CH3H2 Electrochemistry and Interfaces

What is an interface/surface

a region of finite thickness where two homogeneous bulk phases meet

how do physical properties change around the interface region

they change rapidly over the length of a few molecules but it is not instant

what properties of interfaces influence

• optical appearance

• wetting behaviour

• chemical reactivity

draw a density diagram for this interface

what size of particle allows for surface chemistry to dominate reactivity and why

nanoscale (aka nanoparticles nad mesoporous materials) due to high surface area to volume ratios

give examples of important interfaces

catalytic converter

• solid-gas interface on surface of Pt, Rh or Pd catalysts that remove harmful gases from car exhaust via oxidation and reduction reactions

lungs

• air-liquid interface in alveolar sacs, mediated by the layer of alveolar fluid

describe bulk liquids. draw a diagram

• molecules subject to attractive interactions in all directions

• these all lower the overall potential energy

• net force is zero for each molecule

describe what is happening at a gas-liquid interface. draw a diagram

• molecules at the surface have fewer near neighbours so fewer attractive interactions than bulk liquid

• potential energy increased relative to molecules in the bulk

• unbalanced forces results in net inward pull and contraction of the surface

how does a liquid minimise the energy of the system

reduces the surface area as it is energetically unfavourable for liquid molecules to reside at the surface

a ball has the smallest surface area (lower than cube or prism)

how do you extend an interface

supply energy/do work

what are the equations for the work needed to extend the interface

dA = L dx

dw = 𝛾 dA

• dA = change in surface area

• L = length of film

• dx = change in length

• dw = work needed to extend the interface

• 𝛾 = surface tension (Jm^-2 = Nm^-1)

  • constant of proportionality

what unit is equivalent to Jm^-2

Nm^-1

what are cohesive forces

intermolecular interactions in a liquid

what kind of molecule will have large surface tensions and why

molecules with strong intermolecular forces, such as H-bonding or dipolar interactions

• there is a large energetic cost to placing them at the surface

• the molecules at the surface are strongly bound to each other

what are examples of cohesive forces

• van der Waals

• H-bonding

• metal bonding

• dipole-dipole interactions

rank these liquids from highest to lowest surface tensions: water, hexane, mercury, ethanol

highest → lowest

1. mercury

2. water

3. ethanol

4. hexane

bubble definition

a thin film of liquid containing a gas

what is the difference between a bubble and a cavity

bubbles have gas on either side

cavities is when a gas pocket is in a liquid (e.g. carbonated drinks)

does a bubble have a section of bulk liquid

yes - the thickness of the liquid film is several hundred water molecules thick so the inner part of the film is bulk liquid

why doesn’t a bubble shrink to reduce surface area

shrinking the bubble causes pressure of the gas inside to increase, which requires work to be done

size of the bubble at equilibrium is the balance between energetic cost of creating a surface with the energetic cost of compressing the gas

What does the Young-Laplace equation tell you

links the radius of a bubble, r, to the difference in pressure between the inside, p_in, and outside, p_out, surfaces of the bubble

what is the Young-Laplace equation for a spherical bubble (2 surfaces

∆p = 4𝛾 / r

• ∆p = change in pressure

• r = radius of the bubble

• 𝛾 = constant of proportionality

how to calculate work released due to decreased surface area of a bubble

Area of the original, larger bubble = 2 × 4πr² = 8πr²

Area of the smaller bubble = 2 × 4π(r-dr)² = 8πr² - 16πrdr + 8πdr² = 8πr² - 16πrdr

• as dr² is very small

∴dw_A = 𝛾dA = 𝛾[8πr² - (8πr² - 16πrdr)] = 16𝛾πrdr

how to calculate work required to compress gas in a bubble

p_out exerts force on outer surface, F₂

• F₂ = p_outA = p_out x 4πr²

p_in exerts force on inner surface

• F₁ = p_inA = p_in x 4πr²

net force (F) is due to pressure difference

• F = F₁ - F₂ = p_in - p_out (4πr²)

work required to compress the bubble by distance dr:

• dw_p = -Fdr = -(p_in - p_out)(4πr²)dr = -∆p x 4πr²dr

what is the Young-Laplace equation for a liquid drop or a cavity

∆p = 2𝛾 / r

what does the Young-Laplace equation show for the relationship between pressure difference and radius of curvature of the surface?

inverse relationship to smaller bubbles support larger pressure differences

when ∆p drops to 0, the radius of curvature becomes infinite (a flat surface)

what does it mean if a solute is negatively surface adsorbed

concentration of solute molecules is higher in the bulk

what does it mean if a solute is positively surface adsorbed

concentration of solute is higher at the interface

what is an example of a negatively surface adsorbed solute?

aqueous solutions of simple salts (NaCl)

how do negatively adsorbed solutes affect the surface tension and why?

increase surface tension as hydration shells form around each of the ions

interactions between ions and water molecules are stronger than water-water hydrogen bonds so the closer a water molecule is to an ion, the more it is stabilised (free energy is reduced)

water molecules at the surface are destabilised by having fewer near neighbours and by being further away from any ions

this effect tends to be small

why are salts at much higher concentrations in the bulk than at the surface

hydration shells form around each of the ions

• placing an ion at the interface means it loses part of the hydration shell, which costs more energy than having a solvent molecule at the interface instead

• concentration of ions at the interface is therefore much lower than in the bulk

do negatively surface adsorbed solutes have larger or smaller effects on surface tension than positively surface adsorbed solutes

smaller

what makes a solute positively surface adsorbed

if solute-solvent interactions are weaker than solvent-solvent interactions then the solute will migrate to the surface

how do positively surface adsorbed solute molecules affect surface tension and why

decrease surface tension

• adding organic alcohols to water disrupts the energetically favourable H-bonding network of water. this disruption is minimised by moving alcohol molecule to the surface where there are fewer H-bonds to break

• by concentrating the alcohol molecules at the interface, H-bonding network (which gives water its high surface tension) at the surface is disrupted which reduces the cohesive forces holding the surface together, translating to a reduction in surface tension

what does the Gibbs adsorption isotherm do

relates the rate of change in surface tension with bulk concentration of the solute to the concentration of solute molecules adsorbed at the interface

what is the Gibbs adsorption isotherm equation

𝚪_S = - (c / RT) (d𝛾 / dc) = -(1 / RT) (d𝛾 / d lnc)

• 𝚪_S = concentration of solute molecules adsorbed at the interface

• c = bulk concentration of solute

• d𝛾 / dc = rate of change of 𝛾 (when linear)

• d𝛾 / d lnc = rate of change of 𝛾 (when its plotted as function of lnc)

what value of 𝚪_S would you expect for negatively adsorbed solutes

𝚪_S < 0

what value of 𝚪_S would you expect for positively adsorbed solutes

𝚪_S > 0

• d𝛾 / dc < 0

what are surfactants

a class of molecule that are strongly adsorbed at the air-water interface

what is the structure of surfactants

molecules that contain both polar (hydrophilic) and non-polar (hydrophobic) segments, represented by a polar head group and a line for the non-polar tail

what are the 4 classes of surfactant molecules

anionic

• anionic head with hydrocarbon tail

cationic

• cationic head with hydrocarbon tail

non-ionic

• polar head (e.g. -OCH₂CH₂OH) with hydrocarbon tail

zwitterionic

• zwitterionic head (contains both charges) with hydrocarbon tail

how do surfactants react to being in aqueous solutions

at the interface, the polar head group maximises the favourable electrostatic interactions by being immersed in the water

the non-polar tail groups are largely in the vapour phase to minimise unfavourable solute-solvent interactions and maximising the favourable tail-tail interactions

what happens at the critical micelle concentration (CMC)

as surfactant concentration is increased, surface tension of water decreases

above a certain concentration (the CMC), 𝛾 no longer decreases and instead remains nearly constant

what happens to surface tension at low concentration of surfactant

surface tension decreases linearly w.r.t. surfactant concentration so d𝛾 / dc is constant

what happens to surfactant molecules below the CMC

surfactant molecules mainly exist as monomers at the air water interface

what happens to surfactant molecules around the CMC

the air water interface is saturated with surfactant molecules

what happens to surfactant molecules above the CMC

they can no longer migrate to the interface as it is full and instead begin to self assemble into aggregates known as micelles

what is the structure of micelles

polar shell surrounding the hydrocarbon core