Synthesis and Aggregation Behavior of a Hexameric Quaternary

ARTICLE pubs.acs.org/Langmuir

Synthesis and Aggregation Behavior of a Hexameric Quaternary Ammonium Surfactant Yaxun Fan,† Yanbo Hou,† Junfeng Xiang,‡ Defeng Yu,† Chunxian Wu,† Maozhang Tian,† Yuchun Han,† and Yilin Wang*,† †

Key Laboratory of Colloid and Interface Science and ‡Center for Physiochemical Analysis & Measurement, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

bS Supporting Information ABSTRACT: A star-shaped hexameric quaternary ammonium surfactant (PAHB), bearing six hydrophobic chains and six charged hydrophilic headgroups connected by an amide-type spacer group, was synthesized. The self-assembly behavior of the surfactant in aqueous solution was studied by surface tension, electrical conductivity, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, and NMR techniques. The results reveal that there are two critical aggregate concentrations during the process of aggregation, namely C1 and C2. The aggregate transitions are proved to be caused by the changes of the surfactant configuration through hydrophobic interaction among the hydrocarbon chains. Below C1, PAHB may present a starshaped molecular configuration due to intramolecular electrostatic repulsion among the charged headgroups, and large aggregates with network-like structure are observed. Between C1 and C2, the hydrophobic interaction among the hydrophobic chains may become stronger to make the hydrophobic chains of the PAHB molecules curve back and pack more closely, and then the networklike aggregates transfer to large spherical aggregates of ∼100 nm. Beyond C2, the hydrophobic interaction may become strong enough to cause the PAHB molecular configuration to turn into a pyramid-like shape, resulting in the transition of the spherical large aggregates to spherical micelles of ∼10 nm. Interestingly, the PAHB displays high emulsification ability to linear fatty alkyls even at very low concentration.

’ INTRODUCTION Surfactants are compounds that possess both hydrophobic and hydrophilic moieties in a molecule, and their amphiphilic properties are responsible for their unique functions. For a long time, this field was focused on conventional monomeric surfactants. Subsequently, the search for novel surfactants with high efficiency and effectiveness gave birth to the concept of gemini surfactants.1
5 The structures of gemini surfactants endow them with unique properties and various self-assembly behaviors compared to the corresponding monomeric surfactants.6 Hence, oligomeric surfactants, which are made of three or more amphiphilic moieties chemically connected by spacer groups, are expected to exhibit special properties much more different from monomeric and gemini surfactants. To explore the superior properties of oligomeric surfactants is necessary, and it will be possible to bridge the gap between gemini surfactants and polymeric surfactants. Since the early investigations of oligomeric surfactants were well reviewed by Laschewsky,7 oligomeric surfactants have been extensively promoted.8
16 They were classified as linear, ringtype, and star-shape according to the features of their spacer groups. Zana’s group17,18 synthesized two linear oligomeric surfactants, trimeric and tetrameric, and branched thread-like r 2011 American Chemical Society

and closed-looped micelles were observed in aqueous solutions. Laschewsky et al.19,20 prepared three series of linear oligomeric surfactants with rigid spacers, and indicated that their aggregation behaviors resulted from both the effects of the degree of oligomerization and the nature of the spacer groups. Furthermore, a series of ring-type trimeric surfactants with different lengths of alkyl chains were synthesized by Yoshimura and Esumi,21 and it was found that large aggregates exist in aqueous solution below the critical micelle concentration (CMC), which attributes to the strong interaction between the multiple hydrocarbon chains. Menger and co-workers22 synthesized three series of star-shaped compounds based on pentaerythritol, dipentaerythritol, and adamantine, named “multiarmed” surfactants. The surfactants with eight-carbon hydrocarbon chains have a high propensity to aggregate, which leads to the formation of small micelles consisting of only a few molecules. In brief, people have made great efforts to create plenty of oligomeric surfactants and investigate their aggregation behaviors in aqueous solution, despite the difficulties of their synthesis and purification. Received: June 29, 2011 Revised: July 28, 2011 Published: July 28, 2011 10570

dx.doi.org/10.1021/la202453c | Langmuir 2011, 27, 10570–10579

Langmuir

ARTICLE

ethylammonio)ethylamino)-3 -oxopropyl)-N1,N1,N19,N19-tetramethyl4,16-dioxo-3,7,10,13,17-pentaazanonadecane-1,19-diaminium hexabromide (PAHB) was synthesized according to Scheme 1 and characterized by 1H NMR, 13C NMR, mass spectrum, and elemental analysis. The Krafft temperature of PAHB is below 0 °C.

Hexamethyl 30 ,300 ,3000 ,30000 ,300000 -(2,20 ,200 -Nitrilotris(ethane2,1-diyl)tris(azane-triyl)) Hexapropanoate (1). The solution of

methyl acrylate (47.8 g, 0.56 mol) and tris(2-amino-ethyl)amine (8.82 g, 0.06 mol) in methanol was stirred in room temperature overnight. Then methanol and excessive methyl acrylate were removed in vacuo, and the pure hexaester compound was obtained with 100% yield. 1H NMR (CDCl3, 400 MHz): δ 2.40 (24H, t, CH2), 2.73 (12H, s, CH2), 3.62 (18H, s, CH3O). MS-ESI (m/z): 663 (M+H).

3,3 0 ,3 00 ,3 000 ,3 0000 ,3 00000 -(2,2 0 ,2 00 -Nitrilotris(ethane-2,1-diyl) tris(azanetriyl))hexakis(N-(2-(dimethylamino)ethyl)propanamide) (2). The solution of N,N-dimethylethylenediamine (9.45

Figure 1. Chemical structure and of PAHB.

1

H NMR signal assignments

Our group23
25 synthesized two star-shaped trimeric and one tetrameric cationic quaternary ammonium surfactants with amide-type spacer groups and studied their aggregation behaviors in aqueous solution. Interestingly, both the trimeric surfactants (DTAD and DDAD) form vesicles just above the critical aggregate concentration (CAC), and then transfer to spherical micelles gradually with the increase of the concentration. More specially, the tetrameric surfactant PATC generates network-like premicellar aggregates well below the CMC and changes to small spherical micelles at high concentration. It was proved that the variation of molecular configuration driven by hydrophobic interaction among the hydrophobic chains plays an important role in these aggregate transitions. Because of the rigid spacer and the intramolecular electrostatic repulsion among the quaternary ammonium headgroups, the hydrophobic chains of DTAD and DDAD pack loosely, and the PATC molecule presents a stretched configuration at low concentrations. With the increase of concentration, the hydrophobic interaction of the hydrocarbon chains becomes strong enough to convert the molecular configuration into a pyramid-like shape, leading to the transitions of aggregates. In order to gain further insight into the effect of the degree of oligomerization on their aggregation behaviors in aqueous solution, in the present work, we synthesized a star-shaped hexameric cationic ammonium surfactant PAHB (Figure 1), which has six quaternary ammonium headgroups and six 12-carbon hydrocarbon chains. Its aggregation behavior in aqueous solution was investigated. Special transitions of the aggregates were revealed, and the transition mechanism was discussed.

’ EXPERIMENTAL SECTION Materials. Tris (2-amino-ethyl) amine, dodecyl bromide, and N, Ndimethylethylenediamine were purchased from Alfa Aesar. Methyl acrylate and all organic solvents were purchased from Beijing Chemical Co., and all the organic solvents were dried and distilled. Triply distilled water was used in all experiments. Synthesis. 10-(2-(Bis(3-(2-(dodecyldimethylammonio)ethylamino)3-oxopropyl)amino) ethyl)-N1,N19-didodecyl-7,13-bis(3-(2-(dodecyldim-

g, 0.10 mol) and the above hexaester compound (3 g, 4.5 mmol) in methanol was stirred for 7 days at 35.0 °C. Excessive N,N-dimethylethylenediamine was removed in vacuo, and yellow oil was obtained with 100% yield. 1H NMR (CDCl3, 400 MHz): δ 3.30 (q, 12H, N
CH2), 2.73 (t, 12H, NH
CH2), 2.45 (t, 12H, N
CH2
CH2
N), 2.39 (t, 12H, CH2
N (CH3)2), 2.31 (t, 12H, (CH2)
CdO), 2.22 (s, 36H, N (CH3)2). MS-ESI (m/z): 999 (M+H). PAHB. The solution of dodecyl bromide (18.0 g, 72.3 mmol) and the above polyamine compound (3 g, 3.0 mmol) in acetone/methanol 1:3 (30 mL) was heated to 40 °C under nitrogen atmosphere and was stirred for 3 days. Solvent was removed in vacuo, and excessive dodecyl bromide was washed off with petroleum ether. The crude product was repeatedly recrystallized from ethyl acetate/methanol to afford 3.50 g (46.8%) as white powder. 1H NMR (D2O, 400 MHz): δ 3.71 (t, 12H, HN
CH2), 3.56 (t, 12H, CH2
N+), 3.50 (t, 12H, N+
CH2), 3.25 (s, 36H, CH3
N+), 2.97(t, 24H, N
CH2
CH2
CdO), 2.57(t, 12H, N
CH2
CH2
N), 1.85 (m, 12H, CH2
CH2N+), 1.43(m, 12H, CH3
(CH2)8
CH2), 1.34 (m, 96H, CH3
(CH2)8), 0.93 (t, 18H, CH3-CH2). 13C NMR (D2O, 600 MHz): δ = 174.9, 66.2, 63.3, 52.9, 52.5, 51.4, 50.3, 34.5, 34.0, 33.0, 30.8, 30.7, 30.6, 30.5, 30.3, 27.4, 23.8, 23.6, 22.9, 14.1; MS-ESI (m/z): Calcd, 2492; Found 419 ([M-5Br
]5+/ 5, 10), 543 ([M-4Br
]4+/4, 20), 751([M-3Br
]3+/3, 100); Anal. Calcd for C120H252Br6N16O6 3 1HBr: C, 55.76; H, 9.79; N, 8.70; Br, 21.76. Found: C, 55.79; H, 9.89; N, 8.49; Br, 21.47. Preparation of the PAHB Solution. The PAHB sample was ultrasonically dissolved in water for 2 min at 400 W by a JY92-II N Ultrasonic cell crusher (Ningbo Xinzhi) fitted with a dispersing tool with 2.76 mm outer diameter. The PAHB solution was kept at 25 °C for more than 1 day before measurements. Surface Tension Measurements. Surface tension measurement was carried out using the drop volume method.26 Every drop was kept for more than half an hour, and each surface tension value (γ) was determined from at least five consistent measured values. The surface tension curve was repeated three times. The measurement temperature was controlled at 25.00 ( 0.05 °C using a thermostat. Electrical Conductivity Measurement. The conductivity of the surfactant solutions was measured as a function of concentration using a JENWAY model 4320 conductivity meter. The measurement was performed in a temperature-controlled, double-walled glass container with a circulation of water. Sufficient time was allowed to the system equilibrium between successive additions. The temperature of the solution was controlled at 25.0 ( 0.1 °C. Isothermal Titration Microcalorimetry (ITC). A TAM 2277201 isothermal titration microcalorimeter (Thermometric AB, J€arf€alla, Sweden) was used to measure the value of the CAC and the enthalpy change for aggregation of PAHB in aqueous solution. The sample cell and the reference cell of the microcalorimeter were initially loaded with 700 μL and 815 μL of pure water, respectively. Five millimolar PAHB 10571

dx.doi.org/10.1021/la202453c |Langmuir 2011, 27, 10570–10579

Langmuir

ARTICLE

Scheme 1. Synthetic Procedure of PAHB

solution was injected consecutively into the stirred sample cell in each portion of 10 μL using a 500 μL Hamilton syringe controlled by a Thermometric 612 Lund pump until the desired concentration range had been covered. During the whole 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. The reproducibility of experiments was within (4%. All the measurements were performed at 25.00 ( 0.01 °C. NMR. 1H NMR measurement was carried out at 20.7 ( 0.3 °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 solution of PAHB in D2O. The center of the HDO signal (4.79 ppm) was used as the reference in the D2O solutions. In all the NMR experiments, the number of scans was adjusted to achieve good signal-to-noise ratios depending on the surfactant concentration, and was recorded with a digital resolution of 0.04 Hz/data point. Spin
spin relaxation time (T2), 2D NOESY, and 2D DOSY NMR experiments were all performed on a Bruker Avance 600 spectrometer at 25 °C. The T2 values were measured by the Carr
Purcell
Meiboom
Gill (CPMG) sequence (PD-90°x-[τ-180°y-τ]2n-AC). The 2D NOESY experiments were carried out with the standard three-pulse sequence with a mixing time of 800 ms. 2D DOSY NMR spectra were obtained with the stebpgp1s pulse program and a maximum gradient strength of 50 G cm
1. The gradient field was linearly increased in 32 steps, resulting in an attenuation of 1H NMR from 2% to 95%. Dynamic Light Scattering (DLS). Measurements were carried out using an LLS spectrometer (ALV/SP-125) with a multi-τ digital time correlater (ALV-5000). Light of λ = 632.8 nm from a solid-state He
Ne laser (22 mW) was used as the incident beam. The measurements were conducted at a scattering angle of 90°. All of the solutions were filtered through a 0.45 μm membrane filter of hydrophilic PVDF before the measurements. The correlation function was analyzed from the scattering data via the CONTIN method to obtain the distribution of diffusion coefficients (D) of the solutes. The apparent hydrodynamic radius Rh was deduced from D by the Stokes
Einstein equation Rh = kBT/(6πηD) for spherical particles, where kB represents the Boltzmann constant, T is the absolute temperature, and η is the solvent viscosity. All the measurements were performed at 25.00 ( 0.01 °C.

Cryogenic Transmission Electron Microscopy (Cryo-TEM). The PAHB samples were embedded in a thin layer of vitreous ice on freshly carbon-coated holey TEM grids by blotting the grids with filter paper and then plunging them into liquid ethane cooled by liquid nitrogen. Frozen hydrated specimens were imaged by using an FEI Tecnai 20 electron microscope (LaB6) operated at 200 kV with the lowdose mode (about 2000 e/nm2) and the nominal magnification of 50 000. For each specimen area, the defocus was set to 1
2 μm. Images were recorded on Kodak SO163 films and then digitized by Nikon 9000 with a scanning step 2000 dpi corresponding to 2.54 Å/pixel.27 Preparation and Characterization of Emulsions. The volume ratio of the PAHB aqueous solution to oil was always kept at 1:1 in this study. Heptane, dodecane, toluene, and xylene were chosen as oil phase separately. The oil was added into 0.005 mM PAHB aqueous solution, and then the mixture was ultrasonically homogenized for 2 min at 700 W by a JY92-II N Ultrasonic cell crusher (Ningbo Xinzhi) fitted with a dispersing tool with 2.76 mm outer diameter. The mixtures were kept at 25 °C and were observed in 1 day and 2 weeks. The emulsion type was identified by the drop test.28 A drop of emulsion was added to a small volume of the oil phase and the aqueous phase separately. An emulsion that dispersed in the aqueous phase but not in the oil phase was assessed as water continuous (oil-in-water (O/W)) and vice versa. The degree of emulsification was determined by the height of emulsion layer.29 Microscope images of the emulsion droplets were obtained using a XSP-BM-13C biological microscope system. Emulsion droplets were placed either on a slide covered by a coverslip or directly on a slide when the droplet size was large.

’ RESULTS AND DISCUSSION CAC of PAHB. The surface tension (γ), the observed enthalpy change (ΔHobs), and the electrical conductivity (k) are plotted as a function of the PAHB concentration (C) as shown in Figure 2. The CAC from the break in the surface tension curve is 1.31 mM, while the CAC from differentiating the calorimetry curve is 0.05 mM, much lower than the former. Especially, two breakpoints at 0.11 mM and 1.13 mM are found by differentiating the electrical conductivity curve, which are very close to the critical concentrations from the calorimetry curve and the surface 10572

dx.doi.org/10.1021/la202453c |Langmuir 2011, 27, 10570–10579

Langmuir

ARTICLE

Table 1. CAC, Aggregation Ionization Degree (r), and Enthalpy Change for Aggregation (ΔHagg) of PAHB in Aqueous Solution at 25.0 °C CAC (mM) R

ΔHagg (kJ mol
1)

0.11

0.29


83.4

1.13

0.24

surface tension calorimetry conductivity C1 C2

Figure 2. Variations of (a) surface tension (γ), (b) observed enthalpy change (ΔHobs), and (c) electrical conductivity (k) of the PAHB aqueous solution with the concentration (C) at 25.0 °C. The red lines in panels a and b are the corresponding differential curves.

tension curve, respectively. Thus it can be concluded that PAHB displays two CACs in the concentration region studied. Herein, they are defined as C1 and C2. The first transition C1 could not be observed in the surface tension curve, probably because the value of surface tension at C1 is close to that of pure water, and the concentration value is very low, and thus the breakpoint could not appear obviously. Surprisingly, the enthalpy change for the aggregation of PAHB (ΔHagg) (Figure 2b) exhibits a very large exothermic value,
83.4 kJ/mol, which is much larger than those of other surfactants.23,25,30
32 Here we compare it with some surfactants comprising very similar cationic ammonium amphiphile moiety. ΔHagg(monomeric DTAB) is
1.01 kJ/mol, ΔHagg(dimeric C12C6C12 3 2Br
) is
3.67 kJ/mol, ΔHagg(trimeric DDAD) is
18.9 kJ/mol, and ΔHagg(tetrameric PATC) is
30.2 kJ/mol. From the monomeric to the present hexameric surfactant, the enthalpy change per amphiphilic moiety increases gradually from
1.01,
1.84,
6.3,
7.6 to
17.4 kJ/mol. It is indicated that, during the PAHB aggregation, the contribution of each hydrocarbon chain to inter- and intramolecular hydrophobic interaction becomes much stronger than that of other surfactants. That is to say, the cooperative hydrophobic interaction becomes

0.05 1.31

stronger with the increase of the number of the hydrophobic chains in a surfactant molecule. Of course, hydrogen bonding between amide groups can also increase the enthalpy change per amphiphilic moiety for DDAD, PATC, and PAHB. However, each amphiphilic moiety of DDAD, PATC and PAHB has one amide group, and the significantly enhanced enthalpy change per amphiphilic moiety for PAHB confirms that the contribution of each hydrocarbon chain to inter- and intramolecular hydrophobic interaction in the PAHB aggregation becomes much stronger than that for other surfactants. ΔHagg (
83.4 kJ/mol) should concern the entire aggregation process including both the first and the second aggregation processes at C1 and C2. Because we cannot see an obvious enthalpy change at C2, the ΔHagg (
83.4 kJ/mol) should be mainly contributed by the first aggregation process. In addition, two aggregation ionization degrees (R) for the two aggregation processes characterized by C1 and C2 have been evaluated from the electrical conductivity curve, which are taken from R1 = (dk/dC)C1