Langmuir 1994,10,3485-3487
3485
Surface Chemistry in Monoethylene Glycol: Aggregation Behavior of Sodium Dodecyl Sulfate P. D. Sawant, B. K. Mishra, and C. Manohar* Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400085,India Received January 31, 1994. I n Final Form: June 23, 1994@ Aggregation behavior of sodium dodecyl sulfate in water-monoethylene glycol (MEG) mixtures is investigated by surface tension, electrical conductivity, light scattering, and fluorescence techniques. It is concluded that loose micelle-like aggregates, with a large amount of solvent penetration, are formed beyond 65%of MEG.
Introduction Monoethylene glycol (MEG)is an industrially important solvent with many applications. While the largest use is in the automobile industry as a n antifreeze, i t is also used in paints, inks, etc., and manufacture of polyester fibers.l In these applications MEG is mixed with surfactants, polymers, pigments, and other materials. The complex interaction between these constituents is of great interest to industries and the surface chemistry would be challenging to investigate. But unfortunately MEG has not been a favored solvent in research to the same extent as other solvents like water or alcohol. In this paper we have undertaken to investigate the surfactant aggregation behavior using sodium dodecyl sulfate (SDS) as the surfactant. MEG is a solvent completely miscible with water in all proportions and has a dielectric constant of 38.66. This enables one to tune the polarity of the medium continuously by mixing water and MEG and study the adsorption, aggregation, and other phenomena of surfactants in these systems. Some reports of such investigations have begun to appear recently in l i t e r a t ~ r e . ~ . ~ Materials and Methods We have used pyrene and SDS from Fluka Chemie AG Puriss grade and MEG from Qualigens Fine Chemicals, L.R.grade. The fluorescence spectrum of pyrene was monitored in the perpendicular geometry using a fluorescence spectrometer, Hitachi Model 4010. The excitation wavelength was chosen to be 336 nm. Conductivity was measured by conductivity meter; manufactured by Systronics. Surfacetension was measured by a Hardson tensiometer. Dynamic light scattering experiments were done using a Brookhaven InstrumentsModel BI-90particle sizer. Results Figure 1 shows the surface tension measured as the concentration of SDS is increased in MEG. It can be seen that there is a slight reduction in surface tension initially and the value remains constant as the surface appears to get saturated around 80 mM. Normally this is a n indication of the fact that micellization starts. It can be seen that the reduction in surface tension is not much as in the case of water. @
Abstract published inAdvame ACSAbstracts,August 15,1994.
(1)Glycols; Curme, G. O., Johnston, F., Eds.; Reinhold Publishing Corp.: New York, 1952. (2)Garibi, H.; Palepu, R.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. Langmuir 1992,8,782.Zha, R.; Ahluwalia, J. C. J.Phys. Chem. 1991, 95,7782. (3)Sjoberg, M.;Hendrikson, U.; Warnheim, T. Langmuir 1990,6, 1205. Sjoberg, M.;Jansson, M.; Henrikson, U. Langmuir 1992,8,409.
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Figure 1. Surface tension of SDS in MEG. Note the flattening off of the surfacetension beyond 80 m M indicatingthe formation of aggregates. The electrical conductivity of SDS solutions in MEG showed that the values were very low and with the increase in the concentration the conductivity showed almost a linear rise with a very sight change of slope around 80 mM. The low conductivity is indicative of the low dissociation constant of the surfactant. Since the MEG was miscible with water in all proportions, we thought that it would be possible to tune the polarity of the medium continuously and the conductivity experiments were performed varying the relative ratios of MEG/water. Figure 2 shows the set of results for 0,40,60,70,85 and 100% MEG. Both the lowering of the conductivity and the reduction in the change of slope are dramatically seen in Figure 2 indicating the smooth transition in the polarity of the medium. If one takes the concentration a t which the rise in the conductivity changes slope as the critical micelle concentration (cmc), then this dependence of cmc with content of MEG is shown in Figure 3. The linear variation of the log of cmc has also been seen in a cetyltrimethylammonium bromide (CTAB) ~ y s t e m . ~ The polarity of a medium can also be measured using the pyrene as a fluorescent probe. It has been shown that the fluorescence spectrum of the pyrene consists of five peaks and that the ratio of third peak to first peak shows a linear variation with the polarity of the m e d i ~ m This .~ property has also been used to show the solubilization of pyrene inside a SDS micelle. A pyrene molecule in water (4)Kalyanasundaram, K.; Thomas, J. K. J.Am. Chem. SOC.1977, 99,2039.
0 1994 American Chemical Society
Sawant et al.
3486 Langmuir, Vol. 10,No.10,1994
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is always surrounded by water molecules and senses the polarity of water. On adding SDS to the system, and increasing the concentration beyond the cmc, the pyrene gets transferred from water to interior of the micelle. Since the polarity inside the micelle is more like that of a hydrocarbon a t the center of the micelle, the ratio of the intensities changes smoothly from that of water to that of a h y d r ~ c a r b o n . ~ Figure 4 shows the intensity ratio of the third peak to the first peak in the fluorescence intensity of pyrene as the MEG content is increased. The SDS concentration was fixed a t 150 mM and the pyrene concentration was fixed at 1.5 pM. At this concentration of pyrene there was no self-quenching and no excimer band. The drastic change in the ratio of intensities between 60 and 80% MEG is to be noted.
Discussions A slight decrease in the surface tension observed in Figure 1 indicates that there is a poor adsorption of the surfactant a t the interface compared to that in water. This seems to suggest tha the lyophobic effect in MEG is very much less compared to hydrophobic effect. This is also reflected in the fact that the saturation of the surface tension occurs around 80 mM a s compared to 10 mM for SDS in water. However, the occurrence of saturation around 80 mM indicates that micelle-like aggregates-
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Figure 4. Ratio of intensities of III/I lines in the fluorescent spectra of pyrene in MEG/water mixtures. Note the sudden increase in the local polarity sensed by pyrene indicating the solvent penetration into the micelle beyond 65 MEG.
whose nature we will speculate below-are formed beyond this concentration. The variations of the conductivity with concentration of SDS for various combinations of water and MEG are shown in Figure 2. This figure demonstrates several interesting consequences. The first one is the dramatic reduction in the conductivity, as pointed earlier, with increase in the percentage of MEG in water. This shows that the dissociation of SDS decreases dramatically with the increase in MEG percentage. Therefore one should expect that the micelle, if and when formed, would not have a high surface charge. This is consistent with the observation in Figure 2 that change in the slope a t cmc reduces as the MEG content increases. A high surface charge of the micelle would have caused the condensation of the counterions on the micellar surface making them unavailable for electrical conduction. The increase in the cmc as monitored by the change in the slope ofconductivity is again consistent with the weak surface activity of SDS in MEG. These results seem to suggest that the micelle formed would be a loosely aggregated structure in the MEG because of the reduced lyophobic effect and considerable solvent penetration should exist in these micelles. Similar conclusions have been arrived at in ref 3 on CTAB micelles in MEG and other similar solvents. Figure 4 suggests that as the concentration of MEG increases, the polarity remains constant initially, but around 65% MEG the polarity starts increasing rapidly and the change is nearly complete around 80%. The value of around 0.95 for the intensity ratio, observed below 65% MEG, is characteristic of aromatic solvents and indicates a local dielectric constant of about 4. This value is consistent with similar studies in aqueous media4 and indicates that the pyrene probe resides inside a fairly well defined micelle whose inner portion is sufficiently hydrophobic. Around 65%MEG the conductivity still shows a fairly sharp break a t cmc and the micelles still seem to have a sufficiently well defined hydrophobic core. About 65%MEG there seems a sudden influx of the solvent into the micelle and the local polarity increases rapidly to a local dielectric constant of about 33 (similar to methanol). This picture is consistent with the conductivity results. Attempts to perform dynamic light scattering studies above 65%MEG failed because of the very low intensity of the scattered light. This seemed consistent with the solvent penetration in the micelle proposed above. Under the conditions of solvent penetration in micelle, the
Surface Chemistry in MEG difference between the refractive indicies of the solvent and the micelle is expected to be small and hence a low intensity of the scattered light.
Conclusions The investigation of SDS behavior in water-MEG mixtures leads to the following conclusions: (a) The polar head of surfactant shows a decreased dissociation as the concentration of MEG in water is increased. (b) The surface activity of SDS is considerably reduced in MEG giving an indication that surfactants are less effective in their action.
Langmuir, Vol. 10, No. 10, 1994 3487
(c) Micelle-likeaggregates are formed with a low surface charge. (d) The solvent MEG seems to penetrate inside the micelle around 65% MEG and above formingmicelles with loosely aggregated structures. The above conclusionsare consistent with those arrived by Sjoberg et al.3 using the NMR and other techniques complementary to those used by us.
Acknowledgment. We are grateful to Mr. C. Subba Rao for pointing out the importance of MEG in industries and for discussions.
