Surface and Colloid Chemistry PDF

SURFACE and COLLOID CHEMISTRY Prof. Dr. Orhan OZDEMIR November 2024  Contact Angle ▪ When we drop a liquid on a solid surface, the liquid either disperses on the solid or takes place in the form of drops. Liquid disperses on solid surface Liquid...

SURFACE and COLLOID CHEMISTRY Prof. Dr. Orhan OZDEMIR November 2024  Contact Angle ▪ When we drop a liquid on a solid surface, the liquid either disperses on the solid or takes place in the form of drops. Liquid disperses on solid surface Liquid does not disperse on solid surface (no drops are formed) (drops are formed) ▪ If there is a strong interaction between the liquid molecules and the solid surface, the liquid will be completely dispersed on the solid surface. Air Water Air / Mineral surface Mineral ▪ Such surfaces are called hydrophilic (water-loving) surfaces. ▪ If there is a weak interaction between the liquid molecules and the solid surface, the liquid is placed on the solid in the form of drops. Air Su Air / Mineral surface Mineral ▪ Such surfaces are called hydrophobic (water-repellent) surfaces. ▪ An angle is formed between a liquid surface and a solid surface which is called contact angle. ▪ Size of this angle depends on the relative size of the attraction forces (cohesion forces) among the liquid molecules and the attraction forces (adhesion forces) between solid and the liquid layer. ▪ If the magnitude of the cohesion forces is larger than that of the adhesion forces, the contact angle between the liquid and solid will be bigger. ▪ In other words, a big contact angle indicates the low solid-liquid attraction force, a small contact angle indicates that these forces are greater. Big contact angle Water Small contact angle Water ▪ Contact angle is one of the most important parameters that determine the behavior of a liquid on the solid. ▪ It is a measure of how much the solid surface can be wetted by a liquid. ▪ It is an easy parameter to measure. ▪ It is used to characterize the surface free energy of the solid. ▪ Equilibrium contact angle is determined by the molecular interactions at the liquid / air, solid / air, and solid / liquid interfaces. ▪ The degree of wetness of a mineral is measured by its contact angle. ▪ The contact angle is the angle between the mineral surface and the tangent line to the bubble at the point where the air bubble touches the mineral. ▪ It is always measured by the liquid phase. ▪ Equlibrium contact angle based on Young formula; γWA 𝛾𝑀𝐴 − 𝛾𝑀𝑊 = 𝛾𝑊𝐴. 𝐶𝑜𝑠𝜃 Air: A Mineral Water: W Air θ γMA Water γMW Mineral: M ▪ On hydrophilic surfaces, the liquid drop will be completely dispersed on the solid surface, which means a low contact angle. ▪ On hydrophobic surfaces, the contact angle will be high. Hydrophilic surface Hydrophobic surface ▪ If a liquid gives a contact angle between 0 and 30o on the solid surface, the solid is wettable, ▪ If it gives a contact angle between 30 and 89o, it is partially wettable. ▪ If it is over 90o, the liquid cannot wet the solid. ▪ Many minerals are hydrophilic, water droplets give a contact angle between 0 and 30o. ▪ If the mineral is hydrophobic, it gives a contact angle of around 90o.  Molecular Dynamics Simulation (MDS) of a Drop on Hydrophilic and Hydrophobic Surfaces Hydrophilic Surface Hydrophobic Surface Ɵ ~ 10o Ɵ ~ 70o Atluri et al., 2019. “The hydrophobic surface state of talc as influenced by aluminum substitution in the tetrahedral layer”. Journal of Colloid and Interface Science, 536, 737-748.  ϴ 0o 50-75o 90o 180o Cos ϴ 1 0.64-0.25 0 -1 Fully Non-Wetting Partial Not wetting Fully Wetting Wetting MINERAL SURFACE  Contact Angle Hysteresis ▪ Contact angle varies depending on advancing and receding on a surface. ▪ Hysteresis is the difference between the maximum (θA) and minimum (θR) values of the contact angle. ▪ Advancing contact angle is always bigger than receding contact angle.  Effect of Roughness on Contact Angle ▪ Mineral surface can show microscopic changes. Effect of Heterogeneity on Contact Angle ▪ Mineral surface can contain different chemical groups (it may get wet or not). Contact angles of some selected substances  Contact Angle and Flotation ▪ Wetting of minerals is one of the important parameters that affect many mineral processing processes such as flotation, agglomeration, and solid-liquid separation. ▪ Mineral particles in a flotation system are in constant contact with air bubbles dispersed inside of suspension. ▪ In the flotation method, hydrophobic (water- repellent) particles are enriched using air bubbles in suspension. Attachment of Flotation Cell bubble-particle  Contact Angle and Flotation ▪ The flotability of a mineral depends on the feature of wetting the mineral with water. ▪ Any non-wetting particle (hydrophobic) which is suitable for flotation tends to attach to the air bubble surface. ▪ Hydrophobic minerals are with low surface energy minerals (coal, graphite, sulfur, talc, etc.) ▪ The purpose of flotation is to make the desired mineral floatable by using various chemicals (collectors) and to provide the situation that will prevent other minerals from floating. ▪ The non-polar particle (hydrophobic) is well held by the air bubble. Mineral Surface Collision of bubble-particle ▪ The buoyancy of the bubble must be greater than the gravity force. ▪ The large contact angle makes mineral float easier. ▪ Thus, the flotation recovery increases. ▪ By increasing the contact angle with the collectors, it becomes stronger to hold the air bubble to the mineral. ▪ In this respect, the contact angle is a measure of the suitability of a mineral for flotation. ▪ For a good flotation, the contact angle should be 50o < θ < 75o.  Super-hydrophobic ve Hydrophobic Surfaces Super-hydrophobic surface Hydrophobic surface  Contact Angle Measurement Methods a) Sessile drop b) Sessile bubble c) Captive bubble d) Tilting plate  Contact Angle Measurement Methods ▪ The contact angle formed by a drop of liquid on a solid surface is measured by an optical tensiometer called Goniometer. ▪ This equipment mainly includes light source, lens, video camera, sample positioner.  Contact Angle Measurement Methods ▪ Contact angle measurement is the simplest and easiest Lens Cell methods. Camera Φ Light ▪ A liquid through the syringe drop is placed on sample Syringe Goniometer surface. ▪ The shape of the drop formed on the surface can be Air analyzed either by eye or a Liquid Syringe picture taken with a high resolution camera. ▪ Lotus Effect is based on surface roughness caused by different microstructures together with the hydrophobic properties of the epicuticular wax. Lotus leaf (Contact angle ~170o) Micro-nanosize structures on Lotus leaf (SEM picture)  Lotus Effect in action: Self-cleaning surfaces ▪ Water drop is rolling ▪ Water drop takes up ▪ Lotus leaf over the dirt particles on the dust covering a a smooth hydrophobic super hydrophobic surface. surface.  Lotus Effect 1× 1×1 Artificial super hydrophobic surface Lotus leaf Lotus Effect Hydrophobic Coating CONTACT ANGLE EXPERIMENT ❑ Table 1 presents the results for the contact angle measurement of quartz with respect to the DAH concentration. DAH Concentration Contact Angle (M) (o) 1.10-6 (water) 25.3 5.10-6 29.7 1.10-5 35.4 2.10-5 42.8 5.10-5 48.3 1.10-4 54.6 2.10-4 59.5 5.10-4 65.2 1.10-3 70.1 Su Damlası Water Droplet 2.10-3 72.3 59.6 59,6o 59.5 59,5o 5.10-3 68.4 1.10-2 6.8 TalkSurface Quartz Yüzeyi 2.10 -2 4.7 4.10-2 3.9  80 70 CA 60 Contact Angle (o) 50 40 30 20 10 0 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 DAH Concentration (M) CONTACT ANGLE EXPERIMENT ❑ Table 2 presents the results of the flotation experiments of the quartz sample as a function of DAH using a micro-flotation column DAH Concentration Flotation Recovery (M) (% ) 1.10-6 (water) 15.6 5.10-6 21.4 1.10-5 30.7 2.10-5 42.4 5.10-5 53.6 1.10-4 64.7 2.10-4 75.2 5.10-4 80.9 Mikro-f lotasyon Micro-flotation cell Flowmeter Manometre hücresi 1.10-3 82.7 Köpük Float particles toplama kabı 2.10 -3 83.1 Air Hava 5.10 -3 76.8 Source Tankı MagnetManyetik 1.10 -2 11.2 Sink balık 2.10-2 9.7 Flowmeter Debimetre 4.10-2 8.3 Manyetik karıştırıcı Magnetic Mixer  90 80 70 FR 60 Contact Angle (o) 50 40 30 20 10 0 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 DAH Concentration (M)  + + + + + + + + + CA: Contact Angle FR: Flotation Recovery 80 100 70 80 + + + + + + + Flotation Recovery (%) 60 Quartz (SiO2) Contact Angle (o) 50 60 40 30 40 20 CA FR 20 10 0 0 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 DAH Concentration (M)  Correlation of Quartz-Amine Adsorption / Contact Angle / Flotation (a) Between 1.10-6 and 1.10-5 M Insufficient amine concentration, Low contact angle (~35-40o), Low flotation recovery (max. 40%). (a) (b) (c) (b) Between 1.10-4 and 1.10-3 M 100 Quartz-amine flotation Sufficient amine concentration, High contact angle (~70o), 80 (%) (%) (b) High flotation recovery (max. 90%). Recovery Flotation Recovery 60 (c) At 1.10-2 M Excessive amine concentration, Flotation 40 (a) No contact angle (0o), No flotation (0%). 20 (c) 0 1,E-07 1.10-7 1,E-06 1.10-6 1,E-05 1.10-5 1,E-04 1.10-4 1,E-03 1.10-3 1,E-02 1.10-2 1,E-01 1.10-1 DAH Concentration DAH Concentration (M) (M) Nex Week Lab 3: Contact Angle Experiment December 6, 2024 at 9:00-11:20