Surface Tension of Liquids A short presentation
Liquids Static Methods Du Noüy Ring Method The traditional method used to measure surface or interfacial tension. Wetting properties of the surface or interface have little influence on this measuring technique. Maximum pull exerted on the ring by the surface is measured. Wilhelmy Plate Method A universal method especially suited to check surface tension over long time intervals. A vertical plate of known perimeter is attached to a balance, and the force due to wetting is measured. Spinning Drop Method This technique is ideal for measuring low interfacial tensions. The diameter of a drop within a heavy phase is measured, while both are rotated. Pendant Drop Method Surface and interfacial tension can be measured by this technique, even at elevated temperatures and pressures. Geometry of a drop is analysed optically.
Liquids Dynamic Methods Bubble Pressure Method A measurement technique for determining surface tension at short surface ages. Maximum pressure of each bubble is measured. Drop Volume Method A method for determining interfacial tension as a function of interface age. Liquid of one density is pumped into a second liquid of a different density and time between drops produced is measured.
Solids Sessile Drop Method This optical contact angle method is used to estimate wetting properties of a localised region on a solid surface. Angle between the baseline of the drop and the tangent at the drop boundary is measured. Dynamic Wilhelmy Method A method for calculating average advancing and receding contact angles on solids of uniform geometry. Wetting force on the solid is measured as the solid is immersed in or withdrawn from a liquid of known surface tension. Single Fibre Wilhelmy Method Dynamic Wilhelmy method applied to single fibres to measure advancing and receding contact angles. Powder Contact Angle Method Enables measurement of average contact angle and sorption speed for powders and other porous materials. Change of weight as a function of time is measured.
Surface Tension of Liquids σ = σ P + σ D σ P = Polar Parts of Surface Tension Dipole-Dipole-Interaction Hydrogen bonding Lewis Acid-Base-Interaction σ D = Disperse Parts of Surface Tension Van der Waals-Interaction
Short Excerpt of Liquid Database
Liquid Name Surface Tension Disperse Part Polar Part N,N-dimethyl-Formamid n-decane n-heptane n-hexane n-octane n-tetradecane nitro-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) Water Water (Busscher) Water (Rabel) 22 Water (Ström) 20 α-bromo-naphthalene (Busscher) α-brom-naphthalene (Ström)20 37.1 23.9 20.4 18.4 21.8 25.6 31.9 36.8 37.0 49.7 36.3 27.0 28.4 40.9 72.8 72.1 72.3 72.8 44.4 44.6 29.0 23.9 20.4 18.4 21.8 25.6 27.5 29.8 30.0 49.7 36.3 26.7 26.1 39.2 26.0 19.9 18.7 21.8 44.4 44.6 8.1 0.0 0.0 0.0 0.0 0.0 4.4 7.0 7.0 0.0 0.0 0.3 2.3 1.7 46.8 52.2 53.6 51.0 0.0 0.0
Forces Between Molecules in the Bulk and at the Interface Phase 1 Phase 2 Interface
Methods to Determine Surface Tension and Interfacial Tension of Liquids
Du Noüy Ring Method
F = Force Air Ring made of Pt-lr L = Wetted Length θ = contact angle Liquid
Forces During Ring Measurement F 1 F max F 3 θ = 0 at F = Fmax
10 Force During Ring Measurement F 3 F max Force (mn) 5 F 1 Lamella Breaks Water at 20 C 0 0 1 2 3 4 5 6 Distance, d, of ring above surface (mm)
Ring σι = F max F V L cosθ σl = Liquid Surface Tension L = Wetted Length cos θ =1 FV = Force Due to Liquid Weight Fmax = Total Force
Wilhelmy Plate Method Plate made of roughened Pt Air F = Force, mn L = Wetted Length, mm Liquid θ =0 o Liquid Plate σ = F L cos θ
Wilhelmy Plate Method σ = L FW cosθ FW = Wetting Force L = Wetted Length of the Platinum Plate cos θ = 1
Ring σ = F max F V L cosθ Plate σ = L F cosθ
Time Dependence of Surface Tension 35 mn/m stirring 72 mn/m 55 mn/m 35 mn/m
Surface-Tension [mn/m] Gold Plating Solution Containing Fluorosurfactants Plate Method Surface Tension versus Time Data 40.00 39.00 38.00 37.00 36.00 Low Surfactant Concentration 35.00 34.00 33.00 High Surfactant Concentration 0 100 200 300 400 500 600 700 800 900 1000 1100 Time [sec]
Micelle Formation
75 Determination of Critical Micelle Concentration 70 Surfactant Molecule Hydrophobic Portion Hydrophilic Portion 65 60 Surfactant in Water at 20 C Surface Tension(mN/m) 55 50 45 40 Critical Micelle Concentration (CMC) Air Surfactant at Surface Surface Saturated Micelles Formed 35 Surface 30 Water 25 0.1 1 10 100 1000 10,000 Log Concentration (mg/l)
A Spherical Micelle
Surfactant Bilayer
50 Critical Micelle Concentration Determination Surface Tension (mn/m) 45 40 35 30 CMC = 33 mg/l (CHCH) 2 2 Sample: Nonylphenol Ethoxylate CH3 (CH) 2 8 O Solvent: Water Temperature: 24.2 ± 0.4 C Method: Plate Analysis Time: 92 minutes 9.5 OH 25 0.1 1 10 100 1000 Concentration (mg/l)
60 Surface Tension of Aqueous Sodium Dodecyl Sulfate 55 Surface Tension (mn/m) 50 45 40 99% Pure Purified by passage through HPLC column containing 300 m 2 /g octadecylsilanized silicon gel 35 Sodium Dodecyl Sulfate in water at 25 C 30-3.5-3.0-2.5-2.0 Log Concentration (mm)
Surface Tension [mn/m] 60.0 57.5 55.0 52.5 50.0 47.5 45.0 42.5 40.0 37.5 35.0 32.5 30.0 27.5 25.0 22.5 20.0 Critical Micelle Concentration Data CH 3 (CH 2 ) 5 CH 2 CH CH 2 (CH 2 ) 5 CH 3 O CH 2 CH 2X OH C 15 E 7 C 15 E 9 C 15 E 12 1 5 10 50 100 500 1e3 5e3 1e4 Concentration [mg/l]
Polymer / Surfactant Interaction 1 SURFACE TENSION CAC 2 3 4 SURFACTANT CONCENTRATION 5 6 CMC 1 2 3 4 5 6
Synergistic Effects of Surfactant Mixtures
10 1 0.0 0.2 0.4 0.6 0.8 1.0 Aqueous CMC Data for SDS / DTAB Solutions 10 5 CMC ( micromolar of total surfactant ) 10 4 10 3 10 2 MOLAR RATIO (SDS/Total Surfactant)
Spinning Drop Method Heavy Phase, ρ H r ω Light Phase, ρ L Capillary Wall σ i = r 3 ω 2 (ρ H 4 ρ ) L
Spinning Drop Tensiometer Diagram Ocular Window Inlet for Heavy Phase Septum Heavy Phase Illumination Droplet of Light Phase
Spinning Drop σ i σ i =kr 3 ω 2 (ρ H - ρ L ) = Interfacial Tension k = Constant r = Drop Radius ω = Angular Velocity ρh = Density Heavy Drop ρl = Density Light Phase
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 coming in air or nitrogen 4-6 Bar Tank ( 0 Bar at beginning of measurement) Pressure Sensor PTFE Probe ( Diam. 1.5mm ) Liquid
Diffusion of Surfactants to Bubble Surface P c Tensid >> c CMC
Dynamic Behaviour of 2 Surfactants
Maximum Bubble Pressure Method σ = P - P r max 0
Dynamic Surface Tension [mn/m] Dynamic Data for Nonylphenol Ethoxylate (9.5) in Water 70.00 65.00 60.00 55.00 500 mg/l 100 mg/l 30 mg/l 10 mg/l 50.00 1000 mg/l 45.00 40.00 35.00 8000 mg/l 30.00 10 100 1000 10,000 50,000 Surface Age [ms]
Drop Volume Technique
Light Phase to Heavy Phase Setup Glass Sample Tube Liquid Level Drop LED Photodiode Capillary Capillary Holder Tubing to syringe pump
Heavy Phase to Light Phase Setup Tubing to syringe pump Bleed Capillary Holder Capillary LED Photodiode Drop Heavy Phase Glass Sample Tube
DROP VOLUME σ i = V drop ρ π d ρ ( H - L) g σ i = Interfacial Tension Vdrop = Drop Volume g = Acceleration Constant ρh = Density Light Phase ρl = Density Light Phase d = Drop Diameter
Drop Volume Method Heavy phase, ρ H Balance of Forces at the Tip V (ρ H - ρ L )g = Separation Force Light phase, ρ L d σ i π d = Adherence Force σ i = Vdrop (ρ - ρ)g H L πd
Drop Volume Method Tip at Drop Separation Orifice tip thin d 1 = d 2
Drop Volume Method Sequence of Drop Detachement Orifice tip too thick d 1 d 2 d 2 d 1 Time
Dynamic Interfacial Tension (mn/m) 25 20 15 10 5 Slow Surfactant Dynamic Interfacial Tension Data for Alcohol Ethoxylate Solutions CMC = 100 mg/l Flow Rates 1 ml/hr 0.75 ml/hr 0.50 ml/hr 0.25 ml/hr 0.10 ml/hr Dense phase: Water withalcohol ethoxylate 3 (density = 0.998 g/cm) CH CH - O - CH CH - OH 3 2 2 2 11 10 Light phase: Canola Oil 3 (density = 0.891 g/cm) Temperature: 23 ±1 C Method: Drop Volume 0 10 100 1000 10000 Concentration (mg/l)
SUMMARY Characterization of surfactant dynamics is an important tool for process-near optimization 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