Article pubs.acs.org/Langmuir
Self-Assembled Structures of Anionic Hydrophobically Modified Polyacrylamide with Star-Shaped Trimeric and Hexameric Quaternary Ammonium Surfactants Yaxun Fan,† Chunxian Wu,† Meina Wang,† Yilin Wang,*,† and Robert K. Thomas‡ †
Key Laboratory of Colloid and Interface Science, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China ‡ Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom ABSTRACT: The self-assembly of a 1% hydrophobically modified and 30% hydrolyzed polyacrylamide (C12PAM) with cationic star-shaped oligomeric surfactants has been investigated by isothermal titration microcalorimetry, turbidimetry, ζ potential, scanning electron microscopy, and 1H NMR techniques. The oligomeric surfactants are composed of quaternary dodecyldimethylammonium ions with three or six hydrophobic chains connected by a polyamine spacer at the headgroup level, abbreviated as DTAD and PAHB, respectively. DTAD/C12PAM and PAHB/C12PAM mixed systems undergo the same aggregate transitions with increases in surfactant concentration from soluble networklike aggregates to precipitated denser and more cross-linked structures and then to soluble spherical aggregates. The networklike aggregates are generated at very low surfactant concentration. However, at the corresponding surfactant concentration without C12PAM, DTAD cannot form aggregates and PAHB forms only networklike aggregates with a very loose structure. The strong electrostatic and hydrophobic interaction of DTAD and PAHB with C12PAM and the hydrophobic interaction between the alkyl chains of DTAD and PAHB themselves evidently promote the formation of networklike aggregates. As the surfactant concentration increases, cationic surfactants become excessive. The molecular configuration is changed by the stronger hydrophobic association among the DTAD and PAHB molecules and the enhanced electrostatic repulsion between the mixed aggregates. Thus, the networklike aggregates transfer to spherical aggregates.
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INTRODUCTION Hydrophobically modified water-soluble polymers (HM polymers) have amphiphilic properties in aqueous solutions because they contain both hydrophobic and hydrophilic moieties.1−10 Despite the degree of substitution often being lower than 5 mol %, they often have superior performance with respect to their unmodified relatives. In particular, mixtures of HM polymers with surfactants in aqueous solution give rise to the formation of associated structures that can modify solution and interfacial properties in ways that are interesting for both fundamental and applied purposes.11−24 Hydrophobic and electrostatic interactions both contribute to the driving force for the association of surfactant/HM polymer mixtures in aqueous solution, leading to intricate phase behavior and rich selfassembling morphologies. Polyacrylamide (PAM) is a widely applied water-soluble polymer. Due to the presence of functional group −CONH2, PAM can undergo many chemical modifications. Hydrophobically modified polyacrylamide (HMPAM) can be synthesized by introducing hydrophobic alkyl groups into the PAM backbone, and the surfactant/HMPAM mixtures have been widely studied.25−28 Biggs and Candau29 investigated the mixed system of sodium dodecyl sulfate (SDS) with a copolymer of acrylamide and N-(4-ethylphenyl) acrylamide and found that © 2014 American Chemical Society
the rheological properties of the system were very sensitive to the amount of SDS. Dramatic changes in viscosity were explained in terms of the balance between inter- and intrachain interactions and their effects on chain dimensions. Kwak et al.16,17 reported the interactions of HMPAM with SDS or alkylbenzenesulfonates (ABS). When the degree of substitution of HMPAM is larger than 2%, gels may form in SDS/HMPAM and ABS/HMPAM mixtures, resulting from surfactant binding with the hydrophobic side chains of HMPAM. Our previous work30 found that HMPAM shows a stronger ability for forming middle-phase microemulsions with SDS than with unmodified PAM, which indicates a stronger interaction of SDS with HMPAM than with PAM. Our group31 also studied the interactions of HMPAM and unmodified PAM with SDS and cationic surfactant tetradecyltrimethylammonium bromide (TTAB) by using flow microcalorimetry. The mixing enthalpy curve showed that an increase in the hydrophobicity of the PAM induces a decrease in the peak height of the endothermic curves but has no obvious effect on the critical aggregation concentration (cac) of surfactants. Kwak et al.15 and PenottReceived: March 15, 2014 Revised: May 11, 2014 Published: May 27, 2014 6660
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Figure 1. Molecular structures of DTAD, PAHB, and C12PAM and 1H NMR signal assignments of PAHB and C12PAM.
Chang et al.32 found that the addition of ionic surfactants to HMPAM induces an increase in viscosity due to intermolecular bridging between the hydrophobic groups of HMPAM and the surfactant in the mixed micelles. However, a higher SDS concentration leads to a viscosity reduction because electrostatic repulsion among the charged mixed aggregates can inhibit the association of the aggregates. On the other hand, the solution viscosity of HMPAM with cationic cetyltrimethylammonium p-toluenesulfonate (CTAT) is enhanced by entanglements between the polymers and wormlike micelles in the mixed solution.32 Compared to conventional single-chain surfactants, mixtures of gemini surfactants with HM polymers have shown a wider range of properties. Bai et al.33 studied the mixed systems of HMPAM with cationic ammonium gemini surfactant C12CnC12Br2 (n = 3 and 6). The results showed that the C12CnC12Br2/HMPAM interaction is much stronger than for the corresponding single-chain surfactants and strongly depends on the spacer length. Our group34,35 showed that the interactions of cationic gemini surfactants C12CnC12Br2 (n = 3, 6, and 12) with HMPAM and unmodified PAM are strong and the geminis may disrupt the self-assembled aggregates of HMPAM and form mixed micelles. Moreover, C12C6C12Br2/ PAM was found to be more efficient at reducing interfacial tension than C 12 C 6 C 12 Br 2 /HMPAM, indicating that C12C6C12Br2 molecules preferentially bind to the hydrophobic chains of HMPAM and form bulk C12C6C12Br2/HMPAM aggregates. Recently, interactions of the HM polymer with oligomeric surfactants have been studied for the first time. Oligomeric surfactants are made of three or more amphiphilic moieties chemically connected by spacer groups. Zana’s group36 studied the interaction of linear cationic trimeric surfactants (12-3-12-312 and 12-6-12-6-12) with hydroxypropyl guar (HPG) and its modified derivative (HMHPG) and compared the behavior to that of corresponding monomeric surfactant dodecyltrimethylammonium bromide (DTAB) and gemini surfactant C12CnC12Br2. The apparent viscosity of the dilute mixed solution increases in the order monomeric surfactant < gemini
surfactants < trimeric surfactants, and this is related to the degree of oligomerization of the surfactants. It was found that the gemini and trimeric surfactants form threadlike micelles or vesicles, leading to gel formation with polymer. Cationic trimeric, tetermeric, and hexameric quaternary ammonium surfactants with a star-shaped amide-type spacer have recently been synthesized by our group.37−39 These oligomeric surfactants with a star-shaped spacer provide extensive possibilities for fabricating versatile aggregate structures through the variation of the surfactant configurations controlled by hydrophobic interaction of the chains. The starshaped trimeric surfactants form vesicles first which change to spherical micelles with increasing concentration, whereas the star-shaped tetrameric and hexameric surfactants form networklike premicellar aggregates far below their critical micelle concentration and change to small spherical micelles in concentrated solution. So far, there are no reports on the interaction between polymers and oligomeric surfactants with star-shaped spacers. The present work explores mixtures of cationic quaternary ammonium trimeric and hexameric surfactants (DTAD and PAHB) with 1% hydrophobically modified and 30% hydrolyzed polyacrylamide (C12PAM). The molecular structures of DTAD, PAHB, and C12PAM are presented in Figure 1. The aggregation behavior of mixtures of C12PAM with DTAD and PAHB has been studied by isothermal titration microcalorimetry (ITC), turbidity, ζ potential, scanning electron microscopy (SEM), and 1 H NMR techniques.
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EXPERIMENTAL SECTION
Materials. Trimeric cationic surfactant tri(dodecyldimethylammonioacetoxy)-diethyltriamine trichloride (DTAD) and hexameric cationic surfactant 10-(2-(bis(3-(2-(dodecyldimethylammonio)ethylamino)-3-oxopropyl)amino)ethyl)-N1,N19-didodecyl-7,13-bis(3(2-(dodecyldimethylammonio)ethylamino)-3-oxopropyl)N 1 ,N 1 ,N 19 ,N 19 -tetramethyl-4,16-dioxo-3,7,10,13,17-pentaazanonadecane-1,19-diaminium hexabromide (PAHB) were synthesized and purified according to previous work.39,40 Their structures were confirmed by mass spectroscopy and 1H NMR, and the purity was verified by elemental analysis and surface tension measurements. 6661
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Figure 2. I1/I3 ratio of pyrene and light-scattering intensity at 90° as a function of C12PAM concentration. C12PAM was prepared as described previously.17 The average molecular weight of C12PAM estimated from the capillary viscosimetry of dilute solution is approximately 10K. The pKa value of the acrylic acid group of C12PAM is about 5.0. This leads to a degree of dissociation for the acrylic acid group of nearly 100% at pH 7.0, which was the pH for this work, i.e., all of the 30% hydrolyzed acrylic acid groups of C12PAM carry negative charges. Pyrene was from Aldrich and recrystallized from ethanol. Milli-Q water (18 MΩ cm) was used in all experiments. Steady-State Fluorescence Measurement. Fluorescence was used to monitor the micropolarity change using measurements of the pyrene polarity index (I1/I3) at different C12PAM concentrations. I1/I3 is the ratio between the fluorescence intensities of peaks I (372 nm) and III (384 nm) of the pyrene emission spectrum. The fluorescence intensities were measured using a Hitachi model F-4500 spectrophotometer. Pyrene was excited at 335 nm, and the emission spectra were scanned from 350 to 500 nm. Dynamic Light Scattering (DLS). Measurements of the lightscattering intensity of C12PAM solutions were carried out using an LLS spectrometer (ALV/SP-125) with a multi-τ digital time correlater (ALV-5000). The light-scattering intensity was used to follow the sizes of the C12PAM aggregates. Light of λ = 632.8 nm from a solid-state He−Ne laser (22 mW) was used as the incident beam, and measurements were conducted at a scattering angle of 90°. All solutions were filtered through a 0.45 μm membrane filter of hydrophilic PVDF before the measurements. The measurements were performed at 25.00 ± 0.01 °C. Turbidimetric Titration. The turbidity of the DTAD/C12PAM and PAHB/C12PAM mixed solutions, reported as 100 − %T, was measured at 450 nm using a Brinkman PC920 probe colorimeter thermostated at 25.0 ± 0.1 °C. The surfactants and polymer do not have visible absorption at this wavelength. Turbidimetric titration was carried out by adding equal volumes of concentrated surfactant solution and 0.3 g/L C12PAM solution to a stirred solution of 0.3 g/L C12PAM to increase the surfactant concentration while keeping the C12PAM concentration constant. ζ-Potential Measurements. ζ-potential measurements of the DTAD/C12PAM and PAHB/C12PAM mixed solutions were performed with a Malvern Zetasizer Nano-ZS instrument (ZEN3600, Malvern Instruments, Worcestershire, U.K.) equipped with a 4 mW He−Ne laser at a wavelength of 633 nm. Disposable capillary cells (DTS1060C) were used for the measurements. The ζ potentials were calculated from the mobility measured during an electrophoretic lightscattering (ELS) experiment using the Hemholtz−Smoluchowski relationship. All experiments were performed at room temperature. Isothermal Titration Microcalorimetry (ITC). The ITC measurements were made in a TAM2277-201 microcalorimetric system (Thermometric AB, Järfälla, Sweden) with a stainless steel sample cell of 1 mL at 25.00 ± 0.01 °C. The sample cell of the microcalorimeter was initially loaded with 700 μL of pure water or C12PAM solution. Concentrated DTAD or PAHB solution was injected consecutively into the stirred sample cell using a 500 μL
Hamilton syringe controlled by a Thermometric 612 Lund pump until the desired concentration range had been covered. During the titration process, the system was stirred at 60 rpm with a gold propeller, and the interval between two injections was long enough for the signal to return to the baseline. The observed enthalpy (ΔHobs) was obtained by integrating the areas of the peaks in the plot of thermal power against time. Scanning Electron Microscopy (SEM). The morphology of the DTAD/C12PAM and PAHB/C12PAM samples was imaged with a field-emission scanning electron microscope (Hitachi S-4800). The samples were prepared by freezing a small drop of the surfactant/ C12PAM solution on a clean silica wafer with liquid nitrogen so that the microstructures of the mixtures in aqueous solution can be well retained. Immediately afterward, the frozen sample was lyophilized under vacuum at about −50 °C. Finally, a 1−2 nm Pt coating completed the sample preparation. 1 H NMR. 1H NMR measurement was carried out at 23 ± 2 °C on a Bruker AV400 FT-NMR spectrometer operating at 400.1 MHz. Deuterium oxide (99.9%) was purchased from CIL Cambridge Isotope Laboratories and used to prepare the stock solutions of PAHB, DTAD, and C12PAM in D2O, respectively. The solutions of the mixtures were prepared by mixing the stock solutions to reach the desired concentrations. The center of the HDO signal (4.79 ppm) was used as the reference in the D2O solutions. In all of the 1H NMR experiments, the number of scans was adjusted to achieve good signalto-noise ratios depending on the surfactant concentration and was recorded with a digital resolution of 0.04 Hz/data point.
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RESULTS AND DISCUSSION Self-Aggregation of C12PAM. Figure 2 shows the variations of the pyrene polarity ratio I1/I3 and light scattering intensity at 90° in the C12PAM solution from steady-state fluorescence and DLS measurements, respectively. The sharp decrease in I1/I3 for the C12PAM system reflects the formation of aggregates. The critical aggregation concentration of C12PAM (Cp) is ∼3.2 g/L, determined from the intercept between the linear extrapolations of the rapidly varying portion of the curve and of the almost-horizontal portion at high concentration.41 A corresponding breakpoint exists at ∼2.0 g/L in the light-scattering intensity curve, confirming that the critical aggregation concentration of C12PAM (Cp) is between 2.0 and 3.2 g/L. The result indicates that C12PAM alkyl chains associate to minimize the exposure of hydrophobic chains to water. In the following studies, the C12PAM concentration is fixed at 0.3 g/L, which is below Cp, in order to focus on how oligomeric surfactants DTAD and PAHB interact with the nonaggregated C12PAM and induce the formation and transition of the aggregates in the mixtures. 6662
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Figure 3. (A1) ITC curves for titrating 5 mM DTAD into 0.3 g/L C12PAM solution (■) and water (□). (B1) ITC curves for titrating 5 mM PAHB into 0.3 g/L C12PAM solution (■) and water (□). Turbidity and ζ potential as a function of DTAD (A2, A3) and PAHB (B2, B3) concentrations in the presence of 0.3 g/L C12PAM.
changes. The observed enthalpy ΔHobs values for the mixtures are mainly contributed by the electrostatic binding and hydrophobic association of DTAD or PAHB with C12PAM and the demicellization of the surfactants, which are exothermic and endothermic, respectively. According to the ITC curves for titrating DTAD or PAHB into water, the demicellization enthalpy of PAHB is much larger than for DTAD. Therefore, the observed enthalpy of PAHB/C12PAM is endothermic, while the observed enthalpy of DTAB/C12PAM is close to zero. Between C1 and C2 for DTAD or C1′ and C2′ for PAHB, precipitation takes place in both systems, as shown by the high turbidity. In this stage, the ITC curve for DTAD presents exothermic enthalpy changes (Figure 3A1), but the ITC curve between C1′ and C2′ for PAHB presents the endothermic enthalpy changes (Figure 3B1). However, both ITC curves show the same varying tendency, decreasing to the more exothermic direction and rising up beyond an extreme value. Herein, the enthalpy changes are also mainly contributed by electrostatic binding between the oppositely charged surfactants and polymer, hydrophobic association between the hydrophobic chains of the surfactants and polymer, and demicellization of the surfactants. The electrostatic binding and hydrophobic association are exothermic, while the demicellization is endothermic. The observed enthalpy changes depend on the relative strength of these interactions and vary with the processing of the interactions. The difference in the ITC curves between DTAD and PAHB results from the different strength of these interactions. The exothermic
Effects of DTAD and PAHB on the Aggregation of C12PAM. The observed enthalpy changes, the turbidity, and ζpotential values for the titrations of the concentrated DTAD or PAHB solution into C12PAM solution are plotted against the final DTAD and PAHB concentrations in Figure 3. For comparison, the enthalpy changes for titrating DTAD or PAHB solution into water are also plotted (A1, B1) DTAD/C12PAM and PAHB/C12PAM exhibit similarly changing patterns in their ITC, turbidity, and ζ-potential curves. There are three transitions upon increasing the surfactant concentration. The two critical aggregation concentrations are C1 (∼0.24 mM) and C2 (∼0.45 mM) for DTAD/ C12PAM and C1′ (∼0.07 mM) and C2′ (∼0.23 mM) for PAHB/C12PAM. In the region of CDTAD < C1 or CPAHB < C1′, when the DTAD or PAHB solution is titrated into the C12PAM solution, the ITC curve displays a very small ΔHobs value that is close to zero for DTAD but endothermic for PAHB. Correspondingly, the turbidity gradually increases and reaches a larger value at C1 or C1′. Besides, the ζ-potential values below C1 and C1′ are large and negative, reflecting the charge state of C12PAM. With the addition of DTAD and PAHB, the ζpotential values become less negative, suggesting the binding of cationic quaternary ammoniums of DTAD and PAHB with anionic carboxylic groups of C12PAM. For both surfactants, the ΔHobs values are lower than the corresponding ΔHobs values for the dilution of the surfactants. This means that the added DTAD and PAHB bind with C12PAM mainly through electrostatic interaction, leading to exothermic enthalpy 6663
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Figure 4. SEM images of the DTAD/C12PAM and PAHB/C12PAM aggregates formed in the presence of 0.3 g/L C12PAM at DTAD concentrations of 0.10 mM (a1), 0.35 mM (b1), and 0.80 mM (c1) and at PAHB concentrations of 0.05 mM (a2), 0.20 mM (b2), and 0.80 mM (c2).
Figure 5. 1H NMR spectra and proton assignments of PAHB in D2O at different concentrations (above C2′) in the presence of 0.3 g/L C12PAM (A) and in the absence of C12PAM (B).
enthalpy changes for DTAD indicate that the contribution from the demicellization of DTAD is weaker than that from the electrostatic binding and hydrophobic association of DTAD with C12PAM, while the endothermic enthalpy changes for PAHB indicate that the contribution from the demicellization of DTAD is stronger than that from the electrostatic binding and hydrophobic association. Above C2 and C2′, the turbidity value decreases sharply to a constant value and the solutions have a semitransparent bluish appearance, which is typical of soluble large aggregates. The ITC curves merge into the dilution curves of DTAD and PAHB in water, suggesting that the interaction of DTAD and PAHB with C12PAM has reached saturation. In parallel, the ζ potentials change from negative to positive values larger than 40 mV, showing that the surfactant
molecules have reversed the charges of the DTAD/C12PAM and PAHB/C12PAM aggregates. The net positive charges from the cationic surfactants enhance the electrostatic repulsion between the aggregates and lead to the redissolution of the precipitates. The ζ potential in precipitation cannot be measured because the aggregates are too large to be stably dispersed in solution. However, the fact that the ζ potential changes sign across the precipitation region suggests that the aggregates in precipitation are close to charge neutralization. SEM with freezing sample preparation was used to identify the aggregate transitions in DTAD/C12PAM and PAHB/ C12PAM. The SEM images of 0.3 g/L C12PAM with DTAD and PAHB in different concentration regions are shown in Figure 4. The SEM results indicate that DTAD/C12PAM and 6664
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Figure 6. Schematic diagrams describing the interaction of C12PAM with DTAD and PAHB.
PAHB/C12PAM have similar aggregation processes. Before C1 or C1′, the DTAD/C12PAM and PAHB/C12PAM mixtures form loose networklike aggregates (Figure 4a1,a2). Between C1 and C2 or between C1′ and C2′, the networklike aggregates become denser and more cross-linked (Figure 4b1,b2), especially for the PAHB/C12PAM mixture. Beyond C2 or C2′, the mixed aggregates change to large uniform spherical aggregates (Figure 4c1,c2). The DTAD/C12PAM spherical aggregates (∼1 μm) are much larger than the PAHB/ C12PAM spherical aggregates (∼200−500 nm). Interactions of Oligomeric Surfactants with C12PAM. To understand the interaction and aggregation of surfactants DTAD and PAHB with C12PAM, 1H NMR was used to study the microenvironment of the DTAD and PAHB molecules during the transitions in the DTAD/C12PAM and PAHB/ C12PAM mixed systems. The 1H resonance peaks of the surfactant protons are broadened and the fine peak structures are lost below C2′ for the PAHB/C12PAM mixture and over the whole concentration region for the DTAD/C12PAM mixture. Generally speaking, the limited motions of protons make the dynamics or the exchange rate of deuterium much slower than the time scale of the 1H NMR peak width. Thus, a very strong homonuclear dipolar coupling is involved in 1H detection,42 and the signal is presumably broadened to an extent that cannot be detected above the background.43 Thus, that the proton signals of the DTAD and PAHB molecules with C12PAM cannot be detected below C2 or C2′ indicates that the binding of DTAD and PAHB with C12PAM limits the movements of the surfactant molecules and almost all the surfactant molecules are bound to C12PAM through both electrostatic attraction and hydrophobic interaction. Even when the concentration of DTAD is above C2, the 1 H NMR signals of DTAD cannot be observed for the DTAD/ C12PAM mixture. Figure 5 shows the plot of the chemical shifts of the PAHB protons versus the PAHB concentration beyond C2′ with and without 0.3 g/L C12PAM. Although the 1H NMR signals of the PAHB/C12PAM mixture can be detected above C2′, the signal-to-noise ratios are much weaker than those of PAHB without C12PAM at the same concentrations. In addition, the 1H NMR signals of the PAHB protons in hydrophobic chains (Ha, Hb, Hc, and Hd) overlap all of the C12PAM protons at δ < 2.0 ppm and thus cannot be distinguished. In this range, the PAHB/C12PAM mixtures form spherical aggregates, as observed above. The 1H NMR
result indicates that the binding of surfactants with C12PAM in the spherical aggregates also restricts the motion of PAHB molecules. However, the PAHB molecules in the PAHB/ C12PAM spherical aggregates may form looser arrays, and excess PAHB molecules should be located in a freer microenvironment than for the networklike aggregates below C2′. In contrast, almost all of the proton signals of the DTAD molecules are screened, probably because larger spherical aggregates are formed in the DTAD/C12PAM mixture and more DTAD molecules may remain inside the larger spherical aggregates with the strongly limited motion of their protons. In addition, as the PAHB concentration increases, the PAHB protons shift downfield up to a concentration of 0.8 mM, which corresponds to the saturation point for interaction in the ITC curves. The downfield shifts of the PAHB protons indicate that their protons sense a less-polar microenvironment in the spherical aggregates than in the networklike aggregates. This suggests that the alkyl chains of the PAHB molecules are extended below C2′ but gradually coil beyond C2′. Beyond this point, the binding of PAHB with C12PAM has reached a saturation state and the 1H NMR signals of PAHB do not change anymore. On the basis of all of the results above, Figure 6 presents a possible interaction model of the aggregate transitions in the DTAD/C12PAM and PAHB/C12PAM mixtures at different surfactant concentrations. The aggregate transitions take place in succession with increasing DTAD or PAHB concentration. The interaction of DTAD and PAHB with C12PAM generates loose networklike aggregates at surfactant concentrations well below the initial aggregation concentrations of DTAD and PAHB without C12PAM, which are respectively 0.29 and 0.10 mM.38,39 At low surfactant concentration, the hydrophobic chains of DTAD and PAHB are extended because of the rigid spacers and the strong intramolecular electrostatic repulsion among their headgroups. In the absence of C12PAM, the stretched star-shaped configuration of DTAD prevents it from forming aggregates, and PAHB forms only networklike aggregates with a loose structure. C12PAM therefore must play an important role in the formation of the present networklike aggregates. The noncooperative electrostatic binding of the multiply charged headgroups of DTAD or PAHB with anionic carboxylic groups of C12PAM and the hydrophobic interaction of the stretched alkyl chains of the surfactant molecules with the alkyl chains of C12PAM promote cross-linking between the 6665
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Langmuir two entities. Moreover, the hydrophobic association of the stretched multiple alkyl chains of DTAD and PAHB molecules already bound with C12PAM helps the cross-linking. Hence, the electrostatic and hydrophobic interaction between the surfactants and C12PAM as well as the star-shaped molecular configuration of oligomeric surfactants all contribute to the formation of large networklike aggregates. With increasing DTAD and PAHB concentrations, the negative charges of C12PAM are gradually neutralized by the positive charges of DTAD and PAHB. With the assistance of strong cooperatively hydrophobic interaction between the multiple alkyl chains of DTAD and PAHB, the almost neutralized mixed aggregates may further associate and grow to a very large size. Thus, the denser and more cross-linked networklike aggregates with ζ potentials of around zero are generated and precipitated. For the PAHB/C12PAM system, more amphiphilic moieties in the PAHB molecules lead to a stronger hydrophobic interaction among the alkyl chains of PAHB and even denser and more cross-linked networklike aggregates. This means that the binding intensity of the starshaped oligomeric surfactants with C12PAM becomes stronger with an increase in the degree of oligomerization. With a further increase in DTAD and PAHB concentrations, more DTAD and PAHB molecules bind to the mixed aggregates due to the hydrophobic interaction among the alkyl chains of DTAD and PAHB, changing the sign of the ζ potential. The increase in the number of positive charges strengthens the electrostatic repulsion among the mixed aggregates and results in a stretching of the C12PAM backbone, and thus the precipitates are redissolved. The enhanced hydrophobic interaction between the alkyl chains of DTAD and PAHB becomes strong enough for the alkyl chains to associate with each other, and then the stretched star-shaped configuration changes to a closed configuration, resulting in the formation of the soluble spherical aggregates. A variation of the molecular configuration also takes place in pure DTAD and PAHB aqueous solution without C12PAM. DTAD forms vesicles above the critical aggregation concentration and then changes to spherical micelles,38 while PAHB forms loose networklike premicellar aggregates before changing to spherical ones.39 In brief, DTAD cannot form networklike premicellar aggregates before its critical aggregation concentration. Therefore, the present results indicate that C12PAM greatly assists in the formation of networklike aggregates and the transitions of the molecular configurations of the surfactants. The size of the DTAD/C12PAM spherical aggregates is much larger than those of PAHB/C12PAM. The sizes of the spherical aggregates from DLS measurements are about 200 nm for DTAD/C12PAM and about 800 nm for PAHB/C12PAM. Since the ζ potential for DTAD/C12PAM and PAHB/C12PAM shows almost the same value at the same surfactant concentration but the number of positive charges of DTAD is only half of those of PAHB, it is probably because fewer PAHB molecules with six amphiphilic moieties are required to induce a strong enough hydrophobic interaction with DTAD and bring about the change in molecular configuration and the resultant aggregate transition. The 1H NMR results also reveal that almost all of the proton signals of the DTAD molecules are screened, indicating that more DTAD molecules take part in the formation of the spherical aggregates. As a result, a smaller number of PAHB molecules interact with C12PAM to form smaller spherical aggregates.
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CONCLUSIONS
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AUTHOR INFORMATION
Article
Mixed systems of 1% hydrophobically modified and 30% hydrolyzed polyacrylamide (C12PAM) with trimeric or hexameric cationic surfactant (DTAD or PAHB) have been investigated. These oligomeric cationic surfactants with a starshaped spacer promote the formation of mixed aggregates of disaggregated C12PAM. At a fixed C12PAM concentration, the aggregation behavior of the DTAD/C12PAM and PAHB/ C12PAM mixtures goes through three aggregation regions with increasing surfactant concentration: soluble networklike aggregates, precipitated denser and more cross-linked aggregates, and then soluble spherical aggregates. As reported previously, DTAD molecules do not form aggregates and PAHB forms only networklike aggregates with a loose structure below their critical aggregation concentrations. Thus, the present work indicates that the strong binding of oligomeric surfactants with C12PAM, attributed to the electrostatic attraction of the high charged cationic headgroups of oligomeric surfactants with the anionic charge units of C12PAM, hydrophobic interaction between the alkyl chains of the surfactants and C12PAM, and the hydrophobic interaction among the alkyl chains of the surfactants themselves, greatly improves the formation of networklike aggregates at very low concentrations. In addition, the variation of the surfactant molecular configurations is an important factor inducing the morphology and structure transitions of the mixed aggregates, which is controlled by the adjustments of hydrophobic interaction among the alkyl chains of the oligomeric surfactants and C12PAM and the charge density of the mixed aggregates. The combination of star-shaped oligomeric surfactants with oppositely charged and hydrophobically modified polymers provides interesting possibilities for constructing and tuning the novel structures and properties of surfactant/polymer mixed aggregates. So far, only a few studies have been reported on the interaction between polymer and linear oligomeric surfactants, but we have not found any reports on the interactions between star-shaped oligomeric surfactants and polymers. The aggregate transitions of the systems studied in the present paper are unique and quite different from those of other surfactant/ polymer systems. In particular, they form stable and clear networklike aggregates at very low concentration, which has not been found in other surfactant/polymer mixtures. Therefore, this paper provides additional new observations and understanding of polymer/surfactant mixtures in aqueous solutions and may enrich the properties and applications of this kind of system.
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We are grateful for financial support from the Chinese Academy of Sciences and the National Natural Science Foundation of China (grants 21025313, 21021003, and 21361140353). 6666
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