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Comparative Studies on the Crystalline to Fluid Phase Transitions of Two Equimolar Cationic/Anionic Surfactant Mixtures Containing Dodecylsulfonate and Dodecylsulfate Fu-Gen Wu,† Ji-Sheng Yu,† Shu-Feng Sun,‡ and Zhi-Wu Yu*,† †
Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China ‡ Lab of Bio-imaging, the Core Facilities, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, P. R. China ABSTRACT: In this work, a cationic surfactant, dodecyltrimethylammonium bromide (DTAB), and an anionic surfactant, sodium dodecylsulfonate (SDSO3) or sodium dodecylsulfate (SDSO 4 ), were mixed in an equimolar ratio to prepare SDSO 3 DTAB and SDSO 4 DTAB binary mixtures. The phase behavior, structure, and morphology of these two surfactant mixtures were investigated by differential scanning calorimetry, synchrotron X-ray scattering, freeze-fracture electron microscopy, and Fourier transform infrared spectroscopy. It was found that upon heating, both of the two systems transform from multilamellar crystalline phase to liquid crystalline (or fluid) phase. It is interesting to find that, although SDSO3 has a lower molecular weight, the crystalline phase of SDSO3 DTAB shows much higher thermostability as compared with that of SDSO4 DTAB. Other than this, we observed a large difference in the repeat distances of the two crystalline phases. More interestingly, at 60 °C in the fluid phases, cylindrical micelles formed in the SDSO3 DTAB system, while spherical micelles were observed in the SDSO4 DTAB system. Our present work demonstrates that a subtle difference in the headgroup structure of the anionic component markedly affects the thermostability, packing structure, and morphology of the surfactant mixtures, which suggests the importance of the match of the head head and tail tail interactions between the cationic and anionic surfactants.
1. INTRODUCTION Mixtures of cationic and anionic surfactants possess certain unique properties due to the strong synergistic interactions between the two components. For example, large reductions in the area of surfactant headgroups and critical aggregation concentrations (CACs) can be realized in these surfactant mixtures relative to the single surfactants. Besides, the strong attractive electrostatic interactions between the oppositely charged headgroups allow the formation of various microstructures. These self-assembled aggregates formed by the cationic/anionic surfactant mixtures provide a good platform for studies involving structural transformation, phase behavior and phase polymorphism, and the nucleation and growth mechanisms of the crystalline phases. The cationic/anionic surfactant mixtures (including catanionic surfactants, usually referring to the equimolar cationic/ anionic surfactant mixtures having low amounts of counterions) are known to form spherical micelles, wormlike micelles, hydrogels, vesicles, giant vesicles, reverse vesicles, bilayers, disks, tubules, ribbons, and so forth.1 9 The morphology, structure, and phase behavior of the cationic/anionic surfactant mixtures have been studied from various factors such as temperature,10 21 concentration,18,22 26 the molar ratio of the anionic and cationic surfactant,13,19,26 34 and additives.35 40 Besides, the influences of headgroup15,20,38,41 43 and alkyl chain22,24,44,45 on the properties of the cationic/anionic surfactant mixtures are also investigated. r 2011 American Chemical Society
Despite the extensive investigations on various properties of the cationic/anionic surfactant mixtures, details on the phase transition mechanisms, especially the influences of the headgroup structure and configuration on the macroscopic thermodynamic behavior, microscopic morphology, and molecular/ submolecular packing state are still rarely addressed. Besides, since the attractive electrostatic interactions between the cationic and anionic headgroups and the van der Waals forces in the alky tail region are the two main intermolecular forces governing the interactions between the cationic and anionic surfactants, it is important to see how the match of these head head and tail tail interactions determines the properties of the formed aggregates. In this work, we selected a cationic surfactant, dodecyltrimethylammonium bromide (DTAB), and two anionic surfactants, sodium dodecylsulfonate (SDSO3) and sodium dodecylsulfate (SDSO4), to form two equimolarly mixed cationic/anionic surfactants, i.e., SDSO3 DTAB and SDSO4 DTAB, with an aim to explore the detailed mechanisms of the transition from lamellar crystalline phase to fluid phase. Particular attention has been paid to the influence of the intermolecular interactions in the head and Received: June 13, 2011 Revised: October 26, 2011 Published: November 07, 2011 14740
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Figure 1. Molecular structures of SDSO3, SDSO4, d25-SDSO4, and DTAB.
tail regions on the properties of the cationic/anionic surfactant mixtures. The SDSO3 DTAB and SDSO4 DTAB systems are similar to catanionic surfactants, except for the presence of counterions. Although the aggregation behavior of SDSO4 DTAB systems has been extensively investigated,1,29,33,34,42,46 49 a comparative study on the SDSO3 DTAB and SDSO4 DTAB systems can provide some new insights into the effect of headgroup chemistry on the properties of the formed aggregates. Here, we studied the structural and phase behavior aspects of SDSO3 DTAB and SDSO4 DTAB by using differential scanning calorimetry (DSC), synchrotron small- and wide-angle X-ray scatterings (SAXS and WAXS), freeze-fracture electron microscopy (FFEM), and Fourier transform infrared (FTIR) spectroscopy. In the FTIR studies, in order to clearly monitor the structural changes of the head and tail regions of the cationic and anionic surfactants individually at the submolecular level, we used the deuterated sample, d25-SDSO4, instead of the hydrogenated sample, SDSO4. The structures of these molecules are shown in Figure 1. The results show that a slight difference in the headgroup structure of the anionic component markedly affects the phase behavior, structure, and morphology of the cationic/ anionic surfactant mixtures.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. SDSO3 and SDSO4 were purchased from Acros Organics. The perdeuterated SDSO4 (d25-SDSO4) was obtained from Aldrich. DTAB was purchased from TCI. Equimolar binary mixtures of the cationic (DTAB) and anionic (SDSO3, SDSO4, or d25-SDSO4) surfactants were premixed by sonication in a chloroform solution for 5 min and the solvent was evaporated under nitrogen. The mixtures were then kept under vacuum overnight to remove the residual solvent. After that, double deionized H2O with a resistivity of 18.2 MΩ 3 cm was used for the suspension of the thus obtained mixed surfactants to the required surfactant concentration (25 wt %). Homogeneous dispersions were prepared by vortexing the samples at 70 °C for 15 min, and were then subject to repeated thermal cycles between 0 and 70 °C. 2.2. DSC. Calorimetric data were obtained with a differential scanning calorimeter DSC821e equipped with the high-sensitivity sensor HSS7 (Mettler-Toledo Co., Switzerland).
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2.3. Synchrotron X-ray Scattering. SAXS and WAXS experiments were performed at the beamline 1W2A of the Beijing Synchrotron Radiation Facility (BSRF) (λ = 1.54 Å). A standard silver behenate sample was used for the calibration of diffraction spacings. To obtain the SAXS and WAXS data simultaneously, we fixed the sample-to-detector distance at 473.6 mm, and to further acquire the finer SAXS data, we adjusted the sample-to-detector distance to 1805.0 mm. A Linkam thermal stage (Linkam Scientific Instruments, UK) was used for temperature control ((0.1 °C). The X-ray powder diffraction intensity data were analyzed using the program Fit2D. 2.4. FFEM. Freeze-fracture electron microscopy (FFEM) technique was used to characterize the morphology of the two samples (SDSO3 DTAB and SDSO4 DTAB) at 60 °C. Samples and tools used for sample manipulation were equilibrated at the desired temperature for at least 5 min. A small amount of the suspension was mounted onto the sample holder and manually plunged into the liquid ethane cooled by the liquid nitrogen in advance. After that, the samples were stored in the liquid nitrogen. The freeze-fracture procedure was carried out in a Balzers BAF 400D freeze-fracture apparatus at 120 °C, 3 10 7 mbar. Fractured surfaces were shadowed with platinum/carbon. Platinum and carbon evaporation was done from 45° and 90° angles, with the thickness of 20 Å and 150 Å, respectively. Replicas were cleaned by floating onto water or water methanol solvent. Images were then examined and recorded with a JEOL 2010 transmission electron microscope (JEOL Ltd., Tokyo, Japan). 2.5. FTIR Spectroscopy. FTIR spectra were recorded using a Nicolet 5700 Fourier transform infrared spectrometer with a DTGS detector in the range of 4000 900 cm 1 with a spectral resolution of 2 cm 1 and a zero filling factor of 2. The precision of the frequency is better than 0.1 cm 1. Samples were coated onto the inner surfaces of a pair of CaF2 windows, which were mounted on a Linkam heating cooling stage for temperature control ((0.1 °C). Spectra were recorded every ∼30 s and each spectrum consists of 16 scans.
3. RESULTS AND DISCUSSION 3.1. DSC. Figure 2 shows the DSC results of SDSO3 DTAB and SDSO4 DTAB dispersed in H2O; the transition temperatures and enthalpies are summarized in Table 1. From the DSC results, we can see that the onset and peak temperatures of SDSO3 DTAB are markedly higher than those of SDSO4 DTAB during the initial heating process, indicating that lowtemperature state of SDSO3 DTAB has higher thermostability than that of SDSO4 DTAB. The phase transition enthalpy of SDSO3 DTAB is almost twice that of the SDSO4 DTAB system, further suggesting that the former is much more stable than the latter. Upon cooling, the onset temperatures of the exothermic events of the two systems are about 22 and 14 °C lower than those recorded during heating, indicating that the high-temperature phases in both cases are supercooled. The fact that SDSO3 DTAB has greater larger hysteresis than SDSO4 DTAB also implies that nucleation in the former system is more difficult. Figure 3 is the DSC result of d25-SDSO4 DTAB dispersed in H2O. The onset and peak temperatures of the phase transition during heating are 37.9 and 38.6 °C, respectively, which are close to the corresponding transition temperatures (39.5 and 40.5 °C) observed in the SDSO4 DTAB system. Upon cooling, a hysteresis of ∼9 °C was recorded. Such feature and the abruptness of the exothermic peak suggest that the nucleation in this system is also difficult. In comparison with the results in Figure 2, we can see that the d25-SDSO4 DTAB and SDSO4 DTAB systems behave similarly during the phase transitions, and it is thus 14741
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Figure 2. DSC results of SDSO3 DTAB (A) and SDSO4 DTAB (B) dispersed in H2O. The scan rate is 0.5 °C/min.
Table 1. Calorimetric Data of the SDSO3 DTAB and SDSO4 DTAB Systems upon Heating and Cooling at 0.5 °C/min SDSO3 DTAB Tonset (°C)
Tpeak (°C)
SDSO4 DTAB ΔH (J/g)
Tonset (°C)
Tpeak (°C)
ΔH (J/g)
Heating
43.3
45.4
133 ( 3
39.5
40.5
70 ( 1
Cooling
21.6
20.5
125 ( 6
25.3
24.2
68 ( 2
Figure 3. DSC result of d25-SDSO4 DTAB dispersed in H2O. The scan rate is 0.5 °C/min.
reasonable for us to use d25-SDSO4 DTAB system instead of the SDSO4 DTAB system for the FTIR investigations. To sum up, the DSC data show that the two systems (SDSO3 DTAB and SDSO4 DTAB) show large differences in the phase transition temperature, enthalpy, and hysteresis. 3.2. SAXS and WAXS. To investigate the structural changes of SDSO3 DTAB and SDSO4 DTAB during the phase transitions, we carried out SAXS and WAXS measurements at two temperatures (20 and 55 °C), and the results are shown in Figures 4 and 5. In Figure 4A and B, at 20 °C, we can see periodical Bragg peaks (marked with arrows) with ratios of 1:2:3 in SDSO3 DTAB and 1:(2):(3):4:5 in SDSO4 DTAB (Note that the intensities of the
second- and third-order peaks are too low to be observed in the SDSO4 DTAB system. Missing diffraction peaks in a lamellar system is not uncommon and has been reported before.50), indicating that the two systems are both multilamellar. From the q values, we can obtain the repeat distances (the d values) of these two lamellar structures using the equation d = 2π/q. The thus obtained d value for SDSO3 DTAB is 2.2 nm and that for SDSO4 DTAB is 3.2 nm. Hence, these two lamellar structures show a large difference in repeat distance. Besides the periodical Bragg peaks, many other sharp peaks are also observed in both systems. These peaks mainly residing in the wide-angle region reflect a highly ordered packing state, indicating the formation of crystalline phases. The different peak positions suggest that the two crystalline phases have different packing states. At 55 °C, both systems show a broad diffuse band at around 13.8 nm 1 in the wide-angle region. The corresponding d value, 0.46 nm, is the same as that of the liquid crystalline (or fluid) phase of lipids.51 54 The results show that the two systems at 55 °C are in the fluid phase with loosely packed alkyl chains. The SAXS results at q < 2 nm 1 for the SDSO3 DTAB and SDSO4 DTAB systems at 55 °C are shown in Figure 5. For both systems, only a broad, diffuse peak centered at 0.81 nm 1 (SDSO3 DTAB) or 0.89 nm 1 (SDSO4 DTAB) was observed. Besides, the absence of the periodic high-angle peaks (at q > 1.5 nm 1 in Figure 4) indicates that multilamellar structures are absent in the two systems at this temperature. The results suggest that micelles or unilamellar structures may form in both systems. Besides, the difference in peak positions of SDSO3 DTAB and SDSO4 DTAB may reflect different structures of the two aggregates. 3.3. FFEM. FFEM is perhaps the most suitable technique to investigate the microstructures of the highly concentrated samples (the total surfactant concentration is 25 wt %) of SDSO3 DTAB 14742
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Figure 4. SAXS and WAXS data of SDSO3 DTAB (A) and SDSO4 DTAB (B) dispersed in H2O at 20 and 55 °C. The q values of the lamellar peaks are shown in brackets.
Figure 6. FFEM images of SDSO3 DTAB (A) and SDSO4 DTAB (B) at 60 °C.
Figure 5. SAXS data of SDSO3 DTAB and SDSO4 DTAB dispersed in H2O at 55 °C.
and SDSO4 DTAB in the fluid phases (60 °C). In SDSO3 DTAB (Figure 6A), long rod-like structures with a width of 12 ( 3 nm were observed, while in SDSO4 DTAB (Figure 6B), spherical structures with sizes of 21 ( 3 nm predominate in the picture. By considering the sizes and shapes of these two structures, we propose that cylindrical micelles form in SDSO3 DTAB and spherical micelles form in SDSO4 DTAB. The formation of these two structures can be explained by the critical packing parameter, which will be discussed in section 3.5. 3.4. FTIR. To gain submolecular information on the phase behavior of the surfactant mixtures, we carried out time-resolved FTIR measurements on d25-SDSO4 DTAB, SDSO3 DTAB, and SDSO4 DTAB systems. Using the deuterated sample d25SDSO4 instead of the hydrogenated sample SDSO4, we are able to monitor the changes of the alkyl chains of the anionic surfactant (d25-SDSO4) as well as the cationic surfactant (DTAB) separately based on the same set of spectroscopic data. Moreover, by monitoring the characteristic IR vibrations of the polar headgroups of the anionic surfactant (the SO4 part of SDSO 4 ) and the cationic surfactant (the N(CH 3 )3 + part of DTAB), we can simultaneously detect the changes of the head and tail regions of the two surfactants during the phase transition process upon heating.
Shown in Figure 7A are the temperature-dependent changes of the FTIR contours of the alkyl tail groups (the CH2s in DTAB and the CD2s in d25-SDSO4) of the d25-SDSO4 DTAB system observed during the crystalline to fluid phase transition upon heating at 0.5 °C/min. The 3000 2800 cm 1 region contains the CH2 asymmetric and symmetric stretching vibrations of DTAB, which reflect the conformational order of the alkyl chains. In the crystalline phase (20 °C), the CH2 asymmetric and symmetric stretching bands (νasCH2 and νsCH2) of DTAB center at 2920.8 and 2851.0 cm 1, respectively, which shift to higher frequencies at 2924.3 and 2853.7 cm 1 in the fluid phase at 50 °C. Similarly, in the 2300 2000 cm 1 region, the CD2 asymmetric and symmetric stretching bands (νasCD2 and νsCD2) of d25-SDSO4 centering at 2195.1 and 2090.3 cm 1, respectively, in the crystalline phase at 20 °C shift to higher frequencies at 2196.9 and 2096.8 cm 1 in the fluid phase at 50 °C. These band shifts have been used frequently to follow the conformational order and the trans gauche isomerization of the hydrocarbon groups in lipid tails.55 61 The shifts toward high frequencies and the increases in bandwidth indicate that the alkyl chains of DTAB and d25-SDSO4 change from an all-trans conformation in the crystalline phase at 20 °C to a state with increased gauche conformers in the fluid phase at 50 °C. Figure 7B summarizes the temperature-dependent band positions of the CH2 symmetric stretching vibration in DTAB and the CD2 symmetric stretching vibration in d25-SDSO4 during the phase transition. Evidently, both alkyl chains change cooperatively upon heating. 14743
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Figure 7. (A) Time-resolved FTIR absorbance spectra of d25-SDSO4 DTAB in the regions of 3000 2800 cm 1 (the left panel) and 2300 2000 cm 1 (the right panel) upon the initial heating at 0.5 °C/min. The spectra are 1 °C apart. (B) Dependency of wavenumbers on temperature of the crystalline fluid transition.
Figure 8. (A) Time-resolved FTIR absorbance spectra of d25-SDSO4 DTAB in the region of 1530 1100 cm 1 upon the initial heating at 0.5 °C/min. (B) Time-resolved FTIR absorbance spectra of SDSO3 DTAB and SDSO4 DTAB in the region of 1520 1440 cm 1 upon the initial heating at 0.5 °C/min. The spectra are 1 °C apart.
In order to unveil the rearrangement cooperativity in the headgroup region during the phase transition, three important IR absorption bands of the d25-SDSO4 DTAB systems upon heating from 20 to 50 °C are shown in Figure 8A. We can see that a sharp peak at 1469.1 cm 1 from the CH2 scissoring vibration (δCH2) becomes a much less intensive peak at 1468.2 cm 1, typical of the changes from a crystalline phase to a fluid phase. The phase transition temperature is ∼38 °C, in good agreement with the DSC result in Figure 3. This band is very sensitive to the intermolecular forces, and can be served as a key band for examining the state of alkyl chain packing in various phases.55 61 In the crystalline phase, the single peak at 1469.1 cm 1 indicates that the alkyl chains are packed in the close-to-hexagonal/triclinic lattice. While in the fluid phase, the band at 1468.2 cm 1 suggests that the alkyl chains are in the fused state.
The band in the region of 1508 1474 cm 1 is ascribed to the asymmetric deformation of the methyl groups attached to the N+ atom (δasCH3 N+) in the head.57,62 65 It is a key band for monitoring the structural behavior of the hydrophilic part of the amphiphilic substance, which is known to be sensitive to the extent of disorder and the packing of the headgroups.63,65 As can be seen in the figure, the band at 1483.7 cm 1 in the crystalline phase at 20 °C splits into two bands at 1491.7 and 1480.4 cm 1 in the fluid phase at above 38 °C. As compared with the solid state trimethylammonium bromides (TABs) whose δasCH3 N+ is sharp and resides at around 1487 cm 1,63 the relatively broader feature and relatively lower frequency of δasCH3 N+ at 1483.7 cm 1 in the crystalline phase may indicate a certain degree of hydration of this polar group. The occurrence of the two bands at above 38 °C can be attributed to the disordered, well-hydrated headgroups in the fluid state.63 The strong and 14744
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Langmuir heavily overlapped bands in the region of 1300 1150 cm 1 arise from the asymmetric stretching of SO3 in the SO4 group (νasOSO3 ). They become broader as the sample converts from the crystalline phase to the fluid phase, indicating the increases in the hydration and mobility of this group. From Figure 8A, we can also see that the polar groups of CH3 N+ (in DTAB) and SO4 (in d25-SDSO4) and the apolar CH2 tails of DTAB change simultaneously at the transition temperature (38 °C). Together with the fact the CH2 in DTAB and CD2 in d25-SDSO4 change synchronously during the transition (Figure 7), we can now draw a conclusion that the rearrangements of the different parts of d25-SDSO4 and DTAB are highly cooperative during the crystalline to fluid phase transition. However, due to the presence of electrostatic screening caused by counterions, the above result may not definitively indicate the formation of a charge-pair between the two components. Figure 8B contains the vibrations of δasCH3 N+ and δCH2 in the SDSO3 DTAB and SDSO4 DTAB systems. The δCH2 bands reside at 1467.7 cm 1 in SDSO3 DTAB and 1469.2 cm 1 in SDSO4 DTAB. The difference in peak positions indicates that the alkyl chains of the two systems are somewhat different in their packing states. Besides, by careful comparison of these two bands, we can find that the former is narrower than the latter, which suggests that the alkyl tails in SDSO3 DTAB is more tightly packed than those in SDSO4 DTAB. The larger peaks of δCH2 observed in the SDSO3 DTAB and SDSO4 DTAB systems as compared with that in the d25-SDSO4 DTAB system (which is more evident at high temperatures) are due to a summation of the absorptions from the alkyl chains in both the cationic and anionic surfactants. In the crystalline phases at low temperatures, an unexpected large difference was observed for the δasCH3 N+ of DTAB in the two mixtures. In the SDSO3 DTAB system, there are two bands residing at 1493.4 and 1482.3 cm 1, while in the SDSO4 DTAB system, only one band at 1482.4 cm 1 is seen. The results indicate that the intermolecular interactions between the headgroups of the cationic and anionic surfactants and thus the headgroup arrangements are very different in these two mixtures. However, when heated to the fluid phase at high temperatures, the δasCH3 N+ bands of DTAB in the two systems are almost identical, which indicates that the CH3 N+ groups of DTAB molecules in the two systems have the same configuration and hydration degree. The loosely packed and disordered alkyl tails together with the high mobility of the headgroups of these two systems in the fluid phase make it possible for the headgroups to adopt the same or similar configuration. Schematic models showing the possible conformation/packing states of the two systems in crystalline and fluid phases may be helpful to explain the above IR results. As shown in Figure 9A, the additional oxygen atom lowers the charge density of the polar head, which weakens the electrostatic interactions between the cationic and anionic surfactants. Besides, the presence of this additional oxygen atom destroys the good match between electrostatic interaction in the head region and the van der Waals interaction in the tail region. Thus, a closer packing of the two surfactants can be expected in SDSO3 DTAB system. Actually, such a close packing is evidenced by the doublet feature of the δasCH3 N+ band and the narrower δCH2 band in SDSO3 DTAB system, while in the fluid phases, due to the good hydration and high mobility of the polar headgroups, no detectable difference of the δasCH3 N+ band was observed for the two systems.
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Figure 9. (A) Possible conformational states of SDSO3 DTAB and SDSO4 DTAB in the crystalline phase. (B) Schematic drawings showing the packing states of SDSO3 DTAB and SDSO4 DTAB in the fluid phase.
From the IR results, we can see that, during the crystalline to fluid phase transition processes of the two surfactant mixtures, the alkyl tails change from an ordered state to a loosely packed, disordered state, while the polar head groups increase in their hydration degree and mobility. The rearrangements of different parts of the surfactants are quite cooperative during the phase transition upon heating, just like a united double-chained amphiphilic molecule. Moreover, the headgroup configurations are very different in the crystalline phase but are almost the same in the fluid phase for SDSO3 DTAB and SDSO4 DTAB. 3.5. Differences between SDSO3 DTAB and SDSO4 DTAB Systems. The equimolar mixture of cationic and anionic surfactants has a high tendency toward precipitation owing to the charge neutralization of oppositely charged surfactant headgroups.66 In our high concentration systems (the total concentration of surfactants is 25 wt %), the surfactant mixtures can readily precipitate and crystallize to form tightly packed multilamellar structures. What is of interest to us is the difference between the two mixtures. First, we observed a big difference in the d-spacings of SDSO3 DTAB (2.2 nm) and SDSO4 DTAB (3.2 nm) systems in the crystalline phases. It is worth noting that both d values are relatively small, since the d value is composed of the length of a surfactant bilayer and the length of the interlamellar hydration layer, and the fully stretched lengths of SDSO3, SDSO4, and DTAB as determined by MM2 simulation are ∼1.8, ∼1.9, and ∼1.9 nm, respectively. This means that the alkyl tails in both of the two systems must be tilted and/or interdigitated. Regarding to the difference in d values, it is unlikely due to the difference in hydration layer, because we are discussing crystalline phases of two systems with only a small difference in surface charge densities (since the thickness of the hydration layer is mainly determined by the surface charge density of the bilayer). Rather, we attribute it to the different headgroup configurations of the two systems in the crystalline phases as revealed by FTIR in Figure 8B. Second, we found the SDSO3 DTAB system has much higher thermostability than the SDSO4 DTAB system, as demonstrated by a higher phase transition temperature and a much larger enthalpy value (see Table 1). As shown in Figure 1, SDSO3, SDSO4, and DTAB have the same alkyl tails, and the only difference between SDSO3 and SDSO4 is that the latter possesses an additional oxygen atom in its polar headgroup. In SDSO3 DTAB, close contact of the oppositely charged 14745
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Langmuir headgroups and the tight arrangement of the alkyl tails can be realized at the same time. This will guarantee strong electrostatic interaction in the headgroup region and strong van der Waals interaction in the tail region, and thus high thermostability. While in SDSO4 DTAB, the additional oxygen atom connecting SO3 and C12H25 disturbs the good match between the electrostatic interaction in the head and the van der Waals forces in the tail. Besides, this additional oxygen atom plays a role in sharing the negative charge of the headgroup, and the thus lowered charge density would weaken the electrostatic interactions between the cationic and anionic surfactants. As shown in Figure 9B, the smaller charge density and larger size of the SO4 group result in a larger separation between the two oppositely headgroups, and the surfactant tail is also loosened especially in the regions close to the head. As a result, the crystalline phase of SDSO4 DTAB is less thermostable than that of SDSO3 DTAB. Third, we observed cylindrical micelles in the SDSO3 DTAB system, but spherical micelles in SDSO4 DTAB system in the respective fluid phases. The heating-induced multilamellar crystalline-to-fluid micelle transition of SDSO3 DTAB and SDSO4 DTAB systems is concomitant with the variations of their membrane curvature driven by the increased headgroup hydration degree and the melting of the alkyl chains. The marked difference in the morphology of the two systems in the fluid phase (rod-like and spherical micelles) may also be related with the different sizes of the SO3 and SO4 groups. The larger SO4 group has a smaller charge density, which weakens the binding of the oppositely charged headgroups. As shown in Figure 9B, the larger SO4 group and the larger separation between the two oppositely charged headgroups together lead to a larger polar size of the surfactant pair. Here, we can use the critical packing parameter Pc to explain the formation of the two different morphologies of the cationic anionic surfactant pair using the equation Pc = V/(al), where V is the surfactant tail volume, a is the effective headgroup area, and l is the extended length of the alkyl chain, respectively.67,68 When Pc < 1/3, spherical micelles form. When 1/3 < Pc
