Interfaces

1
Interfaces
Gas/Liquid
Liquid/Liquid
Liquid/Solid
Gas/Solid
Solid/Solid

• Gas/Liquid
• Bubbles/Foam
• Surfactants
• Adsorption
• Liquid Crystals
• Insoluble monolayers

• Liquid/Liquid
• Emulsions
• Detergency
• Reverse Micelles
• Aerosols

2
Adhesion of Surfaces

• Work of COHESION, W11
• Energy needed to separate two identical
• surfaces from contact to infinite separation
• Work of ADHESION, W12
• Energy needed to separate two dissimilar
• surfaces from contact to infinite separation
• Units of energy per unit area (mJ/m2 erg/cm2)

3
Surface Free Energy

• Process of creating new surface area equivalent
  to separating two half spaces.
• Surface Energy (Solid/Gas)
• For Liquid/Liquid Interface, usually termed
  Interfacial Tension
• For Gas/Liquid interface usually termed
  Surface Tension
• with units of force/unit length (mN/m)
• note mN/m mJ/m2

4
Surface Tension

• Imbalance of forces at the Gas/Liquid Interface

• Surface tension (g) decreases with increasing
  temperature
• g decreases with increasing pressure

5
Contact Angle, q (Young-Laplace Equation)
gLG
?
gSG
gSL
At equilibrium all tensions must be balanced
gSG gSL gLGcosq For a
water droplet on a surface 0 lt q lt 90
hydrophilic , 90 lt q lt 180, hydrophobic
TiO2-Silicone film after UV irradiation
TiO2-Silicone film before UV irradiation
6
Determination of Surface Energy (Zisman Plot)
Contact angle measurement of a solid with
various liquids of known surface tension
Cos q 1 b(gl - gc)
Cos q
q Contact angle b 0.03 0.04 gl Surface
tension of liquid gc Critical surface tension
g1
Extrapolate curve to Cos q 1, obtain ?c ,
characteristic of Surface Energy
(Adamson, 1997)
7
Young-Laplace Equation Equation of Capillarity

• - Interfacial Tension
• P - Pressure
• R - Radius

• Pressure inside a drop or bubble is always
  greater than in the continuous phase.
• The balance between surface tension and external
  forces (e.g. gravity) dictate the shape of drops
  and bubbles.

8
Surface Tension Measurement -- du Nouy ring --
Adamson, Physical Chemistry of Surfaces, 2nd Ed
p. 22 (1976)
wttotal total weight wtring ring weight R
ring radius g surface tension

• Still commonly used but values may be as much as
  25 in error.

9
Surface Tension Measurement -- Wilhelmy Plate --
g surface tension q contact angle wttotal
total weight
wtplate plate weight b buoyancy force l
width of plate

• Normally platinum is used to have q ? 0 and
  plate just touches liquid so buoyancy is small

10
Surface Tension Measurement -- Drop Weight Method
--
Adamson, Physical Chemistry of Surfaces, 2nd Ed
p. 19 (1976)
W 2prg
W weight of droplet r radius of droplet g
surface tension
11
SURFACTANTS
12
BASIC TERMINOLOGY

• Hydrophilic A liquid/surface that has a high
  affinity to water.
• Hydrophobic A liquid/surface that has very low
  affinity to water
• Lipophilic A liquid/surface that has a high
  affinity to oil.
• Lipophobic A liquid/surface that has a very low
  affinity to oil.

13
BASIC TERMINOLOGY
14
INTRODUCTION

• Surfactants are molecules that preferentially
  adsorb at an
• interface, i.e. solid/liquid (froth flotation),
  liquid/gas
• (foams), liquid/liquid (emulsions).
• Significantly alter interfacial free energy
  (work needed to
• create or expand interface/unit area).
• Surface free energy of interface minimized by
  reducing
• interfacial area.

15
SURFACTANT STRUCTURE

• Surfactants have amphipathic structure
• Tail or hydrophobic group
• Little affinity for bulk solvent. Usually
  hydrocarbon (alkyl/aryl) chain in aqueous
  solvents. Can be linear or branched.
• Head or hydrophilic group
• Strong affinity for bulk solvent. Can be neutral
  or charged.

16
SURFACTANT CLASSES

• Carboxylic acids and their salts including
  various fatty acids tall oil acids, and
  hydrolyzed proteins

• Sulfonic acids and their salts, including
  hydrocarbon backbones of alkylbenzene, benzene,
  naphthalene, toluene, phenolm lingin, olefins,
  diphenyloxide, petroleum cuts, succinate esters
  etc.

• Sulfuric acid or salts including sulfated
  primary alcohols, sulfated polyxyalkylenated
  alcohols etc.

• Alkyl xanthic acids

• Alkyl or aryl dithiophosporic acids

• Polymeric anionics involving repeated groups
  containing carboxyl acid functionality

Anionic ( 60 of industrial surfactants)
17
SURFACTANT CLASSES (contd.)

• Long chain amines derived from animal and
  vegetable acids, tall oil and synthetic amines

• Diamines and polyamines including ether amines
  and imidazolines

• Quaternary ammonium salts including tertiary
  mines and imidazolines

• Quaternized and unquartenized polyoxyalkylenated
  long chain amines

• Amine oxides derived from tertiary amines
  oxidized with hydrogen peroxide

Cationic ( 10 of industrial surfactants)
18
SURFACTANT CLASSES (contd.)

• Polyoxyethylenated alcohols, alkyl phenols,
  alcohol ethoxylates including derivatives from
  nonyl phenol, coconut oil, tallow, and synthetic
  alcohols

• Polyoxyethylenated glycols

• Polyoxypropylenated glycols

• Esters of carboxylic acids and alkyene oxides

• Alkanolamine condensates with carboxylic acids

• Polyoxyalkylenated mercaptans

Non-ionic ( 25 of industrial surfactants)
19
SURFACTANT CLASSES (contd.)

• Acrylic acid derivatives with amine
  functionality

• Subsituted alkylamides

• n-Alkyl betaines

• n-Alkyl suffobetaine

• Thio alkyl amines and amides

Amphoteric or zwitterionic ( 10 of industrial
surfactants). Generally expensive specialty
chemicals.
20
HYDROPHILIC-LIPOPHILIC BALANCE

• Griffin (1949) the hydrophilic-lipophilic
  balance (HLB) of a surfactant reflects its
  partitioning behavior between a polar (water) and
  non-polar (oil) medium.
• HLB number, ranging from 0-40, can be assigned
  to a surfactant, based on emulsification data.
  Semi-empirical only.
• Strongly hydrophilic surfactant, HLB ? 40
• Strongly lipophilic surfactant, HLB ? 1
• HLB dependent upon characteristics of polar and
  non-polar groups, e.g. alkyl chain length,
  headgroup structure (charge, polarity, pKa).

21
HYDROPHILIC-LIPOPHILIC BALANCE -- Effect of
Structure --
22
HYDROPHILIC-LIPOPHILIC BALANCE
A value of 10 represents a mid-point of HLB.
23
HYDROPHILIC-LIPOPHILIC BALANCE
Translucent to clear solution
No dispersibility in water
Milky dispersion unstable
poor dispersibility in water
Clear solution
0
2
6
8
10
12
14
16
18
4
HLB
Water in oil emulsifier
Wetting agent
Detergent
Solubilizer
Insecticidal sprays
triglycerol monooleate Cream and ointment
stabilizers
Oil-in-water emulsifier
Polysorbate 20
24
MICELLES

• If concentration is sufficiently high,
  surfactants can form
• aggregates in aqueous solution ?
  micelles.
• Typically spheroidal particles of 2.5-6 nm
  diameter.

Hartley Spherical Micelle McBain Lamellar
Micelle
Oil in Water Micelle Water in Oil Micelle
Surfactant Micelle
(Klimpel, Intro to ChemicalsUsed in Particle
Systems,p. 29, 1997, Fig 21)
25
MICELLES --CMC--

• Onset of micellization observed by sudden change
  in
• measured properties of solution at
  characteristic surfactant
• concentration
• ? critical micelle concentration
  (CMC).

(Klimpel, Intro to ChemicalsUsed in Particle
Systems, p. 29, 1997, Fig 20)
26
MICELLES --CMC Trends--

• For the same head group, CMC decreases with
  increasing alkyl chain length.
• (2) CMC of neutral surfactants lower than ionic
• (2) CMC of ionic surfactants decreases with
  increasing salt concentration.
• (3) For the same head group and alkyl chain
  length, CMC increases with increase in number of
  ethylene oxide groups.
• (4) For mixed anionic-cationic surfactants, CMC
  much lower compared to those of pure components.

27
MICELLES --Driving Force--

• Hydrophobic groups can perturb solvent
  structure and
• increase free energy of system. Surfactant
  will ? concentrate at
• S/G interface to remove hydrophobic groups
  from solution and
• lower DGo.

AIR
WATER
28
MICELLES --Driving Force--

• DGo can also be decreased by aggregation into
  micelles
• such that hydrophobic groups are directed into
  interior of
• structure and hydrophilic groups face solvent.
• Decrease in DGo for removal of hydrophobic
  groups from
• solvent contact by micellization may be opposed
  by
• (i) loss in entropy of surfactant
• (ii) electrostatic repulsion for charged
  headgroups
• Micellization is ? a balance between various
  forces which
• can be influenced by certain phenomena
  (Mukerjee and
• Mysels, 1971).

29
MICELLES --Example Mayonnaise--
lecithin
Water matrix containing fat droplets. The
surfactant (emulsifier) is lecithin. It can
contain up to 12 g of fat in 15 ml
2 µm
http//wilfred.berkeley.edu/gordon/BLOG-images/ma
yo15.jpg
30
MICELLES --Headgroup and Chain Length--

• Klevens (1953) surfactants with linear alkyl
  chains, CMC is related to number of carbons by
• log10CMC b0 - b1mc
• Where
• mc is number of carbons in chain
• b0 and b1 are constants

(Hunter, Foundations of Colloid Science, p. 569,
1993, Fig 10.2.1)
31
MICELLES --Headgroup and Chain Length--

• Branching or addition of double bonds or polar
  groups to alkyl chain
• generally increases CMC.
• Addition of benzene ring equivalent to addition
  of 3.5 carbons
• (methylene groups).
• Replacement of hydrogens in alkyl chain with
  fluorine initially
• increases CMC, followed by marked decrease as
  fluorine substitution goes to saturation.

(Hunter, Foundations of Colloid Science, p. 569,
1993, Fig 10.2.1)
32
MICELLES --Temperature and Pressure--

• For ionic surfactants there exists a critical
  temperature above which
• solubility rapidly increases (equals CMC) and
  micelles form
• ? Kraft point or Kraft temperature (TK),
• Below TK solubility is low and no micelles are
  present.

(Klimpel, Intro to Chemicals Used in Particle
Systems, p. 30, 1997, Fig 22)
33
MICELLES --Temperature and Pressure--
TK
surfactant crystals

• Surfactants much less effective below Kraft
  point, e.g. detergents.
• For non-ionic surfactants, increase in
  temperature may result in
• clear solution turning cloudy due to phase
  separation. This critical
• temperature is the cloud point.
• Cloud point transition is generally less sharp
  than that of Krafft
• point.

34
MICELLES --Electrolyte--

• Addition of electrolyte significantly affects
  CMC, particularly
• for ionic surfactants.
• For non-ionic and zwitterionic surfactants
• log10CMC b2 b3Cs
• where Cs is salt concentration (M)
• b2 and b3 are constants for specific
  surfactant, salt and
• temperature.
• Change in CMC attributed to salting in or
  salting out
• effects. Energy required to create volume to
  accommodate
• hydrophobic solute is changed in electrolyte
  solution due to
• water-ion interactions
• ? change in activity
  coefficient.

35
MICELLES --Electrolyte--

• If energy required is increased by electrolyte,
  activity
• coefficient of solute is increased and salting
  out occurs
• ? micellization is favored
  and CMC decreases.
• Conversely, for salting in, CMC increases.
• Effects of electrolyte depend on radii of
  hydrated anions and
• cations and is greater for smaller hydrated
  ions, i.e. follow
• lyotropic series.
• CMC depression follows order
• F- gt BrO3- gt Cl- gt Br- gt NO3-
  gt I- gt CNS-
• and
• NH4 gt K gt Na gt Li

36
MICELLES --Electrolyte--

• For ionic surfactants
• log10CMC b4 b5log10Cs
• where b4 and b5 are constants for a specific
  ionic head group at a
• particular temperature.
• Depression of CMC with increasing salt due to
  double layer
• compression around charged head group and
  charge screening
• effect between head groups in micelle.
• Different salts vary in their effectiveness,
  e.g. for sodium laurate,
• CMC depression follows
• PO42- gt B4O72- gt OH- gt CO32- gt SO42- gt Cl-

37
MICELLES --Electrolyte--
The effect of added salt on the CMC of SDS and
dodecylamine hydrochloride (DHC). (From Stigter
1975a,with permission)
(Hunter, Foundations of Colloid Science, p. 572,
1993, Fig 10.2.2)
38
MICELLES --Organic Molecules--

• Small amounts of organic molecules can affect the
  CMC, e.g. in
• aqueous solution of SDS, dodecanol (hydrolysis
  product of SDS)
• causes minimum in surface tension measurement.
• Solubilization of impurity in micelles causes
  rise in surface tension.
• Very important for detergency, stabilization
  and dispersion.

go
Surface Tension
CMC
Surfactant Concentration
39
MICELLES --Organic Molecules--

• Solubilization characterized by large increase in
  solubility of lipophilic (hydrophobic) organic
  species above surfactant CMC.
• Lipophilic organics can aid or oppose micelle
  formation. Two
• classes based on mode of action.
• Group A (or Type I)
• Adsorb within micelle and reduce CMC. Typically
  polar
• molecules, e.g. alcohols and amides.
• Effective at low concentrations.
• Short chain members adsorb near micelle-water
  interface.
• Longer chain members adsorb in core
• ? can influence micelle shape.

40
MICELLES --Organic Molecules--

• Free energy of micellization lowered by
  screening repulsion
• between charged head groups (ionic surfactants)
  and/or reducing
• steric hindrance (non-ionic surfactants).
• CMC depression greatest for linear species
• ? maximum when chain length approaches that of
  surfactant.
• Group B (or Type II)
• Modify bulk water structure around surfactant
  or micelle,
• usually at higher concentrations than Group A
  molecules.
• Structure breakers disrupt water structure
  about hydrophobic
• tails and increase entropy. Entropy increase
  upon micelle
• formation ? reduced
• ? CMC is increased.

41
MICELLES --Organic Molecules--

• Examples of structure breakers are urea,
  formamide and guanidinium salts. Most effective
  on non-ionic surfactants of PEO type.
• Structure makers promote structuring of water,
  e.g. xylose and
• fructose. Conversely, CMC is reduced due to
  enhanced entropy
• increase upon micellization.
• At high bulk concentrations, species such as
  dioxane, esters,
• short-chain alcohols and ethylene glycol can
  increase solubility of
• monomeric surfactant, thus opposing
  micellization and raising
• CMC.

42
MICELLES --Aggregation Number--

• Formation of micelles from association of n
  surfactant monomers can be described by
• nS Mn
• where n is number of surfactant monomers needed
  to form a micelle ? aggregation number
• k1 and k-1 are rate constants for forward and
  reverse reactions
• Equilibrium constant, K, can be expressed as

k1
k-1
43
MICELLES --Aggregation Number--
If Cs and Cm are concentrations of surfactant
monomer and micelle, respectively
Variation of dCm/dCT with total surfactant
concentration for different values of aggregation
number, n. C0 is the critical micellization
concentration and Cm the concentration of
micelles.
(Hunter, Foundations of Colloid Science, p. 572,
1993, Fig 10.3.1)
44
MICELLES --Aggregation Number--
t1
t2
Micelle size distribution. Mn is the number of
aggregates of size n. The aggregates on the left
side of the minimum (L) are called submicellar,
those on the right-hand side (proper) micelles
with mean size of n, and the width of their size
distribution is given as s.
(Hunter, Foundations of Colloid Science, p. 572,
1993, Fig 10.7.1)
45
MICELLES --Aggregation Number Trends--

• For same polar group, n increases with increasing
  chain
• length.
• (2) For constant alkyl chain length, n decreases
  with increasing number of ethylene oxide groups
  in surfactant molecule.
• (3) Oils or long chain alcohols increase n.
• (4) For ethoxylated non-ionic surfactants, n
  drastically increases with temperature.
• (5) For anionic surfactants, n increases when
  NaCl is replaced with MgCl2 or CaCl2.
• (6) In aqueous solution, n ranges from 50-5,000,
  while in organic solvents, n usually lt 10.

(Hunter, Foundations of Colloid Science, p. 572,
1993, Fig 10.7.1)