Micellization and Adsorption of Heterogemini Surfactants Containing a

Article pubs.acs.org/jced

Micellization and Adsorption of Heterogemini Surfactants Containing a Hydroxyl Headgroup in Aqueous Solution Tianhua Zhou,*,†,‡ Shengwei Liu,‡ Yi You,† Rong Xu,‡ and Jianxi Zhao*,† †

College of Chemistry, Fuzhou University, Fuzhou 350002, People’s Republic of China School of Chemical & Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459

‡

S Supporting Information *

ABSTRACT: The micellization of a homologous series of heterogemini surfactants, CmOhpNCn (m, n = 10, 8; 12, 8; 14, 8; 16, 8; and 10, 14), in aqueous solution and their adsorption at the air/water interface have been investigated by surface tension, conductivity, and fluorescence techniques. The surface tension curves of C16OhpNC8 and C10OhpNC14 showed two break points, corresponding to the critical concentration of the premicellar aggregation and the general micellization, respectively. The results of conductivity and fluorescence spectra using pyrene as probe confirmed the premicellization behavior in the two cases. This indicated that these surfactants have strong aggregation ability in the solution. The micelle formed nearby the cmc displayed only a small aggregation number but a large ionization degree. The C20 that characters the efficiency in surface tension reduction was quite small in comparison with those of conventional surfactant and even smaller than those of symmetric gemini surfactants such as 12-s-12 homologous. The special molecular packing of CmOhpNCn in aqueous solution was closely related to the intermolecular hydrogen bonding and a weak electrostatic repulsion. aqueous solution.21 They further investigated the solid phase transitions of C12CsC14Br2 with s = 2, 6, or 10 using POM, XRD, and DSC techniques and observed that all the compounds exhibited thermotropic phase transition from crystalline to smectic liquid crystal.16 Bai and his colleagues measured the ΔHmic of CmC6CnBr2 series with m + n = 24, where m = 12, 13, 14, 16, and 18, in aqueous solution using microcalorimetry.17 They observed a small reduction in cmc accompanying a very large decrease in ΔHmic with increasing the m/n ratio, showing significant effect of dissymmetry (m/n) on the micellization in aqueous solution.15 Another type of dissymmetric gemini surfactants was referred to as “heterogeminis” by Alami and Holmberg.22−24 They have two nonidentical polar head groups and two different (or identical) lengths of alkyl tails. Jaeger et al. first synthesized the heterogemini surfactants containing quaternary ammonium and carboxylate head groups and two dodecyl or tetradecyl tails.25

1. INTRODUCTION In the past decade, gemini surfactants have received considerable attention for their unique properties different from those of the single-chain counterparts, such as remarkably low critical micelle concentration (cmc); much higher efficiency in surface tension reduction; unusual morphologies of the aggregates, and so forth.1,2 So far, most investigated gemini surfactants are symmetrical, that is, those having two identical head groups and two alky chains with the same lengths.3−10 Recently, dissymmetric gemini surfactants have also received increasing interest.11−19 One type of dissymmetric gemini surfactants are those with two alkyl tails of different lengths and two identical head groups in a molecule, with a general molecular formula [C m H 2 m + 1 (CH 3 ) 2 N−(CH 2 ) s −N(CH3)2CnH2n+1]Br2, designated as CmCsCnBr2 (where m ≠ n).20Oda and co-workers studied the CmCsCnBr2 series with n and m values of 8−18, and their results indicated that the alkyl tail length and the dissymmetry of surfactants have a strong influence on the micellization and the phase behavior in aqueous solution.20 Sikirić et al. reported the coexistence of three types of aggregates with different sizes in C12C2C14Br2 © 2016 American Chemical Society

Received: September 11, 2015 Accepted: July 27, 2016 Published: August 4, 2016 2915

DOI: 10.1021/acs.jced.5b00778 J. Chem. Eng. Data 2016, 61, 2915−2922

Journal of Chemical & Engineering Data

Article

Pt−Ir du Noüy ring. The circumference of the ring is 5.930 cm. The ratio of the outside radius to the radius of the ring cross section (R/r) is 53.1218. 2.2.2. Electrical Conductivity. Electrical conductivity of the sample solutions was collected using a digital conductometer (Shanghai Weiye Instrument Plant, model DDS-307) with DJS0.1 platinum electrode. The cell constant = 0.928 cm−1 provided by the manufacturer. 2.2.3. Time-Resolved Fluorescence. Time-resolved fluorescence quenching technique with pyrene as probe and CPC as quencher, respectively, was used to measure the aggregation number of the micelle. The fluorescence decay spectra were recorded by Edinburgh FL/FS920 TCSPC fluorescence spectrophotometer (Edinburgh, U.K.) using the quartz cell of 1 cm path length. In the presence of quencher, the emission signal of excited pyrene in a micellar solution has a time dependent profile of the form34

They observed the giant vesicle in the aqueous solutions of these heterogemini surfactants. Menger and his colleagues synthesized another family of heterogeminis CmH2m+1−PO4−− (CH2)2−N+(CH3)2−CnH2n+1, designated as Cm−Cn.23 These surfactants are also found to form the aggregates with various structures at high concentration, such as tubular structure aggregates, giant vesicles and coacervate droplets.26−29Moreover, the vesicles formed by Cm−Cn (m ≠ n) are successfully cohered into a “pearls on a string”, which led to a complex network that rigidified the water.24 From the reported results, one can realize the novel and complex self-assembly properties of dissymmetric gemini surfactant in aqueous solution. So far, however, only a little knowledge is available about the micellization and adsorption of heterogemini surfactants. Recently, we reported a stable thermotropic liquid crystalline formed by a homologous series of the heterogemini surfactants, (N,N-dimethyl-N-[3-(alkyloxy)-2-hydroxypropyl]-alkylammonium bromide (referred to as CmOhpNCn), whose molecular structure is shown in Figure 1. Although the hydroxyl group in CmOhpNCn has only weak

I(t ) = I(0)exp{ −k 0t − ⟨n⟩[1 − exp( −k Q t )]}

(1)

where I(t) and I(0) are the fluorescence intensities at time t and zero, respectively. k0 is the unquenched decay rate constant of the probe and kQ is the first order rate constant for quenching. ⟨n⟩ is the average number of quenchers per micelle. Fitting the curve, one could obtain ⟨n⟩ and thus the aggregation number N is calculated by the formula34

Figure 1. Molecular structure of CmOhpNCn (m, n = 10, 8; 12, 8; 14, 8; 16, 8; and 10, 14).

N=

⟨n⟩(C − cmc) CQ

(2)

where C and CQ are the concentration of surfactant and quencher, respectively. cmc is the critical micelle concentration. 2.2.4. Steady-State Fluorescence. Steady-state fluorescence emission spectra of pyrene (1 × 10−6 mol·L−1) were recorded by the instrument described above at λex = 335 nm. The ratio of emission intensity (I1/I3) of the first to the third peaks was used to character the polarity of the microenvironment located by pyrene35 that was known to solubilize in the palisade layers of the micelles.36,37 All of the surfactant solutions were prepared using Milli-Q water (resistivity = 18.2 MΩ cm), and all of the measurements were performed at 25.0 ± 0.1 °C.

hydrophilic character, it indeed acts as the head function, which induces the formation of hydrogen bonding according to our observation by IR.30 The presence of hydrogen bonding significantly improved the stability of liquid crystal compared to the counterpart symmetrical gemini surfactant.30−32 Furthermore, we have more recently investigated the aggregation and rheological behavior of CmOhpNCn in aqueous solution and found that the hydrogen bonding also affect the formation of aggregates. When increasing m value, the hydrogen bonding induced by the hydroxyl headgroup strengthened, and long rodlike or wormlike micelles are observed.33 Obviously, the hydrogen bonding affects not only the stability of liquid crystal but also the aggregation and rheological behavior of CmOhpNCn in aqueous solution. However, it is still unclear about the micellization properties of CmOhpNCn in dilute aqueous solution. To explore the effects of special structure on micellar parameters, such as premicellization, ionization degree, aggregation number, in this work, we further investigate the micellization properties of CmOhpNCn aqueous solution near the CMC. The results show that CmOhpNCn has high surface activity and complicated micellization behavior near the cmc in aqueous solution except for its various micellar morphologies at high concentration.

3. RESULTS AND DISCUSSION 3.1. Critical Micelle Concentration. Figure 2 shows the semilogarithmic plots of surface tension (γ) against the

2. MATERIALS AND METHODS 2.1. Materials. The CmOhpNCn (m, n = 10, 8; 12, 8; 14, 8; 16, 8; and 10, 14.) was previously synthesized.30 Pyrene (Fluka, AR) was recrystallized three times in ethanol. Cetylpyridinium chloride (CPC, Shanghai Reagent Corporation, AR) was recrystallized five times in acetone−alcohol mixed solvent. 2.2. Measurements. 2.2.1. Surface Tension Measurements. Surface tension of the surfactant aqueous solutions was performed on CHAN DCA-315 tensionmeter equipped with a

Figure 2. Semilogarithmic plots of surface tension (γ) against the concentration (C) of CmOhpNCn at 25.0 °C and pressure 0.1 MPa. (■) m, n = 10, 8; (□) m, n = 12, 8; (▲) m, n = 14, 8; (△) m, n = 16, 8; and (●) m, n = 10, 14; where log C has been shifted left by 0.7 scale units for better distinguishing. 2916



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Table 1. Characteristic Parameters of CmOhpNCn Aqueous Solutions at Pressure 0.1 MPaa and Temperature 25.0 °Ca m, n 105 cmc/mmol·L−1 γcmc/mN·m−1a 105 C20/mmol·L−1 1010 Γmax/mol·cm−2 Amin/nm2 α N

10, 8 108 (109) 30.0 9.12 1.79 0.93 0.362 23.6c

12, 8 20.7 (22.9) 32.7 3.63 2.27 0.73 0.626 26.6c

14, 8 4.25 (4.32) 32.8 0.85 2.36 0.70 0.738 34.8c

16, 8 0.45,a 1.38b (2.56) 33.1 0.43

10, 14 0.47,a 1.59b (2.84) 30.3 0.47

0.798 24.0d

0.823 14.9d

The data correspond to the first break point on the surface tension curve. bThe data correspond to the second break point. cThe measurement is at 4.0 × 10−3 mol·L−1. dThe measurement is at 1.0 × 10−3 mol·L−1. The data in the parentheses come from the intersection point of the two straight lines on the conductivity curves. The standard uncertainties u(γ) = 0.1 mN·m−1, u(κ) = 0.05, u(T) = 0.1 °C, and u(P) = 0.01 MPa. a

concentration (C) of CmOhpNCn, in which the plots of C16OhpNC8 and C10OhpNC14 show two break points as arrow shown, which generally indicates premicellization occurs between the two break points,38−40 and the other three plots only exhibit one inflection. Thus, the critical micelle concentration (cmc) is obtained by the break point (or by the second break point if there are two ones as suggested by Pinazo et al.38 and Sakai et al.39). The data in Table 1 show the cmcs of C16OhpNC8 and C10OhpNC14 are almost 2 orders of magnitude lower than that (8.4 × 10−4 mol·L−1) of symmetrical gemini surfactant ethanediyl-α,ω-bis(dimethyldodecyl ammonium bromide), (designated as 12−2−12), even though the three compounds have the same total carbon atoms in the alkyl tails.41 Comparatively, C10OhpNC8’s cmc is close to that of 12− 2−12, but its alkyl tails are much shorter than the latter. These indicate quite strong aggregation ability of CmOhpNCn with asymmetric structure in aqueous solution in comparison with symmetric 12−2−12. However, if only changing the degree of asymmetry in the alkyl chains, as in the cases of CmCsCnBr2 that has identical quaternary ammonium heads, Wang et al. observed a very small variation of the cmc.15 Even for didodecyldimethylammonium bromide that has a single head but two alkyl tails of the same length, its cmc (7.0 × 10−5 mol/ kg) is also 4−5 times larger than that of C16OhpNC8 and C10OhpNC14.42 Here, the weak hydrophilic headgroup (hydroxyl) of CmOhpNCn perhaps plays an important role in the self-assembly. The hydroxyl group could insert between the quaternary ammonium heads on the surface of the micelle, resulting in the reduction of the electrostatic repulsion between the ionic heads. Consequently, hydrogen bonding could be responsible for the low cmc in the CmOhpNCn system as well.33,43,44 To confirm the intermolecular hydrogen bonding in CmOhpNCn system, IR spectra of C12OhpNC12 in solid form and in CDCl3 were shown in Figure 3. For the case of C12OhpNC10 in CDCl3, two characteristic bands at 3416 and 3274 cm−1 assigned to the hydroxyl group are obviously spread out and slightly shifted to lower values compared with those in solid form, suggesting the existence of hydrogen bonding in CmOhpNCn/CDCl3 system. Similarly, intermolecular hydrogen bonding can also occur in CmOhpNCn aqueous solution between CmOhpNCn and water molecule, resulting in the extremely low cmc value. 3.2. Premicellization. 3.2.1. Deviation from the Linear Relationship of Log (cmc) versus (m + n). Menger and Littau provided the first evidence for the premicellar aggregation of several series of gemini surfactants in aqueous solution.1 They found that the plots of log cmc with the carbon number m of alkyl chain show an upward curvature for m ≥ 16 instead of usual linear dependence. Furthermore, similar behavior is also

Figure 3. FT-IR spectra of C12OhpNC12 in solid form (a) and in CDCl3 (b) at 1.0 mol·L−1.

reported for other series of dimeric surfactants.45−47 In this work, the plot of the log (cmc) versus the total carbon number (m + n) in the two alkyl tails is shown in Figure 4. The variation

Figure 4. Relationship between log(cmc) measured by surface tension method and the total carbon number (m + n) in the two hydrophobic tails.

of log cmc with (m + n) indeed deviates from linear relationship for (m + n) = 24, that is, for the two cases of C16OhpNC8 and C10OhpNC14, probably suggesting the presence of premicellization in the solution. 3.2.2. Fluorescence Evidence. The emission spectra of pyrene at different concentrations of C16OhpNC8 and C10OhpNC14 aqueous solutions are shown in Figure 5, where a broad emission band at 480 nm can be seen at some concentrations besides five characteristic maxima of pyrene 2917



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monomer at 370−400 nm. The broad band is generally attributed to the existence of excimer.48 For the surfactant in the solution, this result means the aggregates are formed, resulting in the solubilization of pyrene and the formation of pyrene excimer. Figure 6 shows the semilogarithmic plots of the intensity ratios of both the first to third peaks, I1/I3, and of the maximum of the excimer to the monomer, IE/IM, against the surfactant concentration C. From Figure 6d and 6e, clear rise of IE/IM is observed at the first perpendicular dotted lines whose concentrations (designated as cpmc) correspond, respectively, to the first inflection points in the surface tension curves of C16OhpNC8 and C10OhpNC14. This supports their premicellization in the solution.39 3.2.3. Conductivity Evidence. Figure 7 shows the plots of specific conductivity versus C for CmOhpNCn aqueous solution. Generally, the intersection point of the two straight lines is also assigned to the cmc.49 The cmcs for C10OhpNC8, C12OhpNC8 and C14OhpNC8 (see the data in the parentheses in Table 1) are very close to those as determined by surface tension. However, Tsubone50 and Quagliotto et al.51 observe that when

Figure 5. Emission spectra of pyrene (5 × 10−6 mol·L−1) in C16OhpNC8 (a) and C10OhpNC14 (b) aqueous solutions with different concentrations at 25.0 °C and pressure 0.1 MPa: from 1 to 7 the surfactant concentration is (a) 8.50 × 10−6, 1.19 × 10−5, 1.98 × 10−5, 2.99 × 10−5, 4.76 × 10−5, 7.54 × 10−5, 1.19 × 10−4 mol·L−1 and (b) 2.97 × 10−6, 5.41 × 10−6, 1.08 × 10−5, 1.79 × 10−5, 3.60 × 10−5, 6.12 × 10−5, and 1.08 × 10−4 mol·L−1, respectively.

Figure 6. I1/I3 (■) and IE/IM (□) versus logC for (a) C10OhpNC8; (b) C12OhpNC8; (c) C14OhpNC8; (d) C16OhpNC8; and (e) C10OhpNC14 in aqueous solution using pyrene as the probe at 25.0 °C and pressure 0.1 MPa. The perpendicular dashed line in each graph corresponds to the cmc, whereas the dotted lines in (d) and (e) correspond respectively to the first break points in the surface tension plots in the cases of C16OhpNC8 and C10OhpNC14, which is designated as the cpmc (critical premicelle concentration). 2918



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Figure 8. Plots of molar conductivity versus C 1/2 for (a) C10OhpNC8, (b) C12OhpNC8, (c) C14OhpNC8, (d) C16OhpNC8 and (e) C10OhpNC14 aqueous solutions at 25.0 °C and pressure 0.1 MPa.

Figure 7. Plots of specific conductivity versus C for (a) C10OhpNC8, (b) C12OhpNC8, (c) C14OhpNC8, (d) C16OhpNC8, and (e) C10OhpNC14 aqueous solutions at 25.0 °C and pressure 0.1 MPa.

the existence of premicellar aggregates, the cmc obtained by conductivity is 2−8 times larger than that obtained by surface tension, which is consistent with the present results for the cases of C16OhpNC8 and C10OhpNC14. Besides, for the case of C16OhpNC8 or C10OhpNC14, the κ vs C plot shows a small upward curvature over the range of low concentration, whereas the molar conductivity Λ vs C1/2 plot exhibits a pronounced maximum (Figure 8), which is indicative of the formation of premicellar.39,52,53 However, similar phenomena do not appear in other three cases. This result again suggests the formation of premicellar aggregates in the cases of C16OhpNC8 and C10OhpNC14. 3.3. Discussion about the Premicellization. In general, premicellar aggregation only occurs in the cases of the sufficiently hydrophobic surfactants. For the quaternary ammonium type gemini surfactants, the spacer has been demonstrated to have significant influence on the premicellar association.42,52 For instance, for alkanediyl-α,ω-bis (alkyldimethylammonium bromide) series that are referred to as m−s− m, where m and s are, respectively, the carbon numbers of the alkyl tail and of the spacer group, the premicellar association occurred for 12−s−12 dimers with s ≥ 12 and for m−8−m demers with m ≥ 14, respectively.52 When p-xylylene or monohydroxypropyl was used as the spacer, premicellar association was present for m−xylyl−m or m−hop−m dimers with m ≥ 14.46,52 In brief, for the symmetric gemini surfactants, premicellar association occurred in those surfactants containing the tails long enough (the total carbon numbers ≥28) when the

spacer was relative short, or containing quite long spacer chain but the tails were slightly short (the total carbon numbers ≥24). For dissymmetric gemini surfactants, the requirement for premicellar association seems to be slightly reduced. Yoshimura et al.54 reported the premicellar aggregation for zwitterionic heterogemini surfactants containing ammonium and carboxylate head-groups and two tails respectively with 12 carbon atoms. In this case, the spacer, though it was complicated in structure, was considerably shorter than the length of 12 methylene groups as in 12−12−12 where the premicellar aggregation occurred.47 Obviously, the attraction between the oppositely charged headgroups was an important factor, leading to their strong aggregation and thus the remarkable premicellization. In the present cases, both C16OhpNC8 and C10OhpNC14 have also total carbon atoms of 24 in their two tails and a quite short spacer. We believe the hydroxyl group plays a key role in their premicellization. As discussed in above section, the hydroxyl inserts between the quaternary ammonium headgroups in aggregate and benefits to reduce the electrostatic repulsion, which strengthens the effect of hydrogen bonding accordingly. The hydrogen bonding formed by hydroxyl groups was favorable for premicellization.55 Summarily, when the spacer is short, the heterogemini-type structure of amphiphile composed of both an ionic surfactant and a nonionic one (containing a hydroxyl headgroup) tend to aggregate in the solution due to weakening in the electrostatic repulsion between the ionic headgroups, which is contribute to enhance premicellization process induced by hydrogen 2919



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C8TABr or C12TABr,49 which results in large apparent area per molecule at the micelle surface and thus promotes the dissociation of the counterions as mentioned by Zana.60 On the other hand, only one ionic headgroup of CmOhpNCn may be a cause producing large α in comparison with 12−s−12. More importantly, the presence of hydrogen bonding in CmOhpNCn system could weaken the electrostatic repulsions between the charged headgroups in micelles (screening effects). As a result, a larger α is observed. For 12−s−12 without significant hydrogen bonding, its two quaternary ammonium headgroups are pulled close by a short spacer (s = 2−6) and concentrated head charges restrict the dissociation of the counterions. Such a large ionization degree of CmOhpNCn micelle is undoubtedly sensitive to the salt added in the solution. Indeed, we have observed a strong influence of added NaBr on the aggregation of CmOhpNCn in the solution.62 3.6. Effectiveness and Efficiency in Surface Tension Reduction. From Figure 2, one can obtain the minimum surface tension (γcmc) at the cmc and the concentration (C20) required to reduce 20 mN·m−1 surface tension of water. The data are listed in Table 1. These parameters are generally used to characterize the effectiveness and the efficiency in surface tension reduction, respectively.63 Similar to the discussion in section 3.1, we also compare the above active parameters with those of the corresponding monomers in order to realize the role of the dissymmetric molecular structure of CmOhpNCn in the adsorption. However, the data of C20 were very lack in the literature in particular requiring their molecular structures similar to the expected monomers. Here, we only find the C20 of C12H25(OC2H4)2OH that may be the closest to C12(OC2H4)OH in molecular structure to be 5.0 × 10−6 mol·L−1 at 25 °C.63 We also estimate the C20 of C8TABr to be 0.34 mol·L−1 using surface tension measurement. The combination of C12H25(OC2H4)2OH and C8TABr is used as the comparison with C12OhpNC8. We find that the C20 of C12OhpNC8 lies between the ones of C12H25(OC2H4)2OH and C8TABr. This means the adsorption behavior of CmOhpNCn at the air/water interface may be similar to those of the equal molar mixing of the corresponding monomers. With increasing m from 10 to 16 at fixed n = 8, the C20 continuously decreases, which can be attributed to the increase in the hydrophobicity of CmOhpNCn molecule. Although gemini surfactant strongly reduces the C20, it cannot generally enhance the effectiveness in surface tension reduction as concluded by Rosen.64 The data of γcmc shown in Table 1 are consistent with this conclusion, where the γcmc does not exhibit a value considerably lower than those of 12−s−12.65 3.7. Surface Excess and Minimum Adsorption Area. In the cases of CmOhpNCn (m, n = 10, 8; 12, 8; 14, 8), no inflection point appears in the γ versus logC curve before the cmc. Thus, the maximum surface excess of surfactant Γmax at the cmc can be calculated by the Gibbs formula

bonding. This will guide us to design and synthesize new surfactants exhibiting strong aggregation in the solution. 3.4. Aggregation Number of the Micelle. As a representative example of CmOhpNCn, Figure 9 shows the

Figure 9. Fluorescence decay of pyrene (6.4 × 10−6 mol·L−1) in C12OhpNC8 (4 mmol·L−1) micellar solution with 1.6 × 10−4 mmol· L−1 CPC (□) and without quencher (■) at 25.0 °C and pressure 0.1 MPa.

fluorescence decay plots of pyrene in C12OhpNC8 micellar solutions with and without quencher at 25 °C. Fitting the curves by eq 1 shown in experimental section, one could obtain ⟨n⟩ and the aggregation number N by eq 2.34 It is well known that gemini surfactant generally forms relatively small micelle at the cmc but the micelle gradually grows with further increasing the concentration.56 For instance, Hattori et al.57 concluded the variation of the N with the C for a series of 12−s−12 and found the N values were between 15 and 25 by extrapolating the C to the cmc. This agrees well with the present result of N as shown in Table 1. For C16OhpNC8 and C10OhpNC14, though the chosen concentrations for determining the aggregation number are about 2 orders of magnitude larger than the cmcs, the Ns are still small (Table 1) compared with those of conventional ionic surfactants, for example, 64 for SDS and 55 for C12TABr.49 Considering the aggregate growth with increasing the surfactant concentration due to the presence of the intermolecular hydrogen bonding as our observed,33 the N in the region of the concentration near the cmc should be small. Small N implies an uncompleted core−shell structure of the aggregate probably due to the formation of premicelles and thus a relatively high polarity of environment for solubilizing pyrene.58 This may also be the reason that in these two cases, the I1/I3 ∼ logC curves do not reach plateau at the concentration of surfactant above cmc (see Figure 6d and 6e). 3.5. Ionization Degree of the Micelle. In general, the specific conductivity κ vs C plot (Figure 7) can be used to estimate the ionization degree of micelle (α) by the ratio of the slopes of the two straight lines above and below the cmc (S2 and S1), that is, α = S2/S1.59 The data shown in Table 1, except C10OhpNC8, are considerably larger than that of C8TABr (0.36) or C12TABr (dodecyltrimethylammonium bromide, 0.23)60 and also than that of 12−s−12 (0.2−0.35 at s = 2− 6).41 In fact, Nyuta and his colleagues even obtained larger values of α (0.83−0.99) for similar heterogemini surfactants containing a quaternary ammonium salt and a sugar moiety by the same estimating method.61 As seen in the above section, the aggregation number of CmOhpNCn micelle is quite small over the range of examined surfactant concentration compared with

Γ=−

dγ 1 2.303nRT d(log C)

(3)

where n is 2 owing to CmOhpNCn having only an ionic headgroup associated with one counterion. From the Γmax, the minimum area per molecule at the interface Amin is calculated by following formula A min = 2920

1 NA Γmax

(4) DOI: 10.1021/acs.jced.5b00778 J. Chem. Eng. Data 2016, 61, 2915−2922


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Figure 10. Molecular packings of 12−2−12 and CmOhpNC8 affected by surfactant structure.

where NA is the Avogardo constant. The data are listed in Table 1. With increasing m from 10 to 14, the Amin gradually decreases. This implies the molecules are tightly packed together at the interface owing to enhancing the hydrophobic interaction between the alkyl tails. These data are comparable with those of 12−s−12 (s = 3−6), which are estimated to be 0.70 nm2 (s = 3), 0.77 nm2 (s = 4), and 0.95 nm2 (s = 6), respectively, using 3 as the n in the eq 3,65 but larger than that of 12−2−12 (0.52 nm2)65 and of C12TABr (0.52 nm 2) 66 measured by surface tension technique. CmOhpNC8 has two alkyl tails and therefore the large area occupied at the interface is comprehensible in comparison with the surfactant having one alkyl chain. Compared with 12−2− 12, CmOhpNC8 possesses stronger hydrophobicity because of its weaker hydrophilic headgroup and higher dissymmetric degree,33 and a smaller Amin (Figure 10) can be observed theoretically. Such an opposite result, however, suggests there are some factors affecting the molecular packing of CmOhpNC8. The structure of CmOhpNC8 is similar to that of 12−2−12 except for the hydroxyl group and the ether oxygen atom that can promote the formation of hydrogen bonding between molecules. Consequently, the hydrogen bonding between the hydroxyl groups, the Br− counterions,30 ether oxygen atom,67 and water molecules44 leads to the folding of the alkoxy chain at the carbon atom, thus increasing the Amin. Obviously, hydrogen bonding not only dominates the molecular arrangement in aggregates but also affects the molecular packing at air/water interface (Figure 10).

short. Furthermore, the hydroxyl headgroup also disturbs the structure of the aggregates of surfactant CmOhpNCn in aqueous solution. Although the cmc values of CmOhpNCn are much lower than those of conventional gemini surfactants, the CmOhpNCn molecules seem to be loosely packed at the interface. This paradox can be ascribed to the hydrogen bonding between the hydroxyl groups and the Br− counterions, ether oxygen atom, and water molecules, which not only decreases the cmc values but also disturbs the tight molecular packing. For the goal of constructing short alkyl chain gemini surfactants with excellent micellization performances, it is important to introduce hydroxyl group or weaken electrostatic repulsion between the ionic heads. This work paves a new clue for designing new types of surfactants with interesting properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00778. Synthesis of CmOhpNCn, sample description table, semilogarithmic data of surface tension against the molality, I1/I3 and IE/IM versus log(molality), specific conductivity versus the molality, and additional references. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

4. CONCLUSIONS The heterogemini surfactants, CmOhpNCn, exhibit very low cmc and remarkable premicellization for those with the total carbon atoms of 24 in the two alkyl chains, that is, C16OhpNC8 or C10OhpNC14, showing the superiority in aggregation in the solution. However, for conventional gemini surfactants containing the same total carbon atoms, no premicellization behavior can be observed because their alkyl chains are not long enough. These special micellization properties are attributed to the unique headgroup. The existence of hydroxyl group in CmOhpNCn molecule weakens the intermolecular electrostatic repulsion and contributes to the formation of hydrogen bonding, which results in strong aggregation ability and large ionization degree even though the alkyl chains are relatively

*E-mail: [email protected]. *E-mail: [email protected]. Funding

Support from the National Natural Science Foundation of China (20673021 and 20873024) and the Foundation of MOE of China (20050386007) is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly thank Prof. Rui-Qing Sun for the photoluminescence experiments and thank Prof. Bing-Lei Song from Jiangnan University for discussion. 2921



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