Rod-like micelles of octadecyltrimethylammonium bromide and their freezing upon solubilised styrene polymerisation

Abstract

The association of octadecyltrimethylammonium bromide (C₁₈TABr) in ethanol/water (10/90 wt%) solvent has been studied using light-scattering measurements. With the help of sodium 2-naphthalenesulfonate (SNphs), C₁₈TABr formed rod-like micelles. Styrene was solubilised into the templating C₁₈TABr micelles with a mole ratio of SNphs to surfactant (n_SNphs/n_C18TABr) of 0.5, which was examined by dynamic light-scattering (DLS) and UV-spectral measurements. Then the solubilised styrene was polymerised in situ. Dilution experiments were performed to test the freezing effect. It was found that the amount of solubilising styrene, represented as the mole ratio of styrene to surfactant (n_st/n_surf), should be greater than 2.7 so as to achieve a good freezing effect. In the case of n_st/n_surf = 4.4, well-frozen rod-like micelles were obtained and their morphologies were evinced by TEM images. These frozen micelles were stable even in the 400% dilution test.

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Introduction

Amphiphilic surfactants can self-assemble into a variety of aggregate structures such as spherical micelles, rod-like or worm-like micelles, vesicles, or lamellar phases in polar or non-polar solvents.^1,2 These organised surfactant aggregates are extensively used in a wide range of applications. However, because surfactant self-aggregation is controlled by thermodynamics,^2,3 changes in concentration, temperature, salinity, pH, etc., modify their microstructures. It is therefore of significance to freeze these microstructures for aggregate applications. Two approaches have been tried: one was to polymerise the monomers, such as styrene, that were previously solubilised in the aggregates, so as to fix the micellar microstructures,^4–8 another was to use polymerisable surfactants.^9–11 The former approach was convenient, and more common than the latter.

In the present work, we were interested in the approach of monomer polymerisation. Here a key problem is how to ensure that the aggregate structure is not destroyed during styrene solubilisation and the ensuing polymerisation. Moreover, it is more difficult to ensure the morphology of unsymmetrical aggregates, such as short rods, than that of symmetrical aggregates such as spherical micelles or circular vesicles.

Rod-like micelles can be formed by cetyltrimethylammonium bromide/chloride (C₁₆TABr or C₁₆TACl) with the help of simple salts^12–15 or hydrotropes.^16–22 Hydrotropes, in which sodium salicylate (NaSal) was used the most frequently, were far more effective than simple salts for this purpose. However, NaSal generally favours the induction of rapid growth of the initial rods into thread-like micelles due to its strong interactions with the surfactant. For certain applications, for example in colloidal crystals, short rods are required when using the frozen aggregates as new colloidal units to obtain ordered periodic arrays.²³ Unfortunately, the aim of only freezing rod-like micelles has not yet been realised using the method of styrene solubilisation and polymerisation.

In this issue, we reported a study on freezing rod-like micelles according to the first approach described above. For this purpose, a cationic surfactant, octadecyltrimethylammonium bromide (C₁₈TABr), and a hydrotrope, sodium 2-naphthalenesulfonate (SNphs), were chosen, by which we obtained rod-like micelles: we then solubilised and polymerised styrene monomers, by which the frozen rod-like micelles were then obtained.

Results and discussion

Aggregation of C₁₈TABr in the presence of SNphs

Dynamic light-scattering was used to characterise the aggregation of C₁₈TABr in ethanol/water (10/90 wt%) in the presence of SNphs. The time correlation functions of scattering intensity were measured at a 90° angle for the samples of C₁₈TABr (15 mmol L⁻¹) with different mole ratios of SNphs to C₁₈TABr, n_SNphs/n_C18TABr. The experimental data were analysed by the CONTIN program to obtain the intensity-fraction distributions around each characteristic aggregate size (Fig. S1, ESI†).^25,26 For the sample of C₁₈TABr alone (without SNphs), only a single intensity-fraction distribution was observed around a large aggregate size (the apparent hydrodynamic radius R_h) of c. 100 nm. With the addition of SNphs, bimodal intensity-distributions appeared at n_SNphs/n_C18TABr ≥ 0.1, besides the original large aggregate size, a new characteristic size of c. 2 nm was detected (Fig. S1, ESI†).

Upon further increasing n_SNphs/n_C18TABr, the large aggregates disappeared at n_SNphs/n_C18TABr > 0.35, while the small particles, after remaining unchanged until n_SNphs/n_C18TABr < 0.47, started to grow. Fig. 1 shows this SNphs effect, where R_h is shown as a function of n_SNphs/n_C18TABr.

Fig. 1 R_h measured at a 90° angle as a function of the molar ratio of SNphs to surfactant for C₁₈TABr (15 mmol L⁻¹) in ethanol/water (10/90 wt%) solvent, where R_h1 and R_h2 represent the hydrodynamic radius of large and small aggregates, respectively, as derived from Dblexp analysis.

To understand the behaviour of C₁₈TABr without, and with, SNphs, it is necessary to review the aggregation of long-tailed quaternary ammonium surfactants such as C₂₂TABr, C₂₀TABr, or C₁₈TABr in ethanol/water solvent. As reported in our previous issue,²⁸ these surfactants, which were dissolved in ethanol in advance, can self-assemble to form micelles, upon the addition of water to a certain extent. However, solvent ethanol was found to swell the micellar cores, which gave rise to gaps between the long-tail surfactant molecules within the micelle, and exposed the alkyl tails to the polar solvent. This resulted in secondary association of the micelle into large aggregates.²⁸ This was only the case for C₁₈TABr without SNphs (C₁₈TABr alone) as shown in Fig. S1† where large aggregates were observed.

The addition of salt can screen the charges of the headgroups of such surfactants and promote the closer packing of these molecules in the micelles. Thereby, secondary aggregation decreased and even disappeared with addition of salt as seen in Fig. 1. The characteristic parameter, β_LD, indicated the critical molar ratio of SNphs after which the large aggregates disappeared. Another parameter was β_C that indicated the critical point after which the core–shell micelles began to grow. The values of both are listed in Table 1.

Table 1 Critical molar ratio of salt to surfactant in ethanol/water (10/90 wt%), β_LD where the large aggregates disappeared, and β_C where the micelles began to grow^a

	SBzs	SNphs	NaSal
a The data in parentheses are the difference between the values of βC and βLD.
βLD	0.95	0.35	0.50
βC	1.3 (0.35)	0.47 (0.12)	0.50 (0)

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Formation of rod-like micelles

Dynamic, and static, light-scattering were used to determine the hydrodynamic radius, R_h, and the radius of gyration R_g, respectively, for which the measurements were performed at a 90° angle.^25,26,29 Fig. 2 plots theoretical R_h versus R_g curves for geometrical models of spheres, oblate ellipsoids, and rigid rods according to the formulae reported elsewhere.³⁰ The experimental data for the C₁₈TABr micelles at n_SNphs/n_C18TABr = 0.5, 0.6, and 0.7 are shown in Fig. 2, which were close to the theoretical rod line. This indicated that at n_SNphs/n_C18TABr > 0.47, the micelles had a rod-like shape.

Fig. 2 Theoretical curves of hydrodynamic radius R_h versus radius of gyration R_g. The solid circles represent experimental data for C₁₈TABr (15 mmol L⁻¹) micelles in the presence of SNphs in ethanol/water (10/90 wt%) at 30 °C, in which n_SNphs/n_C18TABr = 0.5, 0.6, and 0.7, from left to right.

The influence of surfactant concentration was also examined at a fixed ratio of n_SNphs/n_C18TABr (0.5). The results in Fig. 3 show that the experimental points were close to the theoretical rod line except at 100 mmol L⁻¹ at which, deviation from the line was slightly greater. This indicated that the micelles formed by C₁₈TABr over a moderate range of surfactant concentrations were all rod-like in shape.

Fig. 3 Theoretical R_h versus R_g curves and the experimental points for C₁₈TABr/SNphs in ethanol/water (10/90 wt%) at n_SNphs/n_C18TABr = 0.5: the surfactant concentrations were 15, 30, 50, and 100 mmol L⁻¹ from left to right.

Why was SNphs chosen, rather than SBzs or NaSal?

Besides SNphs, sodium benzenesulphonate (SBzs) and sodium salicylate (NaSal) were also candidates; the former was the homologue of SNphs, while the latter has been extensively used in C₁₆TABr solution to promote micellar growth.³¹ Thus, we also tried both salts in the C₁₈TABr system. However, we found that, for the purpose of maintaining the production of rod-like micelles, both SBzs and NaSal may be unsuitable.

For SBzs, as shown in Table 1, its efficiencies, including both inhibition of secondary aggregation and promotion of micellar growth, were all far lower than that of SNphs.

For NaSal, although the values of both β_LD and β_C were comparable with that of SNphs, this salt caused rapid growth of the C₁₈TABr micelles, probably being too strong in its interaction with the surfactant. Thereby, short rod production was hard to maintained, on the contrary, long worm-like micelles were often obtained due to the promotion of NaSal. This conclusion was derived from the comparison of the three salts in the DLS measurements, the micellar shapes (R_h versus R_g plots) and the rheological behaviour (steady-state viscosity curves, which were sensitive to micellar shape³²), which are available in ESI, Fig S2 to S7.†

Herein, we would like to re-emphasise that SNphs, rather than NaSal, was chosen to induce rod-like micelles in the C₁₈TABr system. Indeed, NaSal was used more extensively than SNphs in promoting the micellar growth of quaternary ammonium surfactants, and particularly C₁₆TABr, a cationic surfactant studied extensively.^33–35 NaSal has been demonstrated to be quite effective at inducing the formation of worm-like micelles but it may be unsuitable if only short rods were required. Contrarily, SNphs can interact, to some extent, with C₁₈TABr, which can ensure the formation of rod-like micelles in the solution.

Solubilisation of styrene

The solubilisation of styrene in the rod-like micelles of C₁₈TABr was examined, herein the templating system was that at n_SNphs/n_C18TABr = 0.5. Fig. 4 shows the appearance of the samples of C₁₈TABr (30 mmol L⁻¹) in ethanol/water (10/90 wt%) in the presence of SNphs (15 mmol L⁻¹, i.e., n_SNphs/n_C18TABr = 0.5) and with different mole ratios of styrene to surfactant (n_st/n_surf). With increasing n_st/n_surf, the sample changed from being clear in appearance (shown as the first region with n_st/n_surf < 0.8) to a weak milk-white colour (the second region, 0.8 ≤ n_st/n_surf < 1.7), and then completely white, and even slight cloudy (the third region, 1.7 < n_st/n_surf < 4.5) (Fig. 4). For the first region where four transparent samples are shown, the hydrodynamic radius, R_h, measured at a 90° angle was represented: it had an average value of 9.2 nm.

Fig. 4 Appearance of the samples of C₁₈TABr (30 mmol L⁻¹) in ethanol/water (10/90 wt%) in the presence of SNphs (15 mmol L⁻¹, i.e., n_SNphs/n_C18TABr = 0.5) and with different mole ratios of styrene to surfactant (n_st/n_surf). For the left-hand four clear samples, the hydrodynamic radius, R_h, measured at a 90° angle, and processed by the CONTIN model, was represented.

To confirm the effects of solubilisation, the UV spectra of styrene in the solutions were measured (Fig. 5). The spectrum of styrene (without surfactant) in ethanol/water (10/90 wt%) showed bimodal bands of absorbance over the UV region at 202 nm and 246 nm, respectively: this was similar to that of styrene in methanol–water solvent, which had bimodal bands at 210 nm and 240 nm.³⁶ These have been assigned to the π → π* transition in the vinyl group and the π → π* transition in the aromatic ring. The spectrum of styrene in n-dodecane was a single broad band at around 244 nm (Fig. 5), agreeing with that found in the spectrum of n-decane.³⁶ For styrene in the surfactant solutions, the spectra only showed a single band at 244 nm, which demonstrated that the styrene was solubilised in the micellar cores composed of hydrophobic hydrocarbon tails.

Fig. 5 UV spectra recorded at 30 °C for styrene in the templating micellar system at n_SNphs/n_C18TABr = 0.5. The spectra of styrene in the ethanol/water solvent without surfactant and in n-dodecane are shown for reference (dashed lines).

Styrene polymerisation and micellar freezing

The solubilised styrene monomers were polymerised in situ as described in Experimental Section. Fig. 6 shows the appearance of the samples after polymerisation, in which the samples in the secondary region became completely transparent in comparison with those before polymerisation as shown in Fig. 4, and even those in the third region were also clear. This phenomenon was also observed in other cases, for example, the system of spherical micelles formed by mixed C₂₂TABr/C₁₈TABr (unpublished results). For these clear samples in the secondary region, the value of R_h can be now measured (see Fig. 6). The average value of R_h was almost identical to that in the first region and also to those before polymerisation (Fig. 4), which indicated that the polymerisation of styrene did not change the size of the micelles.

Fig. 6 Appearance of the samples in Fig. 4 after polymerisation and the R_h, measured at a 90° angle and processed by the CONTIN model, for those clear samples as a function of n_st/n_surf.

SAXS is also often used to analyze the morphology of aggregate. Fig. 7 shows the SAXS scattering intensity (I) as a function of scattering vector (q) (q = (4π/λ)sin θ, where λ is the X-ray wavelength and 2θ is the scattering angle) for the samples before and after polymerisation. The slopes of the double logarithmic plots in the low-q region all had a value of −1 as shown by the solid lines, which indicated the formation of cylindrical (i.e. rod-like) micelles and also demonstrated the micellar morphology unchanged after polymerization.

Fig. 7 SAXS spectra (plots of scattering intensity I(q) versus scattering vector q) for the samples at n_st/n_surf = 1.3 before (open) and after (solid) polymerization.

The effect of freezing was tested by a dilution experiment, where the samples after polymerisation were diluted 100-fold, i.e., adding 100 g water to 1 g of the sample. After dilution, the samples at low n_st/n_surf (the first and secondary regions) showed rather wide distributions of scattering intensity as seen in the insets, which yielded relatively greater average values (represented by the star symbols in Fig. 8, top) compared with the original ones (solid circles). This indicated that the polymerisation did not fix these micelles for these samples and they must be fragmented during dilution. At high n_st/n_surf (the third region), however, single, narrow distributions of scattering intensity were obtained for each sample, and in particular, for the last sample (n_st/n_surf = 4.4, see inset). Upon increasing the dilution to 400% (a total of 400 g water added to 1 g of the original sample), these samples still showed a single, narrow distribution of intensity (Fig. S11, ESI†). This indicated that at a large n_st/n_surf (the third region), the polymerisation can freeze the micelles.

Fig. 8 Dilution experiments: (top) R_h, measured at a 90° angle and processed by the CONTIN model, as a function of n_st/n_surf, where the symbols represent samples (●) just after polymerisation, (☆) after 100%, and (○) 400% dilution. The insets are the intensity-fraction distributions obtained by CONTIN analysis. (bottom) Relative variance, μ₂/[capital Gamma, Greek, macron]², obtained by Cumulant analysis for samples (●) just after polymerisation, (☆) after 100%, and (○) 400% dilution. The appearance of the last three samples after dilution is shown in the insets.

The relative variance, μ₂/[capital Gamma, Greek, macron]², obtained by the Cumulant analysis can be used to characterise the polydispersity of aggregates.^25,26 The results in Fig. 8 (bottom) showed an average value of 0.08 for μ₂/[capital Gamma, Greek, macron]² at high n_st/n_surf (the third region), which indicated a good mono-dispersity for the formed aggregates before, and after, dilution.

The aforementioned results suggested an effective approach to freezing the micelles, i.e., a slightly great amount of styrene should be solubilised for this purpose, for example, the sample at n_st/n_surf = 4.4 showed good freezing effect. In our unpublished work, the spherical micelles formed by mixed C₂₂TABr/C₁₈TABr were well fixed at n_st/n_surf ≥ 4.9. This value was comparable with the present 4.4 (n_st/n_surf), which may have entailed a similar mechanism for freezing the aggregates. We also hoped to get some supports from published literature but were disappointed since few reports were available. Here only two cases are shown because they mentioned the values of n_st/n_surf related to micellar freezing. Becerra et al. reported a typical value of the weight ratio of styrene (st) to cetyltrimethylammoniumtosilate (CTAT), w_st/w_CTAT of 0.1, which corresponded to a mole ratio of 0.44, for the CTAT system.⁷ They suggested that the microstructures of aggregates might be preserved but did not show any evidence to that effect, for example, a dilution test. Kurja et al. reported a rather high value for the weight ratio of styrene to surfactant (dioctadecyldimethylammonium bromide, DODAB) of 1.59, which corresponded to a mole ratio of 9.6, in a DODAB-vesicle system.⁴ They confirmed that the vesicle structure was retained after the solubilised styrene was polymerised. Comparatively, our two values of n_st/n_surf, 4.4 for the present system, and 4.9 for mixed C₂₂TABr/C₁₈TABr spherical micelles, may be more useful because their frozen aggregates have been demonstrated thus by dilution tests.

The TEM image of this sample after 100% dilution provided direct observation of the aggregate morphology (Fig. 9), where rod-like micelles were seen. Synthesising all such results, we can now say that, in this system of C₁₈TABr/SNphs (n_SNphs/n_C18TABr = 0.5), well frozen rod-like micelles have been obtained depending on the polymerisation of solubilised styrene monomers.

Fig. 9 TEM images of the polymerised sample at n_st/n_surf 4.4 after 100% dilution.

Experimental

Materials

C₁₈TABr was synthesised in our laboratory and the purity was checked by ¹H NMR and elemental analyses.²⁴ Ethanol (AR, Sinopharm) was used as received. The water used was of Milli-Q grade with a resistivity of 18.2 MΩ cm. SNphs (Acros), sodium benzenesulphonate (SBzs, Aldrich), and sodium salicylate (NaSal, Sinopharm Chemical Reagent Co.) were used as received.

Sample preparation

The surfactant was dissolved in ethanol and the solution was filtered through a 0.22 μm pore-size filter. The Milli-Q water was slowly added to the solution using a drop-rate auto-controllable micro-syringe (LongerPump TJ-1A) at a flow rate of 20 μL min⁻¹ until a 90 wt% water content was reached. This method of preparation ensured both homogeneous solutions of C₁₈TABr, and good dissolution of hydrotropes, particularly SNph. The solutions were stirred for 24 h before testing. All experiments were performed at 30 °C.

Styrene solubilisation and polymerisation

Reagent grade styrene (AR, Sinopharm) was washed with 10 wt% NaOH several times and then washed with water until the solution pH reached 7 to remove the inhibitor and any polymeric residue. Desired amounts of styrene were put into test tubes containing surfactant micelles in ethanol/water solvent in the presence of organic salt. The test tubes were vibrated for 48 h at a constant 30 ± 0.1 °C to ensure the solublilising equilibrium of styrene in the micelles.

The UV absorption spectra of the styrene-containing solutions were recorded on a Hitachi U-3010 (Japan) UV/vis spectrophotometer using a quartz cell with a 5 mm path length.

Polymerisation was carried out in a 250 mL glass reactor with a solution of C₁₈TABr micelles solubilised by styrene. The glass reactor was equipped with a temperature controller and was stirred using a magnetic stirrer. K₂S₂O₈ of 1 wt% with respect to the monomer was added and the solution was heated to 70 °C while being stirred. The reaction was performed for 8 h, after which the temperature was rapidly decreased to room temperature.

Light-scattering measurements

The light-scattering ability of the solutions was measured with a Brookhaven Instrument which was composed of a BI-200SM goniometer, a BI-9000AT digital correlator (522 channels), and a photomultiplier detector. A green laser with 532 nm wavelength and 200 mW output power was used as the light source. The measurement temperature was controlled by a thermostatic circulator (Poly-Science, USA) to an accuracy of ±0.01 °C.

For dynamic light-scattering (DLS), the experimental data were analysed by use of the CONTIN program to obtain the intensity-fraction distributions around each characteristic aggregate size. For those double distributions of scattering intensity, a double-exponential model (Dblexp) was also used to analyse the data to a greater precision.^25,26

For static light-scattering (SLS), the reduced scattering intensity KC/R_θ was measured at different concentrations. Here, R_θ is the Rayleigh ratio obtained by calibration measurements with benzene: R_θ = 8.51 × 10⁻⁶ at 25 °C,²⁷ C is the surfactant concentration, and K is the optical constant which is given by:

K = 4π²n²₀(dn/dC)²/(λ⁴₀N_A) (1)

where n₀ is the solvent refractive index, and dn/dC is the refractive index increment measured by BI-DNDC.

SAXS measurements

The experiments were performed at 25 °C using a NanoStar (Bruker) SAXS equipped with a 2-d detector. The incident X-rays of CuKα radiation (1.54 Å) were monochromated by a cross-coupled Göbel mirror and passed through the sample placed in a 2 mm quartz capillary. The distance between the sample and the detector was 1070 mm, allowing the value of the scattering vector q to range from 0.07 to 2.3 nm⁻¹. The data shown were for the normalised intensity I (arbitrary units) versus q = (4π/λ)sin(θ), where λ is the wavelength of the X-rays and 2θ is the scattering angle.

Cryo-EM measurement

A drop (3.5 μL) of solution was applied onto a 300-mesh GiG carbon grid (Jiangsu Life-Trust) that was pre-treated in plasma cleaner (PDC-32G, Harrick Plasma). The grid was then blotted for 3.0 s at 100% humidity, using an FEI Vitrobot (Mark IV) before it was rapidly frozen in liquid ethane that had been cooled by liquid nitrogen. The samples were then imaged with a FEI Titan Krios cryo-EM that was operated at 300 kV and equipped with a Gatan UltraScan 4000 charge-coupled device camera.

Conclusions

In this study, we prepared rod-like micelles from C₁₈TABr with the help of SNphs, and further froze their microstructure via solubilising styrene monomers into the micelles and then in situ polymerisation thereof. The main conclusions may be drawn as follows:

(1) Depending on assistance from the SNphs, C₁₈TABr formed rod-like micelles. If only a rod-like morphology was required, SNphs were the most suitable for this purpose, and they cannot be substituted by NaSal.

(2) The microstructure of the rod-like micelles formed by C₁₈TABr at n_st/n_surf > 2.7 can be well frozen via the polymerisation of solubilising styrene monomers.

(3) To achieve a good freezing effect, a relatively large amount of solubilised styrene may be required: in the present case, for instance, the sample at n_st/n_surf = 4.4 exhibited good micellar freezing.

Acknowledgements

Support from The National Natural Science Foundation of China (Grant no. 21273040) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18794c

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