Investigation on the behavior of choline-derived cationic surfactant in aqueous solution in the absence and presence of PdCl2

Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 100–107

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Investigation on the behavior of choline-derived cationic surfactant in aqueous solution in the absence and presence of PdCl2 Shili Song ∗ , Peng Fu, Yefang Wang, Zhen Wang, Yuhua Qian College of Chemistry an Environmental Science, Henan Normal University, 46# East of Construction Road, Xinxiang 453007, PR China

a r t i c l e

i n f o

Article history: Received 9 December 2011 Received in revised form 1 February 2012 Accepted 24 February 2012 Available online 5 March 2012 Keywords: Vesicles Choline Nanosphere Palladium chloride

a b s t r a c t A choline-derived cationic surfactant dodecyl-(2-hydroxyethyl)-dimethylammonium bromide (DHDAB) was synthesized. The properties of DHDAB and DHDAB/PdCl2 aqueous solutions were investigated by employing tensiometry, conductance, dynamic light scattering, transmission electron microscopy and turbidity measurements. The fraction of counterion binding (ˇ) of the DHDAB micelle decreases with increase of the temperature. The energetics of the self-assembly process of DHDAB were evaluated, it 0 0 indicated the Hm and Sm values for the micellization exhibited nice compensations between them. Addition of PdCl2 to the DHDAB solutions can significantly affect the interface and aggregation properties of DHDAB solutions due to forming a complex between DHDA+ and [PdClm Br4−m ]2− . The phase behaviors of the DHDAB/PdCl2 aqueous mixture at fixed ratios and at constant concentration of PdCl2 were studied. DLS coupled with TEM studies indicated the DHDAB/PdCl2 aqueous mixtures can form vesicles in a certain CDHDAB range. Moreover, the hollow Pd spheres were obtained by direct reduction (with NaBH4 ) of the DHDAB/PdCl2 aqueous mixture at the control experimental condition. This may advance further understanding of metal-ion induced transformation of organized assemblies in choline-like cationic surfactant solutions and promote its applications in preparation of hollow nanomaterial. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Quat-based ammonium ionic liquids (in particular, quat analogues of choline) have attracted increasing interest recently because they are much less expensive than imidazolium-based ionic liquids [1–12]. Choline chloride (also known as 2-hydroxy ethyltrimethylammonium chloride or vitamin B4 ) is a cheap organic salt, normally produced in human tissues in sufficient amount to meet human needs [13]. It is an important component of phospholipids (lecithin and sphingomyelin), it is required for the synthesis of the neurotransmitter acetylcholine, it acts as a source of labile methyl groups, and it is a component of pulmonary surfactant [14]. Binnemans et al. [15] prepared low toxic choline saccharinate and choline acesulfamate ionic liquids. Abbott et al. obtained ionic liquids by mixing choline chloride with hydrated transition metal salts [16], or with anhydrous zinc(II) chloride or tin(II) chloride [8,9]. They found that choline chloride forms so-called “deep eutectic solvents (DES)” with hydrogen bond donors a mixture of urea and choline chloride in a 2:1 molar ratio is a liquid at room temperature [17]. DESs share many characteristics of conventional ILs (e.g. they are nonreactive with water, nonvolatile, and biodegradable), but their low

∗ Corresponding author. Tel.: +86 373 3326335; fax: +86 373 3326336. E-mail address: [email protected] (S. Song). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2012.02.042

cost makes them particularly desirable (more than conventional ILs) for large-scale synthetic applications. DESs have been the solvent of choice for a number of enzyme-based biotransformations because of their excellent properties for a wide variety of solutes, including enzymes and substrates [18,19]. These ion-liquid like solvents were applied for the synthesis of zeolite analogues [20] and for the functionalization of cellulose [21]. Mixtures of choline chloride and malonic acid were used for the synthesis of iron(III) oxalatophosphates [22]. In addition, complexes between alkyl-(2hydroxyethyl)-dimethylammonium salts and inorganic salts can be used in the electrodeposition of thick, adherent, crack-free films. That type of salt can be used as an effective media for some Diels–Alder reactions [23]. As precursors of choline-like ILs, quat analogues of choline have also been synthesized, their thermodynamic properties and partition coefficients in octan-1-ol/water binary system (log P) have been described, together with their anti-microbial activities [10,11,24]. Rogalski’s group investigated the effect of (2-hydroxyethyl)-dimethylpropylammonium bromide and (2-hydroxyethyl)-dimethylbutylammonium bromide on the properties of CTAB aqueous solutions. It was shown that the stability of mixed micelles is higher as compare with CTAB micelles due probably to the well-known ability of choline-like ILs to form hydrogen bond [25]. Above statement show choline chloride can form ionic liquids with metal halides (e.g. ZnCl2 , SnCl2 and FeCl3 ) because of metal halides forming complex anions with Cl− , as addition of analogues of choline into CTAB solution, more stable mixed

S. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 100–107

micelles compared with CTAB micelles can form. These characteristic properties inspire us to further study the properties of aqueous solutions of quat analogues of choline with longer alkyl chains in presence or absence of metal halides. Vesicles in aqueous solutions are fascinating supramolecular self-assembled structures that have extraordinary potential for applications such as microreactors and nanodevices for encapsulation and controlled release [26–28]. Recently, several groups have noted that some ligands with long alkyl chains can form vesicles upon coordination with metal ions [29]. Metal decorated vesicular structures can mimic not only the active center of the metalloenzyme but also the microenvironment of the metalloenzyme [30]. Moreover, these metal-compound-induced vesicles can be applied as efficient directors for the rapid synthesis of hollow metal spheres [31]. Hence, metal decorated vesicles have become a topic of increasing interest in recent years. So far there are many studies on metal-compound-induced vesicles. However, the vesicle composed of choline-like cationic surfactant and metal salt has not been reported up to now. In this work, physicochemical properties and the phase behaviors of choline-like quat ammonium salt dodecyl-(2-hydroxyethyl)dimethylammonium bromide in the absence or presence of PdCl2 solution were investigated. Furthermore, the hollow Pd spheres were obtained by direct reduction (with NaBH4 ) of the DHDAB/PdCl2 aqueous mixture at the control experimental condition. This may advance further understanding of metal-ion induced transformation of organized assemblies in choline-like cationic surfactant solutions. 2. Experimental 2.1. Materials The quaternary ammonium salt (N-(2-hydroxyethyl)-N,Ndimethylammonium bromide (DHDAB) was synthesized as follows, 1-bromohexadecane and N,N-dimethylethanolamine were taken in the molar ratio 1.2:1 in 30% methanol/acetonitrile and refluxed. After 24 h, the solvent was evaporated on a rotary evaporator and pure products were obtained by crystallization of the reaction mixture from methanol/ethyl acetate. The yield was 78%. The product was characterized with 1 H NMR spectroscopy (Bruker Avance-400, shown in Fig. S5b). 1 H NMR (400 MHz, CDCl3 ): ı = 3.90 (br, 2H), 3.39 (t, 2H), 3.30 (t, 2H), 3.04 (s, 6 H), 1.66 (br, 2H), 1.16–1.24 (m, 18H), 0.65 ppm (br, 3H). All other reagents were products of A.R. grade. Doubly distilled water was used for solution preparation. 2.2. Surface tension measurements The surface tension of aqueous solutions of single and mixed surfactants at various concentrations was determined at 25 ◦ C with the drop volume method. The drop was kept for about 10 min to attain the adsorption equilibrium. Corrections to the drop volume were made according to Harkins and Brown [32]. The surface tension was measured with accuracy ±0.1 mN/m. All experiments were repeated at least three times to ensure consistent results. 2.3. Conductivity measurements The electrical conductivity was measured at 25 ± 0.1 ◦ C using a DDS-12A conductometer from DAPU Instrument Co., Shanghai, China, and the cell constant was obtained by calibration with KCl solutions of known concentration. All measurements were performed in a double-walled glass container with the temperature being controlled by circulation of water.

101

2.4. Turbidity measurements The turbidity of the DHDAB/PdCl2 aqueous solutions, reported as 100-%T, was measured at 800 nm using a Brinkman PC920 probe colorimeter equipped with a thermostated water circulating bath. The final turbidity titration curves were only recorded after the values became stable (about 2–4 min) and were corrected by subtracting the turbidity curve from a metal halide free DHDAB solution. The experiments were performed at a temperature of 25 ◦ C. 2.5. Dynamic light scattering (DLS) The mean droplet size and distribution of aggregates were determined by dynamic light scattering (DLS) at a scattering angle of 90◦ (Zetasizer Nano-ZS90, Malvern, UK) at 298.18 K, employing an argon ion laser ( = 532 nm). The DLS data was analyzed by the cumulants method obtaining the z-average mean of aggregate diameter and the polydispersity index, which is a dimensionless measure of the width of size distribution. Polydispersity indexes lower than 0.2 indicate that polydispersity maintains its significance as an accurate measure. 2.6. Transmission electron microscopy (TEM) TEM samples were prepared from the DHDAB/PdCl2 aqueous solutions using a negative-staining method, and 1% uranyl acetate solution was used as the staining agent. A drop of the solution was placed onto a carbon Formvar-coated copper grid (300 mesh), and the excess liquid was sucked away by filter paper. After drying, the samples were imaged under a JEM-100SX electron microscope at an operating voltage of 80 kV. 2.7. Preparation and characterization of hollow nanomaterial Typical synthetic procedure for Pd hollow spheres: The freshly prepared NaBH4 solution (1.0 mL, 0.1 mol L−1 ) was added quickly to the stock solutions of DHDAB/PdCl2 (5:1, 25.0 mL, CDHDAB = 5 mmol L−1 ). The mixture was stirred for 3 min, and the product was collected by centrifugation and washed with water. Then the products were characterized by TEM and powder XRD (D8 Advance, Bruker). 3. Results and discussion 3.1. CMC, counterion binding and aggregation number The CMCs of DHDAB at different temperature were determined by conductance. As comparison, we also measured the CMC of DHDAB at 25 ◦ C using tensiometry. Table 1 collects some of these results. The measured conductance and surface tension were plotted against concentration to estimate CMC from the breaks in the plots or the transition points therein (Fig. 1). The CMC value of DHDAB determined by the method of conductance at 25 ◦ C is slightly lower than that of dodecyltrimethylammonium bromide [33]. This is probably due to the hydrogen bonding of hydroxyl groups of DHDAB. Therefore, the aggregation of DHDAB is easier than that of dodecyltrimethylammonium bromide in some ways. Careful examination should reveal the presence of two breaks in the conductivity plot of DHDAB at different temperature (Figs. 1b and S1 in supporting information). Two transition points were also detected in the surface tension plot of DHDAB at 25 ◦ C (Fig. 1a). The first break point or transition point indicated the DHDAB began to aggregate but did not micellize, this phenomena is similar to that of short alkyl imidazolium salt ionic liquid [34]. The second break point or transition point meant the DHDAB formed

102

S. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 100–107

Table 1 CMC, counterion binding and related energetic parameters. T (K)

CMC (mol L−1 )

ˇ

G (kJ mol−1 )

H (kJ mol−1 )

S (kJ mol−1 )

293.18 298.18 308.18 313.18 318.18 328.18 338.18

0.01412 0.01429 (0.01370)a 0.01453 0.01464 0.01510 0.01572 0.01658

0.7603 0.7533 0.7255 0.7095 0.7014 0.6844 0.6560

−35.50 −35.91 −36.42 −36.66 −36.93 −37.31 −37.77

−17.02 −18.20 −20.45 −21.53 −22.56 −24.55 −26.41

0.0631 0.0594 0.0519 0.0483 0.0452 0.0389 0.0336

a

CMC value determined by tensiometry.

Fig. 1. (a) Plot of surface tension () vs concentration of DHDAB at 25 ◦ C. (b) Specific conductance () vs concentration of DHDAB at 25 ◦ C.

micelles in aqueous solutions. From the slopes of the pre- (S1 ) and post-micellar (S2 ) courses in the conductance plots, the fraction of counterion condensation/binding (ˇ) of the micelle was obtained from the relation ˇ = (1 − S2 /S1 ) [35]. This is a convenient and fairly accurate method for the determination of ˇ, the other potential methods are ionometry (using ion-selective electrodes) [36], NMR [37], and microcalorimetry to a limited extent [38]. The ˇ values of micelles at different temperature are also presented in Table 1. The fraction of counterion binding of the micelle moderately decreases with increase of temperature. The micellar aggregation number (N) was determined by the fluorescence quenching method with pyrene as the probe and benzophenone as the quencher. The emission spectral results and the linear plot of ln(I0 /I) against the concentration of quencher (Cq ) at 25 ◦ C are presented in Fig. S2. The equilibrium of the DHDAB between the aqueous and micelle phase follows the Poisson distribution. The equation to be applied is [39] ln

I  0

I

=

NCq Ct − CMC

(1)

where I and I0 are the fluorescence intensities with and without the presence of quencher respectively. Ct is the total concentration of DHDAB. According to Eq. (1), value of the average aggregation number for DHDAB has been calculated from the slope of linear plot (Fig. S2b) and the CMC value (0.0137 mmol L−1 , determined by tensiometry). The N value obtained was about 41. 3.2. Process energetics of DHDAB aggregation The energetics of the self-assembly process of DHDAB were evaluated with the following rationale. Considering the pseudophase 0 ) was obtained by micellar model, the standard Gibbs energy (Gm the relation 0 Gm = (1 + ˇ)RT ln XCMC

(2)

where XCMC is the CMC value expressed in the mole fraction scale R and T have their usual significance. The standard state was a hypothetical state of unit activity of the solute at unit mole fraction. The van’t Hoff equation (Eq. (3)) was used to get the enthalpy of 0 ) [40]. Thus micellization (Hm



0 Hm

=

0 /T ) ∂(Gm ∂(1/T )



(3) p

0 /T vs 1/T is presented in Fig. S3 of the supporting The plot of Gm information, The curve was fitted to a polynomial (Eq. (4)) 0 Gm b c =a+ + 2 T T T

(4)

where the coefficients a, b, and c obtained were 0.057 ± 0.047, −87.58 ± 29.49, and 10,337.5 ± 4621.14, respectively. The first 0 ). The derivative of eq4 produced the van’t Hoff enthalpy (Hm energetic parameters obtained for the micellization system of 0 and H 0 values are all DHDAB are presented in Table 1. Gm m negative and decrease with increase of temperature, the negative 0 suggest that the process of micellization of DHDAB values of Hm is exothermic. This means that the major attractive force for micellization of DHDAB molecules is hydrophobic interaction [41]. On 0 , is positive and has a the other hand, the entropy change, Sm decreasing tendency with the increase of temperature, indicating formation of more disordered aggregates. Similar results were also reported by others for DTAB surfactant [42]. From the tempera0 , the specific heat of micellization (C 0 ) ture dependence of Hm pm has been found to be −0.22 kJ K−1 mol−1 for the micelle formation. 0 and T was nicely linear (Fig. S4 The graphic plot between Hm 0 = [∂(H 0 )/∂T ] from the in supporting information) to get Cpm m p slope with a correlation coefficient 0.9991. When plotting entropy 0 ) against the corresponding enthalpy change (H 0 ), a change (Sm m linear relationship was found (Fig. 2). This can be attributed to the enthalpy–entropy compensation effect. When the enthalpy change becomes larger, the micelle becomes more ordered and hence loses entropy during the aggregation. Such compensations are often cited

S. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 100–107

103

Table 2 The values of DHDAB concentration at different break points. Mixture

CDHDAB (mmol L−1 ) Surface tension

DHDAB/PdCl2

Conductance

C1

C3

C1

C2

0.057

0.52

0.13

0.55

break point because the volume of the aggregation particle or the number of vesicle increase. 3.4. Phase behavior of the DHDAB/PdCl2 aqueous mixture

Fig. 2. Enthalpy–entropy compensation plots for micellization.

[43] in the literature for micellization and other physicochemical processes. 3.3. CMC of DHDAB in the presence of PdCl2 The critical micellar concentrations for the systems of DHDAB/PdCl2 aqueous mixture were obtained by conductance and drop volume measurements at fixed ratio. The curves of the  vs CDHDAB showed three distinct transition points (Fig. 3a) and the  vs CDHDAB showed two distinct break points (Fig. 3b). For the system of DHDAB/PdCl2 , the surface tension decreases rapidly at lower concentration up to the first transition point (0.057 mmol L−1 ), then gradually decreases to the second transition point (0.324 mmol L−1 ). Above 0.324 mmol L−1 , the surface tension decreases rapidly up to the third transition point again, then gradually goes down. The slow decrease of  in the region between the first and the second transition point hints a formation of micelles. In the concentration range between the second and the third transition point, it is probably that vesicle coexists with micelle. Above the third point, the solution becomes more turbid with increase of the concentration which indicates the amount of insoluble (DHDA+ )2 [PdClm Br4−m ]2− and the number of vesicle is increasing. In the aqueous mixture of DHDAB/PdCl2 , PdCl2 can complex with the counterion of DHDAB, Br− , which can form metal anion such as [PdClm Br4−m ]2− [8,9,16]. This metal anion can combine with dodecyl-(2-hydroxyethyl)-dimethylammonium ion (DHDA+ ) which forms (DHDA+ )2 [PdClm Br4−m ]2− , it is similar to double chain cationic surfactant. As we know, the CMC of double chain cationic surfactant is usually lower than that of single chain cationic surfactant with same hydrophobic group, and usually forms vesicle at a certain concentration [44]. On the other hand, the volume of the metal anion is larger than the volume of Br− , the aggregation of (DHDA+ )2 [PdClm Br4−m ]2− at the air-solution interface is probably more loose than that of DHDAB, therefore, the surface tension for the system of DHDAB/PdCl2 is higher than that of DHDAB. In the plot of  vs CDHDAB , two break points are found for DHDAB/PdCl2 system. The values of DHDAB concentration at the break points are shown in Table 2. It is clear that the CMC determined by tensiometry is different from that measured by conductance. At low concentrations, below the critical micelle concentration, the conductivity increases with CDHDAB due to the sum of the contributions of the free Br− , [PdClm Br4−m ]2− and DHDA+ ions. Above the CMC, the increase in  is smaller because the micelles or vesicles have lower mobility than the free ions, and a fraction of the Br− or [PdClm Br4−m ]2− are ion-paired with the micelles or vesicles. With further increase of DHDAB concentration, the plot of  vs CDHDAB obtains the second

Fig. 4a and b shows the plots of turbidity vs concentration of DHDAB at fixed ratio of DHDAB/PdCl2 and constant concentration of PdCl2 respectively. At fixed ratio, the solutions of DHDAB/PdCl2 maintain transparent while the CDHDAB < 0.4 mmol L−1 , the turbidity increases rapidly in the concentration range of 0.4–0.5 mmol L−1 , at CDHDAB = 0.5 mmol L−1 , the solution became slightly turbid observed by naked eyes. Continuous increase of DHDAB concentration, the turbidity increases continuously until reach the maximum around 14 mmol L−1 , which indicates the amount of insoluble (DHDA+ )2 [PdClm Br4−m ]2− and the number of vesicle increase. Above 14 mmol L−1 , the turbidity of the system decreases sharply with increase of DHDAB concentration. At constant concentration of PdCl2 , The turbidity hardly changes in the DHDAB concentration range of 2–8 mmol L−1 , it indicates the excess DHDAB in this researching concentration range may mainly exist in monomer. Above 9 mmol L−1 , the turbidity of the system decreases sharply until the solution becomes transparent in the narrow concentration range (10–12 mmol L−1 ). Further increase of DHDAB concentration, the turbidity for two systems maintain constant because the excess DHDAB may form small aggregates. Upon storage, microphase separation occurred and an orangered organic salt precipitated slowly for the turbid solutions of the two researched systems. The organic salt was readily soluble in common organic solvents such as chloroform, toluene and THF. The 1 H NMR spectra of the precipitation (shown in Fig. S5a) argues the precipitation is a complex between DHDA+ and [PdClm Br4−m ]2− . Above a certain DHDAB concentration which is close to the CMC of DHDAB, the apparent solubility of the organic salt increased with increasing surfactant concentration for two systems (shown in the sparse area of Fig. 4a and b). This observation can be explained in terms of solubilization in the classical sense, i.e. the incorporation of the alkyl chains of the preformed organic salt in the hydrophobic zone of the vesicles. Alternatively, binding of the complex anions to the palisade layer of the aggregates may occur. The structure and composition of these aggregates are practically the same in either case, provided that the number of individual palladate-surfactant species incorporated in vesicles, or the number of palladate anions bound to one vesicle, is small in comparison with the aggregation number of the vesicle. 3.5. Size and morphology of aggregates DLS is a versatile tool with which to measure the hydrodynamic diameter (dH ) of the self assemble particles. The plots in Fig. 5a and b show the distribution of the hydrodynamic radius of the DHDAB/PdCl2 aggregates at different CDHDAB for a given composition and for a given CPdCl2 respectively. At the fixed ratio of DHDAB/PdCl2 , monomodal distribution is observed except the CDHDAB = 0.5 and 30.0 mmol L−1 (Fig. 5a). At CDHDAB = 0.5 mmol L−1 , the size of the aggregate has a wide distribution in the range of 20–400 nm. It indicates that the micelle and vesicle could

104

S. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 100–107

Fig. 3. (a) Plots of surface tension () vs concentration of DHDAB and (b) specific conductance () vs concentration of DHDAB at fixed ratio of DHDAB/PdCl2 = 5:1.

coexist in this system and the micelle is predominant. The distribution of the aggregate size becomes narrow with increase of CDHDAB until CDHDAB = 14.0 mmol L−1 . Especially, the distribution of the aggregate size hardly changes as CDHDAB is between 2.0 and 10.0 mmol L−1 . It can be speculated that the number of vesicle increase with CDHDAB and the vesicle is predominant in this concentration range. Further increase of DHDAB concentration, the distribution of the aggregate size becomes wide again. At CDHDAB = 30 mmol L−1 , the distribution is typical bimodal with a predominant small aggregate. As discussed in the turbidity section, the excess DHDAB may “dissolve” the vesicle and form micelle or a certain kind of large micelles. Moreover, we found that the value of dH corresponding to each peak top increases with CDHDAB as CDHDAB is below 30.0 mmol L−1 . However, at constant CPdCl2 = 0.4 mmol L−1 , the distributions of the aggregate size change slightly with the CDHDAB and are all monomodal in our researched range of CDHDAB (Fig. 5b). At the fixed ratio of DHDAB/PdCl2 , the average dH of the self assemble particle increase with the increase of CDHDAB as CDHDAB < 20 mmol L−1 (Fig. 6). Especially, the average dH of the particle maintains nearly constant as CDHDAB is between 2.0 and 10.0 mmol L−1 , it indicates that the aggregates are more stable in this concentration range than the aggregates of the other concentrations. For the systems of different ratio, the similar phenomena were also observed (Fig. 6). To identify the morphology of aggregates, we performed the TEM measurement for DHDAB/PdCl2 aqueous mixtures. Fig. 7 shows a typical morphology of vesicles at CDHDAB = 5.0 mmol L−1 . Although some vesicles collapsing due to the drying of sample, vesicles induced by DHDAB/PdCl2 are relatively uniform spheres, the

average radius is about 160 nm, which agrees with the result of DLS to some extent. The formation of vesicles may be explained by the critical molecular packing parameters p (=v/al) [27] p < 1/3 (spherical micelles), p = 1/3–1/2 (cylindrical micelles), p = 1/2–1 (flexible bilayers and vesicles), and p = 1 (planar bilayers). Here v is the volume occupied by the alkyl chain, l is the length of the chain, and a is the area on the surface of a given aggregate. In the presence of PdCl2 , the DHDA+ can complex with [PdClm Br4−m ]2− , which forms analogues of double-chain surfactant, and their headgroups are not too small, so their p values should be close to 1, which makes them possible to form vesicles or disk like micelles composed of bilayer structures. For in aqueous solutions containing a mixture of Cl− and Br− a series of mixed ligand complexes [PdClm Br4−m ]2− , may coexist to extents depending on the relative concentrations of the two halide ions [45,46]. Measurements of the stability constants of the complexes indicated that Pd2+ binds Br− more strongly than Cl− , the overall stability constant of [PdBr4 ]2− is nearly 104 times higher than that of [PdCl4 ]2− [45,46]. For the present studied DHDAB/PdCl2 solutions, the surfactant concentration, and hence the Br− concentration, was in a large excess over [PdCl2 ]. Therefore, it seemed plausible to assume that [PdBr4 ]2− is predominant counterion in our experimental condition. It is surprising that vesicle was not observed in the wide range of C12 TABr concentration for the dodecyltrimethylammonium bromide (C12 TABr)/K2 [PdCl4 ] solutions [47]. Comparing with C12 TABr, the head volume of DHDAB is obviously larger than that of C12 TABr, therefore, the critical molecular packing parameters p of [DHDA+ ]2 [PdBr4 ]2− should be lower than that of [C12 TA+ ]2 [PdBr4 ]2− , and in range of p value which forms

Fig. 4. The turbidity in the DHDAB/PdCl2 systems. (a) Turbidity vs concentration of DHDAB at DHDAB/PdCl2 = 5:1. Inset: the turbidity plots below CDHDAB = 1.0 mmol L−1 . (b) −1 Turbidity vs concentration of DHDAB at CPdCl2 = 0.4 mmol L . Turbidity is expressed as 100-100 × T (transmittance).

S. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 100–107

105

Fig. 5. Distribution of the hydrodynamic radius for aggregates of the DHDAB/PdCl2 aqueous mixtures. (a) The plots of DHDAB/PdCl2 at fixed ratio (5:1) and (b) represents −1 the plots of DHDAB/PdCl2 at different ratios, CPdCl2 = 0.4 mmol L .

In recent years, hollow metal nanostructures have attracted more attention on account of their numerous potential applications in drug-delivery carriers, biomedical diagnosis agents, and cell imaging [48]. Hollow nanostructures can be obtained by using hard- and soft-template methods. Although the hard-template method [31,49] is an effective route for fabricating size-controlled

hollow nanostructures, the preparation of the template is tedious and post-synthetic treatments are needed to remove them from the products, which might destroy as-prepared hollow nanospheres. Soft-template approaches including use of vesicles, emulsions, micelles, and even gas bubbles have been well developed by several groups [50]. A few studies showed metal ion-induced vesicle was an efficient director for rapid synthesis of hollow nanomaterial [31,51]. Our studies clearly show that DHDAB/PdCl2 aqueous mixture can form vesicles with hollow structures under controlled condition, respectively. The vesicle surface should consist of numerous aggregated DHDA+ cations and metal anions such as [PdClm Br4−m ]2− . When these vesicles are reduced by NaBH4 ,

Fig. 6. Mean hydrodynamic diameters of the DHDAB/PdCl2 aqueous mixtures at different CDHDAB . () The plots of DHDAB/PdCl2 at fixed ratio (5:1); (䊉) represents −1 the plots of DHDAB/PdCl2 at different ratios; CPdCl2 = 0.4 mmol L .

Fig. 7. Transmission electron micrograph (negative-staining method) of vesicles formed from DHDAB/PdCl2 solution (CDHDAB = 5.0 mmol L−1 , DHDAB/PdCl2 = 5:1).

vesicle. Moreover, the hydroxy group of DHDAB molecule probably play a role in the mechanism of vesicle formation to a extent, which deserves further investigations. 3.6. Preparation and characterization of Pd nanospheres

106

S. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 100–107

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21076064), the Fundation of Science and Technology of Henan, China (No. 092300410241) and the Natural Science Fundation of Henan, China (No. 2009A150013). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2012.02.042. References

Fig. 8. TEM micrographs of Pd (CDHDAB = 5.0 mmol L−1 , DHDAB/PdCl2 = 5:1).

the newly formed metal nucleation sites with high reactivity are expected to diffuse quickly to form homogeneous metal on the vesicle template. After growth of these nucleation sites, hollow Pd shells would form. The hollow Pd spheres were obtained by direct reduction (with NaBH4 ) of the turbid solution containing PdCl2 . The structure and morphology of the Pd sample were investigated by TEM. The representative TEM images for Pd sphere obtained by direct reduction of DHDAB/PdCl2 solution are presented in Fig. 8. As shown in Fig. 8, the centers of the spheres are brighter than the edges, which indicate that their interiors are hollow. The average size of the hollow Pd sphere is approximately 200 nm, which is comparable with the size of the vesicles before reduction. The XRD patterns of the as-prepared Pd spheres are presented in the supporting information Fig. S6. The broad peaks in the XRD patterns indicate the small sizes of nanocrystals. Moreover, we found that the average size of the hollow Pd hardly change in the range of CDHDAB 2.0–12.0 mmol L−1 , which was in line with the results of DLS investigations. Template synthesis is not only a powerful means for materials synthesis, but it can also contribute to the determination and analysis of self-organized morphologies [52]. Thus, the TEM images of Pd further confirm that the predominant aggregation morphologies of DHDAB/PdCl2 solution used in preparing the nanomaterial are vesicles.

4. Conclusion In conclusion, we have synthesized choline-like quaternary ammonium salt dodecyl-(2-hydroxyethyl)-dimethyl-ammonium bromide. The properties of DHDAB, DHDAB/PdCl2 aqueous solutions were investigated. For DHDAB aqueous solution, the critical micelle concentration of DHDAB is 0.01370 and 0.01429 mol L−1 determined by the method of conductance and tensiometry respectively at 25 ◦ C. The fraction of counterion binding (ˇ) of the DHDAB micelle decreases with increase of the temperature. The energetics of the self-assembly process of DHDAB were evaluated, which 0 and S 0 values for the micellization exhibindicated the Hm m ited nice compensations between them. PdCl2 can significantly affect the interface properties of the DHDAB solutions at fixed ratio of DHDAB/PdCl2 . The phase behaviors of the DHDAB/PdCl2 solutions at fixed ratio are different from that of the DHDAB/PdCl2 solutions at constant concentration of PdCl2 . DLS coupled with TEM verified DHDAB/PdCl2 aqueous mixture can form vesicles in a certain CDHDAB range. Furthermore, the hollow nanosphere of Pd was prepared using DHDAB/PdCl2 forming vesicles as template.

˛ nska, ´ [1] J. Pernak, A. Syguda, I. Mirska, A. Pernak, J. Nawrot, A. Prazy S.T. Griffin, R.D. Rogers, Choline-derivative-based ionic liquids, Chem. Eur. J. 13 (2007) 6817–6827. [2] A. Hamz, E. Rubi, P. Arnal, M. Boisbrun, C. Carcel, X. Salom-Roing, M. Maynadier, S. Wein, H. Vial, M. Clas, Mono-bis-thiazolium salts have potent antimalarial activity, J. Med. Chem. 48 (2005) 3639–3643. ´ The antielectrostatic effect of N[3] J. Pernak, P. Chwała, A. Syguda, R. Pozniak, alkanol-N-alkoxymethyl-N,N-dimethylammonium chlorides and their ester derivatives, Pol. J. Chem. 77 (2003) 1263–1274. [4] J. Pernak, P. Chwała, Synthesis and anti-microbial activities of choline-like quaternary ammonium chlorides, Eur. J. Med. Chem. 38 (2003) 1035–1042. [5] J. Pernak, P. Chwala, A. Syguda, Room temperature ionic liquids – new choline derivatives, Pol. J. Chem. 78 (2004) 539–546. [6] V. Mzvellec, B. Leger, M. Mauduit, A. Roucoux, Ruthenium-catalyzed stereospecific decarboxylative allylation of non-stabilized ketone enolates, Chem. Commun. (2005) 2838–2839. [7] A.P. Abbott, T.J. Bell, S. Handa, B. Stoddar, O-acetylation of cellulose and monosaccharides using a zinc based ionic liquid electronic, Green Chem. 7 (2005) 705–707. [8] A.P. Abbott, G. Capper, D.L. Davies, H. Munro, R.K. Rasheed, V. Tambyrajah, Preparation of novel, moisture-stable Lewis-acidic ionic liquids containing quaternary ammonium salts with functional side chains, Chem. Commun. (2001) 2010–2011. [9] A.P. Abbott, G. Capper, D.L. Davies, R. Rasheed, Ionic liquids based upon metal halide/substituted quaternary ammonium salt mixtures, Inorg. Chem. 43 (2004) 3447–3452. ´ R. Bogel-Łukasik, Physicochemical properties and solubility of [10] U. Domanska, alkyl-(2-hydroxyethyl)-dimethylammonium bromide, J. Phys. Chem. B 109 (2005) 12124–12132. ´ [11] U. Domanska, R. Bogel-Łukasik, Solubility of ethyl-(2-hydroxyethyl)dimethylammonium bromide in alcohols (C2 –C12 ), Fluid Phase Equilib. 233 (2005) 220–227. [12] M.J. Earle, J.M.S.S. Esperanc, M.A. Gilea, L.J.N. Canongia, L.P.N. Rebelo, J.W. Magee, K.R. Seddon, J.A. Widegren, The distillation and volatility of ionic liquids, Nature 439 (2006) 831–834. [13] G. Panfili, P. Manzi, D. Compagnone, L. Scarciglia, G. Palleschi, Rapid assay of choline in foods using microwave hydrolysis and a choline biosensor, J. Agric. Food Chem. 48 (2000) 3047–3403. [14] G.P. Zaloga, M.D. Bortenschlager, Vitamins, in: G.P. Zaloga (Ed.), Nutrition in Critical Care, Mosby-Year Book, St. Louis, MO, 1994. [15] P. Nockemann, B. Thijs, K. Driesen, C.R. Janssen, K.V. Hecke, L.V. Meervelt, S. Kossmann, B. Kirchner, K. Binnemans, Choline saccharinate and choline acesulfamate ionic liquids with low toxicities, J. Phys. Chem. B 111 (2007) 5254–5263. [16] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, Ionic liquid analogues formed from hydrated metal salts, Chem. Eur. J. 10 (2004) 3769–3774. [17] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Novel solvent properties of choline chloride/urea mixtures, Chem. Commun. (2003) 70–71. [18] S.Z.E. Abedin, F. Endres, Ionic liquids: the link to high-temperature molten salts? Acc. Chem. Res. 40 (2007) 1106–1113. [19] M.C. Gutiérrez, M.L. Ferrer, L. Yuste, F. Rojo, F.D. Monte, Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics? Angew. Chem. Int. Ed. 48 (2009) 60–103. [20] E.R. Cooper, C.D. Andrews, P.S. Wheatley, P.B. Webb, P. Wormald, R.E. Morris, Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues, Nature 430 (2004) 1012–1016. [21] A.P. Abbott, T.J. Bell, S. Handa, B. Stoddart, Cationic functionalisation of cellulose using a choline based ionic liquid analogue, Green Chem. 8 (2006) 784–786. [22] C.Y. Sheu, S.F. Lee, K.H. Lii, Ionic liquid of choline chloride/malonic acid as a solvent in the synthesis of open-framework iron oxalatophosphates, Inorg. Chem. 45 (2006) 1891–1893. [23] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Quaternary ammonium zinc- or tin-containing ionic liquids: water insensitive, recyclable catalysts for Diels–Alder reactions, Green Chem. 4 (2002) 24–26. ´ [24] U. Domanska, E. Bogel-Łukasik, R. Bogel-Łukasik, 1-Octanol/water partition coefficients of 1alkyl-3-methylimidazolium chloride, Chem. Eur. J. 9 (2003) 3033–3041.

S. Song et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 100–107 [25] A. Modaressi, H. Sifaoui, B. Grzesiak, R. Solimando, U. Domanska, M. Rogalski, CTAB aggregation in aqueous solutions of ammonium based ionic liquids; conductimetric studies, Colloids Surf. A: Physicochem. Eng. Aspects 296 (2007) 104–108. [26] D.M. Vriezema, M.C. AragonKs, J.A.A.W. Elmans, J.J.L.M.A.E. Rowan, R.J.M. Nolte, Self-assembled nanoreactors, Chem. Rev. 105 (2005) 1445–1490. [27] M. Antonietti, S. Forster, Vesicles and liposomes: a self-assembly principle beyond lipids, Adv. Mater. 15 (2003) 1323–1333. [28] T. Dwars, E. Paetzold, G. Oehme, Reactions in micellar systems, Angew. Chem. Int. Ed. 44 (2005) 7174–7199. [29] (a) T.A. Waggoner, J.A. Last, P.G. Kotula, D.Y. Sasaki, Self-assembled columns of stacked lipid bilayers mediated by metal ion recognition, J. Am. Chem. Soc. 123 (2001) 496–497; (b) X. Luo, S. Wu, Y. Liang, Vesicle formation induced by metal ions from micelleforming sodium hexadecylimino diacetate in dilute aqueous solution, Chem. Commun. (2002) 492–493; (c) T. Owen, R. Pynn, J.S. Martinez, A. Butler, Micelle-to-vesicle transition of an iron-chelating microbial surfactant, marinobactin E, Langmuir 21 (2005) 12109–12114; (d) J. Hao, J. Wang, W. Liu, R. Abdel-Rahem, H. Hoffmann, Zn2+ -induced vesicle formation, J. Phys. Chem. B 108 (2004) 1168–1172. [30] (a) A. Song, J. Hao, Self-assembly of metal–ligand coordinated charged vesicles, Curr. Opin. Colloid Interface Sci. 14 (2009) 94–102; (b) P.C. Griffiths, T. Fallis, I.A. Tatchell, L. Bushby, A. Beeby, Aqueous solutions of transition metal containing micelles, Adv. Colloid Interface Sci. 144 (2008) 13–23. [31] (a) X.J. Zhang, D. Li, Metal-compound-induced vesicles as efficient directors for rapid synthesis of hollow alloy spheres, Angew. Chem. Int. Ed. 45 (2006) 5971–5974; (b) S. Pevzner, O. Regev, A. Lind, Evidence for vesicle formation during the synthesis of catanionic templated mesoscopically ordered silica as studied by cryo-TEM, J. Am. Chem. Soc. 125 (2003) 652–653. [32] W.D. Harkins, F.E. Brown, The determination of surface tension (free surface energy), and the weight of falling drops: the surface tension of water and benzene by the capillary height method, J. Am. Chem. Soc. 4 (1) (1919) 499–524. [33] S.K. Mehta, K.K. Bhasin, R. Chauhan, S. Dham, Effect of temperature on critical micelle concentration and thermodynamic behavior of dodecyldimethylethylammonium bromide and dodecyltrimethylammonium chloride in aqueous media, Colloids Surf. A: Physicochem. Eng. Aspects 255 (2005) 153–157. [34] (a) I. Goodchild, L. Collier, S.L. Millar, I. Prokeˇs, J.C.D. Lord, C.P. Butts, J. Bowers, J.R.P. Webster, R.K. Heenan, Structural studies of the phase, aggregation and surface behaviour of 1-alkyl-3-methylimidazolium halide + water mixtures, J. Colloid Interface Sci. 307 (2007) 455–468; (b) J. Sung, Y. Jeon, D. Kim, T. Iwahashi, T. Iimori, K. Seki, Y. Ouch, Air–liquid interface of ionic liquid + H2 O binary system studied by surface tension measurement and sum-frequency generation spectroscopy, Chem. Phys. Lett. 406 (2005) 495–500; (c) J. Bowers, C.P. Butts, P.J. Martin, M.C. Vergara-Gutierrez, R.K. Heenan, Aggregation behavior of aqueous solutions of ionic liquids, Langmuir 20 (2004) 2191–2198. [35] (a) N.J. Turro, A. Yekta, Luminescent probes for detergent solutions. A simple procedure for determination of the mean aggregation number of micelles, J. Am. Chem. Soc. 100 (1978) 5951–5952; (b) A. Paul, P.K. Mandal, A. Samanta, On the optical properties of the imidazolium ionic liquids, J. Phys. Chem. B 109 (2005) 9148–9153. [36] K.M. Kale, E.L. Cussler, D.F. Evans, Characterization of micellar solutions using surfactant ion electrodes, J. Phys. Chem. 84 (1980) 593–598. [37] A. Paul, P.C. Griffiths, E. Pettersson, P. Stibs, B.L. Bales, R. Zana, R.K. Heenan, Nucler magnetic resonance and small-angle neutron scattering studies of anionic surfactants with macrocounterions: tetramethylammonium dodecyl sulfate, J. Phys. Chem. B 109 (2005) 15775–15779.

107

[38] A. Chatterjee, S.P. Moulik, S.K. Sanyal, B.K. Misra, P.M. Puri, Thermodynamics of micelle formation of ionic surfactants: a critical assessment for sodium dodecyl sulfate, cetyl pyridinium chloride and dioctyl sulfosuccinate (Na salt) by microcalorimetric, conductometric, and tensiometric measurements, J. Phys. Chem. B 105 (2001) 12823–12831. [39] (a) G.B. Ray, I. Chakraborty, S. Ghosh, S.P. Moulik, R. Palepu, Self-aggregation of alkyltrimethylammonium bromides (C10 -, C12 -, C14 -, and C16 TAB) and their binary mixtures in aqueous medium: a critical and comprehensive assessment of interfacial behavior and bulk properties with reference to two types of micelle formation, Langmuir 21 (2005) 10958–10967; (b) M. Benrraou, B.L. Bales, R. Zana, Effect of the nature of the counterion on the properties of anionic surfactants. 1. CMC, ionization degree at the CMC and aggregation number of micelles of sodium, cesium, tetramethylammonium, tetraethylammonium, tetrapropylammonium, and tetrabutylammonium dodecyl sulfates, J. Phys. Chem. B 107 (2003) 13432–13440. [40] M. Kajari, C. Indranil, C.B. Subhash, K.P. Amiya, P.M. Satya, Physicochemical studies of octadecyltrimethyl ammonium bromide: a critical assessment of its solution behavior with reference to formation of micelle, and microemulsion with n-butanol and n-heptane, J. Phys. Chem. B 111 (2007) 14175–14185. [41] C. Tanford, The Hydrophobic Effect: Formation of Micelles and Biological Membranes, second ed., Wiley, New York, 1980. [42] K. Dibyendu, G. Abhisek, D. Joykrishna, Charaterization of micelle formation of dodecyldimethyl-N-2-phenoxy ethylammonium bromide in aqueous solution, J. Colloid Interface Sci. 298 (2006) 451–456. [43] M. Prasad, S.P. Moulik, A. MacDonald, R. Palepu, Self-aggregation of alkyl (C10 , C12 -, C14 -, and C16 -) triphenyl phosphonium bromides and their 1:1 molar mixtures in aqueous medium: a thermodynamic study, J. Phys. Chem. B 108 (2004) 355–362. [44] O. Yousuke, K. Hideya, A. Masahiko, M. Hiroshi, Effects of micelle-to-vesicle transitions on the degree of counterion binding, J. Colloid Interface Sci. 287 (2005) 685–693. [45] S.C. Srivastava, L. Newman, Mixed ligand complexes of palladium(II) with chloride and bromide, Inorg. Chem. 5 (1966) 1506–1510. [46] S. Feldberg, P. Klotz, L. Newman, Computer evaluation of equilibrium constants from spectrophotometric data, Inorg. Chem. 11 (1972) 2860–2865. [47] V. Bernadett, K. Zoltán, Size-selective synthesis of cubooctahedral palladium particles mediated by metallomicelles, Langmuir 19 (2003) 4817–4824. [48] (a) J. Chen, D. Wang, J. Xi, L. Siekkinen, A. Au, A. Warsen, Z.Y. Li, H. Zhang, Y. Xia, X. Li, Immuno gold nanocages with tailored optical properties for targeted photothermal destruction of cancer cells, Nano Lett. 7 (2007) 1318–1322; (b) S. Guo, S. Dong, E. Wang, A general method for the rapid synthesis of hollow metallic or bimetallic nanoelectrocatalysts with urchinlike morphology, Chem. Eur. J. 14 (2008) 4689–4695. [49] (a) V. Salgueirino-Maceira, M. Spasova, M. Farle, Water-stable, magnetic silica–cobalt/cobalt oxide–silica multishell submicrometer spheres, Adv. Funct. Mater. 15 (2005) 1036–1040; (b) Z. Yang, Z. Niu, Y. Lu, Z. Hu, C.C. Han, Templated synthesis of inorganic hollow spheres with a tunable cavity size onto core–shell gel particles, Angew. Chem. Int. Ed. 42 (2003) 1943–1945. [50] (a) J.X. Huang, Y. Xie, B. Li, Y. Liu, Y.T. Qian, S.Y. Zhang, In situ source–templateinterface reaction route to semiconductor CdS submicrometer hollow spheres, Adv. Mater. 12 (2000) 808–811; (b) C.E. Fowler, D. Khushalani, S. Mann, Interfacial synthesis of hollow microspheres of mesostructured silica, Chem. Commun. (2001) 2028–2029. [51] H.P. Liang, L.J. Wan, C.L. Bai, L. Jiang, Gold hollow nanospheres: tunable surface plasmon resonance controlled by interior-cavity sizes, J. Phys. Chem. B 109 (2005) 7795–7800. [52] H. Hentze, S.R. Raghavan, C.A. McKelvey, E.W. Kaler, Silica hollow spheres by templating of catanionic vesicles, Langmuir 19 (2003) 1069–1074.