Liyan Wangab,
Yue Zhanga,
Limin Dingc,
Jia Liua,
Bing Zhaoa,
Qigang Deng*a and
Tie Yan*b
aCollege of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar, 161006, P. R. China. E-mail: wlydlm@126.com
bKey Laboratory of Enhanced Oil & Gas Recovery, Ministry of Education, Northeast Petroleum University, Daqing, 163318, P. R. China
cCadre Ward, Qiqihaer First Hospital, Qiqihar 161005, P. R. China
First published on 20th August 2015
A series of novel gemini surfactants, namely, phenyl-1,4-bis[(carbamoylmethyl) N,N-dimethylalkyl ammonium chloride)] (a, b and c), was synthesized systematically and characterized by FTIR, 1H NMR, 13C NMR and MS. The surface and bulk properties were evaluated by surface tension, conductivity, viscosity, dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. These surfactants have been found to have low surface tension (γCMC) values as compared to other categories of gemini cationic surfactants, and they form vesicles in solutions at low concentrations. The DLS and TEM studies showed that the aggregations of the above mentioned surfactants changed from larger globular vesicles and smaller globular micelles to network aggregates and then to globular vesicles with an increase in the surfactant concentration. It is assumed that this unusual aggregation behavior is related to the transformation of the molecular conformation of phenyl-1,4-bis(carbamoylmethyl) spacer, along with its rigidity and hydrogen-bonding capability. The thermodynamic parameters of micellization process, namely, standard Gibbs free energy (ΔG0m), enthalpy (ΔH0m) and entropy (ΔS0m), were derived from conductivity measurements at different temperatures. The Krafft points of three gemini surfactants are very low.
The chemical structures of gemini surfactants play an important role in their aggregation behavior.11,12 In particular, the nature of the spacer group (length, flexibility, and polarity) has been shown to be of the utmost importance in determining the solution properties of aqueous gemini surfactants.13,14 These effects lead to the synergism of two alkyl tails of a gemini molecule, change in the charge density of its headgroup, and variation of its molecular geometry due to which rich structures and morphologies of aggregates are obtained. In addition, the flexible spacer influences the above mentioned functions of the gemini mainly depending on its length.6,15 A very long flexible spacer can bend toward the alkyl tails to meet the chemical environment around the molecule, due to which the molecule self-assembly is influenced. The effect of the short rigid spacer is almost identical to that of the flexible spacer having a similar length.16 However, the long rigid spacer yields considerably different effects compared to the flexible spacer owing to the two alkyl tails being inhibited from being too close, which leads to a column-like molecular shape and identical probability for the cis/trans configuration of the two alkyl tails around the spacer.17,18 These factors change the aggregate morphology of the gemini surfactant with long rigid spacer, when the gemini surfactant concentration is increased or few additives are added.19,20 The chemical modification for the spacer is also discussed, which is expected to promote molecular self-assembly, or give some new functions to the aggregates.21–24
The most widely studied gemini surfactants are cationic alkanediyl-α,ω-bis(alkyldimethyl-ammonium) dibromides, which are referred to as CmCsCm, where m and s stand for the carbon atom number in the tail alkyl chain and in the methylene spacer, respectively. Previous studies have shown that the variation of the spacer and the tail alkyl chain usually affects the aggregation behavior of CmCsCm,2,25 and CmCsCm series (4 ≤ s ≤ 12, 12 ≤ m ≤ 16) tends to form higher-curvature aggregates in aqueous solutions such as spherical or elongated micelles.26,27 Therefore, to design and synthesize a series of new gemini surfactants with phenyl-1,4-bis(carbamoylmethyl) as spacer (the phenyl possesses rigidity and N–H can easily form a hydrogen bond) tend to form larger aggregates (vesicles and network aggregations) than micelles in aqueous solutions. Their properties such as surface activity and self-aggregation behavior were investigated, which would contribute to extend their potential application such as templates for the synthesis of nanoparticles, nanorods, and mesoporous materials.
In this study, gemini surfactants phenyl-1,4-bis[(carbamoylmethyl) N,N-dimethylalkyl ammonium chloride)] (a, b and c) have been synthesized. Their surface activity, thermodynamic property and aggregation behavior were evaluated by surface tension, electrical conductivity, transmission electron microscopy (TEM) and dynamic light scattering (DLS) techniques.
Gemini surfactants | a | b | c |
---|---|---|---|
Krafft point (°C) | <5 | <5 | <5 |
A plot of surface tension versus logC is often linear up to a concentration called the critical micelle concentration (CMC), at which the surfactant begins to aggregate into a micelle. From Fig. 1, it can be observed that with increasing hydrophobic chain length, the CMC value gradually decreased because of the enhanced hydrophobic interaction between the longer alkyl chains (n = 12, 14 and 16). The surface parameters, i.e., CMC, the surface tension attained at the CMC (γCMC), the effectiveness of surface tension reduction at CMC (πCMC), the maximum surface excess concentration (Γmax), the area per molecule at the interface (Amin), and the surfactant concentration required to reduce the surface tension of the solvent by 20 mN m−1 (C20) of three gemini surfactants are given in Table 2. The packing densities of surfactants at the air–water interface are very important to interpret the surface activities of various surfactants.14 The maximum surface excess concentration (Γmax)30 is defined as the maximum concentration of surfactant molecules at the interface of their solutions in the saturation state. It can be calculated by eqn (1).
![]() | (1) |
Surfactants | CMC (mol L−1) | γCMC (mN m−1) | πCMC (mN m−1) | Γmax (μmol m−2) | Amin (nm2) | C20 (mol L−1) | CMC/C20 | pC20 |
---|---|---|---|---|---|---|---|---|
a BAC-12: dodecyl dimethyl benzyl ammonium chloride, BAC-14: tetradecyl dimethyl benzyl ammonium chloride, BAC-16: hexadecyl dimethyl benzyl ammonium chloride. The data of corresponding single chain surfactant (BAC) reported in ref. 31. | ||||||||
a | 6.37 × 10−4 | 38.97 | 33.03 | 0.93 | 1.78 | 1.43 × 10−4 | 4.45 | 3.84 |
b | 1.38 × 10−4 | 40.70 | 31.30 | 0.82 | 2.03 | 2.94 × 10−5 | 4.69 | 4.53 |
c | 4.58 × 10−5 | 41.31 | 30.69 | 0.65 | 2.56 | 6.46 × 10−6 | 7.09 | 5.19 |
BAC-12a | 9.10 × 10−3 | 38.0 | 1.64 | 1.01 | 2.850 | |||
BAC-14a | 1.90 × 10−3 | 39.2 | 1.59 | 1.05 | 3.368 | |||
BAC-16a | 4.00 × 10−4 | 39.6 | 1.46 | 1.14 | 4.134 |
The minimum area occupied per surfactant molecule (Amin) at the air–water interface is related to the maximum surface excess concentration (Γmax), as shown in eqn (2).
![]() | (2) |
Table 2 shows that these surfactants have a significantly lower critical micelle concentration (CMC) and a higher efficacy for reducing surface tension compared to other conventional surfactants.31 With increasing hydrocarbon chain length, the gemini surfactant molecules pack more loosely; thus, the γCMC of c compound is more than that of a or b compound.32 It is also quite clear that surface tension values of these surfactants (n = 12, 14, and 16) are lower than the corresponding gemini surfactants with large diethyl ammonium headgroups,14 which indicates that the gemini surfactants (a, b and c) have higher surface activity. The gemini surfactants (a, b and c) have two alkyl chains, dimethyl ammonium headgroups, and a spacer containing a –(C6H4)– and two amide groups. The γCMC values of Et series were slightly bigger than those of Me series; loose arrangement of molecules at the air/water interface induced by a bigger headgroup should account for this phenomenon.15 Close arrangement of gemini surfactants (a, b and c) molecules at the air/water interface, induced by a smaller headgroup and a rigid spacer, should account for the phenomenon of lower surface tension. The CMC values of the gemini surfactants (a, b, and c) are significantly lower than those of conventional surfactants. The lower CMC values of these gemini surfactants are largely attributed to the two alkyl chains and the long rigid spacer. The long rigid spacer of phenyl-1,4-bis(carbamoylmethyl) yields considerably different effects compared to the flexible spacer because two alkyl tails are inhibited from being close, which leads to a column-like molecular shape (Fig. 7a) and the identical probability for the cis/trans configuration of the two alkyl tails around the spacer (Fig. 8). The longer hydrophobic chain is more prone to bending at the air/water interface. These factors make the surfactant molecules to reach the saturated adsorption state easily at the interface. Thus, it can be seen that, for the gemini surfactants, the CMC values decrease and Amin values increase as the tail carbon number is increased from 12 to 16 (Table 2).
The gemini surfactants (a, b and c) have para-phenyl employed as the rigid spacer unit with dimethyl ammonium headgroups. Dimethyl ammonium headgroups are small. Thus, Amin values of the gemini surfactants (a, b and c) are smaller than those of cationic gemini surfactants with diethyl ammonium headgroups.14 Moreover, the longer hydrophobic chain is more prone to bending at the air/water interface, which leads to surfactant molecules occupying a larger area at the air/water interface.33 According to Table 2 data, Γmax decreased but Amin increased with the increase of the hydrocarbon chain length, which suggested that the gemini surfactant molecules (a, b and c) with longer hydrocarbon chains have lower packing densities at the air–water interface.
πCMC indicates the effectiveness of surface tension reduction, and it can be used to evaluate the effectiveness of surfactant to lower the surface tension of water.32 (Here, πCMC = γ0 − γCMC, where γ0 and γCMC are the surface tensions of water and the surfactant solution at the CMC, respectively.) Thus, the larger the value of πCMC is, the higher the effectiveness of the surfactant is. pC20 is obtained using eqn (3):
pC20 = −log![]() | (3) |
The values of pC20 and πCMC are listed in Table 2. The efficiency parameter indicates the adsorption behavior of surfactant molecules at the interface. The larger the value of pC20, the greater is the tendency of surfactants to be adsorbed.34 The variation of pC20 indicated that the pC20 increased with the increasing hydrocarbon chain length. However, the πCMC decreased with the increasing hydrocarbon chain length. The results show that the gemini surfactants (a, b and c) with longer hydrocarbon chain have lesser ability to reduce surface tension. The value of CMC/C20 ratio is used to determine structural factors in the adsorption and micellization process. The surfactant with a larger CMC/C20 ratio has a greater tendency to adsorb at the interface than to form micelles in solution. In Table 2, it can be seen that the surfactant with the increased hydrophobic chain length was more easily adsorbed at the air/water interface than self-assembling in solution. The result is in accordance with the conclusion from the value of pC20.
The chemical structures of the gemini surfactants play an important role for their surface activity. In particular, the nature of the spacer has been shown to be of the utmost importance in determining the properties of the aqueous solutions of gemini surfactants. Gemini surfactant molecules adopt different conformations, depending on the spacer (length and flexibility). Gemini surfactants (a, b and c) have unsaturated and conjugated rigid spacer. The gemini surfactants might assume a linear conformation, which would place only one of its hydrophobic tails into the air, while the other tail would be forced to dip entirely into the water (Fig. 2A). Alternatively, the horseshoe conformation would place both chains into the air (Fig. 2B), but they would be separated from one another by a distance equal to the length of the spacer. Finally, a gemini surfactant might prefer to lie flat on the water surface (Fig. 2C), an orientation that would occupy an extraordinarily large surface area per molecule. Amin values of the gemini surfactants (a, b and c) are very large (1.78–2.56 nm2), which correspond closely to 2.36 nm2.35 We thereby speculate that surfactant molecules are planar at the interface. The surfactant with a longer hydrocarbon chain has a lower surface concentration at the interface and a higher hydrophobic character. Therefore, hydrophobic chain length also has a great influence on the surface activity of the gemini surfactants (a, b and c). These results are in good agreement with the previously obtained surface activity parameters.
The CMC values were obtained from the intersection of two fitted lines in the conductivity region. It is observed that the CMC values of the gemini surfactants (a, b and c) increase with the increase in temperature (Fig. 3). The CMC values obtained by conductivity measurement are in good agreement with the surface tension derived data (Table 3). It is noteworthy that the CMC value of the gemini surfactants (a, b and c) obtained by this method is larger than that obtained by surface tension measurement (Table 2), which is due to the existence of non-surface-active premicellar aggregates in aqueous solutions of gemini surfactants. Similar results have been observed in another study.12
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Fig. 3 Variation in the electrical conductivity (κ) with the concentrations (C) of gemini surfactants (a, b and c) at different temperatures (25, 35, 45 and 55 °C). |
Surfactants | T (K) | CMC (mmol L−1) | β | ΔG0m (kJ mol−1) | ΔH0m (kJ mol−1) | TΔS0m (kJ mol−1) |
---|---|---|---|---|---|---|
a | 298 | 1.17 | 0.69 | −32.48 | −7.38 | 25.11 |
308 | 1.22 | 0.64 | −31.37 | −7.60 | 23.77 | |
318 | 1.35 | 0.63 | −31.76 | −8.02 | 23.74 | |
328 | 1.50 | 0.59 | −31.13 | −8.19 | 22.94 | |
b | 298 | 0.17 | 0.51 | −31.67 | −10.25 | 21.42 |
308 | 0.18 | 0.48 | −31.55 | −10.64 | 20.91 | |
318 | 0.21 | 0.46 | −31.57 | −11.10 | 20.47 | |
328 | 0.25 | 0.43 | −31.30 | −11.53 | 19.77 | |
c | 298 | 0.10 | 0.50 | −32.59 | −11.16 | 21.43 |
308 | 0.13 | 0.44 | −31.20 | −11.22 | 19.98 | |
318 | 0.20 | 0.39 | −29.73 | −11.36 | 18.37 | |
328 | 0.23 | 0.37 | −29.67 | −11.84 | 17.83 |
Two opposite factors operated with increasing temperature:36 (1) the ordered structure of water molecules around the hydrocarbon chain may be broken, which could promote the solubilization of the surfactant monomer, and thus disfavor micellization and (2) the degree of hydrophilic hydration around the polar headgroup may decrease, thus favoring micellization. For the gemini surfactants (a, b and c), the former factor is predominant in the micellization process in the studied temperature range.
In addition, the degree of counterion dissociation (α) was calculated from the ratio of the slopes above and below CMC. The degree of counterion binding (β) could be obtained by eqn (4).33 β values were obtained from conductivity curves and listed in Table 3.
β = 1 − α | (4) |
The decrease of β with the increase of temperature meant the decrease of charge density on the micelle surface, which may be caused by a reduction in aggregation number of the gemini surfactants (a, b and c) micelles.
The thermodynamic parameters for the micellization of the aqueous solutions of gemini surfactants (a, b and c) can be calculated by the phase separation model. The standard Gibbs free energy (ΔG0m), enthalpy (ΔH0m), and entropy (ΔS0m) of micellization at different temperatures were calculated using eqn (5)–(7).
ΔG0m = RT(0.5 + β)ln![]() | (5) |
![]() | (6) |
![]() | (7) |
The thermodynamic parameters, i.e., ΔG0m, ΔH0m and ΔS0m, of the gemini surfactants (a, b and c) at different temperatures are listed in Table 3.
It can be seen that ΔG0m for micellization in all the cases was negative and that the negative values decreased with the increase of temperature. The negative values of ΔG0m indicated that the micellization process was a thermodynamical spontaneous process. It can also be observed that ΔH0m was negative in all the cases and decreased with the increase of temperature. The negative values of ΔH0m indicated that micelle formation is an exothermic process. It is quite clear from Table 3 that the values for ΔH0m are smaller compared to −TΔS0m, thus the micellization process is entropy driven. This is true for all the gemini surfactants investigated in this study.
Fig. 4 shows the size distributions of the gemini surfactant b in solutions as observed at 5 times the CMC, 50 times the CMC and 100 times the CMC by DLS. Fig. 4 shows that the size distribution range is about from 23 nm to 400 nm at different times of the CMC.
Fig. 4b (black curve) shows that there is one obvious peak with an average r of about 400 nm at 50 times the CMC for the surfactant b, reflecting the occurrence of large aggregations. However, there are two peaks with an average r of about 45 nm and of about 200 nm of the surfactant b at 5 times the CMC (Fig. 4a, red curve). Usually, the relative intensity of the larger size distributions is big and the relative intensity of the smaller size distributions is small. This reflects that lots of large aggregations and a small amount of micelles coexist at 5 times the CMC. Fig. 4c (blue curve) shows that there is one peak with an average r of about 170 nm at 100 times the CMC, which is close to the big peak of Fig. 4a. Thus, large aggregations may exist at 100 times the CMC.
The abovementioned results show that the relative intensity of the large size distribution began to increase, and then decreased with the increase of the surfactant concentration (from 5 CMC to 100 CMC).
To attain a direct visualization of the abovementioned results, the morphologies of the gemini surfactant b at different concentrations were studied by TEM (Fig. 6).
It is well known that for a bilayer self-assembly, the hydrophobic chain modulates the phase behavior, whereas the headgroup determines the bilayer surface chemistry. Intermolecular H-bondings between N–H and CO in the amide group of the spacer are responsible for the formation of spherical bilayer vesicles in dilute aqueous solutions, which has also been reported earlier.24 In fact, similar H-bonding interaction has been reported for many vesicles-forming surfactants.37 The spacer of surfactants (a, b and c) that contains an amide group possesses hydrogen-bonding capability. A hydrogen atom with an oxygen atom of an amide group forms an intermolecular hydrogen bonding of the surfactant molecules (Fig. 5). These H-bonding interactions near the headgroup region are able to minimize the repulsive interactions among the cationic Me2N+ groups, thus ensuring higher stability to the bilayer membrane. Thus, spherical vesicles with diverse sizes were observed for the surfactant b at 5 times the CMC (Fig. 6a, the left arrow marks the small vesicle, while the right arrow marks the large vesicle), 50 times the CMC (Fig. 6c, the arrow mark), and 100 times the CMC (Fig. 6e, the arrow mark). Moreover, visibly smaller globule micelles were also present at 5 times the CMC (Fig. 6b, the arrow mark). We thereby have reached the conclusion that the vesicles and micelles coexist at low concentrations. The abovementioned results are in agreement with the DLS outcome.
The different spacers used will influence the molecule in a variety of ways. The distance between the charged nitrogen atoms will affect the charge density of the micelle and thus the degree of ionization. Thus, we assume that the formation of vesicles is also due to the chemical structure of the spacer of the gemini surfactant. Phenyl-1,4-bis(carbamoylmethyl) as a long rigid spacer constrains the distance between the headgroups, which inhibits the two alkyl tails from being close. Consequently, the surfactant molecules present a mainly column-like molecular configuration (Fig. 7a), which can form vesicles. In addition, a small amount of surfactant molecules also pack the hydrophobic tails tightly (Fig. 7b), which can form micelles at low concentration.12
The DLS and TEM studies also show that the vesicles (Fig. 6a) of various sizes and smaller micelles (Fig. 6b) transform into network aggregates (Fig. 6d) with increasing concentration of the gemini surfactant (from 5 CMC to 50 CMC). Vesicles can be cross-linked together (Fig. 6c, the arrow mark) by trans configuration (Fig. 8b) and then network aggregates could be formed at 50 times the CMC (Fig. 9). Usually, surfactants first form small aggregates at low concentrations, then the aggregates become larger with the increase of the surfactant concentration. In general, network aggregation should become larger at 100 times the CMC. However, the results obtained by this study are exactly opposite. Vesicles exist in lots of diverse sizes (Fig. 6e) rather than as larger aggregates at 100 times the CMC. This is an abnormal phenomenon. With the increase of the surfactant concentration (from 50 CMC to 100 CMC), trans configuration of alkyl tails (Fig. 8a) of surfactant molecules will transform into the cis configuration of alkyl tails (Fig. 8a). The cis configuration of alkyl tails causes the network aggregates to disappear and transform into vesicles.
We thereby have reached the conclusion that diverse sized vesicles may arise at low concentrations as well as at high concentrations.
To further study the aggregation behavior, relative viscosity (relative to the water) of the gemini surfactant b was measured at different concentrations (Fig. 10).
Fig. 10 shows that the relative viscosity increased with rapid growth from 5 times the CMC to 50 times the CMC and with very slow growth from 50 times the CMC to 100 times the CMC. At low concentrations, the adhesion and fusion of vesicles by the trans configuration of alkyl tails led to rapid growth of viscosity. The results agree with the TEM outcome.
The effect of salt (NaCl) on network aggregates was investigated at the same time. In combination with the DLS results, it can be concluded that network aggregates were destroyed and transformed into vesicles [size distributions (about 172 nm)] and micelles [size distributions (about 23 nm) (Fig. 4d, green curve)] in the aqueous solution of these gemini surfactants with the addition of 0.1 M NaCl. The TEM and DLS results showed that the network aggregations indeed disappeared at 50 times the CMC with the addition of 0.1 M NaCl (Fig. 6f). The results show that the network aggregates have a poor salt tolerance. In addition, adding NaCl increases the concentration of counterion (Cl−), which inhibits the ionization of vesicles. Thus, vesicles find it difficult to form network aggregates.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13616d |
This journal is © The Royal Society of Chemistry 2015 |