Effect of Branched Alcohols on Phase Behavior and Rheology of

Article pubs.acs.org/jced

Effect of Branched Alcohols on Phase Behavior and Rheology of Nonionic Surfactant Systems Cristina García-Iriepa and Mercedes Valiente* Departamento Química Analítica, Química Física e Ingeniería Química, Universidad de Alcalá, E-28871 Madrid, Spain ABSTRACT: The effect of single- and double-branched alcohols on the phase and rheological behavior of the laurylsulfobetaine (LSB)/water system was studied, with special attention to the reverse micelles region. In our case, reverse micelle phases behaved as Newtonian fluids where alcohols play the role of solvent. Indeed, it can be concluded that the extension of the reverse micelles phase is determined by the alcohol miscibility in water. Regarding liquid crystals, it is observed that the lamellar phase exhibits gel-like behavior and is the most sensitive to the type of alcohol, affecting the elastic properties and the yield stress values. On the contrary, the hexagonal liquid crystal is barely sensitive to the kind of alcohol. In addition, a comparative study of the phase and rheological behavior of two dodecyl surfactants with different head groups (sulfobetaine LSB, and poly(oxyethylene) Brij-35) as a function of surfactant and tert-butyl alcohol concentrations was performed, revealing two completely different phase diagrams. Whereas the hexagonal liquid crystal appeared only for the LSB/tert-butyl alcohol/water system, the cubic liquid crystal phase appeared for both systems, but the addition of salt was required to stabilize it in the Brij-35 system. DLS7 in water-rich mixtures. In the same way those for ionic surfactants, more hydrophilic alcohols form only one isotropic region (L phase) from the water corner to the alcohol corner in the ternary phase diagrams. The formation of two well-defined micelle regions (L1 and L2 phases) is observed with octanol. Other kinds of surfactants are zwitterionic ones. They have been intensively studied due to their role in biology and their excellent biodegradability. These are used in the form of betaines or sulfobetaines, but the phase behavior of alkyldimethylbetaines8 has been less studied than the one of ionic surfactants. In addition, these compounds are milder on the skin than the anionics and have an especially low eye-sting effect, which leads to their use in toiletries and baby shampoos. We had already investigated9 the effect of a branched alcohol (3,3-dimetyl-1-butanol) on a laurylsulfobetaine system regarding to the corresponding linear alcohol (butanol). The results demonstrated a significant phase behavior change from the branched alcohol to the linear one, especially the micelle phases. Up to now, systems with linear alcohols have been extensively studied, in particular, those with short and medium chain, due to their properties as cosurfactants. However, branched alcohols have been much less studied even though they may also act as cosurfactants. The effect of branched alcohols on water solubilization in nonionic microemulsions of Brij-97 has been studied by Garti el al.10,11 They concluded that many factors participate in the process such as location of the

1. INTRODUCTION Phase diagrams of ternary surfactant/alcohol/water systems are of both technological and theoretical interest. Alkanols act as cosurfactants in mixed micelle formation of ionic and nonionic surfactants. With hydrophilic alcohols (from methanol to butanol and benzyl alcohol) in ionic surfactant systems, only one micelle region (L phase) extends from the water corner to the alcohol corner.1,2 On the contrary, two regions of direct (L1 phase) and reverse (L2 phase) micelles exist with more hydrophobic alcohols1 Thus, the nature of the alcohol employed affects strongly the phase behavior of the surfactant systems. But, this is not the only region of interest in surfactant systems. With an increase in the surfactant concentration, different kinds of lyotropic liquid crystals (lamellar, hexagonal, and cubic) are also formed. Among these liquid crystals, cubic phases are the least understood. Nonionic surfactants such as Brij-35 [poly(oxyethylene (∼23)-lauryl ether, C12E∼23) have been extensively studied in water solutions.3−5 Brij-35 is a mixture of poly(oxyethylene)lauryl ether molecules with an average of 23 oxyethylene units. The self-assembled structures built up from this kind of surfactant are much more sensitive to temperature than the ones derived from ionic surfactants. The first ones are used extensively in low-temperature detergency. The structure of micelles below 200 g·L−1 has been studied by small angle neutron scattering (SANS).6 It was concluded that when the surfactant concentration increases, the mean aggregation number rises, whereas the core radius increases only slightly and the polymer chains adopt a more stretched conformation. Mixed micelles with alkanols (from ethanol to 1-decanol) or microemulsions have also been characterized by SAXS and © 2014 American Chemical Society

Received: May 8, 2014 Accepted: June 18, 2014 Published: June 27, 2014 2634

dx.doi.org/10.1021/je5004109 | J. Chem. Eng. Data 2014, 59, 2634−2641

Journal of Chemical & Engineering Data

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butanol and 3,3-dimethyl-1-butanol) or with longer main chain (2-methyl-1-pentanol). We also studied a water miscible double-branched alcohol, tert-butyl alcohol. Experiments were carried out at a constant ratio % LSB/% water of 1.5, while the alcohol content was varied from 0 to 100 %. It was observed that the phase behavior of LSB in these alcohols is different. The phase boundaries of the uniphasic region for each alcohol at % LSB/% water of 1.5 are set in Table 1.

alcoholic OH group along the carbon backbone, the number of branches along the alkyl chain, the components nature, etc. But, there are not enough studies of branched alcohols in ternary surfactants systems to understand the effect of alcohol branching in the whole phase behavior. In this work, we have investigated the influence of the alcohol structure on the phase behavior and viscoelastic properties of the laurylsulfobetaine (LSB) and water system. Single-branched alcohols (2-methyl-1-butanol and 2-methyl-1pentanol) and double-branched alcohols (3,3-dimethyl-1butanol and tert-butyl alcohol) were used. Furthermore, the whole ternary phase diagrams of two surfactants (LSB and Brij35) in tert-butyl alcohol and water have been also studied. So, a comparative study between two surfactants containing the same hydrophobic group but different head groups, one zwitterionic and another nonionic, was performed. Viscoelastic properties of micelles and liquid crystals phases for LSB/tert-butyl alcohol/ water and Brij-35/tert-butyl alcohol/water have been also studied. Knowledge about the formation and rheological properties of nonionic surfactant systems is important not only to obtain a better theoretical understanding of the systems, but also for practical applications such as in cosmetics and toiletry products, because of the absence of charged species and improved mildness to the skin.

Table 1. Water Content (%) in Phase Boundaries of the Isotropic and Liquid Crystal Phases for % LSB/% water = 1.5 at 30.0 ± 0.1 °C. (L = isotropic phase; H1 = hexagonal phase; Lα = lamellar phase) 1-butanol

2-methyl1-butanol

2-methyl1-pentanol

0 to 26 20 to 39 26 to 46 77 0

0 to 24 24 to 36 26 to 48 31 0.75

0 to 19 24 to 36 26 to 48 8.1 0.8

3,3dimethyl1-butanol

tert-butyl alcohol

phase L Lα H1 s (g L−1) FB

0 to 19 17 to 45 30 to 48 1.0

0 to 33 26 to 45 ∞ 2.0

As we have moved along the line with constant ratio % LSB/ % water of 1.5, the water content was always lower than 40 %. So that, the micelles are not direct micelles as they should appear at lower % LSB/% water ratios. The extension of reverse micelles is in line with the alcohol solubility in water. The higher content of water-soluble in reverse micelles was found for tert-butyl alcohol, even more than for 1-butanol.9 Whereas, the more hydrophobic alcohols (3,3-dimethyl-1-butanol and 2methyl-1-pentanol) decrease water solubilization in reverse micelles (Table 1). Therefore, it can be concluded that the alcohols nature and structure determine the extent of micelle phases: the larger reverse micelles region corresponds to the more hydrophilic alcohol systems (see alcohols solubility in water values in Table 1). With regard to the liquid crystal phases, the hexagonal phase is hardly sensitive to the type of alcohol. Whereas, the lamellar phase boundaries vary considerably with the type of alcohol even, this phase disappears with tert-butyl alcohol at this % LSB/% water ratio. To rationalize the effect of branched alcohols on phase behavior, we have used an extended form of the geometrical branching parameter, FB, as it has been already suggested by Garti et al.10 The FB parameter is closely related to the distance between the branch and the headgroup, and to the size of the branch itself. Both quantities are normalized to the length of the main chain of the alcohol’s hydrophobic part. This factor may be evaluated using the equation 1 FB = ∑ liti d i (1)

2. EXPERIMENTAL SECTION 2.1. Materials. LSB (laurylsulfobetaine, C12H25N+(CH3)2CH2CH2CH2SO3−), Brij-35 (C12H25(OCH2CH2)∼23OH), tert-butyl alcohol, 2-methyl-1butanol (> 99 %), and 2-methyl-1-pentanol (99 %) were purchased from Aldrich, and 1-butanol z.a. was purchased from Merck. All materials were used as supplied. 2.2. Apparatus and Procedure. The samples were prepared by mixing all the components at the given weight percentage. After homogenization by shaking in a Heidolph REAX 2000 vibrator, the samples were left in a 30 °C bath. Each sample was observed first, by direct inspection and then, between crossed light polarizers. Polarized light microscopy observations were obtained with a Nikon Eclipse 50 i optical microscope. Rheological measurements were performed at 30 °C, in a Carri-Med CSL2100 controlled stress rheometer with cone− plate configurations (4 cm 1° and 2 cm 2°, depending on the samples viscosity). Two different experiments, steady flow (shear stress weep mode) and oscillation were performed. In flow experiments a logarithmic series of increasing stress for 10 min was applied. Apparent viscosity values (η) were calculated as the ratio of shear stress to shear rate. The measurements were carried out after 10 min of thermal equilibration on the plate of the rheometer. In the oscillation experiments, the storage modulus G′ and the loss modulus G″ were measured as a function of stress to obtain the linear viscoelastic region. When the linear viscoelastic region was established, measurements were performed as a function of frequency at constant stress.

where d is the length (i.e., the number of C atoms) of the amphiphile main chain to which the side groups (or chains) are attached; li is the length of the main chain section measured from the free end (i.e., not attached to the headgroup) to the point where it joins to the side chain; and ti is the length of the i branch (i.e., the side chain). FB values for single- and doublebranched alcohols are given in Table 1. Based on eq 1, branched alcohols are more branched, that is, higher FB values,

3. RESULTS AND DISCUSSION 3.1. Phase Behavior. A comparative study was performed using different alcohols as cosurfactants and LSB as surfactant. The linear alcohol, 1-butanol, was taken as reference. In particular, the alcohols used in this work were single- and double-branched with the same main chain (2-methyl-12635



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when they have their branches attached to the alkyl chain at points as near as possible to the headgroup. The branching parameter does not affect the extension of the L phase as it is determined by the alcohol solubility in water. Alcohols seem to mainly play the role of dispersing the medium of reverse micelles. The most important change in phase behavior takes place with tert-butyl alcohol where the lamellar phase does not appear at this % LSB/% water ratio. The more branched alcohols reduce the range of concentrations of lamellar liquid crystal phase; therefore, the entire phase diagram was studied in detail for this alcohol (Figure 1). Phase boundaries were delimited Figure 2. Phase diagram of the Brij-35/tert-butyl alcohol/water system at 30.0 ± 0.1 °C and a polarizing micrograph of the gel phase. Dashdot lines correspond to the phase boundaries with salt.

stability, changing to a micelle phase after one month. The addition of salt stabilized both the cubic and the gel phases. The effect of electrolytes addition on the phase behavior of nonionic surfactants in water is highly dependent on the types of ions, especially anions.13 Salting-out electrolytes, for example, NaCl decrease the mutual solubility between water and nonionic surfactants. The cubic phase found in the binary system C12E25 seems to be built of rodlike micelles.13 Smallangle X-ray scattering (SAXS) measurements showed that NaCl induces a slight reduction of the effective surface area per surfactant molecule due to a change in hydration of the poly(oxyethylene) I chains.14 Similar surfactants with the same hydrophobic part (like C12−14E12) but less hydrophilic head groups, also show cubic phases in their binary systems15 but at temperatures lower than 30 °C. The melting point of the cubic phase is around 27 °C. With respect to Brij-35 (C12E∼23), previous studies in buffer solutions have demonstrated that it self-assembles as a function of its concentration into a micelle solution in water, a packed micellar cubic phase and a region of lamellar structures.16 Increasing the temperature does not cause the the L1 phase boundary to change, but it does destabilize the liquid crystal structures, causing them to transform into a reverse micelle phase. Over 40 °C, the cubic phase in the binary Brij-35/water system disappears. We can conclude that for these surfactants C12Ex, increasing the hydrophilicity of nonionic heads groups (higher x values) and/or decreasing temperature, favor the formation of the cubic phase. Brij-35 is not pure surfactant C12E23 but mixed C12E∼23. So that the presence of C12Ex with x values lower than 23 could destabilize the formation of the cubic phase. 3.2. Rheological Study. The flow behavior of reverse micelles with 12 % LSB and 80 % alcohol composition has been studied for every alcohol. These reverse micelles behave like Newtonian fluids and are lowly viscous (Table 2). Their viscosities are predominantly dependent on the solvent viscosity and follow the same order as the alcohol viscosities.

Figure 1. Phase diagram of the laurylsulfobetaine/tert-butyl alcohol/ water system at 30.0 ± 0.1 °C. Dot lines correspond to % LSB/% water ratios of 1.5 and 4.

with errors smaller than 3 % in weight. Phase diagram shows an extensive isotropic phase, L phase, which goes from the water to alcohol corner and three liquid crystal phases at LSB contents higher than 50 % and tert-butyl alcohol contents lower than 25 %. Moreover, a small region of cubic liquid crystal phase was found, which solubilizes a low amount of alcohol. The hexagonal liquid crystal phase exists in a wide range of surfactant contents, and the lamellar liquid crystal phase solubilizes a greater amount of alcohol than the other liquid crystals. Finally, the study was completed with the Brij-35/tert-butyl alcohol/water system. Although Brij-35 has the same hydrophobic group as LSB (lauryl), it is a very hydrophilic surfactant due to its poly(ethylene oxide-23) hydrophilic group. Brij-35 has a relatively high HLB value (16.9) given that HLB values range from 1 to 20 for nonionic surfactants. Its phase diagram can be observed in Figure 2. It is a very simple phase diagram with two uniphasic regions, one consisting of a great isotropic region and the other one of a milky white gel which is located in the Brij-35 corner. This gel corresponds to a lamellar structure as we can view in the micrograph (Figure 2). The appearance between crossed polarizers in the microscope texture shows the characteristic mosaic texture of the lamellar phase. Moreover, a cubic phase appears at intermediate concentrations of Brij-35 in the absence of tert-butyl alcohol. As the micellar phase, the cubic phase is optically isotropic and transparent. However, we can distinguish one from the other because the cubic phase is very viscous due to its threedimensional structure. This cubic phase showed short kinetic

Table 2. Viscosity of the LSB Micelles with Different Alcohols at 30.0 ± 0.1 °C 12 % LSB and 80 % alcohol

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alcohol

viscosity (mPa·s)

tert-butyl alcohol 1-butanol 2-methyl-1-butanol 3,3-dimethyl-1-butanol

3.5 4.0 6.0 10.5



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In this case, the alcohol basically plays the role of the solvent. The water content, which primarily determines the size of the reverse micelles, is the same for every sample. A more detailed investigation was performed for the L phase with tert-butyl alcohol. The relative viscosities (the ratio of the viscosity of micelles to the viscosity of the alcohol) for two different % LSB/% water ratios are plotted in Figure 3. The

Figure 4. Flow curves for lamellar phases at 45 % LSB and 25 % alcohol with different alcohols at 30.0 ± 0.1 °C.

vesicles to unilamellar vesicles.23 Multilamellar vesicles exhibit a more complex rheological behavior than unilamellar vesicles24 due to the larger size and more complex architecture. Lamellar phases built from nonionic and zwitterionic surfactants lead to flexible surfactant bilayers and the curvature and bending properties of the uncharged bilayers can be influenced by various additives.25 Given that no plateau region exists at lower shear rates we cannot define any zero-shear-rate viscosity so, we consider the viscosity at a fixed shear rate to compare the flow properties for the different alcohols (Table 3). The results show that the

Figure 3. Relative viscosity against the amount of water at different % LSB/% water ratios at 30.0 ± 0.1 °C: a ○, 1.5; b □, 4 (solid lines just eye guidelines).

results show that the viscosity is dependent on the content of water. This observation is consistent with the presence of reverse micelles, in which the water pool size largely determines the viscosity values. The density and viscosity of tert-butyl alcohol−water mixtures are well described in ref 17. The change in viscosity as a function of water mole fraction permits a calculation of the excess viscosity which was large and positive. We have also estimated the relative viscosity taking into account the viscosity of the solvent.17 Relative viscosity depends more strongly on the water content than that predicted by the hard sphere droplet model. This model fits well the results only when the density of micelles is so low than they can move freely under shearing. As the water volume fraction increases, the number of reverse micelles and/or the size of micelles also increases. In such a way, the interactions among the micelles become stronger. Moreover, an isotropic phase exists from direct to reverse micelles without phase separation for this system. The transition from direct to reverse micelles with the water content should go through highly disordered and interconnected structures (bicontinuous structures), with domains of alcohol and water separated by a surfactant layer. The lamellar liquid crystal phase behaves like a plastic fluid with every alcohol. The flow behavior when the LSB content is 45 % can be observed for different alcohols (excepting tert-butyl alcohol) in Figure 4. Micelles, rather than lamellar phases, exist under these conditions with tert-butyl alcohol. The apparent viscosity decreases with increasing shear rate for all samples according to a power law shear thinning behavior which in general can be described by η ≈ (shear rate)−a. No plateau region is found within the measured range. The exponent has a value between 0.83 and 0.85 for every flow curve at least until shear rate values reach 200 s−1. Exponents of the same order have been found often for multilamellar vesicles.18−21 For a pure hard sphere system the exponent a should be about 0.5.22 Recent investigations have shown that the exponent a changes from 0.8 to 0.5 for multilamellar

Table 3. Apparent Viscosity at the Shear Rate of 1 s−1 for Samples with % LSB/% water = 1.5 at 30.0 ± 0.1 °C and Different Alcohols % LSB 45 45 54 59

(Lα) (L) (H1) (H1)

BuOH

t-BuOH

23 1000 3000

2Me-1BuOH 60

3,3dime-1BuOH 90

34 1100 2600

sequence of apparent viscosity with the type of alcohol for the lamellar phase is of the same type as that observed for the micelle phase (Table 2). The alcohol structure affects the viscosity values. The lower viscosity corresponds to the linear alcohol and the viscosity increases with raising the branching parameter given in Table 1. On the other hand, plastic fluids are characterized by a yield stress value. The yield stress is difficult to define. Approximate yield stress measurements can be gained by plotting the shear stress values for a range of shear rates, fitting a curve to the data, and extrapolating through the stress axis. The intersect on the stress axis gives us the dynamic yield stress. This is not an exact method, but it permits to estimate a value (Figure 5). The yield stress value depends on the alcohol structure. The lowest value corresponds to the linear alcohol and it increases with increasing branching parameter. Oscillatory measurements were also carried out. All the lamellar liquid crystal samples show elastic gel-like rheograms (Figure 6). The storage and loss moduli are nearly frequencyindependent and the storage modulus is higher than the loss modulus within the measured frequency range. This implies that G′ and G″ do not intersect each other; hence, we cannot define any finite relaxation time. This observation is characteristic of a gel-type structure. Densely packed multilamellar 2637



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Figure 5. Shear stress against shear rate at low shear rates at 45% LSB and 25% alcohol with different alcohols at 30.0 ± 0.1 °C (□, 3.3dimethyl-1-butanol, ○, 1-butanol, △, 2-methyl-1-butanol).

Figure 7. Effect of alcohols on flow curves of hexagonal phase at 54 % LSB and 36 % water at 30.0 ± 0.1 °C.

independent of the kind of alcohol (Figure 7, Table 3). Its high viscosity, together with its low alcohol content, could explain this behavior. With regard to its viscoelastic properties, both the loss and storage moduli depend on frequency within the measurement range. The storage and loss moduli increase with increasing frequency as can be seen for 1-butanol (Figure 8). Storage

Figure 6. Effect of alcohols on elastic modulus of lamellar phases of the systems with LSB (45 % LSB/25 % alcohol) and with Brij-35 (80 % Brij-35/20 % tert-butyl alcohol) at 30.0 ± 0.1 °C (opened symbols correspond to the LSB system and closed symbols to the Brij-35 system). Figure 8. Storage (□), loss (○) and complex viscosity (△) moduli as a function of frequency for the hexagonal liquid crystal phase with 1butanol (54% LSB/10% alcohol) at 30.0 ± 0.1 °C.

vesicles often form a vesicle gel, but such behavior is not necessarily a unique property of a multilamellar vesicle phase. Some authors found a frequency-independent storage modulus for multilamellar systems26−30 as well as for open and extended bilayers.31−33 The rheological behavior cannot allow distinguishing in a safe way between the two different topologies of the lamellar phase. But, taking into account the elastic behavior and the flow curves we can dare to say that the lamellar phase consists of multilamellar vesicles. With respect to the effect of alcohols in rheograms, we observe that the storage modulus slightly depends on the type of alcohol and the loss modulus is nearly independent of the type of alcohol. The hexagonal phase is highly viscous, as usual. This can be attributed to the hexagonal structure, which allows the cylindrical aggregates to move freely only along their length. This phase shows a shear-thinning behavior, similar to cylindrical micelles, that is related to the alignment of the cylindrical aggregates (Figure 7). At low shear rates, viscosity tends to a plateau that corresponds to the zero-shear rate viscosity but it is not well-defined. Contrary to the lamellar phase, the viscosity of the hexagonal phase is nearly

modulus is more dependent on frequency than is the loss modulus, such that G′ and G″ intersect each other. Similar G′ and G″ values were found for each alcohol. Hexagonal liquid crystal phases usually exhibit viscoelastic behavior, in which the values of dynamic moduli increase with increasing frequency with different slopes.31 More information regarding the network structure of the cubic phase can be obtained from oscillatory shear frequency sweep measurements. The cubic phase shows a Maxwell-type behavior (Figure 9), reaching G′ a plateau at high frequencies which corresponds to the instantaneous storage modulus (G°) and G″ drops at high frequencies. This kind of rheogram is typical of bicontinuous cubic liquid crystals.31,34,35 The micellar cubic phase shows storage modulus independent of frequency.36,37 A finite value of the relaxation time (τ) can be estimated as the inverse of the frequency where G′ and G″ intersect. In this case, τ is 60 s, a very high relaxation time. This corresponds to an instantaneous storage modulus, G°, which is 2638



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Figure 9. Oscillating rheograms for the cubic phases of the systems with LSB and with Brij-35 with tert-butyl alcohol at 30.0 ± 0.1 °C (opened symbols, 59 % LSB/2 % tBuOH; and closed symbols, 47 % Brij-35/5 % tBuOH with salt).

Figure 11. Effect of % Brij-35/% water ratio on tert-butyl alcohol content dependence of viscosity at 30.0 ± 0.1 °C.

slightly higher than 106 Pa, which is of the same order of magnitude as previously reported.34−36 In addition, the viscosity of the cubic phase decreases with increasing frequency as usual. Flow measurements were obtained for the binary Brij-35/ water system without salt after one month. The viscosity of all samples was independent of shear rate. It is found that the viscosity passes through a maximum at 60 % in Brij-35 (Figure 10). SAXS and DLS studies have demonstrated a slight

tert-butyl alcohol is related to the decrease in the surfactant aggregation number by the addition of the alcohol. Alcohols which are miscible7 in water, such as ethanol, propanol, and tert-butyl alcohol, break the micelles. As observed through the polarization microscope, the milky white phase shows a lamellar structure and behaves like a gel (Figure 5), similar to the lamellar liquid crystals of the LSB system. The cubic phase stabilized by salt addition is optically transparent and show elastic properties. At low frequencies, G′ and G″ increase with increasing frequency with different slopes and intersect at a characteristic frequency. G′ reaches a plateau value at high frequency and G″ decreases with increasing frequency (Figure 9). The complex viscosity tends to a constant value in the region of low frequencies, corresponding to a zero shear viscosity. This is the Maxwell-type behavior characteristic of a viscoelastic system with a finite structural time. As we have already said, it is related to a bicontinuous cubic structure, while the cubic phase made up of individual sphere micelles has infinite relaxation time. In this case, the relaxation time is shorter than the one of the cubic phase in the LSB system with a value close to 3 s. The instantaneous elastic modulus, G°, is also lower (2·105 Pa) and the elastic and viscous properties are slightly lower for this cubic phase.

Figure 10. Viscosity against the amount of Brij-35 at 30 ± 0.1 °C.

4. CONCLUSIONS The effect of branched alcohols on the phase behavior of the LSB/water system has been studied. The solubility of alcohols determines the extent of the reverse micelle phase. Regarding the liquid crystal, the hexagonal liquid crystal phase is only slightly sensitive to the alcohol type, not affecting the phase and the viscoelastic behavior. On the contrary, the location and extension of the lamellar liquid crystals are modified by the specific alcohol’s nature and especially, by the branching parameter. The lamellar liquid crystal show elastic gel-like properties. Storage moduli and yield stress increase when the branching parameter of the alcohol increases. The greatest effect on both the phasic and rheological behavior is caused by tert-butyl alcohol. The absolute solubility of tert-butyl alcohol in water and its compact structure are apparently the main reasons for this behavior. We have compared phase diagrams of the LSB/tert-butyl alcohol/water and Brij-35/tert-butyl alcohol/water systems. Brij-35 is a surfactant with the same hydrocarbon chain but

elongation of the micelles at concentrations above 10 mass %.5 This study employed concentrations up to 20 % Brij-35. A change in shape from globular to slightly elongated micelles together with the increase in the number of micelles may explain the increase in viscosity with Brij-35 concentration. For surfactant contents higher than 40 %, the viscosity is nearly constant except for contents close to the phase boundary. The concentration-dependence of the viscosity can be partly explained from the transition from discrete micelles to bicontinuous structure where the viscosity keeps nearly constant. Although the viscosity increases until 0.5 Pa·s, samples behave as Newtonian fluids in all tested concentrations and conditions. The addition of tert-butyl alcohol decreases the viscosity of the micelles for each % Brij-35 /% water ratio (Figure 11). The tert-butyl alcohol effect is more significant than the surfactant effect on viscosity. The decrease in the viscosity with increasing 2639



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with a more hydrophilic and larger in size headgroup than LSB. The first consequence is the more extensive micelle region. The micelles exhibit Newtonian behavior and the viscosity decreases by addition of tert-butyl alcohol, as it breaks the micelles. Hexagonal liquid crystals which appear at high concentrations of LSB did not appear in the system with Brij-35. The rheology properties of the cubic liquid crystal are similar in both systems showing a Maxwellian behavior.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.G.-I. is grateful to Alcalá University for a research fellowship REFERENCES

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