Interfacial tension, measurement, effect of surfactant on oil/water interface

Transcription

1 Interfacial tension, measurement, effect of surfactant on oil/water interface

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4 infusion mounting, solutions prepared from Na-oleate (M = 304,5), NaDS (Na dodecyl sulfate, M=288,4), NaDBS (Na dodecyl benzene sulphonate M=348,5) Prepare 500 ml stock solution from one of the surfactants. Its concentration should be 10 mmol/l, if NaDS was selected, while 5 mmol/l, if Na-oleate or NaDBS was chosen. Make dilution series by repeated double dilution of stock solution in such a way that measure 200 ml of stock solution into the 500-ml graduated cylinder, dilute it by 200 ml distilled water, homogenize it and pour 200 ml into a 200-ml beaker; than dilute the 200 ml remained in cylinder again by 200 ml distilled water, homogenize it and pour 200 ml into an other 200-ml beaker. Repeat this process once again. Pour 200 ml stock solution and distilled water into two pieces of 200-ml beaker, respectively. Fix the Donnan s pipette cleaned and dried, previously,

5 infusion mounting twice for oil water interface., which disturb repetition of drop counting. the logarithm of surfactant Summarize the measured and calculated data in a Table: Surfactant concentration c, mol/l log c Number of drops n 1 Number of drops n 2 Average drop numbers Interfacial tension γ ow, mn/m (F)

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14 20x20 cm glass plate CaCl 2 Measure one of the oil/aqueous soap solutions with volume quotients 15/35, 20/30 or 25/25 given in ml/ml into a 100-ml shaking cylinder. Shake it strongly for half a minute. Spill a small portion of emulsion on the glass plate and determine the type of emulsion by means of colouring test (see below). Calculate how many milliliters of 0.1 mol/l CaCl 2 solution are required to form Ca-soap from Nasoap (either Na-stearate (M = 306,5) or Na-oleate (M = 304,5)) in principle. Consider the following stoichiometric equation: 2 R-COO - Na + + CaCl 2 W (R-COO) 2 Ca + 2 NaCl Titrate the content of shaking cylinder with the portions first 1 ml then 0.5 ml of 0.1 mol/l CaCl 2 solution and shake the cylinder powerfully after each. Follow with attention the change of emulsion type. Watch the colour and viscosity of emulsion and the separated phases (lower part is milky, upper part is oily). Continue the titration until the emulsion becomes homogeneous, greasy and viscous system.

15 Record the observations (foaming, colour, density, flowability, number of phases) taking place during titration, and the results of colouring test, as well the types of emulsions before and after titration). Compare and explain the calculated amount of CaCl 2 solution with that required to phase inversion in practice. (Consider that soap solutions were prepared from chemicals of technical quality.) test-tube with stopper Materials: toluene, o-xylene, penthanol (iso or normal isomer), Na-oleate, hexadecyl- pyridinechloride (HDPCl) Instruction: Make one of the w/o microemulsions: 1) Either measure a given amount (between 0.5 and 1 g) of Na-oleate into a test-tube with stopper and pipette a given volume between 1 and 3 ml of distilled water and 5 ml of toluene or o-xylene into it, then try to homogenize the heterogeneous system with strong shaking. 2) Or measure a given amount (between 0.25 and 1 g) of HDPCl into a test-tube with stopper and pipette a given volume between 0.5 and 1.5 ml of distilled water and 5 ml of either toluene or o-xylene into it, then try to homogenize the heterogeneous system with strong shaking. After shaking, titrate the content of test-tube with penthanol first, then distilled water in the following way: i) Titrate with the 0.2 ml portions of penthanol to the formation of microemulsion, which is indicated by the sudden clarification of liquid. Homogenize the test-tube after each portion! Check the presence of small droplets with laser-pointer! Record the observation and the volume of penthanol required to forming microemulsion (V penthanol ). ii) Titrate with the ml portions of distilled water to the end of microemulsion, which is indicated by formation of either milky emulsion or translucent jelly like material, continue titration of the latter to formation o/w microemulsion. Record the volumes of water (V water ) belonging to the emulsion, gel and microemulsion! iii) Then titrate again with penthanol to the formation of microemulsion again. Record the volume of penthanol (V penthanol )! Observe and record the changes in the properties of test-tube contents such as appearance (homogeneous/inhomogeneous), optical feature (clear/turbid), flowability (thin/thick). Calculate the volume fraction of penthanol (φ penthanol = V penthanol /V total liquid ) in both w/o microemulsions, microemulsion gel phase and o/w microemulsion.

16 Table 3. Comparison of the properties of micro- and macroemulsions PROPERTIES MICROEMULSION MACROEMULSION Formation spontaneous, without work investment with work investment Stability Interfacial tension Stabilizing agent Visual observation Type Phase inversion (change in type) in real thermodynamical equilibrium state γ ow ~ 0 surfactant and co-surfactant transparent, translucent o/w and w/o types bicontinuous (preparation of o/w type is more difficult) It can take place through liquid crystal gel state, but only w/o 6 o/w non-equilibrium systems, only kinetic permanency exists, creaming, coalescence of droplets takes place. γ ow > 1 mn/m amphiphilic material (surfactant, macromolecule, fine powder) milky, turbid o/w and w/o types (preparation of o/w type is more difficult), o/w/o and w/o/w multiple emulsions can be also prepared It can take place through destabilization in general. w/o W o/w Questions: 1. Define the interfacial tension between oil and water phases! 2. Explain the orientation of surface active molecules at oil/water interface! 3. How can you determine interfacial tension of an aqueous solution, if you know the interfacial tension of water and you have a Donnan pipette? 4. What is emulsion? And what is it made of? And how to prepare? 5. Explain the different stabilization possibilities of emulsions! 6. How does the HLB value of emulsifiers influence the type of emulsion formed in the presence of the given emulsifier? 7. Explain the phase inversion of emulsion! 8. Write the reaction equation of Na-soap with CaCl 2! 9. How can you determine the type of emulsions? Explain one of the methods! 10. What are multiple emulsions? And how to prepare them? 11. How do microemulsions form? And what are they made of? 12. Explain the most important properties of microemulsions? 13. Explain the phase inversion of microemulsion! 14. Compare the formation of macro- and microemulsions! 15. Compare the appearance of macro- and microemulsions!

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