Article pubs.acs.org/JPCB
Pressure-Induced Transition of Bilayers in a Nonionic Surfactant Solution Tetsuo Takano, Youhei Kawabata,* Takuro Suzuki, and Tadashi Kato Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan S Supporting Information *
ABSTRACT: Pressure effects on the bilayers of polyoxyethylene type nonionic surfactant in water have been investigated by means of smalland wide-angle X-ray scattering. It has been found that the Krafft transition from the micellar phase to the lamellar gel phase (Lβ) is induced by pressure. By further pressurizing, the lamellar structural parameters, such as the repeat distance d and Caillé parameter η, discontinuously decrease after taking a maximum. All the SAXS and WAXS results revealed that the Lβ phase is transformed into the higher-ordered lamellar crystal phase (Lc). On the basis of these observations, we have made the T−C and T−P phase diagrams.
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narrow, vesicles are formed to be polydispersed and deformed.7 In the case of the deep quench, bilayers become rigid, and vesicles can not deform themselves. Therefore, thermodynamic parameters, such as temperature or pressure, are important for the vesicle and bilayer formations. For the pressure effects on the bilayer or vesicle structures, some researchers have reported pressure-induced structural transition in lipid or surfactant systems. Kaneshina et al. have reported pressure effects on the lipid bilayers and shown the various kinds of pressure-induced structures such as Lβ (lamellar gel phase) and LβI (interdigitated lamellar gel phase).11−13 Goto et al. presented the pressure-induced transition from Lα (lamellar liquid-crystal phase) to the Lc via the Lβ phase.14 These previous reports are in terms of dialkyl chains, and there are few studies for bilayer structures composed of monoalkyl surfactant or amphiphilic systems.15 In this study, we have investigated the pressure-induced bilayer structure by means of small and wide-angle X-ray scattering (SAXS and WAXS). We have found not only the Krafft transition induced by the lower pressure but also newly observed transition under the higher pressure, where intermembrane or intermolecular structures discontinuously change. All of the experimental evidence lead us to the conclusion that the transition is induced from Lβ to Lc and that the fluctuation of lamellar structure is enhanced and the lamellar gel structure swollen near the transition temperature and pressure. On the basis of this result, we could also summarize T−P (temperature−pressure) and T−C (temperature−concentration) phase diagrams.
INTRODUCTION Surfactants in water self-assemble to form various kinds of structures such as micellar, hexagonal, lamellar, and cubic phases. These structures can be varied with changing temperature, concentration, or pressure. Krafft transition is one of the important phenomena in the surfactant aqueous solutions involving these self-assemblies. The Krafft phenomenon has been known as the deposition of hydrated solids by decreasing temperature below the Krafft temperature, which can be interpreted as the melting phenomenon of hydrated solid surfactants.1−3 The hydrated solids are considered to be surfactant crystals, which are lamellar crystal (Lc) or gel phase (Lβ) coexisting with excess water; the Krafft transition can also be induced by pressure. Nishikido et al. have measured the pressure dependence of the Krafft temperature in typical ionic and nonionic surfactants and water systems.4,5 They showed that the Krafft temperature increases with increasing pressure, and that the Krafft transition can also be induced by increasing pressure. In our previous studies, we have found a transition corresponding to the Krafft phenomenon, i.e., transition from the micellar to the interdigitated lamellar phase in a polyoxyethylene-type nonionic surfactant, C16E7 (C16H33(OC2H4)7OH)/water system, below the Krafft temperature.6−10 After the temperature jump across the Krafft temperature, micelles are transformed to the lamellar gel phase (Lβ) in excess water. It has been also found that multilamellar vesicles with a hollow including excess water are spontaneously formed through the Krafft transition. This vesicle formation is one of a few reported cases in a nonionic surfactant system. Therefore, we have paid much attention to the conditions, structures, and kinetics of the vesicle formation. In the previous reports, we have shown that the vesicle and bilayer structures depend on temperature even below the Krafft point. For example, when the temperature quench depth is © XXXX American Chemical Society
Received: May 21, 2015 Revised: July 6, 2015
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DOI: 10.1021/acs.jpcb.5b04880 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry B
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EXPERIMENTAL SECTION Sample Preparation. Samples were prepared by mixing C16E7 with purified water so that the concentration became 10 wt %. C16E7 were purchased from Nikko chemicals, Inc. in crystalline form (>98%) and used without further purification. Before mixing, water was bubbled by nitrogen to avoid oxidation of the ethylene oxide group of surfactants. The cloud and Krafft temperatures are about 52 and 12 °C, respectively.10,16 Experimental Method. The samples were loaded into a high-pressure microheat stage (Hikari High-Pressure Machinery Co. Ltd.), which was customized for X-ray scattering experiments with two CVD diamond windows, shown in Figure 1. The CVD diamond, whose thickness was 1 mm, was made
spectra were obtained simultaneously with the each SAXS spectrum by PILATUS 100 K/R arranged at the front of vacuum tube. The q range of the WAXS experiments is 10−30 nm−1. At the NANO-Viewer (Rigaku Inc.), the scattered X-rays were detected with a PILATUS 100 K/R. The scattering vector (q) range is 0.5−3.0 nm−1.
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RESULTS AND DISCUSSION Stepwise Pressure Increment Experiments. Figure 2a shows SAXS profiles obtained at 2 min after increasing each pressure. Pressure was varied stepwise from ambient pressure to 100 MPa, with a fixed temperature of 13 °C. In the profiles at 0.1 and 5 MPa, a broad peak corresponding to the micellar phase was observed. At 10 MPa, the Bragg peaks around q = 0.8 and 1.6 nm−1, which are originated from lamellar gel phase (Lβ), were observed. With increasing pressure, the Bragg peaks became weak, gradually shifted to low q, and finally divided into two peaks. Figure 2b shows WAXS profiles measured simultaneously with the SAXS experiments. At 10 MPa, the peak around q = 15 nm−1 corresponding to the hexagonal arrangement of surfactant molecules in plane was observed. From these SAXS and WAXS results, we found that the Krafft transition can also be induced by pressure between 5 and 10 MPa. Next, we have conducted the same experiment at 4 °C, as shown in Figure 2c,d. Because the temperature was below the Krafft temperature, the Bragg peak due to Lβ lamellar phase was observed at ambient pressure. With increasing pressure, the peak was divided into two peaks, and at the pressure above 70 MPa, the Bragg peaks became sharp, and the peak position shifted to higher q. Figure 3 indicates the pressure dependence of the repeat distance d. The graph of pressure against d is also shown in the Supporting Information.23 The repeat distance increases with adding pressure and decreases abruptly at 70 MPa. Note that the lamellar structure with d ≈ 7.7 nm coexists with the swollen lamellar structure from 10 to 60 MPa. Figure 1d shows WAXS profiles, where the peak around q = 15 nm−1 gradually becomes sharp above 70 MPa. Compared to the profiles at the lower pressure, the peak at 100 MPa is obviously sharp. The discontinuous change in d between 60 and 70 MPa has not been observed in the temperature-quench experiments. These results suggest that the existence of another transition from Lβ phase at the higher pressure region. The peak splitting was observed between 70 and 100 MPa at 13 °C and between 10 and 60 MPa at 4 °C, which indicates the lamellar−lamellar coexistence in addition to excess water, i.e., the three-phase coexistence. According to the Gibbs phase rule, the system has only one degree of freedom (f) when three phases coexist (f = c − p + 2 = 1, c = 2, and p = 3, where c is the number of components and p is the number of phases.). Therefore, if either the temperature or pressure is fixed, the system is invariant. This means that the peak splitting should be observed not in a pressure range but at a fixed temperature only at one pressure point. The results in Figure 2 are in apparent contradiction to the Gibbs phase rule. We assume that this apparent contradiction would be due to the fact that the rate of pressure increment is too fast for the system to reach the equilibrium state. To solve this problem and possibility of any transitions at the higher pressure, we conducted pressure-jump experiments. Pressure-Jump Experiments. Figure 4a shows SAXS profiles multiplied by the fourth power of q to analyze them by using the scattering function of the lamellar phase. They were
Figure 1. Photograph (a) and drawing (b) showing a detail of the high-pressure microheat stage.
by Diamond Materials Inc. The diamond windows are supported by a stainless-steel block, whose contact surface to the diamond is polished, and they are fixed by a rubber cap. Pressure can be generated up to 100 MPa by hand-operated hydraulic pump through a capillary tube. Temperature was controlled using an external water circulator. For the SAXS and WAXS experiments, we used two spectrometers: the synchrotron radiation SAXS spectrometer installed in BL-6A at the photon factory (PF) of the HighEnergy Accelerator Research Organization (KEK), Tsukuba, and a Nano-Viewer (Rigaku Inc.). At BL-6A, the scattering Xrays were detected with a PILATUS 300 K/R and 1M/R. The scattering vector q (q = 4π sin θ/λ, where 2θ is the scattering angle and λ is the X-ray wavelength, 0.15 nm) range is 0.5−3.0 nm−1. The energy resolution (ΔE/E) is 3 × 10−3. WAXS B
DOI: 10.1021/acs.jpcb.5b04880 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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Figure 2. SAXS (a) and WAXS (b) profiles obtained immediately after adding pressure from ambient pressure to the indicated pressure at 13 °C (above the Krafft temperature). SAXS (c) and WAXS (d) profiles obtained immediately after adding pressure from ambient pressure to the indicated pressure at 4 °C (below the Krafft temperature).
obtained at 5 min after a pressure jump from ambient pressure. In these P-quench experiments, we could not find any peak splitting. Figure 4b shows the pressure dependence of the lamellar repeat distance d obtained from the peak position of the profile in Figure 4a. The pressure against d graph is also shown in the Supporting Information.23 The d discontinuously changes between 35 and 50 MPa, as shown by the dotted line. Below 35 MPa, the repeat distance increased with increasing pressure, while it decreased slightly above 50 MPa. To investigate the structural change further, we have analyzed the SAXS profiles using eq 1 proposed by Nallet et al. to explain the scattering data for lamellar bilayer structures,17 Figure 3. Pressure dependence of the repeat distance (d) at 4 °C. The red circles indicate the repeat distance of the coexisting lamellar phase.
Iq 4 = C
2π P(q)S(q)q2 d
(1) DOI: 10.1021/acs.jpcb.5b04880 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry B
Figure 4. (a) SAXS profiles obtained at 120 min after P-quenching from ambient pressure to the indicated pressures at 5 °C. The red lines are fitting results based on eq 1. (b) Pressure dependence of the lamellar repeat distance (d). (c) Pressure dependence of Caillé parameter obtained from the fitting analysis.
Figure 5. (a) WAXS profiles simultaneously obtained with the SAXS experiment. (b) Pressure dependence of the repeat distance dl corresponding to the intermolecular distance in bilayers.
Here, q is the scattering vector, P(q) is the form factor for bilayers, and S(q) is the structure factor for a lamellar structure. P(q) is written as P(q) =
where vh, veo, and vw are volumes of the hydrophobic chain, hydrophilic chain, and water, respectively, and h is the hydration number per ethylene oxide group. These three parameters, vh, veo, and vw are influenced not only by temperature but also by pressure. If the pressure effects on veo and vw are negligible,15 the temperature dependence of these parameters can be written as
2 ⎧ ⎛ δh ⎞⎫ ⎛ δ h ⎞⎤ 4⎡ ⎨ ⎬ ⎜ ⎟ ⎜ ⎟ ⎢ ⎥ ( ρ − ρ ) sin q + δ + ( ρ − ρ ) sin q eo w h eo ⎠⎭ ⎝ 2 ⎠⎦ ⎩ ⎝2 q2 ⎣ eo
(2)
where ρh, ρeo, and ρw are electron densities of the hydrophobic layer, the hydrophilic layer, and water, respectively. δh and δeo are the half-thickness of the hydrophobic layer and the hydrophilic layer, respectively. Thus, ρh, ρeo, and ρw are obtained from the relationships18 8n − 7 ρh = vh
ρeo =
185 + 10h veo + h
ρw =
10 vw
veo/nm 3 = 7(0.0613 + 0.000059(T /°C − 25)) + 0.0458 + 0.000027(T /°C − 25) vw /nm 3 = 0.0300 + 0.0000077(T /°C − 25)
(6) (7)
For vh, we should take into account the pressure effect, and we used the following equation on the basis of pressure dependence of molar volume of hexadecane:19
(3)
v hc/nm 3 = 0.3767(1.0806 + 0.0004(25 − T /°C)
(4)
− 0.0002P /MPa) (5)
(8)
S(q) is written as D
DOI: 10.1021/acs.jpcb.5b04880 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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S(q) = 1 + 2
⎛
∑ ⎜⎝1 − i=1
attractive forces between bilayers. As a result, the intermembrane distance might increase due to the Helfrich repulsive interaction. It has been reported that the surfactant parameter of nonionic surfactant increases progressively with increasing temperature because area per polar headgroup decreases.22 This may be due to the decrease in the hydration number of water around the headgroup area with increasing temperature. According to our fitting results, the hydration number per ethylene oxide group increases with increasing pressure (see Figure S1). Furthermore, the intermolecular distance dl increases at the higher pressure (see Figure 5). Therefore, we can infer that the packing of polar headgroups becomes loose due to the increase of the hydration number with increasing pressure. Finally, we have performed the pressure-jump experiment from 95 to 0.1 MPa at 4 °C (see Figure S8). The SAXS profile corresponding to the Lc phase at 95 MPa changes to that of the Lβ phase after the pressure jump. This result means that the Lβ/Lc transition is reversible, and that the Lβ phase here is stable. In summary, we imaged the structure formation process from Lβ to Lc phase with increasing pressure, as shown in Figure 6.
qdi i ⎞⎟ cos ⎠ N 1 + 2Δq2d 2α(i)
⎡ 2q d α(i) + Δq2d 2i 2 ⎤ 1 ⎥ exp⎢ − ⎣ 2(1 + 2Δq2d 2α(i)) ⎦ (1 + 2Δq2d 2α(i))1/2 2 2
(9)
where N is the number of bilayers in a domain, and Δq is the width of the resolution function. The α(i) is defined as η α (i ) = {ln(πi) + γ } (10) 2π 2 where γ is Euler’s constant, and η is Caillé parameter in terms of the elastic constants of the smectic phase. The width of the resolution function Δq was estimated to be 0.006 nm−1 by using the equation given by Glinka et al.20 The red lines are fitting results based on eq 1. In this analysis procedure, we especially focus on the parameters η and δh. From the fitting results, the Caillé parameters were obtained as shown in Figure 4c. η increases with increasing pressure below 35 MPa, while it decreases above 50 MPa. This result means that fluctuation of membranes is enhanced with increasing pressure and suppressed above 50 MPa. Figure 5a shows the WAXS profiles simultaneously measured with the SAXS. Above 50 MPa, the Bragg peak at around q = 15 nm−1 became sharper than that below 50 MPa. This tendency with increasing pressure is consistent with that represented in Figure 2d. This indicates that the molecules are closely packed in plane with higher order. Figure 5b shows the pressure dependence of the repeat distance dl, which was obtained from the profile in Figure 5a. The results of these SAXS and WAXS obtained by pressure jump to 35−50 MPa at 5 °C are summarized as follows. With increasing pressure: • The intermembrane distance decreases discontinuously. • The Caillé parameter decreases discontinuously. • The Bragg peak of the WAXS profile becomes sharp. In general, it has been known that bilayers in the lamellar gel phase Lβ have lower-order structures than those in the lamellar crystal phase, Lc.21 Therefore, these results strongly suggest that the phase transition to the Lc phase, which has a higher degree of order than that of Lβ, occurs between 35 and 50 MPa. If the Lc phase is induced by pressure, bilayers should become rigid because of their closely packed membrane structures. Thus, bilayers can not fluctuate, which leads to the decrease in the lamellar repeat distance. This is consistent with our experimental results. In our preliminary observation via microscope, angular-shaped vesicles have been formed at the Lc phase (see the Supporting Information). This would also consistent with the results here, and it is considered that the membranes of the vesicles are buckled due to their rigid bilayers. All these results lead us to conclude that the phase transition from Lβ to Lc is induced by pressure. Below 35 MPa, however, the pressure dependence of each parameter is summarized as follows: • The intermembrane distance increases. • The Caillé parameter increases. • The SAXS peaks become weak. From these results, it can be deduced that the packing of molecules in bilayers becomes loose, and the fluctuation of bilayers is enhanced with increasing pressure. In this case, the lamellar phase coexists with the excess water. Therefore, the lamellar spacing is determined by the balance of repulsive and
Figure 6. Schematic model of the transition process from Lβ to Lc for the C16E7/water system. The upper parts correspond to the molecular arrangement in a bilayer, the larger circles indicate surfactant molecules, and the smaller ones indicate hydrated water. The lower figures correspond to the variation of the lamellar repeat distance with increasing pressure.
The upper and lower parts in this figure represent the schematic molecular arrangement in the bilayer membrane and the lamellar structure, respectively. The large purple circles and the small blue circles are the headgroup and hydrated water, respectively. In the Lβ region, hydrated water increases, and the lamellar structure is swollen due to the enhancement of the thermal fluctuation of bilayers. When the pressure increases further, hydrophobic interaction between alkyl chains overcomes the steric repulsive force between polar heads, and the surfactant molecules make highly ordered arrangement corresponding to the transition to Lc. Thus, d decreases discontinuously at the Lβ/Lc transition pressure. T−C and −-P Phase Diagrams. To determine the boundary of Lβ/Lc transition in T−P phase diagram and the boundary of Lβ/Lβ + W (excess water), and Lc/Lc + W in T−C phase diagram, we have performed temperature variation experiments at 35, 50, 65, 80, and 95 MPa. Typical results are shown in Figures7 and 8, and the other results are shown in Supporting Information. Figure 8a shows the profiles at 65 MPa, observed at 20 min after increasing temperature continuously from 3 to 20 °C. The red lines represent the fitting result based on eq 1. Because the double Bragg peaks corresponding to the lamellar−lamellar coexistence appeared at E
DOI: 10.1021/acs.jpcb.5b04880 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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Figure 7. (a) Temperature variation of the SAXS profiles observed at 20 MPa, (b) 95 MPa, and (c) after applying the indicated pressure at 25° C.
Figure 8. (a) Temperature variation of the SAXS profiles observed at 20 min after increasing temperature at 65 MPa. (b) Temperature dependence of the repeat distance (d) obtained from the positions of SAXS peak shown in panel a. (c) Temperature dependence of Caillé parameter obtained from fitting analysis.
Figure 9. T−C phase diagrams of the C16E7/water system at 0.1 (a), 35 (b), and 65 (c) MPa. The dotted line represents the expected phase boundary.
Caillé parameters obtained from the fitting results, which also supports that the transition from the Lβ to the Lc phase occurs between 7 and 11 °C. Figure 9c represents the T−C phase diagram at 65 MPa, with the Lβ−Lβ + W phase boundary. The shaded area shows that the Lβ/Lc transition should occur in the region between 7 and 11 °C. The lower boundary of the Lβ or Lc phase was estimated, taking into account the lamellar swelling law ϕh = δh/d, where ϕh is the volume fraction of the
9 °C, we did not analyze the profile. The SAXS profiles are multiplied by the fourth power of q to estimate the bilayer thickness at each temperature and pressure by the curve fitting described above. At 9 °C, another Bragg peak appears at q ≈ 0.7 nm−1, although the main peak at q ≈ 0.9 nm−1 still exists. This indicates that the transition temperature should be 9 °C at 65 MPa. Figure 8b is the pressure dependence of the repeat distance d obtained from Figure 8a. The discontinuous change in d is observed between 7 and 11 °C. Figure 8c represents the F
DOI: 10.1021/acs.jpcb.5b04880 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry B hydrophobic part of bilayers, which was calculated using the following equation.18 1 ϕh = veo 1 − ws Mh + Meo vw 1+ v + w Mw vh (11) h s where Mh, Meo, and Mw are molecular weights of the hydrophobic chain, hydrophilic chain, and water, respectively. δh was obtained by the fitting analysis using eq 1. To consider the T−C phase diagram in detail, we have made the T−C phase diagrams at 35 MPa as well and compared it with that at 65 MPa, as shown in Figure 9. The dotted line represents an expected phase boundary. We have found that the boundary of Lβ/Lβ + W shifts to the lower concentration with increasing pressure. It should be noted that at 65 MPa, the lower boundary of Lβ rapidly decreases as the temperature approaches the transition temperature of Lβ/Lc. That is to say, the Lβ phase can be swollen down to 30 wt %, which is close to the swelling limit of the Lα phase of the C16E7/water system, 20−30 wt %. This swollen Lβ phase may be formed due to the Helfrich interaction enhanced by thermal fluctuation of disordered bilayers with increasing pressure, as mentioned before (see Figure 5) . After the transition to the Lc phase, the boundary moves to higher concentration, i.e., 50−60 wt %, because bilayers have the highly ordered arrangement of surfactant molecules, and they can not be swollen due to the suppression of the Helfrich interaction. Finally, we made the T−P phase diagram. To determine the boundary of Krafft transition (L1/Lβ) in the T−P phase diagram, we performed three experiments: temperature variations at 20 and 95 MPa and pressure variation at 25 °C. Figure 7 shows the SAXS profiles obtained in these three experiments. In Figure 7a,b, the Bragg peaks appear below 16 °C at 20 MPa and below 35 °C at 95 MPa. In Figure 7c, the Bragg peaks were observed at 55 MPa, while the broad peaks corresponding the L1 phase were obtained. Furthermore, using the result of Figure 2, we determined the transition pressure of L1/Lβ at 4 and 13 °C. The transition temperature at ambient pressure was already known to be 11.6 °C from our previous study.10 The estimated transition temperature and pressure were summarized in Table 1. Also using Figures 2−5, 7, 8, and
Figure 10. T−P phase diagram of C16E7/water system at 10 wt % at 0.1 (a), 35 (b), and 65 (c) MPa. The circles, diamonds, and stars indicate the L1 (micelle), Lβ, and Lc phases, respectively. The red and blue solid lines are L1/Lβ and Lβ/Lc boundaries, respectively.
surfactant systems (1.0−2.0 or 2.4 × 10−7 K/Pa) for the L1/Lc transition.4,5 However, the slope of the Lβ/Lc transition is about dT/dP = 2.0 × 10−7 K/Pa, which is a little smaller than that of the L1/Lβ transition. This difference in the value of dT/dP between both transitions is similar to that reported by Goto et al., who have showed the value to dT/dP for the Lα/Lβ transition is larger than that for the Lβ/Lc transition.
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CONCLUSIONS In this study, we have investigated the pressure effects on the bilayers of polyoxyethylene type nonionic surfactant in water by means of SAXS and WAXS. The discontinuous changes in repeat distance d and the Caillé parameter η of the lamellar structures with increasing pressure are observed in the SAXS profile analysis. For the WAXS results, the Bragg peak corresponding to the molecular packing appears, and it becomes sharp at the higher pressure. These results reveal that the transitions from the L1 to the Lβ phase and from the Lβ to the Lc phase are induced by pressure. Furthermore, the phase structures and boundaries were shown as the T−C and T−P phase diagrams. This is the first observation of the Lβ/Lc transition in a nonionic surfactant system, and pressure effects on the vesicle formation should be a future work.
Table 1. Estimated Transition Temperature and Pressure for L1/Lβ and Lβ/Lc L1/Lβ
Lβ/Lc
T /°C
P/MPa
T /°C
P/MPa
11.6 16−18 25 35−36
0.1 20 50−55 95
5 7−11 10−15
