People-Courses-26-Thermodynamics of Surfaces and Interfaces

7/23/2019 People-Courses-26-Thermodynamics of Surfaces and Interfaces

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Thermodynamics of

Surfaces and Interfaces



What is thermodynamics dealing

with?

• Thermodynamics is the branch of science that idconcerned with the principles of energytransformation in macroscopic system.

•

Macroscopic properties of matter arise from thebehavior of a very large number of molecules.

• Thermodynamics is based upon experiments andobservation , summaried and generalied in thelaws of thermodynamics.

• These laws are not derivable from any otherprinciples! they are in fact improvable andtherefore can be regarded as assumptions only.



Some "e#nitions

• Intensive variables

• $xtensive variables

•

System, isolated, open, closed• Surroundings

• %oundary

•

$&uilibrium• 'rocess

• Thermodynamic state



$&uilibrium between phases inheterogeneous closed system

• What does e&uilibrium mean? – ( system is in e& if no further spontaneous changes

ta)es place and if the same state can be reached byother directions.

• ( phase?

• ( phase e&uilibrium is de#ned when the samespecies are present in di*erent phases.

•

( heterogeneous closed systems is composedof two or more phases, with each phase isconsidered as an open system within an overallclosed system.




• If the system initially, is not in aninternal e&, then any process shouldoccur in irreversible direction.

• So, according to #rst law!

• (nd, combining with +lausiusine&uality! for both reversible and irreversible

processes-




• r #nally!



Wettability and +ontact(ngleReference:



Wettability and contactangle



• In the case of a li&uid that forms a uniform#lm i.e., where / 0-, the solid is said tobe completely wetted by the li&uid, or that

the li&uid wets the solid.• Where a nonero angle is formed, there

exists some controversy as to how todescribe the system. If a #nite contact

angle is formed 1 0-, someinvestigators describe the system asbeing partially wetted.



• thers prefer to ma)e a distinction based onthe sie of the contact angle. 2or example, agiven wor)er may de#ne as 33wetting44 any

li&uid that produces a contact angle of 50 orless on a solid. %etween 50 and 67 thesystem would be 33partially wetting,44 and 70and above nonwetting.

•

(lternatively, any system with 088 960would be partially wetting, and only for 960would the nonwetting term be applied.



• While the contact angle of a li&uid on a solid may beconsidered a characteristic of the system, that will betrue only if the angle is measured under speci#edconditions of e&uilibrium, time, temperature,

component purity, and other parameters.• The great utility of contact angle measurements

stems from their interpretation based on e&uilibriumthermodynamic considerations. (s a result, moststudies are conducted on essentially static systems in

which the li&uid drop has presumably- been allowedto come to its #nal e&uilibrium value under controlledconditions.



• (s an application, contact angles, for example, canbe extremely useful as aspot test of the cleanlinessof sensitive surfaces such as glass or silicon wafersfor microelectronics fabrications.

• %oth surfa es are 33high energy44 and are completelywetted by pure water.

• If the surface is contaminated by something suchas an oil that interferes with the processing of the

material e.g., the coating of a photoresistpolymer-, a drop of water will have a nonerocontact angle, and the contamination will beimmediately apparent.



+ontact angle hysteresis

• 2or systems that have 33true44 nonerocontact angles, the situation may befurther complicated by the existence of

contact angle hysteresis.• Thus, the contact angle one observes may

vary depending on whether the li&uid isadvancing across fresh surface the

advancing contact angle, (- or recedingfrom an already wetted surface thereceding contact angle, :- 2ig. 9;.5-.



• (s an operational convenience, many, if notmost, static contact angles measured andreported are in fact advancing angles. 2or agiven system, it will be found that ( :.

• In practice, very few systems exhibit a completelac) of hysteresis, so that the problem can beoperational as well as philosophical.

• ne should )eep in mind that when discussingcontact angle data, one must always be aware ofhow the angle has been measured in order tointerpret its signi#cance properly.



Why hysteresis

• In dynamic contact angle studies, additionalcomplications arise because the movement of thewetting line is not always a steady, continuous process.

• It is often observed that the movement is 33<er)y,44 with

the drop or li&uid front holding a position for a time andthen <umping to a new con#guration.

• This phenomenon is often referred to as a 33stic)=slipprocess44 and is not fully understood as yet. It has alsobeen observed that in dynamic systems, the values of

( and : will vary as a function of the velocity ofwetting line movement, with ( increasing with

velocity and : decreasing.



Why hysteresis

• When used with >oung4s e&uation and other suchrelationships, the contact angle provides a relativelysimple yet sensitive insight into the general chemicalnature of a surface through such thermodynamic

&uantities as the wor) of adhesion. nfortunately, asalready mentioned, contact angles often exhibithysteresis and cannot be de#ned unambiguously byexperiment.

• It is always important to )now as much as possible

about the cleanliness, topography, homogeneity, andother characteristics of a solid surface, as well as thepurity and composition of the li&uid employed, whenattempting to interpret contact angle data.



Why hysteresis

• (lthough the existence of contact anglehysteresis has been recognied for atleast 900 years, the root of the 33evil44 has

not always been understood. In additionto the physicochemical adsorptionprocess already mentioned, which leadsto di*erences in advancing and receding

contact angles, it is recognied thatseveral physical and )inetic factors alsocontribute to the overall problem.



Contact Angle MeasurementTechniques

• There are a variety of simple and inexpensivetechni&ues.

• The most common direct methods 2ig. 9;.@- includethe sessile drop a), the captive bubble (b), the sessile

• bubble c), and the tilting plate (d). Indirect methodsinclude tensiometry and geometric analysis of theshape of a meniscus.

• 2or solids for which the above methods are notapplicable, such as powders and porous materials,methods based on capillary pressures, sedimentationrates, wetting times, imbibition rates, and otherproperties, have been developed.



The Eects of Surface Roughnesson Contact Angles and Wetting

FIGRE !"#$# The a%%arent contact angle of a liquid on asurface ma& dier fromthat expected, the 33true44 contact angle a), due to irregularities—either physical or

chemicalAincluding surface roughness b) or chemicalheterogeneity (c).



The Eects of Surface Roughnesson Contact Angles and Wetting

• The theoretical discussion of contact angle and wetting to thispoint has assumed implicitly that the solid surface in &uestionis a smooth, ideal plane.

• In fact, of course, very few solid surfaces even begin toapproach such a state.

• The #nest polished glass surface, for example, will usuallyhave asperities of B nm or more.

• +ommonly encountered polished surfaces, will be muchrougher

• by factors of 90=9000.

• The earliest, and still most useful, &uantitative attempt tocorrelate the observed contact angle of a li&uid on a solid withthe surface roughness is the Wenel relationship whichproposes a thermodynamic relationship such that



• Wenel relationship!

• where Rw is defned as the suraceroughness actor, the ratio o the trueand apparent surface areas of the

solid 2ig. 9;.Bb). Defning theapparent contact angle as yields!



:ecall >oung e&uation

with new notations-

FIGRE !"#"# 'oung(sequation fordeterminingthe contactangle )asoriginall&based on ananalysis of theforce balanceamong thethree surfacetensions



The last e&uation may be ta)en as a fundamental de#nition ofthe e*ect of surface roughness on wetting and spreading phenomena.



• (s a #nal note on the e*ects of surface roughness,examination of the $&uation, leads to a useful ruleof thumb for some important applications ofwetting and spreading phenomenaC that is!

• If the 33true44 contact angle of a li&uid an adhesive,say- is less than 70 on the smooth surface, theangle will be even smaller on a rough surface.

• 2or a true contact angle 70, roughness will

increase the apparent angle. Mathematically thesituation can be described as!



*eterogeneous Surfaces

• :oughness represents <ust one aspect of thee*ects of the nature of the solid surface oncontact angles and wetting phenomena.

• ( second potentially important factor is that of

the chemical heterogeneity of the surface 2ig.• 9;.Bc).

• It is possible to develope the followingrelationship relating apparent contact angle to

the chemical composition of a surface!




• where and ! are the ractions o the suracehaving inherent contact angles 9 and D.

• Since !" #, the e$uation can be written interms o one component.

• Theoretically, if the inherent contact angles of atest li&uid on the homogeneous surfaces are)nown, then the composition of a compositesurface can be determined from a simple contactangle measurement.




• $xperiments employing speciallyprepared composite surfaces haveshown that contact angle data can

give results that agree reasonablywell 9BE- with more sophisticatedsurface composition data obtained

using, for example, FGrayphotoelectron spectroscopy F'S-.

People-Courses-26-Thermodynamics of Surfaces and Interfaces

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