The surface phenomenon of liquid

Applications to life science

Ass. Falfushynska Halina

Surface tension• The surface of a liquid acts as a tensional film which always tends to contract to a minimum area. It proves that surface of liquid has tension. It is called surface tension.

Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders) to run on the water surface.

Surface tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent—but when referring to energy per unit of area, people use the term surface energy —which is a more general term in the sense that it applies also to solids and not just liquids.

The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension.

The surface tension of water is 72 dynes/cm at 25°C . It would take a force of 72 dynes to break a surface film of water 1 cm long. The surface tension of water decreases significantly with temperature as shown in the graph. The surface tension arises from the polar nature of the water molecule.

Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension.

The work done by increasing a unit area of liquid surface is called surface energy.

Surface energy

Surface tension F can be described by the surface tension coefficient (ratio between energy E and square S). The relation between them is LFtension where L is the length of a line on the liquid surface.

Molecules liquid state experience strong intermolecular attractive forces. When those forces are between like molecules, they are referred to as cohesive forces. For example, the molecules of a water droplet are held together by cohesive forces, and the especially strong cohesive forces at the surface constitute surface tension.

Cohesion and Adhesion

http://www.youtube.com/watch?v=OuDP9-TA7dghttp://www.youtube.com/watch?v=VHnFMPxteGo&feature=related

The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals bonds.

When the attractive forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces lead to an upward turning meniscus at the walls of the vessel and contribute to capillary action.

convex meniscus

concave meniscus

Capillarity

The tube with very short diameter is called a capillary tube. When a capillary tube is put in a liquid, the liquid surface in it will change.

If liquid can wet the wall of the capillary tube, the liquid surface in the tube will go up and if the liquid cannot wet the wall, the surface will go down.

This phenomenon is called capillarity. Let’s have a look at the case of liquid surface going up.

The surface-active agent is called surfactant or a wetting agent. Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants. For example, organic acid and soap are active agent of water; salt is non-active agent of water. When the surface-active agent dissolves in solvent (the liquid in which another substance (solute) is dissolved to form a solution), the attraction of solvent molecules is greater than attraction between solvent molecule and solute molecule.

Detergents in biochemistry and biotechnology

In solution: detergents help solubilize molecules by dissociating aggregates and unfolding proteins. These include SDS, CTAB. Detergents are key reagents to extract protein by lysis of the cells and tissues: they disorganize the membrane's lipidic bilayer (SDS, Triton X-100, X-114, CHAPS, DOC, NP-40), and solubilize proteins. Milder detergents such as (OctylThioGlucosides) are used to solubilize sensible proteins (enzymes, receptors). Non-solubilized material is harvested by centrifugation or other means. For electrophoresis for example, proteins are classically treated with SDS to denature the native tertiary and quaternary structures, allowing the separation of proteins according to their molecular weight.Detergents have also been used to decellularise organs. This process maintains a matrix of proteins that preserves the structure of the organ and often the microvascular network. The process has been successfully used to prepare organs such as the liver and heart for transplant in rats. Pulmonary surfactants are also naturally secreted by type II cells of the lung alveoli in mammals.

What is Adsorption?Adsorption is the phenomenon of accumulation of large number of molecular species at the surface of liquid or solid phase in comparison to the bulk.How Adsorption occurs?The process of adsorption arises due to presence of unbalanced or residual forces at the surface of liquid or solid phase. These unbalanced residual forces have tendency to attract and retain the molecular species with which it comes in contact with the surface. Adsorption is essentially a surface phenomenon.

Adsorption is a term which is completely different from Absorption . While absorption means uniform distribution of the substance throughout the bulk, adsorption essentially happens at the surface of the substance. When both Adsorption and Absorption processes take place simultaneously, the process is called sorption.

http://www.youtube.com/watch?v=djIzXvwIz5U&feature=related

Oxygen molecules (red) adsorb on a bimetallic surface of platinum (purple) and cobalt (green).

Adsorption process involves two components Adsorbent and Adsorbate. Adsorbent is the substance on the surface of which adsorption takes place. Adsorbate is the substance which is being adsorbed on the surface of adsorbent. Adsorbate gets adsorbed.

Adsorbate + Adsorbent gives rise to Adsorption

Some modern techniques have been used to study surface.1.Low energy electron diffraction (LEED).2.Photo electron spectroscopy (PES).3.Scanning Tunneling microscopy (STM).

Adsorption is a spontaneous processFor reaction or process to be spontaneous, there must be decreases in free energy of the system i.e. ΔG of the system must have negative value.Also we know, ΔG = ΔH – TΔSAnd during this process of adsorption, randomness of the molecule decreases which ΔS is negative. We can rewrite above equation as

Types of AdsorptionForces of attraction exist between adsorbate and adsorbent. These forces of attraction can be due to Vanderwaal forces of attraction which are weak forces or due to chemical bond which are strong forces of attraction. On the basis of type of forces of attraction existing between adsorbate and adsorbent, adsorption can be classified into two types: Physical Adsorption or Chemical Adsorption.Physical Adsorption or PhysisorptionWhen the force of attraction existing between adsorbate and adsorbent are weak Vanderwaal forces of attraction, the process is called Physical Adsorption or Physisorption. Physical Adsorption takes place with formation of multilayer of adsorbate on adsorbent. It has low enthalpy of adsorption i.e. ΔHadsorption is 20-40KJ/mol.It takes place at low temperature below boiling point of adsorbate.As the temperature increases in, process of Physisorption decreases.

Chemical Adsorption or ChemisorptionWhen the force of attraction existing between adsorbate and adsorbent are chemical forces of attraction or chemical bond, the process is called Chemical Adsorption or Chemisorption. Chemisorption takes place with formation of unilayer of adsorbate on adsorbent. It has high enthalpy of adsorption

Physical Adsorption vs T and Chemical Adsorption vs T

Applications of Adsorption

1. Charcoal is used as a decoloriser as it adsorbs the coloring matter from the coloured solution of sugar.

2. Silica gel adsorbs moisture from the desiccators.

3. Silica and alumina gels are used as adsorbents for removing moisture and for controlling humidity of rooms.4. Activated charcoal is used in gas masks as it adsorbs all the toxic gases and vapours and purifies the air for breathing.

5 .Adsorption processes are useful in carrying out heterogeneous catalysis.

Factors on which Adsorption Depends

Temperature. Adsorption increases at low temperature conditions.Adsorption process is exothermic in nature. According to Le Chatleir principle, low temperature conditions would favour the forward direction.PressureAs depicted by Adsorption Isotherm, with the increases in pressure, adsorption increases up to a certain extent till saturation level is achieved. After saturation level is achieved no more adsorption takes place no matter how high the pressure is applied.Surface Area. Adsorption is a surface phenomenon therefore it increases with increase in surface area.Activation of AdsorbentActivation of adsorbent surface is done so as to provide more number of vacant sites on surface of adsorbent. This can be done by breaking solid crystal in small pieces, heating charcoal at high temperature, breaking lump of solid into powder or other methods suitable for particular adsorbent.