lecture on contact angles, may 4, 2005 a) liquid and solid ... angles05... · liquid solid the...

Lecture on Contact Angles, May 4, 2005

A) Liquid and Solid Surface EnergiesB) Liquid Contact Angles on Solid Surfaces

Prof. Allan S. Hoffman

(with some slides by Buddy Ratner)

The unusual propertiesof surfaces and interfacesare due to unbalancedintermolecular interactionforces (or energies) acrossthe surface or interface.

Forces within liquids and solids and across their interfaces

include:Polar Interactions:

Non-Polar interactions:

Ion-ion (+ -)Ion-dipole (H-bond)Dipole-dipoleDipole-induced dipole

Hydrophobic (dispersion forces)

These forces exist between individual molecules

θθ

vapor

liquid

solid

Polar

Non-polar

Higher energy

Lower energy

stronger

weaker

The energy to bring one molecule to the surface and then to volatile it

Break 6 nearest neighbor bondsand move 1 molecule to surface

Remake 4 nearest neighbor bonds in surface: Net cost-->2 bonds

Break 4 nearest neighbor bondsand volatilize molecule: Net cost-->4 bonds

Fill surface vacancywith another molecule

Thus, the energy to evaporate a molecule is composed of two parts:1) The energy to bring the molecule to the surface (approx 1/3 of total)

2) The energy to volatilize the molecule (approx 2/3 of total)Conclusions: 1) The surface is always higher energy than the bulk.

2) The surface is only a few molecular layers thick (or thin).

High energy surfaces in air

H2O, hydrogels, clean glass, and metaloxides. They are all hydrophilic and havesignificant ionic, H-bonding and polarforces. BUT they also have importanthydrophobic forces.

These surfaces form low energy interfacesunder water.

Low energy surfaces in air

P.E., Teflon, S.R., HC oils. They are allhydrophobic and most have highhydrophobic or dispersion forces and low orzero polar forces.

These surfaces form high energy interfacesunder water.

These surfacesare difficult tocontaminatein air…

…but may becomecontaminatedunder water

These surfacesare readilycontaminatedin air…

…but difficult to contaminateunder water

1) What are "high" energy groups?

Polar, H-bonding, ionized, hydrophilicgroups as:

-OH -NH2 -COOH -OSO3H

-NH3+ -COO- -OSO3-

2) What are "low" energy groups?

Non-polar, hydrophobicgroups as:

-CmH2m+1 hydrocarbons

CH3

-(Si -O)- silicones CH3

-CmF2m+1 fluorocarbons

In order to

MINIMIZESURFACE ENERGY,

polar groups orientaway from the surface

in air,

or

toward the more polar phase,at aqueous interfaces.

Always remember this:Mother Nature (and thermodynamics) drives all

systems to a minimum energy state.

Simple liquids illustrate surface orientationof polar and non-polar groups to minimize surface energies

Water 72.8 dynes/cmEthanol (CH3CH2-OH) 22.1 dynes/cmn-Octanol (C8H17-OH) 27.5 dynes/cmn-Octane (C8H18) 21.8 dynes/cm

H HO

H HO HO HO

octane

H H H

O O O

Surface tensions

The non polar, HC portion of the molecule tries to escape the highly polar water

environment by orienting into the air phase.

High Energy Surfaces in Air:

Low Energy Surfaces in Air:

Clean metals or metal oxides (very high)Clean glasses or ceramics (high)Clean surface oxidized polymers (moderately high)Clean water (high)Clean mercury (very high)

Hydrocarbon-based polymersFluorocarbon-based polymers (very low)Silicone-based polymers & silicone oils

These generalizations may be the opposite under water, ie, high energy surfaces are not readily contaminated while the opposite is the case for low energy surfaces. The exception is when surface groups can “flip” to expose polar groups and lower interfacial energy under water.

readilycontaminated

in air

not readilycontaminated

in air

AirAirAny metal oxideHigh surface energy

Teflon®Low surface energy

Poly(hydroxyethyl methacrylate)

Low surface energy

WaterWaterTeflon®

High interfacial energy


Low interfacial energy

Low interfacial energy

Any metal oxide

HO

HO

HO

HO

HO

HO

HO

HO

HO

HO

HO

HO

OH

OH

OH

OH

OH

OH

OH

OH

OH

OH

OH

OH

Poly(HEMA)

To minimize surface energy in air, non-polar groups are exposed,and to minimize interfacial energy in water, polar groups are exposed.


Low surface energy in air Low interfacial energy in water

CH3CH3CH3CH3CH3

CH3CH3CH3CH3CH3

Poly(HEMA)

Poly(HEMA)--soft contact lens(CH2C)

CH3n

C=OOCH2CH2OH

In 1972, we were funded to modify the blood contacting surfaces(eg, silicone rubber) of the AEC artificial heart by radiation grafting PHEMA and immobilizing biological agents such as Heparin onto the -OH groups.

Diagram by Allan Hoffman

Grafted polyHEMA hydrogel layer on silicone rubber

Hoffman, et al., Trans Amer Soc Artif Int Organs, 18, 10 (1972)

Schematic showing poly(HEMA) grafting on PDMS followed by immobilization of

biological agents such as Heparin or PGE1 onto thePoly(HEMA) -OH groups

0

500

1000

1500

2000

2500

280282284286288290292

Poly(HEMA)

C-H

C-OO=C-O

Poly(2-hydroxyethyl methacrylate) C1s ESCA Spectrum

(CH2C)CH3

n

C=OOCH2CH2OH

Si - O

CH3

CH3

Poly(dimethyl siloxane) (PDMS or “Silicone Rubber”)

Surface Mobility of HEMA-g-PDMS Studied by ESCAPolyHEMA grafted to PDMS

(frozen at -120°C while hydrated)PolyHEMA grafted to PDMS

(dried at ambient temperature)

Binding Energy (eV)285 280290295

285 280290295

silicone rubber C1s

coun

ts285 280290295

0

500

1000

1500

2000

2500

280282284286288290292

hema_c1s.r01

42

p-HEMA C1s

coun

ts

.................. .... .................

...........

................. ........... ..................................................

Ratner,BD; Weathersby,PK; Hoffman,AS; Kelly,MA; Scharpen,LH (1978): Radiation-grafted hydrogels for biomaterial applications as studied by the ESCA technique. J. Appl. Polym. Sci.22, 643-664.

Surface Mobility of HEMA-g-PDMS Studied by ESCAPolyHEMA grafted to PDMS

(frozen at -120°C while hydrated)PolyHEMA grafted to PDMS

(dried at ambient temperature)

Binding Energy (eV)285 280290295

285 280290295

silicone rubber C1s

coun

ts285 280290295

0

500

1000

1500

2000

2500

280282284286288290292

hema_c1s.r01

42

p-HEMA C1s

coun

ts

.................. .... .................

...........

................. ........... ..................................................

Ratner,BD; Weathersby,PK; Hoffman,AS; Kelly,MA; Scharpen,LH (1978): Radiation-grafted hydrogels for biomaterial applications as studied by the ESCA technique. J. Appl. Polym. Sci.22, 643-664.

Another example of how Mother Naturedrives all systems to a minimum energy

Summary1) Surfaces and interfaces are at a higher free energy leve l than

bulk phases.

2) There is always a thermodynamic driving force to minimizesurface a nd interfacial energies.

a) Surface/volume ratios minimize (e .g., l iquid droplets arespherical )

b) High energy surfaces are hard to keep clean and areeasily contaminated wi th low energy substances.

c) Surface molecules wi ll orient so as to minimize surfaceor interfacial energy (polar groups inward, nonpolargroups outward in air -- the opposite in w ater.)

d) "Surface-active" molecules will adsorb and similarlyorient at high energy surfaces o r high energy in terface s.

3) High energy surfaces in a ir tend to form low energy interfacesin aqueous solutions, and vice v ersa.

4) Sur faces and interfaces are usua lly only a few molecu lar layers thick (or thin).4) Surfaces and interfaces are usual ly only a few molecula rlayers thick (or thin).

Summary of Surface Fundamentals

PTFE

Pure water Contaminated water

Contaminated Surface? The Teflon® “Film” Test

Teflon® Teflon®

Lecture on Contact Angles, May 4, 2005

A) Liquid and Solid Surface EnergiesB) Liquid Contact Angles on Solid Surfaces

Prof. Allan S. Hoffman

(with some slides by Buddy Ratner)

static SIMS

ESCA

ATR-IR

Contact angles

0 Å

100 Å

200 Å

300 Å

400 Å

500 Å

x 40

1 cm

AFM, STMContact angles,AFM, STM,Static SIMS ESCA, Auger

ATR-IREach techniqueprobes a uniquedepth into the surface. Spatialresolution of the technique is also important.

SURFACE ANALYSISTECHNIQUES

Contact angles and water wettability are important for:

Industrial/CommercialAdhesionPrintingPaintsAbsorbancyTextile processingStatic dissipationWater repellancyStain repellancyLaundering

BiomedicalProtein adsorptionBlood compatibilityCell-surface interactions

Contact Angles

θθ

vapor

liquid

solid

The CONTACT ANGLE (θ) is the angle that a small drop of liquid makes as it meets the surface or interface of another phase, usually a solid.

The contact angleis always measuredthrough the droplet

(or air bubble)θ θ

Contact Angles (θ)

solidsolid

water

air bubbleor oil droplet

θ

θwater

solid

vaporNote that θis alwaysmeasured

through thedroplet or air

bubble

Contact Angles (θ)

solidsolid

water

air bubbleor oil droplet

θ

θwater

solid

vapor

Note that θis alwaysmeasuredthrough thedroplet.

What is θ for a very hydrophilic solid surface,

using an air bubble to measure it?

The contact angle of water on skin is about 90 degrees. (If it were zero, external water could penetrate the pores).

The contact angle of water on Teflon® is about 110 degrees.

A bird’s feather has a contact angle of water as high as 150 degrees.

Some interesting observations about contact angles

In measuring contact angles, you are creatingand altering the free total surface of a drop:

solid

solid

solid

Non-wettingsurface -- a sphere hasminimum surfaceto volume

Drop volumeis constant

Thus, drop surface areamust increase

Why does the drop spread

on some surfaces?

In measuring contact angles, you are creatingand altering the free total surface of a drop:

solid

solid

solid

Non-wettingsurface -- a sphere hasminimum surfaceto volume

Drop volumeis constant

Thus, drop surface areamust increase

Why does the drop spread

on some surfaces?

Same liquid

Solid surface energy

increasing

Same solid

Liquid surface tension

decreasing

There are many techniques for measuring the contact angle

Figure 5

γsv

γlv

γslθ

θ

a.

c. d.

θ

air oroctane

b.

θ

Capillary rise Wilhelmy Plate

Droplet or bubble contact angles

Add liquid drop here

Goniometer

View here

Eyepiece has acrosshair +Protractor

This drop will appearupside down in the

goniometer objective.

Above the LCST

Below the LCST

Phase Transition Behavior of PNIPAAm-PDMS

20oC 40oC

Photographs of Water Drops on PNIPAAm-grafted PDMS Surfaces

Data of M. Ebara, UW, 1/05 (Stayton/Hoffman group)

Capillary rise of water in the grafted channel is reduced as temperature is raised above the LCST.

1 mm x 5 mm

Advancing contact angles (in air) sense the hydrophobic portion of surface properties

waterOH

OH

OH

OH

OH

OH

OH

CH3 CH3 CH3 CH3CH3 CH3 CH3 CH3

waterOH

OH

OH

OH

OH

OH

OH


Drop probes hydrophobic regions and θadv is high.Add water to the drop (to make it advance)

Water drop on surface at equilibrium

Receding contact angles (in air) sense the hydrophilic portion of surface properties

Drop is held back by hydrophilic regions and θrec is low.

Remove water from the drop (to make it recede)

CH3 CH3 CH3CH3 CH3 CH3

Water drop on surface at equilibrium

waterOH

OH

OH

OH

OH

OH

OH


CH3 OH

OH

OH

OH

OH

OH

OH

OH

waterOH

Drumheller and Hubbell, J. Bioed. Mater. Res. 29, 207 (1995)PEG molecular weight

1000 10,000 100,000

Advancing θ

Receding θ

Largehysteresis

Contact angles on hydrogels made by crosslinking three-arm PEG acrylates of different PEG MWs

θ

θ

b.θ

a.

Advancing (θadv)and receding (θrec)

contact angles

Tilting plane gives bothadvancing and receding

contact angles in same drop.

θadv θadv

θrec

θrec

Concerns and considerations in contact angle measurements

•Liquid penetrating the surface•Liquid dissolving the surface•Contamination of liquid•Size of drop and effect of gravity•Evaporation•Usually limited to ambient temperature•Usually limited to flat surfaces •Surface roughness•Surface heterogeneity•Surface group mobility and kinetics•Advancing vs. receding contact angles•Limited to relatively low energy solids (polymers)•Individual eyeball “bias”

Concerns and considerations in contact angle measurements

•Liquid penetrating the surface•Liquid dissolving the surface•Contamination of liquid•Size of drop and effect of gravity•Evaporation•Usually limited to ambient temperature•Usually limited to flat surfaces •Surface roughness•Surface heterogeneity•Surface group mobility and kinetics•Advancing vs. receding contact angles•Limited to relatively low energy solids (polymers)•Individual eyeball “bias”

NOTE: Contact angles do not directly

analyze surface composition

Force Balance at the Contact Angle (θ)

liquid

solid

vaporof liquid γ sv

γ lv

γ sl

γ lv is for liquid in equilibrium with its own vapor

γ sv is for solid in equilibrium with vapor of liquid

is for solid-liquid interfaceat equilibrium (assume noswelling of solid by liquid)

γ

γ l/s

θ

l/s

γsv γso πe “Equil. Spreading Pressure”

(Young-Dupré Eqn)γsv = γ l/s + γlvcosθAt equilibrium:

Young-Dupre Equation

γ sv=

γ ls

γ lvcosθ

-

θ = 180° cos θ = -1θ = 90° cos θ = 0θ = 0° cos θ = 1

Molecules in liquid and molecules on the surface have very different forces!

Molecules in liquid and molecules on the surface have similar forces!

γ svγ ls

Note that for solving this equation, we know

θ and , but we are missing two values!

We want

We do not know

lvγ

aθ vs. cos θ, and θadv vs. θrec

Practical application of the Young-Dupre equation

γ dγ p +γ =s

γ bγ a +γ =s γ d+

Critical surface tension(Zisman)

Polar + Dispersive(Good, Fowkes)

Acid-Base + Dispersive(Fowkes)

γc

We will use contact angles to estimate solid surface energies:

γs = γi + γH + γp + γd + γm

γs = γi + γa + γb + γd + γm

A concept developed in the 1960’s by Walter Zisman at the NRL

Critical surface tension, γc = surface tension of a liquid that would just completely wet a solid being tested.

When the liquid just spreads on the solid, then one could saythat the solid surface energy is just matched by the liquid

surface tension*. However, Zisman never believed γc was a measure of solid surface energy, but only an empirical

approximation of it, and this is the best way to view γc .

*(Note that the act of wetting and spreading is not an equilibrium phenomenon).

Critical Surface Tension, γc

Critical Surface Tension, γc

Measure θadv on one surface for a series of liquids varying in surface tension.

Plot cos θadv vs. γlv

Extrapolate to cos θadv = 1.0 (θadv = 0°)

Define γc γlv at cos θadv = 1.0 (θadv = 0°)=

Liquids used in critical surface tension measurements

WaterGlycerolFormamideThiodiglycolMethylene iodideTetrabromoethane1-bromonaphthaleneDibromobenzene1-methyl naphthaleneDicyclohexylHexadecaneDecane

72.963.758.453.551.749.845.042.938.932.727.624.1

Surface tension, dynes/cm (22°C)

Why these liquids?They’re stable, and can be purified. They have relatively low viscosity and low volatility. Only pure liquids will have a well-known surface tension.

Note γc range is relatively

narrow

(Baier, 1969)

(Baier, 1969)

25

γ dγ p +γ =s





γc




Some Polar and Dispersive Contributions to Liquidand Solid Surface Energies @ 20° C

LIQUIDS

γ = γd + γp

water 72.8 = 21.8 + 51.0 (dynes/cm)

fomamide 58.2 = 39.5 + 18.7

CH2I2 50.8 = 48.5 + 2.3

C16H34 27.6 = 27.6 + 0.0

SOLIDS

γ = γd + γp γc

Nylon 66 41.4 = 33.6 + 7.8 46

Dacron 39.5 = 38.4 + 2.2 43

P.E. 32.4 = 31.3 + 1.1 31

PDMS 22.1 = 20.5 + 1.6 22

PTFE 15.6 = 14.6 + 1.0 18.5

Contact angle, θ vapor ads’n. π

NOTE THAT EVEN OXIDES AND SALTS OF METALS, AS WELL ASGLASSES, HAVE AN IMPORTANT

DISPERSION COMPONENT OF SURFACE ENERGY

γ dγ p +γ =s





γc




Brønsted-Lowry acid is defined as a proton donor. Brønsted-Lowry base is defined as a proton acceptor.

The older acid-base principle applied to proton interactions:

HCl + NH3 HNH3+ + Cl-

B-Lbase

B-Lacid

The more modern acid-base principle applied to electron inter’ns:

Lewis acid is an electron pair acceptor. Lewis base is an electron pair donor.

HCl + :NH3 H:NH3+ + Cl-

F3B + :N(CH3)3 F3B:N(CH3)3

L. baseL. acid

It’s clear that Fowkes’ theory relating surfaceenergies and contact angles through acid-base

(plus hydrophobic) interactions is just another way to characterize polar (plus hydrophobic)

interactions between molecules at interfaces.

or acid-base interns

Conclusions

Definition of surface tension, γ : force in the surface of a liquid, to be applied to a barrier perpendicular to it in order to increase surface area.

Thus, surface tension, γ = f/L = (f•2)/(L•2) = f/L

force = f

L

force = f/2

L/2

is the same as:xx/2

Definition of surface energy per unit area, ε: force applied to a barrier of length L, perpendicular to it, acting over a

distance x, to increase surface area by x•L.The work to do this is the surface energy/area, ε.ε = (f•x)/(L•x) = (f•2x)/(L•2x) = (2f•2x)/(2L•2x) = f/L

THUS, SURFACE TENSION SURFACE ENERGY/AREA

lecture on contact angles, may 4, 2005 a) liquid and solid ... angles05... · liquid solid the...

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