Ch18 Surface Chemistry PChem 353.pdf

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Chem 353 Physical Chemistry II

Ch.18 Surface Chemistry and ColloidCh.18 Surface Chemistry and ColloidCh.18 Surface Chemistry and ColloidCh.18 Surface Chemistry and Colloid

2013 Fall Semester

Okan Esentürk METU Chemistry Department

Section 2

Intro

• Interface: a boundary that separates two phases

• Surface: a special case where one of the phase is a gas.

• Surface plays a significant role in many applications

- Painting lubrication detergency corrosion adhesion rusting

- r!ns in electrochemical cells r!ns in biological cells

- wal" of a gec"o pic"ing up tiny particles with finger tips• Properties of molecules at surfaces and interfaces are #uite different than the ones in

the bul" due to the imbalance of forces at the boundary region.

Intro (Cont’d)

$nterfacial layerSurface layer$nterphase region

−INTERFACE

−

%hic"ness form one atomic layer tose&eral molecular diameters

'!( )/* interface of a pure li#uid is +3molecular diameter thic".

and are two different regions

• S/S S/) or S/* transitions are more abrupt than )/) or )/, interfaces.• ompare to bul" or -phase of molecules at the interfacial layer is e!tremely small

thus its effect on system bul" properties are negligible.• %he properties become important when the process happens at the interface.

18.1 Adsorption%wo main types of adsorption(1. Physisorption physical adsorption &an der aals adsorption2. hemisorption chemical adsorption acti&ated adsorption

Physisorption• Forces are physical in nature• $nteraction strengths are relati&ely wea" and mainly go&erned by &an der aals forces• any layers of adsorption is possible• o acti&ation -4 rapid process• 5eat e&ol&ed is similar to the enthalpy of condensation

'!( heat of physisorption of a gas &an der aals is +20 "6/mol

• Δ is small enough to be dissipated by thermal motions phonons• an be measured by measuring the rise of substrate temperature during ads.

Δ H"7/mol

5 8 -21

2 -21

5 29 -:;

5 2 -<8

www.horiba.com



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18.1 AdsorptionChemisorption• First considered by $r&ing )angmuir

• olecules adsorbed to surface with chemical bonds

• =dsorbed molecules find sites that ma!imi>e the coordination number with the substrate

• ?sually ha&e acti&ation energy -4 slow process

• Δ is large +100-:00 "6/mol

•

'!cept few cases 52 on glass its e!othermic• 9nly one layer of chemisorption would occur.

• 9nce all the a&ailable sites are co&ered its saturated

• =dditional adsorption onto the first layer is usually wea".

Δ H"7/mol

9 on Fe -1;2

52 on -1<<

www.scm.com

18.1 AdsorptionPhysisorptionand Chemisorption of H2• Physisorption occurs first.• Physisorption has shallow stabili>ation

energy when 5 2 adsorbed.• hemisorbed atoms ha&e much deeper

minimum corresponding to a strongerbonding.

• rossing of two cur&es shows the small

'a for chemisorption.• $nitial physisorption is an essential future

for chemisorption.@arrier through a physisorbed stage is lower than diss.of 52.

Figure 18.1

Physisorptiondetermined by:

Temperature!as pressureInteraction bet"een

surface and #as $e%#&apor pressure'

Surface area

18.2 Adsorption IsothermsIsotherm: an e#n relating one property to another at constant %.

Adsorption isotherm: an e#n relating amount of adsorbate on the absorbent to theconcentration of adsorbing molecules in gas or li#uid phase mo&ing phase at a fi!edtemperature.

=n e#uation that relates the amount of a substance to a surface to its concentration in the gas phase or in solution at afi!ed temperature is "nown as an adsorption isotherm.

Surface =dsorbent

=dsorbate

$ A9P9A'S

9 P9A9?S '=BS?@S%A=%'

'=BS?@S%A=%'

'S9P9A'S(=PP$)=AC9 D' S=%$9 )=C'A$ *

S$E %CP'S 9F S9AP%$9 -$S9%5'A S by $?P= 1;<:

Langmuir Isotherm (Ideal adsorption)18.2 Adsorption Isotherms

• Simplest isotherm de&eloped by $r&ing )angmuir

=ssumptions(1 =ll parts of the surface are e#ual and beha&e e!actly the same no preference of

adsorption on sites2 onolayer co&erage3 =dsorption of a molecule to a site is independent of neighbors

no interaction btw adsorbed molecules

Surface =dsorbent

=dsorbate

%CP'$

%CP' $(Pores are typically microporus with the e!posed surface residingalmost e!clusi&ely inside the microspores which once filled withadsorbate lea&e little or no e!ternal surface for further adsorption.

()* )+ mo,ecu,es co&er ) cm2 of the surface



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Langmuir Isotherm18.2 Adsorption Isotherms

=dsorption(• Suppose that a dynamic e#uilibrium is established

+ − − ⇔ − −

a fraction of the surface is co&ered anda fraction

−will not be co&ered.

e!tend of co&erage

! "# $ "% &'"( '& "))* ' $! "# $ "% &'"( '& ,' -,

Assumptions:)' A,, parts of the surface are e-ua, and

beha&e e.act,y the same2' /ono,ayer co&era#e0' Adsorption of a mo,ecu,e to a site is

independent of nei#hbors

.//// ////

Surface =dsorbent

=dsorbate


=dsorption(

Aate of adsorption( 0 1 2 −

and rate of desorption( 0$ 1 $

=t e#( 0 0 $ thus

1 2 − 1 $ 3−

11 $

2 4 2

4 2

+ 4526

Similarly for a gas adsorption at a pressure of P

4 7+ 4 7

($ 8

8 9

Assumptions:)' A,, parts of the surface are e-ua, and

beha&e e.act,y the same2' /ono,ayer co&era#e0' Adsorption of a mo,ecu,e to a site is

independent of nei#hbors

Surface =dsorbent

=dsorbate

,olume correspond to complete co&erage

Simp,e1an#muirIsotherm


4 2+ 4526

:; +4526

Surface =dsorbent

=dsorbate

Figure 18.2 2& 2 < 3 <

2& 2 = 3

>?@A BCD EC>?@FG?5 6 I J

KFLM@FL?N KM@OFE? EC>?@FG? P J

'!ample 1<.1

en3ene adsorbed on #raphite is found to obey the 1an#muir isotherm to a #ood appro.imation% At a pressure of )%** Torrthe &o,ume of ben3ene adsorbed on a samp,e of #raphite "as found to be 4%2 mm0 at STP$*°°°°C and ) atm pressure'5 at0%** Torr it "as 6%+ mm0% Assume a ben3ene mo,ecu,e to occupy 0* 72 8 estimate the surface area of the #raph ite%

SolutionRemember: 8

8 Q ($ 4 2

R4526S 1et the saturated surface co&era#e is 8 9 = X mm05

at ) torr TUV W

U< 4R U< 4

and at 0 torr XUY W

ZU< 4RZU< 4

TU VXU Y

U< 4[\ + U<4]

ZU<4[\ + ZU< 4] 3 4 <U Z ($ 8 9 ^UT __ Z

The ma.imum amount of ben3ene adsorbed is thus )9%4 mm0%

) mo, at STP occupies 22%4 1 2%24××××)* 9 mm0

)9%4 mm0 is thus )9%4;$2%24××××)* 9' mo, 9%99××××)* 9mo,

9%99××××)* 9×××× <%*22××××)* 20 4%<6××××)* )9 mo,ecu,es%

The estimated area of the surface is thus

4%<6××××)* )9 ××××0* 72 )%4*××××)* )=72 *%)4* m2



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Prob,em )6%)

A surface is ha,f co&ered by a #as "hen the pressure is ) bar% If the simp,e 1an#muir isotherm app,ies:a% >hat is ?@b% >hat pressures #i&e 9+ 8 =* 8 == 8 ==%= co&era#e@c% >hat co&era#e is #i&en by pressures of *%) bar8 *%+ bar8 )*** bar@

So,ution

[A]1[A]

θK

K ++++

====

a%

bar11bar1

5.0××××++++

××××====

K K 1bar1 −−−−

====K

b%

1

1

bar11bar1

θ −−−−

−−−−

××××++++

××××====

PP

θ1θ−−−−

====P

bar999999.0θ

bar9999.0θ

bar990.0θ

bar375.0θ

====→→→→====

====→→→→====

====→→→→====

====→→→→====

P

P

P

P

1

1

bar11bar1

θ−−−−

−−−−

××××++++

××××====

PP

c%

999.0θbar1000

33.0θbar5.0

091.0θbar1.0

====→→→→====

====→→→→====

====→→→→====

P

P

P

Prob,em )6%0

The fo,,o"in# resu,ts "ere reported by 1an#muir for the adsorption of nitro#en on mica at 2* °°°°C:

Pressure/atm 2.8 4.0 6.0 9.4 17.1 33.5

Amount of gasadsorbed/mm 3

at 20 °°°°C and 1 atm12.0 15.1 19.0 23.9 28.2 33.0

a% /aBe a ,inear p,ot of these &a,ues in order to test the 1an#muir isotherm% If it app,ies8 e&a,uate theconstant K %

b% Suppose that )*)+ mo,ecu,es co&er ) cm2 of the surface% /aBe an estimate of the effecti&e area in1an#muir s e.periment%

So,utionThe amount x adsorbed is proportiona, to θθθθ and therefore8

[A]1[A]

θ K aK

a x ++++========

To con&ert atmospheres to concentrations:

3dmmol15.29308205.0

/atm −−−−

××××========

P RT

PV n

To con&ert amount of #as adsorbed to mo,es:

mol15.29308205.010

/mmmoldmatm15.29308205.0

(atm)16

3

13 ××××××××====

××××========

−−−−

V V RT PV

n

Concentration/mol dm -3 0.116 0.166 0.249 0.391 0.711 1.39

Amount adsorbed/10 -7 mol 4.99 6.28 7.90 9.94 11.7 13.7

a% A ,inear p,ot may be obtained by p,ottin# ); x a#ainst );DA :

[A]1[A]

K aK

x++++

====aaK x1

[A]11

++++====

[A] -1 /mol dm -3 8.62 6.02 4.02 2.56 1.41 0.719

x -1 /10 6 mol 2.00 1.59 1.27 1.01 0.85 0.73

6

6

10161.01

10615.01

××××====

××××====

aK

a13

6

moldm82.3

mol1063.1−−−−

−−−−

====

××××====

K

a

b.Complete coverage, a, corresponds to

1.63 ××××10 -6 mol = 9.82 ××××1017 molecules

Using the knowledge that 10 15 molecules covers 1 cm 2 of the surface, the surfacearea was thus about

9.82 ××××1017 /10 15 = 10 3 cm 2=10 -1 m2

y 0.1G11! H 0.G18I

AJ 0.;;;G0

0.:

1

1.:

2

2.:

0 : 10



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Adsorption ith !issociation18.2 Adsorption Isotherms

• `abcdefghidfj k[ldeefmdnidfj fo n cfpbmqpb\.lefghidfj fo n edjrpb cfpbmqpb fmmqhs cfgb edibe]

t ⇔u v wxyz {zx

| } J ~t F•N | }~t=t e#. | | thus

~J ~

}}

€[t C@ 4 [V 2 [V

4[V 2 [V

. .//// //// //// ////

4 24526

=. 5ussain et al.Topics in Catalysis, :8 81:-823 2011.

Adsorption ith !issociation18.2 Adsorption Isotherms

4 [V 2 [V4[V 2 [V "% 4 [V 2 [V

Fi#ure )6%2

2& 2 < 3 < 2& 2 = 3

>?@A BCD EC>?@FG?€[t €[tI J

KFLM@FL?N KM@OFE? EC>?@FG?€[t

€[tP J

Simi,ar to nondissociati&eco&era#e

Competiti"e Adsorption18.2 Adsorption Isotherms

• =dsorption of two substance to the same surface

⇔ and ‚ ƒ⇔ƒ

.//// //// ‚//// ////

|u

}u J ~u ~„ |u }u ~u |

„}„… J ~„ ~u |„ }„ ~„

=t e#. |u | for both = and @

}u ~u }u J ~u ~„ ~u

J ~u ~„}u †‡ ‡̂ .

2 4 2 2‰ŠŠ ‰‹5‹6

}„ ~„ }„ … J ~„ ~u~„

J ~u ~„}„ …†Œ Œ̂‚

4‹ ‹4ŠŠ 4‹5‹6

F. Fang and $. S>leifer6. hem. Phys. 11; 10:3 2003

$f K@L 0 or B@ 0 thenit is the same as singleadsorption

#ther Isoterms18.2 Adsorption Isotherms

De&iation from ideality(

• Surfaces are not uniform or homogeneous

• =dsorption of one species may fa&or or inhibit the adsorption of the others.

Freund,ich Isoterm• onsiders no saturationMMM

• Fit of a system beha&ior to an empirical formula

• *ood for heterogeneous surface energies.

Ž 1 )( \ ‘ ’“ “” –]=mount ofsubstanceabsorbed

'mpiricalconstants

)7 [)Vc) and c2 are constants Ž 1 )[( —(˜ *(# +"% -, - "% &'"( # +"% -, $ "% &'"(



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#ther Isoterms $ %&'18.2 Adsorption Isotherms

• =llows physisorption of additional layers abo&e

the monolayer

• = multilayer model

• =ssumes )angmuir model applies to each layer.http(//www.7hu.edu/+chem/fairbr/9)DS/deri&e.html

™™š ™› ™

Jš

+™

š ›

šœ >CBM ? CO GFK FNKC@ž?N™œ ™@?KKM@?

š › œ šCBM ? CO C•CBFA?@ EC>?@FG?™› œ FLM@FL?N Ÿ@?KKM@?

• $t is better to use to estimate the surface area• @'% method can be used to measure %CP' $$ nonporous $, wea"

substrate and ,$ layering isoterms.

=pplications(N PharmaceuticalsN atalysts

N Pro7ectile propellantsN edical implantsN FiltersN ements

'!ample 1<.2

The fo,,o"in# data re,ate to the adsorption of nitro#en at 99 ? on a )%** # samp,e of si,ica #e,:

P ;BPa )+%2 +4%6V ;cm0 $STP' )0+ 249

At 99 ? the saturation &apor pressure P * of nitro#en is )*)%0 BPa% Estimate the surface area of the #e,8 taBin#the mo,ecu,ar area of nitro#en to be )%<2××××)* )=m2%

Solution

$nsertion of the data into the @'% e#uation gi&es the following two simultaneous e#uations(

00

2.151132.0

V K V ++++====

00

8.541483.0

V K V ++++==== )(kPa17.3)(cm113 13

0−−−−

−−−−=========>=>=>=> K and V

=t S%P 22.8 ) 22800 cm3 is the &olume occupied by 1 mol.

= &olume of 113 cm 3 thus contains 113/22800 0.00:08 molO 0.00:08 ×G.022×1023 3.08×1021 molecules.

Since each molecule occupies 1.G2 ×10 -1; m2 the estimated surface area is

3.08 ×1021×1.G2×10-1; 8;2 m 2

18. 'hermodynamics of Adsorption

• = simplest way to distinguish physisorption and chemisorption is by comparing their

enthalpy changes. Figure 1<.1

• During an adsorption heat is liberated in general Δ ?¡CL¢?@ £E

• Degree of freedom decreases w/adsorption especially with chemisorption Δ• =dsorption is a spontaneous process Δ¤• $fΔH¥for adsorption then Δ¦must also be 4 0 for adsorption to occur.

• Aemember &an t 5off e#n(

§– 1§¨

©ª«¬;¨V

18. 'hermodynamics of Adsorption• =s a simple e!ample lets consider ads. 9f a non-dissociati&e substance and let

® ! CO LCLFB CB?EMB?K £• L¢? GFK Ÿ¢FK?

! CO FNKC@ž?N CB?EMB?K ! CO CŸ?• K£L?K FL ?¯U

°¢MK ® ± ® ®š

±

F•N ±

°¢?• w ±

±®±[

®[š [

4 ) ² 8

² ³ ²

´K£•G ŸF@L£L£C•K OM•EU ebmJµU¶

¯

¯® ¯ ? ·¸¹ º [»¼

½abj

±±

± ® ± ®¯

¯® ¯ ? ·¸¹ º [»¼

.pef

±±

¾J − ¾

¾J − ¾

± ® ¯¯® ¯

? ·¸¹ º [»¼

.eeqcb

¯ J •C O@??NC CO CL£C•

¯ ž \fjps ¿dÀU njl gfinidfjnp ogbblfc]

ÁÂ ÃÄ } „ ½ Å[t

¢ Å ÀÂ

Æ− Æ

Ç ³

Z

VÈ_1 ¨ Z[V

-- ³

·©É [;¨



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18. Chemical *+ns on Surfaces

• %he surface reactions are usally

?nimolecular i.e. surface cataly>ed ammonia decomposition@imolecular i.e. acetaldehyde decomposition

nimo,ecu,ar reaction

• ?nimolecular r!ns can be treated in terms of )angmuir adsorption isotherm.

• Simplest case( rate of r!n is proprtional to surface co&erage ~

0 1 14 2+ 4526

• Deri&ed from an assumption of rapid adsorption e#uilibrium formationfollowed by slow chemical reaction.

- Similar to the dissociation- Similar to ichaelis enten '#n.

,nimolecular Surface *+ns18. Chemical *+ns on Surfaces

nimo,ecu,ar reaction

%hus the rate depends on the surface concentration in other words co&erage.

- =t high concentrations BK=L 44 14 0 >eroth order

- =t low concentrations BK=LQQ 14 0 † ˆ . first order

0 1 14 2+ 4526

Figure 18.3

,nimolecular *+ns ith Inhi-itor18. Chemical *+ns on Surfaces

• =dsorption of a substance other than the reactant on the surface result in decrease of

effecti&e surface area and therefore the rate.

• Suppose that a substance = is undergoing a unimolecular reaction on a surface and that

a nonreacting substance $ "nown as an inhibitor or a poison is also adsorbed.• @oth reactant molecule = and the inhibitor $ do compete for a&ailable sites similar to

competeti&e r!ns then

~H

J + H + H {5Ê6 3 | }

HJ + H + H {5Ê6

,nimolecular *+ns ith Inhi-itor18. Chemical *+ns on Surfaces

~H

J + H + H {5Ê6 3 | }

HJ + H + H {5Ê6

ase $( H{ Ê P J + H 3 | } Ë u

Ë Ì5Í6

ase $$( H{ Ê I J + H 3 | }Ë u

€RË Ì5u6no inhibition

'!ample( Decomposition of ammonia on platinum

- Å Î€t

- t +Åt t F•N Δ t ¥ Δ - Å ¥ Δ - t

%hus as the reaction proceeds 52 poisons the system. | }Ë ÏÐ Ñ

Ë Ì5ÐÒ6

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%imolecular *+ns18. Chemical *+ns on Surfaces

• %here are two possible mechanisms

1an#muir Hinshe,"ood /echanism

• Aate is proportional to the probability of = and @ absorbing on neighboring sites.

• %he possibility of = and @ absorbing to the neighboring site is proportional to the

fractions of surface co&erages of = and @ ~uand ~„ competiti&e ads thus

| } ~u ~„ }u

J + u + „ …„ …

J + u + „ …

0 14 2 4 2

+ 4 2 2 + 4 V@oth " and B are

temperature dependent


ases when K@L is "ept constant(

=t low K=L 0 Ó 526=t high K=L 0 Ó [ 526• %he ma!imum rate corresponds to the

presence of the ma!imum number ofneighboring =-@ pairs on the surface.

0 14 2 4 2

+ 4 2 2 + 4 V

Figure 18.3

• $f @ absorbs strongly than = H„ … PJ u

Ô Õ‰ŠŠ4

'!( ±Ö€tÖt Î ±Öt C• ¯MF@L×

• $f both K=L and K@L are low thenJ P u + „ …

Ô Õ‰Š 4 Š


1an#muir Ridea, /echanism

• A!n between a molecule that is not absorbed = and theone adsorbed @

• %he rate of reaction is proportional to concentration of =and surface co&erage of @

0 Ó 2

0 14

+ 4 2 2 + 4

Figure 18.3

• o ma!imum R different than )angmuir-5inshelwood mech.

'!( ombination of hydrogen atoms

~ Ë Ð€RË5Ð6

and | } ~

3 | } t

J + 5 6• =t low % surface is fully co&ered ~ Ø J P J 3| }5 6• =t high % low co&erage J P 3 | } t


E.amp,e

Decomposition of phosphine P53 on tungsten is first order at low P and >erothe order athigh P. Find the rate e#n.

=ssuming the rate is proportional to the surface co&erage

| }~ }™J ™

• hen P is low JP ™ 3| } ™ S • J

• hen P is high ™ P J 3 | } S •

Aeading 1<.: and 1<.G

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Problem 1<.10

A first order surface reaction is proceedin# at a rate of )%+××××)* 4mo, dm 0s ) and has a rate constant 2%*××××)*0 s )% >hat "i,, be the rate and the rate constant if

a% the surface area is increased by a factor of )*@

The rate and the rate constant are both increased by a factor of )*:

v )%+××××)* 0mo, dm 0s )5k 2%*××××)* 2s )

b% the amount of #as is increased tenfo,d at constant pressure and temperature@

The rate of con&ersion $mo, s)' remains the same8 but since the &o,ume is increased by a factor of )*the rate is reduced by a factor of )*8 as is the rate constant:

v )%+××××)* +mo, dm 0s )5k 2%*××××)* 4s )

So,ution

If these &a,ues ofv and k app,y to a reaction occur rin# on the surface of a spherica, &esse, of radius )* cm:c% >hat "i,, be the rate and rate constant in a spherica, &esse,8 of the same materia,8 of radius )** cm8 at the samepressure and temperature@

Increasin# the radius by a factor o f )* increases the surface area by a factor of )** and the &o,ume by a factorof )***% The rate and the rate constant are thus reduced by a factor of )*:

v )%+××××)* +mo, dm 0s )5k 2%*××××)* 4s )

d% Gefine a ne" rate constantk that is independent of the #as &o,umeV and the area S of the cata,yst surface%

Sincek is proportiona, to S and in&erse,y proportiona, toV 8 the constant k kV ; S is independent of V and S %

e% >hat "ou,d be its SI unit@

Its SI unit is m s)%

Problem 1<.1G

Su##est e.p,anations for the fo,,o"in# obser&ations8 in each case "ritin# an appropriate rate e-uation based ona 1an#muir isotherm:

a% The decomposition of phosphine $PH0' on tun#sten is first order at ,o" pressures and 3ero order at hi#herpressures%

b% The decomposition of ammonia on mo,ybdenum is retarded by the product nitro#en8 but the rate does notapproach 3ero as the nitro#en p ressure is increased%

Solution

][PH1][PH

3

3

K kK

v++++

====

a. low pressure][PH 3kK v ====

k v ==== high pressure

][N][NH1][NH

23

3

iK K kK

v++++++++

====

b.

c% n certain surfaces $e%#%8 Au' the hydro#en o.y#en reaction is first order in hydro#en and 3ero order in o.y#en8"ith no decrease in rate as o.y#en pressure is #reat,y increased%

d% The con&ersion of para hydro#en into ortho hydro#en is 3ero order on se&era, transition meta,s%

][O][H1]][O[H

2O2H

22O

22

2

K K kK

v++++++++

====

c.smallis][H 2H 2K

largeis][O 2O 2K

][H 2k v ====

d.

2 / 12

2 / 1 ][H1

θ1K

====−−−−

22 θ)1]([H −−−−==== k v

K k

v ====

Solution



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11

/ethods to measure surface tension18. Surface 'ension and Capillary

a' Static or Sessi,e Grop /ethod

b' >i,hem,yP,ate /ethod

c' Capti&e Air ubb,e /ethod

d' Capi,,ary Rise /ethod

e' Ti,ted drop /easurement

http(//www.uweb.engr.washington.edu

18. Surface 'ension and Capillary

E.: Determine the minimum road length btw 8 town located at the corner of a s#uare.

r

/ethods to measure 0 i) Capillary rise18. Surface 'ension and Capillary

ase $( omplete wetting A r no angle btwthe meniscus and the wall surface ~is >ero.

• et force on the surface is

æ VÈ% Ù• et force on the li#uid due to gra&ity is

æV UG š ê G È%V ë ³

• Aise stops when forces are balanced

ì € ì t

%hus ÃÄ@ Ü Ä@t ¢ ê G

Ü %V

ë ³

Figure 18.9

Figure 1<.; illustrates the simple andcommonly employed is the capillary-rise method.

r

/ethods to measure 0 i) Capillary rise18. Surface 'ension and Capillary

ase $$( hen it does not completely wetthe surface í î @Û ~ is non>ero.

• et force on the surface is

æ VÈ% Ù Ç"

• et force on the li#uid due to gra&ity is

æV UG š ê G È%V ë ³

• Aise stops when forces are balanced

ì € ì t

%hus ÃÄ@ Ü ±CK~ Ä@t ¢ ê G

Ü %

V Ç" ë ³

F

fg < 3 `fe Î Ç"_ Uï &Figure 18.9

Figure 1<.; illustrates the simple andcommonly employed is the capillary-rise method.

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12/23/2013

12

/ethods to measure 0 ii) ilhelmy late /ethod18. Surface 'ension and Capillary

Plate( rough Pt or paper

Ù Ç" ï&"& , \ï, &-]V ,

~œEC•LFEL F•GB? K FBBDz z ðœLCLFB D?£GL¢ D?LL?N

Dñð zxœŸBFL? D?£GL¢ £• F£@žœžCMAF•EA OC@E? \@F£K£•G L¢? ŸBFL?]BœB?•GL¢ CO ŸBFL?

www."eywordpicture.comO www."s&nima.com

18. Surface 'ension and Capillary

Compound é _²_ VòX 4=cetone 23.I

@en>ene 2<.<<

'thanol 22.2I

ethanol 22.G

-he!ane 1<.8

Diethylether 1I

ater I2.I:

a l a# G <2.::ercury 8I2

T $oC' éó ô“” _²_0 23.I

2: 2<.<<

:0 22.2I

100 22.G

Surface tension• Depends on intermolecular forces• $n&ersely depends on %

£U?U õ Ü J °°w • Should be >ero at critical temperature• etallic li#. 4 ionic li# 4 co&alent li#.

18. Surface 'ension and Capillaryur&ed surfaces(

• System will wor" for minimi>ation of the surface area which leads to the formation ofcur&ed surfaces as in spheres

Surfacetension

Cur&edsurfaces

Chan#e in &apor pressure of ,i-uid

Capi,,ary rise

a @ubble( a region where &apor H air is trapped by a thin film and ha&e two surfacesinside Houside

b a&ity( &apor-filled hole in a li#uid on surface insidec Droplet( small &olume of li#uid at e#uilibrium by its &apor Hair

3oung$Laplace &4uation (&4uation of Capillarity)18. Surface 'ension and Capillary

http(//en.wi"ipedia.org/

www.docstoc.com

• P inside a drop or bubble is always greater than outside aircontinuous phase

• =s the radius becomes smaller P becomes larger• @alance btw Üand e!ternal forces i.e. gra&ity determines the shape

r; µµµµm

∆∆∆∆P;atm

1000 0.0018

100 0.018

1 1.83G

0.01 183.G

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13

Comparing "apor of sphere to "apor of linear surface18. Surface 'ension and Capillary

• First considered by illiam %homson )ord Bel&in

N¤ N• ö°pj ™™ ÷Δ¤ • ö°B• HH ™[™›

• dn( number of moles of li#uid transfered from plane surface to a spherical droplet

• From surface area and surface energy consideration

šCBM ? CO N• CB B£¯Ûš ø N•[êO M@OFE? F@?F CO N@CŸB?LÛ ù Ä @t • =n increase in radius by dr resuls in &olume as T È %V$%thus

ø N•ê ùÄ@

t

N@3 N@

ø

ùÄ@tê

N• F•N N ùÄ @ + N@ t − ùÄ@t úÄ@N@

3 N¤ ÜN úÄ@ Ü N@ 3 N¤ Ã Üø@ ê

N• N• ö° pj™™›

–7

7 <

VÙû%ë

?e,&in E-n%

Comparing "apor of sphere to "apor of linear surface18. Surface 'ension and Capillary

–7

7 <

VÙû%ë

?e,&in E-n%

The &apor P of the drop,et is hi#herthan that of the ,i-uid

Problem 1<.1I

The surface tension of "ater at 2* °°°°C is 9%29××××)* 2N m ) and its density is *%==6 # cm0% Assumin# a contactan#,e θθθθ of 3ero8 ca,cu,ate the rise of "ater at 2* °°°°C in a capi,,ary tube of radius $a' )mm and $b' )* 0cm%

Solution

2ρ

γ grh

====

a

gr h

ργ2

====

m1049.1)s(m81.9)m(kg998(m)10

)m(N1027.72 2233

12−−−−

−−−−−−−−−−−−

−−−−−−−−

××××====××××××××

====h

r )* 0m

b

m49.1)s(m81.9)m(kg998(m)10

)m(N1027.72235

12

====××××××××

====−−−−−−−−−−−−

−−−−−−−−

h

r )* +m

Problem 1<.20

The t"o arms of a tube ha&e radii of *%*+ cm and *%)* cm% A ,i-uid of density *%6* # cm0is p,aced in the tube8and the hei#ht in the narro" arm is found to be 2%2* cm hi#her than that in the "ider arm% Ca,cu,ate the surfacetension of the ,i-uid8 assumin# θθθθ *%

Solution

gr

h

ρ

γ2====

−−−−====

21

11

ρ

γ2∆

r r g

h

−−−−××××

====−−−−−−−− 001.0

10005.01

)s(m81.9)m(kg108.0γ2

m022.0 233

12 mN086.0skg086.0γ −−−−−−−−========

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18

Problem 1<.1;

The density of "ater at 2* °°°°C is *%==6 # cm0and the surface tension is 9%29××××)* 2N m )% Ca,cu,ate the ratio the&apor pressure of a mist drop,et ha&in# a mass of )*)2 # and the &apor pressure of "ater at a p,ane surface%

Solution

331831212

π34

m10002.1cm10002.1998.0

10dropletof Volume r ====××××====××××========

−−−−−−−−

−−−−

m1021.6 7−−−−××××====r

0017.0(K)15.298)Kmol(J3145.8(m)1021.6)m(kg10998.0

)mol(kg1002.18)m(N1027.72

ργ2

ln

11733

1312

0

====

××××××××××××××××××××

××××××××××××××××====

====

−−−−−−−−−−−−−−−−

−−−−−−−−−−−−−−−−

rRT M

PP

0017.10

====

PP

18.8 Li4uid films on surfaces $ #mitted#mitted#mitted#mitted

= )angmuirR@lodgett troughcrann.tcd.ie

en.wi"ipedia.org

%ransfer of = )angmuirR@lodgettfilm to a solid substrate

miner&a.union.edu

Coherent fi,ms:• 5a&e a critical area per molecule• Pressure increase with dec. in area per

molecule• )ong-chain al"anes usually ha&e 0.2

nm2 /molecule

Noncoherent fi,ms:• 5a&e no critical area per molecule• @eha&es li"e ideal gas and obeys

Ä }„°• %end to occur at high temperatures

18.5 Colloidal Systemsolloid( glue li"e

• Particles in solution%rue solutions homogeneous mi!ture

i.e. Sugar.. ot possible to see by eye

Suspensions( heterogeneous mi!turecontaining more than one moleculeand possible to see by eye or microscope

i.e. flour in water

olloidal dispersions( may ha&e more than one molecule but not big enough to see by eye or microscope.

• Dispersion medium( medium where the particles are dispersed.• Disperse phase( particles that are present in dispersion medium.

Gispersionmedium

Gispersephase

Name ofsystem

E.amp,es

*as )i#uid =erosol Fog mist clouds

*as Solid =erosol Smo"e dust

)i#uid *as Foam hipped cream

)i#uid )i#uid 'mulsion il" mayonnaise

)i#uid Solid Sol *old in water paint blood

Solid )i#uid *el 6elly

Solid Solid *el Auby glass gold in glassSolid *as Solid foam Pumice syrofoam

Lyopho-ic and Lyophilic Sols18.5 Colloidal Systems

%wo types of sols(

1 )yophobic fear of dissol&ing Sols• =lso called hydrophobic sols when dispersion medium is water• 5a&e low affinity to sol&ent thus they are relati&ely unstable•

=ddition of electrolyte to sols usually results in coagulation and precipitation2 )yophilic )i#uid lo&ing Sols

• 5a&e strong affinity between disperse phase and medium• uch more stable• @eha&e generally li"e a true solution

'!amples( strach in water micelles protein in water blood

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1:

Light Scattering -y Colloidal articles18.5 Colloidal Systems

• %he solution is considered optically clear if the particle si>e is lessthan 10 -; m in diameter.

• olloidal particles will scatter the light i.e. fog smo"e dust

• %yndall effect scattering ( light scattering by particles in colloid.

• Fraction of scattered light increases with number and si>e of theparticles

Ê Ê›?·üð kabgb ýœLM@ž£N£LAÛ Bœhniapbjria

• $ntensity of light scattered w.r.t. to scattering angle ~ and radius rof spherical particles is

Êþ H@

J mfe t ~ \Jú¶J C@N öFAB?£G¢]

• @lue short wa&elength scattered more blue s"y sunset orange-red

milesmathis.com

hyperphysics.phy-astr.gsu.edu

Documents

Ch18 Surface Chemistry PChem 353.pdf