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Phase behavior of surfactant mixtures and the effect of alkyl chain and temperature on lyotropic liquid crystal
Journal Pre-proof Phase behavior of surfactant mixtures and the effect of alkyl chain and temperature on lyotropic liquid crystal Jinpeng Liu, Lushi Guan, Zhongni Wang
PII:
S0927-7757(19)31010-6
DOI:
https://doi.org/10.1016/j.colsurfa.2019.124019
Reference:
COLSUA 124019
To appear in:
Colloids and Surfaces A: Physicochemical and Engineering Aspects
Received Date:
19 April 2019
Revised Date:
24 September 2019
Accepted Date:
25 September 2019
Please cite this article as: Liu J, Guan L, Wang Z, Phase behavior of surfactant mixtures and the effect of alkyl chain and temperature on lyotropic liquid crystal, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124019
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Phase behavior of surfactant mixtures and the effect of alkyl chain and temperature on lyotropic liquid crystal Jinpeng Liu1, Lushi Guan2*, Zhongni Wang1* 1. College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, PR China 2. Marine Biomedical Research Institute of Qingdao, Qingdao 266073, PR China. Corresponding auther: Zhongni Wang; E-mail: [email protected]
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Lushi Guan; E-mail: [email protected]
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Graphical Abstract
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In order to study lyotropic liquid crystal (LLC) which have good stability to strong electrolytes, inorganic salts and pH, the mixtures of Polyoxyethylene-10-oleyl ether
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(Brij 97) with different Polyoxyethylene (20) sorbitol fatty acid ester (Tweens®) were studied. Firstly, the phase diagrams had been constructed for Brij 97/Tween 20/H2O (Tween 20 system), Brij97/Tween 40/H2O (Tween 40 system), Brij 97/Tween 60/H2O (Tween 60 system) and Brij 97/Tween 80/H2O (Tween 80 system). It showed that the difference in the alkyl chain length and insaturation of Tweens® had significant effect on the lyotropic liquid crystal phase region size. 1
Secondly, the system containing
Tweens® had smaller viscous moduli (G''), elastic moduli (G') and larger lattice parameter (α) than the system without Tweens®, which indicated that the latter has higher stability than the former.
It is worth noting that the sample (Brij 97/Tween
40/H2O=52.5/10/37.5) appeared as a mixed phase of hexagonal phase (H1) and micellar cubic phase (I1) at 37 oC, while the other samples behaved as hexagonal phase. It should also be noted that with the increase in temperature, the rheological properties of sample (Brij 97/Tween 60/H2O=0/55/45) showed changes from viscous dominated to elastic dominated to viscous dominated, which provided a basis for thermosensitive materials
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and drug carriers. Keywords: Brij 97, Tweens®, surfactant mixtures, phase behavior, lyotropic liquid
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crystal, temperature
1. Introduction
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Lyotropic liquid crystal (LLC) is self-assembling aggregate formed by surfactants under the induction of solvent, behaving as liquid and crystal properties [1]. In addition,
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LLC has various structures, which can be classified into three types: hexagonal, lamellar or cubic phases [2]. Because of unique properties and variable structures, LLC
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is identified as potential materials and have been extensively studied in many fields, such as electronics, nanotechnology, solar cells energy, biomedical, food technology, pharmaceutical (drug delivery) and structural biology [3]. Due to the order and
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uniformity of the structures, LLC is used in templates to synthesize nanomaterials and nanostructured membrane [4-6]. However, strong electrolytes, inorganic salts and pH
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conditions sometimes limit the application of LLC, so it is necessary to find surfactants that are resistant to the above conditions. Polyoxyethylene (EO)-based nonionic surfactant (EO-SAA) is commonly used
surfactant, which has good stability against strong electrolytes, inorganic salts and pH. It is widely used in industrial applications, agricultural formulations, and petroleum industry applications [7]. In addition, EO-SAA is generally safe, low toxic and low2
irritating, which has been widely used in food field and pharmaceutical field [8-10]. It is found that EO-SAA can form various LLC phase in solvents, such as hexagonal (H1), reversed hexagonal (H2), normal “bicontinuous” cubic (V1), reversed “bicontinuous” cubic (V2), lamellar (L), and micellar cubic (I1) phases [11]. The LLC structures that EO-SAA may construct can be predicted by surfactant packing parameter (Rp) [12]. The value of Rp can be adjusted by changing the length of the alkyl chain and the number of EO. However, due to structural limitations, a single surfactant sometimes fails to form the desired LLC phase. Therefore, the use of two types of surfactants to construct
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LLC phase is a good solution. Up to now, researchers have extensively studied in detail the aggregates formed by two types of EO-SAA in the dilute solution [13-16]. While,
two types of EO-SAA were used to construct LLC phase have been seldom studied [1720]. Therefore, it is of great significance for the application of LLC to study the
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behavior of LLC formed by two kinds of EO-SAA simultaneously [25- 27].
Polyoxyethylene-10-oleyl ether [C18:1(EO)10 or Brij 97] with 10 (EO) chain and long
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alkyl chain is common EO-SAA, having good hydrophilicity and lipophilicity. Brij 97 can form hexagonal and lamellar phase in water at 25 oC [21]. As the introduction of
(bmimPF6)
and
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two ionic liquids (hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate hydrophilic
1-butyl-3-methylimidazolium
tetrafluoroborate
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(bmimBF4)), the types of LLC for Brij 97 aqueous solution were not changed [22]. With the introduction of isopropyl myristate (IPM), a new LLC phase (cubic phase) was formed in Brij 97 aqueous solution at 25 oC [12]. Polyoxyethylene (20) sorbitol fatty
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acid ester (Tweens®) is another EO-SAA with 20 (EO) chain, and often used as solubilizer and stabilizer [8]. Tweens® was found to be able to form lamellar, hexagonal
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and cubic LLC in the aqueous or organic solvents [23-25]. In addition, alkyl chain of Tweens® and temperature have an important effect on the types of LLC phase [23, 25]. In this paper, in order to enrich the study of LLC constructed by EO-SAA, we
explored the EO-SAA/EO-SAA/water system. Brij 97 and Tweens® (Tween 20, Tween 40, Tween 60 and Tween 80) were selected as EO-SAA. Phase behavior of Brij 97/Tweens®/H2O was studied by phase diagrams. The effect of mass ratio for Brij 97 3
and Tweens®, Bri 97 concentration, Tween 40 concentration, alkyl chain and temperature on LLC was studied by rheology and small angle X-ray scattering (SAXS). 2. Material and methods 2.1. Materials Brij 97 was purchased from Sigma–Aldrich China (Shanghai, China). Polyoxyethylene (20) sorbitan monolaurate (Tween 20), polyoxyethylene (20) sorbitan monostearate (Tween 60) and polyoxyethylene (20) sorbitan monooleate (Tween 80) were offered by Sinopharm Chemicals Reagent Company (Shanghai, China, CP).
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Polyoxyethylene (20) sorbitan monopalmitate (Tween 40) was purchased from Shanghai Macklin Biochemical Co., Ltd (Shanghai, China, CP). Deionized water was used after doubly distilled. All chemicals were used directly. 2.2 Phase diagram determination
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The ternary phase diagram of Brij 97/Tween 20/H2O was determined using a previous method [26]. Firstly, Bri 97 and Tween 20 were weighed according to the mass
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ratio of 10:0 to 0:10, and the surfactant mixtures were stirred well at 45 oC. Then, the deionized water was added drop by drop to the surfactant mixtures and thoroughly
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mixed. Finally, the resulting mixtures were balanced in a water bath at 37 oC. The phase equilibrium was recorded by visual observation under normal light.
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The ternary phase diagrams of Brij 97/Tween 40/H2O, Brij 97/Tween 60/H2O and Brij 97/Tween 80/H2O were determined according to the above method. 2.3 Preparation of samples
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Samples were prepared in Brij 97/Tweens®/H2O system, and kept at 37 oC to achieve structural equilibrium. The water content was fixed as 45 wt%, and the mass ratio of
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Brij 97/Tweens® (wt%/wt%) was changed as 55/0, 45/10, 35/20, 25/30 and 0/55. The content of Tween 40 was fixed as 10 wt%, and the mass ratio of Brij 97/H2O (wt%/wt%) was changed as 30/60, 37.5/52.5, 45/45 and 52.5/37.5. The content of Brij 97 was fixed as 37.5wt%, and the mass ratio of Tween 40/H2O (wt%/wt%) was changed as 0/62.5, 10/52.5, 20/42.5and 25/37.5. 2.4. Characterization 4
Polarization microscopy (POM) observations were performed using an BK-POL microscope (Chongqing Aote optician Co) with a maximum magnification of 1000. The texture pictures were photographed with computer software. Small angle X-ray scattering (SAXS) was carried out in SAXSess high-flux SAXS instrument (Anton-Paar, Graz, Austria) with a Cu Kα radiation (0.1542 nm). The operating voltage, current, and sample to detector distance were 40 KV, 50 mA and 264.5 mm, respectively. Rheological measurements were measured on an American AR-2000ex rheometer
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(TA Instruments, New Castle, DE, USA) with a cone–plate sensor (diameter of 35 mm and cone angle of 2o). The thickness of samples was 0.105mm. The stress sweep
measurements were carried out at 6.286 rad/s to determine the linear viscoelastic region. The frequency sweep (0.03–300 rad/s) and temperature sweep measurements were
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conducted in the linear viscoelastic region. 3. Results and discussion
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3.1. Phase behavior
The effect of Tweens® with different alkyl chain on the phase behavior of Brij
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97/H2O system was studied at 37 oC. The phase diagrams of Brij 97/Tween 20/H2O (Tween 20 system), Brij97/Tween 40/H2O (Tween 40 system), Brij 97/Tween 60/H2O
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(Tween 60 system) and Brij 97/Tween 80/H2O (Tween 80 system) in the whole concentration range were shown in Fig.1. In All maps, two single-phase regions are identified: one lyotropic liquid crystal (LLC) phase and one isotropic solution phase
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(L1 + L2). In addition, the mixed phase of LLC and isotropic solution is found in the phase diagrams of each system, and marked as “2Φ”. It shows that Brij 97 can form
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LLC phase in water with the water content of 17-67 wt%. As the introduction of Tweens®, the LLC phase moves towards higher water content, and the LLC phase zone shrinks as the increase of Tweens® content. It may be that Tweens® has more (EO) chain than Brij 97, showing greater hydrophilic capacity. Owing to the different alkyl chain of Tweens®, the LLC region of different systems shows various size. Comparing the size of the LLC region in the phase diagrams by visual analysis, the region size is 5
Tween 60 system > Tween 40 system > Tween 80 system > Tween 20 system. The Tweens® content of LLC region for each system is different. It may be caused by the altered hydrophobic capacity of Tweens®. Tween 60/H2O system can form LLC in Tweens®/H2O binary system. While, the LLC region of Tween 60 system with higher Tween 60 content shows opaque appearance, and the other systems behave as transparent solid state in the whole LLC region, which may be due to the relatively high melting point of Tween 60. Aggregates formed by surfactants can be predicted by surfactant packing parameter
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(Rp) [12]. The packing parameter which can be defined as Rp = vL aS lc , where aS is the interfacial area occupied by a surfactant headgroup, lc and vL are the length and
volume of the hydrophobic group, respectively. In our previous work, the Rp of Brij 97
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was estimated to be 0.45 [22]. The lc and vL of Tweens® were calculated using the
previous method [27]. Results are shown as Table 1. The value of aS is approximately
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equal to the maximum adsorption capacity of Tweens®, which is about 100 Å2 [28]. Therefore, the Rp of Tween 20, Tween 40, Tween 60 and Tween 80 are 0.29, 0.28, 0.28
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and 0.27. The values of Rp are between 0.25 and 0.45 for mixture of Brij 97 and Tweens®. Thus, the mixed systems tend to form hexagonal LLC and micellar cubic
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LLC in aqueous.
Therefore, the representative samples were selected in the phase diagrams to explore the microstructure of LLC.
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3.2 the effect of mass ratio for Brij 97 and Tweens® 3.2.1 Microstructures
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Firstly, the effect of mass ratio for Brij 97 and Tweens® on the microstructures of
LLC were studied. The total content of Brij 97 and Tweens® was fixed as 55 wt%, and the mass ratio of Brij 97/Tweens® was changed as 55/0, 45/10, 35/20 and 25/30. The LLC samples were characterized by SAXS. The representative SAXS spectra are shown in Fig. 2 and Fig. S1. It can be seen from Fig. 2, The relative peak positions of sample at 55/0 correspond 6
to the 100, 110, and 200 planes, and follow the relationship of
1 3 , showing as
hexagonal phase (H1) [29]. The lattice parameter (α) of hexagonal LLC can be
calculated according to the following formula: 4
3 q100 , where q100 is the
scattering vector of the first reflection [30]. The α of sample at 55/0 is 73.68 Å. Introducing Tween 40 into the samples, the relative peak positions also correspond to
1 3 , indicating that those samples remain as hexagonal LLC. But the Bragg
peaks move to the lower q. The samples of Tween 20, 60 and 80 system also show this
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behavior. The α of samples was calculated, as shown in Fig.3a. The values of α increase with the increasing Tweens® content, which indicated that the introduction of Tweens®
continuously swells the hexagonal lattices [31]. As the Tweens® content increases to
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10 wt%, the samples of Tween 40, 60 and 80 system have the same α. It may be due to that the difference of alkyl chain between Tween 40, Tween 60 and Tween 80 cannot
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be expressed in lower Tweens® content. As Tweens® content increases to 20 wt% and 30 wt%, the differences of alkyl chain of Tween 40, Tween 60 and Tween 80 were
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reflected, showing various α values for different system. The α values of samples at 35/20 and 25/30 follows the order: Tween 60 system > Tween 40 system > Tween 80 system > Tween 20 system. The saturated alkyl chain structure of Tween 20, Tween 40
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and Tween 60 is same except for different length. Compared the α values of Tween 20, 40 and 60 system, Tween 60 system has the largest α values, which is caused by the
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longest alkyl chain. Tween 80 and Tween 60 have the same length of alkyl chain, but the α value of Tween 80 system is less than Tween 60 system, which may be attributed
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to the double bond in the alkyl chain of Tween 80. The microstructures of LLC also can be explained by some characteristic parameters
calculated from SAXS models [30]. Apolar volume fraction, f is defined as
f v v . L
P
(1)
P
ϕP is the volume fractions of surfactant mixture. vL is the volume of hydrophobic tail, 7
and vP is the volume of surfactant molecule. For Brij 97, vL and vP are 500 Å and 1158.6 Å3, respectively [22]. For Tweens®, vP was calculated by molar mass and density. The density of Tween 20, Tween 40, Tween 60 and Tween 80 are 1.01 g/mL, 1.08 g/mL, 1.04 g/mL and 1.08g/mL respectively. The results of vP can be seen in Table 1. The radius of the cylindrical aggregates, dH, is deduced as
dH 3 2 f
(2)
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The dH values of samples are shown in Fig. 3b. It can be found that the values of dH decrease with the increase of Tweens® content. The dH values for Tween 40, 60, and
80 system are same when Tweens® content increases to 10 wt%, and are larger than
Tween 20 system. As Tweens® content increases to 20 wt% and 30 wt%, the dH values
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of samples follow the order: Tween 60 system < Tween 80 system < Tween 40 system < Tween 20 system.
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The radius of water channels, dw, is deduced as
(3)
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dW 2dH
The dw can be seen in Fig. 3c. The dw value of sample at 55/0 is 35.78 Å. With the increase of Tweens® content, dw values gradually increase to about 45 Å, which may
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be due to that Tweens® has larger hydrophilic head than Brij 97. Although Tweens® has the same hydrophilic head group, dw also shows a change in the samples containing
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different Tweens® results from the difference in alkyl chain. The effective cross-sectional area of each surfactant molecular at the interface, aS, is
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calculated as
aS 2vP f dH P
(4)
The calculated results of aS for sample are shown in Fig. 3d. The introduction of
Tweens® increases the value of aS, which due to that Tweens® has a larger molecular volume than Brij 97. 3.2.2 Rheological behavior 8
In order to reveal the viscoelasticity of the systems, rheological measurements were carried out. Rheology can provide real-time macroscopic characteristics of LLC and reflect the changes of internal microstructures [32, 33]. Stress sweep measurements were carried out at a set frequency of 6.286 rad/s and a temperature of 37 oC. As shown in Fig. 4a, for sample without Tweens®, the critical stress (σc) can reach 973 Pa, and the complex moduli (G*) can reach 31588 Pa, indicating that the sample is weakly dependent on the applied stress. Next, the Tween 40 system is used as example to illustrate the effect of Tweens® on the viscoelasticity
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of LLC. When Tween 40 is introduced into the sample, G* and σc continue to decrease as the Tween 40 content increases. The results indicate that the introduction of Tween 40 weakens the resistance of LLC to external force.
Frequency sweep curves of samples are shown in Fig. 4b. At lower frequencies, the
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viscous moduli (G'') are larger than the elastic moduli (G'), showing as viscous
properties. When the frequency exceeds a certain value (intersection frequency of G'
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and G''), G' is larger than G'', showing as elastic properties. The dynamic moduli (G' and G'') behave as frequency dependent. The values of G' increase with the increasing
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frequency. While, the values of G'' increase with the increase of frequency at lower frequency, and gradually approaches the platform at higher frequency. The behavior is
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similar to our previous system [34, 35]. Dynamic moduli of samples increase with the increase of Tween 40 content, which is the same as the behavior of G* in stress sweep curves. The frequency dependence of the hexagonal LLC in linear viscoelastic region
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can be further explained by multiple Maxwell models [36]. The solid lines in Fig. 4b are model fitted, and all samples are consistent with multiple Maxwell models.
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Generally, aggregates composed of nonionic surfactants are affected by temperature,
showing as temperature sensitivity [37]. Thus, the relationship between microstructures of LLC and temperature was studied by temperature sweep measurements. The temperature response of storage moduli (G') and loss moduli (G'') can be seen in Figure 4c. It can be found that the viscoelastic moduli values of all samples drop rapidly at a certain temperature (Tc). The reason is that the ordered structure of LLC is destroyed 9
and converted into a poorly ordered structure of micelle due to the increase in temperature. The introduction of Tween 40 reduces Tc from 60 oC to about 50 oC. This may be that Tween 40 leads to the loosening of LLC, which can be explained by the increased lattice parameter (α). Stress sweep curves, frequency sweep curves and temperature sweep curves of the Tween 20, 60, 80 system have similar phenomenon to the Tween 40 system, which can be seen in Fig. S2. 3.3 The effect of alkyl chain for Tweens® on rheological properties
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From the SAXS spectra, it can be found that the effect of alkyl chain for Tweens® can be revealed when Tweens® content is 20 wt% and 30 wt%. Therefore, the stress
sweep measurements were carried out to study the influence of alkyl chain on rheological properties, as shown in Fig. 5a. The stress sweep curves show that the
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order of σc and G* are Tween 60 system > Tween 80 system > Tween 40 system > Tween 20 system. This may be that the larger values of α lead to the looser
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microstructure, so the structure of LLC is less stable and less resistant to external forces. From the above research, it can be found that Brij 97 and Tweens® content
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have effects on the microstructure of LLC. So next, the effects of Brij 97 or Tweens® concentration on the LLC was investigated.
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3.4 The effect of Bri 97 concentration
The Tween 40 content was fixed as 10 wt%, and Brij 97/H2O mass ratio was changed as 30/60, 37.5/52.5, 45/45 and 52.5/37.5. The SAXS spectra of samples (Brij
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97/H2O = 30/60, 37.5/52.5 and 45/45) at 37 oC are shown in Fig. 6a. The relative peak
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positions correspond to 1
3 , which confirmed as hexagonal LLC (H1). While, the
sample (Brij 97/H2O = 52.5/37.5) has five scattering peaks. The relative peak positions correspond to 1
3 , showing the structure of hexagonal LLC. The other relative
peak positions correspond to the 111, 220, 311 planes and follow the relationship of
3 4 11 , behaving as face-centered cubic space group Fd3m (micellar cubic LLC, I1). It indicates that the sample is a mixture of H1 and I1 at 37 oC. As shown in 10
Fig. 6a, with the increase in Brij 97 concentration, the values of α and aS increase gradually indicating that the microstructure of LLC is tighter. While, the values of dH increase gradually and values of dW decrease gradually. It maybe that the increase in Brij 97 concentration promotes the winding degree of surfactant, which is conducive to the overlap of adjacent layers [38]. The temperature sweep curves of samples are shown in Fig. 7b. the samples with the Brij 97/H2O mass ratio of 30/60, 37.5/52.5 and 45/45 have one phase transition
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temperature (Tc 1) of 56 oC, 54 oC and 50 oC, respectively. While, For the sample with the Brij 97/H2O mass ratio of 52.5/37.5, the temperature sweep curve has two phase transition temperature of Tc
2
(38 oC) and Tc
1
(48 oC). It is worth noting that the
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viscoelastic moduli in the lower temperature range are significantly low than the
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viscoelastic moduli in the higher temperature range. To make clear of this phenomenon, the SAXS measurement was performed on the sample at 43 oC. The SAXS spectrum
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(dotted line) has three scattering peaks, and the relatively peak positions correspond to
1 3 , shown as hexagonal LLC. However, the peak intensity is greater than 37 oC,
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indicating that its structural order is enhanced.
The frequency sweep curves of samples are shown in Figure 6c. As the increase in
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shear frequency, the values of G' and G'' increase with different slop, showing as frequency dependence. The frequency sweep curves of samples conform to multiple
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Maxwell models. The relaxation spectra are shown in the inset of Figure 8c. The samples with Brij 97/H2O mass ratio of 30/60, 37.5/52.5 and 45/45 show that the relaxation moduli decrease as the increased relaxation time. While, the sample (Brij 97/H2O=52.5/37.5) has a maximum relaxation modulus. The number of Maxwell elements required to make up the function is shown in the inset. The Maxwell element number of samples decreases as the increase of Brij 97 concentration. The minimum 11
relaxation time is usually the time required for water molecules to transition between disturbance and equilibrium states. The minimum relaxation time of samples with Brij 97/H2O mass ratio of 0/60, 37.5/52.5and 45/45 is substantially the same. While, the sample with Brij 97/H2O mass ratio of 52.5/37.5 has significantly larger values. It may be that the larger viscoelastic moduli are not conducive to the movement of water molecules. The maximum relaxation time is usually the time required for surfactant molecules to transition between disturbance and equilibrium states. The maximum relaxation time of sample (Brij 97/H2O=52.5/37.5) shows as the smallest value, which
3.5 The effect of Tween 40 concentration
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maybe that the sample is the mixed LLC phase.
The Brij 97 content was fixed as 37.5 wt%, and the Tween 40/H2O mass ratio was changed as 0/62.5, 10/52.5, 20/42.5 and 25/37.5. All samples have three scattering
1 3 , confirmed as hexagonal
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peaks, and the relative peak positions correspond to
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LLC. It can be seen from Fig. 7a, the α values become large as Tween 40 concentration increases, indicating that the cylinder of hexagonal LLC becomes more closely packed.
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It can be found that dH remains basically unchanged, while dW values appear to be reduced. The aS values increase as the Tween 40 concentration increases, which may be due to the larger molecule volume of Tween 40.
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Temperature sweep curves of samples are shown in Fig. 7b. As Tween 40 concentration increases, the Tc 1 of samples decreases. As the introduction of Tween
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40, there is a reduction in G' before reaching Tc 1. As the concentration of Tween 40 increases, the temperature at which G' decreases is dropping. It indicates that the
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increase in Tween 40 concentration promotes the sensitivity of LLC to temperature. The frequency sweep curves of samples can be seen in Fig. 7c. The viscoelastic
moduli of samples all appear to be frequency dependent. The relaxation spectra can be seen in the inset of Fig. 7c. Observing the relaxation spectra, for the samples with Tween 40/H2O mass ratio of 10/52.5, 20/42.5 and 25/37.5, the relaxation moduli decrease with the increase of relaxation time. While, the sample (Tween 40/H2O = 0/62.5) shows a maximum value. Observing the number of Maxwell elements, it can 12
be found that sample with Tween 40/H2O mass ratio of 0/62.5 has 5 Maxwell elements, and the other samples have 6 Maxwell elements. The minimum relaxation time, of with Tween 40/H2O mass ratio of 0/62.5 and 10/52.5 are significantly larger, which may be related to their greater viscoelastic moduli. For maximum relaxation time, the sample (Tween 40/H2O = 0/62.5) shows as the largest maximum relaxation time value, which maybe that the sample do not have Tween 40. 3.4 The effect of temperature on Tween 60/water binary system As shown in Fig. 1, LLC can be constructed in Tween 60/H2O binary system at 37 °C.
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While, the appearance of LLC is more transparent at 37 °C than at 25 °C. It may be that the microstructures were changed under the effect of temperature. Therefore, a representative sample (Tween 60/H2O=55/45) was selected to study the effect of temperature on its microstructures.
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Firstly, the temperature sweep measurement was carried out at 6.286 rad/s. The change of G' and G'' is reflected by the parameter tan δ (G''/G'). As shown in Fig. 8a,
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G' and G'' are significantly affected by temperature. Between 20 oC and 28 oC, G'' is greater than G'. While, the values of tan δ are basically unchanged and greater than 1.
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It indicates that the temperature interval (I interval) is dominated by viscosity. Between 28 oC and 35.5 oC, G'' is still greater than G'. But tan δ is gradually decreasing, showing
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that the elastic component in the sample gradually increases. The temperature interval (II interval) exhibits a relative transition of viscous dominance and elastic dominance. Between 35 oC and 38.4 oC, G' is greater than G'', and tan δ is less than 1. It shows that
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the temperature interval (III interval) is elastic dominated. As the temperature continues to rise, between 38.4 oC and 50.0 oC, the sample shows as G' > G'', and the tan δ is
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greater than 1. It indicated that the temperature interval (IV interval) is viscous dominance. The sample behaves as various viscoelastic properties at different temperature interval, indicating that the microstructure has changed under the effect of temperature. In order to investigate the specific structure of sample at different temperature, the intermediate temperature of 25 oC in the I interval and 37 oC in the III interval was 13
selected. Polarization microscopy observations (Fig. 8b and Fig. 8c) show that the sample has polarized texture at 25 oC and 37 oC, exhibiting birefringence. Therefore, the sample is both anisotropic at 25 oC and 37 oC. However, the birefringence at 37 oC is stronger than that at 25 oC, indicating that the microstructures of sample are tighter at 37 oC. In order to illustrate the specific structure of sample at 25 oC and 37 oC, SAXS measurements were carried out, as shown in Fig. 8d. At 25 oC, sample has three Bragg peaks with a relative position of
1 2 3 , which is consistent with lamellar LLC (L).
At 37 oC, there are also have three Bragg peaks, but the relative position of Bragg peaks
1 3 , showing as hexagonal LLC. The frequency sweep spectra of
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corresponds to
sample are shown in Fig. 8e. At 37 oC, G' and G'' have an intersection at 0.08 rad/s.
Below 0.08 rad/s, G'' > G', the sample is viscosity dominated; above 0.08 rad/s, G' >
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G'', behaving as elastic dominance. The sample behaves as viscoelastic fluid, and the frequency sweep curve conforms to the behavior of hexagonal LLC. At 25 oC,
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Throughout the test range, G' and G'' of sample increase with increasing frequency, and
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G'' is greater than G', which showed viscosity dominance. By extending the curve, G' and G'' have an intersection point at 622.7 rad/s, indicating that the elastic dominant region of the sample is at out of test range.
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Conclusion
In this paper, the mixtures of EO-based nonionic surfactant (EO-SAA) in water were
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studied. The phase diagrams at 37 oC of the Brij 97/Tween 20/H2O (Tween 20 system), Brij 97/Tween 40/H2O (Tween 40 system), Brij 97/Tween 60/H2O (Tween 60 system)
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and Brij 97/Tween 80/H2O (Tween 80 system) in the whole concentration range show two single phase: one lyotropic liquid crystal (LLC) phase and one isotropic solution phase (L1 + L2). Compared with previous mixture of EO-SAA/EO-SAA systems, only one LLC phase region appears in the phase diagrams. The difference in the alkyl chain length and insaturation of Tweens® leads to significant changes in the phase region size and microstructure of LLC. The phase size order of LLC region on the phase diagrams and lattice parameter (α) order of samples whose Brij 97/Tweens® was 35/20 and 25/30 14
was Tween 60 system> Tween 40 system >Tween 80 system > Tween 20 system. As the increase of Tweens® content, the critical stress (σc) and moduli (G*, G' and G'') decrease continuously. At 37 oC, most of LLC samples are hexagonal phase (H1) and show the characteristics of viscoelastic fluids. However, sample (Brij 97/Tween 40/H2O=52.5/10/37.5) appears as a mixed phase of hexagonal phase (H1) and micellar cubic phase (I1) at 37 oC. Under the influence of temperature, the sample transformed from mixed phase of H1+ I1 at 37 oC to H1 at 42 oC. In a word, the study of the behavior for Brij 97 and Tweens® simultaneously forming
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LLC can have a good guiding for the application of LLC.
Declaration of Interest Statement
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The authors report no conflict of interest.
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Acknowledgments
Support of this work by the National Natural Science Foundation of China (31271933
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Figure captions
Fig. 1 The phase diagrams for Brij 97/ Tweens® /H2O system at 25 oC. (a) Brij 97/Tween 20/H2O system; (b) Brij 97/Tween 40/H2O system; (c) Brij 97/Tween 60/H2O system; (d) Brij 97/Tween 80/H2O system. Fig. 2 The SAXS spectra of investigated LLC samples in Brij 97/Tween 40/H2O
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system at 37 oC. The total content of Brij 97 and Tweens 40 was fixed as 55 wt%, and the Brij 97/Tweens 40 mass ratio was changed as 55/0, 45/10, 35/20 and 25/30.
Fig. 3 The results of the characteristic parameters obtained from the SAXS spectra: (a) lattice parameter (α); (b) the radius of the cylindrical aggregates for hexagonal
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phase (dH); (c) the radius of the water channels (dW); (d) the effective cross-sectional area per surfactant at the interface (aS).
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Fig. 4 The water content was fixed as 45 wt%, and the Brij 97/Tween 40 mass ratio was changed as 55/0, 45/10, 35/20 and 25/30. (a) The complex moduli (G*) versus
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stress. (b) Elastic (G' filled) and viscous (G'' hollow) moduli as a function of angular frequency for samples. (c) Temperature evolution of the storage (filled square G') and
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loss (hollow square G'') moduli upon heating for samples. The temperature at which the dotted line is located is the phase transition temperature. Fig. 5 The water content was fixed as 55 wt%, and the Brij 97/Tween 40 mass ratio
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was 35/20 (a) and 25/30 (b). The complex moduli (G*) versus stress. Fig. 6 The Tween 40 content was fixed as 10 wt%, and the Brij 97/H2O mass ratio
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was changed as 30/60, 37.5/52.5, 45/45 and 52.5/37.5. (a) The SAXS spectra of samples at 37 oC (solid line); the SAXS spectra of sample whose Brij 97/H2O = 52.5/37.5 at 43 oC (dotted line). The Insert table is the characteristic parameter obtained from SAXS. (b) Temperature evolution of the storage (filled square G') and loss (hollow square G'') moduli upon heating for samples. The temperature at which the dotted line is located is the phase transition temperature (Tc). (c) Elastic (filled 21
square G') and viscous (hollow square G'') moduli as a function of angular frequency for samples at 37 oC. Inset: the discrete relaxation spectra of samples at 37 oC. Fig. 7 The Brij 97 content was fixed as 37.5wt%, and the Tween 40/H2O mass ratio was changed as 0/62.5, 10/52.5, 20/42.5 and 25/37.5. (a) The SAXS spectra of samples at 37 oC. The Insert table is the characteristic parameter obtained from SAXS. (b) Temperature evolution of the storage (filled square G') and loss (hollow square G'') moduli upon heating for samples. The temperature at which the dotted line is located is the phase transition temperature. (c) Elastic (filled square G') and viscous
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(hollow square G'') moduli as a function of angular frequency for samples. Inset: the discrete relaxation spectra of samples at 37 oC.
Fig. 8 (a) Temperature evolution of the storage (filled square G'), loss (hollow square G'') moduli and tanδ (filled circular) upon heating for sample (Brij 97/Tween
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60/H2O=0/55/45). Polarized textures of sample after constant for 10 minutes at 25 oC (b) and 37 oC (c). (d) The SAXS spectra of the investigated LLC sample at 25 oC and
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37 oC. (e) Elastic (filled square G') and viscous (hollow square G'') moduli as function
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of angular frequency for sample at 25 oC and 37 oC.
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Fig. 1 The phase diagrams for Brij 97/ Tweens® /H2O system at 25 oC. (a) Brij
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97/Tween 20/H2O system; (b) Brij 97/Tween 40/H2O system; (c) Brij 97/Tween
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60/H2O system; (d) Brij 97/Tween 80/H2O system.
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Fig. 2 The SAXS spectra of investigated LLC samples in Brij 97/Tween 40/H2O
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system at 37 oC. The total content of Brij 97 and Tweens 40 was fixed as 55 wt%, and
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the Brij 97/Tweens 40 mass ratio was changed as 55/0, 45/10, 35/20 and 25/30.
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Fig. 3 The results of the characteristic parameters obtained from the SAXS spectra: (a) lattice parameter (α); (b) the radius of the cylindrical aggregates for hexagonal
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phase (dH); (c) the radius of the water channels (dW); (d) the effective cross-sectional
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area per surfactant at the interface (aS).
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Fig. 4 The water content was fixed as 45 wt%, and the Brij 97/Tween 40 mass ratio
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was changed as 55/0, 45/10, 35/20 and 25/30. (a) The complex moduli (G*) versus stress. (b) Elastic (G' filled) and viscous (G'' hollow) moduli as a function of angular
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frequency for samples. (c) Temperature evolution of the storage (filled square G') and loss (hollow square G'') moduli upon heating for samples. The temperature at which the dotted line is located is the phase transition temperature.
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Fig. 5 The water content was fixed as 55 wt%, and the Brij 97/Tween 40 mass ratio
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was 35/20 (a) and 25/30 (b). The complex moduli (G*) versus stress.
Fig. 6 The Tween 40 content was fixed as 10 wt%, and the Brij 97/H2O mass ratio was changed as 30/60, 37.5/52.5, 45/45 and 52.5/37.5. (a) The SAXS spectra of samples at 37 oC (solid line); the SAXS spectra of sample whose Brij 97/H2O = 52.5/37.5 at 43 oC (dotted line). The Insert table is the characteristic parameter obtained from SAXS. (b) Temperature evolution of the storage (filled square G') and loss (hollow square G'') moduli upon heating for samples. The temperature at which 27
the dotted line is located is the phase transition temperature (Tc). (c) Elastic (filled square G') and viscous (hollow square G'') moduli as a function of angular frequency
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for samples at 37 oC. Inset: the discrete relaxation spectra of samples at 37 oC.
Fig. 7 The Brij 97 content was fixed as 37.5wt%, and the Tween 40/H2O mass ratio was changed as 0/62.5, 10/52.5, 20/42.5 and 25/37.5. (a) The SAXS spectra of
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samples at 37 oC. The Insert table is the characteristic parameter obtained from SAXS. (b) Temperature evolution of the storage (filled square G') and loss (hollow
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square G'') moduli upon heating for samples. The temperature at which the dotted line is located is the phase transition temperature. (c) Elastic (filled square G') and viscous (hollow square G'') moduli as a function of angular frequency for samples. Inset: the discrete relaxation spectra of samples at 37 oC.
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Fig. 8 (a) Temperature evolution of the storage (filled square G'), loss (hollow square
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G'') moduli and tanδ (filled circular) upon heating for sample (Brij 97/Tween 60/H2O=0/55/45). Polarized textures of sample after constant for 10 minutes at 25 oC (b) and 37 oC (c). (d) The SAXS spectra of the investigated LLC sample at 25 oC and
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37 oC. (e) Elastic (filled square G') and viscous (hollow square G'') moduli as function
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of angular frequency for sample at 25 oC and 37 oC.
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Table 1 The results of lc (the length of hydrophobic group), vL (the volume of hydrophobic group), Rp (surfactant packing parameter) and vP (the volume of surfactant molecule) for Tweens® Name
vL (Å3) 325 433 487 473
Rp 0.29 0.28 0.28 0.27
vP (Å3) 2018.8 1974.3 2095.1 2014.4
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lP
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Tween 20 Tween 40 Tween 60 Tween 80
lc 11.32 15.37 17.39 17.48
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PII:
S0927-7757(19)31010-6
DOI:
https://doi.org/10.1016/j.colsurfa.2019.124019
Reference:
COLSUA 124019
To appear in:
Colloids and Surfaces A: Physicochemical and Engineering Aspects
Received Date:
19 April 2019
Revised Date:
24 September 2019
Accepted Date:
25 September 2019
Please cite this article as: Liu J, Guan L, Wang Z, Phase behavior of surfactant mixtures and the effect of alkyl chain and temperature on lyotropic liquid crystal, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124019
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Phase behavior of surfactant mixtures and the effect of alkyl chain and temperature on lyotropic liquid crystal Jinpeng Liu1, Lushi Guan2*, Zhongni Wang1* 1. College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, PR China 2. Marine Biomedical Research Institute of Qingdao, Qingdao 266073, PR China. Corresponding auther: Zhongni Wang; E-mail: [email protected]
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Lushi Guan; E-mail: [email protected]
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Graphical Abstract
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In order to study lyotropic liquid crystal (LLC) which have good stability to strong electrolytes, inorganic salts and pH, the mixtures of Polyoxyethylene-10-oleyl ether
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(Brij 97) with different Polyoxyethylene (20) sorbitol fatty acid ester (Tweens®) were studied. Firstly, the phase diagrams had been constructed for Brij 97/Tween 20/H2O (Tween 20 system), Brij97/Tween 40/H2O (Tween 40 system), Brij 97/Tween 60/H2O (Tween 60 system) and Brij 97/Tween 80/H2O (Tween 80 system). It showed that the difference in the alkyl chain length and insaturation of Tweens® had significant effect on the lyotropic liquid crystal phase region size. 1
Secondly, the system containing
Tweens® had smaller viscous moduli (G''), elastic moduli (G') and larger lattice parameter (α) than the system without Tweens®, which indicated that the latter has higher stability than the former.
It is worth noting that the sample (Brij 97/Tween
40/H2O=52.5/10/37.5) appeared as a mixed phase of hexagonal phase (H1) and micellar cubic phase (I1) at 37 oC, while the other samples behaved as hexagonal phase. It should also be noted that with the increase in temperature, the rheological properties of sample (Brij 97/Tween 60/H2O=0/55/45) showed changes from viscous dominated to elastic dominated to viscous dominated, which provided a basis for thermosensitive materials
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and drug carriers. Keywords: Brij 97, Tweens®, surfactant mixtures, phase behavior, lyotropic liquid
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crystal, temperature
1. Introduction
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Lyotropic liquid crystal (LLC) is self-assembling aggregate formed by surfactants under the induction of solvent, behaving as liquid and crystal properties [1]. In addition,
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LLC has various structures, which can be classified into three types: hexagonal, lamellar or cubic phases [2]. Because of unique properties and variable structures, LLC
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is identified as potential materials and have been extensively studied in many fields, such as electronics, nanotechnology, solar cells energy, biomedical, food technology, pharmaceutical (drug delivery) and structural biology [3]. Due to the order and
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uniformity of the structures, LLC is used in templates to synthesize nanomaterials and nanostructured membrane [4-6]. However, strong electrolytes, inorganic salts and pH
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conditions sometimes limit the application of LLC, so it is necessary to find surfactants that are resistant to the above conditions. Polyoxyethylene (EO)-based nonionic surfactant (EO-SAA) is commonly used
surfactant, which has good stability against strong electrolytes, inorganic salts and pH. It is widely used in industrial applications, agricultural formulations, and petroleum industry applications [7]. In addition, EO-SAA is generally safe, low toxic and low2
irritating, which has been widely used in food field and pharmaceutical field [8-10]. It is found that EO-SAA can form various LLC phase in solvents, such as hexagonal (H1), reversed hexagonal (H2), normal “bicontinuous” cubic (V1), reversed “bicontinuous” cubic (V2), lamellar (L), and micellar cubic (I1) phases [11]. The LLC structures that EO-SAA may construct can be predicted by surfactant packing parameter (Rp) [12]. The value of Rp can be adjusted by changing the length of the alkyl chain and the number of EO. However, due to structural limitations, a single surfactant sometimes fails to form the desired LLC phase. Therefore, the use of two types of surfactants to construct
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LLC phase is a good solution. Up to now, researchers have extensively studied in detail the aggregates formed by two types of EO-SAA in the dilute solution [13-16]. While,
two types of EO-SAA were used to construct LLC phase have been seldom studied [1720]. Therefore, it is of great significance for the application of LLC to study the
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behavior of LLC formed by two kinds of EO-SAA simultaneously [25- 27].
Polyoxyethylene-10-oleyl ether [C18:1(EO)10 or Brij 97] with 10 (EO) chain and long
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alkyl chain is common EO-SAA, having good hydrophilicity and lipophilicity. Brij 97 can form hexagonal and lamellar phase in water at 25 oC [21]. As the introduction of
(bmimPF6)
and
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two ionic liquids (hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate hydrophilic
1-butyl-3-methylimidazolium
tetrafluoroborate
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(bmimBF4)), the types of LLC for Brij 97 aqueous solution were not changed [22]. With the introduction of isopropyl myristate (IPM), a new LLC phase (cubic phase) was formed in Brij 97 aqueous solution at 25 oC [12]. Polyoxyethylene (20) sorbitol fatty
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acid ester (Tweens®) is another EO-SAA with 20 (EO) chain, and often used as solubilizer and stabilizer [8]. Tweens® was found to be able to form lamellar, hexagonal
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and cubic LLC in the aqueous or organic solvents [23-25]. In addition, alkyl chain of Tweens® and temperature have an important effect on the types of LLC phase [23, 25]. In this paper, in order to enrich the study of LLC constructed by EO-SAA, we
explored the EO-SAA/EO-SAA/water system. Brij 97 and Tweens® (Tween 20, Tween 40, Tween 60 and Tween 80) were selected as EO-SAA. Phase behavior of Brij 97/Tweens®/H2O was studied by phase diagrams. The effect of mass ratio for Brij 97 3
and Tweens®, Bri 97 concentration, Tween 40 concentration, alkyl chain and temperature on LLC was studied by rheology and small angle X-ray scattering (SAXS). 2. Material and methods 2.1. Materials Brij 97 was purchased from Sigma–Aldrich China (Shanghai, China). Polyoxyethylene (20) sorbitan monolaurate (Tween 20), polyoxyethylene (20) sorbitan monostearate (Tween 60) and polyoxyethylene (20) sorbitan monooleate (Tween 80) were offered by Sinopharm Chemicals Reagent Company (Shanghai, China, CP).
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Polyoxyethylene (20) sorbitan monopalmitate (Tween 40) was purchased from Shanghai Macklin Biochemical Co., Ltd (Shanghai, China, CP). Deionized water was used after doubly distilled. All chemicals were used directly. 2.2 Phase diagram determination
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The ternary phase diagram of Brij 97/Tween 20/H2O was determined using a previous method [26]. Firstly, Bri 97 and Tween 20 were weighed according to the mass
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ratio of 10:0 to 0:10, and the surfactant mixtures were stirred well at 45 oC. Then, the deionized water was added drop by drop to the surfactant mixtures and thoroughly
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mixed. Finally, the resulting mixtures were balanced in a water bath at 37 oC. The phase equilibrium was recorded by visual observation under normal light.
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The ternary phase diagrams of Brij 97/Tween 40/H2O, Brij 97/Tween 60/H2O and Brij 97/Tween 80/H2O were determined according to the above method. 2.3 Preparation of samples
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Samples were prepared in Brij 97/Tweens®/H2O system, and kept at 37 oC to achieve structural equilibrium. The water content was fixed as 45 wt%, and the mass ratio of
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Brij 97/Tweens® (wt%/wt%) was changed as 55/0, 45/10, 35/20, 25/30 and 0/55. The content of Tween 40 was fixed as 10 wt%, and the mass ratio of Brij 97/H2O (wt%/wt%) was changed as 30/60, 37.5/52.5, 45/45 and 52.5/37.5. The content of Brij 97 was fixed as 37.5wt%, and the mass ratio of Tween 40/H2O (wt%/wt%) was changed as 0/62.5, 10/52.5, 20/42.5and 25/37.5. 2.4. Characterization 4
Polarization microscopy (POM) observations were performed using an BK-POL microscope (Chongqing Aote optician Co) with a maximum magnification of 1000. The texture pictures were photographed with computer software. Small angle X-ray scattering (SAXS) was carried out in SAXSess high-flux SAXS instrument (Anton-Paar, Graz, Austria) with a Cu Kα radiation (0.1542 nm). The operating voltage, current, and sample to detector distance were 40 KV, 50 mA and 264.5 mm, respectively. Rheological measurements were measured on an American AR-2000ex rheometer
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(TA Instruments, New Castle, DE, USA) with a cone–plate sensor (diameter of 35 mm and cone angle of 2o). The thickness of samples was 0.105mm. The stress sweep
measurements were carried out at 6.286 rad/s to determine the linear viscoelastic region. The frequency sweep (0.03–300 rad/s) and temperature sweep measurements were
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conducted in the linear viscoelastic region. 3. Results and discussion
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3.1. Phase behavior
The effect of Tweens® with different alkyl chain on the phase behavior of Brij
lP
97/H2O system was studied at 37 oC. The phase diagrams of Brij 97/Tween 20/H2O (Tween 20 system), Brij97/Tween 40/H2O (Tween 40 system), Brij 97/Tween 60/H2O
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(Tween 60 system) and Brij 97/Tween 80/H2O (Tween 80 system) in the whole concentration range were shown in Fig.1. In All maps, two single-phase regions are identified: one lyotropic liquid crystal (LLC) phase and one isotropic solution phase
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(L1 + L2). In addition, the mixed phase of LLC and isotropic solution is found in the phase diagrams of each system, and marked as “2Φ”. It shows that Brij 97 can form
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LLC phase in water with the water content of 17-67 wt%. As the introduction of Tweens®, the LLC phase moves towards higher water content, and the LLC phase zone shrinks as the increase of Tweens® content. It may be that Tweens® has more (EO) chain than Brij 97, showing greater hydrophilic capacity. Owing to the different alkyl chain of Tweens®, the LLC region of different systems shows various size. Comparing the size of the LLC region in the phase diagrams by visual analysis, the region size is 5
Tween 60 system > Tween 40 system > Tween 80 system > Tween 20 system. The Tweens® content of LLC region for each system is different. It may be caused by the altered hydrophobic capacity of Tweens®. Tween 60/H2O system can form LLC in Tweens®/H2O binary system. While, the LLC region of Tween 60 system with higher Tween 60 content shows opaque appearance, and the other systems behave as transparent solid state in the whole LLC region, which may be due to the relatively high melting point of Tween 60. Aggregates formed by surfactants can be predicted by surfactant packing parameter
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(Rp) [12]. The packing parameter which can be defined as Rp = vL aS lc , where aS is the interfacial area occupied by a surfactant headgroup, lc and vL are the length and
volume of the hydrophobic group, respectively. In our previous work, the Rp of Brij 97
-p
was estimated to be 0.45 [22]. The lc and vL of Tweens® were calculated using the
previous method [27]. Results are shown as Table 1. The value of aS is approximately
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equal to the maximum adsorption capacity of Tweens®, which is about 100 Å2 [28]. Therefore, the Rp of Tween 20, Tween 40, Tween 60 and Tween 80 are 0.29, 0.28, 0.28
lP
and 0.27. The values of Rp are between 0.25 and 0.45 for mixture of Brij 97 and Tweens®. Thus, the mixed systems tend to form hexagonal LLC and micellar cubic
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LLC in aqueous.
Therefore, the representative samples were selected in the phase diagrams to explore the microstructure of LLC.
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3.2 the effect of mass ratio for Brij 97 and Tweens® 3.2.1 Microstructures
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Firstly, the effect of mass ratio for Brij 97 and Tweens® on the microstructures of
LLC were studied. The total content of Brij 97 and Tweens® was fixed as 55 wt%, and the mass ratio of Brij 97/Tweens® was changed as 55/0, 45/10, 35/20 and 25/30. The LLC samples were characterized by SAXS. The representative SAXS spectra are shown in Fig. 2 and Fig. S1. It can be seen from Fig. 2, The relative peak positions of sample at 55/0 correspond 6
to the 100, 110, and 200 planes, and follow the relationship of
1 3 , showing as
hexagonal phase (H1) [29]. The lattice parameter (α) of hexagonal LLC can be
calculated according to the following formula: 4
3 q100 , where q100 is the
scattering vector of the first reflection [30]. The α of sample at 55/0 is 73.68 Å. Introducing Tween 40 into the samples, the relative peak positions also correspond to
1 3 , indicating that those samples remain as hexagonal LLC. But the Bragg
peaks move to the lower q. The samples of Tween 20, 60 and 80 system also show this
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behavior. The α of samples was calculated, as shown in Fig.3a. The values of α increase with the increasing Tweens® content, which indicated that the introduction of Tweens®
continuously swells the hexagonal lattices [31]. As the Tweens® content increases to
-p
10 wt%, the samples of Tween 40, 60 and 80 system have the same α. It may be due to that the difference of alkyl chain between Tween 40, Tween 60 and Tween 80 cannot
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be expressed in lower Tweens® content. As Tweens® content increases to 20 wt% and 30 wt%, the differences of alkyl chain of Tween 40, Tween 60 and Tween 80 were
lP
reflected, showing various α values for different system. The α values of samples at 35/20 and 25/30 follows the order: Tween 60 system > Tween 40 system > Tween 80 system > Tween 20 system. The saturated alkyl chain structure of Tween 20, Tween 40
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and Tween 60 is same except for different length. Compared the α values of Tween 20, 40 and 60 system, Tween 60 system has the largest α values, which is caused by the
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longest alkyl chain. Tween 80 and Tween 60 have the same length of alkyl chain, but the α value of Tween 80 system is less than Tween 60 system, which may be attributed
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to the double bond in the alkyl chain of Tween 80. The microstructures of LLC also can be explained by some characteristic parameters
calculated from SAXS models [30]. Apolar volume fraction, f is defined as
f v v . L
P
(1)
P
ϕP is the volume fractions of surfactant mixture. vL is the volume of hydrophobic tail, 7
and vP is the volume of surfactant molecule. For Brij 97, vL and vP are 500 Å and 1158.6 Å3, respectively [22]. For Tweens®, vP was calculated by molar mass and density. The density of Tween 20, Tween 40, Tween 60 and Tween 80 are 1.01 g/mL, 1.08 g/mL, 1.04 g/mL and 1.08g/mL respectively. The results of vP can be seen in Table 1. The radius of the cylindrical aggregates, dH, is deduced as
dH 3 2 f
(2)
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The dH values of samples are shown in Fig. 3b. It can be found that the values of dH decrease with the increase of Tweens® content. The dH values for Tween 40, 60, and
80 system are same when Tweens® content increases to 10 wt%, and are larger than
Tween 20 system. As Tweens® content increases to 20 wt% and 30 wt%, the dH values
-p
of samples follow the order: Tween 60 system < Tween 80 system < Tween 40 system < Tween 20 system.
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The radius of water channels, dw, is deduced as
(3)
lP
dW 2dH
The dw can be seen in Fig. 3c. The dw value of sample at 55/0 is 35.78 Å. With the increase of Tweens® content, dw values gradually increase to about 45 Å, which may
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be due to that Tweens® has larger hydrophilic head than Brij 97. Although Tweens® has the same hydrophilic head group, dw also shows a change in the samples containing
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different Tweens® results from the difference in alkyl chain. The effective cross-sectional area of each surfactant molecular at the interface, aS, is
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calculated as
aS 2vP f dH P
(4)
The calculated results of aS for sample are shown in Fig. 3d. The introduction of
Tweens® increases the value of aS, which due to that Tweens® has a larger molecular volume than Brij 97. 3.2.2 Rheological behavior 8
In order to reveal the viscoelasticity of the systems, rheological measurements were carried out. Rheology can provide real-time macroscopic characteristics of LLC and reflect the changes of internal microstructures [32, 33]. Stress sweep measurements were carried out at a set frequency of 6.286 rad/s and a temperature of 37 oC. As shown in Fig. 4a, for sample without Tweens®, the critical stress (σc) can reach 973 Pa, and the complex moduli (G*) can reach 31588 Pa, indicating that the sample is weakly dependent on the applied stress. Next, the Tween 40 system is used as example to illustrate the effect of Tweens® on the viscoelasticity
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of LLC. When Tween 40 is introduced into the sample, G* and σc continue to decrease as the Tween 40 content increases. The results indicate that the introduction of Tween 40 weakens the resistance of LLC to external force.
Frequency sweep curves of samples are shown in Fig. 4b. At lower frequencies, the
-p
viscous moduli (G'') are larger than the elastic moduli (G'), showing as viscous
properties. When the frequency exceeds a certain value (intersection frequency of G'
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and G''), G' is larger than G'', showing as elastic properties. The dynamic moduli (G' and G'') behave as frequency dependent. The values of G' increase with the increasing
lP
frequency. While, the values of G'' increase with the increase of frequency at lower frequency, and gradually approaches the platform at higher frequency. The behavior is
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similar to our previous system [34, 35]. Dynamic moduli of samples increase with the increase of Tween 40 content, which is the same as the behavior of G* in stress sweep curves. The frequency dependence of the hexagonal LLC in linear viscoelastic region
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can be further explained by multiple Maxwell models [36]. The solid lines in Fig. 4b are model fitted, and all samples are consistent with multiple Maxwell models.
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Generally, aggregates composed of nonionic surfactants are affected by temperature,
showing as temperature sensitivity [37]. Thus, the relationship between microstructures of LLC and temperature was studied by temperature sweep measurements. The temperature response of storage moduli (G') and loss moduli (G'') can be seen in Figure 4c. It can be found that the viscoelastic moduli values of all samples drop rapidly at a certain temperature (Tc). The reason is that the ordered structure of LLC is destroyed 9
and converted into a poorly ordered structure of micelle due to the increase in temperature. The introduction of Tween 40 reduces Tc from 60 oC to about 50 oC. This may be that Tween 40 leads to the loosening of LLC, which can be explained by the increased lattice parameter (α). Stress sweep curves, frequency sweep curves and temperature sweep curves of the Tween 20, 60, 80 system have similar phenomenon to the Tween 40 system, which can be seen in Fig. S2. 3.3 The effect of alkyl chain for Tweens® on rheological properties
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From the SAXS spectra, it can be found that the effect of alkyl chain for Tweens® can be revealed when Tweens® content is 20 wt% and 30 wt%. Therefore, the stress
sweep measurements were carried out to study the influence of alkyl chain on rheological properties, as shown in Fig. 5a. The stress sweep curves show that the
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order of σc and G* are Tween 60 system > Tween 80 system > Tween 40 system > Tween 20 system. This may be that the larger values of α lead to the looser
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microstructure, so the structure of LLC is less stable and less resistant to external forces. From the above research, it can be found that Brij 97 and Tweens® content
lP
have effects on the microstructure of LLC. So next, the effects of Brij 97 or Tweens® concentration on the LLC was investigated.
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3.4 The effect of Bri 97 concentration
The Tween 40 content was fixed as 10 wt%, and Brij 97/H2O mass ratio was changed as 30/60, 37.5/52.5, 45/45 and 52.5/37.5. The SAXS spectra of samples (Brij
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97/H2O = 30/60, 37.5/52.5 and 45/45) at 37 oC are shown in Fig. 6a. The relative peak
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positions correspond to 1
3 , which confirmed as hexagonal LLC (H1). While, the
sample (Brij 97/H2O = 52.5/37.5) has five scattering peaks. The relative peak positions correspond to 1
3 , showing the structure of hexagonal LLC. The other relative
peak positions correspond to the 111, 220, 311 planes and follow the relationship of
3 4 11 , behaving as face-centered cubic space group Fd3m (micellar cubic LLC, I1). It indicates that the sample is a mixture of H1 and I1 at 37 oC. As shown in 10
Fig. 6a, with the increase in Brij 97 concentration, the values of α and aS increase gradually indicating that the microstructure of LLC is tighter. While, the values of dH increase gradually and values of dW decrease gradually. It maybe that the increase in Brij 97 concentration promotes the winding degree of surfactant, which is conducive to the overlap of adjacent layers [38]. The temperature sweep curves of samples are shown in Fig. 7b. the samples with the Brij 97/H2O mass ratio of 30/60, 37.5/52.5 and 45/45 have one phase transition
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temperature (Tc 1) of 56 oC, 54 oC and 50 oC, respectively. While, For the sample with the Brij 97/H2O mass ratio of 52.5/37.5, the temperature sweep curve has two phase transition temperature of Tc
2
(38 oC) and Tc
1
(48 oC). It is worth noting that the
-p
viscoelastic moduli in the lower temperature range are significantly low than the
re
viscoelastic moduli in the higher temperature range. To make clear of this phenomenon, the SAXS measurement was performed on the sample at 43 oC. The SAXS spectrum
lP
(dotted line) has three scattering peaks, and the relatively peak positions correspond to
1 3 , shown as hexagonal LLC. However, the peak intensity is greater than 37 oC,
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indicating that its structural order is enhanced.
The frequency sweep curves of samples are shown in Figure 6c. As the increase in
ur
shear frequency, the values of G' and G'' increase with different slop, showing as frequency dependence. The frequency sweep curves of samples conform to multiple
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Maxwell models. The relaxation spectra are shown in the inset of Figure 8c. The samples with Brij 97/H2O mass ratio of 30/60, 37.5/52.5 and 45/45 show that the relaxation moduli decrease as the increased relaxation time. While, the sample (Brij 97/H2O=52.5/37.5) has a maximum relaxation modulus. The number of Maxwell elements required to make up the function is shown in the inset. The Maxwell element number of samples decreases as the increase of Brij 97 concentration. The minimum 11
relaxation time is usually the time required for water molecules to transition between disturbance and equilibrium states. The minimum relaxation time of samples with Brij 97/H2O mass ratio of 0/60, 37.5/52.5and 45/45 is substantially the same. While, the sample with Brij 97/H2O mass ratio of 52.5/37.5 has significantly larger values. It may be that the larger viscoelastic moduli are not conducive to the movement of water molecules. The maximum relaxation time is usually the time required for surfactant molecules to transition between disturbance and equilibrium states. The maximum relaxation time of sample (Brij 97/H2O=52.5/37.5) shows as the smallest value, which
3.5 The effect of Tween 40 concentration
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maybe that the sample is the mixed LLC phase.
The Brij 97 content was fixed as 37.5 wt%, and the Tween 40/H2O mass ratio was changed as 0/62.5, 10/52.5, 20/42.5 and 25/37.5. All samples have three scattering
1 3 , confirmed as hexagonal
-p
peaks, and the relative peak positions correspond to
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LLC. It can be seen from Fig. 7a, the α values become large as Tween 40 concentration increases, indicating that the cylinder of hexagonal LLC becomes more closely packed.
lP
It can be found that dH remains basically unchanged, while dW values appear to be reduced. The aS values increase as the Tween 40 concentration increases, which may be due to the larger molecule volume of Tween 40.
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Temperature sweep curves of samples are shown in Fig. 7b. As Tween 40 concentration increases, the Tc 1 of samples decreases. As the introduction of Tween
ur
40, there is a reduction in G' before reaching Tc 1. As the concentration of Tween 40 increases, the temperature at which G' decreases is dropping. It indicates that the
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increase in Tween 40 concentration promotes the sensitivity of LLC to temperature. The frequency sweep curves of samples can be seen in Fig. 7c. The viscoelastic
moduli of samples all appear to be frequency dependent. The relaxation spectra can be seen in the inset of Fig. 7c. Observing the relaxation spectra, for the samples with Tween 40/H2O mass ratio of 10/52.5, 20/42.5 and 25/37.5, the relaxation moduli decrease with the increase of relaxation time. While, the sample (Tween 40/H2O = 0/62.5) shows a maximum value. Observing the number of Maxwell elements, it can 12
be found that sample with Tween 40/H2O mass ratio of 0/62.5 has 5 Maxwell elements, and the other samples have 6 Maxwell elements. The minimum relaxation time, of with Tween 40/H2O mass ratio of 0/62.5 and 10/52.5 are significantly larger, which may be related to their greater viscoelastic moduli. For maximum relaxation time, the sample (Tween 40/H2O = 0/62.5) shows as the largest maximum relaxation time value, which maybe that the sample do not have Tween 40. 3.4 The effect of temperature on Tween 60/water binary system As shown in Fig. 1, LLC can be constructed in Tween 60/H2O binary system at 37 °C.
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While, the appearance of LLC is more transparent at 37 °C than at 25 °C. It may be that the microstructures were changed under the effect of temperature. Therefore, a representative sample (Tween 60/H2O=55/45) was selected to study the effect of temperature on its microstructures.
-p
Firstly, the temperature sweep measurement was carried out at 6.286 rad/s. The change of G' and G'' is reflected by the parameter tan δ (G''/G'). As shown in Fig. 8a,
re
G' and G'' are significantly affected by temperature. Between 20 oC and 28 oC, G'' is greater than G'. While, the values of tan δ are basically unchanged and greater than 1.
lP
It indicates that the temperature interval (I interval) is dominated by viscosity. Between 28 oC and 35.5 oC, G'' is still greater than G'. But tan δ is gradually decreasing, showing
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that the elastic component in the sample gradually increases. The temperature interval (II interval) exhibits a relative transition of viscous dominance and elastic dominance. Between 35 oC and 38.4 oC, G' is greater than G'', and tan δ is less than 1. It shows that
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the temperature interval (III interval) is elastic dominated. As the temperature continues to rise, between 38.4 oC and 50.0 oC, the sample shows as G' > G'', and the tan δ is
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greater than 1. It indicated that the temperature interval (IV interval) is viscous dominance. The sample behaves as various viscoelastic properties at different temperature interval, indicating that the microstructure has changed under the effect of temperature. In order to investigate the specific structure of sample at different temperature, the intermediate temperature of 25 oC in the I interval and 37 oC in the III interval was 13
selected. Polarization microscopy observations (Fig. 8b and Fig. 8c) show that the sample has polarized texture at 25 oC and 37 oC, exhibiting birefringence. Therefore, the sample is both anisotropic at 25 oC and 37 oC. However, the birefringence at 37 oC is stronger than that at 25 oC, indicating that the microstructures of sample are tighter at 37 oC. In order to illustrate the specific structure of sample at 25 oC and 37 oC, SAXS measurements were carried out, as shown in Fig. 8d. At 25 oC, sample has three Bragg peaks with a relative position of
1 2 3 , which is consistent with lamellar LLC (L).
At 37 oC, there are also have three Bragg peaks, but the relative position of Bragg peaks
1 3 , showing as hexagonal LLC. The frequency sweep spectra of
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corresponds to
sample are shown in Fig. 8e. At 37 oC, G' and G'' have an intersection at 0.08 rad/s.
Below 0.08 rad/s, G'' > G', the sample is viscosity dominated; above 0.08 rad/s, G' >
-p
G'', behaving as elastic dominance. The sample behaves as viscoelastic fluid, and the frequency sweep curve conforms to the behavior of hexagonal LLC. At 25 oC,
re
Throughout the test range, G' and G'' of sample increase with increasing frequency, and
lP
G'' is greater than G', which showed viscosity dominance. By extending the curve, G' and G'' have an intersection point at 622.7 rad/s, indicating that the elastic dominant region of the sample is at out of test range.
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Conclusion
In this paper, the mixtures of EO-based nonionic surfactant (EO-SAA) in water were
ur
studied. The phase diagrams at 37 oC of the Brij 97/Tween 20/H2O (Tween 20 system), Brij 97/Tween 40/H2O (Tween 40 system), Brij 97/Tween 60/H2O (Tween 60 system)
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and Brij 97/Tween 80/H2O (Tween 80 system) in the whole concentration range show two single phase: one lyotropic liquid crystal (LLC) phase and one isotropic solution phase (L1 + L2). Compared with previous mixture of EO-SAA/EO-SAA systems, only one LLC phase region appears in the phase diagrams. The difference in the alkyl chain length and insaturation of Tweens® leads to significant changes in the phase region size and microstructure of LLC. The phase size order of LLC region on the phase diagrams and lattice parameter (α) order of samples whose Brij 97/Tweens® was 35/20 and 25/30 14
was Tween 60 system> Tween 40 system >Tween 80 system > Tween 20 system. As the increase of Tweens® content, the critical stress (σc) and moduli (G*, G' and G'') decrease continuously. At 37 oC, most of LLC samples are hexagonal phase (H1) and show the characteristics of viscoelastic fluids. However, sample (Brij 97/Tween 40/H2O=52.5/10/37.5) appears as a mixed phase of hexagonal phase (H1) and micellar cubic phase (I1) at 37 oC. Under the influence of temperature, the sample transformed from mixed phase of H1+ I1 at 37 oC to H1 at 42 oC. In a word, the study of the behavior for Brij 97 and Tweens® simultaneously forming
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LLC can have a good guiding for the application of LLC.
Declaration of Interest Statement
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The authors report no conflict of interest.
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Acknowledgments
Support of this work by the National Natural Science Foundation of China (31271933
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lP
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Figure captions
Fig. 1 The phase diagrams for Brij 97/ Tweens® /H2O system at 25 oC. (a) Brij 97/Tween 20/H2O system; (b) Brij 97/Tween 40/H2O system; (c) Brij 97/Tween 60/H2O system; (d) Brij 97/Tween 80/H2O system. Fig. 2 The SAXS spectra of investigated LLC samples in Brij 97/Tween 40/H2O
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system at 37 oC. The total content of Brij 97 and Tweens 40 was fixed as 55 wt%, and the Brij 97/Tweens 40 mass ratio was changed as 55/0, 45/10, 35/20 and 25/30.
Fig. 3 The results of the characteristic parameters obtained from the SAXS spectra: (a) lattice parameter (α); (b) the radius of the cylindrical aggregates for hexagonal
-p
phase (dH); (c) the radius of the water channels (dW); (d) the effective cross-sectional area per surfactant at the interface (aS).
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Fig. 4 The water content was fixed as 45 wt%, and the Brij 97/Tween 40 mass ratio was changed as 55/0, 45/10, 35/20 and 25/30. (a) The complex moduli (G*) versus
lP
stress. (b) Elastic (G' filled) and viscous (G'' hollow) moduli as a function of angular frequency for samples. (c) Temperature evolution of the storage (filled square G') and
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loss (hollow square G'') moduli upon heating for samples. The temperature at which the dotted line is located is the phase transition temperature. Fig. 5 The water content was fixed as 55 wt%, and the Brij 97/Tween 40 mass ratio
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was 35/20 (a) and 25/30 (b). The complex moduli (G*) versus stress. Fig. 6 The Tween 40 content was fixed as 10 wt%, and the Brij 97/H2O mass ratio
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was changed as 30/60, 37.5/52.5, 45/45 and 52.5/37.5. (a) The SAXS spectra of samples at 37 oC (solid line); the SAXS spectra of sample whose Brij 97/H2O = 52.5/37.5 at 43 oC (dotted line). The Insert table is the characteristic parameter obtained from SAXS. (b) Temperature evolution of the storage (filled square G') and loss (hollow square G'') moduli upon heating for samples. The temperature at which the dotted line is located is the phase transition temperature (Tc). (c) Elastic (filled 21
square G') and viscous (hollow square G'') moduli as a function of angular frequency for samples at 37 oC. Inset: the discrete relaxation spectra of samples at 37 oC. Fig. 7 The Brij 97 content was fixed as 37.5wt%, and the Tween 40/H2O mass ratio was changed as 0/62.5, 10/52.5, 20/42.5 and 25/37.5. (a) The SAXS spectra of samples at 37 oC. The Insert table is the characteristic parameter obtained from SAXS. (b) Temperature evolution of the storage (filled square G') and loss (hollow square G'') moduli upon heating for samples. The temperature at which the dotted line is located is the phase transition temperature. (c) Elastic (filled square G') and viscous
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(hollow square G'') moduli as a function of angular frequency for samples. Inset: the discrete relaxation spectra of samples at 37 oC.
Fig. 8 (a) Temperature evolution of the storage (filled square G'), loss (hollow square G'') moduli and tanδ (filled circular) upon heating for sample (Brij 97/Tween
-p
60/H2O=0/55/45). Polarized textures of sample after constant for 10 minutes at 25 oC (b) and 37 oC (c). (d) The SAXS spectra of the investigated LLC sample at 25 oC and
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37 oC. (e) Elastic (filled square G') and viscous (hollow square G'') moduli as function
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lP
of angular frequency for sample at 25 oC and 37 oC.
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Fig. 1 The phase diagrams for Brij 97/ Tweens® /H2O system at 25 oC. (a) Brij
lP
97/Tween 20/H2O system; (b) Brij 97/Tween 40/H2O system; (c) Brij 97/Tween
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60/H2O system; (d) Brij 97/Tween 80/H2O system.
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Fig. 2 The SAXS spectra of investigated LLC samples in Brij 97/Tween 40/H2O
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system at 37 oC. The total content of Brij 97 and Tweens 40 was fixed as 55 wt%, and
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na
lP
the Brij 97/Tweens 40 mass ratio was changed as 55/0, 45/10, 35/20 and 25/30.
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Fig. 3 The results of the characteristic parameters obtained from the SAXS spectra: (a) lattice parameter (α); (b) the radius of the cylindrical aggregates for hexagonal
lP
phase (dH); (c) the radius of the water channels (dW); (d) the effective cross-sectional
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area per surfactant at the interface (aS).
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Fig. 4 The water content was fixed as 45 wt%, and the Brij 97/Tween 40 mass ratio
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was changed as 55/0, 45/10, 35/20 and 25/30. (a) The complex moduli (G*) versus stress. (b) Elastic (G' filled) and viscous (G'' hollow) moduli as a function of angular
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frequency for samples. (c) Temperature evolution of the storage (filled square G') and loss (hollow square G'') moduli upon heating for samples. The temperature at which the dotted line is located is the phase transition temperature.
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Fig. 5 The water content was fixed as 55 wt%, and the Brij 97/Tween 40 mass ratio
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lP
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was 35/20 (a) and 25/30 (b). The complex moduli (G*) versus stress.
Fig. 6 The Tween 40 content was fixed as 10 wt%, and the Brij 97/H2O mass ratio was changed as 30/60, 37.5/52.5, 45/45 and 52.5/37.5. (a) The SAXS spectra of samples at 37 oC (solid line); the SAXS spectra of sample whose Brij 97/H2O = 52.5/37.5 at 43 oC (dotted line). The Insert table is the characteristic parameter obtained from SAXS. (b) Temperature evolution of the storage (filled square G') and loss (hollow square G'') moduli upon heating for samples. The temperature at which 27
the dotted line is located is the phase transition temperature (Tc). (c) Elastic (filled square G') and viscous (hollow square G'') moduli as a function of angular frequency
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lP
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-p
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for samples at 37 oC. Inset: the discrete relaxation spectra of samples at 37 oC.
Fig. 7 The Brij 97 content was fixed as 37.5wt%, and the Tween 40/H2O mass ratio was changed as 0/62.5, 10/52.5, 20/42.5 and 25/37.5. (a) The SAXS spectra of
ur
samples at 37 oC. The Insert table is the characteristic parameter obtained from SAXS. (b) Temperature evolution of the storage (filled square G') and loss (hollow
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square G'') moduli upon heating for samples. The temperature at which the dotted line is located is the phase transition temperature. (c) Elastic (filled square G') and viscous (hollow square G'') moduli as a function of angular frequency for samples. Inset: the discrete relaxation spectra of samples at 37 oC.
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Fig. 8 (a) Temperature evolution of the storage (filled square G'), loss (hollow square
lP
G'') moduli and tanδ (filled circular) upon heating for sample (Brij 97/Tween 60/H2O=0/55/45). Polarized textures of sample after constant for 10 minutes at 25 oC (b) and 37 oC (c). (d) The SAXS spectra of the investigated LLC sample at 25 oC and
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37 oC. (e) Elastic (filled square G') and viscous (hollow square G'') moduli as function
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of angular frequency for sample at 25 oC and 37 oC.
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Table 1 The results of lc (the length of hydrophobic group), vL (the volume of hydrophobic group), Rp (surfactant packing parameter) and vP (the volume of surfactant molecule) for Tweens® Name
vL (Å3) 325 433 487 473
Rp 0.29 0.28 0.28 0.27
vP (Å3) 2018.8 1974.3 2095.1 2014.4
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lP
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Tween 20 Tween 40 Tween 60 Tween 80
lc 11.32 15.37 17.39 17.48
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