Effects of Chirality on the Aggregation Properties of Amide-Bonded

Article Cite This: Langmuir 2019, 35, 8968−8976

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Effects of Chirality on the Aggregation Properties of Amide-Bonded Pyridinium Gemini Surfactants Maximilian E. Franke* and Heinz Rehage Lehrstuhl für Physikalische Chemie II, Technische Universität Dortmund, D-44227 Dortmund, Germany

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S Supporting Information *

ABSTRACT: In a series of experiments, we synthesized and characterized a new type of cationic gemini surfactant, the chiral and the achiral forms, which were compared regarding their surface-active properties. These surfactants show interesting aggregation processes, which are affected by the interplay of different structural characteristics. By substituting the counterions, we found a way to control the solubility of these compounds in order to investigate the behavior of solutions as well as insoluble monolayers. The comparison of the chiral with the meso compound instead of the racemic mixture is a major advantage for the analysis of the effects of chirality because racemates can be understood as a mixed surfactant system with one additional degree of Gibbs freedom. Furthermore, we investigated the viscoelastic behavior of concentrated aqueous surfactant solutions which formed elongated wormlike micelles. A strong effect of the stereochemistry on the aggregation properties of Langmuir layers as well as elongated micelles was found.

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fibers.9,19 Furthermore, self-assembled aggregates are of interest in nanotechnology,20,21 drug delivery,22 fracture fluids, or dragreducing agents.23 Chiral surfactants are also of great importance in enantiomeric separation24 and chiral synthesis25 and as templates for chiral nanostructures.26 Covalent dimerization of this kind of divalent surfactant ion, with short spacers and the amide bonds next to the cationic pyridinium ring systems, often leads to poorly soluble compounds, as we often found in our studies. The solubility of surfactant compounds is a crucial factor for all applications27 and is the result of a complex interplay of various factors.28 By exchange of the halide counterion into the formate, we achieved good solubility of the compounds (3a and 3b). Although the bromides 2a and 2b form stable Langmuir layers, which were investigated with Brewster angle microscopy, formates 3a and 3b are soluble even at high concentrations. The critical micelle concentrations (cmc’s) were determined by different methods. Thermodynamic parameters of micellization were determined, and the rheological response of the semidilute aqueous solutions was characterized by oscillation experiments. These measurements pointed to the formation of elongated, threadlike micelles for both isomers.

INTRODUCTION Amide-bonded pyridinium surfactants represent a barely researched class of cationic surfactants, which often exhibit interesting aggregation behavior that is affected by the ability to form hydrogen-bonding networks.1 Dimerization by covalent bonds near the polar headgroups leads to gemini surfactants2−4 which are known for their remarkable surface-active properties5 because they show a far stronger aggregation tendency6 resulting from enhanced hydrophobic interaction7 of the two chains within the molecule.8 The orientation of the monomers in aggregates can be influenced by functional groups, which can enhance interactions and therefore affect the stability of the aggregates.9 Structural features, such as conformational restrictions caused by rigid moieties, for example, by amide groups bound to aromatic rings10 and ionic groups11 in combination with the ability to form intermolecular forces12 can lead to a more complex assembly of the monomers and determine the shape of the aggregates.13,14 In this context, the presence of asymmetric carbon centers in the vicinity decisively contributes to the assembly and is known to promote the formation of larger aggregates.15 In biological systems, chirality and the ability to form hydrogen bonds is the essential prerequisite of natural building blocks16 and determines the assembly of higher aggregates, which is also true for synthetic assemblies.17 By analogy to phospholipids, which consist of two hydrophobic chains that are linked through a chiral center and two charged headgroups, divalent ionic gemini surfactants can be synthesized, being linked by a chiral spacer. Such systems have been proven to be suitable for mimicking biological systems such as membranes,18 vesicles, helical structures, and micellar © 2019 American Chemical Society

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EXPERIMENTAL SECTION

Synthesis. Compounds 1a, 1b−3a, and 3b were synthesized as shown in Scheme 1. The chiral form was based on L-tartaric acid as a

Received: February 28, 2019 Revised: May 25, 2019 Published: June 12, 2019 8968

DOI: 10.1021/acs.langmuir.9b00592 Langmuir 2019, 35, 8968−8976

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Langmuir Scheme 1. Synthesis and Designation of the Compoundsa

a

(a) Chiral form and (b) meso form.

Figure 1. Surface pressure−area (δ−A) isotherms and Brewster angle micrographs of the surfactant ion bromides (chiral form (a, b), meso form (c, d)). Compression rate: 10 cm2/min.

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Langmuir precursor. The syntheses of compounds 1a and 1b were conducted according to a modified procedure by Chen et al.29 The quaternization reaction with 1-hexadecyl bromide was performed in dimethylformamide at 110 °C to yield 2a and 2b. Ion exchange was performed by the utilization of strongly basic ion-exchange resin to yield 3a and 3b. The specific rotation of 3a was determined. A detailed synthesis approach and all analytical data are presented in the Supporting Information. Methods. Brewster angle microscopy (wavelength of the incident LASER beam, 690 nm) and interfacial characterizations were performed on an Accurion Nanofilm Ultra BAM equipped with a Langmuir−Blodgett trough, including a Wilhelmy plate and a Kelvin probe (Kelvin probe KP1). Stock solutions of the samples were prepared by solvating the amides in a 1/9 methanol/chloroform (v/v) mixture and spread via gastight glass syringe. Before starting the measurements, 20 min passed to let the solvent evaporate. All measurements were performed at 22 °C. Tensiometric measurements were performed on a DCAT 11 tensiometer (Dataphysics) equipped with a platinum−iridium Wilhelmy plate at 22 °C. The Wilhelmy plate was cleaned before each measurement by annealing on a Bunsen burner. Dilute solutions were stored over 2 days in the covered sample vessel and carefully placed in the tensiometer without concussion. The measurements were performed until no significant change in the surface tension with time was observed, and the average plateau value was used. All measurements were performed twice to ensure reproducibility. Conductivity measurements were performed in a sealed vessel with a WTW Cond 7310 probe. High-purity water with a conductance of 18.2 MΩ cm was always used. Measurements on solutions of less than 1 mM surfactant were accompanied by flushing with argon before sealing. The probe was cleaned in hot water by ultrasonic treatment. All samples were stirred during the measurement. Reproducibility was verified on a random basis. Rheology measurements were performed in a TA Instruments HR-3 Discovery Hybrid rheometer, equipped with doublewall concentric steel Couette geometry. Dynamic light scattering experiments were performed on a Malvern Zetasizer ZS nano instrument. The supplementary measurements were performed to confirm the first micellization data. The results indicate the formation of micelles with hydrodynamic radii typical of that of spherical micelles. Further details are presented in the Supporting Information. All organic solvents were distilled before use, and high-purity water (Elga PURELAB flex) was always used. Glassware was pyrolized at 450 °C and rinsed with high-purity water.

the surfactants change substantially. The surfactants are no longer water-insoluble, and strong thickening effects can be found at higher concentrations. The verification of the formation

Figure 2. Measurement of the surface tension of dilute aqueous surfactant solutions.

of micelles was obtained by tensiometry, conductivity, dynamic light scattering, and small-angle X-ray scattering. Tensiometry. The results of measurements of the surface tension with concentration are presented in Figure 2. The surface tension at concentrations higher than the cmc is lowered only to about 41 mN/m, which is explicable by the strong electrostatic repulsion of the dimeric headgroups.30 Dilute aqueous solutions of the surfactants indicate a strong lowering of the surface tension. The lowering is pronounced even at very low concentrations. The cmc data and the adsorption behavior of the chiral form and the meso form do not show a significant difference. Conductometry. By using conductivity data, we identified transition ranges that are large compared to the cmc value. Two breaks in the specific conductivity with the concentration were found and described cmc1 and cmc2. Because of the extension of the transition range at the first cmc and the second break, we used Carpena’s method,31 which was modified for two cmc’s. The basic assumption of this method is that the first derivative of the conductivity versus concentration plots can be described by a Boltzmann-type sigmoid, which takes the transition range of the micellization process32 into account. The experimental data were evaluated by using the expression

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RESULTS AND DISCUSSION Monolayers of the Chiral and Achiral Bromides. The bromides (2a and 2b) form Langmuir layers with surface pressure−area (δ−A) isotherms typical3 for insoluble gemini surfactants and without visible phase transitions (Figure 1a,c). The isotherms of the chiral and the meso forms are similar, although the film pressure of the chiral form increases at lower areas per molecule. At film pressures above 5 mN/m, the formation of different structures can be observed. The textures that are formed by the chiral form are especially remarkable (Figure 1b). The reflectivity of the Brewster angle micrographs shows a dependency of the orientation of the structures which could be explained by two-dimensional crystalline aggregates. In contrast, the meso form indicates the formation of noncrystalline, fractal-like aggregates (Figure 1d), which were found at higher film pressures. The comparison of the isotherms (Figure 1a,c) indicates that the chiral form tends to form more compressed films. An explanation might be that the packing of two-dimensional crystalline aggregates is favored only in the case of the chiral compound, which is affected by the conformation of the surfactant headgroups, including the arrangement of the bromide counterions of the two isomers. Characterization of Aqueous Solutions of Formats. After ion exchange from bromides to formates, the properties of

( ) yzzzzzzz z ( ) zzzz{

ij jj 1 + exp j F(x) = σ0 + (σ 1́ x) + Δx1(σ 2́ − σ 1́ ) lnjjjj jj jj 1 + exp k

ij jj 1 + exp j + (σ 2́ x) + Δx 2(σ 3́ − σ 2́ ) lnjjjj jj jj 1 + exp k

( (

) zyzzzzzz z ) zzzz{

x − cmc1 Δx1 −cmc1 Δx1

x − cmc2 Δx 2 −cmc2 Δx 2

(1)

where x designates the surfactant concentration; σ0 is the specific conductivity of the solvent; and σ′1, σ′2, and σ′3 are the limiting slopes of the conductance versus concentration plot at 8970


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Figure 3. Specific conductivity at different temperatures of (a) formates of the chiral compound and (b) formates of the meso compound with the modified Carpena fit functions. Coefficient sof determination: 0.999874 for 3a and 0.999765 for 3b.

Figure 4. Phase diagrams of (a) formates of the chiral compound and (b) formates of the meso compound in dilute solutions. The coloring corresponds to deviations from linear behavior in conductivity measurements. Red: strong deviation from linear behavior. Blue: no deviation from linear behavior.

very low concentrations, at concentrations between the two breaks in the diagram, and at concentrations beyond the second break. The widths of the first and second transitions are designated by Δx1 and Δx1, and the first and second cmc’s are cmc1 and cmc2, respectively. The fit function displays very good correlation with the measured values (Figure 3) and was therefore used to identify the transition ranges at different temperatures. Through the application of the second derivate, it is possible to illustrate the process of micelle formation more precisely. The meaning of the extremum of the second derivate of the specific conductivity can be described as the concentration at which the maximal deviation from linear behavior of specific conductivity with the concentration is present, which is understood to be the cmc.32 The second derivative of eq 1 yields

f (́ x) =

(σ 3́ − σ 2́ )b (σ 2́ − σ 1́ )a + Δx1(1 + a)2 Δx 2(1 + b)2

ij x − cmc 2 zy ji x − cmc1 zyz zz zz, b = expjjj a = expjjjj zz j Δx z Δ x 1 2 { k { k

(2)

To visualize the transition width in a phase diagram, we used the data deduced from 4 |f (́ x)| , which is very sensible even to small deviations from the linear range. The deduced phase diagram is shown in Figure 4. The temperature-dependent data was used to estimate thermodynamic parameters for the process of micellization. The counterion dissociation of micelles can be estimated by33

α=

σ 2́ σ 1́

(3)

According to the pseudophase model of micelle formation, the Gibbs energy of micellization ΔGm can be determined by8 8971


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Langmuir Table 1. Data of Micellization Obtained by Conductometry cmc1 chiral isomer [mmol/L]

T [°C] 15 20 25 30 35 40 45 50 55 60

cmc1 meso isomer [mmol/L]

cmc2 chiral isomer [mmol/L]

cmc2 meso isomer [mmol/L]

T [°C]

0.189 0.188 0.185 0.191 0.196 0.202 0.226 0.256 0.286 0.334 ΔGm chiral isomer [kJ/mol]

0.182 0.182 0.177 0.182 0.189 0.197 0.211 0.223 0.246 0.268 ΔGm meso isomer [kJ/mol]

ΔHm chiral isomer [kJ/mol]

ΔHm meso isomer [kJ/mol]

15 20 25 30 35 40 45 50 55 60

−74.1 −75.4 −76.8 −78.1 −79.2 −80.4 −80.9 −81.3 −82 −82.1

−77.3 −79.2 −80.5 −81.7 −82.8 −83.7 −84.8 −85.3 −86 −86.4

−2.5 −4.7 −7.9 −12.2 −17.6 −24.3 −32.3 −41.9 −53.1 −66.1

−2.6 −4.9 −8.3 −12.8 −18.4 −25.4 −33.8 −43.8 −55.4 −68.6

ΔGm = 2RT (1.5 − α) ln(Xcmc)

7.5 7.7 8.1 8.4 8.8 9 9.5 10.1 10.4 10.4

δ ln(X

(5)

)

cmc The term can be determined more accurately by using a δT fit function to describe the temperature dependence of the cmc. We used a fit function of the type y = a + bxc. The entropy of micellization ΔSm was calculated by

ΔSm =

ΔHm − ΔGm T

250 240 230 220 200 180 150 120 90 50

0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27

α meso isomer

0.22 0.22 0.21 0.21 0.21 0.21 0.21 0.21 0.21 0.22 ΔSm meso isomer [J/mol K] 260 250 240 230 210 186 160 130 90 50

aggregates. Temperature-dependent oscillation experiments were performed, showing that the thickening of the chiral form (Figure 6a) is much stronger, even at much lower concentrations, than the thickening of the meso form (Figure 6b). The temperature dependence of the sol−gel transition in amplitude sweeps is significantly affected by stereochemistry, which can be recognized by a comparison of plots a and b of Figure 6. The transition temperature of the chiral form (Figure 6a, black curve), which increases with the concentration, is higher than the transition temperature of the meso form (Figure 6b, black curve), although the concentration is considerably lower. Viscoelastic surfactant solutions have often been treated as “living” polymers because the micellar systems can break and recombine.34,35 The processes of stress relaxation of a viscoelastic surfactant solution occur on characteristic time scales. These processes are mainly considered to be the breaking and recombination of the micelles and reptation, which is regarded as curvelinear diffusion along a tube that is formed by the entanglement of other chains and was included in the model by Cates.36 The rheological properties are dependent on the relative magnitude of the relaxation times for each process, λBREAK and λREP·. If λBREAK ≪ λREP, then the breaking kinetics are predominant and the solution behavior follows the Maxwell model. The relaxation time λ is then the geometrical average, hence λ = λBREAK λREP . In oscillation experiments, the storage modulus G′ and the loss modulus G″ are described by the equations

(4)

were Xcmc denotes the mole fraction of gemini surfactant, R is the gas constant, and T is the temperature in Kelvin. The enthalpy of micelle formation ΔHm can be derived by the van’t Hoff equation: i δ ln(Xcmc) yz zz ΔHm = −2RT 2(1.5 − α)jjjj z δT k {p

10.9 11 11.1 10.5 10.9 11 11.1 11.1 11.1 10.5 ΔSm chiral isomer [J/mol K]

α chiral isomer

(6)

The cmc1 values are very low, typical of gemini surfactants. As shown in Figure 4, both compounds have a wide transition range at cmc1, which covers the submicellar regime. By a comparison of the chiral form and the meso form, it can be seen that the parameters for the formation of the first species of micelles, including thermodynamic data, do not differ significantly (Figure 5). It is noticeable that the counterion dissociation of micelles of the meso form is about 5% lower than for the chiral compound. A significant difference can be found for the formation of the second species of aggregates: the cmc2 values of the chiral compound are substantially lower at lower temperatures and approximate the values of the meso compound with increasing temperature. Rheology. Both the chiral and achiral forms show a significant thickening effect in higher-concentration solutions. In general, the thickening of surfactant solutions correlates with the formation of large intermolecular assemblies. The solutions show a strong viscoelastic response within a broad temperature range, which provides information about the stability of the

G′(ω) =

G0(ωλ)2 1 + ω 2λ 2

(7)

and G″(ω) = 8972

G0ωλ 1 + ω 2λ 2

(8) DOI: 10.1021/acs.langmuir.9b00592 Langmuir 2019, 35, 8968−8976

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Figure 5. Micellization data at different temperatures deduced from conductometric measurements.

where G0 is the plateau modulus and ω is the angular frequency. The Cole−Cole plot, in which G″(ω) is plotted against G′(ω), provides a precise investigation of the relaxation behavior. If monoexponential stress relaxation is present, then a semicircular curve shape will be obtained. If the observed time scales include

polymer motion as breathing or local Rouse-like37 motion, then upturns in G″(ω) and G′(ω) in the Cole−Cole plot for higher frequencies can be found. The resulting minimum G″min can be used to estimate the average contour length L of the micelles38 8973


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Figure 6. Amplitude sweeps of (a) a 12.5 wt % solution of the chiral compound at 1 rad/s and (b) a 16.4 wt % solution of the meso compound at 1 rad/ s.

kBT 9/5 6/5 le lp

(10)

with the persistence length lp. The Cole−Cole plots (Figure 8), which were obtained from frequency sweep experiments (Figure 7), indicate for both isomers monoexponential stress relaxation. If we assume lp to be 20 nm as found for similar surfactant systems,39 then the average contour length L can be roughly estimated. By evaluating the Cole−Cole plots at different temperatures, we estimated the effect on the micellar parameters as shown in Table 2. The values of roughly estimated average contour length L agree with the results of comparable viscoelastic cationic gemini surfactants.40 Evaluating the data at the same temperatures, we find that contour length L of the chiral form is longer, the entanglement density is higher (cf. le), and the plateau moduli G0 are higher than those of the meso form.

Figure 7. Frequency sweeps of aqueous solutions of 4 wt % of chiral form 3a and 16.4 wt % of meso form 3b at 0.4% strain and 22.5 °C.

l G″min ≈ e G0 L

≈ G0

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CONCLUSIONS The effect of chirality has been investigated for both interfacial and bulk properties of a novel type of pyridinium surfactant.

(9)

Figure 8. Normalized Cole−Cole plots of aqueous solutions of (a) 4 wt% of the chiral form 3a and (b) 16.4 wt % of 3b at 0.4% strain. 8974


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aggregates has been investigated by rheological measurements within the framework of this study. Additional experiments and investigations of supramolecular chirality as well as a prospective investigation of surfactant systems utilizing more complex counterions could enable further interesting findings regarding the structural effects on self-assembly. These results also provide a promising approach for different fields of research, for example, as templates for the synthesis of chiral nanostructures.

Table 2. Parameters of Elongated Micelles Obtained from Temperature-Dependent Rheological Measurements chiral form [3a] T [°C]

G″min [Pa]

G0 [Pa]

le [nm]

L [nm]

15 17.5 20 22.5

0.37 0.39 0.4 0.41

1.54 1.99 2.83 3.46

496 432 365 320

2100 2200 2500 2700

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ASSOCIATED CONTENT

S Supporting Information *

meso form [3b] T [°C]

G″min [Pa]

G0 [Pa]

le [nm]

L [nm]

17.5 20 22.5 25 27.5

0.4 0.39 0.41 0.46 0.43

1.17 1.6 2.28 2.29 3.28

580 490 400 405 330

1700 2000 2300 2000 2500

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00592. Experimental details, synthesis and characterization of compounds, and additional results (PDF)

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AUTHOR INFORMATION

Corresponding Author

Counter ion effects have been used to control the solubility of the compounds. Surfactant ion bromides 2a and 2b form Langmuir layers with similar δ−A isotherms. The chiral form tends to form more highly compressed films than the meso form and shows the formation of two-dimensional crystalline aggregates, which have not been obtained for the meso form. Ion exchange to the formate leads to soluble compounds 3a and 3b, which form threadlike aggregates at higher concentrations. Both types of surfactant formates have a low cmc1 and therefore high stability of the micellar form compared to the monomeric form. The transition ranges are wide compared to cmc1.32 This indicates that the aggregation process begins well below cmc1. Chirality has a minor effect on the aggregation properties at the cmc1, which is primarily dominated by hydrophobic interactions. However, the formation of higher aggregates is dependent on the stereochemistry of the compounds: cmc2 of the chiral compound is considerably lower than that of the achiral isomer. A remarkable thickening effect and a Maxwelltype rheological response can be found for semidilute aqueous solutions. The formation of long entangled micelles is assisted by the low degree of ionization of the aggregates, which turned out to be higher in the case of the chiral surfactant. However, the presence of chiral centers leads to a stronger thickening effect at lower concentrations and higher temperatures. The aggregation of surfactants in general is affected by their stereochemistry. The orientation of the hydrophobic groups is given by the hydrophobic effect, and some similarity between the surface of micelles and the water-facing interface of a Langmuir layer can be found. In many cases, chirality leads to twisted twodimensional aggregates for Langmuir layers of insoluble surfactants, which results from the arrangement of the surfactants with a given orientation of the hydrophobic moieties and the resulting arrangement of the hydrophilic groups.41 Because the aggregates of Langmuir layers of the chiral form of bromide do not indicate the formation of two-dimensional chiral aggregates, some consideration of the orientation of surfactants at interfaces within aggregates may be indicated. However, these considerations are not transferable to the arrangement of the surfactant formates within larger micellar aggregates because the natures of formate and bromide as counterions are comparable only to a limited extent. It is therefore difficult to make a reliable statement about the structural differences between larger aggregates of the chiral and meso forms in solution on the basis of these data. The stabilizing effect of chirality9,15 on larger

*E-mail: [email protected]. ORCID

Maximilian E. Franke: 0000-0001-7774-3636 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Samy Al-Ayoubi for performing SAXS measurements, confirming the cmc data. REFERENCES

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