 Alright, in this first module on introduction, we continue the general introduction, definitions, examples, challenges and then come to the concept of surface tension. In order to complete this lecture and make it self-sufficient, let us have a brief recapitulation of some of the basic definitions we had started with. We had seen interfacial science could be defined as the branch of knowledge, the branch of knowledge concerned with the study of phenomena occurring at various types of interfaces encompassing their formation, modification and destruction as well as of the properties of stable or dynamic interfaces between different phases, while the interfacial engineering could be defined as the branch of applied knowledge concerned with the interfacial processes and having enhancement, separation or control of interface formation as its primary goals and establishment of its connections with the other engineering and scientific disciplines being the assimilative and augmentative objectives. Then we saw an elementary sketch of the connections of interfacial engineering with various identified disciplines of science and engineering and one glance at this would indicate a strong connection between interfacial engineering and many components of chemical engineering such as heat transfer, mass transfer, with or without reaction, thermodynamics, process modeling, kinetics, particle mechanics, fluid mechanics and rheology. Besides, there would be connections with surface physics and chemistry, materials, science and engineering, solid state physics, optics and electromagnetism and biochemical engineering. This diagram is only suggestive, the students should think about modifying this adding the connections they see in practice. Coming to the typical challenges posed by interfacial engineering specifically to the chemical engineers, these are as follows. First challenge is to understand the mechanisms and to formulate mathematical models so that these could be used for process analysis, design and scale up of interfacial processes. Then the second challenge is to develop molecular level structure property relations for guiding the production of materials with specified physical and chemical properties. And third is to build up an improved understanding of the elementary chemical and physical transformations occurring at these phase boundaries. We could consider several examples of interfacial processes and to give you some idea, we look at a few of these, the use of immobilized animal cell cultures for production of desired chemicals, these could be specific antibodies, special purpose proteins and ethanol. Production and preparation of solid materials and process analysis, design and scale up. This area offers chemical engineers a variety of new and exciting challenges. The second example of interfacial process could be the use of the sol-gel process to produce microstructured ceramic parts for replacement of bone and tooth. The interfacial issues include drying, consolidation and shaping the initial colloidal dispersion of ceramic particles and finally surface treatment of the parts to assure biocompatibility. Third example is the use of surfactants as bilayered vesicles for microencapsulation and site specific drug delivery and for conducting artificial photosynthesis. This one is the use of micelles for separation of protein products from fermentation broths. The landfill disposal of garbage involves microbial reactions at solid interfaces and movement of fluid through soil driven by interfacial tension. Our next example is controlled processes to produce molecularly tailored reliable microstructured or nanostructured materials for high technology electronic computer and communication industries. Crystal growth, thin film growth, thin film coatings, production of thin films on solids. For example by electron beam evaporation, molecular beam epitaxy, ion beam bombardment sputtering, DC arc deposition, plasma spraying and chemical vapor deposition processes. The next example is production of advanced ceramic materials such as optical fibers or alternatives to alloys in the form of thin films, micro or nanostructured materials etc. Our eighth example here is processes involving coatings like in photographic films, paper products, magnetic memory and optical discs, photoresist, conducting films, piezoelectric films, optical waveguides and membranes for separations. We then look at some interesting examples of products of interfacial engineering and we have listed out here a large number of them like carbon paperless copying using latex of micro encapsulated ink drops, pressure sensitive micro encapsulated adhesives, thin films for welding with a matchstick, electronic inks, pollution preventing lithographic inks, solid foams, bi-liquid foams or polyathrons, computer chips, micro capsules for controlled release of drugs, micro-encapsulated phase change materials for augmentation of heat transfer, photocopying products like wet and dry toners, detergents, low VOC paints, self cleaning glass panes, pollution abating surface coatings for roots based on TiO2, disinfectants infect the insecticides, herbicides and a range of cosmetic and personal care products, contact lenses, all kinds of medicines, specialized food products like milk products, bakery products like breads, biscuits and cookies and yet other products like sauces, spreads, cold drinks and food supplements of all kinds. Even newer fuels like biodiesels or ethanol blended petrol, newer lubricants based on biodegradable formulations, fruit preservatives and polishes, flour and wood polishes, fabric whiteners and brighteners, all kinds of adhesives and tapes, plastic products like wrappers, packings, polyurethane foam, packaging materials, bags and bottles etc. Glasses especially high quality refractive index glasses, optical quality glasses, photochromatic glasses, suspended particle devices, tempered and float glasses, specialty ceramics like monoliths and zeolites, Teflon based products like coatings and lubricants, photo catalyst and photo electrochemical cells, lithium batteries, solar cells etc. and nucleating agents for artificial rains. Obviously a number of specialized techniques and methods and instruments are required to be used in interfacial engineering to understand the surfaces and interfaces and their behavior. Some of these would include microscopy, both optical microscopy and electron microscopy. In recent years much use is made of scanning electron microscopes, transmission electron microscopes, atomic force microscope, scanning probe microscope, scanning tunneling microscope, magnetic force microscope, field ion microscope and acoustic microscope. Many of the interfaces and surfaces deal with liquids, so tensiometry is an integral part of measurement of properties of surfaces and interfaces and there are a number of methods for measuring surface and interfacial tensions such as based on willy me plate, ring method, rock weight method, sassile rock method, pendant rock method and excess bubble pressure method. We will be looking into many of these in later parts of this course. Measurements of viscosity and rheometry as well as metrology for surface elastic moduli, these would be necessary for measuring surface viscosity, surface compressional modulus and other surface properties. Then one needs measurements on electrostatics and electrokinetics and to name a few one would require zeta potential measurement, Debye Huckel length measurement, surface and interfacial potentials, both contact and distribution potentials, electrophoretic mobilities. Then one needs to measure film thicknesses, optical constants, surface compositions, electrical anisotropy and roughness and one uses interferometry, ellipsometry especially variable angle spectroscopic ellipsymetry could be used. Particle size distributions based on either laser scattering method which in turn is based on me scattering theory and Coulter principle or electrical sensing zone method. Now we come to the concept of interface and its definition once again, boundary between two homogeneous phases is not a simple geometrical plane as we had seen earlier upon either side of which extend the homogeneous phases is not a plane of zero thickness. We should rather regard the interface as a lamina or film of a characteristic thickness. The material in this lamina or film which will refer to a surface phase would show different properties from those of materials in the contacting homogeneous phases and we are especially interested in the properties of matter in this surface layer. Just as in the case of bulk phases, the matter in the interfacial phase displays behavior such as of solid state, liquid state and gas or vapor state. We concerned with the boundaries between liquid and gas or vapor, a liquid and a liquid and a liquid and solid. The liquid is often seen to behave as if it were surrounded by an elastic skin. It would exhibit a tendency to contract. So drops of liquid if uninfluenced by gravity or other external force field would show a truly spherical shape. Some of you might have come across a common laboratory experiment which concerns determination of the contractile behavior and tension of a soap film stretched across a wire framework. Historically it was in 1805 that Young explained the surface tension in terms of the attractive and repulsive forces between molecules of liquid. According to him, the cohesion between the molecules of liquid must surpass their tendency to separate under the influence of thermal motion. This net attraction between neighboring atoms of or molecules is obviously fulfilled most completely in the interior of the phase. The atoms or molecules at the surface are attracted less completely than their counterparts in the bulk when we could show this as in this figure. The surface molecules are attracted by the other surface molecules whereas a bulk molecule will be attracted by the molecules which are not only in their plane but also above or below them. So we see a basic asymmetry existing in the surface compared to the relatively symmetric bulk of liquid. The atoms or molecules at the surface are attracted less completely than the molecules in the bulk. The consequence of this asymmetry is the following. The energy of molecules at the surface is greater than the molecules in the bulk. But the free energy of every system attempts to be a minimum. Consequently the surface of a pure phase will always tend to contract spontaneously and that is the thermodynamic rational behind the spontaneous contract contraction exhibited by all pure liquids. Now we go back to the fundamental equation of thermodynamics to relate this contractile tendency and its spontaneity to certain quantitative measure and our attempt is to get at the surface tension. So if we have gamma 0 represent force per centimeter tending to contract such a surface and S is entropy, T is absolute temperature, capital P is pressure, capital V is volume and capital A is the surface area, mu is chemical potential and N the number of molecules in the system. Then according to the fundamental equation of thermodynamics we have dF is equal to minus S dT minus P dV plus gamma 0 dA plus mu dN where F is the Helmholtz free energy, total Helmholtz free energy of the system. At constant temperature volume and given number of moles we will have the first second and the last term on the right hand side vanishing and that would mean dF is equal to gamma 0 dA. So at constant temperature volume and for a given number of moles of the system dF is equal to gamma 0 dA or we could rearrange this to write gamma 0 is equal to dou F by dou A at constant T, V and N. What can we infer from here? We have gamma 0 the force per centimeter trying to contract the surface and that is expressed as the partial of the total Helmholtz free energy of the system with respect to area at constant temperature volume and N. Let us try to ascribe a physical significance to this equation. In view of what we have been discussing if we say that there is a spontaneous contraction it means the area tends to decrease spontaneously. So if dou A is negative and that is a spontaneous contraction then the corresponding dou F or delta F should be negative. And therefore in this spontaneous change some characteristic force contractile force per unit length which we have denoted here by gamma 0 should be a positive quantity. Spontaneous contraction of surface area or delta A negative will decrease F that is delta F is less than 0 provided that gamma 0 is positive. And since the surface of a stable liquid always tends to decrease in area gamma 0 is always positive and this is what we call surface tension. This is the perspective one should have of surface tension. So many pure liquids the values of surface tensions have been listed out in the following table 2.1. We will make a few comments on some of the values here. These are surface tensions of pure liquids against air. So for water at 20 degree centigrade the value is 72.8 dynes per centimeter. At a slightly higher temperature of 25 degree centigrade for water the surface tension is 72 dynes per centimeter. So one sees here a decrease in surface tension with increase in temperature and increase in temperature results in decrease in surface tension. And we see that this value of about 72 dynes per centimeter is one of the highest values that one comes across for ordinary liquids. Then we have a number of organic liquids for which surface tensions have been listed out here for Bromo benzene. The surface tension is 35.75 dynes per centimeter about half the value for water. For benzene it is about 28.88 dynes per centimeter. At a higher temperature of 25 degree centigrade it drops further to 28.22 dynes per centimeter. For toluene we have a 20 degree centigrade surface tension of 28.43 for octanol 27.53 and so on. For carbon tetrachloride we have about 26.9 for octane it is 21.8. For ethyl ether there is a significant decrease in the value of surface tension compared to what we have seen over here for many organic liquids it is about 17 dynes per centimeter. And the highest surface tension for the normal liquid at room temperature is one observed for mercury about 485 dynes per centimeter coupled with the fact that mercury is the only liquid metal at room temperature this is not unexpected. For oleic acid the surface tension is about 32.5 and for hexane it is about 18.43 for cast oil the value is 39 dynes per centimeter. One may want to observe here that normal hexane has a surface tension which is quite low 18.43. Besides being a good solvent hexane is also able to penetrate the pores in the oil cake in the solvent extraction for removal of residual oil in production of the vegetable oils. Olive oil has a surface tension of 35.8 cotton seed oil similar about 34 35.4 and liquid petrol atom has about 33.1 dynes per centimeter. We return to this table and talk about water and the relatively high surface tension that we find. As we will see later water molecules have a distinct dipole moment which makes for a greater cohesion among water molecules compared to what we observe for other common liquids. It is the strong association among water molecules because of a significant polar nature of water molecules the reason why the asymmetry in the surface results into a pronounced large magnitude for surface tension. This cohesive tendency surpasses the tendency to separate under the influence of thermal motion for water molecules much more compared to the similar balance for organic liquids at corresponding temperatures. I would want you to also read little more into the effect of temperature that we seen this table at the level of 5 degree centigrade increase in temperature the surface tension drops from 72.8 to 72 dynes per centimeter for water and similarly for benzene we see a drop from 28.88 to 28.22 dynes per centimeter. Why is there is why is there a decrease in surface tension with increase in temperature obviously the asymmetry between the surface and bulk molecules is reduced the differences rising out of asymmetry between the surface molecules and the bulk molecules is reduced as we increase the temperature as we have higher temperature the surface energies for the surface molecules and the energy associated bulk molecules both increase in their differences diminish. So, surface tensions here being referred to the adjacent medium air this is understandable but a similar argument as we will see later for interfacial tensions will also hold good the asymmetry which is of residual nature for interfacial tension between two liquids that again has a less pronounced effect on the contractile tendency of the interfacial region. So, as we increase the temperature not just the surface tension but also the interfacial tension would tend to decrease or one may say in the jargon of the surface phase versus bulk phase again this we will address in more detail later the differences between the surface and bulk phases will become lesser at higher temperatures. One may go on to say that it is as if the surface or interfacial phase becomes more miscible or more alike the bulk phase just as one would talk about the increase in the miscibility of two liquids when the temperature goes up just as bulk phases become more miscible at higher temperatures. So could we say that surface phase becomes more miscible with the bulk phase at higher temperature or likewise the interfacial phase becomes more miscible with the bulk phase on either side of the interface. So, in the light of this discussion we would conclude that the interfacial science and interfacial engineering impacts a large number of practical systems poses intricate problems for application of mathematical and range of experimental techniques so that we understand better what happens what happens at the surfaces and interfaces and are in a position to better manipulate or control their formation and if required their destruction if so desired. End of time I would like to also make a suggestion here that what might appear as very gentle changes in the context of a surface phase or interfacial phase actually can translate into very high magnitudes in bulk terms. So, it is possible to achieve certain conversions or processes in the surface or interfacial regions with much gentler looking changes in surface pressures when compared to the corresponding magnitudes of equivalent bulk pressures. Similarly the properties at the surfaces might show small differences, but in bulk terms those changes could be very significant. At this point it is sufficient to say that viscosities of some of the surface phases with slight orientation of molecules could be equivalent of very high viscosities representative of let us say butter or toffee in bulk terms. Small changes in surface pressures could be expressed as very large changes in equivalent bulk pressures. Relatively small looking surface concentrations would be equivalent to very high bulk concentrations. So, under these premises it might be possible to achieve certain chemical conversions at phase boundaries in a much more convenient in easier fashion compared to achieving those conversions in bulk phase. This also leads us to the notion that the dispersions with very high interfacial areas are a productive line of investigation and for industrial operations for achieving conversions for many chemical and biochemical processes which otherwise would require very intensive environment and very large magnitudes of the operating pressures or concentrations. To take this further because of the peculiar nature of surface and interface region it may be possible to even make use of the surface properties to guide conversions of some chemical processes in a selective manner. It might be possible to carry out some reactions in preference to other side reactions using the peculiar properties that the surface phases offer and this is a rich area we will be able to make use of the special distinctive properties of surface phases to achieve many other important effects like for example, the catalytic action of surface phases or interfacial phases because of their unique structural and energetic states. We conclude this lecture 2 of module 1 here and continue from here next time. So, we stop here for today.