Colloidal Materials: Part III

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 Colloidal Materials: Part III Dr. Pallab Ghosh Associate Professor Department of Chemical Engineering IIT Guwahati, Guwahati 781039 India Joint Initiative of IITs and IISc Funded by MHRD 1/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 Table of Contents Section/Subsection Page No. 1.4.1 Determination of molecular weight by light scattering 3 1.4.2 Experimental characterization of colloids 6 9 1.4.2.1 Transmission electron microscopy 6 1.4.2.2 Scanning electron microscopy 7 1.4.2.3 Dynamic light scattering 8 1.4.2.4 Small-angle neutron scattering 8 1.4.3 Electrical properties of colloids 10 19 1.4.3.1 Electrostatic double layer 10 1.4.3.2 Electrokinetic phenomena 12 1.4.3.2.1 Zeta potential 14 1.4.3.2.2 Reciprocal relationship 19 Exercise 20 Suggested reading 22 Joint Initiative of IITs and IISc Funded by MHRD 2/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 1.4.1 Determination of molecular weight by light scattering In liquid dispersions, the scattering of light is due to fluctuations in the solvent density and fluctuations in the particle concentration. The total intensity of the scattered light in all directions is given by, is 2 Is I0 2 r sind (1.4.1) 0 I0 The second term inside the integral in Eq. (1.4.1) represents an area on the surface of a sphere, where r is the radius of the sphere and is the angle with the horizontal axis. From the theory of Rayleigh scattering in a solution, the quantity is I 0 can be determined as shown below. The Rayleigh scattering equation for a solution (at constant temperature) is expressed by, is I0 2 2 dn 2 r nr ktc dc 2 4d r o dc 2 1cos (1.4.2) where i s is the intensity of the light scattered per unit volume of solution. The gradient of refractive index of the solution n r, is given by dnr dc. The osmotic pressure is given by, c B 2 c 2 o RT c RT Bc M RT M, B B (1.4.3) RT Where M is the molecular weight of the solute, c is the concentration, and B is the second virial coefficient. Therefore, the osmotic pressure gradient is given by, d o 1 RT 2Bc dc M From Eq. (1.4.2), we can write, 2 i Kc 1 cos s I 2 0 r 1 M 2Bc The quantity K is a constant, which is given by, (1.4.4) (1.4.5) Joint Initiative of IITs and IISc Funded by MHRD 3/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 2 2 dn 2 r nr dc K 4 N A From Eqs. (1.4.1) and (1.4.6) we obtain, 2 I 2 Kc 1 cos sind s I 0 0 1 M 2Bc 2 The value of the integral: where H 16 K 3 (1.4.6) (1.4.7) sin 1 cos d is 83. Therefore, Eq. (1.4.7) becomes, 0 Is 16 Kc Hc I0 31 M 2Bc 1M 2Bc (1.4.8) is a constant. It is related to the refractive index, its gradient and the wavelength of light in the medium by the following equation. 3 2 2 where is the wavelength of light in the solution and 32 nr dnr dc H 4 3N A (1.4.9) N A is Avogadro s number. The quantity Is I 0 is the turbidity,. Therefore, from Eq. (1.4.8) we have, Hc 1 2Bc M (1.4.10) Equation (1.4.10) is known as Debye equation. It predicts that the plot of Hc versus c should be a straight line. From the intercept, the molecular weight M can be determined. Example 1.4.1: The variation of Hc with concentration for a polymeric colloid in benzene is given below. c (kg/m 3 ) 2.5 4.0 6.5 8.0 10.0 3 Hc 10 (mol/kg) 5.6 6.1 6.6 7.2 8.0 From these data, calculate the molecular weight of the polymer. Joint Initiative of IITs and IISc Funded by MHRD 4/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 Solution: The plot of Hc vs. c is shown in the following figure. The data were fitted by a straight line, as shown in Fig. 1.4.1. The intercept is, 1 0.0048 M mol/kg M 208.333 kg/mol Fig. 1.4.1 Variation of Hc with concentration. If the system is polydispersed, Eq. (1.4.10) is applicable for each molecular weight fraction. For dilute solutions, we can neglect the second term on the right side of Eq. (1.4.10). For the j th fraction, we can write, Hc j 1 (1.4.11) j M j The experimentally measured concentration, turbidity and the average molecular weight are correlated by Equation (1.4.11) as, Hcexp exp 1 (1.4.12) M where, cexp cj (1.4.13) and Joint Initiative of IITs and IISc Funded by MHRD 5/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 exp j (1.4.14) From Eqs. (1.4.11) (1.4.14), we obtain, exp j H cm j j cm j j M Hc exp H c j H c j c j (1.4.15) Since cj njm j Vd, we have, M 2 nm j j (1.4.16) nm j j This average molecular weight (i.e., M ) is known as the weight-average molecular weight. 1.4.2 Experimental characterization of colloids Colloids are characterized by various methods. Some of these are, transmission electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS), and small-angle neutron scattering (SANS). Depending on the properties of the colloidal matter, the appropriate characterization method is selected. 1.4.2.1 Transmission electron microcopy Many colloid particles are too small to be viewed in an optical microscope. The numerical aperture of an optical microscope is generally less than unity, which can be increased up to 1.5 with oil-immersion objectives. Therefore, for light of wavelength 600 nm, the resolution limit is of the order of 200 nm. To increase the resolving power of a microscope so that colloidal dimensions can be directly observed, the wavelength of the radiation must be reduced considerably below that of visible light. Electron beams can be produced which have wavelengths of the order of 0.01 nm. These are focused by electric or magnetic fields, which act as the equivalent of lenses. A resolution of the order of 0.2 nm can be attained after smoothing the noise. Single atoms appear to be Joint Initiative of IITs and IISc Funded by MHRD 6/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 blurred irrespective of the resolution, owing to the rapid fluctuation of their location. The TEM can be used for measuring particle size between 1 nm and 5 m. Due to the complexity of calculating the degree of magnification directly, calibration is done using pre-characterized polystyrene latex particles. The use of electron microscopy for studying colloid systems is limited by the fact that electrons can travel without any hindrance only in high vacuum. Therefore, the samples need to be dried before observation. A small amount of the sample is deposited on an electron-transparent plastic or carbon film (10 20 nm thick) supported on a fine copper mesh grid. The sample scatters electrons out of the field of view, and the final image can be viewed on a fluorescent screen. The amount of scattering depends on the thickness and the atomic number of the atoms of the sample. The organic materials are relatively transparent for electrons whereas, the heavy metals make ideal samples. To enhance contrast and obtain three-dimensional effect, various techniques (e.g., shadow-casting) are generally employed. 1.4.2.2 Scanning electron microcopy In scanning electron microscopy, a fine beam of medium-energy electrons scans across the sample in a series of parallel tracks. These electrons interact with the sample to produce various types of signals such as secondary electron emission, back-scattered-electrons, cathodoluminescence and X-rays. These are detected, displayed on a fluorescence screen and photographed. In the secondary-electron-emission mode, the particles appear to be diffusely illuminated. Their size can be measured and the aggregation behavior can be studied. In the back-scattered-electron mode, the particles appear to be illuminated from a point source, and the resulting shadows can provide good impressions of height. The resolution limit in SEM is about 5 nm, and the magnification achieved is generally less than that in a TEM. However, the depth of focus is large, which is Joint Initiative of IITs and IISc Funded by MHRD 7/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 important for studying the contours of solid surfaces, particle shape and orientation. 1.4.2.3 Dynamic light scattering If the light is coherent and monochromatic (e.g., a laser), it is possible to observe time-dependent fluctuations in the scattered intensity using a suitable detector such as a photomultiplier capable of operating in photon-counting mode. These fluctuations arise because the particles are small, and they undergo Brownian movement. The distance between them varies continuously. Constructive and destructive interference of light scattered by the neighboring particles within the illuminated zone gives rise to the intensity fluctuation at the detector plane. From the analysis of the time-dependence of the intensity fluctuation, it is possible to determine the diffusion coefficient of the particles. Then, by using the StokesEinstein equation (see Lecture 3, Module 1), the hydrodynamic radius of the particles can be determined. An accurately known temperature is necessary for DLS because knowledge of the viscosity is required (because the viscosity of a liquid is related to its temperature). The temperature also needs to be stable, otherwise convection currents in the sample will cause non-random movements that will ruin the correct interpretation of size. Dynamic light scattering is also known as photon correlation spectroscopy (PCS) or quasi-elastic light scattering (QELS). It is a well-established technique for the measurement of the size distribution of proteins, polymers, micelles, carbohydrates, nanoparticles, colloidal dispersions, emulsions and microemulsions. 1.4.2.4 Small-angle neutron scattering Neutrons from reactors or accelerators are slowed down in a moderator to kinetic energies corresponding to room temperature or less. The wavelength probed by SANS is quite different from the visible light. A typical range of wavelength is Joint Initiative of IITs and IISc Funded by MHRD 8/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 0.33 nm. This range is much smaller than that of visible light (i.e., 400700 nm). The usefulness of SANS to colloid and polymer science becomes evident when one considers the length scales and energy involved in neutron radiation. Light scattering is indispensable for studying particles having size in the micrometer range. For very small particles, neutrons are useful. The energy of a neutron with 0.1 nm wavelength is 1.3 10 20 J. Due to such low energy, neutron scattering is useful for sensitive materials. For colloidal dispersions, where the distance between the particles is large, the scattering angle is small (~ 20 rad or less). Light and X-rays are both scattered by the electrons surrounding atomic nuclei, but neutrons are scattered by the nucleus itself. There is, however, no systematic variation of the interaction with the atomic number. The isotopes of the same element can show significant difference in scattering. For example, neutrons can differentiate between hydrogen and deuterium. This has useful applications in biological science in the technique known as contrast matching. The interaction of neutrons with most substances is weak, and the absorption of neutrons by most materials is very small. Neutron radiation, therefore, can be very penetrating. Neutrons can be used to study the bulk properties of samples with path-length of several centimeters. They can also be used to study samples with somewhat shorter path-lengths but contained inside an apparatus (e.g., cryostat, furnace, pressure cell or shear apparatus). The SANS technique provides valuable information over a wide variety of scientific and technological applications such as chemical aggregation, defects in materials, surfactant assemblies, polymers, proteins, biological membranes, and viruses. Joint Initiative of IITs and IISc Funded by MHRD 9/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 1.4.3 Electrical properties of colloids The electrical properties of colloidal materials lead to some of the most important phenomena in interfacial engineering. The presence of electrostatic double layer surrounding the particles results in their mutual repulsion so that they do not approach each other closely enough to coagulate. An increase in the size of the particles by coagulation would lead to a decrease of total area, and hence to a decrease of free energy of the system. Therefore, union of colloid particles would be expected to occur, were it not for the repulsion caused by the electrostatic double layer. The stability of the charged colloid particles depends on the presence of electrolytes in the dispersion. 1.4.3.1 Electrostatic double layer Let us consider a solidliquid interface. Suppose that the interface is positively charged and the atmosphere of the negatively charged counterions is around it. This visualization of the ionic atmosphere near a charged interface originated the term electrostatic double layer. The Coulomb attraction by the charged surface groups pulls the counterions back towards the surface, but the osmotic pressure forces the counterions away from the interface. This results in a diffuse double layer. The double layer very near to the interface is divided into two parts: the Stern layer and the GouyChapman diffuse layer. The compact layer of adsorbed ions is known as Stern layer. This layer has a very small thickness (say, 1 nm). The counterions specifically adsorb on the interface in the inner part of the Stern layer, which is known as inner Helmholtz plane (IHP) (see Fig. 1.4.2). The potential drop in this layer is quite sharp, and it depends on the occupancy of the ions. Joint Initiative of IITs and IISc Funded by MHRD 10/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 Fig. 1.4.2 Electrostatic double layer. The outer Helmholtz plane (OHP) is located on the plane of the centers of the next layer of non-specifically adsorbed ions. These two parts of the Stern layer are named so because the Helmholtz condenser model was used as a first approximation of the double layer very close to the interface. The diffuse layer begins at the OHP. The potential drop in each of the two layers is assumed to be linear. The dielectric constant of water inside the Stern layer is believed to be much lower (e.g., one-tenth) than its value in the bulk. The value is lowest near the IHP. The diffuse part of the electrostatic double layer is known as GouyChapman layer. The thickness of the diffuse layer is known as Debye length (represented by 1 ). This length indicates the distance from the OHP into the solution up to the point where the effect of the surface is felt by the ions. is known as DebyeHückel parameter. The Debye length is highly influenced by the concentration of electrolyte in the solution. The extent of the double layer decreases with increase in electrolyte concentration due to the shielding of charge Joint Initiative of IITs and IISc Funded by MHRD 11/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 at the solidsolution interface. The ions of higher valence are more effective in screening the charge. 1.4.3.2 Electrokinetic phenomena The phenomena associated with the movement of charged particles through a continuous medium, or with the movement of a continuous medium over a charged surface are known as electrokinetic phenomena. There are four major types of electrokinetic phenomena, viz. electrophoresis, electroosmosis, streaming potential and sedimentation potential. There is a common origin for all the electrokinetic phenomena, i.e., the electrostatic double layer. Electrophoresis refers to the movement of particles relative to a stationary liquid under the influence of an applied electric field. If a dispersion of positively charged particles is subjected to an electric field, the particles move towards the cathode. Electrophoresis is perhaps the most important electrokinetic phenomenon. Three types of electrophoresis are usually used, viz. microelectrophoresis, moving-boundary electrophoresis and zone electrophoresis. Electrophoresis is widely used in biochemical analysis for separation of proteins. Another very important application of electrophoresis is electrodeposition. A cylindrical microelectrophoresis cell for studying the movement of air bubbles under electric field is shown in the Fig. 1.4.3. Fig. 1.4.3 Setup for microelectrophoresis (source: A. Phianmongkhol and J. Varley, J. Coll. Int. Sci., 260, 332, 2003; reproduced by permission from Elsevier Ltd., 2003). Joint Initiative of IITs and IISc Funded by MHRD 12/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 Electroosmosis refers to the movement of the liquid of an electrolyte solution past a charged surface (e.g., a capillary tube or a porous plug) under the influence of an electric field. The pressure necessary to balance the electroosmotic flow is known as electroosmotic pressure. To understand how electroosmosis occurs, consider a glass capillary containing an aqueous electrolyte solution. The charge on the wall of the tube can develop from either the dissociation of the surface SiOH groups or adsorption of the OH ions on the wall. This charge is balanced by an equal and opposite charge in the solution, as shown in Fig. 1.4.4. Fig. 1.4.4 Electroosmosis. When the electric field is applied, the ions in the diffuse part of the double layer move towards one of the electrodes depending on their charge. The motion of these hydrated ions imparts a body-force on the liquid in the double layer. This force sets the liquid in motion. Electroosmosis has been used in many applications related to environmental pollution abatement. Suppose that the electrolyte solution is forced to pass through a capillary under pressure applied from outside. An electrical potential is generated between the ends of the capillary. This is called streaming potential. The same phenomenon can be observed when the solution is forced through a porous medium. If a dispersion of charged particles is allowed to settle, the resulting motion of the particles causes the development of a potential difference between the upper and lower parts of the dispersion. It is known as Dorn effect and the potential is known as sedimentation potential. Therefore, the situations which give rise to streaming potential and sedimentation potential are opposite to those of electroosmosis and electrophoresis, respectively. Joint Initiative of IITs and IISc Funded by MHRD 13/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 1.4.3.2.1 Zeta () potential The surface charge of the colloid particles is expressed in terms of the zeta potential. It is the potential at the surface of shear. When a particle moves in an electric field, the liquid layer immediately adjacent to the particle moves with the same velocity as the surface, i.e., the relative velocity between the particle and the fluid is zero at the surface. The boundary located at a very short distance from the surface at which the relative motion sets in is known as the surface of shear. The precise location of this surface cannot be determined exactly, but it is presumed that it is located very close to the surface of the particle, may be a few molecular-diameters apart. The magnitude of -potential provides an indication of the stability of the colloid system. The ph of the medium strongly affects the - potential. 1.4.3.2.1.1 Determination of -potential from electrophoresis Let us consider the motion of a small spherical colloid particle moving with velocity u in an electric field E. In a dilute dispersion, the mobility is given by the Hückel equation, u E 2 0, R s 0.1 (1.4.17) 3 where is the DebyeHückel parameter, is the dielectric constant of the medium, 0 is permittivity of the free space, is the viscosity of the liquid and R s is the radius of the sphere. For large values of Rs and -potential is given by Smoluchowski HelmholtzSmoluchowski equation),, the relationship between the electrophoretic mobility equation (also known as Joint Initiative of IITs and IISc Funded by MHRD 14/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 u E 0, R s 100 (1.4.18) Equation (1.4.18) is applicable for relatively high salt concentrations for which is large. Apart from these two limiting conditions, the zeta potantials for the other values of Rs can be calculated by the following equation. u 20 f Rs (1.4.19) E 3 where f R s can be calculated from either Henry s or Ohshima s equation. The latter equation is more convenient for computation. 1 f Rs 1 3 2.5 21 Rs12expRs This equation is valid for any value of than 1%. (1.4.20) Rs with maximum relative error less 1.4.3.2.1.2 Determination of -potential from electroosmosis When an electric field is applied across a capillary containing electrolyte solution, the double layer ions begin to migrate. After some time, a steady state is reached when the electrical and viscous forces balance each other, i.e., the force exerted on the medium by the ions is balanced by the force exerted by the medium on the ions. If the steady state volumetric flow rate in a fine capillary due to electroosmosis is V, then the -potential is given by, V, R c 1 (1.4.21) 0EA where R c is the radius and A is the cross-sectional area of the capillary. Therefore, by measuring the volumetric flow rate through the capillary, the - potential can be determined. Joint Initiative of IITs and IISc Funded by MHRD 15/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 Example 1.4.2: An aqueous solution of sodium chloride is placed inside a capillary in an electroosmosis apparatus and subjected to an electric field of 100 V/m. The electroosmotic velocity in the capillary is observed to be 10 m/s. Calculate -potential from these data. Solution: The electroosmotic velocity is given by, V 6 10 10 m/s A For water, 78.5, 3 1 10 Pa s, and the permittivity of free space is, 12 0 8.854 10 C 2 J 1 m 1. From Eq. (1.4.21) we obtain, 6 3 V 1010 110 0.1439 12 0EA 78.58.85410 100 V = 143.9 mv 1.4.3.2.1.3 Determination of -potential from streaming potential The -potential can be correlated with streaming potential as follows. A pressure difference p across a capillary is applied which sets the liquid in motion inside it. The charge of the double layer moves with the surrounding liquid generating an electric current, which is known as the streaming current. On the other hand, the charge transferred downstream generates an electric field in the opposite direction. After a short time, the two currents due to pressure gradient and reverse electric field balance each other. The streaming potential E s is the potential drop associated with this electric field. The following equation gives the -potential. ks Es 0 p, R c 1 (1.4.22) where k s is the conductivity of the electrolyte solution. Equation (1.4.22) is valid for the large values of Rc (where R c is the radius of the capillary). Therefore, it is likely to give erroneous results when the concentration of salt is low (which would result in a low value of ). Joint Initiative of IITs and IISc Funded by MHRD 16/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 1.4.3.2.1.4 Determination of -potential from sedimentation potential An electric field is developed during the settling of charged particles. This is known as sedimentation potential. Smoluchowski presented the first theoretical estimate of the magnitude of the field. For a dispersion of solid non-conducting spheres of radius R s, immersed in an electrolyte solution of conductivity k s, dielectric constant, and viscosity, the sedimentation potential is predicted to be, 3 40 gnr E s sed (1.4.23) 3kV s d where n is the number of particles and V d is the volume of dispersion. This equation is valid in those situations where the thickness of electrostatic double layer is small with respect to the radius of the particles R s 1. It can be observed from Eq. (1.4.23) that the sedimentation potential is proportional to the amount of the dispersed phase. The settling of fine droplets can generate high sedimentation potentials. For example, the settling of water drops in the gasoline storage tanks can produce a very high sedimentation potential owing to the low conductivity of the oil phase, which can be dangerous. The value of sedimentation potential can be as high as 1000 V/m or above, even for a moderate value of the -potential (e.g., 25 mv). If the diameter of the droplets is larger than 100 m, they settle down completely and the sedimentation potential becomes zero. If they are smaller than 1 m, the sedimentation potential gradient reduces the rate of settling and a haze of water drops floats in the electric field. The sedimentation velocity is reduced by the sedimentation potential gradient. If the steady state settling velocity (i.e., terminal velocity) of the particle is v t when the particle is uncharged, and v c is the velocity when the particle carries a surface charge, then for a single sedimenting particle, Joint Initiative of IITs and IISc Funded by MHRD 17/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 2 1 1 0 vc v t ks Rs, R s 1 (1.4.24) The experimental data agree with this equation within an order of magnitude. If Rs 0.1 m, 25 mv and 4 k s 110 1 m 1, it can be shown that the velocity of the particle in water will be reduced by 30%. This causes a haze of fine water drops in oil. 1.4.3.2.1.5 A comment on the -potential determined by various methods The -potential depends only on the properties of the phases in contact. Therefore, its value must be independent of the experimental method employed for its determination. Several scientists, using the Smoluchowski theory, have found that -potential obtained from streaming potential or electroosmotic measurements is quite smaller than the value obtained from electrophoretic mobility or sedimentation potential measurements. These discrepancies can be due to the influence of surface conductivity on streaming potential and electroosmotic flow. If the condition R 1 (where R is the radius of sphere, or the radius of capillary) is not satisfied, then the Smoluchowski equation can yield inaccurate values of the -potential. Two conditions must be satisfied to justify comparison between the values of - potential obtained by different electrokinetic experiments: the effect of surface conductivity must be taken care of, unless it is negligible, and the surface of shear must divide comparable double layers in all the cases. Fulfillment of the second requirement depends upon the experimental procedure. For example, if the same capillary is used for electroosmosis and streaming potential studies, the second condition can be satisfied. On the other hand, the surfaces of a capillary and a migrating particle can be quite different. Sometimes, the surfaces are coated with a protein, and the characteristics of both surfaces are governed by the adsorbed protein. Joint Initiative of IITs and IISc Funded by MHRD 18/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 1.4.3.2.2 Reciprocal relationship The cause and effect are interchanged in streaming potential and electroosmosis. Similarly, the situation in sedimentation potential is the opposite of electrophoresis. From c s, we can write, V 0 Ic ks (1.4.25) From Eqs. (1.4.22) and (1.4.25) we can write, Es V 0 p Ic ks (1.4.26) The coupling of two different electrokinetic ratios, i.e., Es p and V I c through Eq. (1.4.26) is an example of the law of reciprocity of Lars Onsager. Eq. (1.4.21), putting E I Ak Joint Initiative of IITs and IISc Funded by MHRD 19/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 Exercise Exercise 1.4.1: Calculate the sedimentation potential for 50 m radius water drops in an oil (dielectric constant = 2, viscosity = 0.5 mpa s, and conductivity = 110 9 1 m 1 ) if the -potential is 0.025 V. The density of the oil is 700 kg/m 3 and the volume fraction of the dispersed aqueous phase is 0.05. Exercise 1.4.2: Calculate the electrophoretic mobility of a 50 nm diameter spherical colloid particle in an aqueous solution of NaCl at 298 K. The -potential is 0.02 V. The concentration of NaCl in the solution is 100 mol/m 3. Given: The Debye length is ~1 nm at this concentration of the salt. Exercise 1.4.3: Application of 101.325 kpa pressure produces a streaming potential of 0.4 V in an experiment using aqueous NaCl solution. Calculate the -potential. Given: k s 1 1 0.01 m and 1 mpa s. Exercise 1.4.4: Answer the following questions clearly. a. What are the advantages and limitations of transmission electron microscopy in the characterization of colloidal materials? b. What are the different types of signals produced in a scanning electron microscope? c. Explain how dynamic light scattering can be used to measure the size of a colloid particle. d. For what types of colloids does the dynamic light scattering have advantage over TEM or SEM? e. What are the advantages of neutron scattering over visible light scattering? What is small-angle neutron scattering? Where is it used? f. Explain what you understand by electrokinetic phenomena. g. What are the four major electrokinetic phenomena? h. Explain electrophoresis. What are the major uses of electrophoresis? Joint Initiative of IITs and IISc Funded by MHRD 20/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 i. What is zeta potential? Explain its significance. j. Discuss the applicability criteria of Hückel and Smoluchowski equations. k. What is electroosmosis? How does it differ from osmosis? What is electroosmotic pressure? l. Explain how you would calculate the zeta potential from electroosmosis. m. What is streaming potential? How is it developed? n. What is sedimentation potential? o. Explain Onsager s reciprocal relationship. Joint Initiative of IITs and IISc Funded by MHRD 21/22

NPTEL Chemical Engineering Interfacial Engineering Module 1: Lecture 4 Suggested reading Textbooks D. J. Shaw, Introduction to Colloid and Surface Chemistry, Butterworth- Heinemann, Oxford, 1992, Chapters 3 & 7. J. C. Berg, An Introduction to Interfaces and Colloids: The Bridge to Nanoscience, World Scientific, Singapore, 2010, Chapters 5 & 6. P. C. Hiemenz and R. Rajagopalan, Principles of Colloid and Surface Chemistry, Marcel Dekker, New York, 1997, Chapters 5 & 12. P. Ghosh, Colloid and Interface Science, PHI Learning, New Delhi, 2009, Chapter 2. Reference books D. F. Evans and H. Wennerström, The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet, Wiley-VCH, New York, 1994, Chapters 4 & 8. R. J. Hunter, Foundations of Colloid Science, Oxford University Press, New York, 2005, Chapters 5 & 8. Journal articles A. Phianmongkhol and J. Varley, J. Colloid Interface Sci., 260, 332 (2003). E. W. Anacker, J. Colloid Sci., 8, 402 (1953). F. Booth, J. Chem. Phys., 22, 1956 (1954). H. Ohshima, J. Colloid Interface Sci., 168, 269 (1994). H. V. Tartar and A. L. M. Lelong, J. Phys. Chem., 59, 1185 (1955). J. B. Peace and G. A. H. Elton, J. Chem. Soc., 2186 (1960). L. Onsager, Phys. Rev., 37, 405 (1931). Joint Initiative of IITs and IISc Funded by MHRD 22/22