interactions are stronger in the bulk phase where ions can be fully solvated and surrounded by water molecules.
At the liquid-air interface, solvation is less favorable due to the asymmetry of the environment (fewer water molecules above the surface) leading to the negative adsorption (i.e., lower ion concentration at the surface compared to the bulk).
Due to the depletion of ions at the surface enhances the hydrogen-bonding network of water molecules at the interface, making the surface more tightly bound and increasing surface tension which is exactly opposite of surfactants (surface-active agents) which accumulate at the surface (γ > 0) and reduce surface tension.
Test Your Knowledge
1. Why do electrolytes generally increase the surface tension of a liquid?
a) Their ions are preferentially attracted to the liquid-air interface.
b) Their ions are preferentially repelled from the liquid-air interface.
c) They increase the hydrogen bonding within the bulk solution.
d) They reduce the cohesive forces between liquid molecules.
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Their ions are preferentially repelled from the liquid-air interface.2. The phenomenon where electrolyte ions are depleted from the liquid-air interface is called
a) Positive adsorption
b) Negative adsorption
c) Surface saturation
d) Interfacial tension
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Negative adsorption.3. According to the Gibbs adsorption isotherm, what does a positive value of dγ/dC indicate for electrolytes?
a) Surface tension decreases with increasing concentration.
b) Surface tension remains constant with increasing concentration.
c) Surface tension increases with increasing concentration.
d) The electrolyte is acting as a surfactant.
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Surface tension increases with increasing concentration.4. If for an electrolyte, dγ/dC > 0, what can be inferred about the surface excess concentration (Γ)?
a) Γ > 0 (positive adsorption)
b) Γ < 0 (negative adsorption)
c) Γ = 0 (no adsorption)
d) Γ is unrelated to dγ/dC
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Γ < 0 (negative adsorption).5. How do electrolyte ions affect the hydrogen-bonding network of water molecules at the interface?
a) They enhance it due to stronger ion-dipole interactions at the surface.
b) They disrupt it, leading to a weaker surface.
c) Their depletion from the surface enhances the hydrogen-bonding network of water.
d) They form new hydrogen bonds with water molecules.
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Their depletion from the surface enhances the hydrogen-bonding network of water.6. Which of the following is true regarding surfactants compared to electrolytes in terms of surface activity?
a) Surfactants exhibit negative adsorption and increase surface tension.
b) Surfactants exhibit positive adsorption and increase surface tension.
c) Surfactants exhibit positive adsorption and reduce surface tension.
d) Surfactants exhibit negative adsorption and reduce surface tension.
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Surfactants exhibit positive adsorption and reduce surface tension.7. The cohesive forces between molecules at the surface of pure water are primarily due to
a) Ionic bonds
b) Covalent bonds
c) Hydrogen bonding
d) Van der Waals forces
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Hydrogen bonding.8. Why do ions prefer the bulk phase over the liquid-air interface in an electrolyte solution?
a) They are less solvated in the bulk phase.
b) They are better solvated and surrounded by water molecules in the bulk.
c) The interface has a higher concentration of other ions.
d) There are no ion-dipole interactions at the surface.