Surface and Interfacial Phenomenon PDF

SURFACE AND INTERFACIAL PHENOMENON Types of boundaries: – Gas/ Gas (vapour/vapour) v/v – Liquid/ Liquid (L/L); e.g. oil and water in emulsion – Liquid/ Vapor (L/V); e.g. water exposed to air – Solid/ Vapor (S/V); e.g. table top exposed to air – Solid/ Liquid (S/L); e.g. suspe...

SURFACE AND INTERFACIAL PHENOMENON Types of boundaries: – Gas/ Gas (vapour/vapour) v/v – Liquid/ Liquid (L/L); e.g. oil and water in emulsion – Liquid/ Vapor (L/V); e.g. water exposed to air – Solid/ Vapor (S/V); e.g. table top exposed to air – Solid/ Liquid (S/L); e.g. suspension Forces of Attractions: - Cohesive force: are the forces that exist between molecules of one phase (i.e. between like molecules). - Adhesive force: are the forces that exist between molecules of two different phases (i.e. between unlike molecules). - MISCIBILITY: occurs when cohesive forces are less than adhesive forces, e.g. water and alcohol. - IMMISCIBILITY: occurs when cohesive forces are more than adhesive force e.g. water and oil  WETTABILITY - Why does one fabric absorb water well while another seems to refuse it? - Why does water collect into large drops on a greasy surface and instead form an adherent film on a clean surface? - When the forces of adhesion are greater than the forces of cohesion, the liquid tends to wet the surface, when instead the forces of adhesion are less by comparison to those of cohesion, the liquid tends to "refuse" the surface. - We are dealing with wettability between liquids and solids. For example, water wets clean glass, but it does not wet wax.  CAPILLARITY - It is due to wettability i.e. the adhesive forces between liquid and capillary tube wall is higher that cohesive forces. So, the liquid wets the wall and rises in the tube. - In case of mercury, the cohesive force between its molecules is very high compared to adhesive force, so there is no wetting at all. - The rising or the lowering of the level of the liquids into thin tubes is named capillarity. - Liquids which wet the walls make concave surfaces (eg: water/glass), those which do not wet them, make convex surfaces (eg: mercury/glass).  INTRODUCTION TO SURFACE PHENOMENA Why do some insects succeed in skating on water instead of sinking?? Why in some cases, does the water sprinkled on a glass surface collect into drops (beads) and in other cases spread like a thin film?? Why does water climb up a thin tube??  INTRODUCTION The surface of a substance has special properties. These surface properties are what allow these strange phenomena we have mentioned. Not only that, but the surface is also the place of contact among different substances. When phases exist together, the boundary between two immiscible phases is termed an interface. Whereas that presents between two phases one of which is air or vapor known as surface. The properties of molecules forming the interface are different from those in the bulk of each phase. Surface and interfacial tension: Interface: is the boundary between 2 phases in contact with each other. Surface: is the boundary between 2 phases, one of them is gas or vapor. No interface between two gases because they mix in all proportions.  SURFACE TENSION AND SURFACE FREE ENERGY Attractive forces at the surface and in the bulk of a liquid  SURFACE TENSION (Y) In the surface of a liquid, there is no molecules to counteract the forces of attraction exerted by molecules in the bulk for molecules in the surface. The inward force of attraction exerted on the surface molecules results in the tendency of the surface to contract. This inward pull tends to diminish the surface area, and in this respect a liquid surface resembles a stretched elastic membrane. Thus the liquid squeezes itself together until it has the locally lowest surface area possible These centrally directed forces cause the droplet to assume a spherical shape, thereby minimizing both the free energy and surface area as a sphere has the smallest possible surface area to volume ratio. The interfacial tension is the force between two immiscible liquids phases.  SURFACE TENSION (Y) Therefore, the surface tension phenomenon is a property of liquids arising from unbalanced molecular cohesive forces at or near the surface, as a result of which the surface tends to contract and has properties resembling those of a stretched elastic membrane. surface tension“ defined as force per unit length that must be applied parallel to the surface so as to counterbalance the net inward pull. It has the unit of dyne /cm.  Surface tension: force acting over the surface of liquid per unit length= F/L (dyne/cm) Interfacial tension: force per unit length exciting at the interface between two immiscible liquids.  Surface Energy(surface free energy The free energy is defined as: The work required to increase the area by 1cm2 ү=W/∆A W= ү ∆A , its unit is dyne.cm Effects of surface tension in everyday life Beading of rain water on the surface of a waxed car or leave’s surface. Standing of insect on the surface of a pond. Floating needle effect. Effects of surface tension in everyday life Formation of drops occurs when a mass of liquid is stretched. Effects of surface tension in everyday life Separation of oil and water is caused by a tension in the surface between dissimilar liquids. ( interfacial tension)  INTERFACIAL TENSION It is the force per unite length existing at the interface between two immiscible liquids. Its unit is also dyne/cm e.g. oil and water. Interfacial tension is usually less than Surface tension as the adhesive forces is greater than when between liquid and gas.  1 Measurement of surface & interfacial g = hrdg 2 tension A. Capillary rise method:  the upward force due to the surface tension against the downward force due to the liquid weight in the tube: ү = ½ hrdg Where: γ is the surface tension of the liquid (dyne/cm). h is the height of the liquid in the capillary tube (cm). r is the radius of the capillary tube (Cm). Rise of liquid in capillary tube. d is the density of the liquid (g/Cm3). g is the acceleration due to gravity (980 Cm/sec2).  B. Ring Method (du nuoy tensiometer) measure the force necessary to detach a platinum iridium (attached to a balance) immersed at the surface or interface, which is proportional to the surface or interfacial tension. B. Ring Method (du nuoy tensiometer) (cont.) B. Ring Method (du nuoy tensiometer) (cont.)  B. Ring Method (du nuoy tensiometer) (cont.) The detachment force equals to the surface or interfacial tension F=2πrY By rearrangement Y= F 2πr Where: F is the detachment force r is the ring radius  problem 1- Using du nuoy tensiometer having a ring of surface area of 153.86 cm2 , it was found that the detachment force required to detach the iridium ring from the surface of a liquid is 3200.29 dyne calculate the surface tension of the liquid(π=3.14) Solution 153.86= πr2 r=7cm Y= F/2 π r Y=3200.29dyne/2x3.14x7 = 72.8dyne/cm D. Drop weight and drop volume method: The mass m of an ideal drop of a liquid having surface tension ү , falling from a tube of external radius r is given by Tate,s equation m.g = 2 π r ү Where g is the gravitational constant For relative measurement , it is easier to determine the number of drops formed by a given-volume tube instead of determining the weight of the drops Therefore , ү1/ү2 = m1/m2 = n2d1/n1d2 Stalgmometer is used for this determination.  Significance of surface tension surface tension manifests itself in various effects such as: Spreading of liquids and Contact angles of liquid with solids. The physical and chemical behavior of liquids cannot be understood without taking surface tension into account. It governs the shape that small masses of liquid can believe and the degree of contact a liquid can make with another substance.  Significance of surface tension (cont.) Wetting or non-wetting of solids by liquids. The capillary rise of liquids in fine tubes and wicks, and the curvature of free-liquid surfaces. The action of detergents and anti-frothing agents depend upon the surface tensions of liquids.  SPREADING  The ability of one liquid to spread over another can be assessed in terms of the spreading coefficient (S): S = yA - (yB + yA/B) Where  yA is the surface tension of sub layer liquid.  yB is the surface tension of spreading liquid.  y A/B is the interfacial tension between the two liquids.  A positive or zero value of spreading coefficient is required for spreading to occur.  If spreading coefficient is negative value, the substance forms globules or a floating drop.  SPREADING If a small quantity of an immiscible liquid is placed on the surface of a second liquid, it may spread to cover the surface with a film or remain as a drop or lens. S = Wa - Wc Where S is the spreading coefficient Wa is the work of adhesion Wc is the work of cohesion  problem A liquid B of a surface tension 32dyne/cm Mixed with a liquid A of a surface tension 72.8dyne/cm , if the interfacial tension between the two liquids was found to be 48.3 dyne/cm. Calculate the spreading coefficient (S) and show if liquid B spread over liquid A or forms globules. Solution S = y - (yB + yA/B) 72.8-(32+48.3)=-7.5 A Therefore,liquid forms globules.  Contact Angle (Ө) The contact angle is the angle at which a liquid/vapor interface meets the solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Contact angle (Ө) contact angle is a function of the surface energies (i.e. surface tension) of the system Contact angle at interface of three phases. Forces at contact point shown for contact angle greater than 90° (left) and less than 90° (right)  Some liquid-solid contact angle Liquid Solid Contact angle Water o° glass Ethanol Glycerol Acetic acid water Paraffin wax 107 ° water silver 90 ° mercury glass 140 °  Contact angles It is difficult to clean the floor if liquids with contact angle ≈ 0°spills, like petrol, kerosene, benzene, etc Contact angle of 180° occurs when the liquid-solid surface tension is exactly equal to the liquid-air surface tension.  Pharmaceutical Application The surface tension, and consequently the contact angle, of liquids are very important. Preparation of Emulsion, mixing of immiscible liquids. Preparation of suspension; wetting of insoluble powders so as to prepare suspension. Granulation, prior to tableting, involve the mixing of powder with a liquid binder. Film coating of tablets.  Pharmaceutical Application (cont.) Successful dissolution of tablet or capsules necessitates penetration of liquid into pores in the dosage form. Spreading of lotion on the skin surface. Spreading of ointment in the eye. Spreading of disinfectant on micro-organism surface.  CONCLUSION The physical and chemical behavior of liquids cannot be understood without taking surface tension into account. It governs the shape that small masses of liquid can assume and the degree of contact a liquid can make with another substance. Therefore, to perform the previous pharmaceutical process we need to reduce the surface tension, so as to reduce the contact angle and increase the spreading of the material HOW CAN WE REDUCE THE SURFACE TENSION??????????????