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Chemistry Lab 462
01/98A Study of Surfactants by Tensammetry
Surface active substances adsorb onto the surface of a mercury drop electrode and change the capacitive (charging) current during a DPP ‘run’. Peaks in polarograms which are due to adsorption/desorption processes are referred to as tensammetric peaks.
The charging current decays exponentially with time during the lifetime of the drop, while the faradaic current decays as t-1/2 . With the PAR 174, the pulse is applied for 57 ms and the current is sampled for 16.7 ms. The current is measured before application of the voltage pulse and again at the end of the pulse.
Thus, the two current measurements are made at different potentials. This has very little effect in the absence of surfactant. However, the charge on the electrode decreases in the presence of adsorbed species.
Thus, if the potential is pulsed from a value where there is significant adsorption to a potential where desorption occurs, Dic will be relatively large.
Thus the differential pulse polarogram,in the absence of reduction/oxidation processes, is essentially a differential capacity curve.
At low concentrations, the peak height increases with concentration, i.e., as the concentration of surfactant increases, more can absorb onto the drop. But once the surface is covered, there will be no change with concentration.
Desorption peaks shift to more –ve potential with increasing surfactant concentration.
If there is a faradaic process (reduction) which occurs at a similar potential to a particular tensammetric peak, the tensammetric peak will be less well-defined.
Supporting electrolytes are thus chosen to help minimize interference from faradaic processes, e.g., PEG desorbs at such a negative potential that Na+ would be reduced at a similar potential. Thus, Li salts are used as supporting electrolytes with PEG rather than Na salts, as Li(I) is reduced at a more negative potential than Na(I) and thus interferes less.
Oxygen must be removed from the solutions or we would see its reduction peaks. However, bubbling a solution of surfactant would lead to foaming and therefore changes in concentration. Thus, the supporting electrolyte is purged with nitrogen before the addition of the surfactant. In the case of the SDS, Na2 SO3 is used in the supporting electrolyte to react with any remaining O2.
Canterford & Taylor did not have the same DME controller that we do. For them, as the drop time was increased, their drop size increased. In our system, the drop size is controlled by how long the solenoid switch stays open to let Hg through – as controlled by our choice of S, M & L drop.
The drop hangs until knocked off and for most of its lifetime is the same size.
Differential Pulse Tensammetry at the HMDE
For small concentrations of surfactant, we can allow time for adsorption onto the drop before running the voltage scan. This is also important for polymers whose diffusion to the electrode surface is slow. This requires that we use a single drop (the HMDE). This can be likened to stripping voltammetry.
n - Octanol
Experimental conditions:
Scan rate 5m Vs-1E = 25mV
Current range 10 A
Place 10 mLs of 0.1M Na Cl O4 in a sample cup and bubble with N2 for 4 mins. Then add a drop of n-octanol (through the side flap) to give a saturated solution of n-octanol. Scan from 0.2 V to – 1.5 V.
Note the base current is lower between the two peaks because the capacitive current is lower when a surface-active species is adsorbed. i.e., the charge on the electrode is lowered by the presence of adsorbed species.
Polyethylene glycol (PEG)
Polyethylene glycols of a variety of MW’s have many commercial and industrial applications. Use 10 mL 0.1 M Li2 SO4 as supporting electrolyte. Bubble 4 mins. with N2. Add 100 L of 10-3M PEG.
E 25 mV
Scan rate 5 mV/s
Scan from +0.2 V to –2.0V (peaks ~ O and –1.8 V)
SDS
Only the negative tensammetric peak (desorption) is seen. It comes at ~ -1V. The peak die to adsorption is at too positive a potential to see using a Hg electrode. The supporting electrolyte is 0.1 NaOH/0.1 M Na2 SO3.
E = 25 mV5 A current range
Scan rage 2mV/s
Prepare a 10 mM solution of SDS in water. Put 10 mL of the supporting electrolyte into the sample cup. Bubble with N2 for 4 mins. Add 10 L of 10mM SDS. What if we use different droptimes but keep the product of droptime and scan rate constant?
Run a series of concentrations, then plot peak height (current) versus the surfactant concentration.
Try using different E values for one particular concentration.
Try using different droptimes.
Try using different scan rates.
Try using different dropsizes.
Try using an HMDE with different absorption times.
Each group will not have time to do all these variations, so each group should try varying different parameters and results can then be pooled.
Discuss the effect of varying each parameter. Say what happens and suggest why. See the paper by Canterford & Taylor to help with this discussion. This gives a final cone of 10-4 M. The paper indicates we should be able to see peaks using 10–5M.