Anionic surfactants/DTAB/water systems

The influence of the cationic counterion of the anionic surfactant on the Krafft point and vesicle stability of a catanionic mixture was first investigated in the systems SDS/DTAB and LiDS/DTAB, i.e. by replacing a sodium counterion Na⁺ by a lithium one Li⁺. The experimentally determined Krafft temperatures of the single anionic systems SDS and LiDS at 1wt% were measured to be 11.5°C and <0°C respectively. This pronounced difference in TK makes sense because the more hydrated lithium ion (15) binds less strongly to the dodecylsulfate (DS) than the sodium ion. This is reflected in the CMC (critical micellar concentration) of LiDS (16) (8.92mM at 25°C) which is higher than the cmc of SDS (8.32mM) at 25°C. The binding of Li⁺ ion with the polar head of the surfactant being weaker, the hydrophilicity of the anionic surfactant in water is dramatically increased. The question was thus to know whether this difference in binding and solubility would be reflected in the stability on lower temperatures of the catanionic systems with DTAB. Krafft temperature measurements were thus performed on these systems (Fig. 3) at different ratios from pure cationic surfactant to pure anionic surfactant. It appears that the two curves are similar for the cationic rich solutions whereas the Krafft temperatures are lower for the LiDS/DTAB mixtures when the surfactant ratio exceeds 50wt% of the anionic surfactant. For solutions having a ratio over 62wt% anionic surfactant, the difference becomes more significant since Tk<0°C for the LiDS/DTAB solutions, while Krafft points remain higher than 11°C for SDS/DTAB. This difference is directly linked with the Krafft temperature differences of the single anionic surfactants SDS and LiDS. Anyway the influence of the replacement of the counterion on the vesicle stability remains limited (Fig. 4) since the vesicle solutions precipitate for both systems at lower temperatures (5°C) and the vesicle zone is as extended for SDS/DTAB as for LiDS/DTAB. However the precipitate zone is as expected slightly more reduced for the system LiDS/DTAB.

LES differs from SDS by the presence of 2 ethylene oxide (EO) groups between the hydrocarbon chain and the sulfate polar head. This difference in structure leads to a major difference in their Krafft points (17), since TK (SDS) = 11.5°C and TK (LES) < 0°C. The presence of two EO groups enables a better hydration of the polar head of the surfactant, increasing thus strongly the surfactant’s hydrophilicity. The lower Krafft point of LES has its consequences for the LES/DTAB system. As can be seen from Fig. 3, the Krafft points of the LES/DTAB mixtures are lower than those corresponding to the SDS/DTAB systems over nearly the whole composition range. At low LES contents (0-48%), this effect is particularly pronounced with Krafft temperatures below 0°C. Surprisingly the zone of precipitation between 48 and 56% LES does not vanish even at temperatures higher than 70°C. This temperature is much higher than the Krafft temperatures of the SDS/DTAB or LiDS/DTAB mixtures in the same composition region. Between 56% and 70%, TK is lower than 20°C and above this temperature the solutions appear isotropic and blue as are solutions for anionic surfactant ratios between 32% and 47%. At last, the ratios ranging from 70% LES to pure LES give way to isotropic micellar solutions with Krafft temperatures below 0°C.

Consequently the vesicle zone for the system LES/DTAB appears much larger at all observed temperatures than in the system SDS/DTAB/water (Fig. 4, 5, 6 and 7) and can be observed in both cationic rich and anionic rich samples. An alkylethersulfate surfactant such as LES appears thus to possess the appropriate balance between chain length and hydrophilicity to enable the formation of vesicles at room temperature and even below over a wide range of cationic/anionic ratios. This property of LES over SDS could be confirmed in comparing both phase diagrams of the SDS/DTAB and LES/DTAB systems (Fig. 5) where the two-phase area composed of precipitate and isotropic solution is much reduced in the LES/DTAB system, whereas it occupies an important part of the phase diagram of the SDS/DTAB system.

Moreover, the overall vesicular zone is much larger for the LES/DTAB system than it is for

the SDS/DTAB system. Dynamic Light Scattering measurements were performed on the LES/DTAB system in the vesicular area at 0.8 wt% overall surfactant concentration for mass ratios of LES/DTAB between 30/70 and 40/60. All vesicle sizes ranged between 110 and 180nm for polydispersity indexes of about 0.4.

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Figure 3: Krafft temperatures of the systems SDS/DTAB, LiDS/DTAB, LES/DTAB at a total surfactant concentration of 1wt% at different anionic/cationic mixing ratios (in weight %).

Figure 4: Phase behaviour of the same systems at 1 wt% at 5°C, 25°C and 45°C.

I: isotropic colourless solution; D1: demixing isotropic colourless solution/white precipitate;

D2: demixing bluish solution/white precipitate; V: vesicular solution.

Figure 5: Phase diagrams of the SDS/DTAB and LES/DTAB systems at 25°C.

The phase equilibria can therefore be dramatically changed by incorporating ethylene oxide groups into the polar head group of a surfactant molecule. It is possible, for equimolar mixtures, to obtain an extended isotropic solution phase at the cost of the multiphase regions if a sufficient number of ethylene oxide groups are present in the polar head group (18).

Figure 6: Visual observations made for the system SDS/DTAB at 1wt% overall surractant concentration for SDS/DTAB mass ratios from 10/90 to 80/20.

Figure 7: Visual observations made for the system LES/DTAB at 1wt% overall surractant concentration for LES/DTAB mass ratios from 10/90 to 80/20.