Solid-liquid interface

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1 Lecture Note #9 (Spring, 2017) Solid-liquid interface Reading: Shaw, ch. 6

2 Contact angles and wetting Wetting: the displacement from a surface of one fluid by another. A gas is displaced by a liquid at the surface of a solid Wetting agent: a surface-active substance which promotes this effect 3 types of wetting: (1) spreading wetting, (2) adhesional wetting, (3) immersional wetting Spreading wetting a liquid already in contact with the solid spreads so as to increase the solid-liquid and liquid-gas interfacial areas and decrease the solid-gas interfacial area spreading coefficient (S) (cf. ch.4): S = - G s /A = γ SG (γ SL + γ LG ) (6.1) G s : the free energy increase due to spreading S 0: liquid spreads spontaneously S < 0: liquid remains as a drop having contact angle θ

3 Consider a liquid making an equilibrium contact angle, θ, to spread an infinitesimal amount further so as to cover an extra area, da, of the solid surface the increase in liquid-gas interfacial area is da cosθ and the increase in the free energy of the system dg = γ SL da + γ LG dacosθ γ SG da at equilibrium, dg = 0 γ SL + γ LG cosθ γ SG = 0 (6.2) (Young s equation) γ S γ SG = π SG γ S : surface tension of the solid against its own vapor, γ SG : surface tension of the solid in equilibrium with the vapor of the wetting liquid π SG : the spreading pressure. The reduction of the surface tension of the solid due to vapor adsorption in general, π SG is small for moderately large value of θ, but it can become significant as θ approaches zero

4 γ SL γ S + γ LG cosθ + π SG = 0 (6.3) Using Fowkes s interfacial tension theory (ch.4) γ SL = γ S + γ LG 2(γ Sd + γ LGd ) ½ (6.4) Combining ((6.2) and (6.4) cosθ = (γ Sd + γ LGd ) ½ / γ LG (6.5) Or, for non-polar liquids, where γ LGd = γ LG cosθ = (γ Sd / γ LG ) ½ (6.6) For non-polar liquids on a given solid, θ should decrease as γ LG decreases and become zero below a certain value of γ LG critical surface tension (γ c ) for the solid a parameter for characterizing the wettability of solid surface

5 Value of γ LG which corresponds to θ = 0 (i.e., cosθ = 1)

6 Adhesional wetting a liquid which is not originally in contact with the solid substrate makes contact and adheres to it In contrast to spreading wetting, the area of liquid-gas interface decreases The work (free energy) of adhesion is given by the Dupre equation (ch.4) W a = - G a /A = γ SG + γ LG γ SL (6.7) combining with Young s equation (6.2), W a = γ LG (1 + cosθ) Young-Dupre equation (6.8) For zero contact angle (cosθ = 1), W a = 2 γ LG = W c solid is completely wetted by the liquid if the contact angle is zero and only partially wetted if the contact angle is finite Completely non-wetting (θ = 180 ) unrealistic since W a = 0 or infinite γ LG required always some solid-liquid attraction e.g., water droplets on a paraffin wax surface: θ ~ 110

7 Immersional wetting the solid, which is not originally in contact with the liquid, is immersed completely in the liquid. The area of liquid-gas interface, therefore, remains unchanged The free energy change of immersion of a solid in a liquid - G i /A = γ SG γ SL = γ LG cosθ (6.9) If γ SG > γ SL, then θ < 90 and immersional wetting is spontaneous. If γ SG < γ SL, then θ > 90 and immersional wetting is nonspontaneous work must be done to immerse the solid in the liquid Free energy, enthalpy and entropy of immersion G i = H i - T S i

8 Factors affecting contact angles and wetting 3 types of wetting are summarized reduction of γ SL facilitates all of these wetting, reduction of γ LG is not always helpful ( 종류에따라다름 ) contact angle between water and glass is increased by adsorbed greasy materials such as fatty acid W a since some of glass-water interface is replaced by hydrocarbon-water interface θ (by W a = γ LG (1 + cosθ) )

9 The wetting of a hydrophobic solid surface by an aqueous medium is considerably helped by the addition of surface-active agents: W a and γ LG θ (by W a = γ LG (1 + cosθ) ) Surface roughness has the effect of making the contact angle further removed from 90 : if θ < 90, the liquid will penetrate and fill up most of the hollows and pores in the solid and so form a plane surface which is effectively part solid and part liquid since liquid has zero θ with liquid, θ (wetting 이쉬워짐 ). If θ > 90, the liquid will not penetrate and part solid and part air since no adhesion between the liquid and the entrapped air, θ (wetting 이어려워짐 ). Preparation of solid surface may affect the contact angle: crystallization in water has a lower θ than that in air owing to penetration and entrapment of traces of water in the surface layers Fluorocarbon surfaces have characteristically low critical surface tension (γ c (γ LG ), Table 6.1): the production of non-stick surfaces more non-wetting characteristics of fluorocarbon than hydrocarbon surfaces: larger size of -CF 2 - group than -CH 2 - group (fewer -CF 2 - groups than -CH 2 - groups can be packed into a given area) W a and θ for the fluorocarbon surface

10 Wetting agents Surface-active materials, particularly anionics, are used as wetting agents e.g., -insecticide for sheep & cattle or horticultural sprays: 동물의털등은 greasy, wax-like 잘스며들지않음, 독성때문에너무스며들어도안됨 wetting agents -textile industry 에의응용 : scouring( 정련. 불순물제거, 침투성과염색성향상시키는공정 ), bleaching( 표백 ), mercerizing( 머서화. 광택증진 ), dyeing Cationic surfactants to promote oil-wetting in processes such as dry-cleaning and road making In addition of lowering γ LG, it is important that the wetting agent lowers γ SL irregularly shaped surfactant molecules (e.g., sodium di-n-octyl sulphosuccinate (Aerosol OT)) are often good wetting agents since micelle formation is not favored (owing to steric consideration) high concentration of unassociated surfactant molecules & greater lowering of γ LG and γ SL Non-ionic surfactants are also good wetting agents

Water repellency The converse of wetting agent to make the contact angle as large as possible Textile fabrics: water-repellent by treatment with a long-chain cationic surfactant ( 땀복, 비옷등 ) A

11 Water repellency The converse of wetting agent to make the contact angle as large as possible Textile fabrics: water-repellent by treatment with a long-chain cationic surfactant ( 땀복, 비옷등 ) A condition of negative capillary action: the pressure required to force water through the fabric depends on the surface tension and inversely on the fibre spacing, so that a moderately tight weave is desirable. The passage of air through the fabric is not hindered Duck s feathers owe water-repellent characteristics: consist of fine, waxcovered barbules( 작은깃, 8 μm diameter, separated by air gaps of 30 μm) Dimethyldichlorosilane: a good hydrophobizing agent for silica and glass surfaces react with OH groups

12 Ore flotation solid particle (e.g., ores( 광물 )) to float on water can be modified by surfactants The floatation of solid depends on the contact angle After mining crushing slurry in water + collector oil (adsorbed on surfaces) θ flotation (at least are required) Collector oil molecules to create hydrophobic particle surface on adsorption ( 광물의종류에따라 anionic, cationic, non-ionic, amphiphilic 사용 ) A foaming agent to lower zeta potentials and to minimize electrostatic repulsions Flotation is also used to enrich fuels (e.g., coal & oil) and as a purification procedure for chemical process

13 Detergency dirt removal from solid surfaces soap: consist of the sodium or potassium salts of various long-chain fatty acids and is manufactured by the saponification of glyceride oils and fats with NaOH or KOH, giving glycerol as a by-product Drawbacks of soap (a) It does not function very well in acid solutions because of the formation of insoluble fatty acid (b) It forms insoluble precipitates with Ca 2+ and Mg 2+ ions in hard water synthetic detergents: biodegradable linear isomer and other softer detergent 로대체되어감

14 Mechanism of detergency A satisfactory detergent 1. Good wetting characteristics 2. Ability to remove dirt 3. Ability to solubilize or to disperse removed dirt and to prevent it from being redeposited on to the cleaned surface Wetting wetting of fabrics is not a critical issue in detergency, since γ c of fabric surfaces is usually excess of 40 mn/m and it is easy matter to reduce the surface tension of the aqueous bath to below this value Dirt removal The removal of solid dirt can be considered in terms of the surface-energy changes involved. The work of adhesion between a dirt particle and a solid surface is given by W SD = γ DW + γ SW γ SD (6.13)

The action of the detergent is to lower γ DW and γ SW W SD and detachment of dirt particle If the dirt is fluid (oil or grease), its removal can be considered as a contact angle phenomenon the

15 The action of the detergent is to lower γ DW and γ SW W SD and detachment of dirt particle If the dirt is fluid (oil or grease), its removal can be considered as a contact angle phenomenon the addition of detergent lowers the contact angle at solid-oil-water boundary: if θ = 0, the oil will detach spontaneously from the solid surface, If 0 < θ < 90, the oil can be removed entirely by mechanical means, If 90 < θ < 180, only part of the oil can be detached by mechanical means ( 나머지는온도를올리는등 solubilization 메카니즘을사용해야함 )

17 Adsorption from solution adsorption of material from solution on the solid surface e.g., modification of solid surface, clarification of sugar solutions with activated charcoal, ion adsorption, chromatography, Solution adsorption isotherms polar adsorbent tends to adsorb polar adsorbates strongly & nonpolar adsorbates weakly Polar solutes tend to be adsorbed strongly from non-polar solvents (low solubility) and weakly from polar solvents (high solubility) Fig.6.12(a) polar solid, amphiphilic solutes, non-polar solvent: non-polar chain adsorption (acetic > propionic > butyric) Fig.6.12(b) non-polar solid, amphiphilic solutes, polar solvent: non-polar chain adsorption (acetic < propionic < butyric)

18 Isotherm equations, surface areas Langmuir and Freundlich equations are frequently applied to adsorption from solution data x: the amount of solute adsorbed by a mass m of solid, c: the equilibrium solution concentration, a, k, n: constants 기체흡착처럼표면적계산가능

The Origins of Surface and Interfacial Tension

The Origins of Surface and Interfacial Tension Imbalance of intermolecular forces exists at the liquid-air interface γ la= the surface tension that exists at the liquid-air interface Suppose we have a