Aggregation Behavior of Fluorocarbon and Hydrocarbon Cationic

Jul 10, 2008 - The chemical shifts of proton Δδ (1H) for −CH3 in the mixed systems of DEFUMACl/CnTACl (n = 12, 14, 16, and 18) have different vari...
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J. Phys. Chem. B 2008, 112, 9371–9378

9371

Aggregation Behavior of Fluorocarbon and Hydrocarbon Cationic Surfactant Mixtures: A Study of 1H NMR and 19F NMR Shuli Dong,† Guiying Xu,†,* and Heinz Hoffmann‡ Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan 250100, P. R. China, and UniVersity of Bayreuth, Bayreuth Center for Colloids and Interfaces, D-95446 Bayreuth, Germany ReceiVed: February 11, 2008; ReVised Manuscript ReceiVed: May 7, 2008

The aggregation behavior and the interaction of four mixed systems for a cationic fluorocarbon surfactant, diethanolheptadecafluoro-2-undecanolmethylammonium chloride (DEFUMACl), mixing with cationic hydrocarbon surfactants, alkyltrimethylammonium chloride, CnTACl (n ) 12, 14, 16, and 18; where n ) 12 is DTACl, n ) 14 is TTACl, n ) 16 is CTACl, and n ) 18 is OTACl), were studied by 1H and 19F NMR in more detail. The results of 19F NMR measurements strongly indicate that in the three mixed systems of DEFUMACl/DTACl, DEFUMACl/TTACl, and DEFUMACl/CTACl at different molar fractions of fluorocarbon surfactant (RF ) (cDEFUMACl/cDEFUMACl + cCnTACl)), with an increase of the total concentration of fluorocarbon and hydrocarbon surfactants (cT ) cF + cH), the mixed micelles at the first break point and the individual DEFUMACl micelles at the second break point form. However, three different types of micelles were determined in DEFUMACl/OTACl mixtures by 19F NMR measurements, OTACl-rich and DEFUMAClrich mixed micelles and individual DEFUMACl micelles, respectively. The chemical shifts of proton ∆δ (1H) for -CH3 in the mixed systems of DEFUMACl/CnTACl (n ) 12, 14, 16, and 18) have different variation trends from the 19F NMR measurements. For the two systems of DEFUACl/DTACl and DEFUMACl/TTACl, the mixed micelles form at the first break point. At the second break point, for lower RF values the DTAClrich and TTACl-rich mixed micelles form with a strong downfield shift and for higher RF values DEFUMAClrich mixed micelles form with a strong upfield. For the other two systems of DEFUMACl/CTACl and DEFUMAC/OTACl, the chemical shifts of proton ∆δ (1H) of -CH3 increase with an increase of the total concentration of DEFUMACl/CTACl or OTACl, and mixed CH- and CF-surfactant micelles form. At higher total concentration, the greater effect of fluorinated chains of DEFUMACl on CH-chains was obvious, resulting in the strong upfield chemical shifts. In cationic fluorocarbon and hydrocarbon surfactant mixtures, the different kinds of micelles observed by 19F and 1H NMR measurements could be caused by the increase in alkyl chain length of hydrocarbon surfactants with different critical micelle concentrations. Combining two theoretical models for mixing, for the four different chain-length hydrocarbon surfactants studied, one can conclude that the two components of mixtures interact with each other and form mixed micelles in two completely different ways according to their molecular properties and cmc values in a certain range of total concentrations. One is close to an ideal mixing case with the formation of one type of mixed micelles, such as the DEFUMACl/ DTACl and DEFUMACl/TTACl systems. The other is a demixing case with the formation of two types of micelles, i.e., fluorocarbon-rich and hydrocarbon-rich mixed micelles, such as DEFUMACl/CTACl and DEFUMACl/OTACl systems. However, as the total concentrations of the mixed systems are high enough, the four systems tend to demix and to form individual micelles of corresponding components due to the initial respective interaction between fluorocarbon and hydrocarbon chains. That is to say, at high total concentration, the individual DEFUMACl micelles in all four systems could form. These results may be primarily directed toward acquiring an understanding of the mechanism of CF-CH mixtures in aqueous solution and secondarily directed toward providing more detailed information on nonideal mixing. Introduction Fluorocarbon and hydrocarbon (CF-CH) surfactant mixtures in aqueous solutions are fascinating topics in surfactant sciences,1 and the nonideal mixing of fluorocarbon and hydrocarbon surfactants in solutions has received much attention,2,3 which is not only because the CF-CH mixtures are wide-ranging applications but also because they provide * Corresponding author. Phone: +86-531-88365436. E-mail: xuguiying@ sdu.edu.cn. † Shandong University. ‡ University of Bayreuth.

a useful test for a variety of models. The deviation from ideal mixing in micelle formation is sorted according to either positive (repulsive) or negative (attractive) interaction. According to the natures of the mixed components, CF-CH mixed surfactants in aqueous solutions can self-assemble into either the mixed micelles or the coexistence of CF-rich and CH-rich micelles experimentally and theoretically.1,2 Mukerjee et al.4 investigated the mixed solutions of CF-CH surfactant mixtures by using electric conductivity measurements, in which they obtained the possibility of demixing into two different types of micelles, i.e., the existence of CHrich and CF-rich micelles in aqueous solutions. Micelles were

10.1021/jp801216e CCC: $40.75  2008 American Chemical Society Published on Web 07/10/2008

9372 J. Phys. Chem. B, Vol. 112, No. 31, 2008 theoretically predicted for the coexistence of two different types of micelles.5 Different techniques have been employed to experimentally verify the proposed microscopic demixing of CF-CH mixed surfactants in solutions. Haegel and Hoffman6 demonstrated by ultracentrifugation that both types of micelles are present in sodium perfluorooctanoate (SPFO)/tetradeyldimethylamineoxide (C14DMAO) aqueous solution. Asakawa et al.7 clearly separated by gel filtration two types of micelles in lithium perfluorooctanoate (LiPFO)/lithium tetraecysulfate (LiTES) mixtures. Subsequently, by using fluorescence measurements,8–10 NMR measurements including 1H and 19F data,11–15 and SANS measurements,16,17 etc., different types of micelles of CF-CH surfactant mixtures in solutions were obtained from different groups including our previous results.14,15 Conflicting results were noted to be obtained according to different techniques. Mukerjee et al.4 and Shinoda et al.18 reported the demixing conclusion from the mixed solutions of SPFO/decanoate or sodium dodecanoate. Caponetti et al. concluded that mixed micelles of SPFO/sodium dodecanoate mixtures are formed by SANS19,20 and NMR12 measurements, in which domains of fluorinated and hydrogenated chains form with an internal segregation of the fluorinated chains in the mixed micelles.12 Kadi et al.16 also suggested that an internal segregation of the fluorinated chains in the mixed micelles occurs in cetyltrimethylammonium chloride (CTACl)/cetylpyridium chloride (HFDePC) in addition to the demixing. Recently, cryo-transmission electron microscopy (cryo-TEM) observations on the equilibrium among bilayer cylinders, spheres, and discs in the mixtures of cetyltrimethylammonium bromide (CTABr) and sodium perfluorooctanoate (FC7) in aqueous solutions21 were reported, in which ionic strength is much higher because of the formation of NaBr. In salt-free cationic and anionic CF-CH surfactant mixtures in aqueous solutions, vesicles were also determined by cryo-TEM observations.22 Among the experimental techniques available for the study of fluorinated surfactants and their mixed systems with hydrocarbon surfactants, 19F NMR measurements offer the advantages of being able to observe independently the behavior of the fluorinated derivatives in the mixtures because of no overlapping of 1H signals of hydrocarbon components. Thus, 19F NMR measurements allow one to determine the concentration at which the fluorinated amphiphiles form micelles and give information on the composition of the micelles in CF-CH surfactant mixtures.11,12 Following our recent results15 of 19F NMR and surface tension measurements on the aggregation behavior of DEFUMACl mixed with DTACl in aqueous solutions, herein the totally comparable studies of cationic DEFUMACl mixed with cationic CH surfactants having different chain lengths, i.e., C12-, C14-, C16-, and C18-alkyltrimethylammonium chloride (DTACl, TTACl, CTACl, and OTACl), were performed in more detail by19F and 1H NMR measurements. The interaction, the behavior, and the structural information on these mixed CF-CH systems in aqueous solutions may be primarily directed toward acquiring a better understanding of the mechanism of CF-CH mixtures in aqueous solution and secondarily directed toward providing more detailed information on nonideal mixing.

Dong et al. described in detail elsewhere.15 The 19F NMR spectrum and chemical structure of DEFUMACl were shown in Figure 1 of our recent report,15 and the assignments of each group for DEFUMACl molecule have been indicated.15 Cationic hydrogenatedsurfactants,dodecyltrimethylammoniumchloride(DTACl), tetradecyltrimethylammonium chloride (TTACl), cetyltrimethylammonium chloride (CTACl), and octadecyltrimethylammonium chloride (OTACl) were purchased from Sigma Chemical Co., USA, and used without further purification. NMR Measurements. All the samples were dissolved in D2O (Aldrich product, g99.9%). 1H and 19F NMR spectra were recorded on a Bruker Avance 400 spectrometer equipped with pulse field gradient module (Z axis) using a 5 mm BBO probe. The 19F 1D spectra were reported in the range from +30 to -170 ppm (digitized points ) 32000, 90° pulse ) 7.4 ms, relaxation delay ) 2 s) operating at 376.72 MHz. The 19F NMR chemical shifts are reported relative to the external trifluorotulene at -62.73 ppm. All spectra reported in this work solely display the -CF3 terminal regions of the 19F NMR spectra. 1H NMR spectra were run at 400.13 MHz, and chemical shifts were referred to 4,4-dimethyl 4-silapenfane sodium sulfonate (DSS) as the external standard. All the experiments were operated at 25.0 ( 0.1 °C. Results and Discussion The physicochemical environment of a surfactant molecule surrounded by other surfactant molecules in self-assembled structures such as micelles is expected to be quite different from that of a free monomer surrounded by water molecules. The spectrum of DEFUMACl at T ) 25.0 ( 0.1 °C shows a set of sharp resonance signals observed in the 19F NMR spectrum,15 indicating that the exchange of DEFUMACl molecules between different states is invariably fast on the NMR time scale and reflecting the time-averaged environment of the cationic fluorocarbon DEFUMACl. The shifts of the signals can be explained by the variation of the local dielectric constant experienced by each monomer,15 which could be used to determine the important parameters such as critical micelle concentration (cmc) values of surfactants.23 The polarity of the CF- and CH-chains in CF-CH surfactant mixtures should be completely different from that of individual CF- or CH-surfactant in solutions. Consequently, in the mixtures consisting of CF-CH surfactants, if contacts occur between the two types of tails within mixed micelles, the chemical shift of the -CF3 group of CF-surfactants will be affected by the presence of the CH-tails. Thus, compared with the pure CFsurfactant micelles, the presence of the hydrocarbon moieties within the micelles induces an upfield shift of the signals.11,24–26 On the contrary, for the CH-surfactant micelles, a downfield chemical shift could be observed. Depending on the degree of compatibility or incompatibility between both surfactants, two cases must be distinguished: (i) the formation of single type of completely miscible micelles and (ii) the formation of two types of partially demixed micelles. In a mixed system consisting of CF- and CH-surfactants, because of the incompatibility between CH- and CF-tails, comicellization frequently induces either a negative or a positive deviation from the ideal case.27–29 The observed 19F NMR chemical shift at a given RF value can be expressed as11,12,15

Experimental Section Materials. Unless noted by exceptional circumstance, all reagents used in this study were of analytical grade. Cationic fluorocarbon surfactant, CF3(CF2)7CH2CH(OH)CH2N+(CH2CHOH)2(CH3)Cl- (DEFUMACl), was prepared by the same procedures

δobs )

cFmo (δ - δ′mic) + δ′mic cTRF mo

(1)

where cFmo represents the concentration of the CF-surfactant in the monomer state and δ′mic is the 19F NMR chemical shift of

Behavior of Cationic Surfactant Mixtures

J. Phys. Chem. B, Vol. 112, No. 31, 2008 9373

Figure 1. 19F and 1H chemical shifts for -CF3 of DEFUMACl15 and -CH3 of DTACl in the DEFUMACl and DTACl mixtures versus cT-1 values at different RF values. (a) 19F data:15 0.05 (2), 0.1 (1), 0.3 ([), 0.5 (left-pointing triangle), 0.7 (right-pointing triangle), and 1.0 (b). (b) 1H data: 0 (9), 0.05 (b), 0.1 (2), 0.3 (1), and 0.5 ([). T ) 25.0 ( 0.1 °C.

Figure 2. 19F and 1H chemical shifts for -CF3 of DEFUMACl and -CH3 of TTACl versus cT-1 in DEFUMACl and TTACl mixtures at different RF values. (a) 19F data: 0.1 (b), 0.2 (2), 0.3 (1), 0.5 ([), 0.7 (left-pointing triangle), and 1 (9). (b) 1H data: 0 (9), 0.1 (b), 0.2 (2), 0.3 (1), 0.5 ([), and 0.7 (left-pointing triangle). T ) 25.0 ( 0.1 °C.

the -CF3 group in mixed micelles. The 1H NMR chemical shift of the -CH3 group has the same expression as that of 19F data. DEFUMACl and DTACl Mixed System. The chemical shift differences of the fluorine atom ∆δ (19F) and proton ∆δ (1H) for -CF3 and -CH3 in the end of the chains of DEFUMACl and DTACl in D2O at different RF values are plotted in Figure 1 (parts a15 and b) as a function of cT-1. Figure 1 provides much richer information on the CF- and CH-surfactant mixtures in solutions. From Figure 1a, one can see that below the cmc, the 19F chemical shifts of -CF of DEFUMACl in DEFUMACl 3 and DTACl mixtures are constant and coincident with that observed in pure DEFUMACl in D2O (RF ) 1.0), indicating that fluorinated monomers are really not affected by the existence of the hydrocarbon partners. However, at different RF values, with an increase of total concentration (cT ) cF + cH), one can find in Figure 1a that two distinct break points were determined in ∆δ (19F) vs cT-1 curves. As a drop in chemical shift values generally reflects micelle formation, these two break points can be tentatively attributed to two cmc values, implying that DEFUMACl and DTACl mixed micelles formed at the first break point and the individual DEFUMACl micelles formed at the second break point, for which the detailed discussion has been presented in our previous paper.15 From Figure 1b, at different RF values, with an increase of cT one can also observe that below the cmc, the 1H chemical shifts of -CH3 in the end of DTACl are constant and coincident with that observed in pure DTACl in D2O (RF ) 0), indicating that hydrogenated monomers are also not affected by fluorocarbon surfactants, which is the same situation with the results of ∆δ (19F) for -CF3 observed in Figure 1a. However, above cmc, at the different RF values, with an increase of cT the ∆δ (1H) values, as shown in Figure 1b, do not follow the same

trends as those of the ∆δ (19F) data. Two completely opposite change trends were observed. At RF < 0.3, a strong downfield chemical shift was observed. One can find that the DEFUMACl and DTACl mixed micelles are formed at the first break point from 1H chemical shifts. At the second break point, DTAClrich mixed micelles tend to form because the DTACl composite is much higher than the DEFUMACl amount in solutions at lower RF. When the molar fraction RF is higher than 0.3, the mixed DEFUMACl and DTACl micelles form at the first break point and the DEFUMACl-rich mixed micelles can form at the second break point. The chemical environment variation as a consequence of the DEFUMACl composite and the micelles demixing induces the strong upfield shifts, indicating that the CH component DTACl molecules are greatly affected by the presence of the DEFUMACl-rich mixed micelles at higher molar fraction RF values. This kind of observation was rarely obtained from 1H NMR measurements in cationic CF- and CH-surfactant mixtures in solutions. DEFUMACl and TTACl Mixed System. The chemical shifts of fluorine atoms ∆δ (19F) for -CF3 and proton ∆δ (1H) for -CH3 in the end of the chains of DEFUMACl and TTACl in DEFUMACl and TTACl mixtures are collected in Figure 2 (parts a and b) as a function of cT-1, respectively. Completely similar to the observations of DEMUACl and DTACl mixtures in Figure 1a, with analysis by the ∆δ (19F) chemical shifts of -CF3 of DEFUMACl in DEFUMACl/TTACl mixtures, two kinds of micelles can be monitored, i.e., DEFUMACl and TTACl mixed micelles formed at the first break point and the individual DEFUMACl micelles at the second break point. As for the chemical shifts of proton ∆δ (1H) for -CH3 of TTACl in DEFUMACl and TTACl mixtures, as shown in Figure

9374 J. Phys. Chem. B, Vol. 112, No. 31, 2008

Figure 3. F NMR chemical shifts of -CF3 of DEFUMAC versus cT-1 in DEFUMAC and CTACl mixtures at different RF values: 0.1 (b), 0.3 (2), 0.5 (1), and 1 (9). T ) 25.0 ( 0.1 °C. 19

Figure 4. 1H NMR chemical shifts of -CH3 of CTACl versus cT-1 in DEFUMAC/CTACl mixtures at different RF values: 0.1 (b), 0.3 (2), 0.5 (1), and 1 (9). T ) 25.0 ( 0.1 °C.

2b, at lower RF values the downfield shifts were noticeable, which is similar to the observations in Figure 1b. In these cases, the mixed DEFUMACl and TTACl micelles form at the first break point and then the TTACl-rich mixed micelles form at the second break point. When the molar fraction RF reaches a value of greater than 0.5, an obvious upfield shift can be induced by the DEFUMACl-rich mixed micelles formed at the second break point and the hydrocarbon chains of CH-surfactants are greatly affected by the presence of DEFUMACl, exhibiting an obvious change from downfield to upfield shifts. DEFUMACl and CTACl Mixed System. Figure 3 shows the chemical shifts of fluorine atoms ∆δ (19F) for -CF3 in the end of the chains of DEFUMACl in DEFUMACl and CTACl mixtures. The 19F NMR data in Figure 3 clearly show that the chemical shifts of fluorine atoms ∆δ (19F) exhibit a change trend similar to that in the mixed systems of DEFUMACl/DTACl and DEFUMACl/TTACl; i.e., with an increase of cT, two distinct break points were observed. At different RF values, with an increase of cT, one can clearly observe that mixed micelles of DEFUMACl and CTACl form at the first break point and the individual DEFUMACl micelles form at the second break point. Below cmc the chemical shifts for -CF3 in the end of the chains of DEFUMACl in DEFUMACl and CTACl mixtures are constant and coincident with the value of pure DEFUMACl in D2O (RF ) 1.0). The chemical shifts of the proton ∆δ (1H) for -CH3 in the end of the chains of CTACl in DEFUMACl/CTACl mixtures are shown in Figure 4. The change of chemical shifts from downfield to upfield observed in Figures 1b and 2b was not observed. Above the mixed cmcH-F values of DEFUMACl and CTACl, with an increase of the total surfactant concentration

Dong et al.

Figure 5. 19F chemical shifts of -CF3 of DEFUMACl versus cT-1 in DEFUMAC/OTACl mixtures at different RF values: 0.1 (b), 0.2 (2), 0.3 (1), 0.5 ([), and 1 (9). T ) 25.0 ( 0.1 °C.

(cT), the ∆δ (1H) values increase linearly. Only one break point was observed, meaning that only one type of mixed micelle forms in DEFUMACl and CTACl mixtures. The obvious difference in 1H NMR data between the DEFUMACl/CTACl system and those systems of DEFUMACl/DTACl and DEFUMACl/TTACl could be caused by the large difference in cmc values between CTACl and DEFUMACl; namely, the cmc value of CTACl (1.16 mmol · L-1) being only one-third the cmc value of DEFUMACl (3.40 mmol · L-1) makes sense. In this case, the individual CTACl micelles seem to form prior to the formation of the DEFUMACl and CTACl mixed micelles. However, in our 1H measurements, no such responding break point was identified. The mixed micelles could be CTACl-rich in composition. A distinct break point was observed with a continuous increase of cT; as shown in Figure 4, the δ (1H) chemical shifts follow an upfield trend, indicating the large effect of CF-chains of DEFUMACl on CH-chains of CTACl in mixtures. DEFUMACl and OTACl Mixed System. Figure 5 shows the chemical shifts of fluorine atoms ∆δ (19F) for -CF3 in the end of the chains for DEFUMACl in DEFUMACl and OTACl mixtures, clearly demonstrating trends quite different from those of the other systems of DEFUMACl/DTACl, DEFUMACl/ TTACl, and DEFUMACl/CTACl. Three break points were clearly noticeable on the 19F chemical shifts of -CF3 of DEFUMACl versus cT-1 curves in DEFUMACl and OTACl mixtures, indicating that three different types of micelles formed, OTACl-rich mixed micelles, DEFUMACl-rich mixed micelles, and individual DEFUMACl micelles. Now, we encounter the question of why there are three different micelles formed in DEFUMACl and OTACl mixtures? Much greater differences between the cmc values of DEFUMACl and OTACl might make a great contribution toward the explanation of this problem. Two instances of the CF-surfactants and CH-surfactants studied in this work, for the typical chemical shifts ∆δ as a function of cT-1 curves, are shown in Figure 6. From the break points of the curves, the cmc value of OTACl with cmcOTACl ) 0.30 mmol · L-1 was obtained, which is much lower than (almost 1/10th) that of DEFUMACl with cmcDEFUMACl ) 3.40 mmol · L-1. With an increase of cT, it can be safely postulated that the mixed DEFUMACl and OTACl micelles comprised of predominating OTACl but a minor amount of DEFUMACl form first; i.e., hydrocarbon-rich mixed micelles at the first break point can be identified. Compared with the pure CF-surfactant micelles, the microenvironment of the F3C-groups within the CH-rich micelles is less apolar; hence, one can easily understand why only a small decrease in ∆δ (19F) is observed after the first break point. The second inflection point is identified with the CF-rich surfactant

Behavior of Cationic Surfactant Mixtures

Figure 6. Relative 19F and 1H chemical shifts (∆δ, ppm) of -CF3 and -CH3 groups of DEFUMACl and OTACl vs the reciprocal of DEFUMACl and OTACl concentrations, respectively. T ) 25.0 ( 0.1 °C.

Figure 7. 1H NMR chemical shifts of -CH3 of OTACl versus cT-1 in DEFUMACl and OTACl mixtures at different RF values: 0 (9), 0.1 (b), 0.2 (2), 0.3 (1), and 0.5 ([). T ) 25.0 ( 0.1 °C.

mixed cmc; the DEFUMACl predominated in the mixed micelles with a minor amount of OTACl; i.e., fluorocarbonrich mixed micelles should form. With a further increase of the total concentration, the third break point should occur, indicating that the individual DEFUMACl micelles form. On the other hand, the cmc values of other three CHsurfactants, DTACl, TTACl, and CTACl, determined by 1H NMR, are 22.48, 4.68, and 1.16 mmol · L-1, respectively (the 1H chemical shifts vs c -1 curves in the present report are not T shown), much higher than or close to the cmc value of DEFUMACl. This fact may explain the difference in aggregation behavior of DEFUMACl/CnTCl mixed systems (n ) 12, 14, and 16) and the DEFUMACl/OTACl mixtures, while only two break points were observed in the DEFUMACl/DTACl, DEFUMACl/TTACl, and DEFUMACl/CTACl mixtures. The 1H NMR chemical shifts of proton ∆δ (1H) for -CH3 in the end of the chains of OTACl in the mixtures of DEFUMACl and OTACl are shown in Figure 7, clearly demonstrating the same variation trend of the DEFUMACl and CTACl mixtures but different from those of the DEFUMACl/ DTACl and DEFUMACl/TTACl systems. Above the mixed cmc value, with an increase of the cT, the ∆δ (1H) values increase. At higher cT, a distinct break point of the ∆δ (1H) values was observed, as lined out in Figure 7, the δ (1H) chemical shifts follow an upfield shift. The phenomenon has been observed in DEFUMACl/CTACl system, as shown in Figure 4, which could clearly be contributed to the larger effect of fluorinated chains of DEFUMACl on CH-chains of OTACl.

J. Phys. Chem. B, Vol. 112, No. 31, 2008 9375 To sum up, from the results of the 19F and 1H NMR measurements for the four systems of DEFUMACl/DTACl, DEFUMACl/TTACl, DEFUMACl/CTACl, and DEFUMACl/ OTACl mixtures, several important conclusions can be obtained: (i) The chemical shifts of fluorine atoms ∆δ (19F) for -CF3 in the mixed systems of DEFUMACl/CnTACl (n ) 12, 14, 16) have the same variation trend. Two distinct break points of the ∆δ (19F) vs cT-1 curves give the mixed micelles of DEFUMACl and CnTACl (n ) 12, 14, 16) at the first break point and the individual DEFUMACl micelles at the second break point. (ii) The chemical shifts of fluorine atoms ∆δ (19F) for -CF3 in the mixed systems of DEFUMACl/OTACl show three different types of micelles, hydrocarbon-rich mixed micelles, fluorocarbonrich mixed micelles, and individual DEFUMACl micelles, respectively. (iii) The chemical shifts of proton ∆δ (1H) for -CH3 in the two systems of DEFUMACl/DTACl and DEFUMACl/TTACl have the same variation trend. The mixed DEFUMACl/DTACl and DEFUMACl/TTACl micelles formed at the first break point. At the second break point, for the mixtures of lower RF values, the DTACl-rich and TTACl-rich mixed micelles form with a strong downfield shift; however, for the mixtures with higher RF values, DEFUMACl-rich mixed micelles form with a strong upfield shift. (iv) For the two systems of DEFUMACl/CTACl and DEFUMAC/OTACl, completely different change trends in ∆δ (1H) vs cT-1 curves from those of DEFUMACl/DTACl and DEFUMACl/TTACl mixtures were observed. The chemical shifts of proton ∆δ (1H) of -CH3 increase with an increase of the total concentration of DEFUMACl and CTACl or OTACl. At higher total concentration, the greater effect of fluorinated chains of DEFUMACl on CHchains was obvious, resulting in the strong upfield chemical shifts. To our knowledge, the aggregation behavior and the interaction of cationic fluorocarbon and hydrocarbon surfactant mixtures in aqueous solutions investigated by 1H and 19F NMR measurements in more detail has rarely been reported.15 Two break points that occurred in nonionic CF-surfactant and CHsurfactant mixtures were observed by Barthe´le´my et al.11 It should be noted that such peculiar curve shapes of DEFUMACl and OTACl mixtures were not previously observed when studying other fluorocarbon and hydrocarbon surfactants mixed systems by the 19F NMR method, although the presence of two types of micelles was also recognized in the literature.11 To identify the compatibility and the difference of the four mixed systems for fluorocarbon surfactants, DEFUMACl, and hydrocarbon surfactants, DTACl, TTACl, CTACl, and OTACl mixtures, two assumptions on the types of micelles formed in CF- and CH-surfactant mixed solutions were employed. One assumption is that one type of mixed micelles formed for ideal mixing. cmcF-H values of the mixed systems could be calculated by employing the relation between cmcF-H of the mixture and the cmci value of single surfactant (i ) CF or CH-surfactant) for ideal mixtures. With this supposition, the binding constants of counterions are the same for monomers and micelles30

1 1+βF-H cmcF-H

)

RF cmcF1+βF

+

RH cmcH1+βH

(2)

where cmcF-H represents the cmc value of the mixed micelles and cmcF and cmcH represent the cmc values of the CFsurfactant and CH-surfactant, respectively. βF-H is the binding constant of counterions of mixed micelles, and βF and βH are the binding constants of counterions of single surfactant micelles, respectively. RH is the mole fraction of hydrocarbon surfactant in solution which is defined as RH ) (cCH/cCF + cCH).

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Dong et al.

TABLE 1: cmcF-H Values of the DEFUMACl/DTACl System at Different rF Valuesa cmcF-H (mmol · L-1) (nonideal mixing) DEFUMACl

a

cmcF-H (mmol · L-1) (ideal mixing)

DTACl

DEFUMACl/DTACl

RF

calculated

experimental

calculated

experimental

calculated

0 0.1 0.3 0.5 0.7 1.0

12.97 6.85 5.09 4.18

10.31 6.62 5.05 3.89 3.40

22.03 25.64 31.40 41.71

22.48 20.16 18.69 14.79 13.83

10.93 6.49 4.94 4.03

T ) 25.0 ( 0.1°C.

TABLE 2: cmcF-H Values of the DEFUMAC/TTACl System at Different rF Valuesa cmcF-H (mmol · L-1) (nonideal mixing) DEFUMACl

a

cmcF-H (mmol · L-1) (ideal mixing)

TTACl

DEFUMACl/TTACl

RF

calculated

experimental

calculated

experimental

calculated

0 0.1 0.2 0.3 0.5 0.7 1.0

12.97 8.67 6.85 5.09 4.18

4.98 4.71 4.41 4.06 3.78 3.40

4.98 5.35 5.80 7.10 9.67

4.68 4.68 4.65 4.67 4.51 4.45

4.37 4.23 4.11 3.89 3.69

T ) 25.0 ( 0.1 °C.

TABLE 3: cmcF-H Values of DEFUMACl/CTACl System at Different rF Valuesa cmcF-H (mmol · L-1) (nonideal mixing) DEFUMACl RF 0 0.1 0.3 0.5 1.0 a

calculated 12.97 6.85 5.09

cmcF-H (mmol · L-1) (ideal mixing)

CTACl

experimental 4.24 4.05 3.67 3.40

DEFUMACl/CTACl

calculated

experimental

calculated

1.24 1.44 1.76

1.16 1.14 1.28 1.67

1.22 1.37 1.60

T ) 25.0 ( 0.1°C.

The other assumption is for the formation of two types of micelles, i.e., hydrogen-rich and fluorocarbon-rich micelles, for nonideal demixing cases, which can be expressed as27

cmcF-H )

cmci0 Ri(1⁄(β0,i+1))

(3)

where Ri represents the molar fraction of the component i (CFsurfactant and CH-surfactant, respectively) in the mixtures; β0,i is the binding constant of counterions for single surfactant i; cmcF-H is the cmc value of mixtures calculated by the component i, and cmc0i is the cmc value of corresponding single component i surfactant. In our previous report,15 we calculated cmcF-H values of the DEFUMACl and DTACl mixtures at different RF values by the ideal mixing model (eq 2) and nonideal mixing model (eq 3). For the DEFUMACl and DTACl mixed system, values of βF (DEFUMACl) ) 0.72 and βH (DTACl) ) 0.66 were employed.15 The calculated results are tabulated in Table 1, including the experimental data from 1H and 19F NMR measurements, respectively. For the sake of comparison, the cmcF-H values calculated for the mixed systems of DEFUMACl/TTACl, DEFUMACl/ CTACl, and DEFUMACl/OTACl at different RF values by ideal mixing model (eq 2) and nonideal mixing model (eq 3) are also collected and listed in Tables 2–4 where values of βH (TTACl)

) 0.66, βH (CTACl) ) 0.67, and βH (OTACl) ) 0.65 were used for these calculations. From the above Tables 1–4, important conclusions can be obtained: (i) For the DEFUMACl and DTACl mixtures, experimental cmcF-H values determined from 19F NMR measurements are in good agreement with the calculated ones for ideal cases at different RF values, implying a good compatibility of the two surfactants to form the mixed micelles at a range of surfactant concentration. (ii) For the DEFUMACl and TTACl mixtures, there was an obvious difference between the calculated data by nonideal demixing case and the experimental data both from 19F and 1H NMR measurements, whereas the experimental data were coincident with the calculated data for the ideal case, from which we could speculate that one type of mixed micelles was formed in DEFUMACl and TTACl mixtures. The reason for the formation of one type of mixed micelles might lie in the fact that CF-surfactant and CH-surfactant have almost the same cmc values. (iii) For DEFUMACl and CTACl mixtures, the experimental cmcF-H values were quite close to the ones calculated from CH-surfactant component for two types of micelles and for one type of mixed micelles by the ideal mixing case. The experimental cmc values were a bit less than those of the calculated values. But the experimental data determined from 19F NMR measurements were quite close to the cmc value of pure DEFUMACl in solution. The possible interpretation for

Behavior of Cationic Surfactant Mixtures

J. Phys. Chem. B, Vol. 112, No. 31, 2008 9377

TABLE 4: cmcF-H Values of DEFUMACl/TACl System at Different rF Valuesa cmcF-H (mmol · L-1) (nonideal mixing) DEFUMACl RF 0 0.1 0.2 0.3 0.5 1.0 a

calculated

12.97 8.67 6.85 5.09

cmcF-H (mmol · L-1)(ideal mixing) OTACl

experimental cmc1

cmc2

0.69 0.75 1.45 1.50

4.82 4.74 4.42 3.44 3.40

DEFUMACl/OTACl

calculated

experimental

calculated

0.32 0.34 0.37 0.46

0.30 0.32 0.35 0.38 0.37

4.37 4.23 4.11 3.89

T ) 25.0 ( 0.1°C.

the results could be that two types of micelles should form. One is hydrocarbon-rich micelles comprised of a large amount of CTACl and a minor amount of DEFUMACl, giving the cmcF-H values from 1H NMR for different ratios close to the cmc value of pure CTACl. The other type is fluorocarbon-rich micelles, whose cmcF-H value was close to that of DEFUMACl. The cmc value of CTACl (1.16 mmol · L-1) being one-third that of the DEFUMACl cmc (3.40 mmol · L-1) could make the cause. (iv) For DEFUMACl and OTACl mixtures, there are two groups of similar data in Table 4, where the second break point determined from 19F NMR measurements was very close to the calculated data by ideal mixing, and the cmc data determined from 1H NMR plots were close to the calculated ones by the OTACl component using nonideal mixing. The cmc values of the first break point were larger than the cmc values of OTACl but much smaller than that of DEFUMACl. These results imply that two types of micelles are formed. One is hydrocarbon-rich mixed micelles with a large amount of OTACl and a minor amount of DEFUMACl, and the other one is DEFUMACl-rich mixed micelles having a predominant amount of DEFUMACl and a minor amount of OTACl. The third break point observed by 19F NMR measurements indicated the formation of the individual DEFUMACl micelles. (v) From 19F and 1H NMR measurements, we can safely conclude that two types of micelles formed in four studied mixtures of fluorocarbon and hydrocarbon surfactants, i.e., completely mixed micelles and CF-rich mixed micelles or CH-rich mixed depending on the nature and the cmc values of the two components in mixtures. One can determine the concentrations of the CF-rich micelles by 19F chemical shifts and the CH-rich concentrations by 1H chemical shifts. The value of total concentration cT for CF-rich micelles and CH-rich micelles at the first change in slope should be different. (vi) The formation of one type of mixed micelles at a certain range of total concentration of mixed systems, as DEFDUMACl/DTACl and DEFUMACl/TTACl, could be attributed to the chemical structure of DEFUMACl, i.e., the proper proportion between CF-chains and CH-chains in DEFUMACl. The existence of CH-chains having hydrophilic groups (-OH) in DEFUMACl could endow the novel properties of DEFUMACl such as high solubility in water and the compatibility with CH-components, which provides a better model for studying the phase behavior and interaction between CFsurfactant and CH-surfactant or protein in solutions. Conclusions In cationic fluorocarbon/hydrocarbon surfactant mixtures, the different types of micelles observed by 19F and 1H NMR measurements could be caused by the increase in alkyl chain length of hydrocarbon surfactants with different critical micelle concentrations. In combinations of the two theoretical models

for mixing, it could be concluded that the two components of mixtures interacted with each other in a certain range of total concentration for the four different chain-length hydrocarbon surfactants studied, forming mixed micelles in two completely different ways according to their properties and cmc values. One is close to ideal mixing with the formation of one type of mixed micelles, such as the DEFUMACl/DTACl and TTACl systems; the other is of the demixing type with the formation of two types of micelles, i.e., fluorocarbon-rich mixed micelles and hydrocarbon-rich mixed micelles, such as the DEFUMACl/ CTACl and OTACl systems. However, as the total concentrations of the mixed systems were high enough, because of the initial respective interaction between fluorocarbon/hydrocarbon chains, the four systems tended to demixing, forming individual micelles of corresponding components. That it to say, at high concentration, individual DEFUMACl micelles were found in all the four systems. These results may be primarily directed toward acquiring an understanding of the mechanism of CF-CH mixtures in aqueous solution and secondarily directed toward providing more detailed information regarding nonideal mixing. References and Notes (1) Knodo, Y.; Yoshino, N. Curr. Opin. Colloid Interface Sci. 2005, 10, 88. (2) Funasaki, N. In Mixed Surfactant Systems; Ogino, K., Abe, M. Eds.; Surfactant Science Series 46; Marcel Dekker: New York, 1993; p 145. (3) Kissa, E. Fluorinated Surfactants; Surfactant Science Series 50; Marcel Dekker: New York, 1994. (4) Mukerjee, P.; Yang, Y. S. J. Phys. Chem. 1976, 80, 1338. (5) Mysels, K. L. J. Colloid Interface Sci. 1978, 66, 331. (6) Haegel, F. H.; Hoffman, H. Prog. Colloid Polym. Sci. 1988, 76, 132. (7) Askawa, T.; Miyagishi, S.; Nishida, M. Langmuir 1987, 3, 821. (8) Askawa, T.; Amada, K.; Miyagishi, S. Langmuir 1997, 13, 4569. (9) Askawa, T.; Miyagishi, S. Langmuir 1999, 15, 3464. (10) Asakawa, T.; Ishino, S.; Hansson, P.; Almgren, M.; Ohta, A.; Miyagishi, S. Langmuir 2004, 20, 6998. (11) Barthe´le´my, P.; Tomao, V.; Selb, J.; Chaudier, Y.; Pucli, B. Langmuir 2002, 18, 2557. (12) Amato, M. E.; Caponetti, E.; Chillura, M. D.; Pedone, L. J. Phys. Chem. B 2003, 107, 10048. (13) Mats, A.; Vasil, G. M. J. Phys. Chem. B 2005, 109, 11348. (14) Song, A.; Dong, S.; Hao, J.; Liu, W.; Xu, G.; Wang, H. J. Fluorine Chem. 2005, 126, 1266. (15) Dong, S. L.; Li, X.; Xu, G. Y.; Hoffman, H. J. Phys. Chem. B 2007, 111, 5903. (16) Kadi, M.; Hansson, P.; Almgren, M. Langmuir 2004, 20, 3933. (17) Almgren, M.; Garamus, V. M. J. Phys. Chem. B 2005, 109, 11348. (18) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365. (19) Caponetti, E.; Chillura Martino, D.; Floriano, M. A.; Triolo, R. Langmuir 1993, 9, 1193. (20) Pedone, L.; Chillura Martino, D.; Caponetti, E.; Floriano, M. A.; Triolo, R. J. Phys. Chem. B 1997, 101, 9597.

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