Article pubs.acs.org/Langmuir
Magnetic-Field-Induced Orientational Phase Structure Transition Yingying Dou, Shuli Dong, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry and Key Laboratory of Special Aggregated Materials (Shandong University), Ministry of Education, Jinan 250100, People’s Republic of China S Supporting Information *
ABSTRACT: Magnetic field effect on the phase transition at high temperature (from 50 °C) inside the magnetic field has been found in C14G2 (N-tetradecyllactobionamide)/C12EO4 (tetraethylene glycol monododecyl ether)/D2O system. The phase was transited quickly from lamellar phase to isotropic phases [bottom, micellar phase (L1 phase) and top, sponge phase (L3 phase)] induced by a magnetic field, which was demonstrated by 2H NMR and FF-TEM measurements. The isotropic phases induced by magnetic field were not stable, and the upper L3 phase can recover to lamellar phase after being restored in a 55 °C thermostat outside the magnetic field for about one month. During the mechanism study, the C12EO4 molecule was proved to be the dominant component for the phase transition induced by the magnetic field, while the C14G2 molecule was the auxiliary and just affected the transition speed. The breaking and rebuilding of hydrogen bonds could play an important role in the phase transition and recovering. Moreover, the surfactant concentration had an effect on the speed of phase transiting and phase recovering. These observations could provide an understanding of the phase transition and also the applications for the controlled drug delivery system of bilayer membranes driving, induced by the magnetic field. field is from 0° to 90°. With the influence of the magnetic field, the angle tends to be 90° (i.e., the lamellae are preferred to be parallel to the magnetic field due to the average direction of the hydrocarbon chains of surfactant molecules being perpendicular to the magnetic field),10−15 which is illustrated by Firestone16 and Chmelka.17 In 1999, Briganti studied the magnetic field effect on the phase transition of C12EO6/D2O system induced by cooling in a magnetic field10,11 and observed that the Pake doublet for the parallel orientation of the surfactant chains to the magnetic field disappeared with the cycle of increasing the temperature from 300 K to be above the phase boundary, and then decreasing to the original temperature in a strong magnetic field (14.1 T) due to the orientation effect of a magnetic field. In the system of C14G2/C12EO4/D2O, the magnetic-field effect on the phase structure transition at a given temperature in the magnetic field far below the original phase transition boundary was observed. To the best of our knowledge, it is the first report of the surfactant mixtures in solution.
1. INTRODUCTION Deuterium nuclear magnetic resonance (2H NMR) spectrum has been considered a good way to study phase behaviors of amphiphiles in solution, such as micelles, vesicles, planar lamellar phases, and hexagonal phases. The mechanism is that the degree of deuteron nuclei (spin I = 1) quadrupole splitting [Δ(2H)] relates to the hydration of surfactant aggregates, including the fraction of water molecules oriented by the aggregate surfaces and their average degree of orientation.1−3 For anisotropic aggregates, the deuteron quadrupole splitting average is not zero, so two peaks are generally observed on 2H NMR spectra,4−7 while for isotropic aggregates, the deuteron quadrupole splitting average is zero, and the 2H NMR spectra show only one single peak.1,4−8 Besides, the peaks of different aggregates are superimposed, such as two different anisotropic aggregates1,4 and the coexistence of anisotropic and isotropic aggregates,1,5,8 except the coexistence of two different isotropic aggregates, which only shows one single peak. Herein, 2H NMR spectra have been used to characterize the phase structure transition of the surfactant system with the variation of surfactant concentration, 3 temperature, 4,5 and shearing forces,6,7,9 etc. At high temperature, magnetic effect will be significant for 2H NMR characterization of some special surfactant aggregates in solution.10 In the magnetic field, the angle between the normal of bilayers of lamellar phases and the direction of the magnetic © 2014 American Chemical Society
Received: December 13, 2013 Revised: January 20, 2014 Published: January 21, 2014 1266
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2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Tetraethylene glycol monododecyl ether (C12EO4, >99%) was purchased from Acros Organics. Ntetradecyllactobionamide (C14G2, high purity) was provided by Heinz Hoffmann’ lab, Bayreuth University, Germany. Heavy water (D2O, ≥99.9%) was purchased from Aldrich. These chemicals above were all directly used without further purification. The water used in the experiments was prepared by a UPHW-III-90T-type apparatus, with a resistivity of 18.25 MΩ cm. 2.2. Deuterium Nuclear Magnetic Resonance (2H NMR) Measurements. 2H NMR measurements were carried out on a Bruker Avance 400 spectrometer equipped with a pulsed-field gradient module (z axis). The samples (1.0 mL) were prepared with heavy water (D2O) in NMR tubes of 0.5 cm diameters. Before the 2H NMR measurement, the samples were stayed in 55 °C thermostat for two weeks to equilibrate. During the phase recovery process, the temperature was kept as 55 °C in a thermostat outside the magnetic field. 2.3. Freeze-Fracture Transmission Electron Microscopy (FFTEM) Observations. Approximately 4.0 μL aliquots of the viscous sample solutions were dropped onto the specimen carrier and frozen quickly in liquid ethane at −175 °C. After frozen, the sample was fractured and replicated by Leica EM BAF 060 equipment at −175 °C. For the replication, the Pt/C (45°) film was sprayed for 2.5 nm and the C (90°) film for 18 nm. The replica was loaded on a copper grid and observed using a JEOL JEM-1400 electron microscope operating at 120 kV. 2.4. Fourier Transform Infrared Spectroscopy (FT-IR) Measurements. The changing of the hydrogen-bonding after 2H NMR measurements was proven with the use of a VERTEX-70/70v FT-IR spectrometer at 55 °C, with the accuracy of ±4 cm−1.
Figure 1. 2H NMR spectra of 1.0 wt % C14G2/15 wt % C12EO4 sample, with an increase in one cycle of 8 scans from bottom to top at 9.4 T and 50 °C.
Supporting Information. However, keeping the model sample in the 2H NMR spectrometer for only 40 s (23 magnetic scans), a remarkable phase transition from anisotropic phase to isotropic phases with only one splitting peak (Figure 2b) is obtained. FF-TEM images distinguish the isotropic phase growth in the magnetic field, which consists of two types of isotropic phases. The upper phase is the sponge L3 phase (bilayers), and the bottom is the micellar phase with a diameter of about 20 nm, as demonstrated by FF-TEM images in Figure 3 (panels a and b). The inserted micrographs of the model sample with or without polarizers in Figure 2 can further confirm the phase transition progress. Figure 2a shows obvious birefringent textures between the crossed polarizers, suggesting the existence of lamellar phase, while Figure 2b shows two isotropic phases without birefringent textures between the polarizers. The 1.0 wt % C14G2/20 wt % C12EO4 sample was kept at 55 °C without stirring for one month to investigate the stability of the isotropic phase grown in the magnetic field. The 2H NMR spectrum, as shown in Figure 2c, shows that the upper phase is mostly recovered after around one month, with three splitting peaks, in which the central single peak represents the remaining isotropic phase, and the two side splitting peaks for the lamellar phase, confirmed by FF-TEM (Figure 3c). The optical micrographs, as shown in Figure 2c, also demonstrate the lamellar phase by the birefringent textures (upper phase). Meanwhile, the bottom L1 phase, no change is found, still remains transparent. These 2H NMR spectra and FF-TEM images clearly suggest that the isotropic phases induced by the magnetic field effect are thermodynamically unstable. However, as is well-known, both L1 phase19 (micelles) and L3 phase (sponges)20,21 of surfactants in solution are thermodynamically stable. So far, the reason for this abnormal phenomenon is unclear and much analysis is needed. 3.2. C12EO4 Concentration Effect on the Phase Transition Speed Induced by Magnetic Field. C12EO4 concentration was observed to play a role in the phase transition speed induced by the magnetic field. For the sample of 1.0 wt % C14G2/15 wt % C12EO4 (Figure 4a), after the initial
3. RESULTS AND DISCUSSION 3.1. Investigation of the Phase Transitions Induced by Strong Magnetic Field (9.4 T). In our previous work of the system C14G2/C12EO4/H2O without a magnetic field,18 when the C14G2 concentration was fixed to be 1.0 wt % and the C12EO4 concentration was larger than 15 wt %, lamellar phases such as vesicle, planar lamellar phase, or their coexistence were obtained from 25 to 70 °C (Figure S1 of the Supporting Information). The lamellar phases were transited to isotropic phases when the temperature was high enough. For the sample of 1.0 wt % C14G2/15 wt % C12EO4, its transition temperature from lamellar phase to isotropic phase was determined to be ∼58 °C, and for 1.0 wt % C14G2 mixed with 20 wt % C12EO4, it was ∼58 °C. For 25 wt % C12EO4, the transition temperature was ∼65 °C, and for 30 wt % C12EO4, it was ∼68 °C (see Figure S2 of the Supporting Information). More interesting, during the 2H NMR measurements of this system, we observed that the phase structure transition temperatures of lamellar phase to isotropic phases decreased greatly to be nearly 50 °C (Figure 1) under magnetic field. This suggested that the magnetic field leads to an 8 °C decrease of phase structure transition temperature for the samples of 1.0 wt % C14G2 mixed with 15 wt % C12EO4 and 20 wt % C12EO4, and for 1.0 wt % C14G2/30 wt % C12EO4 sample, it is up to 18 °C. To observe the phase transition induced by the magnetic field more clearly (as shown in Figure S1 of the Supporting Information, only planar lamellar phases exist without the magnetic field at 55 °C), T = 55 °C was kept in this study. Take the sample, 1.0 wt % C14G2/20 wt % C12EO4, as a model system for the study. Before the magnetic field has a real effect, the 2H NMR spectrum (Figure 2a)18 of the sample shows two obvious splitting peaks for the planar lamellar phase. The FF-TEM image clearly confirms the lamellar phase to be the planar lamellar phase, as shown in Figure S4 of the 1267
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Figure 2. 2H NMR spectra and the micrographs of 1.0 wt % C14G2/20 wt % C12EO4 sample at (a) 55 °C, at (b) 9.4 T, and (c) stillness for 1 month at 55 °C without the magnetic field. The scanning for each sample is 8 times. The micrographs of the samples inserted were taken without (left) and with (right) polarizers.
Figure 3. The FF-TEM images of 1.0 wt % C14G2/20 wt % C12EO4 sample after the phase transition induced by the magnetic field (a, the upper phase and b, the bottom phase, at 9.4 T) and after one month recovery outside the magnetic field (c, lamellar phase). T = 55 °C.
Figure 4. 2H NMR spectra of three samples, increase one cycle of 8 scans from bottom to top at 9.4 T and 55 °C at 1.0 wt % C14G2. The C12EO4 concentration is (a) 15 wt %, (b) 20 wt %, and (c) 25 wt %.
scanning ∼25 times (i.e., about 44 s) to demonstrate a novel fast phase transition process. For the sample of 1.0 wt % C14G2/20 wt % C12EO4 (Figure 4b), the phase transition was found to be about 23 scans (about 40 s), while for the sample of 1.0 wt % C14G2/25 wt % C12EO4 (Figure 4c), only 16 scans (about 28 s) are needed, which clearly indicates that phase structure transition is easier for the samples at a higher C12EO4 concentration. Speculating the reason for this novel observation, we take the magnetic field during 2H NMR measurements into consideration, which caused the hydrogen bonds in these systems to be broken. To separate the magnetic field effect
three cycles (24 scans, each of the eight scans as a cycle, which takes about 14 s), only double splitting peaks (i.e., lamellar phase) were observed. However, after the fourth cycle, three splitting peaks appear, the outside double peaks for the lamellar phase while the inside is a single acute peak for the isotropic phases, indicating the onset of the phase structure transition. After further scanning, 2H NMR spectra show only a single acute peak (i.e., isotropic phase forms completely). These observations indicate that the phase structure transition from lamellar phase to isotropic phases induced by the magnetic field for the sample of 1.0 wt % C14G2/15 wt % C12EO4 occurs after 1268
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Figure 5. The 2H NMR spectra of C12EO4 samples: (a) 20, (b) 30, (c) 40 wt % with 8, 64, and 128 magnetic scans from the bottom to the top at 9.4 T and 55 °C.
Figure 6. The FT-IR spectra of samples of (a) 20 wt % C12EO4, (b) 30 wt % C12EO4, and (c) 1.0 wt % C14G2/20 wt % C12EO4 without (black line) or with (red line) 256 magnetic scans at 9.4 T and 55 °C.
from the radio wave during the scanning, we put these three samples in the NMR instrument and took them out after 5 min without scanning at 55 °C, respectively. The phase transition from lamellar phase to two isotropic phases is also observed. So it is clear that the magnetic field causes the phase transition other than the radio wave during scanning. As the magnetic field affects the unpaired electron’s movement, for the sample 1.0 wt % C14G2/25 wt % C12EO4, which has more hydroxyl group (−OH) thus more unpaired electrons, responses to the high magnetic field (9.4 T) are quicker and thus easier for the phase transition. 3.3. Mechanism of the Phase Structure Transition Induced by Magnetic Field. Initially we tried to determine
which was the acting composition of C14G2 or C12EO4. Three samples of 20, 30, and 40 wt % C12EO4 in D2O at 55 °C were characterized by using 2H NMR measurements, respectively (Figure 5). Due to the slight solubility of C14G2 in water, we did not get the 2H NMR spectra of C14G2/D2O system. From Figure 4, we found that C12EO4 in D2O can also be influenced by the magnetic field, transforming from lamellar phases to isotropic phases, just like the C14G2/C12EO4/H2O system, which indicates that C12EO4 is the acting component for the magnetic field effect. However, we cannot prove whether C14G2 is the acting component or not because of its lower solubility. We suppose the reason is similar to that C14G2/C12EO4/H2O system, where during 2H NMR measurements, the magnetic 1269
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Figure 7. The 2H NMR spectra of (a) 1.0 wt % C14G2/30 wt % C12EO4 and (b) 30 wt % C12EO4 samples after keeping 1 month recovery at 55 °C without a magnetic field. Eight magnetic scans at 9.4 T.
field affects the unpaired electrons of −OH, which leads to the breakage of the hydrogen bonds formed by −OH. Along with the breakage of hydrogen bonds, the lamellar phase disappears and isotropic phases form. In addition, the FT-IR data provide indirect evidence for the breakage of hydrogen bonds during the phase structure transition. From Figure 6 (panels a and b), one can detect that after 256 magnetic scans, the FT-IR peak of both 20 wt % C12EO4 and 30 wt % C12EO4 samples have a blue shift, similar with the 1.0 wt % C14G2/20 wt % C12EO4 sample (Figure 6c). It is because breaking the hydrogen bonds leads the electric density between oxygen and hydrogen of −OH to increase, resulting in the force constant of the O−H bond increasing. Hence, the FT-IR results indirectly prove the breakage of hydrogen bonds as the reason for the phase structure transition process induced by the magnetic field. 3.4. Recovery Process of the Phases Induced by Magnetic Field. After the 2H NMR measurements of the samples of C12EO4 in D2O, their phase recovery speed was observed, which was much slower than the model sample of the C14G2/C12EO4/D2O system at 55 °C. For both 30 wt % C12EO4 and 1.0 wt % C14G2/30 wt % C12EO4 in D2O, three splitting peaks were observed (Figure 7) after one month recovery. However, the inside single peak representing the isotropic phase is much smaller for the 1.0 wt % C14G2/30 wt % C12EO4 sample (Figure 7a) than for that of the 30 wt % C12EO4 sample (Figure 7b), indicating that the 1.0 wt % C14G2/30 wt % C12EO4 sample is easier to be phase recovered than 30 wt % C12EO4 sample without C14G2; in other words, adding C14G2 facilitates the phase recovery. One of the reasons for this phenomenon may be that C14G2 molecules provide more hydrogen-bond forming sites, hence the hydrogen bonds are easier to be formed, leading to easier phase recovery. Meanwhile, it is easier for C12EO4 molecules to interact with C14G2 molecules than water molecules due to the hydrogen bonds and hydrophobic interactions. At the same time, higher C12EO4 concentration is also beneficial for the phase recovery [i.e., the phase recovery of the 1.0 wt % C14G2/30 wt % C12EO4 sample (Figure 7a) is easier than that of the 1.0 wt % C14G2/20 wt % C12EO4 sample (Figure 2c)]. On the other hand, when we just put the sample in the NMR instrument without 2H NMR scanning, we observed the lamellar phase recovery from the upper isotropic phase quickly (less than 1 min) after taking the sample out of the magnetic field. This phenomenon provides more evidence to the partial reversibility of the phase transition induced by the magnetic field: just the upper isotropic phase back to the lamellar phase. Second, this experimental result indicates that the phase transition induced by only the magnetic field is more unstable than adding 2H NMR scanning with radio wave energy. So, we propose that with the magnetic field effect on the movement of unpaired electrons of −OH, the hydrogen bonds are broken.
Then the phase transition from the lamellar phase to the isotropic phases occurs, and when the magnetic field effect disappears out of the NMR instrument, the unpaired electrons of −OH could form hydrogen bonds again, so phase recovery occurs. However, adding the 2H NMR scanning (i.e., the radio wave energy), the electron spinning makes the magnetic field effects disappear more difficult, along with the longer phase recovery time.
4. CONCLUSIONS In conclusion, observations on the phase structure transition of C12EO4 and C14G2 mixtures in water induced by magnetic field are presented in this study. An obvious rapid phase transition from lamellar phase to isotropic phases induced by a high magnetic field (9.4 T) at a relatively lower temperature, which is 8−18 °C lower than the phase transition temperature without magnetic field, was obtained. The phase structure transition was determined by 2H NMR spectra and FF-TEM images. With higher C12EO4 concentration, the samples can more easily be influenced by a magnetic field. The induced phases were proven to be unstable and can be recovered after one to several months, and it was easier to recover for the C12EO4/D2O system with higher C12EO4 concentration or adding C14G2 molecules, indicating more hydrogen-bond sites, more easily influenced by the magnetic field. Moreover, C12EO4 molecules played a critically important role in this process. Even though enough measurements and proposed explanation for the phase transition induced by the magnetic field were presented, much more work is still needed to understand and profoundly master. On the other hand, the phase transition from the lamellar phase to isotropic phases induced by the magnetic field plays a significant potential role in controlled drug delivery and drug release, as the lamellar phase has been reported to be very promising for drug delivery.22 So, we believe that the phase transition induced by the magnetic field is worth much more detailed study.
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ASSOCIATED CONTENT
S Supporting Information *
The phase diagram and the FF-TEM observations before the phase transition induced by the magnetic field and the 2H NMR and FF-TEM characterization of the phase transition induced by the magnetic field of the 1.0 wt % C14G2/30 wt % C12EO4 sample. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +86-531-88363532. Fax: +86531-88564750. 1270
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Notes
(20) Merclin, N.; Bender, J.; Sparr, E.; Guy, R. H.; Ehrsson, H.; Engstrfm, S. Transdermal delivery from a lipid sponge phase iontophoretic and passive transport in vitro of 5-aminolevulinic acid and its methyl ester. J. Controlled Release 2004, 100, 191−198. (21) Beck, R.; Abe, Y.; Terabayashi, T.; Hoffmann, H. A novel L3phase from a Ca-salt of an anionic surfactant and a cosurfactant. J. Phys. Chem. B 2002, 106, 3335−3338. (22) Hosmer, J. M.; Steiner, A. A.; Lopes, L. B. Lamellar liquid crystalline phases for cutaneous delivery of paclitaxel: Impact of the monoglyceride. Pharm. Res. 2013, 30, 694−706.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the NSFC (Grants 21033005 and 21273134).
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REFERENCES
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