Article pubs.acs.org/Langmuir
Lamellar Liquid-Crystalline System with Tunable Iridescent Color by Ionic Surfactants Zhenhua Cong,† Bowen Lin,† Weiqing Li,† Jian Niu,*,‡,§ and Feng Yan*,§ †
Nano and Heterogeneous Materials Center, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China ‡ Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China § Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, 999077 Kowloon, Hong Kong S Supporting Information *
ABSTRACT: Liquid crystals formed by the self-assembly of small molecules are very promising smart materials because of their unique properties, such as self-assembled multivalency, biocompatibility, and fast response to external stimuli. Here we report an iridescent liquid-crystal system composed of water layers, which is sandwiched by two bilayer membranes. Such membranes are composed of a self-assembled nonionic surfactant, which is called hexadecylglyceryl maleate (HGM), and only a small amount of ionic surfactants. It is found that the iridescent color of the liquid crystal system is very sensitive to the concentration of ionic surfactants, even if a trace of change of the ionic surfactants’ concentration will induce the color change of liquid-crystal system. The result shows that with the increase in ionic surfactant concentration, the flat bilayer membrane tends to be curved to form some edge-dislocation defects. The appearance of such defects in the lamellar system leads to the decrease in spacing distance between adjacent bilayer membranes. This is because some vacant spaces emerged inevitably during this process. The ionic surfactant-sensitive HGM system also shows the thermal response. It is because the phaseseparation results in the increase in local concentration of SDS in the bilayer membrane, which has the same effect as increasing the SDS concentration in the whole system. they can change their self-assembled structures17 to respond to the environmental change or external stimuli. Moreover, the dissociation and association of noncovalent interactions can likewise take the same effect.13,15 Therefore, the research of complex hierarchical structures using liquid crystals pushes the dynamically functional properties to the molecular level. For example, besides sensing1 and transportation of electrons, ions and molecules,18,19 templating is also evolved.20 Here we report an iridescent solution composed of water layers, which is sandwiched by two bilayer membranes. Such membranes are composed of a self-assembled nonionic surfactant, which is called hexadecylglyceryl maleate (HGM), and only a small amount of ionic surfactants such as sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium bromide (CTAB). This work demonstrated that the iridescent color of HGM solutions can change from red to purple by adding ionic surfactant with different concentration, which is insignificant compared with the total concentration of
1. INTRODUCTION The natural world consists of a large number of well-organized functional nanostructures, which are self-assembled by monomeric building blocks, such as the DNA double helix, tubulin-assembly, and lipid bilayer.1 Enlightened by such nanostructures in the nature, recently, so many supramolecular nanomaterials have been developed by self-assembly with simple small molecules.2−7 Among various supramolecular structures, the liquid-crystalline (LC) state has received more and more attention. The molecules in liquid-crystalline state arrange themselves in an orientational order that relies on the external conditions, such as temperature, electrical field, and so on,8,9 so the molecules in liquid crystalline state are still mobile. Even though the essential ordering of a liquid-crystalline material depends on the external parameters, some internal conditions can also influence the ordering by weak interactions to form more complex structures, which provide more functionalities to the liquid-crystalline materials.8,10−16 Some intermolecular interactions have played important roles in the formation process of self-assembled liquid-crystalline structures, such as hydrogen bonding and ionic interactions.12,13,15,16 Such supramolecular materials have an excellent characteristic that © XXXX American Chemical Society
Received: May 15, 2017 Revised: June 24, 2017 Published: June 26, 2017 A
DOI: 10.1021/acs.langmuir.7b01626 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. (a) Reflection spectra of HGM solutions with different SDS concentration. (b) Schematic of HGM solution with alternative structure of HGM lamellar bilayer and water layer. (c) Structure formula of HGM molecule.
the electrostatic repulsive force between ionic surfactants’ headgroup first separates the HGM bilayers and then makes the swollen lamellar structure formed. Figure 1a shows the effect of different SDS concentration on the reflection spectra of HGM solutions. It can be found that the position of reflection peak is blue-shifted when the concentration of ionic surfactant SDS increases. According to Bragg’s equation (2d sin θ = nλ), the corresponding periodic spacing between adjacent HGM bilayer membranes is decreased from 258 to 156 nm when the molar ratio of SDS to HGM increased from 1/1000 to 1/40. The undulation stabilization theory22,23 is commonly used to explain the blue shift of the iridescent color of the liquid crystalline with lamellar structure. For a nonionic surfactant C12E5 in the systems with lamellar bilayer membranes, a tiny amount of ionic surfactants takes great effect on the performance of dilute bilayer solutions.24,25 It will not only restrain the thermal undulations but also make the bilayers flattened and decrease the interplanar distance. The undulation stabilization theory, as shown in eq 1, has been used widely to explain the color shift to blue side
surfactant. Moreover, the HGM liquid crystal system has presented a rapid thermal responsibility. The iridescent color of the HGM liquid-crystal system shows the dramatic blue shift with the temperature decreasing from 50 °C to room temperature.
2. EXPERIMENTAL SECTION 2.1. Synthesis. HGM was synthesized by the same procedures as our previous work.20 The crude product was purified by silica gel column and eluted with a hexane/ethyl acetate mixture (3/2 by volume). The collected HGM fraction was purified twice by recrystallization from an acetone/hexane mixture (1/1 by volume). The final product (mp. 50−52 °C) was proven to be >99% pure by NMR analysis (Figure S1). The detailed synthesis procedure is demonstrated in the Supporting Information. 2.2. Characterization. Transmission electron microscopy (TEM) was measured on a Tecnai G2 F20 S-Twin field-emission transmission electron microscope. Nuclear magnetic resonance (NMR) analysis results were obtained on a Varian 400 MHz. Rheological studies were performed on a TA Instruments AR2000 rheometer equipped with a temperature-control chamber. Reflection spectra was measured by a UV−vis spectrometer (PerkinElmer, Lambda 750)
Φd /δ = (A + ΔA)/A
3. RESULTS AND DISCUSSION 3.1. Color Change of HGM Solution with Different Concentrations of Ionic Surfactants. The HGM molecules can form an ordered lamellar structure in the form of bilayer membranes in water. This kind of liquid crystalline usually exists as a sandwiched structure in the way of bilayer membrane/water/bilayer membrane when the temperature is beyond its Krafft point (37 °C), as schematically shown in Figure 1b.20,21 When the concentration of HGM is in the range of 1.2 to 2.4 wt %, the regular spacing is tunable on the submicrometer scale, and the solution displays iridescent color due to the Bragg’s diffraction of visible light on the lamellar plane arranged periodically. We found that besides the concentration of HGM molecule, the iridescent color of HGM liquid crystal system also can be modulated by the concentration of cosurfactant SDS. Actually, in the HGM system, the ionic surfactants such as SDS and CTAB are found to be indispensable to form the iridescent liquid-crystalline phase. Without any ionic surfactants, the HGM molecules tend to form floccules structure, which is rather stable for a week even the temperature is above the Krafft point. It indicates that
(1)
where d is the distance between the midplanes of the neighboring bilayer membranes, which is also called interplanar distance. Φ is the volume fraction of surfactant. A and (A + ΔA) are the projected area on the (x, y) plane and the average of the actual area of the undulating membrane, respectively. For simplicity, we take the bilayer thickness δ of 4 nm and the volume fraction Φ of 0.016 as a typical example,20 and the blue shift in the interplanar distance is only ∼20 nm in the normal nonionic iridescent systems. As Figure 1a shows, the HGM system has a much larger blue shift of ∼102 nm by the addition of ionic surfactants. For such a large blue shift, the thermal undulation theory obviously is not sufficient to explain it, and another new mechanism should be proposed for complement. 3.2. Effect of Ionic Surfactants on the Iridescent Color of HGM Solution. It is common knowledge that ionic strength in the solution increases by the increase in the ionic surfactants’ concentration, such as SDS or CTAB. Despite the fact that the increase in ionic strength will reduce the electrostatic repulsive force between neighboring bilayer membranes, it is not the root cause of blue shift in the HGM iridescent system. That is because the midplane distance between the neighboring bilayer B
DOI: 10.1021/acs.langmuir.7b01626 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. (a) Schematic of curve formation in the lamellar structure with the increase in SDS concentration. (b) Schematic of decrease in interlayer space with the emergence of the vacant defects.
Figure 3. (a) Plot of the interplanar distance against molar ratio of SDS to HGM. (b) Interplanar distance change against (1 − c)/c (where c is the weight fraction of HGM). (c) Freeze-fracture electron microscopy images of HGM solutions with different SDS concentration.
membranes does not depend on the minimum value of DLVO potential. For iridescent system, the distance between neighboring bilayer membranes is between 200 and 400 nm, the van der Waals attractive force does not work due to the large spacing distance, and only the electrostatic repulsive force can take effect; the initial spacing distance is only determined
by the concentration of bilayer membrane. As a result, the spacing distance will not vary even if the electrostatic repulsion is reduced by the addition of SDS, only when the repulsion becomes so small that the lamellar periodic structure cannot be maintained. We notice that when the self-assemble system is consisted of two or more complementary surfactants, the shape C
DOI: 10.1021/acs.langmuir.7b01626 Langmuir XXXX, XXX, XXX−XXX
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Figure 4. (a) Reflection spectra of iridescent HGM solutions with the decrease in temperature. (b) Dynamic elastic moduli of HGM system as a function of frequency at different temperature.
which are connected by a dense network of bilayer sheets. During this process, we can see the decrease in interlayer space and the emergence of some vacant spaces, which is coincident with the above discussion. Continually increasing the SDS concentration will make the bilayer membrane more curved, and the separated onion-like vesicles are formed. Under this situation, the color of the HGM system will change from iridescent to a little turbid because the whole system changed from ordered structure to disordered phase. (Figure S2). 3.3. Thermal Response of HGM Solution. Figure 4a plots the reflection spectra of iridescent solutions with different HGM concentration against temperature from 50 °C to room temperature. It can be seen that the color of the solutions with different HGM concentration all shifts from red to blue. Taking the solution with 1.6 wt % HGM as an example, the peak at 618 nm corresponds to a 235 nm periodic spacing between neighboring HGM bilayers membrane, when the temperature is 50 °C, but at room temperature, the peak position blue shifts to 455 nm corresponding to the periodic spacing of 173 nm. The large blue shift indicated that with the decrease in temperature, a small quantity of water was squeezed out from interlayer of HGM bilayers. A possible reason for the color change is proposed that the SDS molecules with natural radius are aggregated in certain regions and cause the curvature of the HGM bilayer membrane in virtue of the in-plane phase separation with temperature decreasing. It takes the same effect with increasing the SDS concentration. As shown in Figure 4b, the appearance of edge dislocation defects has improved the viscoelasticity of the HGM system effectively because of the resistance of water flow normal to the membranes layers by the dislocation defects. The color shift of the HGM system to the blue side is very fast when the temperature is below 50 °C. Figure S3-b shows the reflection spectrum for three consecutive tests. We can see for the second test, the reflection peak has moved to the position as the final test shows. As discussed in our previous work, the unique feature of HGM system is that it can be changed to anisotropic lamellar hydrogel even the temperature approach the 0 °C.20 So the thermal response and its capability maintain the lamellar structure in a wide temperature range (from 0 to 90 °C), which makes the HGM liquid crystal system as the good candidate of thermal sensor.
of the surfactant aggregates is mainly controlled by the critical packing parameter (CPP). It can be estimated by the equation of CPP = v/S0L, where v is the ratio of effective volume, S0 is the headgroup area, and L is chain length, respectively. The HGM molecule has a particular feature that the area of headgroup is similar to the tail, resulting in the value of CPP close to 1, which makes the HGM molecules favor to form planar bilayer structure. Unlike HGM molecule, the headgroup area is obviously larger than the tail area for the SDS molecule, resulting in the value of CPP down to ∼0.33. So the preferred structure for SDS is vesicle in water. Accordingly, as seen in Figure 2a, it makes sense for the HGM bilayer structure to vary from flat lamellar sheets to multilayer vesicles composed of curved bilayer membranes when the proportion of SDS increases. Once such structure is formed, the bilayer membranes are not able to occupy the whole space of solution, causing the decrease in midplane distance and the emergence of some vacant spaces, as shown in Figure 2b. The specific experimental phenomenon for this kind of structural change will be shown below. Figure 3a shows the change of equilibrated midplane distance d with the molar ratio of SDS to HGM. It can be found that when the concentration of SDS is in the range of 0.01 to 0.02, the relationship shows a good linear regression, which is a straight line, while beyond this range, the relationship between d and molar ratio of SDS to HGM becomes a little complex and some tricky curves appear. As shown in Figure 3b, when the concentration of SDS is low enough to MSDS/MHGM 1:1000, the interplanar distances are linear to (1 − c)/c in these solutions, where c is the weight fraction of HGM. For this case, it is thought the perfect lamella sheets occupied the whole space of the solution. Nevertheless, if the concentration of SDS exceeds 1:40, then the linear relationship between the interplanar distance and (1 − c)/c is not able to be be unchanged any more. It reveals that even though the whole system keeps the periodically ordered structure in general, some defects more or less exist in these solutions. The freeze-fracture electron micrograph (FF-TEM) observation is directly employed to examine the microstructures of HGM solutions with different SDS concentration, as shown in Figure 3c. The well-ordered and gently curved lamellae can be observed from the image of HGM solution with low SDS concentration, as shown in Figure 3c-1. In the areas without defects, the long and parallel sheets of membranes dominate the micrograph. With the increase in SDS concentration, the edge dislocation defects are formed (Figure 3c-2). The high curvature spherulite defects emerged,
4. CONCLUSIONS The effects of ionic surfactants SDS on the self-assemble behaviors of nonionic surfactants HGM have been studied D
DOI: 10.1021/acs.langmuir.7b01626 Langmuir XXXX, XXX, XXX−XXX
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systematically. We have found the iridescent color of the liquidcrystal system of nonionic surfactant changes greatly with a trace of change of the ionic surfactants’ concentration. The experimental results indicate that ionic surfactants are freely diffuse in the HGM bilayer membranes. The curvature of HGM bilayer membranes is highly sensitive to the concentration of ionic surfactants. When the concentration of ionic surfactants increases, the flat lamellar membranes become more curved and some edge dislocation defects are formed. Such defects in the lamellar structures account for the decrease in spacing distance between neighboring bilayer membranes because of the appearance of vacant spaces among such structures. The ionic surfactant-sensitive HGM liquid-crystal system also shows the thermal response because the phase-separation results in the increase in local concentration of SDS, which has a similar effect of increasing the SDS concentration in the whole system. From this point of view, the unique thermal response of HGM lamellar liquid-crystal system can be taken as a promising thermal sensor candidate.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01626. Figure S1. 1H NMR spectrum of HGM. Figure S2. The photographs of HGM system with the increase of SDS concentration and corresponding freeze-fracture electron microscopy (FF-TEM) image. Figure S3. The photographs of 2.0 wt % HGM system with the decreasing of temperature from 50 °C to room temperature and the reflection spectrum for three consecutive tests. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*J.N.: E-mail:
[email protected]. *F.Y.: E-mail:
[email protected]. ORCID
Jian Niu: 0000-0001-6274-7722 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the Hong Kong Scholars Program (grant no. XJ2015023), the National Natural Science Foundation of China (nos. 51401107 and 21401100), the Natural Science Foundation of Jiangsu (no. BK20140384), the China Postdoctoral Science Foundation (no. 2015M571759), the Jiangsu Planned Projects for Postdoctoral Research Funds (no. 1402007A), the Fundamental Research Funds for the Central Universities (no. 30916011346), and the NUPT Scientific Foundation (NY215013).
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REFERENCES
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DOI: 10.1021/acs.langmuir.7b01626 Langmuir XXXX, XXX, XXX−XXX
