表面化学5 The solid-liquid interface.pptVIP

5 The solid-liquid interface Contact angle ?SG=?SL+?LGcos? Wetting Types of wetting Spreading wetting Adhesional wetting Immersional wetting Spreading wetting In spreading wetting, a liquid spreads spontaneously on the solid to form a liquid film. The solid-gas interface disappears and The solid-liquid interface and liquid-gas interface form. the spreading coefficient S=Wa-Wc=?SG-?SL-?LG is positive or zero. The contact angle should be zero. Adhesional wetting When a liquid combines with a solid as shown in the following figure, the process is termed adhesional wetting. The work of adhesion Wa=-?G/A= ?SG +?LG -?SL = ?LG (1+cos?) Immersional wetting In immersional wetting, the solid, which is not originally in contact with the liquid, is immersed completely in the liquid. The work of adhesion Wa=-?G/A= ?SG -?SL = ?LG cos? When ?90?, immersional wetting is spontaneous. Measurment of contact angles Factors affecting contact anlges measurment Contamination of the liquid Solid surface (degree of heterogeneity, impurities) Advancing angle or receding angle The extent to which the drop is vibrated. Drop of the liquid/ bubble tan?/2=2h/d The volume of the drop should be less than 10-4ml, or ?90?. Tilting-plate method The angle of the plate equals the contact angle when the liquid surface remains perfectly flat right up to the solid surface. Determination of contact angle of powder Packing the powder into a tube Measureing the pressure (p) required to prevent the liquid in question from entering the powder. p=2?cos?/r r is found from a similar experiment using a liquid which completely wets the solid. r=2 ?’/p’ Then cos?= ?’p/ (?p’) Factors affecting contact angles and wetting Nature of the solid and liquid Surface roughness The method used to prepare the solid surface Wetting agents Wetting agents are used to adjust the wetting in many practical situations. For example, increasing wetting of pesticide with plants and insects c