SURFACE TENSION • The surface tension is denoted by the Greek letter gamma (γ) • The surface tension γ is the magnitude F of the force exerted parallel to the surface of a liquid divided by the length L of the line over which the force acts: – γ =F/L
• SI Unit of Surface Tension: N/m • Liquid: – relatively high density – fixed volume (posses a mobility at the molecular level than solid) – Interfaces between 2 phases - homogenous mobility
• Liquid surface: – Having an “elastic” skin
Surface Tensions of Common Liquids Liquid Surface Tension γ (N/m) Benzene (20 °C) 0.029 Blood (37 °C) 0.058 Glycerin (20 °C) 0.063 Mercury (20 °C) 0.47 Water (20 °C) 0.073 Water (100 °C) 0.059
Attraction forces between molecules at the surface and at the interior of the liquid
Attracted inwards
Attracted inwards
Move upward Move upward surface tension decreases with the increase
of
Reduce the number of molecules at surface
temperature, reaching a value of 0 at the critical temperature. Pressure normally makes the surface tension of liquid decrease because increase the pressure increases the density of vapor.
Interuption?!!!!
Lateral tension
T, [], P?!!! (STRONGER INTERMOLECULAR ATTRACTIONS GREATER SURFACE TENSION T increase -----increase total entropy (disorganisation of molecules arrangement)---reduce free energy (molecules used the free energy for interaction)
Shape distortion due to Gravity, magnetic, electrical or other forces
• T increase -----increase total entropy (disorganisation of molecules arrangement)--reduce free energy (molecules used the free energy for interaction)
• At the same time, the gravitational force is balanced by the repulsive force of nearby molecules. So the molecules can stay on the surface. • If we want to move the lower molecules to the surface, we have to do some work opposing the gravitation from lower molecules. • Therefore the potential energy of molecules increase. Obviously, the molecules on the surface layer have higher potential energy than the molecules in the inner liquid. • As any system always tends to the smallest potential energy, the molecules on the surface tend to move into the liquid. Then the surface area will reduce to minimum. Contrarily, if we want to increase the surface area, we have to do work to move molecules to the surface while the potential energy of the surface also increases.
• Surface tension of solution changes with variation of solute (a substance is dissolved in another substance.) • Some solutes can decrease surface tension of solution and some can increase surface tension. • The former is called surface-active agent and the latter is called non-surface-active agent. (solution = solute + solvent )
• Surface Tension, o of a liquid is often defined as the force acting at right angles to any line of unit length on the liquid surface. •The Short-range intermolecular forces which are responsible for surface/interfacial tension include van derWalls forces and hydrogen bonds (e.g.in water) and metal bonding (e.g. in mercury).
• The surface-active agent is called surfactant or a wetting agent. • For example, organic acid and soap are active agent of water; – When the surface-active agent dissolves in solvent, the liquid in which another substance (solute) is dissolved to form a solution), the attraction of solvent molecules is greater than attraction between solvent molecule and solute molecule. – In such a case, solvent molecules in the surface layer tend to move into the liquid because the greater attractions between solvent molecules. – The solvent molecules move into the liquid as much as possible as in this way, the surface energy will be lower and the system will be more stationary. – In the surface layer, the solute molecules will increases in order to stabilize the system.
• salt and gluside are non-active agent of water.
ADSORPTION AT A WATER SURFACE
•Pure water • high surface tension • sum of dispersion force (DF) + hydrogen bonding (HB) • Different substances: different intensities of attractive forces, different molecules volumes and shape • Molecules with high force---pass into the interior….smaller force field---remain at the surface • The different [] of molecules (surface and bulk) in solution is adsorption. • Interface: adsorption is partitioning of chemical species between bulk and interface. • Hence, adsorption is a the amount of molecules (moles or number of molecules, weight and volume) accumulated per unit area (1m2) . • ST is the minimum amount of work needed to create a new unit area of interface resulting thin film . • Desorption: molecules leaving the interface • Water Surface tension: 72.8 mNm-1. • Water: stronger intermolecular attractive force-fields than other solute. • water moves inwards more rapidly than solute molecules •Organic substances (hydrocarbon) : weak intermolecular forces •When these organic substances are brought into solution : reduces the surface tension of water • ST of water higher than HC, water move inwards more rapid than HC • Leaving surface rich with HC • therefore water field force at surface has been declined. •Due to longer the HC chain the greater tendency for the OH molecules (hydrophilic part) adsorbs at the air-wate .r surface---lower ST
The surface phenomenon of solution: Is solution homogeneous? surface adsorption
A
A Solvent
A
A
B Solute
B
A
B
the difference in the intermolecular interaction
A B A B A B
B
B
B A B
B
The less interaction enriches solute molecules at the surface (positive adsorption), which also causes decrease in surface tension.
A B A B A B
A B A A A A A
The greater interaction lessens the concentration of solute at the surface (negatvie adsorption), which also causes increase in surface tension.
interaction
concentration Surface tension adsorption
A-B > A-A
A-B< A-A
C < Cb
C > Cb
increase
decrease
negative
positive
The concentration difference between surface and bulk solution is named as surface adsorption. The excess surface concentration (): the concentration difference of solute per unit area in surface layer and in the bulk solution. (mol m-2)
The Gibbs adsorption isotherm
S’
a b
interface
ni ni ni ni
S
Interphase Interfacial region
a’ b’
ni ni ni ni ni A
SURFACE EXCESS
• Defined as the moles of solute per cm2 of surface region in excess of the number of moles that would be associated with a portion of bulk solution having the same number of moles of solute. • Symbol, • +ve surface excess- [] of solute is higher at surface than bulk. Increase the chemical potential will decrease the surface tension. • , -ve surface excess- [] of solute in bulk is higher than surface. Increase the chemical potential will increase the surface tension. (harder to pull compound from bulk)
• If A is the area of the interface, the area surface excess concentration, is defined as – = ni /A = (na-ni)/A
• Where: na = the amount of the solute at or close to the surface. • ni= the amount of the solute in the presence of a physically and chemically inert surface • A = surface area s
– = ni total – (ni + ni ) /A
• Solute (surfactant) dissolved in solvent solution OR – Solute dissolved partially 2 layers
• By preferential accumulation of solute (surfactant)molecules at the surface of the solution formed surface layer decrease in free energy (decrease of attraction force of water) +ve adsorption surface conc. of solution > bulk conc.
• How to predict the amount of substance adsorb on the surface? • 2 components system (for an ideal solution) – – – – – –
= -c/RT . d/dc Where = surface excess moles of component (moles/cm2) = surface tension (dynes/cm) Bulk [] R= 8.314x 107 dynes.cm/mol. R = 8.314 J/mol
• You add 0.5mmol of sodium dodecylsulfate to pure water at 25℃. This leads to a decrease in the surface tension from 71.99 mJ m-2 to 69.09 mJ m-2. What is the surface excess of sodium dodecylsulfate? • Solution Because the concentration is so low that we take an approximation of d 0.06909 0.07199 5.80mN 2 mol1 dcB cB 0.0005 0
cB d 0.0005 B (5.80) 1.17 106 mol m2 RT dcB 8.3145 298.15
• For 1:1 ionic surfactant (CTAB: CTA+:Br_)-----2 ions are involved (anions and cations) adsorb at the surface to maintain the local neutrality. • Therefore Gibbs equation will be modified to: – = -c/2RT d/dc OR integration with ln – = -1/2RT d/d ln c – = -c/2RT d/dc (provided surface and bulk system in equilibrium. C- concentration of solute
=-cRT. D/dc -1/RT . d/d ln c 1/2.303RT . d/dlogc
• Determination of dɣ/dc by 2 ways: 1. Graphical 2. Empirical 1) Graphical: • Plot a graph of ɣ vs c. • At any given c, the slope of the curve (dɣ/dc) can be found. • Knowing also R, T and c, can be calculated.
Prediction of Surface tension • Empirical equation (estimation equation) for variation of surface tension with concentration can be obtained. • Example; aqueous solution of aliphatic compounds – o/ = 1- B log (c/a +1) – B is constant (~0.41) for many compounds – A decreases rapidly as a homologous series is ascended. (For A particular compound a is constant) – Differentiating the above equation will give /dc
• Surface Pressure () can be defined as (The change of interfacial tension caused by addition of a given species to a base solution. When an area of liquid covered with a spread substance is separated from a clean area of surface by a mechanical barrier, the force acting on unit length of the barrier is called the surface pressure, or, and is equal to where is the surface tension of the clean surface and that of the covered surface: – = o - – Where- o = surface tension of the solvent
• Therefore:
– = -c/RT. d/dc = c/RT . d/dc
Determination of surface excess
-Plot a graph surface tension as a function to concentration, c. - surface concentration can be obtained from the slope of the graph. - From the relationship of: - = -1/2RT d/d ln c - = -c/2RT d/dc
example • Surface tension of aqueous solution of nonionic surfactant CH3(CH2)9(OCH2CH2)5OH at T = 25oC is given in table. Calculate the surface excess and area occupied per molecules C (×10−1molm−3) (10−3Nm−1)
0.1 0.3 1.0 2.0 5.0 8.0 10.0 20.0 30.0 63.9 56.2 47.2 41.6 34.0 30.3 29.8 29.6 29.5
2) Empirical: • Empirical equation for variation of surface tension with concentration can be obtained. • Example: for aqueous solutions of aliphatic compounds. – /o = 1-B log (c/A +1)
• Differentiating the above equation will give dɣ/dc.
• Surface pressure, – = o - – d = -d – o = surfcae tension of pure liquid – = -c /RT. D/dc = c/RT. d/dc – For dilute solution: • • • • •
c = RT OR = RT = surface area per mole = 1/
VARIATION OF SURFACE TENSION WITH CONCENTRATION FOR VARIOUS TYPES OF COMPOUNDS IN AQUEOUS SOLUTION Type II:
II
I
salts, non-volatile acids and bases, sucrose etc. Type I: nonionic solvable organic molecule with low molecular weight / short chains and containing polar groups such as hydroxyl, amine groups, etc.
III
The surface tension of solution decreases by 3.2 times for the increase of per CH2 group in the chain of fatty acid.
Type III: As the C increase, the of the solution decrease sharply. Ionic / nonionic solvable organic molecule with high molecular weight / long chains and containing polar ionic groups such as –COO-, -SO3- NR4+, etc. For example, the sodium salts of long-chain fatty acids (n > 8) and sodium dodecyl sulfate. The substances that can drastically lower the surface tension of water even at low concentrations are called surface-active compounds / agent or surfactants.
1. Simple Organic Solutes • Slope: -ve ** = +ve. So, +ve adsorption. 2. Simple Electrolytes (Inorganic) and Highly Hydrated Organic Compounds • Slope: +ve ** = -ve. So, -ve adsorption. 3. Amphipatic Solutes (especially ionic ones) • Here there is a sudden break in the curve (Micelle Formation). • Slope: -ve ** = +ve. So, +ve adsorption.
ADHESION
= G
equation 2
LIQUID-LIQUID INTERFACE WORK OF COHESION, (WAA): Definition: work required to pull a column of liquid A apart. ɣ = half the work of cohesion. = free energy change involved when molecules from the bulk of a liquid are moved to the surface. So, WAA = ΔG = 2 ɣA ----------------------------- 1 [cohesion: no surface 2A (or B) surfaces]
WORK OF ADHESION, (WAB): Definition: work required to separate two liquids, A and B. It measures the attraction between the two different phases, A and B. So, WAB = ΔG = ɣfinal - ɣinitial = ɣA + ɣB - ɣAB ----------------------------- 2 [adhesion: 1 AB surface 1A + 1B surface]
MONOLAYER ADSORPTION • Adsorption in which a first or only layer of molecules become adsorbed at an interface. • In monolayer adsorption, all of the adsorbed molecules are in contact with the surface of the adsorbent. • The adsorbed layer is termed as monolayer or monomolecular film.
PHYSICAL STATES OF MONOMOLECULAR FILMS: Monolayers can be roughly classified as: 1. Condensed (solid) Films. 2. Expanded Films.- liquid 3. Gaseous or Vapor Films.
Monolayer adsorption on a homogeneous surface at equilibrium pressure p/p0 . The heat of adsorption of the first monolayer is much stronger than the heat of adsorption of the second (and all following) layers (typical chemisorption case)
Multilayer adsorption/condensation on a homogeneous surface at equilibrium pressure p/p0 . The heat of adsorption of the first (blue) layer is comparable to the heat of condensation of the subsequent (red) layers. Often observed during physisorption.
1. CONDENSED FILMS • • • •
Here molecules are closely packed. Strong lateral adhesion between the film molecules. Cohesion between hydrocarbon chains is strong. Example: palmitic, stearic and higher straight-chain fatty acids at r.t. • At high films area: film molecules group together in small clusters or islands. • They do not separate into individual molecules due to strong cohesion between hydrocarbon chain. P increase
Top view As P is applied to a condensed film, the adsorbed molecules can rearrange to a small extent by achange in packing structure. Beyond that point added P wil result in film “buckling” (bending)
• Initial compression – very low surface pressure, • With > in compression – very rapid rise in (steep slope) molecules become tightly packed together .
Tightly packed
A/nm2 molecules-1
2. EXPANDED FILMS • • • • • • •
Monolayer still coherent, rigid, incompressible and densely packed. Molecular orientation is still perpendicular to the surface, but the tails are less rigidly packed Cohesion between hydrocarbon chains is less due to the presence of more than one polar group in the molecule . Example: double bond. Here, there is an attraction between the second polar group and the aqueous substrate. Compression forces the = above the surface + eventually orientates the hydrocarbon chains in a vertical position. Process is gradual.
Gradual process
A/nm2 molecules-1
3. GASEOUS FILMS. • • • •
•
•
Molecules – negligible size, molecules are relatively far apart and have significant surface mobility. Molecules act essentially independently, molecules orientation is random. No lateral adhesion between them Random movement of molecules on the surface obey an ideal two-dimensional gas equation. (PV= nRT) – A =kT ( = RT) Example: cetyltrimethyl ammonium bromide molecules in the film ionised to (C16H33N(CH3)3+. (CTAB)---repel each other in aqueous phase---- is relatively large at all points Film P are greater at oil/water interface than air-water interface (oil penetrates between the HC chain of the film molecules and remove most of the inter-chain attraction. – = o – bc -----linear relationship between c
A/nm2 molecules-1
•
FACTOR INFLUENCING THE PHYSICAL STATE OF MONOMOLECULAR FILMS.
1.
2.
3. 4.
Physical state of amonolayer depends on lateral cohesive forces between the constituent molecules.The factors exhibit the various monlayer states. Head groups – bulky or not.(prevent efficient packing and hence maximum cohesion between the HC chains. Number of polar groups in molecule – ex. unsaturated fatty acids (existence of =). A film P is required to overcome the attraction between the second polar group and the aqueos substrate before the molecules can orienttaed vertically. Molecule having more than one hydrocarbon chain orientated in different directions from polar part – ex. esters, glycerides. Straight or bent hydrocarbon chain ex. brassidic acid + erucic acid.
– –
Trans CH3(CH2)7CH=CH(CH2)11COOH (straight) gives condensed film Cis CH3(CH2)7CH=CH(CH2)11COOH (bent) gives a very expanded film
5. Branched hydrocarbon chain. 6. pH of aqueous substrate (if monolayer is ionisable). 7. Dissolved electrolytes ex. Ca2+.
GASEOUS FILMS
• Monolayer of soluble material are normally gaseous. • For dilute surfactant solutions, allow solute-solute interactions at surface to be neglected, the surface tension will be approximately linear with []. – – – – – – –
•
= o – bc (b is constant) = o - = bc d /dc = -b Substituting Gibbs equation: = -c/RT. d/dc = RT ; = 1/
the 2-dimensional gas law is obeyed. – = RT
• σ = surface area per mole.
• is also known as the spreading pressure.
EXAMPLE: A 2 x 10-4 M aqueous solution of a surface-active is being studied. With a device known as a microtome, it is possible to skim off a thin layer of known area, and by this means, it is determined that the surface excess of the surfactant is 3 x 10-10 mole/cm2. Calculate the surface tension of the solution, making reasonable assumptions. The temperature is 25OC. (Surface tension is a linear function of concentration: = o – bc) R = 8.314 x 107 dynes cm/mol.K ɣ0 = 72 dyne/cm at 25OC
• • • •
d/dc = b = -c/RT. d/dc = bc/RT bc= RT = o - RT
