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Isotope effects on the phase behaviour of cetyltrimethylammonium bromide in H2O and D2O studied by time resolved X-ray diffraction
Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) 171–176
Isotope effects on the phase behaviour of cetyltrimethylammonium bromide in H2O and D2O studied by time resolved X-ray diffraction Bin Yang a,∗ , John W. White b a
Institute of Material and Engineering Sciences, Australian Nuclear Science and Technology Organization, New Illawarra Road, Lucas Heights, NSW 2234, Australia b Research School of Chemistry, The Australian National University, ACT 0200, Australia Received 10 May 2005; received in revised form 13 November 2005; accepted 25 November 2005 Available online 6 January 2006
Abstract The liquid-crystalline phases of C16 TAB/H2 O and C16 TAB/D2 O lyotropic systems, at high concentrations (60.0–90.0 wt% surfactant) have been investigated in the temperature range of 30–120 ◦ C. Time resolved X-ray diffraction (TRXRD) with synchrotron radiation was applied to record the diffraction spectra at different temperatures over this range. We have obtained highly resolved phase diagrams of the two systems and identified hexagonal, cubic, lamellar and 2D monoclinic (deformed hexagonal) phases. Isotope effects appear when H2 O is substituted by D2 O at comparable molar concentrations of surfactant. Particularly where: (1) about 2 ◦ C higher (hexagonal–monoclinic) phase transition temperature, and (2) about 25% larger size hexagonal phase at 31 ◦ C. The isotope effects were reduced at higher concentrations and at higher temperatures. The swollen of hexagonal phase in DS, i.e. the secondary isotope effects, is induced by the fact that D2 O is a more polar solvent than H2 O. © 2005 Elsevier B.V. All rights reserved. Keywords: Surfactants; Phase diagram; Time resolved X-ray diffraction; Isotope effects
1. Introduction The mesophases of the C16 TAB/H2 O system (‘HS’) have long attracted scientific interest with particular focus on their structures and functionalities [1–4]. Mesoporous bulk [5], thin film silicates and transition metal oxides [6–9] can be synthesized by using liquid-crystalline aggregates of the CTAB with polar solvent as templates to direct the growth of the inorganic network. In order to choose optimum conditions for these reactions, templating knowledge of the phase behaviour is needed in great detail, especially the formation of the hexagonal and cubic phases. Previous studies have demonstrated that CTAB forms micelle, hexagonal (H␣ ), two dimensional monoclinic (also refer as distorted hexagonal, M), cubic (Q␣ ) and lamella (L␣ ) phases in water. The unit cell sizes of the hexagonal and cubic phases were studied by Husson [1], Luzzati [2] and Ekwall [3].
∗
Corresponding author. Tel.: +61 2 9717 9563; fax: +61 2 9717 9630. E-mail address: [email protected] (B. Yang).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.11.058
Recently, the phase behaviour of CTAB in water, formamide and glycerol were reported by Auvary et al. using X-ray diffraction techniques [10]. In Auvary’s paper, the phase diagram of HS system was obtained by following equilibrium solid crystal/liquidcrystalline phases from the micelle to lamellar phase. However, very few measurements performed in the studies of temperature effects for a given concentration. Besides, there is very limited information about the structure and phase behaviour of the HS system at temperature higher than the equilibrium line. In the present study, we chose to use novel X-ray diffraction techniques for an extensive study, so that more complete and detailed phase diagrams can be achieved. Particularly, we have focused on the structural and thermal behaviour by using the time resolved X-ray diffraction (TRXRD) technique employing synchrotron radiation. Isotope effects have been observed in many binary lyotropic liquid-crystalline systems such as APFO/D2 O [11], CsPFO/D2 O [12], mixed alkylglucoside/D2 O [13] and DOPE/D2 O [14]. Our investigation focuses on the concentration range of 60.0–90.0 wt% surfactant by weight, and temperatures between 30 and 120 ◦ C. It is interesting to verify and quantify the
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existence of an isotope effect in the C16 TAB/D2 O system (‘DS’) by comparing to that of the HS. 2. Materials and methods 2.1. Preparation of samples Cetyltrimethylammonium bromide, C16 TAB (C16 H33 N (CH3 )3 + Br, 98%), was purchased from Fluka, without further purification. Samples were prepared by mixing the appropriate amount of salt with 0.5 mL of twice distilled water or deuterium oxide (D2 O) of 99.75% isotropic purity. The mixtures were homogenized at room temperature about 25 ◦ C, and then were flame sealed in Lindemann glass capillaries (diameter 1 mm). For comparison of HS and DS systems, six pairs of samples were prepared for TRXRD experiments. Each of the pairs contains a HS sample and a DS sample with the same molar concentration of CTAB. The concentrations of the six HS samples were 70.0, 75.0, 78.0, 80.0, 85.0 and 90.0 wt%; and the concentrations of the six comparable DS samples were 67.8, 73.0, 76.2, 78.3, 83.6 and 89.1 wt%. 2.2. Measurements TRXRD experiments were performed with the SAXS beamline at the ELETTRA synchrotron facility. The beamline optics contains a flat double crystal monochromator and a double focusing toroidal mirror. An incident beam with photo energy of 8 keV (λ = 0.154 nm) was employed. X-ray diffraction in the small (0.31–12.6 nm−1 in Q space, Q = 4π sin θ/λ) and wide (8–50 nm−1 ) angle regimes were recorded simultaneously by two linear one-dimensional position sensitive detectors. The dspacing of the diffraction peak was calibrated by the diffraction peaks of CTAB salt. The temperature was controlled by a Mettler FP52 controller that allows linear heating and cooling, with temperature scans at rates of 0.2–10 ◦ C/min. For each sample, a set of ninety X-ray diffraction spectra were recorded during the heating of the sample from 30 to 120 ◦ C, 2 ◦ C/min. The recording time for each spectrum was 0.5 min; hence, the temperature resolution is 1 ◦ C. During the heating, the X-ray diffraction patterns were recorded by following two different routes in the phase diagram. The first route was along the salt-solution equilibrium line in the temperature region between 30 ◦ C and Tc (Tc refers the lowest temperature at that the salt is completely dissolved in the solvent). In this temperature region, the surfactant was partially dissolved into the solvent, and the sample was formed by a mixture of solution and CTAB salt. The concentration of the solution is lower than the “prepared concentration” of the sample, and it changes as a function of the temperature along the equilibrium line. The “prepared concentration” is referring to the percentage of salt/(salt + solvent) in weight, as the sample was prepared. In this case, the X-ray diffraction patterns were characterized in the presence of both the diffraction peaks of the salt and the diffraction peaks of liquid-crystalline phases along the equilibrium line. When the temperature rises to Tc or higher, the measurement turned to the second route that is a vertical
line along the “prepared concentration” in the phase diagram. In this route, the salt was completely dissolved into the solvent and the concentration of the solution was equal to the “prepared concentration” and diffraction peaks of the salt disappear from the X-ray diffraction spectra. In fact, each sample had a Tc that can be easily determined from the diffraction patterns. During the cooling process, both HS and DS are quasi-stable with a strong tendency of re-crystallization that was characterized by the formation of colorless crystallites through out the transparent solution. We found that re-crystallization was also evident after a few hours after the samples were prepared, thereby reducing the homogeneity of the sample. Therefore, in order to obtain reliable and reproducible data, X-ray diffraction spectra were recorded as soon as the homogeneous sample had been prepared, during the heating process. Before the TRXRD experiment, the samples were also studied primarily by small angle X-ray scattering (SAXS) with a linear position sensitive Gabriel detector and a rotating anode Xray generator Elliott GX-13 [8]. The SAXS data and the TRXRD data are consistent, in spite of differences in experiments set-up and the heating rate were applied. The fact indicates that all TRXRD measurements are done in equilibrium samples and the heating rate of the samples was reasonably slower comparing with the equilibrium time of the systems. 3. Results and discussion 3.1. Phase diagrams The phase diagrams of HS and DS, based on the result of the TRXRD, are given in Fig. 1(a) and (b), respectively. Hexagonal phase (H␣ ), 2D monoclinic (M), cubic (Q␣ ) and lamella phases (L␣ ) are displayed in the range between 70.0 and 90.0 wt%. Fig. 1(a) and (b) shows that the phase diagram of the DS system is similar to that of HS. 3.2. Time resolved X-ray diffraction At 70.0 wt% (for HS) and 67.8 wt% (for DS), the systems form a H␣ (P6m). Fig. 2(a) shows the diffraction spectra of DS as a function of temperature ranging from 31 to 94 ◦ C. In this paper, for convenience, the 2θ angles were converted into d-spacings of Bragg peak by using the equation: 2d sin θ = λ. For this sample and the sample of HS, the measurement was following directly the second route, the concentration being constant because of the absence of the peak of the salt. Two peaks of H␣ are shown in the diffraction spectra: peak (1 0) at 6.1 nm and peak (1 1) about 3.6 nm, at 31 ◦ C. The intensity of peak (1 1) is very weak, but it is visible. It is also noted in Fig. 2(a) that the value of d(1 0) and the width of the peak decrease while temperature is rising. This fact implicates that the reducing of the size of the H␣ and increasing of the correlation length of the H␣ phase during the heating. Fig. 2(b) presents the d-spacing of the peak (1 0) variation as a function of temperature for HS and DS. Large isotope effect appears by comparing the size of H␣ of the two systems. Particularly, at 31 ◦ C, DS has about 25% larger d(1 0) value than that of HS, with the difference decreasing rapidly
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Fig. 1. The phase diagrams of (a) HS and (b) DS.
as the temperature approaches nearly 50 ◦ C. In the temperature range of 50–84 ◦ C, the two systems have approximately equal d(1 0) values and, at temperatures higher than 84 ◦ C, HS has a slightly larger d(1 0) value. The difference increases with further increasing of temperature. Our experimental data also shows that the phase behaviours of 75.0 wt% HS and 73.0 wt% DS are similar to that of 70.0 wt% HS and 67.8 wt% DS, respectively, displaying only the hexagonal phase. Fig. 3(a) and (b) shows the diffraction spectra of the HS (78.0 wt%) and DS (76.2 wt%), respectively. These figures clearly indicate the following transitions of liquid-crystalline
Fig. 2. (a) The diffraction patterns of DS (67.8 wt%) during heating process. (b) The d-spacing of d(1 0) peak as a function of temperature. The concentration of HS and DS are 70.0 and 67.8 wt%, respectively.
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Fig. 4. The lattice parameter a of the cubic phase as a function of temperature. The concentration of HS and DS are 78.0 and 76.2 wt%, respectively.
Fig. 3. The diffraction patterns of (a) HS with the concentration of 78.0 wt%, and (b) DS with the concentration of 76.2 wt%.
phases as a function of temperature: HS:
(30 ◦ C) Hα (42 ◦ C) ⇒ M (62 ◦ C) ⇒ Qα (120 ◦ C)
DS:
(30 ◦ C) Hα (44 ◦ C) ⇒ M (56 ◦ C) ⇒ Qα (120 ◦ C)
For HS, the hexagonal phase appears along the first route because of the presence of the H␣ peak (1 0) and (1 1), from 30 to 42 ◦ C.
The ratio of d(1 0)/(d(1 1) is about 1.73. This confirms the presence of H␣ , although the intensity of the peak (1 1) is weak. The d-spacing of the peak (1 0) changes from 5.3 to 4.3 nm, and the d-spacing of peak (1 1) various simultaneously from 2.95 to 2.6 nm, with increasing temperature. For DS, H␣ forms at a temperature range of 30–44 ◦ C, while the d-spacing of peak (1 0) changes from 6.3 to 4.3 nm and the d-spacing of peak (1 1) decreases from 3.7 to 2.6 nm. Concerning the route of the measurements, a sharp peak of CTAB salt at 2.6 nm is shown in Fig. 3(a) and (b), at a temperatures below 44 ◦ C (for HS) and 46 ◦ C (for DS), respectively. It indicates that the Tc ’s of these two samples are, respectively, equal to 44 and 46 ◦ C. The measurements are following the equilibrium line below the temperatures. H␣ phase transfers to M phase, at 42 ◦ C (for HS) and 44 ◦ C (for DS). This phase transition is evidenced by splitting the peak (1 0) of H␣ into the two peaks (1 0) at 4.0 and (0 1) at 5.0 nm of the 2D M [10]. At 62 and 56 ◦ C for HS and DS, respectively, the M transforms into a Q␣ (Ia3d) that is characterized by the presence of two sharp peaks at 3.4 nm (2 2 0) and 4.0 nm (2 1 1). The Q␣ phase remains stable going through 120 ◦ C with its slight size reduction. Fig. 4 shows the lattice parameter a of the Q␣ as a function of the temperature. This parameter can be fitted to the linear function; a = 10.28–0.0089T for DS; and a = 10.32–0.0095T for HS, where T is temperature in ◦ C, and the unit of a is nanometer. In fact, the differences between the two systems in the cubic phase region are minimal.
B. Yang, J.W. White / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) 171–176
DS:
175
(30 ◦ C) Hα (44 ◦ C)
⇒ M (51 ◦ C) ⇒ Qα (63 ◦ C) ⇒ Lα (120 ◦ C) The peak of CTAB salt displays till temperatures reach 68 and 96 ◦ C, for HS and DS, respectively. This implies that the diffraction spectra provide information about the phase transitions along the equilibrium line. For HS, from 30 to 42 ◦ C, H␣ forms and the d-spacing of peak (1 0) decreases from 4.5 to 4.3 nm with rising temperature. For DS, the H␣ forms in the temperature range of 30–44 ◦ C, while the d-spacing of peak (1 0) changes from 5.2 to 4.3 nm. M appears in the range 42–49 ◦ C for HS, 44–51 ◦ C for DS, while the peak (10) of Q␣ splits into two peaks: (1 0) and (0 1) of M. From 49 to 61 ◦ C (for HS) and 51 to 63 ◦ C (for DS), the formation of Q␣ is indicated by the appearance two reflections: (2 2 0) and (2 1 1). Q␣ transfers to L␣ with presence of the peak at 3.5 nm, at temperatures higher than 61 ◦ C (for HS) and 63 ◦ C (for DS). The layer thickness (same as the d-spacing of the peak) of L␣ reduces slightly down to 3.4 nm at 120 ◦ C. This experimental data does not show any evidence of the existence of the second lamella phase L␣ (II) at higher temperatures. It is noted from our experiments that Q␣ could be also formed directly from H␣ at a concentration of about 76 wt% in HS. In this case, the cubic phase coexists with the hexagonal phase in a temperature range, with the presence of three peaks: (2 2 0), (2 1 1) of Q␣ and (0 1) of H␣ . This fact suggests the possibility of existence of a small biphasic region near the boundary between Q␣ and H␣ in the phase diagram. Comparing the phase diagram in Fig. 1(a) with that of reported by Auvray et al. [10], the differences lie in the region of M. Unlike other phases, the phase transition M–Q␣ occurs along the constant concentration line (see Fig. 3), therefore the phase boundary between M and Q␣ is not a quasi-vertical line in the phase diagram, and M can be considered as pre-transition from H␣ to Q␣ . 3.3. Isotope effect
Fig. 5. The diffraction patterns of (a) HS with the concentration of 90.0 wt%, and (b) DS with the concentration of 89.1 wt%.
Fig. 5(a) and (b) illustrate the diffraction spectra of the HS 90.0 wt% and DS 89.1 wt%, respectively. These figures clearly indicate the liquid-crystalline phase transitions: HS:
(30 ◦ C) Hα (42 ◦ C)
⇒ M (49 ◦ C) ⇒ Qα (61 ◦ C) ⇒ Lα (120 ◦ C)
Solvent isotope effects can be observed in the lyotropic systems by comparing the diffraction patterns in Figs. 3 and 5: (1) the phase transition temperatures in DS are approximately 2 ◦ C higher than the corresponding temperature in HS; (2) DS has a smaller area of M phase in the phase diagram compared to that of HS; (3) DS has larger size of H␣ phase than that of HS at lower temperature. It is interesting to note that the swollen of hexagonal phase are shown in both first and second routes in our measurements (see Figs. 2b and 3). The swollen of hexagonal phase is a kind of secondary isotope effect, is induced by the stronger polar force in D2 O. Base on the self-assembly theory [15], the area per head group is determined by a balance between attractive and repulsive force of ionic head groups. Moreover, it is known that D2 O is a more polar solvent than H2 O [16]. Thus, changing the solvent induces a more effective screening of the electrostatic repulsion between the ionic heads, and consequently, reduces the average area per head group, and increases the spontaneous radius of the curvature of
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the monolayer and the size of hexagonal phase. The result of this effect is that the size of H␣ in DS is about 25% larger than that in HS, at 31 ◦ C. Similar isotope effects, the swollen of micelles significantly by replacing H2 O with D2 O, was also observed in other lyotropic systems, such as APFO/D2 O [11] and alkylglucoside surfactant/D2 O [13]. An extent example is that size of hexagonal phase in a sodium dodecyl sulfate (SDS) and water/pentanol/cyclohexane system can be increased for more than 10 times by modifying ionic force of the polar medium in the solution [17]. It indicates that the slightly rising the polar force of solvent (such as replace H2 O by D2 O) can induce a relatively large decrease of spontaneous curvature for the surfactant film in the phase H␣ , thus increasing the diameter of the phase H␣ , as what happened in DS. 4. Conclusion The phase diagrams of HS and DS have been determined by using TRXRD. The following isotope effects were observed by substitution of H2 O for D2 O: (1) the swollen lyotropic hexagonal phase at lower temperature; (2) the temperatures of phase transition shift up about 2–3 ◦ C. The isotope effect is mainly caused by the fact that D2 O is slightly stronger polar solvent than H2 O.
References [1] F. Husson, H. Mustacchi, V. Luzzati, Acta Crystallogr. 13 (1960) 668. [2] V. Luzzati, A. Tardieu, T. Gulik-Krzywicki, E. Rivas, F. Reis-Husson, Nature 220 (1968) 485. [3] P. Ekwall, L. Mandell, K. Fontell, J. Colloid. Interface Sci. 26 (1969) 639. [4] T. Wolff, G. von Bunau, Ber. Bunsenges. Phys. Chem. 88 (1984) 1098. [5] Q. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 (1996) 1147. [6] A.S. Brown, S.A. Holt, Thien Dam, M. Trau, J.W. White, Langmuir 13 (1997) 6363. [7] M. Aldissi, S. Henderson, J.W. White, T. Zemb, Mater. Sci. Forum 27/28 (1988) 437. [8] M.J. Henderson, D. King, J.W. White, Aust. J. Chem. 56 (2003) 933. [9] F. Cagnol, D. Grosso, G.J. Soler-Illia, E.L. Crepaldi, F. Babonneau, H. Amenitsch, C. Sanchez, J. Mater. Chem. 13 (2003) 61. [10] X. Auvray, C. Petipas, R. Anthore, I. Rico, A. Lattes, J. Phys. Chem. 93 (1989) 7458. [11] P.J.B. Edwards, K.W. Jolley, M.H. Smith, S.J. Thomsen, N. Boden, Langmuir 13 (1997) 2665. [12] N. Boden, K.W. Jolley, M.H. Smith, J. Phys. Chem. 97 (1993) 7678. [13] C. Whiddon, O. Soderman, Langmuir 17/6 (2001) 1803. [14] K. Kinoshita, M. Yamazaki, BBA 199 (1997) 1330. [15] W.E. McMullen, A. Ben-Shaul, W.M. Gelbart, J. Colloid Interface Sci. 98 (1984) 523. [16] G.C. Kresheck, J. Phys. Chem. B 102 (1998) 6596. [17] L. Ramos, P. Fabre, Langmuir 13 (1997) 682.
Isotope effects on the phase behaviour of cetyltrimethylammonium bromide in H2O and D2O studied by time resolved X-ray diffraction Bin Yang a,∗ , John W. White b a
Institute of Material and Engineering Sciences, Australian Nuclear Science and Technology Organization, New Illawarra Road, Lucas Heights, NSW 2234, Australia b Research School of Chemistry, The Australian National University, ACT 0200, Australia Received 10 May 2005; received in revised form 13 November 2005; accepted 25 November 2005 Available online 6 January 2006
Abstract The liquid-crystalline phases of C16 TAB/H2 O and C16 TAB/D2 O lyotropic systems, at high concentrations (60.0–90.0 wt% surfactant) have been investigated in the temperature range of 30–120 ◦ C. Time resolved X-ray diffraction (TRXRD) with synchrotron radiation was applied to record the diffraction spectra at different temperatures over this range. We have obtained highly resolved phase diagrams of the two systems and identified hexagonal, cubic, lamellar and 2D monoclinic (deformed hexagonal) phases. Isotope effects appear when H2 O is substituted by D2 O at comparable molar concentrations of surfactant. Particularly where: (1) about 2 ◦ C higher (hexagonal–monoclinic) phase transition temperature, and (2) about 25% larger size hexagonal phase at 31 ◦ C. The isotope effects were reduced at higher concentrations and at higher temperatures. The swollen of hexagonal phase in DS, i.e. the secondary isotope effects, is induced by the fact that D2 O is a more polar solvent than H2 O. © 2005 Elsevier B.V. All rights reserved. Keywords: Surfactants; Phase diagram; Time resolved X-ray diffraction; Isotope effects
1. Introduction The mesophases of the C16 TAB/H2 O system (‘HS’) have long attracted scientific interest with particular focus on their structures and functionalities [1–4]. Mesoporous bulk [5], thin film silicates and transition metal oxides [6–9] can be synthesized by using liquid-crystalline aggregates of the CTAB with polar solvent as templates to direct the growth of the inorganic network. In order to choose optimum conditions for these reactions, templating knowledge of the phase behaviour is needed in great detail, especially the formation of the hexagonal and cubic phases. Previous studies have demonstrated that CTAB forms micelle, hexagonal (H␣ ), two dimensional monoclinic (also refer as distorted hexagonal, M), cubic (Q␣ ) and lamella (L␣ ) phases in water. The unit cell sizes of the hexagonal and cubic phases were studied by Husson [1], Luzzati [2] and Ekwall [3].
∗
Corresponding author. Tel.: +61 2 9717 9563; fax: +61 2 9717 9630. E-mail address: [email protected] (B. Yang).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.11.058
Recently, the phase behaviour of CTAB in water, formamide and glycerol were reported by Auvary et al. using X-ray diffraction techniques [10]. In Auvary’s paper, the phase diagram of HS system was obtained by following equilibrium solid crystal/liquidcrystalline phases from the micelle to lamellar phase. However, very few measurements performed in the studies of temperature effects for a given concentration. Besides, there is very limited information about the structure and phase behaviour of the HS system at temperature higher than the equilibrium line. In the present study, we chose to use novel X-ray diffraction techniques for an extensive study, so that more complete and detailed phase diagrams can be achieved. Particularly, we have focused on the structural and thermal behaviour by using the time resolved X-ray diffraction (TRXRD) technique employing synchrotron radiation. Isotope effects have been observed in many binary lyotropic liquid-crystalline systems such as APFO/D2 O [11], CsPFO/D2 O [12], mixed alkylglucoside/D2 O [13] and DOPE/D2 O [14]. Our investigation focuses on the concentration range of 60.0–90.0 wt% surfactant by weight, and temperatures between 30 and 120 ◦ C. It is interesting to verify and quantify the
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existence of an isotope effect in the C16 TAB/D2 O system (‘DS’) by comparing to that of the HS. 2. Materials and methods 2.1. Preparation of samples Cetyltrimethylammonium bromide, C16 TAB (C16 H33 N (CH3 )3 + Br, 98%), was purchased from Fluka, without further purification. Samples were prepared by mixing the appropriate amount of salt with 0.5 mL of twice distilled water or deuterium oxide (D2 O) of 99.75% isotropic purity. The mixtures were homogenized at room temperature about 25 ◦ C, and then were flame sealed in Lindemann glass capillaries (diameter 1 mm). For comparison of HS and DS systems, six pairs of samples were prepared for TRXRD experiments. Each of the pairs contains a HS sample and a DS sample with the same molar concentration of CTAB. The concentrations of the six HS samples were 70.0, 75.0, 78.0, 80.0, 85.0 and 90.0 wt%; and the concentrations of the six comparable DS samples were 67.8, 73.0, 76.2, 78.3, 83.6 and 89.1 wt%. 2.2. Measurements TRXRD experiments were performed with the SAXS beamline at the ELETTRA synchrotron facility. The beamline optics contains a flat double crystal monochromator and a double focusing toroidal mirror. An incident beam with photo energy of 8 keV (λ = 0.154 nm) was employed. X-ray diffraction in the small (0.31–12.6 nm−1 in Q space, Q = 4π sin θ/λ) and wide (8–50 nm−1 ) angle regimes were recorded simultaneously by two linear one-dimensional position sensitive detectors. The dspacing of the diffraction peak was calibrated by the diffraction peaks of CTAB salt. The temperature was controlled by a Mettler FP52 controller that allows linear heating and cooling, with temperature scans at rates of 0.2–10 ◦ C/min. For each sample, a set of ninety X-ray diffraction spectra were recorded during the heating of the sample from 30 to 120 ◦ C, 2 ◦ C/min. The recording time for each spectrum was 0.5 min; hence, the temperature resolution is 1 ◦ C. During the heating, the X-ray diffraction patterns were recorded by following two different routes in the phase diagram. The first route was along the salt-solution equilibrium line in the temperature region between 30 ◦ C and Tc (Tc refers the lowest temperature at that the salt is completely dissolved in the solvent). In this temperature region, the surfactant was partially dissolved into the solvent, and the sample was formed by a mixture of solution and CTAB salt. The concentration of the solution is lower than the “prepared concentration” of the sample, and it changes as a function of the temperature along the equilibrium line. The “prepared concentration” is referring to the percentage of salt/(salt + solvent) in weight, as the sample was prepared. In this case, the X-ray diffraction patterns were characterized in the presence of both the diffraction peaks of the salt and the diffraction peaks of liquid-crystalline phases along the equilibrium line. When the temperature rises to Tc or higher, the measurement turned to the second route that is a vertical
line along the “prepared concentration” in the phase diagram. In this route, the salt was completely dissolved into the solvent and the concentration of the solution was equal to the “prepared concentration” and diffraction peaks of the salt disappear from the X-ray diffraction spectra. In fact, each sample had a Tc that can be easily determined from the diffraction patterns. During the cooling process, both HS and DS are quasi-stable with a strong tendency of re-crystallization that was characterized by the formation of colorless crystallites through out the transparent solution. We found that re-crystallization was also evident after a few hours after the samples were prepared, thereby reducing the homogeneity of the sample. Therefore, in order to obtain reliable and reproducible data, X-ray diffraction spectra were recorded as soon as the homogeneous sample had been prepared, during the heating process. Before the TRXRD experiment, the samples were also studied primarily by small angle X-ray scattering (SAXS) with a linear position sensitive Gabriel detector and a rotating anode Xray generator Elliott GX-13 [8]. The SAXS data and the TRXRD data are consistent, in spite of differences in experiments set-up and the heating rate were applied. The fact indicates that all TRXRD measurements are done in equilibrium samples and the heating rate of the samples was reasonably slower comparing with the equilibrium time of the systems. 3. Results and discussion 3.1. Phase diagrams The phase diagrams of HS and DS, based on the result of the TRXRD, are given in Fig. 1(a) and (b), respectively. Hexagonal phase (H␣ ), 2D monoclinic (M), cubic (Q␣ ) and lamella phases (L␣ ) are displayed in the range between 70.0 and 90.0 wt%. Fig. 1(a) and (b) shows that the phase diagram of the DS system is similar to that of HS. 3.2. Time resolved X-ray diffraction At 70.0 wt% (for HS) and 67.8 wt% (for DS), the systems form a H␣ (P6m). Fig. 2(a) shows the diffraction spectra of DS as a function of temperature ranging from 31 to 94 ◦ C. In this paper, for convenience, the 2θ angles were converted into d-spacings of Bragg peak by using the equation: 2d sin θ = λ. For this sample and the sample of HS, the measurement was following directly the second route, the concentration being constant because of the absence of the peak of the salt. Two peaks of H␣ are shown in the diffraction spectra: peak (1 0) at 6.1 nm and peak (1 1) about 3.6 nm, at 31 ◦ C. The intensity of peak (1 1) is very weak, but it is visible. It is also noted in Fig. 2(a) that the value of d(1 0) and the width of the peak decrease while temperature is rising. This fact implicates that the reducing of the size of the H␣ and increasing of the correlation length of the H␣ phase during the heating. Fig. 2(b) presents the d-spacing of the peak (1 0) variation as a function of temperature for HS and DS. Large isotope effect appears by comparing the size of H␣ of the two systems. Particularly, at 31 ◦ C, DS has about 25% larger d(1 0) value than that of HS, with the difference decreasing rapidly
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Fig. 1. The phase diagrams of (a) HS and (b) DS.
as the temperature approaches nearly 50 ◦ C. In the temperature range of 50–84 ◦ C, the two systems have approximately equal d(1 0) values and, at temperatures higher than 84 ◦ C, HS has a slightly larger d(1 0) value. The difference increases with further increasing of temperature. Our experimental data also shows that the phase behaviours of 75.0 wt% HS and 73.0 wt% DS are similar to that of 70.0 wt% HS and 67.8 wt% DS, respectively, displaying only the hexagonal phase. Fig. 3(a) and (b) shows the diffraction spectra of the HS (78.0 wt%) and DS (76.2 wt%), respectively. These figures clearly indicate the following transitions of liquid-crystalline
Fig. 2. (a) The diffraction patterns of DS (67.8 wt%) during heating process. (b) The d-spacing of d(1 0) peak as a function of temperature. The concentration of HS and DS are 70.0 and 67.8 wt%, respectively.
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Fig. 4. The lattice parameter a of the cubic phase as a function of temperature. The concentration of HS and DS are 78.0 and 76.2 wt%, respectively.
Fig. 3. The diffraction patterns of (a) HS with the concentration of 78.0 wt%, and (b) DS with the concentration of 76.2 wt%.
phases as a function of temperature: HS:
(30 ◦ C) Hα (42 ◦ C) ⇒ M (62 ◦ C) ⇒ Qα (120 ◦ C)
DS:
(30 ◦ C) Hα (44 ◦ C) ⇒ M (56 ◦ C) ⇒ Qα (120 ◦ C)
For HS, the hexagonal phase appears along the first route because of the presence of the H␣ peak (1 0) and (1 1), from 30 to 42 ◦ C.
The ratio of d(1 0)/(d(1 1) is about 1.73. This confirms the presence of H␣ , although the intensity of the peak (1 1) is weak. The d-spacing of the peak (1 0) changes from 5.3 to 4.3 nm, and the d-spacing of peak (1 1) various simultaneously from 2.95 to 2.6 nm, with increasing temperature. For DS, H␣ forms at a temperature range of 30–44 ◦ C, while the d-spacing of peak (1 0) changes from 6.3 to 4.3 nm and the d-spacing of peak (1 1) decreases from 3.7 to 2.6 nm. Concerning the route of the measurements, a sharp peak of CTAB salt at 2.6 nm is shown in Fig. 3(a) and (b), at a temperatures below 44 ◦ C (for HS) and 46 ◦ C (for DS), respectively. It indicates that the Tc ’s of these two samples are, respectively, equal to 44 and 46 ◦ C. The measurements are following the equilibrium line below the temperatures. H␣ phase transfers to M phase, at 42 ◦ C (for HS) and 44 ◦ C (for DS). This phase transition is evidenced by splitting the peak (1 0) of H␣ into the two peaks (1 0) at 4.0 and (0 1) at 5.0 nm of the 2D M [10]. At 62 and 56 ◦ C for HS and DS, respectively, the M transforms into a Q␣ (Ia3d) that is characterized by the presence of two sharp peaks at 3.4 nm (2 2 0) and 4.0 nm (2 1 1). The Q␣ phase remains stable going through 120 ◦ C with its slight size reduction. Fig. 4 shows the lattice parameter a of the Q␣ as a function of the temperature. This parameter can be fitted to the linear function; a = 10.28–0.0089T for DS; and a = 10.32–0.0095T for HS, where T is temperature in ◦ C, and the unit of a is nanometer. In fact, the differences between the two systems in the cubic phase region are minimal.
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DS:
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(30 ◦ C) Hα (44 ◦ C)
⇒ M (51 ◦ C) ⇒ Qα (63 ◦ C) ⇒ Lα (120 ◦ C) The peak of CTAB salt displays till temperatures reach 68 and 96 ◦ C, for HS and DS, respectively. This implies that the diffraction spectra provide information about the phase transitions along the equilibrium line. For HS, from 30 to 42 ◦ C, H␣ forms and the d-spacing of peak (1 0) decreases from 4.5 to 4.3 nm with rising temperature. For DS, the H␣ forms in the temperature range of 30–44 ◦ C, while the d-spacing of peak (1 0) changes from 5.2 to 4.3 nm. M appears in the range 42–49 ◦ C for HS, 44–51 ◦ C for DS, while the peak (10) of Q␣ splits into two peaks: (1 0) and (0 1) of M. From 49 to 61 ◦ C (for HS) and 51 to 63 ◦ C (for DS), the formation of Q␣ is indicated by the appearance two reflections: (2 2 0) and (2 1 1). Q␣ transfers to L␣ with presence of the peak at 3.5 nm, at temperatures higher than 61 ◦ C (for HS) and 63 ◦ C (for DS). The layer thickness (same as the d-spacing of the peak) of L␣ reduces slightly down to 3.4 nm at 120 ◦ C. This experimental data does not show any evidence of the existence of the second lamella phase L␣ (II) at higher temperatures. It is noted from our experiments that Q␣ could be also formed directly from H␣ at a concentration of about 76 wt% in HS. In this case, the cubic phase coexists with the hexagonal phase in a temperature range, with the presence of three peaks: (2 2 0), (2 1 1) of Q␣ and (0 1) of H␣ . This fact suggests the possibility of existence of a small biphasic region near the boundary between Q␣ and H␣ in the phase diagram. Comparing the phase diagram in Fig. 1(a) with that of reported by Auvray et al. [10], the differences lie in the region of M. Unlike other phases, the phase transition M–Q␣ occurs along the constant concentration line (see Fig. 3), therefore the phase boundary between M and Q␣ is not a quasi-vertical line in the phase diagram, and M can be considered as pre-transition from H␣ to Q␣ . 3.3. Isotope effect
Fig. 5. The diffraction patterns of (a) HS with the concentration of 90.0 wt%, and (b) DS with the concentration of 89.1 wt%.
Fig. 5(a) and (b) illustrate the diffraction spectra of the HS 90.0 wt% and DS 89.1 wt%, respectively. These figures clearly indicate the liquid-crystalline phase transitions: HS:
(30 ◦ C) Hα (42 ◦ C)
⇒ M (49 ◦ C) ⇒ Qα (61 ◦ C) ⇒ Lα (120 ◦ C)
Solvent isotope effects can be observed in the lyotropic systems by comparing the diffraction patterns in Figs. 3 and 5: (1) the phase transition temperatures in DS are approximately 2 ◦ C higher than the corresponding temperature in HS; (2) DS has a smaller area of M phase in the phase diagram compared to that of HS; (3) DS has larger size of H␣ phase than that of HS at lower temperature. It is interesting to note that the swollen of hexagonal phase are shown in both first and second routes in our measurements (see Figs. 2b and 3). The swollen of hexagonal phase is a kind of secondary isotope effect, is induced by the stronger polar force in D2 O. Base on the self-assembly theory [15], the area per head group is determined by a balance between attractive and repulsive force of ionic head groups. Moreover, it is known that D2 O is a more polar solvent than H2 O [16]. Thus, changing the solvent induces a more effective screening of the electrostatic repulsion between the ionic heads, and consequently, reduces the average area per head group, and increases the spontaneous radius of the curvature of
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the monolayer and the size of hexagonal phase. The result of this effect is that the size of H␣ in DS is about 25% larger than that in HS, at 31 ◦ C. Similar isotope effects, the swollen of micelles significantly by replacing H2 O with D2 O, was also observed in other lyotropic systems, such as APFO/D2 O [11] and alkylglucoside surfactant/D2 O [13]. An extent example is that size of hexagonal phase in a sodium dodecyl sulfate (SDS) and water/pentanol/cyclohexane system can be increased for more than 10 times by modifying ionic force of the polar medium in the solution [17]. It indicates that the slightly rising the polar force of solvent (such as replace H2 O by D2 O) can induce a relatively large decrease of spontaneous curvature for the surfactant film in the phase H␣ , thus increasing the diameter of the phase H␣ , as what happened in DS. 4. Conclusion The phase diagrams of HS and DS have been determined by using TRXRD. The following isotope effects were observed by substitution of H2 O for D2 O: (1) the swollen lyotropic hexagonal phase at lower temperature; (2) the temperatures of phase transition shift up about 2–3 ◦ C. The isotope effect is mainly caused by the fact that D2 O is slightly stronger polar solvent than H2 O.
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