Phase behavioral changes in SDS association structures induced by cationic hydrotropes

2.1 Materials

Sodium dodecyl sulfate (SDS) 99%, tetra ethyl ammonium chloride >98%, tetra butyl ammonium bromide >98% and heptane >99%, were all obtained from Sigma.

2.2 Methods

3.1 Heptane/water/SDS

Ternary phase diagrams for SDS and water at different temperatures showed liquid crystal formation above 30 °C (Kekicheff et al., 1989). Heptane/water/SDS phase behavior was determined at 25 °C. The micelles region forms with maximum solubility of heptane oil at 8 wt.% (Fig. 1).

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Figure 1

Ternary phase diagram for water/heptane/SDS.

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Micelles formation refers to the hydrophilicity of ${\text{SO}}_4^{-2}$ groups that are in contact with water molecules, while the absence of inverse micelles region is attributed to the long hydrophobic chain tails of SDS, which are packed in a way that does not allow heptane oil molecules to penetrate through.

3.2 Heptane/water/(SDS: TEACl) system

When TEACl cationic hydrotrope was added to SDS, as a co-surfactant, two general effects were noticed. First: the heptane oil solubility in the micelles region increased to 10 wt.%, when the SDS: TEACl molar ratio was 2:1 (Fig. 2) compared to 30 wt.% when the SDS: TEACl ratio was 1:1 (Fig. 3). Second, the micelles region increased in size by shifting more and more toward the mixed-surfactants corner in the cases 2:1 and 1:1 M ratios respectively compared to SDS alone (Fig. 4).

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Figure 2

Phase diagram for water/heptane/(SDS/TEACl) 2:1 M ratio.

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Figure 3

Phase diagram for water/heptane/(SDS/TEACl) 1:1 M ratio.

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Figure 4

Combined phase diagrams of Figs. 1–3.

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The first effect, the increase in oil solubility is due to the increase in spacing between the tails which permits oil to penetrate through these chains.

The second effect, the shift in size of the micelles region, toward the mixed-surfactant corner, means that the cationic hydrotrope increased the solubility of the anionic surfactant in water by increasing the hydrophilicity on head groups. It is clear that increasing the fraction of cationic molecules amongst head groups increases both the hydrophilicity and hydrophobicity of the system.

An additional effect was noticed by increasing the mole fraction of cationic hydrotrope, where the inverse micelles region appeared near the oil corner (Fig. 3) .The micelle and inverse micelle regions were characterized by conductivity measurements at 20 °C.

The results (Fig. 5) showed that in each system the conductivity increased dramatically as the mole fraction of mixed-surfactants increased. On the other hand comparing the conductivities of micelles region (Fig. 5A) with those of the inverse micelles region (Fig. 5B), the former showed conductivity that is 500 orders of magnitude larger than the later. This is because in the inverse micelles region the ionic head groups are oriented inward and surrounded by the oil phase which leads to a large decrease in conductivity.

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Figure 5

Conductivity measurements for (A) micelles and (B) inverse micelles.

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3.3 Heptane/water/(SDS: TBAB) system (1:1 M ratio SDS: TBAB)

The phase diagram for this system at 25 °C showed a larger oil solubility (17 wt.%) (Fig. 6B) compared with SDS alone (8 wt.%) (Fig. 1).

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Figure 6

Phase behavior for (A) SDS/TEACl (1:1 M ratio) and (B) SDS/TBAB (1:1 M ratio) with heptane and water.

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When TBAB cationic hydrotrope was used instead of TEACl, as a co-surfactant with SDS/TBAB (in 1:1 M ratio) the oil solubility decreased to 17 wt.% compared with 30 wt.% for SDS: TEACl (1:1) (Fig. 6).

This decrease in solubility is due to the increase in chain length of the hydrotrope (four carbon atoms in TBAB compared with two carbon atoms in TEACl) making the system more hydrophobic, with less penetration of oil between the hydrophobic chains where TBAB chains occupy some area, hence decreasing the oil solubility in the micelles region.

3.4 Middle phase microemulsion formation

Middle phase microemulsion was formulated at high mixed-surfactants concentration and by adding oil without alcohol or salt addition (Fig. 7).

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Figure 7

Alcohol and NaCl free middle phase microemulsion formed by increasing surfactant concentration.

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The phase diagram (Fig. 6B) shows that at mixed-surfactants in the water range of 35–50 wt.% the addition of oil leads to the formation of Winsor III middle phase microemulsion (ME), without adding salt or alcohol. This ME formation was achieved by increasing the concentration of the cationic TBAB hydrotrope, which works as a co-surfactant and as a salt instead of NaCl salt, and lowers the IFT value to form the middle phase region, that increased with increasing the mixed surfactant concentration (Fig. 7).

3.5 Salinity scans and IFT of SDS/TBAB (2:1 M ratio) system

In this system, salinity scans for 1 wt.% (SDS/TBAB) (2:1 M ratio), in water and heptane (WOR = 1), at 25 °C show Winsor I,III,II sequence (middle phase ME formation) at 8.5 wt.% NaCl (Fig. 8), and equilibrium was reached after 24 h, whereas no middle phase was seen without TBAB, at the same salt concentration. Comparing with previous work, where (Sassen et al., 1989) showed that SDS/water/heptane formed ME when salinity was in the range 4–16 wt.%, by adding alcohol (1-butanol) with (SDS/alcohol) concentration in the range 2–24 wt.%.The solubilization ratio and interfacial tension IFT, at the middle phase can be predicted using Chun Huh equation (Huh, 1979) ${\text{IFT}}_{\text{mo}}=C/{(V_{\text{o}}/V_{\text{s}})}^2\text{,}{\text{IFT}}_{\text{mw}}=C/{(V_{\text{w}}/V_{\text{s}})}^2$

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Figure 8

Inversion by salinity scan for 1% SDS/TBAB (2:1) molar ratio with heptane and WOR = 1 at 25 °C, the arrow indicates the optimal salinity at 8.5 wt.% NaCl.

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• IFT_m/o is the IFT between the middle (surfactant) phase and oil phase.

• IFT_m/w is the IFT between the middle (surfactant) phase and the water phase.

• V_o/V_s is the ratio of the volume of solubilized oil to the volume of total surfactant.

• w/V_s is the ratio of the volume of solubilized water to the volume of total surfactant.

• C is a constant equals 0.3 mN/m.

The results presented in Table 1 show that IFT_m/o falls down with increasing salinity, while IFT_m/w rises up with increasing salinity. Plotting these results (Fig. 9) shows the lower IFT at 0.0062 mN/m, at which the middle phase is formed.

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Figure 9

Diagram represents salt scan versus IFT for SDS/TBAB (2:1) molar ratio.

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The solubility values V_o/V_s showed a decrease with increasing IFT values between each of oil and water with the surfactant phase (Table 1). Plotting these results (Fig. 10) shows that the optimum conditions of high solubility parameters and lower IFT, at the middle phase equal 6.8 mL/g, which correspond to IFT value of 0.0062 mN/m, and we suppose that all surfactants have moved to the middle phase, so the concentration of surfactant in the middle phase is 12 wt.%, which is a good indication of the low quantity of surfactant used to extract a large quantity of oil by water and surfactant and with ultralow IFT.

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Figure 10

Diagram represents salt scan versus solubility for SDS/TBAB (2:1) molar ratio.

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3.6 Salinity scans and interfacial tension IFT of SDS/TBAB (1:1 M ratio)

Phase behavior at ambient temperature of salinity scans containing 8 wt.% mixed surfactants (SDS/TBAB 1:1 M ratio), with equal weight ratios of water and oil (WOR = 1) was observed, in these scans, conventional Winsor I, III, II microemulsion sequence had appeared (Fig. 11), with optimal salinity at 4 wt.%, the equilibrium was reached after 24 h.

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Figure 11

Inversion by salinity scan for 8% SDS/TBAB (1:1) molar ratio with heptane WOR = 1 at 20 °C, the arrow indicates the optimal salinity at 4 wt.% NaCl.

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Solubilization ratio measurements for this system and IFT calculations using Chun Huh equation are shown in Table 2.

Plots of IFTm/w versus salinity, NaCl wt.% in Fig. 12, and plots of solubilization parameters versus salinity NaCl wt.% in Fig. 13, are used to detect the optimum conditions (high solubility parameters SP with lower IFT) at the middle phase equaling 1.8 ml/g, which correspond to IFT = 0.08 mN/m. Comparing the SDS/TBAB systems (2:1) with (1:1) molar ratios, it is noticed that both the optimum salinity S, and the solubilization parameter SP decrease (from 8.5 to 4 wt.%, and from 6.8 to 1.8 ml/g respectively), and that the minimum IFT increases (from 0.0065 to 0.08 mN/m) upon increasing the fraction of cationic hydrotrope (from 1:2 to 1:1 M ratio of TBAB/SDS).

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Figure 12

Diagram represents salt scan versus IFT for SDS/TBAB (1:1) molar ratio, at 20 °C.

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Figure 13

Diagram represents salt scan versus solubility for SDS/TBAB (1:1) molar ratio, at 20 °C.

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Comparing the results of this study with those of (Doan et al., 2003) on mixed surfactants anionic sodium dodecyl sulfate and cationic dodecyl pyridinium chloride (SDS/DPCl) system without alcohol addition, where the lowest IFT values were 0.6, 0.61, and 0.9 mN/m for trichloroethane (TCE), hexane and N-hexadecane respectively. These values are 6–10 times higher than expected for the appearance of a middle-phase microemulsion typically ⩽0.1 mN/m, which indicated that middle phase microemulsions did not occur during these surfactant scans.

While comparing with Witthayapanyanon et al. (2010) results on conventional surfactant (SDS/s-butanol) and extended surfactant systems who found that both surfactant systems exhibit in their microemulsion properties an increase in optimum salinity S, with increasing the oil alkane number (ACN), and the S value (4 wt.%) we obtained for heptane (ACN = 7) in 1:1 SDS/TBAB system is consistent with the typical trend exhibited in extended surfactants.

Witthayapanyanon noticed that although the IFT property of extended surfactant decreases with an increasing oil ACN, the SP value remains relatively constant, an unusual behavior which deviates from the Chun Huh relationship (Huh, 1979) indicates that SP and IFT are inversely proportional .In this study both IFT and SP properties change with changing the SDS/TBAB molar ratio.

In the phase diagrams of this study it is noteworthy the disappearance of liquid crystals, due to the presence of the short chain cationic hydrotropes (TBAB or TEACl), compared with (Marques et al., 1993) where mixed sodium dodecyl sulfate and cationic didodecyl dimethyl ammonium bromide (SDS/DDAB) exhibited liquid crystal formation.

The temperature effect on equilibrium is observed (the clear middle phase becomes turbid) at temperatures above 35 °C in case of 2:1 SDS/TBAB, while this effect is observed above 30 °C in case of 1:1 SDS/TBAB, this indicates the sensitivity of the hydrotrope surfactant to temperature, where increasing its concentration will cause an increase in disorder on one hand, which leads the system to be more sensitive to temperature and will cause a decrease in salt concentration on the other hand.