Effect of pH and Oxygen Atom of the Hydrophobic Chain on the Self

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Effect of pH and Oxygen Atom of the Hydrophobic Chain on the SelfAssembly Property and Morphology of the Pyridyl Boronic Acid Based Amphiphiles Monali Maiti, Aparna Roy, and Sumita Roy* Department of Chemistry and Chemical Technology, Vidyasagar University, Paschim Medinipur 721 102, India S Supporting Information *

ABSTRACT: The surface activity and aggregation behavior of two synthesized boronic acid based anionic surfactants, sodium salt of 2-dodecyl pyridine-5-boronic acid (SDDPB) and sodium salt of 2-oxydodecyl pyridine-5-boronic acid (SODDPB), were studied in buffer solution at pH 9 and 13 containing carbohydrates. The self-assembly formation was investigated by use of a number of techniques including surface tension, conductivity, fluorescence spectroscopy, dynamic light scattering, X-ray diffraction, and transmission electron microscopy (TEM). Both of the amphiphiles exhibit a single break in the surface tension vs log(concentration) plots, indicating existence of one critical aggregation concentration. Steady state fluorescence spectroscopy was used to determine the polarity indexes using pyrene and the rigidity of the microenvironments of the aggregates using 1,6-diphenyl-1,3,5-hexatriene (DPH) as fluorescence probe molecules. The pKa’s of both amphiphiles were determined in buffer solutions of different pH’s. XRD studies were performed to shed light on the morphology of the self-assemblies. TEM micrographs revealed the existence of vesicles for both the amphiphiles in buffer solution of pH 9, but at pH 13, TEM pictures indicate the existence of closed vesicles in SDDPB solution and at concentrated solution the vesicles are fused to form sponge-like micelles. After aging the vesicular solution of pH 13 of SDDPB, the closed vesicles are destroyed. In contrast, for SODDPB at pH 13, TEM pictures suggest the existence of spherical and complex micelles in solution which were further transformed into crystal-like structure upon aging. The average hydrodynamic radii were determined by dynamic light scattering measurement. Therefore, for the first time, we have successfully synthesized two new surfactants containing pyridyl-boronic acid as a headgroup which shows remarkable tuning of morphology in two different pH’s and in the presence of two different carbohydrates.

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INTRODUCTION In recent years, a large number of studies have been devoted to the synthesis and study of the aggregation behavior of amphiphiles for its use in wide ranging chemical and technological areas such as organic, physical chemistry, analytical, biochemistry, pharmaceuticals, petroleum recovery, detergents, cosmetics, paints and coatings, mineral processing, and food science.1 In general, above a critical concentration, the amphiphilic molecules self-assemble to form different types of aggregates depending on shapes and sizes such as micelles (spherical, disks, and rod-like), vesicles (spherical and tubules), and liquid crystals (hexagonal and lamellar) in solutions.2,3 The quarterization of nitrogen and the ionic character of the carboxylic and sulfonic groups are essential components of the well-known families of cationic, anionic, and zwitterionic surfactants.4 Interest continues in developing the new generation of ingeniously designed green-surfactant materials with novel electronic, optical, or physicochemical properties that are likely to play a significant role in the burgeoning area of surfactant chemistry, but there has been little investigation until now on amphiphiles based on boron. However, on the basis of research results and evidence in the literature,5−7 it is both © XXXX American Chemical Society

interesting and worthwhile to study the surface-active properties of these novel synthesized surfactants with hydrophilic heads containing boron atom. Boronic acids contain trivalent boron atoms bonded to one alkyl/aryl substituent and two hydroxyl groups [R−B(OH)2].8 Unlike carboxylic acids, boronic acids are not naturally occurring, though they have appeared in the literature since at least 1860.9 Unique and versatile reactivity10 and stability11 of boronic acids have led to uses in numerous areas, including catalytic C−C bond formation,8 saccharide recognitions,12,13 supramolecular formations,14 asymmetric synthesis, metal catalysis, molecular sensing and as therapeutic agents, enzyme inhibitors, and novel materials.10 However, organo boron compounds are now also finding increasing applications as fuel additives, liquid crystal and agrochemicals, as well as in cosmic and biomedical formulations.15,16 Also of importance for biological applications, many boron compounds exhibit unique neutron bombardment behavior.17 Recently, it has been reported that boronic acids and their esters have become important building blocks in Received: April 22, 2013

A

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Scheme 1. Synthesis Scheme of SDDPB

and its salt shows amphiphilic character in aqueous solution with excellent surface-active properties.30,31 Karpichev and coworkers very recently reported a new approach for creating water-soluble functionalized vesicles employing N-alkyl-3boronopyridinium triflates (alkyl = CH3, C12H25, C16H33) as sensors for monosaccharides.32 However, there is no detailed report until now about the self-assembly properties of the amphiphiles containing pyridyl-boronic acid as a headgroup. With respect to the potential development of amphiphilic molecules having pyridyl-boronic acid as a headgroup to demonstrate the concept of surfactant, the following advantages should be addressed. Boronic acids are stable products of the second oxidation of boranes and relatively benign compounds to humans. Their unique properties as mild organic Lewis acids and their mitigated reactivity profile coupled with their stability and ease of handling make boronic acids a particularly attractive class of synthetic intermediates. Moreover, because of their low toxicity and their ultimate degradation into environmentally friendly boric acid, boronic acids can be regarded as “green” compounds. In our previous report, we demonstrated the interesting selfassembly property of the amphiphile having pyridyl-carboxylic acid (nicotinic acid) as a headgroup.33 The aim of the present work is synthesis and investigation of the self-assembly properties of two amphiphiles having pyridyl-boronic acid as a headgroup in two different pH’s in the presence of different carbohydrates: one having a simple aliphatic chain as the hydrophobic moiety and another one having one oxy-aliphatic chain as the hydrophobic part.

organic synthesis and are key intermediates in the preparation of candidate drugs8 and notably uses as inhibitors or substrates of proteases. 18 Shenvi and Kettner have studied the pharmaceutical research to show that boropeptide (i.e., peptides containing a boronic acid analogue of α-amino carboxylic acid) is a more effective serine protease inhibitor than simple aryl boronic acid.19 Researchers have investigated that phenyl boronic acid with a hydrophobic alkyl chain shows liquid crystalline properties.20−22 O’Donovan et al. in 2011 have shown that boronic acid represents a novel class of bacterial mutagen that may not act by direct covalent binding to DNA.23 Recently, the feature article of Sumerlin and Cambre highlighted research in the field of recent developments in synthesis, processing, and materials development that has enabled the preparation of new biomaterials.24 At first, Kataoka and his research group incorporated boronic acid moieties into smart hydrogels and obtained “totally synthetic” polymer gels that reversibly swell in response to increases in the environmental glucose concentration.25 Horkay et al. have shown copolymerization of tertiary amines into glucose-responsive hydrogels containing boronic acid moieties creates a zwitterionic hydrogel with an enhanced selectivity for glucose relative to fructose, and also creates a hydrogel that shrinks rather than swells in response to an increase in environmental glucose concentration.26 In 2011, McNeil et al. synthesized a series of aryl trihydroxyborate salts with increasing alkyl chain lengths and found them to form gels in benzene with sonication, but the compounds were thermally unstable and readily underwent protodeboronation in solution and solid state.27 Very recently in 2012, Strano and co-workers have designed and synthesized a series of phenyl boronic acid (PBA)-conjugated amphiphilic polyethylene glycol (PEG) polymers.28 They have also examined the interaction of these polymers with semiconducting single-walled carbon nanotubes (SWNTs) using different spectroscopies, and finally examined the ability of these polymer-SWNTs to modulate fluorescence emission in response to different saccharides. Baboulene et al.29 synthesized some amino organo boron amphiphilic molecules that have been shown to have interesting surfactant properties. In recent reports, it was established that long chain amino-boronic acid

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SYNTHESIS OF AMPHIPHILES

Synthesis. We have performed the synthesis of new boronic acid based surfactants, sodium salt of 2-dodecyl pyridine-5-boronic acid (SDDPB) and sodium salt of 2-oxydodecyl pyridine-5-boronic acid (SODDPB), according to Schemes 1 and 2. Synthesis of 5-Bromo-2-dodecyl Pyridine.34 A Grignard reagent was prepared from 1-bromododecane (0.039 mol, 9.79 g) and Mg turnings (0.043 mol, 1.13 g) in 50 mL of anhydrous THF. This freshly prepared n-dodecylmagnesium bromide was added dropwise to a flask containing ZnCl2 (0.05 mol, 6.85 g) in 40 mL of anhydrous THF (ice−water cooling bath), and a white precipitate was obtained. The B

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acetate (25 mL × 2). The organic layers were combined and concentrated to dryness. The resulting crude was purified by silica gel column chromatography (eluent 30% ethyl acetate in hexane) to yield the desired product as an off-white solid. Yield (1.2 g, 42%.). 1H NMR (400 MHz, DMSO-d6): δ 0.84 (t, J = 6.40 Hz, 3H, CH3), 1.22−1.26 (m, 18H), 1.63 (m, 2H), 2.68 (t, J = 7.52 Hz, 2H), 7.19 (d, J = 7.60 Hz, 1H) 7.96−7.98 (dd, J = 1.52 Hz, 7.64 Hz, 1H), 8.21 (s, 2H), 8.76 (s, 1H). MS (ESI+) for C17H30NO2B m/z 291.8 (M + H)+. Synthesis of Sodium Salt of 2-Dodecyl Pyridine-5-boronic Acid. An equimolar solution of NaHCO3 in water was added to a solution of 2-dodecyl pyridine-5-boronic acid (3.44 × 10−3 mol, 1 g) in freshly distilled EtOH (20 mL) at room temperature and left for 24 h for stirring. EtOH was evaporated in a vacuum to afford the sodium salt as white solid. Yield (0.910 g, 96%). Synthesis of 5-Bromo-2-oxydodecyl Pyridine. 5-Bromo-2-oxypyridine (3 g, 17.24 mmol) and Ag2CO3 (5.23 g, 18.96 mmol) were dissolved in 25 mL of dry acetonitrile in a 100 mL two-necked roundbottom flask. Then, 1-iodododecane (6.13 g, 20.69 mmol) was added in the flask via a syringe and the resulting mixture was refluxed for 8 h under a nitrogen atmosphere. The reaction was cooled and filtered over a sintered funnel, and filtrate was concentrated in vacuo. The crude residue was diluted with dichloromethane, the organic fraction was washed with water and dried over MgSO4, and the volatiles were removed in vacuo. The crude residue was purified by silica gel chromatography (1:9, ethyl acetate/petroleum spirits) to afford the desired compound as a light yellow colored oil. Yield (4.12 g, 70%). 1 H NMR (400 MHz, CDCl3): δ 0.86 (t, J = 6.40 Hz, 3H, CH3), 1.24− 1.31 (m, 18H), 1.69−1.73 (m, 2H), 3.86 (t, J = 7.36 Hz, 2H), 6.46 (d, J = 9.20 Hz, 1H), 7.31 (dd, J = 2.68 Hz, 8.72 Hz, 1H), 7.35 (d, J = 2.6 Hz, 1H). MS (ESI+) for C17H28NBrO m/z 342 (M + H)+. Synthesis of 2-Oxydodecyl Pyridine-5-boronic Acid. To a stirred solution of 5-bromo-2-oxidodecyl pyridine (1.0 g, 2.9 mmol) in dry THF (15 mL) was added n-BuLi in THF (2.5 M, 2.3 mL, 5.8 mmol) at −78 °C dropwise under a nitrogen atmosphere. It was stirred at −78 °C for 30 min. Triisopropyleborate (2 mL, 8.5 mmol) was added dropwise at the same temperature. The stirring was continued at −78 °C for a further 30 min. The reaction was then quenched with saturated ammonium chloride solution. The temperature of the reaction mass was gradually increased to room temperature. It was extracted with ethyl acetate (25 mL × 3). The combined residue was washed with brine, dried over Na2SO4, and concentrated under reduced pressure to afford crude boronic acid as a sticky gum. It was further recrystallized in acetone−water to afford boronic acid as a white solid (470 mg, 47%). 1H NMR (400 MHz, DMSO-d6): δ, 0.83 (t, J = 6.84 Hz, 3 H), 1.24−1.37 (m, 18 H), 1.65−1.70 (m, 2 H), 4.20−4.25 (m, 2 H), 6.73 (d, J = 8.28 Hz, 1 H), 7.98 (d, J = 7.08 Hz, 1 H), 8.073 (s, 2H), 8.48 (s, 1 H). MS (ESI+) for C17H30NO3B m/z 308 (M + H)+. Synthesis of Sodium Salt of 2-Oxydodecyl Pyridine-5-boronic Acid. Sodium salt of 2-oxydodecyl pyridine-5-boronic acid was

Scheme 2. Synthesis Scheme of SODDPB

cooling bath was removed and the reaction mixture with a considerable amount of white precipitate was stirred at room temperature for 2 h. The resulting n-dodecylzinc chloride (1.2 equiv) was added dropwise to a presolution of 2,5-dibromopyridine (0.02 mol, 5 g) and Pd(PPh3)4 (0.5 mol %, 0.12 g) in 20 mL of freshly distilled THF. The reaction mixture was stirred overnight at room temperature, poured into 40 mL of water, and 40 mL of CH2Cl2 was added. The resulting solution was vacuum filtered to separate a small amount of insoluble white solid, the organic phase was separated, and the aqueous phase was extracted with CH2Cl2. The organic phases were combined, washed with brine, and then dried over Na2SO4. The solvent was removed by rotary evaporation, and the product was purified by silica gel column chromatography (eluent 20% ethyl acetate in hexane) to afford pure 2-n-dodecyl-5-bromopyridine as light yellow oil. Yield (4.3 g, 62%.). 1H NMR (400 MHz, DMSO-d6): δ 0.82−0.0.86 (t, J = 6.96 Hz, 3H, CH3), 1.21−1.25 (m, 18H), 1.60− 1.64 (m, 2H), 2.65−2.69 (t, J = 7.64 Hz, 2H), 7.22−7.25 (d, J = 8.32 Hz, 1H), 7.90−7.92 (dd, J = 8.28 Hz, 2.36 Hz, 1H), 8.56−8.57 (d, J = 2.2 Hz, 1H). MS (ESI+) for C17H28BrN m/z 326 (M + H)+. Synthesis of 2-Dodecyl Pyridine-5-boronic Acid. Toluene (20 mL) in a 250 mL three-necked flask, equipped with a magnetic stirrer, was cooled down to −78 °C. n-Butyllithium (2.5 M in hexane, 4.8 mL, 12 mmol) was mixed with toluene. After the internal temperature reached −78 °C, a solution of 5-bromo-2-dodecylpyridine (3.26 g, 10 mmol) in toluene (20 mL) was added. The internal temperature was maintained at < −60 °C by controlling the rate of addition. The resulting slurry was aged for 15−20 min, and then, THF (10 mL) was added slowly, keeping the internal temperature at < −60 °C. The mixture was aged for 15 min, and then, triisopropyl borate (2.8 mL, 12 mmol) was added over 2 min. The reaction solution was allowed to warm at −15 °C and was quenched with 2.7 N HCl (10 mL). The phases were separated, and the organic layer was washed with water (50 mL × 3). The aqueous layers were further extracted with ethyl

Figure 1. Plot of γ vs log C of SDDPB and SODDPB in buffer solutions of (A) pH 13 containing sucrose and (B) pH 9 containing fructose. C

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Table 1. Surface Properties of SDDPB and SODDPB at pH 13 Containing Sucrose and pH 9 Containing Fructose pH 13

a

pH 9

properties

SDDPB

SODDPB

SDDPB

SODDPB

CAC (mM) γCAC (mN m−1) ΠCAC (mN m−1) Γmax × 106 (mol m−2) Amin (Å2 molecule−1) P

0.10 ± 0.02, 0.15a 45.20 27.10 3.84 43.20 0.58

0.010 ± 0.005, 0.013a 53.20 19.10 2.45 67.76 0.35

0.31 ± 0.02, 0.37a 38.80 34.70 3.65 45.40 0.55

0.15 ± 0.02, 0.18a 50.50 23.00 3.52 47.16 0.50

Data are obtained from fluorescence measurement.

prepared according to the same procedure of synthesis of sodium salt of 2-dodecyl pyridine-5-boronic acid. 1 H NMR and LC-Mass spectra of all the synthesized compounds are given in the Supporting Information.

The smaller value of Amin in the case of SDDPB suggests the formation of larger aggregates with tightly packed hydrocarbon chains, whereas the Amin value of SODDPB indicates the existence of a micellar type of aggregates. In the case of pH 9 in the presence of fructose, the Amin values for both amphiphiles are similar to those of decyl and dodecyl carboxylic acids which again point out the formation of larger aggregates. Packing Parameter Calculation. Israelachvilli in 1991 differentiates vesicles and other non-vesicular aggregates theoretically in the literature.39 The authors Isrealachvilli,40 Ninham,41 and Evans42 have specified an empirical rationale correlating molecular architecture with possible aggregate morphology. According to this model, the dependence of the morphology of the aggregates on the molecular shape of an amphiphile can be described by the dimensionless critical packing parameter (P) which is related to the geometric properties of the amphiphilic molecules by the equation V/ lcAmin, where V is the volume of the hydrophobic chains of the amphiphile, lc is the length of the hydrocarbon chain in its fully extended conformation, and Amin is the area of cross section occupied by the headgroup of an amphiphile at the aqueous interface.40 The model predicts the formation of spherical micelles at P ≤ 0.33, large cylindrical or rod-shaped micelles at 0.33 ≤ P ≤ 0.50, vesicles or flexible bilayers at 0.50 ≤ P ≤ 1, planar extended bilayers at P = 1, and reversed or inverted micelles at P > 1. Yan et al. have also reported that the P value for tubular structures is much larger than vesicular morphologies.43 They have shown that tubules exist in the region 0.70 ≤ P ≤ 1.0 and vesicles were found in the region 0.50 ≤ P ≤ 0.70. Therefore, to get an idea about the morphology of the aggregates, the packing parameter values for both amphiphiles were determined. Here, we use Edward’s atomic increment method44 for calculation of V values; the V values for SDDPB (433 Å3) and SODDPB (439.2 Å3) were estimated from the molar volume of n-dodecane (368 Å3). The values of lc (17.40 Å for SDDPB and 18.60 Å for SODDPB) were obtained from the energy-minimized structure via the Hartree−Fock method by Gauss View 3.0, and the Amin values were obtained from surface tension measurements. The calculated P values for the aggregates of the amphiphiles are listed in Table 1. In the case of SDDPB surfactant at pH 13, the P value is 0.58, which suggested the formation of vesicles or flexible bilayer aggregates. The P value for the SODDPB, in contrast, was in the range 0.33 ≤ P ≤ 0.50 which means that the aggregates formed by the SODDPB have large cylindrical or rod-like micelles. Packing parameter values in the same range (P = 0.64 for SDDPB and P = 0.38 for SODDPB) were also obtained when the amphiphile tail volumes (V) were obtained from the published relationship,45 V = (27.4 + 26.9n) Å3, where n is the number of C atoms. However, calculated P values in both methods suggest the presence of vesicular type aggregates

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RESULTS AND DISCUSSION Surface Tension Study. The surface tension measurement is a classical and accurate method of studying the critical aggregation concentration (CAC) of the surfactants. Therefore, this method was used to determine the critical aggregation concentration (CAC) of the synthesized amphiphiles. The detailed procedure of surface tension measurement is given in the experimental section of the Supporting Information. The variations of surface tension (γ) with the log(concentration, C) of the amphiphiles SDDPB and SODDPB at two different pH’s are shown in Figure 1. The plots show one sharp break at a concentration corresponding to the CAC values of the amphiphiles, which is indicative of the formation of aggregates. The CAC values and the surface activity parameters of the two amphiphiles at two different pH’s are listed in Table 1. The lowering of the surface tension values compared with that of buffer solutions at the CAC (γCAC) and ΠCAC which is the surface pressure (=γ0 − γCAC, with γ0 being the surface tension of buffer solutions of pH 13 and 9 in the presence of carbohydrates) suggests that the amphiphiles are both good surfactants. The much lower values of γCAC and higher value of ΠCAC of SDDPB than SODDPB for both the buffer solutions indicate that SDDPB has a better surface activity than SODDPB. The greater lowering of the γCAC and much greater ΠCAC values in the case of buffer solution of pH 9 containing fructose suggest both of the amphiphiles are better surfactants at pH 9 than pH 13. The values of surface excess (Γmax) and cross-sectional area per headgroup (Amin) at the interface were calculated by using the Gibbs adsorption equations:2,35,36 Γmax = − (1/2.303nRT )(dγ /d log C)

(1)

A min = 1/(ΓmaxNA )

(2)

where dγ/d log C is the maximum slope, NA is Avogadro’s number, T is the absolute temperature, n is 1 for a 1:1 ionic surfactant in the presence of a swamping amount of 1:1 electrolyte,2 and R is 8.314 J mol−1 K−1. The cross-sectional areas were calculated from Γmax values corresponding to the break of the surface tension plot (Figure 1). The cross-sectional area of SODDPB at pH 13 is 67.76 Å2 molecule−1 which is very close to the value of the surface area of sodium laurate as reported to be 69 Å2.37 However, otherwise/in contrast, SDDPB has a lower Amin value (Table 1) than that of SODDPB and is close to the surface areas of decyl and dodecyl carboxylic acids (45 Å2)38 at pH 13 in a 0.13 mol kg−1 solution. D

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Figure 2. Plot of polarity ratio (I1/I3) vs log C of SDDPB and SODDPB in buffer solutions of (A) pH 13 containing sucrose and (B) pH 9 containing fructose.

Table 2. Self Assembly Properties of SDDPB and SODDPB at pH 13 Containing Sucrose and pH 9 Containing Fructose pH 13 properties I1/I3 r Rh (nm)

Dapp × 1012 (m2 s−1)

Nagg × 10−5

SDDPB 1.52 0.178 66 (0.4 mM) 447 (0.7 mM) 836 (1 mM) 3.35 (0.4 mM) 0.49 (0.7 mM) 0.26 (1 mM) 2.53 (0.4 mM) 116.18 (0.7 mM) 406.40 (1 mM)

pH 9 SODDPB

SDDPB

SODDPB

1.54 0.134 87.10 334 (5 h)

1.33 0.120 585 (2 mM)

1.46 0.078 130 (0.8 mM)

1.90 0.66 (5 h)

0.33 (2 mM)

1.51 (0.8 mM)

2.81 41.36 (5 h)

189.35 (2 mM)

9.01 (0.8 mM)

ordering at the interface and the probe molecule is solubilized in spherical aggregates because the non-polar pyrene molecule generally soluble in the hydrocarbon layer encounters only few or no water molecules around them in the case of spherical aggregates. The polarity indexes of both amphiphiles in buffer solution of pH 9 are lower than those at pH 13, which suggests that the probe molecule is solubilized in the more non-polar region at pH 9. In order to further investigate the rigidity of the microenvironment above the CAC, steady state fluorescence anisotropy (r) was measured using DPH as a fluorescence probe because it is a well-known membrane fluidity probe and different scientists have used it to study many lipid bilayer membranes.47−49 The fluorescence anisotropy, r, is an index of equivalent microviscosity (microfluidity) of the vesicle core. Therefore, r was measured at different amphiphile concentrations above the CAC. It has been observed that the anisotropy value for SDDPB at pH 13 is increased slowly from 0.15 mM and it becomes maximum at a concentration of 0.7 mM (r = 0.178). After this anisotropy value decreases sharply to 0.092 at 1 mM and becomes almost constant thereafter at higher concentrations (Figure S10A, Supporting Information). The large value of r which is greater than the lecithin liposomes (r ∼ 0.098) but less than that of sphingomyelin liposomes (r ∼ 0.247)47 might be due to tight packing of the hydrocarbon chains and perhaps is indicative of the formation of bilayer aggregates.50−52 The low value of polarity index and high steady state fluorescence anisotropy value of SDDPB above the CAC also suggest existence of a closed vesicular structure in the solution. This result is an excellent match to the calculated

for SDDPB and large cylindrical or rod-shaped aggregates for SODDPB in solutions. On the other hand, at pH 9 in the presence of fructose, the packing parameter values of both amphiphiles lie in the range 0.50 ≤ P ≤ 0.70 (Table 1), which predicts the existence of vesicular aggregates for both SDDPB and SODDPB in solutions. Microenvironment Study. It is well established in the literature that pyrene and 1,6-diphenyl-1,3,5-hexatriene (DPH) are used as extrinsic fluorescence probes because both probe molecules bind preferentially to the hydrophobic region of the self-assemblies. Therefore, in order to investigate the microenvironment of the self-assembly, fluorescence studies were performed using pyrene and DPH as fluorescence probe molecules. We know that the ratio of the intensities corresponding to the first and third vibronic bands (I1/I3) is sensitive to solvent polarity46 and its value is maximum in water, and decreases with a decrease in solvent polarity. Therefore, it has been widely used as a micropolarity probe for self-assemblies. The I1/I3 ratios were measured in the presence of various amphiphile concentrations. The plots of I1/I3 versus log(concentration, C) in solution of the amphiphiles at pH 13 containing sucrose and at pH 9 containing fructose are shown in parts A and B of Figure 2, respectively. Both plots show a single step change which is indicative of the presence of a single type of aggregates. The CAC values (Table 1) obtained from the plots are in good agreement with the surface tension measurements. The polarity ratio values corresponding to the CACs thus obtained are also summarized in Table 2. The polarity ratios (I1/I3) of both the amphiphiles at the CAC are much lower than that of water, which indicates more E

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packing parameter value from surface tension measurement. The sudden drop in anisotropy value at higher concentration may be destruction of vesicular structures by fusion of vesicles to form sponge-like micelles. On the other hand, in the case of SODDPB, the r value remains almost constant up to higher concentrations (r = 0.134 to 0.132 from concentration 0.03 to 0.20 mM; Figure S10A, Supporting Information). The low packing parameter value, low polarity ratio, and high constant anisotropy value of SODDPB at pH 13 suggest large cylindrical type aggregates are present in the solution. In case of buffer solution of pH 9, the anisotropy value for SDDPB increases at first and then remains constant up to 4.0 mM (>10 times the CAC), which is indicative of the rigidity of the microenvironment remaining the same even at high concentration (Figure S10B, Supporting Information). In contrast, the “r” value of SODDPB is relatively low and remains constant up to 0.8 mM, showing similar behavior of SODDPB at pH 13. However, the low I1/I3 ratio and high anisotropy values of both amphiphiles (Table 2) in buffer solution of two different pH’s indicate that the microenvironments of the aggregates are very non-polar and rigid. We are unable to measure the anisotropy below the CAC because the intensity of DPH in the solutions was very low. Determination of pKa. In order to investigate the pKa of the amphiphiles in the presence of sucrose and fructose, the fluorescence anisotropy of the DPH probe was measured in surfactant solutions of different pH’s. Figure S11 of the Supporting Information shows that the fluorescence anisotropy of both amphiphiles increases with an increase in pH in the presence of sucrose (Figure S11A, Supporting Information) and fructose (Figure S11B, Supporting Information). The sigmoid change of the amphiphiles indicates a two-state process. From the inflection points of the sigmoid curves, the obtained pKa value of SDDPB was 12.10 and that of SODDPB was 12.25 in the presence of sucrose. Interestingly, the pKa values in the presence of fructose changed to 8.46 for SDDPB and 8.85 for SODDPB due to stronger binding of the fructose molecules to the amphiphiles. TEM. To visualize the actual morphology of the aggregates formed by the amphiphiles SDDPB and SODDPB in the solutions, transmission electron micrographs were taken. TEM pictures of negatively stained specimens prepared from SDDPB and SODDPB in buffer (pH 13) solutions in the presence of sucrose are shown in Figures 3 and 4, respectively (a detailed description of solution preparation is given in the Supporting Information). For SDDPB, the electron micrographs were taken for three different amphiphile concentrations. Figure 3 shows the TEM pictures for 0.4, 0.7, and 1 mM solutions of the amphiphile. At 0.4 mM (Figure 3A), we observed the small unilamellar vesicles are present in the solution having a size in the range 55−195 nm. At a concentration of 0.7 mM, the size of the vesicles increases to ∼460 nm (Figure 3B−D). However, when the TEM micrographs were taken at 1 mM concentration, through careful observation, vesicle-like structures (sponge-like aggregates) were obtained in the large aggregates (Figure 3F, marked by black arrows). The formation of sponge-like aggregates was probably due to the adhesion and fusion of the vesicles at higher concentration. Formation of a similar type of sponge-like micelle by adhesion and fusion of vesicles was reported by Gao and his co-workers.53 All the micrographs were taken after 3 h of the solution preparation. After aging of the solution of concentration 0.7 mM for 6 h, the clear vesicular structures were destroyed (Figure 3E).

Figure 3. Negatively stained (with 2% aqueous uranyl acetate solution) TEM micrographs of SDDPB: (A) 0.4 mM, (B−D) 0.7 mM, (E) 0.7 mM (aging for 6 h), (F) 1 mM.

On the other hand, when the TEM pictures for 0.07 mM SODDPB were captured after 2 h of the solution preparation (Figure 4A−C), the micellar structure was observed. Closer observation revealed that the micelles are also aggregated to form complex micelles which have a size in the range 40−150 nm (Figure 4B,C). After aging of the solution of SODDPB for 5 h, the complex micelles transformed into rod-like crystals with a diameter of 185 nm and triangular crystals with a size of 290 nm (Figure 4D−F). This is due to the agglomeration of the complex micelles to form the rigid structure. At pH 9 in the presence of fructose, the TEM micrographs of both amphiphiles show existence of vesicular aggregates in solutions (Figure 5). The vesicles of SDDPB are in the range 1.1−1.7 μm, which are very big and are called giant vesicles. The size of the vesicles of SODDPB is comparatively small and in the range 75−410 nm. It is interesting to observe that SODDPB aggregated to micelles which upon aging were transformed to rod-like and triangular crystal at pH 13 in the presence of sucrose, whereas, at pH 9 in the presence of fructose, it is able to form vesicular aggregates. This is due to the fact that the fructose molecule enhances the intermolecular hydrogen bonding between the oxygen atom of the hydrophobic chain of one SODDPB molecule to that of the nitrogen atom of another molecule. This type of bilayer at pH 13 in the presence of sucrose is prohibited due to steric hindrance of the sucrose molecules for which the interbilayer separation exceeds the critical distance 9.0 ± 0.5 Å.54 The results are explained later in the XRD section. Dynamic Light Scattering. In order to further investigate the change in aggregate morphology, the dynamic light scattering method was employed to determine the hydrodynamic radius of the aggregates formed by SDDPB and SODDPB in solutions (pH 13 having 12 mM sucrose and pH 9 having 0.5 M fructose) because it is an excellent technique for distinguishing bilayer structure such as vesicles, lamellae, and tubules from micelles based on the size of the aggregates. The instrumentation and experimental procedure is given in the experimental section of the Supporting Information. At first, the z-average hydrodynamic radii (Rh) at pH 13 were determined at three different concentrations of SDDPB (Table 2). Figure S12 of the Supporting Information shows the intensity averaged size distribution of 0.4, 0.7, and 1 mM SDDPB. The Rh for 0.4 mM is approximately 66 nm (Figure F

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Figure 4. Negatively stained (with 2% aqueous uranyl acetate solution) TEM micrographs of SODDPB solution: (A−C) 0.07 mM (after 2 h) and (D−F) 0.07 mM (after 5 h).

Figure 5. Negatively stained (with 2% aqueous uranyl acetate solution) TEM micrographs of (A−C) 2 mM SDDPB solution and (D−F) 0.8 mM SODDPB solution.

Nagg = 8πR h 2/A min

S12A, Supporting Information), at 0.7 mM amphiphile concentration it becomes 447 nm (Figure S12B, Supporting Information), and at 1 mM the z-average hydrodynamic radius was obtained as 836 nm (Figure S12C, Supporting Information). At 0.4 mM, the size of the vesicles is small and also has three types of distributions containing one major distribution. As the concentration of the sample increases to 0.7 mM, the size of the vesicles increases remarkably and the distribution curve becomes monomodal. When again the concentration increases to 1 mM, the vesicles are fused to form sponge-like micelles, as evidenced from TEM micrographs (Figure 3F) and the Rh value increases largely. These results are in good agreement with the results obtained from TEM measurements. Other parameters such as the apparent diffusion coefficient (Dapp) and the mean aggregation number (Nagg) of the aggregates were also estimated by use of the Stokes− Einstein equation48

Dapp = kBT /6πηR h

(4)

Rh and Dapp values thus obtained for 0.4, 0.7, and 1.0 mM concentrations of SDDPB are listed in Table 2. Not only the Dapp values are much smaller than that of normal spherical micelles (∼10−10 m2 s−1),55 indicating that the aggregates formed by SDDPB are very large, but also the Dapp value decreases more and more as the concentration of the amphiphile increases, suggesting aggregates become larger and larger with concentration, as evidenced from the TEM pictures. Consequently, the Nagg value increases progressively and finally it becomes 4.064 × 107. For the sample of SODDPB at the same pH, Rh was measured for 0.07 mM amphiphilic concentration after 2 and 5 h of the sample preparation (Figure S13, Supporting Information). The average radius of the aggregates in 0.07 mM solution after 2 h of the sample preparation is 87.10 nm, which is too large for a normal spherical micelle which has an expected size of average diameter 3−5 nm.56 The size distribution plot shows three distribution

(3) G

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Figure 6. XRD pattern of the cast flim obtained from (A) SDDPB and (B) SODDPB at pH 13 containing sucrose and (C) SDDPB and (D) SODDPB at pH 9 containing fructose.

These results indicate the formation of a bilayer structure of SDDPB at both pH’s where the hydrocarbon tails are in interdigitated form (Figures S16 and S17, Supporting Information). For SODDPB at pH 9, an intense peak at 40.57 Å was obtained (Figure 6D) which is higher than that of twice the length of th ehydrophobic tail (37.20 Å, obtained from energy minimized structure), which is indicative of formation of bilayer structure where the hydrophobic tails are not interdigited (Figure S18, Supporting Information). At pH 13, SODDPB shows an intense sharp peak at 51.39 Å (Figure 6B) which is much larger than twice the hydrophobic chain length (almost 3 times the hydrophobic chain length) which throws outs the existence of any bilayer formation (Figure S19, Supporting Information).

curves corresponding to existence of three types of aggregates (Figure S13A, Supporting Information). This is probably due to the presence of spherical micelles and complex micelles together. After aging of the sample, the distribution curve becomes bimodal having larger average hydrodynamic radius (334 nm), suggesting transformation of complex micelles to very large types of aggregates (Figure S13B, Supporting Information). This result is an excellent match with the TEM micrographs (Figure 4D−F). Corresponding Dapp and Nagg values are also included in Table 2. The remarkable change in the Dapp and Nagg values after aging is also an indicative transformation to very large aggregates from comparatively small aggregates. On the other hand, when the Rh values of the aggregates formed by the same amphiphiles (SDDPB and SODDPB) in buffer solutions of pH 9 in the presence of fructose were determined, the monomodal distributions for both amphiphiles were obtained. The average hydrodynamic radius of 2 mM SDDPB solution is 585 nm and that of 0.8 mM SODDPB solution is 130 nm, respectively. The Rh and the corresponding Dapp and Nagg values are summarized in Table 2, and the size distribution curves are shown in Figures S14 and S15 of the Supporting Information. The very large size of Nagg and the very low Dapp values indicate formation of giant aggregates of SDDPB in buffer solution of pH 9. Also, the Rh value of SODDPB is large which discards the formation of spherical micelles. X-ray Diffraction (XRD). Finally, in order to investigate the nature of the aggregates formed by the amphiphiles at two different pH’s, the XRD experiment was carried out according to the procedure reported by Bhattacharya et al.57 The X-ray diffraction pattern of all the amphiphiles in two different buffer solutions cast films showed periodical diffraction peaks, indicating an ordered layer structure (Figure 6). The XRD data were collected in the range 2θ = 2−12°. The peak position (2θ) values, corresponding planes, and interplanar distances (d) have been listed in Table S1 of the Supporting Information. The aggregate of SDDPB at pH 13 showed main intense peaks at 31.43 Å (Figure 6A) and 30.23 Å at pH 9 (Figure 6C) corresponding to the plane (001). There were also low intensity peaks at higher angles equivalent to the planes (002, 003), indicating a higher order of packing in the arrangement and also suggesting a single type of periodicity in the structure of the aggregates. However, the widths of the bilayer obtained from XRD are 31.43 and 30.23 Å, which are smaller than twice the length of the hydrocarbon chain (34.80 Å, obtained from energy minimized structure in the Hartree−Fock method by Gauss View 3.0) but much larger than a single hydrophobic tail.

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CONCLUSION Therefore, in summary, we have successfully synthesized two single chain boronic acid based green surfactants, which have excellent surface active properties and are also capable of forming interesting morphologies including vesicles and crystallike structures in buffer solution of two different pH’s. To our knowledge, this is the first detailed report of the self-assembly properties of the amphiphiles containing pyridyl-boronic acid as a headgroup. Surface tension measurements at different surfactant concentrations indicate one break-point corresponding to a single CAC value for both amphiphiles. The microenvironments of the self-assemblies are non-polar in nature and are highly ordered. DLS studies suggest existence of larger aggregates for both SDDPB and SODDPB in solutions. The intensity average size distribution curves of SDDPB solution at pH 9 and 13 are monomodal, suggesting the existence of a single type of aggregates. In contrast, the distribution curve for SODDPB has two/three distributions at pH 13, which is indicative of the presence of more than a single type of aggregates. However, at pH 9, SODDPB produces a monomodal size distribution which is pinpointing the presence of a single type of morphology. XRD measurement indicates the presence of interdigited bilayer aggregates for SDDPB at both pH’s, whereas XRD study of SODDPB indicates a noninterdigited bilayer structure at pH 9 and discards formation of any bilayer aggregates at pH 13. TEM pictures revealed the existence of closed bilayer vesicles for SDDPB at 0.4 mM concentration in solution at pH 13 in the presence of sucrose. At higher concentration (0.7 mM), the vesicles are transformed into giant vesicles. Again, when the concentration was increased to 1 mM, the vesicles were fused to form sponge-like aggregates. After aging the same sample of 0.7 mM, the closed vesicular structures were destroyed. Also, from the TEM H

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(11) Tripper, P. C.; McGuigan, C. Boronic Acids in Medicinal Chemistry: Anticancer, Antibacterial and Antiviral Applications. MedChemComm 2010, 1, 183−198. (12) Smoum, R.; Srebnik, M. Contemporary Aspects of Boron: Chemistry and Biological Applications; Abu Ali, H., Ed.; Studies in Inorganic Chemistry 22; Elsevier: Amsterdam, The Netherlands, 2005. (13) Bull, S. D.; Davidson, M. G.; Van Den Elsen, J. M. H.; Fossey, J. S.; Jenkins, A.; Toby, A.; Jiang, Y. B.; Kubo, Y.; Marken, F.; Sakurai, K.; Zhao, J.; James, T. D. Exploiting the Reversible Covalent Bonding of Boronic Acids: Recognition, Sensing, and Assembly. Acc. Chem. Res. 2013, 46, 312−326. (14) Danjo, H.; Hirata, K.; Yoshigai, S.; Azumaya, I.; Yamaguchi, K. Back to Back Twin Bowls of D3-Symmetric Tris(spiroborate)s for Supramolecular Chain Structures. J. Am. Chem. Soc. 2009, 131, 1638− 1639. (15) Garner, C. W. Boronic Acid Inhibitors of Porcine Pancreatic Lipase. J. Biol. Chem 1980, 255, 5064−5068. (16) Illian, G.; Kaibeitzel, A.; Wingen, R.; Schlosser, H. Eur. Pat. Appl. 1993, 40 (EP 541081). (17) Yang, W.; Gao, X.; Wang, B. Biological and Medicinal Applications of Boronic Acids. In Boronic Acids: Preparation and Applications in Organic Synthesis and Medicine; Hall, D. G., Ed.; WileyVCH: Weinheim, Germany, 2005; pp 481−512. (18) Lienhard, G. E.; Koehler, K. A. 2-Phenylethaneboronic Acid, a Possible Transition-State Analog for Chymotrypsin. Biochemistry 1971, 10, 2477−2483. (19) Shenvi, A. B.; Kettner, C. A. U.S. Patent 4499082. (20) Marson, C. M.; Farrand, L. D.; Brettle, R.; Dunmur, D. A. Highly Efficient Syntheses of 3-Aryl-2-cycloalken-1-ones and an Evaluation of Their Liquid Crystalline Properties. Tetrahedron 2003, 59, 4377−4381. (21) Friedman, M. R.; Toyne, K. J.; Goodby, J. W.; Hird, M. The Synthesis and Transition Temperatures of 5-(4-Alkyl- and 4-alkoxyphenyl)-2-cyanobenzo[b]furans and a 5-(4′-Alkylbiphenyl-4-yl)-2cyanobenzo[b]furan: a Comparison with Their Biphenyl and Terphenyl Analogues. Liq. Cryst. 2001, 28, 901−912. (22) Sharma, S.; Lacey, D.; Wilson, P. Synthesis and Characterization of a Range of Heterocyclic Liquid Crystalline Materials Incorporating the Novel Thiophene-Pyrimidine Moiety. Liq. Cryst. 2003, 30, 451− 461. (23) O’Donovan, R. M.; Mee, D. C.; Fenner, S.; Teasdale, A.; Phillips, H. D. Boronic Acids- A Novel Class of Bacterial Mutagen. Mutat. Res., Genet. Toxicol. Environ. Mutagen. 2011, 724, 1−6. (24) Cambre, J. N.; Sumerlin, B. S. Biomedical Applications of Boronic Acid Polymers. Polymer 2011, 52, 4631−4643. (25) Kataoka, K.; Miyazaki, H.; Bunya, M.; Okano, T.; Sakurai, Y. Totally Synthetic Polymer Gels Responding to External Glucose Concentration: Their Preparation and Application in on-off Regulation of Insulin Release. J. Am. Chem. Soc. 1998, 120, 12694− 12695. (26) Horkay, F.; Cho, S. H.; Tathireddy, P.; Rieth, L.; Solzbacher, F.; Magda, J. Thermodynamic Analysis of the Selectivity Enhancement Obtained by Using Smart Hydrogels That Are Zwitterionic When Detecting Glucose with Boronic Acid Moieties. Sens. Actuators, B 2011, 160, 1363−1371. (27) Moy, C. L.; Kaliappan, R.; McNeil, A. J. Aryl Trihydroxyborate Salts: Thermally Unstable Species with Unusual Gelation Abilities. J. Org. Chem. 2011, 76, 8501−8507. (28) Mu, B.; McNicholas, T. P.; Zhang, J.; Hilmer, A. J.; Jin, Z.; Reuel, N. F.; Kim, J. H.; Yum, K.; Strano, M. S. A Structure−Function Relationship for the Optical Modulation of Phenyl Boronic AcidGrafted, Polyethylene Glycol-Wrapped Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2012, 134, 17620−17627. (29) Toumelin, J. B.; Baboulene, M. First Example of Surfactants Enclosing a Semipolar Nitrogen-Boron Bond. Langmuir 1996, 12, 2128−2129. (30) Toumeline, J. B.; Baboulene, M. Organoboron in Organized Molecular Systems. I. Synthesis and Surfactant Properties of Aminoalkylboronic Acid Salts. New J. Chem. 1999, 23, 111−116.

picture at pH 13 in the presence of sucrose, it is revealed that one oxygen atom in the hydrophobic chain is able to change the morphology of the aggregates, and hence for SODDPB, no vesicle was observed in electron micrograph. Instead of vesicular aggregates, the complex micelles of size in the range 40−150 nm were obtained, which after aging transformed into a crystal-like structure. On the other hand at pH 9 in the presence of fructose, TEM micrographs revealed that both amphiphiles are able to form vesicular aggregates in solutions. Therefore, the synthesized boron derivatives can show promise for the constitution of new structured organized media. These new amphiphiles can be used as potential carriers of pharmaceutical drugs at pH 9. Extension of this novel type of boron derivatives is undergoing in our laboratory.

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ASSOCIATED CONTENT

* Supporting Information S

1

H NMR and LC-Mass spectra of the synthesized amphiphiles, experimental section, plots of Tk values, pKa values and anisotropy, size distribution plots, table for XRD data, and schematic representations of SDDPB and SODDPB. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the DST (Grant No. SR/FT/CS-026/2008). M.M. thanks CSIR (09/599(0044)/ 2011-EMR-I) and A.R. thanks UGC (F.17-130/98(SA-I)) for a research fellowship. The authors thank Dr. Sajal Kanti Mal for his valuable suggestion for synthesis of the amphiphile.

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