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J. Phys. Chem. B 2008, 112, 8586–8590
Thermally Changing Lattice Distance of Lamella in the Hydrated Solid of Octadecyltrimethylammonium Chloride Shigeo Sasaki* Department of Chemistry, Faculty of Sciences, Kyushu UniVersity, 33 Hakozaki, Higashi ku, Fukuoka 812, Japan ReceiVed: March 17, 2008; ReVised Manuscript ReceiVed: May 07, 2008
A temperature scanning small-angle X-ray scattering measurement was carried out for the hydrated solids of octadecyltrimethylammonium chloride (OTAC). A gradual change of the lattice spacing of lamella-like structure from 40 nm at 5 °C to 20 nm at 18 °C was observed in the melting process of the hydrated solid that was incubated at 4 °C for a period of 24 h in the aqueous solution, while little change of the lattice spacing of about 20 nm was observed in the same process of the hydrated solid that was incubated at 4 °C for a period about 10 min. This indicates structural changes of the hydrated solid during the incubation at 4 °C and in the melting process. Corresponding to the nanostructure changes, broad endothermic peaks were observed at temperatures from 13 to 22 °C for the former hydrated solid and at temperatures from 15 to 21 °C for the latter hydrated solid in difference scanning calorimetry measurements. The structure change at temperatures below 13 °C is considered to be athermal from the fact that no endothermic peak is observed there. Large dielectric dispersions at frequencies at about 10 kHz were observed for the suspensions of hydrated solids but not for the solutions of dissolved solids. It was found that the electric conductance of the hydrated solid suspensions was much lower than that of the solutions of dissolved solids. The observed electric properties indicate that an amount of the free chloride ion is very small and that the chloride ions binding to the ammonium groups are movable in the hydrated solids by responding to an applied electric field. The electric conductance of suspension of the hydrated solid being incubated at 4 °C for 10 min was 4 times as large as that of a suspension of the hydrated solid being incubated at the same temperature for 24 h. This indicates that the structural change of the OTAC hydrated solid at 4 °C is related to the chloride ion binding to the hydrated solid. The experimental results described above suggest that the lamella in the hydrated solid of OTAC is undulated and that the wavelength of undulation increases with the incubation at a temperature much lower than the melting temperature. 1. Introduction Ionic surfactant molecules are fascinating physical systems, the thermodynamics and kinetics of which result from a delicate interplay of electrostatic, hydrophobic interactions and van der Waals forces. The aqueous solubility of an ionic surfactant as a function of temperature is very peculiar1 because there exists a micellar pseudophase (a small system), in which the alkyl chain is in the liquid state. The insolubilization is usually accompanied with the crystallization of alkyl chains of the surfactant molecules inducing the formation of the hydrated solids in water, which is called the Krafft transition.2 The crystallization of the alkyl chains, which form bilayers composing the multilamella structure, is mainly driven by the van der Waals attraction among them. The concentration of free counterion of the ionic surfactant in the aqueous solution usually decreases with forming the hydrated solid as a consequence of the counterion binding to the headgroup of the ionic surfactant molecule, which induces a decrease of the electric conductance. The Krafft transition from the micelle solution to the hydrated solid suspension of ionic surfactant is accompanied by the discrete reduction of the electric conductance.3 The degrees of counterion binding of micelles, which have been reported to be about 0.7-0.9,3,15 increase with the Krafft transition. Most * Tel. and fax: +81-92-642-2609. E-mail:
[email protected],
[email protected].
of the counterions in the hydrated solid are somehow trapped in the electrostatic potential trough produced by ionized groups of surfactant molecules. The crystallization of alkyl chains caused by the van der Waals attraction and the counterion binding inducing the entropy reduction should be coupled with each other in forming the hydrated solid. The coupling mechanism, however, has not been well-elucidated so far. It has been reported that the two states, the coagel phase (the bilayer lamella with thin water layer) and the gel phase (the bilayer lamella with thick water layer) possibly exist in the hydrated solids of octadecyltrimethylammonium chloride (OTAC).4 The coagel phase stabilized at a lower temperature has been believed to transform to the gel phase with an increase of temperature. If this is the case, the multilamella spacing of the hydrated solid of OTAC should increase with increasing temperature. For the purpose of examining this, we made a measurement of the temperature scanning small-angle X-ray scattering, TS-SAXS, and revealed that the spacing of the hydrated lamella of OTAC gradually decreased with increasing the temperature, which was unexpected from the coagel- and gel-phase transition described above. A question is raised about the structures involved in the Krafft transition of the ionic OTAC molecule. Our recent measurements5 of the TS-SAXS and the differential scanning calorimetry, DSC, for the Krafft transition of cetylpyridinium chloride (CPC) have revealed the existence
10.1021/jp802313c CCC: $40.75 2008 American Chemical Society Published on Web 06/27/2008
Changing Lamella Distance of Hydrated Solid of the metastable ordinary bilayer lamella phase and the stable interdigitated bilayer lamella phase in the hydrated solid. The metastable structure forms at first when decreasing the temperature of the CPC solution and gradually transforms to the stable structure with the elapse of time when holding it at the low temperature.5 The transition from the metastable structure to the stable structure has been found to be a time-consuming process. In the present experiment, the gradual decrease in the lamella spacing with increasing temperature was observed for the hydrated solid of OTAC incubated at 5 °C for 24 h but not for the solid incubated there for 10 min. The lamella spacing in the latter solid was the same as the shortest spacing in the former solid and was not changed with increasing temperature. The mechanism to induce the structure transition of hydrated solids of ionic surfactant such as CPC and OTAC remains still unresolved. The counterion of the ionic surfactant in a suspension of the hydrated solid binds to the headgroup of surfactant molecule. The counterion binding induces the low electric conductance of the hydrated solid suspension.3 The counterions loosely binding to the head groups of surfactant molecules and being over the lamella surface can be easily moved by an applied electric field. The frequency-dependent surface conductivity induced by the motion of bound counterions over the lamella surface can be detected as the large dielectric increment with a dispersion at a frequency lower than MHz.6,7 We measured the dielectric dispersion of the hydrated solid of OTAC and found large dielectric increments at frequencies lower than 100 kHz for the hydrated solid suspension. The dielectric dispersion spectra were found to transitionally change at the Krafft transition temperature, which was detected by the DSC measurement.
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Figure 1. Temperature scanning SAXS 3-D profile as a function of q for the OTAC solution incubated at 5 °C for about 24 h before scanning the temperature. The heating rate of the solution is 0.3 °C/min.
2. Experiment The OTAC purchased from Tokyokasei Co. Ltd. was recrystallized from methanol/acetone and then used. The double distilled water was used in the experiments. The TS-SAXS experiments were carried out for a 100 mM OTAC solution with a SAXS spectrometer of BL45XU (RIKEN Beamline I) installed at Spring8 of Japan Synchrotron Radiation Research Institute, Hyogo, Japan. The observed scattering vectors, q, were ranged from 0.07 to 5 nm-1. About 30 mg of the OTAC solution was encapsulated in an aluminum cell for the SAXS experiment, an optical thickness of which was 2 mm. The cell windows were made of polyimide films. The cell was mounted at a temperature control stage (LTS-350 Japan High Tec Inc.) slightly modified for the TS-SAXS measurements. The cell was incubated for about 24 h or about 10 min at 5 °C before scanning a temperature. The TS-SAXS measurements were made with heating at a rate of 0.3 °C/min. Measurements of the dielectric dispersion were made with using the computer-aided LCR meters (4285A Precision LCR meter Hewlett-Packard Inc. and 3522 LCR HiTESTER Hioki Inc.) for the OTAC solutions in the cell, the temperature of which was controlled within 0.1 °C. The DSC measurements were carried out with heating at a rate of 0.5 °C/min for the 100 mM OTAC aqueous solution using a DSC meter (DSC120 Seiko inc.). About 60 mg of the OTAC solution was encapsulated in an aluminum cell at room temperature and measured. 3. Results TS-SAXS. Figure 1 shows a SAXS 3-D profile as a function of q and T for the 100 mM OTAC solution incubated at 5 °C for about 24 h. The temperature was scanned with
Figure 2. Peak q-values as functions of T. The thick lines are the q-values of the ith peak, qi seen in the SAXS profiles of Figure 1. Broken lines represent the relations of qi(T) ) iq1(T), where q1(T) at T below 15 °C is half of the q-value of the second peak.
heating the solution at a rate 0.3 °C/min. Four peaks observed at temperatures below 18 °C are gradually shifted to high-q positions with increasing the temperature and disappear at 18 °C, above which a broad peak at q ) 0.45 nm-1 appears instead. The q-value of the ith peak, qi at lower temperatures can be described by qi ) iq1(T) as shown in Figure 2. Here, q1(T) is a q-value of the first peak as a function of T. This indicates that the structure of hydrated solid has a crystallike periodic order. The broad peak at the high temperature indicates a short-range ordered liquidlike structure. The lattice spacing, dL, estimated from the relation of dL ) 2π/q1(T) is plotted as a function of T and shown in Figure 3, which demonstrates a gradual decrease in dL from 42 nm at 6 °C to 20 nm at 18 °C with heat. It is noteworthy that dL is much longer than twice the length of a C18 alkyl chain in all-trans configuration, 2.7 nm. This clearly indicates that the structure cannot be identified with the multilamella composed of densely stacked bilayers of OTAC molecules or the lamella with a thin water layer. It is concluded that no pure coagel
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Figure 3. Temperature dependence of the lamella lattice spacing, dL estimated from the relation of dL ) 2π/q1(T), where q1(T) is the q value of the first peak as a function of T shown in Figure 2.
Sasaki
Figure 5. Enthalpy (top), dielectric (middle), and electric conductance (bottom) changes with the Krafft transition of OTAC. The data for solutions L and S, respectively, are represented by red and black colors.
micelles cannot be transformed to the structure with a periodic length of 42 nm. Changes in Enthalpy and the Dielectric Properties of Solution with the Krafft Transition. Figure 5 shows the results of the DSC measurements, the dielectric properties and the electric conductance for 100 mM OTAC solution incubated at 4 °C for a long period of 23 h. This solution will be called solution L. The results obtained for the 100 mM OTAC solution incubated at 4 °C for 10 min, which will be called solution S, are also shown in Figure 5. The electric conductance, κ, at a frequency of f ) 100 Hz shown in Figure 5 is corrected for the viscosity to κcorr of the solution at 25 °C by using
κcorr ) κ(T) κNaCL(25οC)/κNaCl(T)
Figure 4. SAXS 3-D profile as a function of q and T for the OTAC solution incubated at 5 °C for about 10 min after cooling it. The heating rate is 0.3 °C/min.
structure exists in the hydrated solid of OTAC being incubated at 5 °C for about 24 h. Figure 4 shows another SAXS 3-D profile as a function of q and T for the OTAC solution incubated for 10 min at 5 °C after cooling it down from 25 °C at a rate of about 10 °C/min. The feature of the profile is obviously different from the SAXS 3-D profile shown in Figure 1. Four peaks at q ) 0.3 0.44, 0.6, 0.9, and 1.2 nm-1 observed at T below 12 °C disappear at T above 12 °C, where one peak at q ) 0.45 nm-1 is observed. The peaks at q ) 0.3 0.6, 0.9, and 1.2 nm-1 are considered to be a consequence of the ordered periodic array with a lattice spacing of 20.9 nm, which is much longer than the lattice spacing of the multilamella crystal composed of densely stacked bilayers of OTAC molecules. It should be mentioned that no pure coagel phase is detected in the present experiment. The broad peak at q ) 0.45 or 0.44 nm-1 is a consequence of the average interparticle (micelles) interference, indicating the correlation length of the pair distribution function of OTAC micelles, ξC ) 14 or 14.3 nm. During the 10 min incubation at 5 °C, OTAC
(1)
where κNaCl(T) is the κ of an aqueous 10 mM NaCl solution at a given T. The dielectric permittivities, ε at f ) 10 kHz for solutions L and S are plotted against T and are shown in Figure 5. The electrode polarization effect on the dielectric dispersion spectrum,8,9 which gives an additional ε increment being proportional to f -1.6, is not so significant at f ) 10 kHz as shown in Figure 6, which exhibits the spectra for solutions L and S. The DSC curves shown in a top figure in Figure 5 exhibit endothermic transition peaks at about 13 °C for solution L and at about 15 °C for solution S, which demonstrate endothermic melting processes of the hydrated solid. The transition enthalpies estimated from the peak areas for both solutions L and S are about 44 kJ/mol, which is intermediate between the reported enthalpies associated with the coagel-micelle transition and the gel-micelle transition.4 It should be mentioned here that the coincidence between changes of enthalpy, dielectric properties, and electric conductance with the Krafft transition is reasonably good. The broad endothermic peak of solution L reflects on the gradual decrease of the spacing of periodic structure in the hydrated solid with the increase of temperature as shown in Figure 3, while the relatively sharp endothermic peak for solution S reflects on no change of the spacing in the hydrated solid with temperature as shown in Figure 4. The fact that the endothermic process of solution L remains at the temperature where the Krafft transition is almost over in solution S indicates the slow-melting kinetics of the hydrated solid in solution L. The high κcorr at T above 22 °C shown in Figure 5 is a consequence of the highly concentrated free ions derived from
Changing Lamella Distance of Hydrated Solid
Figure 6. Dielectric dispersion spectra for solutions L (closed circle) and S (open circle). A dotted line represents the typical polarization effect of electrodes on the dispersion. Solid and broken lines, respectively, represent the permittivity of solutions L and S corrected by subtracting the polarization effect.
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Figure 8. Cross-polarized light micrograph of a 100 mM OTAC hydrated solid suspension at 12 °C.
κ-value, as shown in Figure 7. It can be concluded from results of the SAXS experiments that the increase in spacing of the periodic structure of the hydrated solid is accompanied with reduction of the κ-value or an increase of the degree of counterion binding and that the bound counterions are movable in the hydrated solid to give ε ∼ 3500 at f ) 10 kHz regardless of the spacing. 4. Discussion
Figure 7. Time courses of the dielectric permittivity (upper curve) and the electric conductance (lower curve) of the 100 mM OTAC solution incubated at T ) 4 °C after cooling down from 25 °C.
the OTAC monomers and micelles in the solution. The low κcorr at T below 13 °C shown in Figure 5 is a consequence of the free ion concentration reduction caused by the binding of Clions to the hydrated solid. The gradual κcorr-change at T between 13 and 22 °C indicates the gradual melt of hydrated solid with increasing T. It should be noted that the κcorr of solution S is much higher than that of solution L. The 10 min incubation at 4 °C is too short to fully transform to the hydrated solids from the OTAC monomers or micelles. This is also observed in the SAXS results for solution S shown in Figure 4, which exhibits the pair distribution function of micelles inducing the broad peak at q ) 0.44 nm-1. Kinetic Change of the Dielectric Properties of Solution with the Krafft Transition. The transition from the micelle solution to the hydrated solid is a time-consuming process as shown in Figure 7, which exhibits time courses of the dielectric permittivity, ε(f)10kHz) and the electric conductance κ(f)100Hz) of the 100 mM OTAC solution, which is rapidly cooled down from 25 to 4 °C at t ) 0. It takes about 500 s that the ε-value reaches a plateau, while a longer time than 1000 s is needed for the κ-value to reach a plateau. As a matter of fact, it takes a longer time than 100000 s to reach the equilibrated
Our findings in the present experiments concerning the Krafft transition and the hydrated solid of OTAC are summarized as follows: (1) The 40 nm lattice spacing of the periodic structure at 5 °C decreases to 20 nm at 18 °C in the melting process of the hydrated solid incubated at 4 °C for the period of 24 h, as shown in Figure 3, while the 20 nm lattice spacing that appeared in the hydrated solid incubated at 4 °C for the period about 10 min changes little in the melting process, as shown in Figure 4. (2) The melt of the former hydrated solid accompanies the endothermic enthalpy change between 13 and 22 °C, and the melt of the latter hydrated solid accompanies the endothermic enthalpy change between 15 and 21 °C, as shown in Figure 5. (3) No enthalpy change with the decrease in the lattice spacing is observed at temperatures below 13 °C. The structure change inducing the lattice spacing is made without changing enthalpy of the system. (4) Forming the hydrated solid accompanies the counterion binding to head groups of a surfactant molecule, which is indicated by the decrease of the electric conductance of solid suspensions, as shown in Figure 5. (5) The increase in the degree of counterion binding indicated by the further reduction of the electric conductance, as shown Figure 7, is accompanied by the increase of the lattice spacing of periodic structure in the hydrated solid. (6) The bound counterion can move in response to the applied electric field, which induces the large dielectric permittivity, for example, ε ∼ 3500 at f ) 10 kHz, as shown Figures 5–7. (7) The large dielectric permittivity observed for the hydrated solid suspension is insensitive to the change of lattice spacing in the hydrated solid. It can be concluded that the change in lattice spacing observed in the TS-SAXS experiments shown in Figure 3 cannot be explained by the coagel- to gel-phase transition.4 The gel phase usually appears as a translucent gellike phase, and the coagel phase appears as an opaque mass.10 The appearance of the aqueous suspension of OTAC hydrated solid is a soft and opaque solid. Figure 8 shows a cross-polarized light micrograph of a
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Figure 9. Schematic leaflet structure of the undulated lamella. The rigid bilayer of the coagel phase is assumed to be interdigitated.12 The neighboring rigid bilayers are connected by a flexible bilayer, from which surfactant molecules represented by the isolated rods can be introduced into the bilayers. The counterions represented by small circles are bound on the surface of rigid bilayer and spread nearby the flexible bilayers.
100 mM OTAC hydrated gel suspension incubated at 12 °C for a period of about 1 h. The characteristic appearance of the lyotropic gel11 can be observed in the micrograph shown by Figure 8, in which a characteristic structure of the coagel phase such as a preferentially oriented structure12 is not exhibited. It has been reported13 that the temperature-dependent infrared spectra of CH stretching and CH2 scissoring bands of the methylene chain in the 20 w/w % water-OTAC system suggests the transformation of trans-zigzag methylene chains arrayed on parallel planes into the rotating and flexible chains with a rise in temperature. The coagel-gel transition has been inferred to correspond to the transformation.13 This is not the present case, because the water content of the 100 mM OTAC solution is more than 90 w/w %, which destabilizes the coagel phase and stabilizes the gel phase. It is reasonable to infer that the gel and coagel phases coexist in the hydrated solid of the present system. The hybridized coagel and gel phases, in which the parallel packed and rotating trans-zigzag methylene chains are mixed together, can form the undulated lamella structure of a hydrated solid. The regular array of bilayers composed of flexible and rigid chains can undulate the lamella structure. The lamella bends at a flexible chain rich domain and flattens at a rigid chain rich domain as shown in Figure 9, which schematically demonstrates the hybrid lamella structures of coagel and gel phases. It is natural that the coagel lamella phase is undulated by inserting the flexible gel phase into it. The thermal motion of methylene chains destabilizes the coagel phase, transforms it to the gel phase and/or dissociates the monomeric unit from the gel phase to the bulk. A rise in temperature decreases a coagel domain and increases a ratio of the gel phase to the coagel phase in the hydrated solid. Distance between neighboring flexible gel phases, which decreases with an increase in the ratio of the gel phase to the coagel phase, reflects on the dL-value, which decreases with a rise in temperature, as shown Figure 3. Therefore we can say that the decrease in the spacing of onedimensional periodic structures in the hydrated solid as shown in Figure 3 is a consequence of the decrease in the coagelphase domain. If a growth rate of the coagel phase is much lower than that of the gel phase, then the wavelength of undulated lamella is short at first when decreasing the temperature, and it increases with growing the coagel phase in the hydrated solid. The present TS-SAXS results for the solutions L and S as shown in Figures 1 and 4 can be explained by much slower growth of the coagel phase than that of the gel phase. The wavelength of undulated
Sasaki lamella slowly increases with incubating the hydrated solid for the long period at the low T. The gel phase might transform to the coagel phase in the hydrated solid over the incubation time. It is possible to say that the gel phase is metastable but kinetically more favored to form than the coagel phase. That is, the surfactant molecules are mainly introduced into the hydrated lamella at the gel phase, as shown in Figure 9. The counterion loosely bound to the arrayed head groups of surfactant molecule consisting of lamella can move in the space of the thick hydration layer on the lamella, which induces the large dielectric increment of hydrated solid, as shown in Figure 5. The periodic density fluctuation of ionic head groups over the lamella plate, which is produced by the periodic distribution of coagel and gel phases in the undulated structure, as shown in Figure 9, makes the electrostatic potential wavy and induces the spatial fluctuation of the counterion distribution, various modes of which are excited by the applied alternating current (AC) electric fields decay with the diffusional relaxation to produce the low-frequency dielectric dispersion of the hydrated solid.14 It should be mentioned that the dielectric increment could diminish when the gel phase disappears in the hydrated solid. We can say that the lamella structure of hybrid coagel and gel phases exists as long as the large dielectric increment of the hydrated solid is observed. 5. Conclusion It is found in the TS-SAXS experiments that the spacing of periodic structure in the hydrated solid of C18 ionic surfactant, OTAC, decreases with an increase of temperature and that the spacing increases with time when incubating it at a low temperature, for example, 4 °C. From the fact that the spacing is much longer than the thickness of the bilayer of C18 alkyl chains, the periodic structure is inferred to be undulated lamella, a leaflet of which consists of the periodically arrayed rigid and flexible bilayers. The rigid lamella portion is coagellike, and the flexible lamella is gellike. From the large dielectric increment of the hydrated solid, it is inferred that the counterion of head groups constrained in the electrostatic potential valley between the lamellae can move by responding to the applied AC electric field. References and Notes (1) Shinoda, K.; Fontell, K AdV. Colloid Interface Sci. 1995, 54, 55– 72. (2) Krafft, F.; Stern, A. Ber. Dtsch. Chem. Ges. 1894, 27, 1747–1761. (3) Vautier-Gingo, C.; Bales, B. L. J. Phys. Chem. B 2003, 107, 5398– 5403. (4) Kodama, M.; Tsujii, K.; Seki, S. J. Phys. Chem. 1990, 94, 815– 819. (5) Sasaki, S. J. Phys. Chem. B 2007, 111, 2473–2476. (6) Schwarz, G. J. Phys. Chem. 1962, 66, 2636–2642. (7) Schurr, J. M. J. Phys. Chem. 1964, 68, 2407–2413. (8) Johnson, J. F.; Cole, R. H. J. Am. Chem. Soc. 1951, 73, 4536– 4540. (9) Scheider, W. J. Phys. Chem. 1975, 79, 127–136. (10) Vincent, J. M.; Skoulios, A. E. Acta Crystallogr. 1966, 20, 432– 441. (11) Cassin, G.; de Costa, C.; van Dyunhoven, J. P. M.; Agterof, W. G. M. Langmuir 1998, 14, 5757–5763. (12) Ambrosi, M.; Nostro, P. L.; Fratoni, L.; Dei, L.; Niham, B. W.; Palma, S.; Manzo, R. H.; Allemandi, D.; Baglioni, P. Phys. Chem. Chem. Phys. 2004, 6, 1401–1407. (13) Kawai, T.; Uemura, J.; Takenka, T.; Kodama, M.; Seki, S. J. Colloid Interface Sci. 1985, 103, 56–61. (14) Oosawa, F. Polyelectrolytes; Dekker: New York, 1971; Chapter 5. (15) Aswal, V. K.; Goyal, P. S. Phys. ReV. E 2003, 67, 051401.
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