Impact of Alkyl Chain Length on the Transition of Hexagonal Liquid

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Influence of alkyl chain length on the transition of hexagonal liquid crystal - wormlike micelle - gel in ionic liquidtype surfactant aqueous solutions without any additive Yimin Hu, Jie Han, Lingling Ge, and Rong Guo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03382 • Publication Date (Web): 04 Nov 2015 Downloaded from http://pubs.acs.org on November 6, 2015

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Impact of alkyl chain length on the transition of hexagonal liquid crystal - wormlike micelle - gel in ionic

liquid-type

surfactant

aqueous

solutions

without any additive Yimin Hu, Jie Han*, Lingling Ge, Rong Guo* School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu, 225002, P. R. China. E-mail: [email protected] KEYWORDS: alkyl chain length; wormlike micelle; hydrogel; hexagonal liquid crystal; surfactant ABSTRACT: The search for functional supramolecular aggregations with different structure has attracted interest of chemists because they have the potential in industrial and technological application. Hydrophobic interaction has great influence on the formation of these aggregations, such as hexagonal liquid crystal, wormlike micelle and hydrogel, et al. So a systematical investigation was done to investigate the influence of alkyl chain length of surfactants on the aggregation behavior in water. The aggregation behavior of 1-hexadecyl-3-alkyl imidazolium bromide and water has been systematically investigated.

These ionic liquid surfactants are

denoted as C16-Cn (n = 2, 3, 4, 6, 8, 9, 10, 12, 14, 16). The rheological behavior and microstructure were characterized via a combination of rheology, cryo-etch scanning electron microscopy, polarization optical microscopy and X-ray crystallography. The alkyl chain has great influence on the formation of surfactant aggregates in water at the molecular level. With

ACS Paragon Plus Environment

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increasing alkyl chain length, different aggregates, such as hexagonal liquid crystal, wormlike micelle and hydrogel can be fabricated: C16-C2 aqueous solution only forms hexagonal liquid crystal; C16-C3 aqueous solution forms wormlike micelle and hexagonal liquid crystal; C16-C4, C16-C6 and C16-C8 aqueous solutions only form wormlike micelle; C16-C9 aqueous solution experiences a transition between wormlike micelle and hydrogel; C16-C10, C16-C12, C16-C14 and C16-C16 only form hydrogel. The mechanism of the transition of different aggregation with increasing alkyl chain length was also proposed.

1 INTRODUCTION The self-assembly of surfactant molecules can create different aggregates, such as sphere micelle, rod micelle, wormlike micelle, vesicle, liquid crystal and gel, et al.1, 2 The assemble process

is

based

on

noncovalent

interactions,

such

as

electrostatic

effect,

hydrophobic-hydrophilic effect, π-π stacking effect, hydrogen bonding effect, or van der Waals forces, et al.3 Sphere micelles are formed firstly with the dissolution of surfactant in a solvent. At dilute regime (above the second critical concentration), the sphere micelles can transit to rod micelles with the monotonical increase of average micellar length as increasing of concentration.4 The counterions of surfactants can reduce the repulsive force between head groups and decrease the spontaneous curvature of micelles. The molecules pack at the hemispherical ends of cylindrical aggregates with the decreasing spontaneous curvature. The hemispherical ends have more free energy than the cylindrical part. The linear growth of cylindrical micelles is thermodynamically driven by “end-cap energy”. By inserting molecules into the cylindrical part, the free energy of the system can be minimized. The surfactants form short rod-like micelles. Above certain concentration, the micelles can grow to several


2

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micrometers long and about several nanometers in diameter, which is named as wormlike micelles. The micelles can entangle with each other and form a transient network which imparts viscoelastic property to the surfactant solution.5, 6 They can relax stress through two mechanisms. One is breaking and recombination process; the other is micellar reputation process. These processes are affected by temperature and solution composition. The average micellar length and relaxation times behaves as a thermodynamic quantity.7 Wormlike micelles are often formed by cationic surfactants, zwitterionic surfactants, nonionics surfactants, anionic surfactants and the mixtures of cationic–anionic8 and ionic–nonionic surfactants.9, 10 In aqueous solution, cationic surfactants which have long alkyl chain are the most popular systems studied for wormlike micelle. The cationic surfactants include cetylpyridinium bromide and cetyltrimethylammonium bromide. Their average micelle length increases by the decrease of potential on the micellar surface via electrostatic screening by adding hydrotrope or inorganic salt.11,

12

Most ionic surfactants self-assemble into cylindrical

aggregates only in the presence of salt. As far as we know, the wormlike micelles composed of cationic surfactant alone without any additive in dilute and semi-dilute regimes is still rare.13,

14

Wormlike micelles have become ubiquitous to a wide variety of

industrial processes and consumer products in recent years.15 The wormlike micelles can be used ranging from fracturing fluid in oil fields16, thickeners for personal and home care products, a size-controlling microreactor

17

and templates for materials synthesis18, 19 to drag-reduction

purposes for district heating and cooling.20 In our previous work, we found that the 1-hexadecyl-3-nonyl imidazolium bromide (C16-C9) aqueous solution without additives shows interesting rheological behavior. The solution can experience the transition between wormlike micelle and hydrogel with concentration and


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temperature.21 In the past decades, ionic liquid–type surfactants have been widely used in the field of electrochemistry, catalysis, material preparation and organic synthesis, et al.22 Meanwhile, as a kind of cationic surfactant, ionic liquid aggregation properties in water have been extensively studied. The molecular structure deeply affects the phase behavior of surfactants in solution. There are two main factors which influence the aggregation process. One is the interaction among the polar heads; and the other is the interaction between the alkyl chains. Electrostatic forces typically exist among the polar heads. The van der Waals force exists between the alkyl chains. Thus, the aggregation behavior can be manipulated by changing the polar head chemistry, number of chains, chain length, chain branching, degree of chain asymmetry, et al. Alkyl chain length of surfactant is one parameter which affects the aggregate process of surfactants. However, few works have studied the rheological properties of wormlike micelles which are affected by the alkyl chain length of surfactant. The reason is that the surfactant chain with more than 16 carbon atoms is often water insoluble. At the same time, most cationic surfactants generally form wormlike micelles in the presence of salt which make the systems more complicated.23, 24 The major motivation of the present work is to generate a series of new ionic liquid-type surfactants. Then their aggregation behavior will be investigated to establish the structure–property relationship. It is interesting that surfactants with longer alkyl chain length than that of C16-C9 can form hydrogels in aqueous solution, such as C16-C10, C16-C12, C16-C14 and C16-C16. However, surfactants with shorter alkyl chain length than that of C16-C9 can form wormlike micelle in aqueous solution, such as C16-C4, C16-C6 and C16-C8. The C16-C3 aqueous solution can transit from wormlike micelle to hexagonal liquid crystal with increasing surfactant concentration. The C16-C2 surfactant can form hexagonal liquid crystal with water. This work indicates that the self-assembly of these imidazolium-based surfactants can be


4

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adjusted by changing the alkyl chain length of the surfactants. It should also be noted that few studies have been done to investigate the effect of the alkyl chain length on the water gelation ability.25, 26 The systematical studies relating the alkyl chain length to the phase transition among the hexagonal liquid crystal, wormlike micelle and hydrogel is seldom seen. The choice of an ionic liquid-type surfactant C16-Cn in this study gives the opportunity to explain the relationship between aggregation behavior and the alkyl chain length of surfactant. This work may shed a light on the study of adjusting the transition among these aggregations by other means.

2. EXPERIMENTAL METHODS 2.1. Materials: The starting materials were offered by the Aladdin Industrial Co and Shanghai Chemical Co. (analytical reagent). C16-Cn (n=2, 3, 4, 6, 8, 10, 12, 14, 16) were synthesized according to the method21. Satisfactory 1H NMR (600 MHz, CDCl3) and melting point were obtained for the C16-Cn (n = 2, 3, 4, 6, 8, 10, 12, 14, 16) (see Supporting Information). Scheme 1 presents the C16-Cn molecular structure. N

N

n

1

Br_

Scheme 1 Molecular structures of the ionic liquid surfactants. n = 2, 3, 4, 6, 8, 9, 10, 12, 14, 16. Sample Preparation. C16-Cn and H2O are mixed at various weight concentration. The mixtures are vortex mixed for several times. Then the samples are centrifugated and equilibrated at 25 oC for 48 hour. Characterization. Rheological Measurements. Rheological properties of the samples are performed on a rheometer Rheostess RS600. The temperature is controlled by oil circulating bath. The


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samples are put on the coneplategeometry. The diameter of the coneplategeometry is 35 mm and the cone angle is 1o). The cone plate is adjusted 52 µm away to the sensor for all measurements. In order to minimize the evaporation of water, a solvent-trapping device is placed above the plate. Cryo-Etch Scanning Electron Microscopy (SEM). Cryo-etch SEM (XL-30E Philips Co., Holland, 20 kV) is used to measure the micro-morphology of the gel and wormlike micelle. The viscoelastic solutions are casted on the Cu slides. The samples are fast cooled by the use of liquid N2. The samples are etched under -90 oC for twenty minutes. Then the samples are cut to produce a plane which is coated with alloy of palladium and gold before measurement. Small-Angel X-Ray Diffraction (SA-XRD): SA-XRD patterns of the samples are measured with a Bruker X-ray diffractometer. The Cu Kα X-ray is generated under a current of 80 mA and a voltage of 40 kV. The xerogel is prepared by evaporated in a vacuum desiccator for 48 h. The hexagonal liquid crystal was spread on the plastic specimen stage with a notch. Polarization Optical Microscopy (POM). Birefringence of the samples are recorded by a Leica DMLP polarization microscope with DFC320 CCD camera and cross polarizers.

3. RESULTS 3.1 C16-C2 At low concentration (< 23 wt.%), the solution is the isotropic and homogeneous. With increasing concentration of C16-C2, the concentrated samples can entrap water. The anisotropic liquid crystalline samples of C16-C2 were firstly determined by optical polarizing microscopy. The C16-C2 solutions appear striated birefringence textures (Figure 1a) giving evidence of the formation of hexagonal liquid crystal.27


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Intensity (a.u)

b

0

2

4

6

8

10 12 14 16

2θ (deg.)

G',G''(Pa)

c

10

5

10

4

10

3

G' G"

10

-2

10

-1

10

0

-1

10

1

10

2

ω (rad⋅s )

e

f G',G''(Pa)

Intensity (a.u)

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0

2

4

6

2θ (deg.)

8

10

10

4

10

3

10

2

G' G"

10

-2

10

-1

10

0

-1

ω (rad⋅s )

10

1

10

2

Figure 1 (a) Polarized optical micrograph of 50.00 wt.% C16-C2 aqueous sample. (b) X-ray diffraction patterns of 35.00 wt.% C16-C2 hexagonal liquid crystal. (c) G’ and G” versus frequency for 55.00wt.% C16-C2 solutions at 25 oC. (d) Optical polarizing micrograph of 55.00 wt.% C16-C3 aqueous system. (e) X-ray diffraction patterns of 40.00 wt.% C16-C3 hexagonal liquid crystal. (f) G’ and G” versus frequency for 45.00wt% C16-C3 sample at 25 oC In hexagonal liquid crystal, the rod micelles packed parallelly in the hexagonal arrangement. The SA-XRD can be used to confirm the hexagonal liquid crystal. As shown in Figure 1b, the 35.00 wt.% C16-C2 aqueous sample shows several distinct diffraction peaks at 2θ = 2.80°, 4.62°, 7.25° and 10.33°. The interlayer distances d = 3.323 nm, 2.067 nm, 1.230 nm and 0.867 nm, in accord with the ratio of 1:1/(3)1/2: 1/(7)1/2: 1/(9)1/2, reveal the phase of inspected samples as a


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hexagonal liquid crystal.27 With increasing C16-C2 concentration, the cylindrical aggregates pack more densely. With the increase of surfactant concentration the interplane distances decrease from 3.411 nm (30 wt.%), 3.323 nm (35 wt.%), 3.309 nm (40 wt.%), 3.253 nm (45 wt.%) to 3.159 nm (50 wt.%). The interlayer distance is the vertical distance from the rod micelle center to the straight line passing through the two centers of the neighboring cylinders. The macroproperties of hexagonal liquid crystal of C16-C2 with different concentration are measured by rheology (Figure 1c). The C16-C2 hexagonal liquid crystals show the analogous rheological curves. One rheogram of 55.00 wt.% C16-C2 sample at 25.0 °C is shown in Figure 1c. The loss modulus (G”) and storage modulus (G’) change with frequency and have a large value. G” and G’ increase gradually with increasing frequency. The slope of G” is less than that of G’. At lower frequencies, the sample shows viscous behavior because G’ < G”. However, at higher frequencies it shows elastic behavior because G’> G”. The rheological results mean the hexagonal liquid crystal has viscoelastic properties, which is same as the typical hexagonal liquid crystal phases. 3.2 C16-C3 The hexagonal liquid crystal phase can also be formed in the concentrated C16-C3 solution. Figure 1d shows the POM pictures of C16-C3 aqueous sample (55.00 wt.%). The typical liquid crystal textures observed for the sample gives evidence of the formation of hexagonal liquid crystal. The SA-XRD can be used to confirm the hexagonal liquid crystal (Figure 1e). The 40.00 wt.% C16-C3 aqueous sample shows several distinct diffraction peaks at 2θ = 2.41°, 4.29°and 7.16°. The interlayer distances d = 3.63 nm, 2.058 nm and 1.232 nm, the ratio of 1:1/(3)1/2:1/(9)1/2 which reveal the phase of inspected samples as a hexagonal liquid crystal.27 With increasing surfactant concentration the interlayer distances decrease form 3.63 nm (40


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wt.%), 3.50 nm (45 wt.%), 3.44 nm (50 wt.%) to 3.38 nm (55 wt.%). The reason is rod micelles stack closer with the increasing surfactant concentration. The rheological properties of the C16-C3 hexagonal liquid crystal were also investigated (Figure 1f). Like C16-C2 samples, G” and G’ increase gradually with increasing frequency. The slope of G” is less than that of G’. However, the C16-C3 hexagonal liquid crystal phase shows more elastic than viscous properties.

33.0 wt.%

a

b

34.0 wt.%

33.0 wt.% 34.0 wt.% 35.0 wt.% 36.0 wt.%

1

35.0 wt.%

2

10

36.0 wt.%

η (Pa.s)

10 G',G''(Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

10

1

10

1

10

2

-1

ω (rad⋅s )

10

-2

10

-1

10

0

1

10 -1 10 ω (rad⋅s )

2

10

Figure 2 (a) G’ (filled symbols) and G” (unfilled symbols) versus frequency for C16-C3 aqueous system at 33.00-36.00 wt.%. (b) Steady shear-rate-viscosity curves as a function of the weight concentration for the C16-C3 wormlike micelles. At low content (< 30 wt.%), the solution is the isotropic and homogeneous. However, at concentration between 30.00 and 36.00 wt.%, the C16-C3 aqueous system can form wormlike micelles. As shown in Figure 2a, the G’ cross over the G” at certain frequency. The crossover frequency becomes lower with the increase of C16-C3 concentration. Figure 2b shows the viscous properties of C16-C3 wormlike micelles. With the increase of concentration, the steady shear-rate-viscosity increases firstly and then decreases. The wormlike micelle solutions undergo a typical shear thinning above the critical frequency. The reason is the micelles alignment induced by shear. This section is a wormlike micelle system.4 However, when the concentration reaches 37.0 wt.%, the frequency at which the G’ cross over the G” becomes much lower and the steady shear-rate-viscosity increases about 6 times of magnitude greater than that of 36.0 wt.%


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C16-C3 solution. At the same time, the 37.0 wt.% C16-C3 exhibits bright streaks (visible under cross polarizers) which means the occurrence of the phase transition. 3.3 C16-C4, C16-C6 and C16-C8 Among the nine ionic liquid- type surfactants, C16-C4, C16-C6 and C16-C8 can form viscoelastic wormlike micelles over a range of surfactant concentration at 25 oC without any additive. The reason of the formation of wormlike micelles without additive may be caused by the large imidazole head with only a positive charge, which facilitates the growth of micelles. The samples were also determined by POM measurements. It is not obtained the marbling and stripe textures here, which usually is observed from the conventional lamellar liquid crystal phases. With increasing alkyl chain length, the lowest surfactant concentration in which wormlike micellar solutions are formed is diminishable and the viscoelastic properties of wormlike micelles is incremental. Among the three wormlike micelles system, we choose the C16-C8 aqueous solution as an example because they have the similar properties. 3.3.1 Rheology Measurements. The rheological measurements were carried out for C16-C8 solutions. The G’ and the G” cross over within the measured frequencies for the 4.00 wt.% C16-C8 (Figure 3a). At low frequency region the sample behaves as liquid because G’ is less than G”. However, the sample behaves as solid matter at high frequency region because G’ is larger than G”. The dynamic rheological measurement gives evidence of a viscoelastic response of the C16-C8 solution at 25 oC. The solution has the viscoelastic properties because the cylindrical micelles can entangle each other to form a transient network.11 At low frequency region, Maxwell model can be used to describe the rheological properties of the wormlike micelle. The Maxwell model is described by the following relations which consider the stress relaxation as a single process.28


10

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G’ (ω) = G0

ω 2τ R2 1 + ω 2τ R2

G” (ω) = G0

ω τR 1 + ω 2τ R2

(1)

(2)

where G0 is the plateau value of G’, τR is the relaxation time. The magnitude of the G0 is related the network density whereas τR is determined by the average micellar length.7 The τR value of the aqueous solution is 184.2 s for 4.00 wt.% sample by τR = 1/ωc. The τR of 3.50 wt.%, 5.00 wt.%, 6.00 wt.%, 7.00 wt.% and 8.00 wt.% C16-C8 solution is 51.2 s, 109.1 s, 25.6 s, 5.0 s and 1.4 s, respectively. The τR increases sharply to the largest and then decreases monotonically with increasing C16-C8 concentration. The change in crossover frequency is caused by the branch of wormlike micelles.29-31 In the dilute region, the electrostatic repulsion which exists between the surfactant head groups slows down the micelles growth considerably. As the surfactant concentration reaches the semidilute regime, the intermicellar screening of the electrostatic interactions occur. The electrostatic interaction goes through a maximum with the increasing concentration. So the electrostatic interactions between the headgroups decrease due to the counterions above the peak concentration. For the surfactant solution with only one component, theoretical calculations suggest that the formation of a junction costs less energy with the decrease of headgroup repulsion.32 However, the endcap energy increases with the decreases of headgroup repulsion. So micelles begin to form branches above the peak composition rather than grow linearly.4 This reflects the structural changes of the average contour micelle length which increases first and rapid decreases of with the increase of C16-C8 concentration. G0 increase monotonically with the C16-C8 concentration which reflects the monotonical increase of the network density.


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6wt.% 1

10

8wt.%

7wt.%

5wt.% 4wt.%

3.5wt.%

0

10

3.5 wt.% 4 wt.% 5 wt.% 6 wt.% 7 wt.% 8 wt.%

2

b

10

η (Pa.s)

G' ,G''/ Pa

a

10

1

0

10

-1

-1

10

-2

-1

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-1

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c

2

d

25 °C 35 °C 45 °C 55 °C 65 °C

10

1

10

10

D (s-1)

ω (rad⋅s ) 2

10

25°C 35°C 45°C 55°C 65°C

1

1

10

10

η (Pa.s)

G',G''(Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

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ω (rad⋅s )

0

10

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10

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10

-1

ω (rad⋅s )

Figure 3 (a) The elastic modulus G’ (filled symbols) and the viscous modulus G” (unfilled symbols) versus frequency for C16-C8 aqueous solution at different weight concentration at 25 o

C. (b) Steady shear-rate-viscosity curves as a function of the weight fraction for C16-C8

wormlike micelle solution. (c) The elastic modulus G’ (filled symbols) and the viscous modulus G” (unfilled symbols) versus frequency for 5.00 wt.% C16-C8 aqueous solution at different temperature. (d) Steady shear-rate-viscosity curves as a function of temperature for 5.00 wt.% C16-C8 systems at 25 oC. Steady rheological measurements were also performed for C16-C8 aqueous solutions. As shown in Figure 3b, the C16-C8 aqueous solution (7.00 wt.%-9.00 wt.%) exhibits a zero-shear viscosity. The solution experiences a typical shear thinning with the increasing frequency because of the micelles alignment induced by shear. The viscosity plateau at low frequency disappears for the 3.50 wt.% -6.00 wt.% C16-C8 aqueous solution. The reason is the more structured wormlike micelles entanglement. With the increasing C16-C8


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concentration, the η0 increase first to the highest value and finally shifts to the lower value again, which indicates the wormlike micelles consisting of network. This behavior can be explained that the branch of wormlike micelles.33 The impact of temperature on the rheological properties is investigated by the dynamic frequency sweep at various temperature. As shown in Figure 3c, the frequency sweep curves show G’-G” crossover below a critical frequency (ωc) for 5.00 wt.% C16-C8 wormlike micelles. The G0 remains about the same at different temperature. So the mesh size (ξ) will increase based on G0 = kBT/ξ3.34 However, the τR decreases exponentially with increasing temperature because ωc increases gradually.35 The exponential decrease of τR is affected by the flow activation energy Ea. The Ea can be obtained by the below equation:7, 36, 37

τ R = A exp (

Ea ) RT

(3)

here A is a pre-exponential factor, R is the gas constant. Semilogarithmic plots of τR vs 1/T (Figure S1) fall on straight lines with the same slopes at different concentration. The Ea calculated by Eq. 3 is listed in Table 1. These values are close to those of the branched wormlike micelles, which further indicates the branched wormlike micelles also exist in the C16-C8 aqueous solution.38 Table 1 The Ea of different C16-C8 aqueous solutions c（wt.%） Ea (kJ•mol-1 )

4.00 183.95

5.00 149.09

6.00 147.29

7.00 144.81

8.00 93.32

The viscous properties of 5.00 wt.% C16-C8 aqueous solutions were also investigated by steady rheological measurements at different temperatures. The η0 decreases


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monotonically with the increase of temperature (Figure 3d). With increasing temperature the average micelle contour length,L, decreases according to the following equation29

L ∝ϕ

1/2

exp [

EC ] 2k BT

(4)

where Ec represents the end-cap activation energy, ϕ represents the worms’ volume fraction and kB represents Boltzmann’s constant. At higher temperature, the transfer of surfactant molecules from the cylindrical body to the hemispherical end-cap of the worm becomes more quickly. Thus, with increasing temperature the average contour micellar length decreases exponentially. For example, L of 5.00 wt.% C16-C8 wormlike micelles is 1.53 µm, 1.21 µm, 0.731 µm and 0.554 µm at 25 oC, 35 oC, 45 oC and 55 oC, ”

respectively. Ec can be calculated from the slope of semilogarithmic plot of G0/G min =L/Lc vs 1/T. The calculated Ec is 88.07 kJ·mol-1 (Figure S2), which is higher than that of other micelles composed of C16 surfactant (ca. 49 kJ·mol-1).8,39 The high Ec is in favor of the growth of micelle in the thermodynamic balance, which can result in extremely long cylindrical micelles. The Eq. 4 is strictly valid only for electric neutral micelles.28 When the micelle surface charges, the net scission energy of a micelle is less than Ec. 3.3.2 Impact the Length of Alkyl Chain on the Wormlike Micelle Properties. With increasing length of alkyl chain, the viscoelastic properties of the three wormlike micelles increases gradually. The maximum viscosity of the three wormlike micelles is 1.1 Pa·s of 17.00 wt.% C16-C4 (Figure S3a), 3.1 Pa·s of 14.00 wt.% C16-C6 (Figure S4a) and 77.9 Pa·s of 5.00 wt.% C16-C8, respectively. The entanglement length Lc can be calculated from G0 by G0 = kBT/L

9/5 c

(5)


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37, 40, 41 From the relationship of G ” min /G0 and Lc of Eq.6, one can obtain theL .

" Gmin L = c G0 L

(6)

So we can estimate the maximumL for the three wormlike micelles, which are 0.11 µm, 0.14 µm, 1.72 µm for C16-C4, C16-C6 and C16-C8, respectively. According to the equation τR = 1/ωc, we can estimate the maximum relaxation time, that is 4.1 × 10-2 s (Figure S3b), 2.0 × 10-1 s (Figure S4b) and 1.8 × 102 s for C16-C4, C16-C6 and C16-C8 wormlike micelles, respectively. From the results of τR as a function of alkyl chain length we can find that micelle formed by surfactant with longer alkyl tails has longer relaxation time which facilitates the formation of transient network. 3.3.3 Cryo-etch-SEM Measurements.

Figure 4 Cryo-etch SEM images of 7.00 wt.% C16-C8 wormlike micelles. Cryo-etch-SEM measurements were employed to gain an insight into the aggregation morphology. Figure 4 shows the images of 7.00 wt.% C16-C8 aqueous solution, which reflect an interconnected structure with long wormlike colloid segments. The branched wormlike micelles can be found in the enlarged picture (Figure 4b). Water viscosity is elevated by the transient networks.


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3.4 C16-C10, C16-C12, C16-C14 and C16-C16 The transition between the wormlike micelle and hydrogel exists in the C16-C9 aqueous solution experiences. With the increasing concentration of C16-C9, the solution forms viscoelastic wormlike micelle, hydrogel and wormlike micelle in sequence.21 However, surfactants with longer alkyl chain length than that of C16-C9 only form hydrogels in aqueous solution. Supramolecular hydrogel is viscoelastic colloidal system which contains abundant water. Hydrogels often consist of long self-assemble micelles which form a 3D network. The interaction among the micelles is supramolecular forces which include hydrophobic-hydrophilic effects, coulombic interactions, van der Waals forces, or hydrogen bonding, et al. The interstices of the 3D network can entrap water which imparts rigidity to the hydrogel. Other hydrogel morphologies have also been identified, such as densely packed vesicles42 and strings of vesicles43. The hydrogel is macroscopically manifestated by inverting the test tube without the observable gravitational flow. The four ionic liquid-type surfactants, C16-C10, C16-C12, C16-C14 and C16-C16, can gel water. The minimum gelator concentration of C16-C10, C16-C12, C16-C14 and C16-C16 is 9.10 wt.%, 7.00 wt.%, 6.30 wt.% and 4.20 wt.%, respectively. 3.4.1 Rheology Measurements. Rheology measurements were used to characterize the macro properties of hydrogels at different concentrations. Because all hydrogels show the similar rheological behaviors, the C16-C12 aqueous system is chosen here as an example for expatiation. At last we will give a property comparison between the three ionic liquid (C16-C10, C16-C12 and C16-C14) at the same molar concentration.


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a

b

3

10

2

G' ( Pa)

G',G''(Pa)

10

1

10

0

10

G' G''

-1

10

2

10

1

10

-2

10

c

-1

10

0

10

-1

1

-1

ω (rad⋅s )

10

10

1

-1

ω (rad⋅s )

10

d

0.05

G',G''(Pa)

-1

0

10

0.04

J (Pa )

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0.03 0.02

10

2

10

1

10

0

G' G''

0.01 0.00 300 400 500 600 700 800 900 1000

80

85

90

t (s)

95 100 105 110 115

T(°C)

Figure 5 (a) Frequency sweep using a constant target strain of 1 Pa for 14.00 wt.% C16-C12 hydrogel at 25.0 oC. (b) Comparison of the G’ values versus frequency at different C16-C12 concentration. T=25.0 oC. (c) Creep and recovery behaviors for C16-C12 hydrogel at T= 25.0 o

C.■8.00 wt.%,●9.00 wt.%,▲10.00 wt.%, ★ 11.00 wt.%, ◆ 12.00 wt.%. (d) Transition

temperature as determined by temperature-sweep oscillation rheology (15.00 wt.% hydrogel of C16-C12) under the applied stress of 5 Pa. The linear viscoelastic region was first determined by the strain sweep measurements. There is a wide plateau for both G’ and G” in the strain sweep line at different concentration (Figure S5a). Both G’ and G” are almost unchanged with gradual increasing of applied stress. They cross over each other at a critical yield stress which indicates the hydrogels breaks. They deviate from linearity beyond the critical yield stress. The yield stress increases gradually with the increasing surfactant concentration. The reason is the 3D network of the gel improves with increasing concentration.44 τ = 1 pa is chosen for the C16-C12 aqueous system when the measurement is carried out. The frequency sweep result of 14.00 wt.% C16-C12 hydrogel is


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shown in Figure 5a. The G’ and G” are virtually frequency-independent, which indicates a viscoelastic properties for the C16-C12 hydrogels. The G’ is greater than G” about an order of magnitude which demonstrates the dominate elastic property of the gel.45 With the increasing C16-C12 concentration the G’ increases monotonically. But the values of G’ and the C16-C12 concentration have no quantitative relation which is due to the not integrated structures of cationic aggregates in the hydrogels (Figure 5b). Figure 5c shows the time dependence of the creep compliance of C16-C12 aqueous solutions at 25.0 oC, which demonstrates the C16-C12 aqueous solutions exhibit the gel behavior. With the increase of C16-C12 concentration the hydrogel exhibits a higher elastic behavior because the J (shear compliance) in the recovery section is lower and decreases nearly to zero.46 All rheological results of the C16-C12 hydrogel indicate the viscoelastic nature for the hydrogel is consistent with that of other gels.47,48 3.4.2 Impact of Alkyl Chain Length on the Property of Hydrogel. Although the four kinds of hydrogels made of different surfactants have similar properties, they have different magnitude modulus at the same molar concentration. Because the gelator concentration of C16-C16 is beyond the gelator concentration of the other three surfactants, we chosen C16-C10, C16-C12, C16-C14 at 250 mM as a comparison. The G’ of hydrogel composed of C16-C10, C16-C12, C16-C14 is 134.3 Pa, 296.1 Pa and 412.4 Pa, respectively (Figure S6a). The elastic property of the three hydrogels increases gradually with increasing alkyl chain length which indicates the highest elasticity of C16-C14 hydrogel. This conclusion can also be confirmed by creep-recovery experiment. We can find that the J of C16-C14 hydrogel in the recovery section is lowest and decrease nearly to zero which is corresponding to the highest elasticity (Figure S6b). G’ of the 5.00 wt.% C16-C16 hydrogel is 9268 pa. So the elasticity of C16-C16 hydrogel is the highest among the four hydrogels.


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3.4.3 Sol-Gel Transition. Thermal stability is important for hydrogel. So we completed our survey by studying 14.00 wt.% C16-C12 hydrogel as example. We chose 5 Pa for the frequency sweep measurement in the linear section. The G’ decreases gradually with the increasing temperature (Figure S5b). This phenomenon may be caused by the decrease of intermolecular hydrogen banding, weakening interactions between the tail groups in the formation of junction zones and the deformation of fiber with the increase of temperature, which changes the complete structure of the hydrogel network. Rheological measurement was used to determine the gel-to-sol transition temperatures (TGS). The equilibrated 15.00 wt.% C16-C12 hydrogel was subject to a oscillating stress (5Pa). At 110.9 o

C, the G” exceeds the G’, that is TGS = 110.9 oC (Figure 5d). When the temperature is higher

than TGS, the hydrogel transits to the sol. Since TGS depends on the stress imposed on the gel (the greater the stress, the lower the TGS) and the gelator concentration, TGS value reported here is useful for showing high thermal stability purpose. Under the same oscillating stress, the TGS of 19.00 wt.% C16-C10, 14.00 wt.% C16-C14, and 7.50 wt.% C16-C16 hydrogel is 106.6 oC, 113.1 oC and 110.6 oC, respectively (Figure S7). 3.4.4 Cryo-etch SEM. In order to gain an insight into the microscopic aggregation morphology, Cryo-etch-SEM was employed. Figure 6 shows the typical images of C16-C12 hydrogel (10.00 wt.%), which reflect an continuous hierarchical network structure with interconnected and branched segments. The morphology is consistent with the hydrogel composed of gemini surfactant. 49 Water viscosity and elasticity are elevated by the delicate molecular network. Water is entrapped in the interspaces of the three-dimensional network by surface tension and capillary forces.


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Figure 6 Cryo-etch-SEM images of 10.00 wt.% C16-C12 hydrogel. 3.4.5 SA-XRD. Recently, several papers tried to ascertain the pack of molecules and clarify the mechanism of gelation in hydrogel by using X-ray crystallographic methodology.50, 51 The X-ray diffraction patterns of 13.00 wt.% C16-C12 xerogels exhibit three diffraction peaks (Figure S8). The spacings (d) are 2.96, 1.46 and 0.98 nm, which indicates C16-C12 molecules indeed assemble into a well-ordered layered structure because the ratio of the spacings is 1:1/2:1/3. The long spacing of gel obtained by X-ray diffraction method is 2.96 nm, which equals nearly to the C16-C12 molecule length (3.14 nm) modeled by MM2 force field simulation.

4. DISCUSSION Mechanism of the aggregation transition with alkyl chain length of surfactant will be discussed in this section. There exist abundant aggregation behaviors in the investigated system as the traditional cationic systems. The hexagonal liquid crystal, wormlike micelle and hydrogel are specially focused here. The visual photographs of surfactants with different alkyl chain length at different concentration at 25 oC are presented in Figure 7. The C16-C2 and C16-C3 samples show transparent solid-like appearance. The mobility of the C16-C4, C16-C6 and C16-C8 transparent solution decreases gradually with increasing alkyl chain length. The C16-C9 sample can sustain its gravitation when the test tube is inversed. When the alkyl chain length increases


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above 9 carbon atoms the solutions appear a little sky-blue. The samples are not transparent which is due to the decrease of solubility.

Figure 7 The photographs of surfactants with different alkyl chain length at different concentration at 25 oC. Their distribution region is shown in Figure 8 for the C16-Cn and water system as a function of concentration. Hexagonal liquid crystal is formed at high surfactant concentration (28.00-57.00 wt.%) in C16-C2 aqueous solution.

Wormlike micelles (30.00-36.00 wt.%) and hexagonal liquid

crystal (40.00-56.00 wt.%) can be formed in C16-C3 aqueous solution. Wormlike micelles can be formed in C16-C4, C16-C6 and C16-C8 aqueous solution without additives. The C16-C10, C16-C12, C16-C14 and C16-C16 aqueous solutions can form the hydrogels. This indicates the alkyl chain length has the important influence on the aggregates. We try to find the mechanism of the transition of the different aggregations. The crucial factor to be the alkyl chain length of surfactant. In surfactant solution there are the hydrophobic effect and electrostatic effect which competes each other to influence the aggregation formation. When surfactant has short chain length, electrostatic effects win and the C16-C2 tends to pack tensely. So the micelles formed by C16-C2 with short alkyl chain manage to aggregate parallelly to form hexagonal liquid crystal accompanied by huge electriferous ion to decrease the energy of system. The hydrophobic free energy changes with increasing alkyl chain length which makes surfactants pack loosely. The micelles can increase which facilitates the formation of wormlike micelles.52 The phase transition of the C16-C3 aqueous solution from


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isotropic wormlike micelle to liquid crystal is induced by the strong interaction and orientational stack among the wormlike micelles.53 So with only one carbon increase the C16-C2 and C16-C3 have different aggregation behavior.

C16-C16

S G P

C16-C14

S

C16-C12 C16-C10 C16-C 9 C16-C 8

G

S

P G

P

S G SWMGWM P S WM P

C16-C 6

S

C16-C 4 C16-C 3 C16-C 2

P

P

WM S

WM

P

S S +H

S

0

10

20

WM WM+H H

30

40

H

P P

50

60

weight concentration (wt.%)

Figure 8 The hexagonal liquid crystal, wormlike micelles and hydrogels regions formed in the binary system composed of C16-Cn and water at different concentration. S = solution; H = hexagonal liquid crystal; WM = wormlike micelles; G = hydrogels; P = precipitation. When surfactant has long chain length, the hydrophobic effects win the electrostatic effects and the micelles increase with increasing alkyl chain length of the surfactant. The elongated micelles are hard to parallelly pack and entangle each other. At this time, the viscoelastic wormlike micelles can be formed. The average contour micelle length increases with the alkyl chain length, which leads to longer breaking times. The maximumL is 0.11 µm, 0.14 µm, 1.72 µm and the breaking time is 4.1 × 10-2 s, 2.0 × 10-1 s and 1.8 × 102 s for C16-C4, C16-C6 and C16-C8, respectively. TheL for 4.00 wt.% C16-C9 wormlike micelles is 4.95 µm, which is much larger than that of C16-C8 aqueous solution. Above the critical concentration of phase transition, the branch of forming networks can be saturated which leads to eventual phase separation instead of forming hexagonal liquid crystal. The breakup of micelles will be hard with the larger alkyl chain length.23 At this time, the solubility of surfactant in water is lower and the mobility becomes worse because of increase of


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the intermolecular force and steric effect. On the other hand, it will be not easy for the exchange of unimer within a single micelle and between different micelles. Thus, the wormlike micelles can grow with the increasing alkyl chain length. It is known from polymer theory that τrep～ L-3.54 The micelles with long average contour length will be very stable. So the wormlike micelles can transit to hydrogel in the end with increasing micellar length. Figure 9 depicts with increasing alkyl chain length the aggregation transition from liquid crystal, wormlike micelles to hydrogels.

Hex Liquid Crystal

Wormlike Micelles

Gel

Figure 9 Schematic mechanism of aggregation transition from liquid crystal, wormlike micelles to hydrogels with the increase of alkyl chain length of surfactants. The shapes of aggregates are related to the surfactant geometrical structures. The relationship of the phase of the aggregates to the molecular geometrical structure is given as the surfactant packing parameter (Rp), which is defined via55

Rp =

V0 Ah L0

(7)

where Vo is the alkyl chain volume in solution，Ah is the surfactant headgroup area of the micelle, and Lo is the completely extended alkyl chain length. The possible aggregation shapes are determined by the value of Rp. The system with larger value of Rp is easy to form larger aggregates with low curvature. For linear surfactant, Vo and Lo can be calculated by: 56 Vo (Å3) = 27.4+26.9N

(8)

Lo (Å) = 1.5+l.265N

(9)


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N represents the carbon atom number on surfactant alkyl chain which is incorporated in the hydrophobic core. Thus, with increasing alkyl chain length (N), Vo increases faster than Lo which leads to the increase of Rp, because Ah becomes about equal for cylindrical micelles. Consequently, the surfactants with longer alkyl chain tend to form larger aggregates. So the hexagonal liquid crystal is transformed into the wormlike micelle and the wormlike micelle is transformed into the hydrogel.

5. CONCLUSION The aggregation behavior of ionic liquid-type surfactant C16-Cn and water was systemically inspected in this work. The system forms hexagonal liquid crystal, wormlike micelle and hydrogel with increasing alkyl chain length. So the length of surfactant alkyl chain has important effect on the aggregates formation. The C16-C2 aqueous solution can form hexagonal liquid crystal. The C16-C3 aqueous solution can form both wormlike micelle and hexagonal liquid crystal. The C16-C4, C16-C6 and C16-C8 aqueous solutions can only form wormlike micelle. The average contour lengthL, the viscosity and the τR increases rapidly with increasing alkyl chain length. The C16-C9 aqueous solution experiences a transition between wormlike micelle and hydrogel. The C16-C10, C16-C12, C16-C14 and C16-C16 only form hydrogel. With increasing alkyl chain length the elasticity of the gel increases gradually. The TGS of the four hydrogels is above 100 oC, which endow them with the potential use in some fields with special requirement of high temperature. The mechanism of the aggregation transition from hexagonal liquid crystal, wormlike micelle to hydrogel is the increase of average contour micellar length with the increasing alkyl chain length. The longer micelle has longer breaking time which facilitates the stability of the network. When the surfactant alkyl chain increases to some extent, the aggregation transition happens. This work may shed a light on the study of the relationship of


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different aggregation and the adjustment of the transition among these different aggregation by other means. ASSOCIATED CONTENT Supporting Information. The characterizations of the different aggregates is given in the additional figures. This information can be found via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Han); [email protected] (R. Guo) ACKNOWLEDGMENTS This work was ﬁnancially supported by the National Nature Science Foundation of China (Nos. 21203162 and 21073156), Research Fund for the Doctoral Program of Higher Education of China (20113250110007) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The characterizations of the samples were supported by the Testing Center of Yangzhou University. REFERENCES (1) Hao, J. C.; Hoffmann, H. Self-assembled Structures in Excess and Salt-free Catanionic Surfactant Solutions. Curr. Opin. Colloid Interface Sci. 2004, 9, 279-293. (2) Qiao, Y.; Lin, Y. Y.; Yang, Z. Y.; Chen, H. F.; Zhang, S. F.; Yan, Y.; Huang, J. B. Unique Temperature-Dependent Supramolecular Self-Assembly: From Hierarchical 1D Nanostructures to Super Hydrogel. J. Phys. Chem. B 2010, 114, 11725-11730. (3) Muthukumar, M.; Ober, C. K.; Thomas, E. L. Competing Interactions and Levels of Ordering in Self-Organizing Polymeric Materials. Science 1997, 277,1225-1232. (4) Raul Zana; Eric Kaler. Giant Micelles-Properties and application; CRC Press: New York, 2007.


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TOC

C16-C16

S G

C16-C14

S

C16-C12

S

C16-C10 C16-C 9 C16-C 8

P P

G

G

P

S G SWM GWM P S

C16-C 6 C16-C 4 C16-C 3 C16-C 2

P

WM S

P

P

WM S

P

WM S

WM WM+H

S

0

10

S+H

20

H

P P

H

30

40

50

weight concentration (wt.%)

60


30

Impact of Alkyl Chain Length on the Transition of Hexagonal Liquid

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