Liquid Interfaces and Surface Tension

• Interfacial phenomena relates to the interfaces and surfaces between phases.
• The phases involved are gas, liquid, and solid.
• Interfaces are boundaries between binary systems.

Examples of Interfaces

• Gas/gas mixtures have no interface.
• Gas/liquid interfaces includes liquid surfaces, such as bodies of water exposed to the atmosphere.
• Gas/solid interfaces examples include solid surfaces such as tabletops in contact with the atmosphere.
• Liquid/liquid interfaces include emulsions (oil/water).
• Liquid/solid interfaces include suspensions (solids suspended in a liquid).
• Solid/solid interfaces includes powder particles in contact.

Liquid/Gas Interface

• The molecular theory of surface tension applies.
• Bulk molecules experience symmetric cohesive forces.
• Surface molecules experience asymmetric adhesive forces.
• Cohesion is the intermolecular attraction between like molecules.
• Adhesion is the force of attraction between molecules of different substances.
• There is an inward pull of surface molecules to the bulk.
• A liquid's surface is under tension due to unbalanced forces and the inward pull.
• Liquids tend to contract in area and have the least number of molecules on the surface
• Spheres have the least surface area for a given volume.
• Liquid drops are spherical to minimize surface area.
• Surface contraction is surface tension.
• Energy is needed to enlarge a contracted surface.
• Transferring molecules from the bulk to the surface requires energy.

Surface Free Energy

• "γ" represents surface free energy.
• Work "w" required to increase the surface area of a liquid by 1 cm², with units in erg/cm².
• Total work "W" is measured in ergs, where W = γ * ΔA.
• γ is calculated as W / ΔA, measured in erg/cm².

Surface Tension

• "γ" represents the force acting at right angles to any line of 1 cm length in the liquid surface, and its units are dyne/cm.
• Surface free energy and surface tension are numerically equal and dimensionally equivalent.
• Water has a surface tension of 72.8 dyne/cm and a surface free energy of 72.8 erg/cm².

Interfacial Tensions

• The force per unit length at the interface between two immiscible liquid phases (A & B), measured in dyne/cm.
• Surface tension measures for Water is 72.8, Benzene is 28.9, Chloroform is 27.1, and Ethanol is 22.
• Interfacial tension measures for Benzene 35.5 and Chloroform is 32.4.

Spreading Coefficient

• Spreading occurs when adhesion is greater than cohesion; this leads to film formation.
• Work of adhesion (Wa) prevents miscibility between two liquids.
• Wa is the extent of attraction betweeen two liquids.
• γLS is destroyed after separation, and γL and γS are created.
• In general, W = γ * ΔA and For 1 cm², W = γ.
• The required work for seperation Wa = γS + γL - γLS

Work of Cohesion

• Work of cohesion (Wc), where Wc = 2 γL because no interface exists between like molecules.

Spreading

• Spreading coefficient "S" = Wa - Wc.
• S = γS + γL - γLS - 2 γL = γS - (γLS + γL).
• γL is surface tension of the spreading liquid
• γS is the surface tension of the sublayer liquid.
• If "S" is positive, spreading occurs, like in oleic acid.
• If "S" is negative, no spreading occurs, as seen with mineral oil.
• Spreading can be described by the likelihood of spreading in terms of cohesion and adhesion.
• It represents a balance betweem adhesional and cohesional forces.
• If Wadhesion > Wcohesion, it would result in spontaneous spreading of B over surface A. Spreading will occur if the surface tension is equal to or greater than 0

Spreading: Two Liquids

• σhexane measures 18.0 dyne/cm.
• σdecane measures 23.9 dyne/cm.
• σoleic acid measures 32.5 dyne/cm.
• σbenzene measures 28.9 dyne/cm.
• σwater measures 72.8 dyne/cm.
• γ water/hexane measures 50.8 dyne/cm.
• γ water/decane measures 52.3 dyne/cm.
• γ water/oleic acid measures 15.6 dyne/cm.
• γ water/benzene measures 35.0 dyne/cm.
• Hexane; decane, benzene, and oleic acid spread on water

Measurement of Surface and Interfacial Tension

• Several methods are available, including capillary rise, drop weight/volume, and du Nuoy tensiometer.

Capillary Rise Method

• Measures the height to which a liquid rises in a capillary tube based wetting.
• Adhesive force between water and glass causes the liquid to rise to a certain distance in the glass capillary.
• The liquid rises until two opposing forces are balanced.
• Upward force (surface tension) is the force of the surface that causes the liquid to rise where γ = F/L, F = γ.L = γ.2Πr
• Downward force is the gravitational pull by the liquid where f = m.g = v.d.g = Πr2h.d.g
• When the liquid stops, the upward force is equal to the downward force.
• γ. = 1\2 r.h.d.g measured in dyne\cm for surface tension.

Drop Weight/Volume Method

• The drop falls when two forces are balanced.
• Surface tension (upwards) occurs when F=γ.2Πr.
• Drop weight (acts downwards) is when F=m.g γ.2Πr=m g 1=2
• Experiments are repeated for two liquids, using water as reference.

Measurements in Experiment

• 2Πr γ₁=m1g and 2Π r γ₂=m2 g γ1/γ2=m1/m2 (m= v.d)
• v= drop volume V /n
• V is the total volume of liquid.
• n is the number of drops.
• γ1/γ2 = (V/n₁)*d₁ / (V/n2)*d2 = n2d1/n1d2
• If density is not given, assume for d₁=d2 simplicity

Ring Method (DuNouy Tensiometer)

• Used for both surface and interfacial tension measurements.
• γ = Dial reading (dyne) / 2 * ring circumference x CF Correction factor (CF) depends on:
  • Radius of the ring
  • Radius of the wire of the ring
  • Volume of liquid raised out of the surface

Surface Active Agents (Surfactants)

• SAA molecules are amphiphilic, possessing both nonpolar (tail) and polar (head) regions.
• An amphiphile is adsorbed at the interface because nonpolar groups have a weak adhesion force with water, causing weak solubility.
• Nonpolar groups interfere with the cohesive force between water molecules, leading to being rejected from water.
• Polar groups are directed to the bulk, satisfying both tendencies at the interface and forms a monolayer.
• Orientation of surfactant molecules varies with concentration.

Surfactant Molecule Orientation

• At low SAA concentration: surfactant molecules orient at the air/oil or water interface, with hydrophobic tails extending outwards.
• At high SAA concentration: surfactant molecules form a monolayer, and excess molecules may remain in the water.
• Examples of surfactants with varying behavior are:
  • Ethanol (CH3-CH2-OH), which is miscible with water in all proportions.
  • Amyl alcohol has reduced water solubility and is surface active at the interface.
  • Cetyl alcohol (C16H33 - OH), which is insoluble in water.
• A relatively large hydrophobic portion is required for significant surface activity.

Traube's Rule

• States, in dilute solutions of homologous compounds, the molar concentration to lower surface tension decreases threefold for each additional CH2 group in the hydrocarbon chain.
• For example, to reduce γ of water from 72.8 dyne/cm to 50 dyne/cm.
• Either of these are needed: 2.03 moles of ethanol (2C), 0.6 moles of propanol (3C), 0.18 moles of butanol (4C), 0.06 moles of pentanol (5C), or 0.02 moles of hexanol (6C).
• The concentration required for equal surface tension lowering decreases by ~3 fo each additional CH₂ group.

Factors Affecting Surface Tension

• Increase in temperature decreases surface tension due to reduced intermolecular forces and structure loss.
• Surface active agents increase adhesive force and decreases surface tension.
• Electrolytes increase inward pull and surface tension.

Explore surface tension, its causes at liquid interfaces, and its relationship to surface free energy. Understand how surface tension affects the shape of liquid drops and the behavior of immiscible liquids. Investigate scenarios with no interface and the forces acting within a liquid's surface.