chm 342 surface

7/31/2019 CHM 342 Surface

1/43

1

CHM 342 (2-46) Surface Chemistry

(. )Surface Tension

Surface tension holds water droplets on theleaves of the Japanese Root Iris.

How can an insect in the abovepicture walk on the water?

( )


2/43

2


(. )

Floating a tack on water.

The tack doesnt really float on the

water, because it is more dense that thewater and should sink to the bottom

The reason that it doesnt sink is because

of the surface tension of the water

What is density?

What is surface tension?


3/43

3


(. )

A tack on the surface of a glass of water.

Poking the surface of the

water with a toothpick is

not sufficient to disturb

the surface tension


4/43

4


(. )

If you dip the toothpick in a soap solution before you poke the water thenthe soak will disrupt the surface tension.


5/43

5


(. )The origin of surface tension.


6/43

6


(. )

Work required for the Formation of Surface

Separation of liquid

requires work

against cohesion

forces

Surface tension ( or ) - increase in free energy as a result offormation of 1 unit (m2 ,cm2) of surface

Formation of surface of square W results in increase in the free

energy G:

)!!positive!(always0;A

G

Tp,

0AGA


7/437


(. )YOUNG-LAPLACE EQUATIONEquation of Capillarity

Pressure inside a drop or bubble is always greater than inthe continuous phase.

The balance between surface tension and external forces(e.g. gravity) dictate the shape of drops and bubbles.

R1

R2

21

R

1

R

1P

- Interfacial Tension

P - Pressure

R - Radius


8/438


(. )Surface Tension is a Force!

Force: mg [N, Newton]

Force: 2L

units: [N/m]

L

Soap film

Surface tension is a vector, i.e. has direction


9/439


(. )Measurement ofSurface Tension

External force

Surface tension

x

fT Force

Ring tearingx

Ring tearing technique

nce}circumfere{ring2

factor)on(correctif T


10/43

10


(. )SURFACE TENSION MEASUREMENT-- du Nouy ring --

)R2(2wtwt ringtotal

wttotal = total weight , wtring = ring weight

R = ring radius , = surface tension

Still commonly used but values may be as

much as 25% in error.

Adamson, Physical

Chemistry

of Surfaces, 2nd Ed

p. 22 (1976)


11/43

11


(. )SURFACE TENSION MEASUREMENT-- Wilhelmy Plate --

2l

b)(wtwtcos()

platetotal

= surface tension

q = contact angle

wttotal = total weight

wtplate = plate weight

b = buoyancy force

l = length of plate

Normal platinum is used to have q 0 and plate just touches

liquid so buoyancy is small


12/43

12


(. )SURFACE TENSION MEASUREMENT-- Drop Weight Method --

W = 2r

W = weight of droplet

r = radius of droplet = surface tension

Ref. Adamson, Physical Chemistry of Surfaces, 2nd Ed , p. 19 (1976)

9See: http://www.erc.ufl.edu/education/courses/intephen 1/_files/lecture1.ppt
http://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppthttp://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppthttp://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppthttp://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppthttp://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppthttp://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppthttp://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppthttp://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppthttp://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppthttp://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppthttp://www.erc.ufl.edu/education/courses/intephen1/_files/lecture1.ppt


13/43

13


(. )Drop on a Solid Surface

Youngs equation relates interfacial tensions and contact angle

q

LV

SV

Solid, S

Liquid, L

Vapor, V

SL

q cosLVSLSV

Contact angle, q(reflects the degree of wetting)


14/43

14


(. )Wetting Phenomena

qq

q

q > 90oq = 90oq < 90o

q = 0o q =180o

Absolute wettingNo wetting


15/43

15


(. )Wettability of Powders

Solid

q

Liquid

Vapor

LV

SV

SL

q cosLVSLSV


16/43

16


(. )

Micelles are not reallydissolved in water, theyare suspended.

These sort ofsuspensions are called

colloids.

Colloids : Particles ofone substance dispersedthroughout another.

Colloids can be detectedby the scattering of lightthat is passed through thesample, this is called theThyndall Effect.

Colloids

The Tyndall effect is evident as a beam oflight passes through a colloidal dispersion

of water droplets in the atmosphere.


17/43

17


(. )Types of Colloidal Dispersions


18/43

18


(. )

All soaps are detergents;all detergents are

surfactants.

()

Surfactants

Surfactants are molecules that preferentially adsorb at aninterface, i.e. solid/liquid (froth flotation), liquid/gas (foams),

liquid/liquid (emulsions). Significantly alter interfacialfree energy(work needed to

create or expand interface/unit area). Surface free energy of interface minimized by reducing

interfacial area.


19/43

19


(. )

If enough soap is added to water the molecules

arrange themselves into a structure called a micelle

Hydrophilic( lyophobic,water-loving) headcontaining a chargedfunctional group

Hydrophobic( lyophilic,water-fearing ) tail

containing ahydrocarbon chain

Soap Molecules

CHM ( ) S f Ch i


20/43

20


(. )Amphiphilic Surfactants

Amphiphilic surfactants contain a non-polar portion(tail) anda polar portion (head).

S O-Na

+

O

O

S

O

OO

OO

O+Na

-O

Aerosol OT

Sodium dodecylsulfate (SDS)

CHM 342 (2 46) S f Ch i


21/43

21


(. )Classification of Surfactants

Anionic

Cationic

Zwitterionic

Nonionic

N+

Br-

S O-Na

+

O

OSodium dodecylsulfate (SDS)

Cetylpyridinium bromide

O

O

P

O

O O

OCH2CH2N(CH3)3+

O-

Dipalmitoylphosphatidylcholine (lecithin)

OO

OO

OH

Polyoxyethylene(4) lauryl ether (Brij 30)

Soap

CHM 342 (2 46) S f Ch i t


22/43

22


(. )Molecular Architecture

Aerosol OTSodium dodecylsulfate (SDS)

CHM 342 (2 46) S f Ch i t


23/43

23


(. )

4 nm

Unimers (monomers) Normal micelles

spherical

cylindrical

Bilayer lamellaReverse micelles

Inverted hexagonal phase

Surfactant aggregates

CHM 342 (2 46) Surface Chemistry


24/43

24


(. )

If concentration is sufficiently high, surfactants can form aggregatesin aqueous solution micelles.

Typically spheroidal particles of 2.5-6 nm diameter.

McBainLamellarMicelle Hydrocarbon

Layer

WaterLayer

WaterLayer

HartleySpherical

Micelle

+

+

+

+

+

+

+

+

- - - - ---

--

- - -

-

--

-

Micelles



25/43

25


(. )

0

2

4

6

8

10

12

14

0 1

Surfactant concentration

CMC

Critical Micelle Concentration

CMC

Below CMC only unimers arepresent

Above CMC there are micelles inequilibrium with unimers

Onset of micellizationobserved by sudden change in

measured properties of solution at characteristic surfactantconcentrationcritical micelle concentration (CMC).



26/43

26


(. ) Onset of micellizationobserved by sudden change in measured

properties of solution at characteristic surfactant concentration

critical micelle concentration (CMC).

Critical Micelle Concentration

( From: Klimpel, Intro to ChemicalsUsed in Particle Systems, p. 29, 1997,



27/43

27


(. )Solution Properties & CMC

02468

101214

0 1Surfactant concentration

CMCConc.

unimers

micelles

02468

101214


CMC

Osmotic pressure

02468

101214

0 1(Surfactant concentration)1/2

CMC

Molar conductivity1/R

02468

101214


CMC

Isc Light scattering



28/43

28


(. )--Driving Force-- Lyophobic (solvent-fearing) groups can perturb solvent structure and

increase free energy of system. Surfactant will concentrate at the

solvent-gas interface to lowerGo.

Gocan also be decreased by aggregation into micellessuch thatlyophobic groups are directed into interior of structure and lyophilicsolvent-loving) groups face solvent.

Decrease in Go for removal of lyophobic groups from solvent contactby micellization may be opposed by:

(i) loss in entropy &

(ii) electrostatic repulsion for charged headgroups

Micellizationis therefore a balance between various forces

AIR

WATER

Micellization Thermodynamics



29/43

29


(. )Micellization Thermodynamics

m.[SURF] [SURF]m

Kmic

mmic

mmic

mic][]SURF[ cmc

ccK

)10030(lnlnm

lnm

lnmm

micomic

mcmcRTcmcRTcRT

KRTG

Go

o

Nonionic surfactant

m - number of aggregation

abovecmc: [SURF] cmc

micmic mm][ cccmcco

T

cmcRTcmcRS

T

cmcRTH

cmcRTG

d

lndln

d

lnd

ln

2o

mic

2omic

omic



30/43

30


(. )--Temperature and Pressure-- For ionic surfactants there exists a critical temperature above which

solubility rapidly increases (equals CMC) and micelles formKrafft pointor Krafft temperature(TK),

Below TK solubility is low and no micelles are present.

(Klimpel,Intro to Chemicals Used in Particle Systems, p. 30, 1997, Fig 22)

Micelles



31/43

31


(. )

surfactantcrystals

TK

Temperature

Surfactants much less effective belowKrafft point, e.g. detergents.

For non-ionic surfactants, increase in temperature may result in

clear solution turning cloudy due to phase separation. This criticaltemperature is thecloud point.

Cloud point transition is generally less sharp than that ofKrafft

point.

Micelles --Temperature and Pressure--



32/43

32


(. )Krafft Point, CMT and Cloud Point (~ temp.)

(I) Krafft Point

Cloud

point

(II) CMT and Cloud Point

1

c

T

Micelles +

solution

Solution

CMC

CMT

Phase

Separation

1Tkrafft

CMC

c

T

Micelles +

solution

Solution

Crystals +

solution

Liquid

crystals

I. Above the Kraft point, the solubility is sharply increase due to the formation of micelles.

II. Some surfactants (e.g. polyoxyethylene (POE)-based) dehydrate and phase separateupon temperature elevation (cloud pint).

III. In Pluronic block copolymer, the micelles form as a result of the dehydration of

polyoxypropylene segments (critical micelle temperature or CMT).


S H d l ?


33/43

33

C 3 ( 6) Su ace C e st y

(. )Soap: How does soap clean?


S H d l ?


34/43

34

( ) y

(. ) soap decreases surface tension of water, making it a better wetting agent.

soap converts greasy and oily dirt into micelles that become dispersed in

water.

soap keeps the greasy micelles in suspension and prevents them from

redepositing until they can be washed away. (repulsion of the charges)

Soap: How does soap clean?



35/43

35

( ) y

(. )

Acid-catalyzed and base-catalyzed hydrolysis.

Through the breakdown of esters by a hydrolysis process.

This bond is broken

How are soap produced?



36/43

36

( ) y

(. )

The saponification of a triglyceride.

Soaps are not produced from simple esters such as methyl acetate

but from more complex esters

animal fat

How are soap produced?



37/43

37

(. )

What are all of these

chemicals ?

What do they do ?

What is in a box

of detergent ?


Wh t i i b f d t t ?


38/43

38

(. )What is in a box of detergent ?



39/43

39

(. )



40/43

40

(. )Solubilization

Spontaneous transfer of a compound insoluble in the bulk solvent into

the solution inside the surfactant micelles.

polar compound

Reverse micelles

non-polar compound

Normal micelles

amphiphilic compound

0

2

4

6

8

10

12

14


CMCSolub

ility

Solubility of a poorly solublecompound increases as aresult of solubilization in themicelles.



41/43

41

(. )HLB and Use of Surfactants Amphiphilic surfactants are characterized by the hydrophilic-lipophilic balance (HLB): a relative ratio of polar and non-polargroups in the surfactant.

HLB number, ranging from 0-40, can be assigned to a surfactant,based on emulsification data. Semi-empirical only.

Strongly hydrophilic surfactant, HLB 40

Strongly lyophilic surfactant, HLB 1

oil

water

Coil

Cwater

C6H13COO- C8H17COO

- C10H21COO-

HLB decreases


HLB d U f S f


42/43

42

(. )

HLB ca. 1 to 3.5: AntifoamsHLB ca. 3.5 to 8: Water-in-Oil Emulsifiers

HLB ca. 7 to 9: Wetting and spreading agents

HLB ca. 8 to 16: Oil-in-Water Emulsifiers

HLB ca. 13 to 16: DetergentsHLB ca. 15 to 40: Solubilizers

HLB and Use of Surfactants

Strongly hydrophilic

Strongly Lipophilic

Question:Why antifoaming or water-in-oil emulsifiers use low-HLBsurfactants & why detergents use hi-HLB surfactants?


HLB needed for emulsification of the oil phase


43/43

(. )HLB needed for emulsification of the oil phase

* Calculate the required HLB for the oil phase of the following o/w emulsion: cetylalcohol 15 g., white wax 1g. Lanolin 2 g, emulsifier (q.s.), glycerin 5 g. water100 g.

HLB (from reference) Fraction

Cetyl alcohol 15 x 15/18 = 12.5White wax 12 x 1/18 = 0.7

Lanolin 10 x 2/18 = 1.1Total required HLB = 14.3

Surfactant blends are commonly used to obtain desired emulsifyingproperties.

If there are several oil ingredients the required HLB is calculated as asum of their respective required HLB multiplied by the fraction of each.

* What is the HLB of the mixture of 40 % Span 60 (HLB = 4.7) and 60 % Tween 60(HLB = 14.9)?

HLB of mixture: 4.7 x 0.4 + 14.9 x 0.6 = 10.8

* In what proportion should Span 80 (HLB = 4.3) and Tween 80 (HLB = 15.0) bemixed to obtain required HLB of12.0?

4.3.(1-x) + 15.x = 12 x = 0.72 ( 72 % Tween 80 and 28 % Span 80)

chm 342 surface

Documents