Temperature dependent phase behavior of Pseudo-ternary Thiourea X-100 + 1-hexanol (1:5 molar ratios)/oil/water systems is reported. The influence of nature of hydrocarbon oil and type of electrolytes (weak as well as strong) has been investigated on the temperature induced phase behavior of the ternary system. At surfactant concentration, Φs = 40%, a “nose shaped” microemulsion region is observed. Below one-phase microemulsion region, Lα phase appears. The presence of NaCl decreases the domain size of 1Φ micellar region whereas oxalic acid first decreases the domain below Φw < 18 and then increases above Φw > 18 in the lower boundry of the phase diagram. The critical weight fraction of water, Φwcri decreases in presence of both electrolytes. However, Φwmax increases in presence of oxalic acid and remains constant in presence of NaCl as compared to salt free system. Furthermore, when cyclohexane was replaced by a longer straight chain hydrocarbon, dodecane, the domain of the one-phase microemulsion region is tremendously increased.
Microemulsions (MEs) are optically transparent, thermodynamically stable, nano-structured mixture of oil and water stabilized by surfactant and co-surfactant [
Phase behavior and structural organization of microemulsions are known to play key roles in its industrial and technological applications. Phase behavior studies provide information on the phase boundaries of different phases as a function of composition and temperature and more important structural organization of surfactant aggregates can also be interred [
The phase behavior and structural organization of surfactant aggregates are highly dependent on the elastic properties of surfactant monolayer separating oil and water domain [
These phase behavior of a ternary system is often studied as a function of temperature and composition and the position of different phases are determined within the phase prism by erecting a vertical section of the prism at constant surfactant concentration. The phase boundaries give rise to characteristic “channel” shown in
Furthermore, nonionic microemulsions are of increasing importance in several industrial applications. They are highly used in liquid detergent formulations [
The nonionic surfactant, Thiourea X-100 (N-heksiloksifenil-N-(4-nitrobenzoil) thiourea) was obtained from Sigma Aldrich, Malaysia. Anhydrous dodecane and 1- hexanol were procured from Schuchardt, Germany. Sodium chloride (AR) was purchased from Fisher Scientific, Singapore. Oxalic acid (AR) was purchased from Sigma Aldrich Malaysia. Double distilled water was used throughout the experiment.
Phase diagram of Thiourea X-100 +1-hexanol (1:5 molar ratio)/oil/water or aqueous electrolyte systems were constructed by titration method and the phase boundaries of one-phase micellar region of the ternary systems were determined at fixed surfactant concentration, γ = 40%. A known amount of surfactant + co-surfactant and oil is taken and subsequently the mixture was titrated with water or aqueous electrolyte solution with the help of Hamilton microsyringe. The final mixture was gently stirred and then immersed in a temperature controlled water bath to equilibrate different phases at desired temperature. Characterization of the resulting isotropic onephase micellar region or anisotropic phases were done visually and through crossed polaroid microscope after allowing sufficient time (typically 4 - 6 hours) for attainment of equilibrium at a particular temperature. The entire phase diagram was mapped in this manner. The detailed procedure for the determination of the phase diagram has been described elsewhere
One of the most common and convenient method 24 to study the phase behavior and related properties of a ternary system, surfactant/oil/water is to characterize the system at the vertical section of the prism at fixed amphiphile concentration. In this respect, phase behavior of pseudo-ternary Thiourea X-100 +1-hexanol (1:5 molar ratio)/cyclohexane/water system was determined as a function of temperature, T and weight fraction of water, Φw and shown in
It is evident from the
Furthermore, it is known that the temperature dependent phase behavior of nonionic surfactant-oil-water system arises in the first place from the variation of local concentration of water in the ethylene glycol layer, which modulates the surfactant film curvature. 45 But it also originates from the solubility variation of the surfactant monolayer in the organic phase occurring as a function of temperature [12,13]. The phase diagram for the system, Thiourea X-100 +1-hexanol/cyclohexane/water plotted in
Okochi and Brimblecombe (2009) had studied temperature dependent phase behavior of C12E4/dodecane/ water and observed similar “nose-shaped” micellar region. However, critical weight fraction of water, Φwcri and maximum weight fraction of water, Φ wmax was 10 and 56 respectively [14,15]. Lα phase was also observed below one-phase microemulsion region in the lower boundary of the phase diagram. In the present studies, Φwcri is equal to ≈15, and Φwmax is equal to ≈25. By comparing this results, it is evident that the temperature dependent phase behavior of nonionic surfactant/oil/water is highly dependent on the amphiphilicity (hydrocarbon chain length) of the surfactant. If the amphiphilicity is decreased, one-phase ME region shrinks. That is what observed in the present investigations.
The influence of 0.1 M NaCl and 0.3 M oxalic acid on the temperature induced phase behavior of the system, Thiourea X-100 +1-hexanol/cyclohexane/water was investigated and shown in
That is, the presence of electrolytes contracts the temperature window of one-phase micellar region but the contraction is more in presence of strong electrolyte (NaCl) than the weak one (oxalic acid). In the lower boundary, at Φw > 10, the presence of NaCl and oxalic acid have opposite effect on the domain size of the micellar region. In the lower boundary, strong electrolyte, NaCl always decreases domain of the 1Φ microemulsion region whereas oxalic acid first decreases the domain of 1Φ micellar region below Φw < 18 and then increases the same above Φw > 18. In the upper boundary, the presence of oxalic acid expands temperature window of 1Φ micellar region above Φw >14 and the presence of NaCl increases the same at Φw > 19. Thus the presence of weak electrolyte, oxalic acid expands 1Φ micellar region more as compared to NaCl in the upper region of the phase diagram. As a result, a bigger domain 1Φ ME region is observed at higher water content.
The changes in the phase boundaries of one-phase micellar region (
the surfactant and thereby coming closure to each other. This would lead more rigid interface, smaller inter-micellar interaction, decrease in size of the core of reverse micelles (RMs) and thus less amount of water is solubilized inside the core RMs in presence of electrolytes and this results in apparent decrease in Φwcrit from 15 (salt free syatem) to 10 (in presence of electrolytes). Further, it can be seen from the
The presence of electrolyte decreases the size of RMs of Thiourea at any temperature 49 due to ion-dipole interaction or complexation between cations and polyoxyethylene chain of TX-100 that results in a closure packing of polar head groups of TX-100 and decreases the extent of solubilization of aqueous electrolyte solution inside the core of RMs. As a result, one-phase micellar region changes into two-phase region in presence of electrolytes relatively at a lower temperature as compared to salt free system. This causes an apparent decrease in upper critical temperature, Tuppercri in presence of electrolyte and the decrease in more in the presence of strong electrolyte (NaCl) than the weak one (oxalic acid). This may be due to a greater extent of iondipole interaction or complexation between the cations and polyoxyethylene chain of TX-100 in presence of the strong electrolyte (NaCl) as compared to the weak electrolyte (oxalic acid). As a result, the upper critical transition temperature, Tuppercri decreases much more in presence of strong electrolyte (NaCl) than the weak one (oxalic acid) and phase transition from one-phase micellar region to two-phase region occurs at relatively lower temperature (~50˚C) in presence of NaCl than oxalic acid (~54˚C). As a result, temperature window of one-phase micellar region becomes narrower in presence of both electrolytes as compared to salt free system.
Furthermore, it is known that the type of ME depends upon water-to-oil ratio and if Φw ≈ 25 - 70 wt%, a sponge like structure 31 is obtained which is known as bicontinuous ME in which surfactant monolayer separates into disordered, interpenetrating domains of oil and water. The size of the domains of water and oil in a bicontinuous ME is highly dependent on amphiphile concentration and water-to-oil ratio. 50 In the middle of phase diagram, at Φw > 18, the presence of electrolytes expands the temperature window of 1Φ micellar region in the upper region of the phase diagram. This may be due to the fact that the presence of electrolytes (weak or strong) makes the surfactant (TX-100) more hydrophobic. 45 That causes a better oil solubility of TX-100 in oil domain and therefore phase transition from 1Φ ME region to two-phase region takes place relatively at a higher temperature and higher water content as compared to salt free system. As a result, temperature window for one-phase micellar and Φmax increase in presence of both electrolytes. Further, at Φw > 18, the presence of oxalic acid expands the temperature window of 1Φ ME region more than NaCl and this may be due the fact that oxalic acid probably increases hydrophobicity of the TX-100 more than NaCl due to higher concentration of oxalic acid. This results an increase in the temperature window of one-phase micellar region in the middle of phase diagram.
The influence of the hydrocarbon chain length of oil on the temperature induced phase behavior of the ternary system. Thiourea X-100+1-hexanol (1:5 molar ratio)/ oil/0.1 M NaCl was investigated. Cyclohexane was replaced by a longer straight chain hydrocarbon, dodecane and domain of one-phase micellar region of Triton X- 100+1-hexanol/dodecane/0.1 M NaCl was determined and shown in
in presence of dodecane as compared to cyclohexane and is equal to 40% whereas it is 25% in presence of cyclohexane. At Φw = 5, the upper critical temperature, Tuppercri is increased and is equal to 70˚C in presence of dodecane. Thus the temperature window of one-phase micellar is very much larger in presence pf dodecane as compared to cyclohexane. In the lower boundary of the phase diagram, three is tremendous increase in domain of one-phase micellar in presence of dodecane. It is quite interesting to note that the one-phase micellar region does not touch the water axis in presence of dodecane also. It means TX-100 +1-hexanol is unable to form normal micelles or oil swollen ME near water axis in presence of oil, dodecane or cyclohexane.
The changes in phase boundary in one-phase micellar region occurs due to different nature of the oils used here in the presence studies. The presence of longer straight chain hydrocarbon, dodecane tremendously increases the domain of 1Φ micellar region as compared to cyclohexane. The critical weight fraction of water Φw cri and Φwmax are larger in presence of dodecane as compared to cyclohexane. Also Tuppercri has been inceased in presence of dodecane. As a result, the temperature window of one-phase micellar region increases. Thus the solubilization efficiency of the amphiphile, TX-100 is much more in presence of dodecane than cyclohexane and this may be attributed to higher hydrophobicity (longer hydrocarbon chain length) of dodecane. It is known that with rise in temperature, the nonionic amphiphile TX-100 becomes hydrophobic [18-20]. As a result, its solubility increases in more hydrophobic oil, dodecane than cyclohexane. Thus phase transition from one-phase micellar region to two-phase region takes place relatively at higher temperature. This causes an apparent increase in Tuppercri in presence of dodecane as compared to cyclohexane. As a result, the temperature window of one-phase micellar region becomes larger in presence of dodecane as compared to cyclohexane.
The temperature dependent phase behavior of Thiourea X-100+1-pentanol (1:5 molar ratio)/cyclohexane or dodecane/water or brine systems were investigated. At surfactant concentration, γs = 40%, a “nose-shaped” microemulsion region is observed. Below one-phase micellar region liquid crystalline, Lα appears. The one-phase micellar region does not touch water axis. At Φw ≈ 5, the temperature window for one-phase micellar region is from below 0˚C - 50˚C in presence of NaCl, below 0˚C - 54˚C in presence of oxalic acid, and form below 0˚C - 60˚C for salt free system in presence of cyclohexane. That is, the presence of electrolytes contracts the domain of Φw micellar region but the contraction is more in presence of strong electrolyte (NaCl) than the weak one (oxalic acid). The presence of NaCl and oxalic acid has opposite effect on the one-phase micellar region. In the lower boundary of the phase diagram, presence of strong electrolyte, NaCl always decreases the domain of 1Φ micellar region whereas oxalic acid first decreases the domain Φw < 18 and then increases the temperature above Φw > 18. The critical weight fraction of water, Φwcri decreases in presence of both electrolytes. Further, when cyclohexane is replaced by dodecane, the temperature window of 1Φ ME region is tremendously increased.
The author thankfully acknowledges the financial support from Malaysian Ministry of Higher Education [MOHE] with Project No. FRGS59165.