Surface Tension of Liquids

A short presentation

Du Noüy Ring Method

The traditional method used to measure surface or interfacial tension.Wetting properties of the surface or interface have little influence onthis measuring technique. Maximum pull exerted on the ring by thesurface is measured.

Wilhelmy Plate Method

A universal method especially suited to check surface tension overlong time intervals. A vertical plate of known perimeter is attached to abalance, and the force due to wetting is measured.

Spinning Drop Method

This technique is ideal for measuring low interfacial tensions. Thediameter of a drop within a heavy phase is measured, while both arerotated.

Pendant Drop Method

Surface and interfacial tension can be measured by this technique,even at elevated temperatures and pressures. Geometry of a drop isanalysed optically.

LiquidsStatic Methods

Bubble Pressure Method

A measurement technique for determining surface tension at shortsurface ages. Maximum pressure of each bubble is measured.

Drop Volume Method

A method for determining interfacial tension as a function of interfaceage. Liquid of one density is pumped into a second liquid of a differentdensity and time between drops produced is measured.

LiquidsDynamic Methods

Sessile Drop Method

This optical contact angle method is used to estimate wettingproperties of a localised region on a solid surface. Angle between thebaseline of the drop and the tangent at the drop boundary is measured.

Dynamic Wilhelmy Method

A method for calculating average advancing and receding contactangles on solids of uniform geometry. Wetting force on the solid ismeasured as the solid is immersed in or withdrawn from a liquid ofknown surface tension.

Single Fibre Wilhelmy Method

Dynamic Wilhelmy method applied to single fibres to measureadvancing and receding contact angles.

Powder Contact Angle Method

Enables measurement of average contact angle and sorption speed forpowders and other porous materials. Change of weight as a functionof time is measured.

Solids

σD = Disperse Parts of Surface TensionVan der Waals-Interaction

σP = Polar Parts of Surface TensionDipole-Dipole-InteractionHydrogen bondingLewis Acid-Base-Interaction

Surface Tension of Liquidsσ = σP + σD

Short Excerpt of Liquid Database

Liquid Name

N,N-dimethyl-Formamidn-Decanen-Heptanen-Hexanen-Octanen-Tetradecanenitro-Ethane (Schultz)nitro-Methane (Schultz)Phthalic-acid-diethylester 22°sym-tetrabromo-Ethane (Ström)sym-tetrachloro-Ethane (Ström)tetrachloro-Methane (Schultz)Toluene (Schultz)Tricresyl-phosphate (Fowkes)WaterWater (Busscher)Water (Rabel) 22°Water (Ström) 20°α-bromo-Naphthalene (Busscher)α-brom-Naphthalene (Ström)20°

Surface Tension

37.123.920.418.421.825.631.936.837.049.736.327.028.440.972.872.172.372.844.444.6

DispersePart

29.023.920.418.421.825.627.529.830.049.736.326.726.139.226.019.918.721.844.444.6

PolarPart

8.10.00.00.00.00.04.47.07.00.00.00.32.31.7

46.852.253.651.00.00.0

Forces Between Molecules in the Bulk and at the Interface

Interface

Phase 1

Phase 2

Methods to DetermineSurface Tension and

Interfacial Tension of Liquids

Du Noüy Ring Method

Ring madeof Pt-lr L = Wetted

Length

θ = contact angle Liquid

Air

F = Force

Forces During Ring Measurement

maxF Fat 0θ = ° =

F3FmaxF1

Fmax

Forc

e(m

N )

Distance, d, of ring above surface (mm)

F1

F3

LamellaBreaks

0 50

5

10

Water at20 °C

1 2 3 4 6

Force During Ring Measurement

Ring

cosθLFF Vmax

ι

⋅−

=σ

Tension Surface Liquid l =σLength Wetted L =

1 cos =θ WeightLiquid toDue Force FV =

Force Total Fmax =

Liquid

Air

LiquidPlate

θ =0o

Plate madeof roughened Pt

L = WettedLength, mm

Wilhelmy Plate Method

σ =F

L cos θ

F = Force, mN

Wilhelmy Plate Method

cosθLFW

⋅=σ

Force Wetting FW =

Plate Platinum theofLength Wetted L =

1 θ cos =

Ring

cosθLFF Vmax

⋅−

=σ

cosθLF

⋅=σ

Plate

35 mN/m stirring

72 mN/m 55 mN/m 35 mN/m

Time Dependence of Surface Tension

Gold Plating Solution Containing FluorosurfactantsPlate Method Surface Tension versus Time Data

Time [sec]

Surf

ace-

Tens

ion

[mN

/m]

0 100 200 300 400 500 600 700 800 900 1000 110033.00

34.00

35.00

36.00

37.00

38.00

39.00

40.00

Low SurfactantConcentration

High SurfactantConcentration

Micelle Formation

Determination of Critical Micelle Concentration

25

30

35

40

45

50

55

60

65

70

75

0.1 1 10 100 1000 10,000

Log Concentration (mg/L)

Surfa

ceTe

n sio

n(m

N/m

)Surfactant Molecule Hydrophobic Portion

Hydrophilic Portion

Surfactant in

Water at 20 °C

Critical MicelleConcentration

(CMC)

Surface

Air

Water

Surfactant at Surface Micelles FormedSurface Saturated

A Spherical Micelle

Surfactant Bilayer

10001001010.125

30

35

40

45

50

Critical Micelle Concentration Determination

Concentration (mg/L)

Surfa

ce T

ensi

on (

mN

/m)

CMC = 33 mg/L

Sample: Nonylphenol Ethoxylate

Solvent: WaterTemperature: 24.2 ± 0.4 °CMethod: PlateAnalysis Time: 92 minutes

CH3 (CH)2 8 (CHCH)2 2 9.5OHO

-3.5 -3.0 -2.5 -2.030

35

40

45

50

55

60

Surface Tension of Aqueous Sodium Dodecyl Sulfate

Surfa

ce T

ensi

on (m

N/m

)

Log Concentration (mM)

Sodium Dodecyl Sulfatein water at 25 °C

99% Pure

Purified by passage throughHPLC column containing300 m2/g octadecylsilanizedsilicon gel

Critical Micelle Concentration Data

Concentration [mg/l]

Surf

ace

Tens

ion

[mN

/m]

1 5 10 50 100 500 1e3 5e3 1e420.022.525.027.530.032.535.037.540.042.545.047.550.052.555.057.560.0

C15E7C15E9

C15E12

CH3 CH2 CH CH2 CH3

O

CH2

CH2X

OH

(CH2)5 (CH2)5

1 2 3 4 5 6

SURFACETENSION

SURFACTANT CONCENTRATION

1

2 3 4

5 6

CAC

CMC

Polymer / Surfactant Interaction

Synergistic Effects of Surfactant Mixtures

105

104

103

102

101

0.0 0.2 0.4 0.6 0.8 1.0

CM

C (

mic

rom

olar

of t

otal

sur

fact

ant )

MOLAR RATIO (SDS/Total Surfactant)

Aqueous CMC Data for SDS / DTAB Solutions

r

Capillary Wall

Heavy Phase, ρH

ω

Light Phase, ρL

4)LρH(ρ2ω3r

iσ−

=

Spinning Drop Method

Inlet for Heavy Phase

Ocular

Spinning Drop Tensiometer Diagram

Window

PhaseHeavy

Illumination

Septum

Droplet ofLight Phase

Tension lInterfaciaiσ = Constant k =

Radius Drop r =VelocityAngular =ω

DropHeavy Density H = ρPhaseLight Density L =ρ

Spinning Drop

σi = kr3 ω2 (ρH - ρL)

Pendant Drop Analysis

SUMMARY

• 4 methods to measure static SFT and IFT– Ring Method of DU NOÜY

– Plate Method of WILHELMY

– Pendant Drop Techniques

– Spinning Drop Techniques

• It is important to split the SFT of a liquid into two or more components

•The advantages of plate vs. ring method:–True static method

– High stability of plate

– Fast

– No correction necessary

•SFT and IFT-techniques are important for surfactant characterization

•CMC, Surface excess and synergistic effect are important parameters which can be determined fully automatic

Dynamic Methods

Dynamic Surface Tension Maximum Bubble Pressure

Pressure Sensor

Tank( 0 Bar at beginning

of measurement)

PTFE Probe( Diam. 1.5mm )

Liquid

Pressure coming in air or nitrogen4-6 Bar

cTensid >> cCMC

PDiffusion of Surfactants to Bubble Surface

Dynamic Behaviour of 2 Surfactants

Maximum Bubble Pressure Method

σ =P - P rmax 0

Dynamic Data for Nonylphenol Ethoxylate (9.5) in Water

Surface Age [ms]

Dyn

amic

Sur

face

Ten

sion

[m

N/m

]

10 100 1000 10,000 50,00030.00

35.00

40.00

45.00

50.00

55.00

60.00

65.00

70.0010 mg/L

30 mg/L

100 mg/L500 mg/L

1000 mg/L

8000 mg/L

Drop Volume Technique

Light Phase to Heavy Phase Setup

GlassSample Tube

Drop

Photodiode

Capillary

CapillaryHolder

Tubing tosyringe pump

LED

LiquidLevel

Heavy Phase to Light Phase Setup

Tubing tosyringe pump

CapillaryHolder

Capillary

Photodiode

Drop

GlassSample Tube

Heavy Phase

LED

Bleed

d πgρρVσ )( L - Hdrop

i =

DROP VOLUME

Tension lInterfaciaiσ = Volume Drop Vdrop = Constanton Accelerati g =

PhaseLight Density H =ρ

PhaseLight Density L =ρ

Diameter Drop d =

Drop Volume Method

V (ρH - ρL)g = Separation Force

σi πd = Adherence Force

σi =Vdrop (ρ - ρ)g

H L

πd

Balance of Forces at the Tip

dLightphase, ρL

Heavy phase, ρH

Drop Volume Method

Tip at Drop Separation

Orifice tip thind1 = d2

Drop Volume Method

Time

d1

d2

Orifice tip too thickd1 ≠ d2

Sequence of Drop Detachement

Dynamic Interfacial Tension Datafor Alcohol Ethoxylate Solutions

100001000100100

5

10

15

20

25

1 mL/Hr0.75 mL/Hr0.50 mL/Hr0.25 mL/Hr0.10 mL/Hr

Flow Rates

Concentration (mg/L)

Dyn

amic

I nte

rfaci

a lTe

nsio

n(m

N/m

)

“Slow Surfactant”

Dense phase: Water withalcohol ethoxylate(density = 0.998 g/cm)

3

Light phase: Canola Oil(density = 0.891 g/cm)

3

Temperature: 23 ±1 °C

CMC = 100 mg/L

Method: Drop Volume

CH CH - O - CH CH - OH3 2 2 211 10

• Characterization of surfactant dynamics is an important tool for process-near optimization

SUMMARY

• The maximum bubble pressure method is a fast and easy to use technique to characterize fast diffusion of surfactants at the liquid / gas interface

• The drop volume technique is a method to characterize diffusions of surfactants at liquid / liquid - interface

• A special capillary tip design increases reproducibility and minimises experimental error