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Part 3: Sorption of liquids and gases on surfaces
Definitions
• Adsorption is a process at surfaces. Molecules settle down on surfaces and get attached through relatively weak forces. They may over time be released again (= desorption). The entire process is dynamic.
• Adsorption is the temporary attachment of particles on surfaces.
• Absorption is the incorporation of a material (e.g. or liquid) in a body (such as a sponge). The absorbed material disappears inside the body.
… even more definitions
Substrate – often used to describe surfaces of solid state bodies, on which adsorbtion occurs (also: adsorbent) the orange bar
Adsorbate – general name of molecular species which is adsorbed onto a substrate the red balls
Adsorption – is a process in which a molecule will be adsorbed at a surface the „down arrow“
Desorption – the opposite of adsorption the „up arrow“
Absorption vs. Adsorption ... =)
Taken from: Carsten Schmuck, Bernd Engels,Tanja Schirrmeister, Reinhold Fink, Chemie für Mediziner,Pearson Studium, 2008, S. 124
How do we describe sorption phenomena
Adsorption isotherms• coverage - measure for the part of the surface to which an adsorbate (species) is
adsorbed. Usually denoted using the ‘theta’ symbol, θ.
• θ=1 indicates complete coverage of the solid surface with a monolayer.
Chemistry is better ;-)
Two major types of adsorption:
• Physisorption
• Chemisorption
Physisorption
Physical adsorption (Physisorption) – attachment through vdW forces. There is no significant change in charge distribution, neither within the molecule nor at the surface. no chemistry involved.
Chemisorption
Chemical adsorption (chemisorption) – attachment through chemical reactions; significant charge redistribution.
Chemical bonds may be anything from ionic to covalent.
Chemisorption vs. Physisorption
Chemisorption Physisorption
Enthalpy of Adsorption
Variable depending on type of attachment: 40 - 800 kJ mol-1
Depends of molar mass and polarity: 5-40 kJ mol-1
Nature of adsorption irreversible reversible
Max. coverage only monolayers multilayers possible
Kinetic aspects activation barrier fast, no activation barrier
Influence on isotherms
Example one: chemisorption of O2 on charcoal monolayer
Example two: physisorption of N2 on silica gel multilayer
chemisorption/physisorption
physisorption
Understanding isotherms
Important factors describing the amount of gas molecules which adsorb to solid
surfaces at constant temperature and constant pressure [discret point on
isotherm].
Interaction energy between adsorbent and adsorbate (adsorption energy)
Interaction between adsorbate molecules (apparent in heat of condensation)
Mobility of the adsorbed molecules
Surface heterogenities
Existence of pores and pore size distribution
Understanding isotherms
Type I: relatively strong adsorption, resulting in monolayer formation
Typ II: polylayer formation with strong interaction
Typ III:polylayer formation with low interaction
Typ IV und V: adsorption at porous surfaces
Quantitative description of adsorption
How to we quantitatively describe isotherms? [Engineers like math]
Literature survey brings about around 50 different mathematical equations, which are based on different models and which are making different assumptions to describe an adsorption process.
At the moment no standard theory of adsorption
Henry isotherm
1903/07 Henry, Dalton
Monolayer adsorption at solid surfaces
Adsorption layer is mobile
No interactions between adsorbed molecules in the adsorbate
Θ = bcΘ: coverage
b : Adsorption coefficient
c: concentration of the volume
Henry
Dalton
Langmuir isotherms
Langmuir 1918
Accounts for footprint of molecules
Initially coverage increases linearily, then levels of to reach a plateau
des
ads
kkK
KcKc
=+
= ,1
θ
Equilibrium:
Adsorption: c + * c* (kads)
Desorption: c* c + * (kdes)
[with c: molecule, *: site, and k: rate constants]
Irving Langmuir (1881 – 1957)
Born in Brooklyn, New York
Nobel price in Chemistry: 1932
Langmuir isotherms
des
ads
kkK
KcKc
=+
= ,1
θ
Maximum coverage: 1
No adsorption beyond monolayer
Langmuir isotherms
des
ads
kkK
KcKc
=+
= ,1
θ
High K or c 1
Low K or c Kc (Henry)
Langmuir isotherms
KcKc+
=1
θ RTGeK /∆−=
Adsorption at a fixed concentration is governed by thermodynamics enthalpy and temperature are key
Langmuir isotherms
Shortcomings of the theory:
Chemists understand it.
Large molecules may occupy more than one adsorption site
Does not account for multilayer builtup.
Coverage does not influence the probability of adsorption to a site.
All sites are equal, i.e. not influenced by the neighbor. No site-dependent rate constants.
des
ads
kkK
KcKc
=+
= ,1
θ
Langmuir isotherms
Example: Adsorption of colloids or nanoparticlesCurrently heavily pursued field of research. Description is difficult because ... reality bites:
Substrates are rough
Bunch of interactions between particles and particle/substrate: electrostatic, vdW or steric arguments (entropic)
Transport mechanisms
Desorption processes? (often: irreversible adsorption)
Langmuir model yields only crude approximation
Random Sequential Adsorption (RSA model)
Sequential adsorption at free sites
No diffusion at the surface, no displacement
Monolayers only
New particle is admitted to a large enough site
θ=86.5%
“Random Car Parking” problem
Random Sequential Adsorption (RSA model)
( )( ) 56.0
56.0
≅∞
≅∞
spheres
squares
θ
θ
At t=0: empty substrate
Monotonic increase in coverage with t
Jammed state: remaining sites are too small
Coverage lower than in close-packed state
Random Sequential Adsorption (RSA model)
(a) Θ = 0.56,
(b) Θ = 0.5 (aspect ratio 4),
(c) Θ = 0.45
(d) Θ = 0.38
RSA – an example
Part 4: Surface tension
Surface tension
Reason for surface tension is the broken symmetry at the transition point from liquid to gas.
Definition:
TPAG
,
==
δδγσ
Surface tension
Measuring surface tension
Ring method (de Noüy)
Wilhelmy plate
drop shape method
Measurement of the force needed to pull a ring out of the liquid
γπ )(2 ai rrF +=
A wettable plate is immersed into the liquid and the force that acts on the plate is measured. This force minus gravity gives the surface tension.
γlFF g 2−=
A droplet falls from a capillary as soon as gravity mg exceeds the surface tension.
γπ krmg 2=
Part 5: Wetting of surfaces
Contact angles
The contact angle Θ is formed at the boundary of the three phases solid/liquid/gas and is a direct measure for the wettability of the surface by the test liquid.
Θ+= coslgσσσ slsg
sgσ Surface tension of solid (s=solid, g=gas)
lgσ Surface tension of liquid
slσ Interfacial tension between solid and liquid
lgσ
sgσ
slσ
Θ Liquid
SolidYoung‘s equation
Gas
Contact angles
Θ+= coslgσσσ slsg
Young‘s equation
Θ=−= coslgσσσσ slsgB
Wetting tension:
The surface tension of the solid could be determined if only we could measure the interfacial tension (surface – liquid).
For Θ<90° (cos Θ >0) we get: σB > 0 wetting
For Θ >90°(cos Θ <0) we get: σB < 0 no or insufficient wetting.
For Θ =180° superhydrophobic
Ist Θ =0° superhydrophilic
Contact angles
Interfacial free energy of solids
Zisman plot Only valid for unpolar interactions
The cosine of the contact angle is plotted as a function of the surface tension of the liquid used.
At the intercept of the regression with cos θ = 1 we get a critical interfacial tension γcrit below which any liquid will spread on the solid.
This value denotes the interfacial free energy of the solid if the interactions between the liquid and the solid are entirely unpolar.
Interfacial free energy of solids
Method of Owens, Wendt, Rabel & Kaelbel(often used for polymeric surfaces)
The advancing contact angle of a liquid with known polar and dispersive fractions of the surface tension γL
p & γLd are measured. The surface free energy is then taken as
the geometric average of the interfacial tensions of the liquid and the solid:
γSL is then calculated as:
Taking this result back into Young‘s equation we can rewrite to generate a linear relation of the type y = ax + b:
pS
dSS
pL
dLL γγγγγγ +=+= &
( )pL
pS
dL
dSLSSL γγγγγγγ +−+= 2
dS
pSd
L
pL
dL
L baxy γγγγ
γγθ
===+
= ,,,2cos1
Interfacial free energy of solids
pSa γ=
dL
pLx
γγ
=
dL
Lyγ
γθ2cos1+
=
dSb γ=
Method of Owens, Wendt, Rabel & Kaelbel(often used for polymeric surfaces)
Interfacial free energy of solids
Different methods lead to different results. Determination of absolute values is difficult if not impossible.
It is not possible to determine the interfacial free energy of a unknown surface using reference measurements on known materials.
The roughness of the surface of interest is very critical
Non-ideal surfaces
Welcome to the real world! Real surfaces teach us that wetting is a rather complex phenomenum.
On real surfaces one usually gets a range of contact angles both at the same spot or at different spots on the sample.
Dynamic contact angles
Advancing contact angle θadv
Measured while liquid is added to the drop
Receeding contact angle θadv
Measured while liquid is taken from the drop
Dynamic contact angles
Dynamic contact angles are measured directly at the moment before the contact line starts to move (i.e. while the drop is still pinned).
The advancing contact angle is usually much larger than the receeding contact angle. Differences are at least 5 – 20°, often a lot more.
Contact angle hysteresis
Reasons for CA hysteresis
Physical roughness
Chemical heterogeneities
Contamination in test liquid
On soft surfaces (some polymers) forces might be strong enough to deform the substrate
Adsorption or desorption of molecules during advancing or receeding motion
Absorption of liquid (e.g. swelling)
Most important: Surface topography and roughness and the respective length scales
Wetting on rough surfaces
Yr r θθ coscos ⋅=
1coscos −+⋅= φθφθ Yr
Wenzel model:
Cassie model:
Young's equation:
• T. Young Philos. Trans. R. Soc. London 1805, 95, 65.• A. Cassie, S. Baxter Trans. Faraday Soc. 1944, 40, 546.• R. N. Wenzel Ind. Eng. Chem. 1936, 28, 988
µ-engineering polymer chemistry
lssgY γγθγ −=⋅coslg
Wenzel wetting
r is a roughness coefficient that relates the actual geometric wetted area to the projected area
Because or r > 1 roughness will always amplify a given wetting behavior (hydrophilic more hydrophilic | hydrophobic more hydrophobic)
Yr r θθ coscos ⋅=
areaprojectedareageometricr =
Cassie & Baxter wetting
Cassie and Baxter assume the air is trapped underneath the droplet and define a wetted fraction φ
( ) ( ) 11cos180cos1coscos * −+=°−+= θφφθφθ SSS
Wetting on rough surface
How well do the theories of Wenzel and Cassie/Baxter describe the wetting of rough/microstructured surfaces?
Lithography + anisotropic siliconetching
10 µm
0.1 µm
1 µm
stru
ctur
e si
ze
Anisotropic siliconetching (nanograss)
PS
PDMAA PFA
PMMAPHEMA
PEGMEM
hydrophilichydrophobic
Static contact angles
PDMAA
0°
180°
90°
flat µ-structured
PFA (Fluoropolymer)
Wenzel wetting
post height = post distance d=s [µm]co
ntac
tang
le
[°] Wenzel theory
Comparison of theory with experiment:
extremely high CA hysteresisWenzel theory has no practical
relevance no thermodynamic equilibrium is
reached
Wenzel theory
Pinning dominates wetting behavior
Strong „pinning“ at post edges dominates receeding CAVariety of „local contact angles“
= Pinning
... More real life
Drop impact!
Printing with misalignment
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desired impact area
actual impact area
spot diameter: 3mmmisalignment: 2mm
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Hydrophobic break valve
CCH3
CH2
C nOO
CH2
CH2
(CF2)7CF3
hydrophobic patch: perfluorinated network
Micronozzles
TopSpot printhead
Collaboration with R. Zengerle, R. Steger, G. Birkle, P. Koltay, T. Brenner, M. Grumann, J. Ducree
a) micronozzle
targetzone
a) micronozzle
targetzone
adhesionforce
a)
low surface energy liquids
b)micronozzle
adhesionforce
targetzone
