PRU Revision PDF

Topic 1 Learning outcome 1: Factors responsible for surface tension Key information (page 2): Molecules in the bulk liquid are subject to equal forces of attraction in all directions, but molecules at the liquid-air interface experience an unbalanced inward force, causing surface tension. Surface te...

Topic 1 Learning outcome 1: Factors responsible for surface tension Key information (page 2): Molecules in the bulk liquid are subject to equal forces of attraction in all directions, but molecules at the liquid-air interface experience an unbalanced inward force, causing surface tension. Surface tension can be considered as the energy per unit area (J/m^2) required to form a surface or the force per unit length (N/m) along the boundary. Surface tension arises from intermolecular forces like Van der Waals forces (London dispersion forces), hydrogen bonding, ionic interactions, and metal bonding that cause an inward attraction of molecules at the liquid interface (page 2). These intermolecular forces result in an unbalanced inward force on molecules at the air-liquid interface compared to those in the bulk liquid, which experience equal forces in all directions. This inward force causes the surface to contract and results in surface tension (pages 2-3). Surface tension can be quantified by dividing the surface energy (J) required to form a unit area (m2) of surface. The resulting value of J/m2 is the numerical value of surface tension (page 3). The types of intermolecular forces responsible for the surface tension in different systems can be expressed as the sum of contributions from Van der Waals forces, hydrogen bonding, metal bonding, and ionic interactions (page 3). Learning outcome 2: Why soap bubbles exist Key information (pages 5-6): A soap bubble is a thin film of liquid with gas inside. The thin film consists of many layers of molecules like the bulk liquid. For a stable bubble, the inward force due to surface tension is balanced by the outward force due to higher gas pressure inside the bubble, as described by the Young-Laplace equation (no text cited). A soap bubble is a thin film of liquid with gas inside. The thin film consists of many layers of molecules like the bulk liquid (page 5). If the bubble were to shrink in size to reduce its total surface area, this would allow more molecules to move from the surface into the inner bulk liquid portion of the film (page 5). However, shrinking the bubble would cause an increase in the gas pressure inside the bubble, as the gas would be compressed into a smaller volume. This increased internal gas pressure would do work to resist the bubble shrinking (page 6). For a stable bubble, the inward force due to surface tension must be balanced by the outward force exerted by the higher gas pressure inside the bubble. This balance is described quantitatively by the Young-Laplace equation derived on pages 6-7. Learning outcome 3: Young-Laplace equation for bubbles and cavities, and how it can be used: Key information (pages 6-7): The Young-Laplace equation states that for a bubble or cavity, the pressure difference Δp across the curved interface is equal to 2γ/r, where γ is the surface tension and r is the radius of curvature. This equation demonstrates the balance between surface tension and internal gas pressure that allows bubbles to exist in stable shapes. The Young-Laplace equation states that for a bubble or cavity, the pressure difference Δp across the curved interface is equal to 2γ/r, where γ is the surface tension and r is the radius of curvature (pages 6-7). This equation is derived by equating the change in surface energy to the work done by the change in internal pressure when a bubble shrinks slightly in size (pages 6-7). For a liquid drop or cavity with only one surface (no internal gas), the equation is Δp = 2γ/r, with the pressure difference equaling the inward force of surface tension (page 7). The Young-Laplace equation demonstrates how surface tension causes an increase in internal pressure for spherical interfaces like bubbles. It quantitatively explains the balance between forces that allows bubbles to exist in stable, curved shapes. The equation can be applied to problems involving relating pressure differences and forces to the geometry of curved liquid interfaces. Learning outcome 4: How surface tension is affected by additives Key information (pages 8-10): The presence of ions or solutes can either increase or decrease surface tension depending on whether they are preferentially excluded from (negatively surface active) or accumulated at (positively surface active) the interface. Surfactant molecules strongly adsorb at the interface, drastically lowering surface tension. Their behaviour leads to a discontinuity in surface tension vs. concentration plots at the critical micelle concentration (CMC). The presence of ions or solutes can either increase (for negatively surfaceactive solutes that are preferentially excluded from the interface) or decrease (for positively surface active solutes that accumulate at the interface) the surface tension (page 8). Surfactant molecules strongly adsorb at the air-water interface due to their amphiphilic structure with both hydrophilic and hydrophobic parts. This drastically lowers the surface tension compared to pure water (pages 8-9). Plots of surface tension versus surfactant concentration typically show a discontinuity at the critical micelle concentration (CMC), above which micelle formation occurs. Below the CMC, surfactants mainly exist as monomers that adsorb to the interface (page 9). The surface pressure Π, which is the expanding pressure exerted by adsorbed surfactant molecules, can be calculated as the reduction of surface tension compared to the pure solvent: Π = γ° - γ (page 10). Dynamic changes in surface tension allow lung alveoli to rapidly adsorb and desorb surfactant molecules during breathing (page 10). Learning outcome 5: Gibb’s Adsorption and its use to predict surface coverage (surface excess) Key information (pages 12-13): Gibbs adsorption isotherm relates the change in surface tension with bulk concentration to the amount adsorbed at the interface. It derives the relationship dγ/dc = -RTΓ/c, allowing calculation of the surface excess Γ from experimental surface tension data. The Gibbs adsorption isotherm relates the change in surface tension (dγ) with changing bulk concentration (dc) of a solute to the amount of solute adsorbed at the interface (the surface excess, Γ) (pages 12-13). It derives the mathematical relationship: dγ/dc = -RTΓ/c. R is the gas constant, T is temperature. (page 12) This allows the surface excess Γ to be calculated from experimental measurements of how surface tension changes with bulk concentration c. (page 13) A worked example is provided to demonstrate using the Gibbs isotherm equation to calculate Γ for n-butanoic acid in water from its measured dγ/dc value at a given concentration. (pages 13-14) The surface excess can then be used to find the average area occupied by each adsorbed molecule at the interface. (page 14) Topic 2 Learning outcome 3 on using a Langmuir trough and Wilhelmy plate to measure pressure-area isotherms: A Langmuir trough contains a liquid subphase (usually water), with movable barriers on each side to allow compression of the floating monolayer (pages 2-3). A Wilhelmy plate (made of filter paper or platinum) is attached to a microbalance and lowered onto the air-water interface. The surface pressure is calculated from the difference in plate force measurements between the clean surface and when the monolayer is present using the equation: Δπ = -Δγ = ΔF/P, where P is the perimeter of the plate (page 3). By slowly compressing the monolayer barriers and measuring the surface pressure exerted using the Wilhelmy plate method, a pressure-area (π-A) isotherm can be obtained. The shape of the isotherm provides information about the phase behavior and interactions within the monolayer (pages 3-4). Learning outcome 4 on the principles of fluorescence: Fluorescence occurs when molecules absorb photons, promoting electrons to a higher energy excited singlet state. The molecule then relaxes to the ground state via internal conversion, emitting a photon of lower energy/longer wavelength in a stochastic process (page 6). This Stokes shift between absorption and emission wavelengths allows emitted photons to be detected against the high background of scattered excitation photons (page 7). Fluorescence emissions are incoherent and radiate out in all directions from the excited fluorophore (page 7). Common fluorescent probes used in monolayer studies include fluorescein and its derivatives, as well as green fluorescent protein (pages 7-8). Learning outcome 5 on visualizing monolayer composition using fluorescence: Fluorescence microscopy can be used to image domains within monolayers containing different compositions based on fluorescence intensity (page 9). An example given is imaging a DPPC phospholipid monolayer containing a small fraction (