Switchable foam control by a new surface-active ionic liquid

Abstract

A new surface-active ionic liquid 4-butylazobenzene-4′-ethoxy-trimethylammonium bis(trifluoromethanesulfonyl)imide ([BAzoTMA][NTf₂]) was designed and synthesized to achieve reversible foaming–defoaming by alternatively adding cucurbit[7]uril (CB[7]) and spermine, which was investigated by measurements of contact angle and conductivity, ¹HNMR, and PM-IRRAS.

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Foams are dispersions of gas bubbles in liquid, stabilized by amphiphilic molecules, polymers, or particles, which are widely applied in fields like petroleum industry, food, cosmetics, mineral processing, etc.¹ For some applications, such as food and cosmetics industry, foams should remain stable for long time; while in some other case, such as mineral processing, both rapid formation of stable foam and defoaming the initially stable foams are required. In the latter case, antifoamer or defoamer should be used to prevent or eliminate foam, which hinders the further re-foaming of the system.² Therefore, new strategies of designing switchable and stimuli-response foams by external stimuli of UV, temperature, magnetic or electric field and CO₂ etc. are appreciated and it has attracted much attention in recent years,³ since Middelberg et al.⁴ used a bio-surfactant for switchable foam control triggered by pH change.

Ionic liquids (ILs), usually defined as liquid salts below 100 °C, are considered as designers' solvents showing unique properties such as wide liquid state range, negligible vapour pressure, and favourable solvation behaviour etc.⁵ Some types of ILs with somewhat long alkyl chains, termed as surface-active ionic liquid (SAIL), can self-aggregate in aqueous and non-aqueous solutions,⁶ and construct new surfactant systems. Due to the combination of both properties of the ionic liquid and the amphiphile, surface-active ionic liquids possess some unique advantages, such as stronger self-aggregation tendency in aqueous and non-aqueous solutions, novel surface properties, and better applications in catalysis and separation processes, etc.⁷ Moreover, it has been reported recently that a new type of SAIL composed of amphiphilic cation and amphiphilic anion presented tuneable self-assemble structures and physicochemical properties with the variation of the amphiphilicity of cation or anion.^6b,8

Cucurbit[n]uril (CB[n] with n = 5–8 or 10) is a family of macrocycles constructed by n glycoluril building blocks and plays a very important role in supermolecular chemistry.⁹ The inner cavities of CB[n]s are negative-charged and hydrophobic, hence can incorporate positively charged molecules and hydrophobic molecules with proper shape and size, which has been utilized to tune the structures of self-assembly,¹⁰ for instance, Zhang et al.^3e utilized CB[7] to change the arrangement of Gemini surfactant 12-2-12 due to the host–guest interaction between the hydrophobic tails and CB[7].

It is commonly accepted that the compactness of hydrophobic tails of amphiphiles arranged at air–water interface plays an important role in the stabilization of foam.^3e,11 By considering merits of SAIL and CB[7], we design a new SAIL 4-butylazobenzene-4′-ethoxy-trimethylammonium bis-(trifluoromethanesulfonimide) ([BAzoTMA][NTf₂]) (Scheme 1) with cation being obvious amphiphilic and able to be closely stacked and anion also presenting somewhat amphiphilic character. The interaction of cation and anion can be tuned by host–guest interaction between cation and CB[7], thus the compactness of the SAIL at the air–water interface and hence the foaming/defoaming process is expected to be controlled. To the authors' best knowledge, this is the first report concerning the usage of SAIL to achieve switchable foam control.

Scheme 1 Chemical structure of [BAzoTMA][NTf₂].

The detailed synthesis process and characteristics of [BAzoTMA][NTf₂] are shown in the ESI.† The melting point of [BAzoTMA][NTf₂] was determined to be 65 °C (MPA100, Opti-Melt, USA), characterizing the feature of the ionic liquid.⁵ As shown in Fig. 1a and b, an aqueous solution of [BAzoTMA][NTf₂] with the equimolar CB[7] results in significantly richer foam as compared to the [BAzoTMA][NTf₂] aqueous solution with the same [BAzoTMA][NTf₂] concentration. Gemini surfactant 12-2-12 has been reported to show good foaming ability,¹² as shown in Fig. 1c, hence we further compared the foaming ability of [BAzoTMA][NTf₂]/CB[7] system with that of Gemini surfactant 12-2-12 with the same surfactant concentration. The relative height of foam (H_R, Fig. 1), the half-time of foam destruction (t_1/2, Fig. 2), and the contact angle (θ, Fig. 3) were measured for aqueous solutions of [BAzoTMA][NTf₂], [BAzoTMA][NTf₂]/CB[7], and 12-2-12, which are summarized in Table 1. It is clearly indicated by Table 1 that the addition of CB[7] to [BAzoTMA][NTf₂] aqueous solution greatly increases the foaming ability, even better than that of 12-2-12.

Fig. 1 Photos of foam generated by (a) [BAzoTMA][NTf₂] (0.05 mM), (b) [BAzoTMA][NTf₂] + CB[7] (0.05 mM), (c) 12-2-12 (0.05 mM), and (d) ([BAzoTMA][NTf₂] + CB[7] (0.05 mM)) + spermine (0.05 mM).

Fig. 2 Plot of relative height of foam H_R against time for the aqueous solutions of [BAzoTMA][NTf₂]/CB[7] and 12-2-2. The insets are the photos of the samples at different time.

Fig. 3 Contact angle measurements of (a) water, (b) [BAzoTMA][NTf₂] (0.05 mM), (c) [BAzoTMA][NTf₂] + CB[7] (0.05 mM) (d) 12-2-12 (0.05 mM), and (e) [BAzoTMA][NTf₂] + CB[7] + spermine (0.05 mM).

Table 1 The values of the relative height of foam (H_R), the half-time of foam destruction (t_1/2), and the contact angle (θ) for different systems

	[BAzoTMA][NTf2]	[BAzoTMA][NTf2] + CB[7]	12-2-12	[BAzoTMA][NTf2] + CB[7] + spermine
HR	0%	76.2%	45.0%	7.5%
t1/2 (min)	0	150	3	0
θ	74.18	63.91	70.32	76.10

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In order to get deep insight into the mechanism of enhancing foaming ability of [BAzoTMA][NTf₂] by adding CB[7], the host–guest interaction between [BAzoTMA][NTf₂] and CB[7] was investigated by ¹H NMR. Fig. 4 shows the down-field shifts of H_a and H_b protons in the [BAzoTMA] cation, indicating that the protons H_a and H_b locate in a more hydrophobic environment due to their incorporation into CB[7]. In addition, H_c and H_d protons in CB[7] show obvious up-field shifts due to the fact that the shielding effect from the benzene ring in [BAzoTMA] cation. These may suggest the formation of the host–guest complex between [BAzoTMA][NTf₂] and CB[7].

Fig. 4 Partial ¹HNMR spectrum profiles of CB[7] (0.05 mM), [BAzoTMA][NTf₂] (0.05 mM), and [BAzoTMA][NTf₂] (0.05 mM) + CB[7] (0.05 mM) in D₂O (for full spectra, please see Fig. S1 in ESI material†).

The host–guest interaction between [BAzoTMA][NTf₂] and CB[7] was further investigated by measurement of conductivity. The conductivities of [BAzoTMA][NTf₂] (0.05 mM) aqueous solution and CB[7] (0.05 mM) aqueous solution were determined to be 14.64 μS cm⁻¹ and 5.96 μS cm⁻¹, respectively; while the conductivity of the aqueous solution of [BAzoTMA][NTf₂] (0.05 mM) + CB[7] (0.05 mM) was 10.96 μS cm⁻¹. This further confirms that the supermolecules composed of CB[7] and [BAzoTMA][NTf₂] are formed and hence reduce the conductivity.

The mechanism of enhancement of the foaming ability may be speculated as follows (Scheme 2). In the [BAzoTMA][NTf₂] aqueous solution not only [BAzoTMA] cation but also [NTf₂] anion adsorb at the interface because of somewhat hydrophobicity of [NTf₂]⁻ and the electrostatic interaction between cation and anion. The presence of hydrophobic [NTf₂]⁻ at air–water interface prevent the close packing of [BAzoTMA] tails due to the steric hindrance. However, after addition of CB[7], the incorporation of [BAzoTMA] cation and CB[7] may shield the repulsive electrostatic interaction between [BAzoTMA] cations and the attractive interaction between [BAzoTMA]⁺ and [NTf₂]⁻, thus some counterions [NTf₂]⁻ escape from the interface, which makes more close packing of [BAzoTMA] tails at the interface and increases the foaming ability. The close packing of [BAzoTMA] tails after addition of CB[7] was also be verified by contact angle measurements on hydrophobic polyethylene substrates as shown in Fig. 3. The close packing of [BAzoTMA] tails resulted in the decrease of contact angle of [BAzoTMA][NTf₂]/CB[7] aqueous solution as compared to that of [BAzoTMA][NTf₂] aqueous solution.^11d,13 Moreover, it should be noted that the ratio of cis-/trans-isomer of [BAzoTMA][NTf₂] in the solution shows no change with the addition of CB[7] and spermine (Fig. S2 in the ESI material†), thus, it may rule out the effect of cis-/trans-equilibrium on the foamability.

Scheme 2 The possible structure change on the interface of aqueous solutions of (a) [BAzoTMA][NTf₂]; (b) [BAzoTMA][NTf₂] with CB[7].

The incorporation of [BAzoTMA] with CB[7] and the decrease of [NTf₂]⁻ quantity at the air–water interface were further investigated by Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS) in the wavelength range of 1000–2000 cm⁻¹ as shown in Fig. 5. The absorbance of C–F bond in [NTf₂]⁻ at about 1268 cm⁻¹ reduces sharply after addition of CB[7], which indicates that a quite large amount of [NTf₂]⁻ is removed from the interface and enters into the bulk solution. Moreover, the presence of large absorbance peak of –CH₂N– in CB[7] at 1520 cm⁻¹ and the slight decrease of the absorbance of –N=N– bond in [BAzoTMA] cation at about 1680 cm⁻¹ further suggest the incorporation of CB[7] and [BAzoTMA] cation.

Fig. 5 Polarization Modulation Infrared Reflection Absorption Spectra.

It is commonly accepted that spermine shows strong host–guest interaction with CB[7],^10b,14 which is supposed to remove CB[7] from [BAzoTMA][NTf₂]/CB[7] complex and thus control the foaming ability of [BAzoTMA][NTf₂] aqueous solution in a reversible way. As it is clearly shown in Fig. 1d, after 1 equiv. of spermine was added into the aqueous solution of [BAzoTMA][NTf₂]/CB[7], the foam eliminates rapidly. Furthermore, the foaming process was recovered by adding more CB[7], thus, the switchable foaming–defoaming process was achieved, and the good reversibility is indicated in Fig. 6 for several cycles.

Fig. 6 Reversible foaming–defoaming process.

In order to verify the importance of the anion [NTf₂]⁻ and the aromatic rings in the hydrophobic tail to this switchable foaming–defoaming process, we also synthesized another two similar compounds 4-butylazobenzene-4′-ethoxy-trimethylammonium bromide [BAzoTMA]Br and dodecyl trimethylammonium bis(trifluoromethanesulfonyl)imide ([DTA][NTf₂]). The changes of foamability for these two systems with the alternative addition of CB[7] and spermine are shown in Fig. 7, both clearly presenting much less effectiveness of switchable foam control.

Fig. 7 Changes of foams with alternative addition of CB[7] and spermine for 0.05 mM (a) [BAzoTMA]Br and (b) [DTA][NTf₂] aqueous solutions.

Furthermore, another experiment has been conducted to show the crucial role of competitive host–guest interaction of [BAzoTMA][NTf₂] and spermine with CB[7] in the switchable foam control. We added 1 equiv. of 1-octanol to aqueous solution of [BAzoTMA][NTf₂] + CB[7] since 1-octanol is usually used as defamer and shows less interaction with CB[7] as compared with [BAzoTMA][NTf₂], which destabilized the foam rapidly. However, the foam cannot be reformed with further addition of CB[7] (see Fig. S3 in ESI material†). Thus, this result clearly indicates the competitive host–guest interaction of [BAzoTMA][NTf₂] and spermine with CB[7] is responsible for the switchable foam control.

To sum up, we have demonstrated an effective way to improve the foaming ability of [BAzoTMA][NTf₂] aqueous solution by adding CB[7], to eliminate the foam by further addition of spermine, and to achieve a switchable foam control with alternatively adding CB[7] and spermine. This work offers a new strategy to design a proper SAIL for the potential application in fields where both stable foam and destabilization of foam are required, e.g. mineral processing. The further work on optimizing SAIL structure for its application is ongoing.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Projects 21173080, 21373085 and 21303055) and the Fundamental Research Funds for the Central Universities (WJ1516001).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic process and characterizations of products; the experimental methods. See DOI: 10.1039/c6ra17245h

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