ADSORPTION AT THE

1
ADSORPTION AT THE SOLID LIQUID INTERFACE
2
Technological Importance of Surfactant and
Polymer Adsorption

• Surfactants
• Froth floatation
• Enhanced Oil Recovery
• Paints
• Stabilization

3
Technological Importance of Surfactant and
Polymer Adsorption

• Polymers
• Lubrication
• Adhesion
• Stabilization
• Flocculation

Silica dispersions with different PEO
concentrations in presence.
Teflon
htttp//www.chm.bris.ac.uk/pt/polymer/dongqiu.sht
ml
4
Pulmonary Surfactant

• Pulmonary surfactant is a surface-active
  lipoprotein formed by type II alveolar cells.
  Their purpose is
• To increase pulmonary compliance.
• To prevent the lung from collapsing at the end of
  expiration

By adsorbing to the air-water interface of
alveoli with the hydrophilic headgroups in the
water and the hydrophobic tails facing towards
the air, the main lipid component of surfactant,
reduces surface tension.
http//oac.med.jhmi.edu/res_phys/Encyclopedia/Surf
actant/Surfactant.HTML
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Langmuir Isotherm
The Langmuir equation can be used to describe
adsorption from dilute solutions, and is based
on the following assumptions (1)
Adsorption is limited to monolayer coverage.
(2) Solvent and solute molecules occupy the same
parking area. (3) Lateral interactions
between solute molecules and solute and
solvent molecules are absent. (4) Adsorbent
surface is uniform with respect to the
enthalpy change, ?H, upon adsorption, i.e.
the surface is homogeneous.
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Langmuir Isotherm

• The Langmuir isotherm can be expressed by the
  following equation
• GA Amount of solute adsorbed (mg/m2 or
  mol/m2)
• at concentration CA (mol/L)
• Gmax Maximum solute adsorbed (mg/m2 or
  mol/m2)
• qA Fraction of coverage
• K Equilibrium constant for
  surfactant/solvent exchange
• K may be related to adsorption free energy by

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Langmuir Isotherm
Linear form of Langmuir isotherm
Parking Area per Surfactant Molecule
Slope 1/K Gmax
1/GA
Intercept 1/Gmax
1/CA
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Adsorption Classification
Adsorption of surfactants can be either physical
or chemical.
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SURFACTANTS
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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.

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Introduction

• Surfactants have amphipathic structure (or 2
  distinct
• structural units).
• Tail or hydrophobic group
• ? little affinity for bulk solvent. Usually
  hydrocarbon
• (alkyl) chain in aqueous solvents. Can be
  linear or
• branched.
• Head or hydrophilic group ? strong
• affinity for bulk solvent. Can be neutral or
  charged.

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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)
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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)
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Surfactant Classes

• 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)
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Surfactant Classes

• 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.
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Micelles

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

(Klimpel, Intro to Chemicals Used in Particle
Systems, p. 29, 1997, Fig 21)
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Manifestations of Micelle Formation
Onset of micellization observed by sudden
change in measured properties of solution at
characteristic surfactant concentration ?
critical micelle concentration (CMC).
How do you rationalize these observations?
18
Driving Force for Micelle Formation
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.
19
Driving Force for Micelle Formation
Beyond a surfactant concentration (CMC) 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 (ii) electrostatic repulsion for charged
headgroups Micellization is ? a balance between
various forces which can be influenced by certain
phenomena
20
Discussion Question
Is Surfactant Micellization driven by Entropy or
Enthalpy?
Argument
Beyond CMC, the surfactant self-assembles to
ordered structures micelles. Isnt self
assembly to form ordered structures reducing
surfactant entropy?
Hence, is micellization Entropy or Enthalpy
driven?
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CMC Effect of Added 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.

22
CMC Values General Trends

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

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Surfactants at the Solid-Liquid Interface

• Adsorption of surfactants at the solid/liquid
  interface is of prime importance in industry and
  agriculture, e.g. to control stability,
  floatability and rheology of particulates.
• Many different forces come into play during
  surfactant adsorption. Dominant forces will
  depend upon not only nature of surfactant (head
  group, chain length, charge, etc.) but also
  nature of surface.
• DGo DGelec DGh-s DGc-c DGc-s
• Behavior of surfactants is different on
  hydrophobic surfaces (e.g. teflon) compared to
  hydrophilic surfaces (e.g. metal oxides).

ads
24
Surfactants at the Solid-Liquid Interface
Low Concentration - Surfactant Adsorption
Moderate Concentration - Hemimicelle Formation
High Concentration - Self-Assembled Surface
Aggregates
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Specific Interactions Chain-Chain

• As surface concentration of ionic surfactant
  increases, if
• attractive hydrophobic interactions between
  alkyl chains
• can compensate for ionic head group repulsion,
• hemimicelles can form.
• At low coverages, patches can form on the
  surface.

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Specific Interactions Chain-Chain

• Hemi-micellization occurs at lower concentrations
  with increasing alkyl chain length

27
Combined Interactions Hydrophilic Surfaces
DGc-c
H
DGh-s
DGelec
Zr
Zr




Bjelopavlic et. al., J. Coll. Int. Sci., 208, p.
183-190, (1998)