Surface chemistry. Liquid-gas, solid-gas and solid-liquid surfaces. Márta Berka és István Bányai, University of Debrecen Dept of Colloid and Environmental Chemistry http://dragon.unideb.hu/~kolloid/ Surfaces and Interfaces • • • • • • Defining of interfacial region Types of interfaces Surface tension Contact angle, wetting, and spread Adsorption Biological interfaces Defining of interfacial region. Types of interfaces Two homogeneous bulk phases meet there is a region of finite thickness where the properties change at a molecular level the thickness of the interfacial region is significant and definitely non-zero Fluid Interface: G-L, L1-L 2 surfactants Here the transition does not follow a smooth monotonic transition. Non-fluid interface: G-S, L-S, S1-S2 The properties of the interfacial region are particularly important when one of the phases is dispersed as many very small particles in the other phase, because of the dramatic increase in surface area. Surface tension The attractive forces acting on molecules at the surface are anisotropic Molecules at the surface are subject to an inward force of molecular attraction, N/m Thermodynamic definition of surface tension = Gibbs free energy of unit area, J/m2 ⎛ dG ⎞ γ =⎜ ⎟ ⎝ dA ⎠ n , p ,T G=γA + other terms Surface tension is the energy required to increase the surface area of a clear liquid by a unit amount, J/m2 γ must be positive, interface tends to a minimum l γ=F/2 l Surface tension, represented by the symbol γ is defined as the force along an imaginary line of unit length, where the force is parallel to the surface but perpendicular to the line, N/m. F .If the gravitational force is less than the surface tension then the object will float on the surface of the water (water strider, needle). http://www.ilpi.com/genchem/demo/tension/index.html example A needle has a length of 3.2 cm. When placed gently on the surface of the water (γ = 0.073 N/m) in a glass, this needle will float if it is not too heavy. What is the weight of the heaviest needle that can be used in this demonstration? three forces act on the needle, its weight W and the two forces F1 and F2 due to the surface tension of the water. The forces F1 and F2 result from the surface tension acting along the length of the needle on either side. http://scipp.ucsc.edu/~haber/ph5B/bubble.pdf Kb 0.47 g 1 g= 0.0098 N Walking on Water Water Striders & Surface Tension Distilled Water (Control) 0.003M 0.001M 0.004M 0.002M 0.005M As detergent concentration increases, surface tension decreases. The lower the surface tension, the deeper the dimple on the water surface in which the insect stands. At an SDS concentration of 0.005M, the water strider is unable to stay above water. http://www.woodrow.org/teachers/bi/1998/waterstrider/student_lab.html Surface tension depends on the intermolecular interactions The stronger interaction of molecules yields a higher surface tension. The interaction between the water and the liquids is stronger than the interfacial tension is lower or diminish. The interfacial tension is about the difference of the surface tension of the liquid saturated with each other. γ AB ~ γ *A − γ B* Measurement of surface tension Wilhelmy plate du Noü ring The maximum force is measured to pull out the ring or plate from the surface Capillary rise (capillary depression) If a narrow capillary tube is dipped into a liquid the level of liquid in the tube is usually different from that in the larger vessel If a tube is sufficiently narrow and the liquid adhesion to its walls is sufficiently strong, surface tension can draw liquid up the tube in a phenomenon known as capillary action. The height the column is lifted to is given by Capillary rise (capillary depression) h = 2γ cos θ / ρ gRc If perfectly wetting 1 2 γ = Δρ ghRc Capillary action is the result of adhesion and surface tension. Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward. The surface tension acts to hold the surface intact, so instead of just the edges moving upward, the whole liquid surface is dragged upward. Influence of temperature on surface tension Eötvös ( Hungarian physicist who introduced the concept of molecular surface tension) and Ramsay and Shields: γV 2/3 ⎛ dγ ⎞ ⎛ dS ⎞ = − ⎜ ⎟ ⎜ ⎟ ⎝ dT ⎠ p ⎝ dA ⎠T = k E (Tc − T ) surface entropy γ V 2 / 3 = k E (Tc − T − 6 ) ( d γ (M / ρ ) dT 2/3 ) = 2.12 ×10 −7 J mol 2 / 3 K −1 Anomalies, association, dissociation V is the molar volume of that substance, TC is the critical temperature Surface tension at a curved interface If a fluid interface is curved the pressures on either side must be different. The forces of surface tension are exactly balanced by the difference in the pressure on the two sides of the interface. the Laplace equation for a spherical liquid surface: ΔP = FzΔP + Fzγ = 0 2γ r rc cos θ = r (P α −P β ) (π r 2 c ) − (2π rc )γ cos θ = 0 ( α ΔP = P − P β ) 2γ = r Projected area =πrc2 rc radius of spherical cup Surface tension at a curved interface the Laplace equation for a spherical liquid surface: A soap bubble has two spherical surfaces (inside and outside) ΔP = 2γ r the Laplace equation for a spherical liquid drop: the Laplace equation for a spherical soap bubble: ΔP = 2γ r ΔP = 4γ r If the bubble and drop had the same radius, we would expect that the pressure difference between the inside and outside of the bubble to be twice as large as that for the drop. The reason is that the bubble has two surfaces, whereas the drop has only one. Thus, the bubble would have twice the force due to surface tension, and so the pressure inside the bubble would have to be twice as large to counteract this larger force. In fact, however, the bubble has twice the radius compared to the drop. The doubled radius means that the bubble has one-half the pressure difference. Consequently, we expect the larger bubble and smaller drop to have the same pressure difference. ? Phenomena at curved interfaces. Kelvin equation • The effect of surface curvature on the vapor pressure of a liquid ⎛ pr ⎞ ⎛ γ VL ln ⎜ ⎟=⎜ ⎝ p∞ ⎠ ⎝ RT Where r>0 ⎞⎛ 2 ⎞ ⎟⎜ ⎟ ⎠ ⎝ rm ⎠ pr , p∞ are respectively the vapor pressures over the curved surface of meniscus radius rm and r∞ of a flat surface r <0 rm> 0 to the radius when it lies in the liquid phase and rm< 0 (negative sign) when it lies in the vapor phase Ostwald ripening Consequences self-nucleation of a new phase Heterogeneous nucleation Capillary condensation Consequences The smaller the radius, the higher the vapor pressure so that there are droplets of various sizes present the smaller ones will tend to evaporate while the larger ones will tend to grow. An important example occurs in clouds where the larger droplets grow until they are heavy enough to fall as rain. A similar mechanism is thought to exist for crystals in a solution. The larger crystals tends to grow at the expense of smaller ones. Ostwald ripening. The equilibrium between a small liquid droplet and its vapor unstable. Self nucleation of a new phase is the formation of very small nuclei or embryos of the new phase inside the old phase. Super saturation critical nuclei size. Contact angle, wetting, and spreading Where the two surfaces meet, they form a contact angle γ 2 = γ 12 cos θ 2 + γ 1 cos θ1 γ SG = γ SL + γ GL cos θ By convention the contact angle is measured in the liquid phase. Why does one fabric absorb water well while another seems to refuse it? Wettability depends on adhesion /cohesion. When the forces of adhesion are greater than the forces of cohesion, the liquid tends to wet the surface, when the forces of adhesion are less by comparison to those of cohesion, the liquid tends to "refuse" the surface. In this people speak of wettability between liquids and solids. For example, water wets clean glass, but it does not wet wax.