The anionic surfactant sodium dodecyl sulphate (SDS) is often chosen for use as a model surfactant despite the difficulty in obtaining the material in high purity. So called pure commercial samples of SDS are contaminated with dodecanol, which has been observed to have a dramatic effect on the properties of SDS solutions66, 78, 144, 158, 159. Even after purification SDS hydrolyzes producing dodecanol, meaning that SDS will decompose over time and self contaminate.
6.2.2.1 Purification
In order to trust any surface or colloid measurements involving SDS it is necessary to thoroughly purify the SDS. Here the purification process used by Casson and Bain66 was followed. The details of this process are described in §2.1.1.2. In addition, like Casson and Bain, after purification the solid SDS was stored in a freezer to minimize hydrolysis. Solutions prepared from this purified SDS were used as quickly as reasonably possible in the experiments to minimize the quantity of dodecanol contaminant present in the solutions.
6.2.2.2 Surface Tension Measurements
Historically the test to determine that SDS is free of contamination has been to measure the surface tension of SDS solutions over a range of concentrations near the CMC.66, 78,
158 Details of the surface tension instrument used in the this work are described in §2.4.
Surface tension measurements performed using purified SDS solutions are shown in Figure 6.19.
Figure 6.19: Surface tension measurements of aqueous SDS at neutral pH.
These measurements show that the surface tension of the SDS solutions decreases as the concentration of SDS is increased up until 7.3 mM. Above this concentration, the surface tension remains constant despite changes in the SDS concentration. A minimum in the surface tension, which is evidence that the SDS is contaminated, is not observed.
6.2.2.3 Adsorption to a Titania Surface
Having purified the SDS, the adsorption to a titania surface was measured using OR. An example of this type of measurement for a 3.0 mM and a 7.3 mM solution at pH 4 is shown in Figure 6.20. 70 60 50 40 Su rf ac e T en si on ( N /m 2 ) 10 8 6 4 2 0 Concentration (mM)
Figure 6.20: Optical reflectometry measurements of the change in surface excess of a titania surface exposed to 3.0 mM SDS solution at pH 4 followed by 7.3 mM solution. After washing the 3.0 mM SDS solution from the surface, the surface excess does not return to baseline. After exposure to the 7.3 mM solution the base line is restored after the wash off phase.
In the first stage of the experiment, a baseline was established. Then at ~4 min 3.0 mM SDS was introduced into the OR cell. The adsorbed SDS was then washed off the surface at ~11 min however the surface excess does not return to the baseline. Repeats of the wash-on/wash-off cycle at 23 min and 42 min also fail to return to the baseline established before SDS was introduced into the cell. At ~62 min the 7.3 mM SDS solution is introduced to the cell then washed-off at 68 min. After exposing the surface to a concentrated SDS solution (above the CMC) the surface excess returns to zero. Subsequent exposure to the SDS solution with a concentration above the CMC also results in a return to the baseline after the wash off. This result indicates that the SDS is contaminated despite the purification process and that this contamination is solubilisied by micelles of SDS.
This is curious as the surface tension measurements indicate the SDS is clean. After cleaning the SDS a second time using the same method the same result was obtained.
0.8 0.6 0.4 0.2 0.0 Su rf ra ce E xc es s (mg /m 2 ) 80 60 40 20 0 Time (min)
that the SDS is contaminated. The conclusion is that the OR instrument is much more sensitive to trace levels of contamination. Since it was not possible to determine the surface excess of SDS on titania surfaces due to the presence of contamination and we were unable to purify the surfactant effectively, this area of study not was explored further.
6.3
Summary
The adsorption of CTAB to titanium dioxide surfaces prepared using Atomic Layer Deposition and the effect of CTAB on the surface forces were examined. Adsorption isotherms were measured in electrolyte concentrations of 10!!!M and 10!!!M NaBr. In both cases CTAB strongly adsorbs to the titania surface at high pH. Below the IEP, where CTAB and the surface have the same charge, there is a significant adsorption of CTAB to the surface. Slow adsorption kinetics of CTAB to titania surfaces were also observed, demonstrating that slow adsorption is not unique to the silica surface. Surface force measurements between titania surfaces with adsorbed CTAB were performed. In general the forces could be consistently described within the DLVO paradigm but only if a surprisingly low value of the Hamaker constant was employed (3 zJ). It is unclear why a larger dispersion force is not observed. At pH values >2 and high surfactant concentrations the forces at <6 nm separation reflected the presence of surfactant layers on the surface, which were displaced at high force.
Attempts to measure the adsorption of SDS to the titania surface were hampered by the presence of contamination. After an extensive purification process, the surface tension measurements indicated that the SDS was free of contaminants. However OR measurements revealed that the SDS still had trace amounts of contaminant which made accurate determination of the surface excess of SDS at the titania surface impossible.