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Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 186–196

Surfactant transition metal chelates

David A. Jaeger a,∗, Robin Jose a, Alvaro Mendoza a, Robert P. Apkarian b,�a Department of Chemistry, University of Wyoming, Laramie, WY 82071, USA

b Integrated Microscopy and Microanalytical Facility, Emory University, Atlanta, GA 30322, USA

Received 9 January 2007; accepted 5 February 2007Available online 13 February 2007

Dedicated to the memory of Dr. Robert P. Apkarian.

bstract

New surfactant octahedral Co(III) chelates 4–7 were prepared from sodium hexanitrocobaltate(III) and EDTA derivatives 8–11, respectively.he molecular compositions of 4–7 were established by combustion analyses and electrospray mass spectrometry, and their structures by 1H and

3C NMR, IR, and UV–vis spectroscopy. Surfactants 4–7 were characterized by measurement of their Krafft temperatures and critical aggregationoncentrations in water. The Krafft temperatures of 4 and 5 are >23 ◦C, and the values of the former are greater than the corresponding values ofhe latter. The Krafft temperatures of 6 and 7 are ≤23 ◦C. Aggregated surfactants were characterized by 1H NMR spectroscopy in D2O, and 5a in
ater was studied by cryo-etch high resolution scanning electron microscopy. The NMR results suggested that 4–7 form small aggregates such asicelles or small vesicles, and 5a displayed characteristic segregation patterns in electron micrographs that are likely formed during the freezing
nd/or cryo-etch processes of sample preparation. 2007 Elsevier B.V. All rights reserved.

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eywords: Cryo-etch high resolution scanning electron microscopy; Surfactan

. Introduction

Surfactant transition metal coordination complexes can dis-lay a wide variety of structural, stereochemical, and derivederformance characteristics [1] that are unavailable to conven-ional surfactants, which do not contain a metal, other than withinounterions. Numerous surfactant transition metal complexesave been prepared and studied in solution without isolation2–4], but only a few have been isolated and characterized5–16].

As part of our studies of transition metal-based surfactants11–16], we previously reported the synthesis and character-zation of surfactant Co(III) chelate 1 [12]. Five of the sixoordination sites of the octahedral Co(III) within 1 are occu-
ied by a ligand corresponding to the trianion of compound 2nd the sixth is occupied by a nitro ligand. Octahedral com-lexes of Co(III) are diamagnetic and kinetically inert towards
∗ Corresponding author. Tel.: +1 307 766 4335; fax: +1 307 766 2807.E-mail address: [email protected] (D.A. Jaeger).

� Deceased on 28 February 2006. s

927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2007.02.021

II) chelate; Surfactant synthesis and characterization

igand substitution, as opposed to paramagnetic and labile, dueo their low-spin d6 electronic configuration [17]. Consequently,hese complexes can be analyzed by NMR without difficultiesuch as line-broadening and/or the absence of signals typicallyncountered with labile or paramagnetic species. Compoundis the mono N-dodecyl amide of ethylenediaminetetraacetic

cid (EDTA, 3), whose tetraanion is a well known hex-dentate ligand that forms stable chelates with metal cations18].

Herein we report the synthesis and characterization of newurfactant Co(III) chelates, including series 4 and 5, and indi-

mailto:[email protected]

dx.doi.org/10.1016/j.colsurfa.2007.02.021

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D.A. Jaeger et al. / Colloids and Surfaces A:

idual surfactant chelates 6 and 7. Compared to surfactant 1,eries 4 and 5 contain long-chain ester and alkyl groups, respec-ively, in place of an N-dodecyl amide group. On going from

to surfactants 6 and 7, the N-dodecyl amide group has beenubstituted with �-hydroxy and �-trimethylammonio groups,espectively. Note that 4, 5, and 6 are anionic surfactants con-aining the same negatively charged Co(III)-based headgroup,hereas 7 is a bola surfactant, since it contains a headgroup

t each of the ends of a hydrocarbon chain. Specifically, sur-actant 7 is an unsymmetrical, zwitterionic bola surfactant,ontaining a quaternary ammonium group at one end of aH2CONH(CH2)12 unit, and the negatively charged Co(III)omplex common to 4, 5, and 6 at the other end. To ournowledge, this is the first example of a bola surfactant con-aining a transition metal coordination complex as a head-roup.

In addition to a nitro ligand, surfactant chelates 4, 5, 6, andcontain pentadentate ligands corresponding to the trianions

f EDTA-derived compounds 8, 9, 10, and 11, respectively. Its the variation within 8–11, compared to 2, that results in theifferences within surfactant chelates 4–7.

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cochem. Eng. Aspects 302 (2007) 186–196 187

. Experimental

.1. General procedures and materials

1H (400 MHz) and 13C (100.6 MHz) NMR spectra (25 ◦C)ere recorded in the following solvents with the indicated inter-al standards (relative to Me4Si): CDCl3, residual CHCl3 (δ.27) and CDCl3 (center line at δ 77.23), respectively; CD3OD,esidual CD2HOD (δ 3.31) and CD3OD (center line at δ 49.15),espectively; CD3SOCD3, residual CD3SOCD2H (δ 2.50) andD3SOCD3 (δ 39.51), respectively; D2O, residual HOD (δ 4.80)nd external Me4Si in CDCl3, respectively. All J values are inz. Electrospray (ES) mass spectra were obtained on a Thermo-innigan LCQ instrument (scan range m/z = 150–2000), using5:5 MeOH–H2O solutions of compounds with direct infusiont the heated capillary (200 ◦C). The Tk values were evaluatedccording to the following protocol, adapted from a literatureethod [19]. A 1.0 mg sample of surfactant is dispersed into

.0 mL of HPLC-grade H2O at 23 ◦C by shaking. If the surfac-ant does not fully dissolve at 23 ◦C, the dispersion is heatedo determine if a clear/translucent mixture is obtained. Theemperature at which a sharp change to clarity is observed cor-esponds to the Tk value. The cac values were obtained fromlots of surface tension versus log[surfactant] at 23 ◦C, usingKibron MicroTrough S; the reported values are averages of

t least duplicate measurements. For surfactants 4a, 5a, andb, the surface tension measurements of the serially dilutedolutions used in a cac determination were made within 8 hf preparation of the initial supersaturated stock solution byeating a mixture of surfactant and H2O above its Krafft tem-erature, until the mixture became transparent. Solutions of therisodium salts of 8, 9, and 10 were prepared by the additionf the compound to H2O containing 3 molar equiv. of NaOH.ryo-etch HRSEM was performed according to literature pro-edures [20,21], including plunge-freezing of a supersaturatedqueous solution of 5a into liquid ethane. Extracts were driedver Na2SO4, and the ratios of solvents are volume:volume. Ele-ental analyses were performed by Atlantic Microlab, Norcross,A.

.2. Synthesis of ethylenediaminetetraacetic acidonoanhydride (12) [12,22]

This compound was prepared by the literature procedure [12]rom ethylenediaminetetraacetic acid dianhydride [12,22].

.3. Synthesis of sodium hexanitrocobaltate(III)

This compound was synthesized by the literature procedure23].

.4. Synthesis of ethylenediaminetetraacetic acid monoctyl ester (8a)

With the procedure used for the preparation of 8c, 5.00 g18.2 mmol) of 12 and 2.38 g (18.3 mmol) of 1-octanol wereonverted into 2.58 g (35%) of 8a: mp 190–192 ◦C (dec); 1H

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88 D.A. Jaeger et al. / Colloids and Surfaces A

MR (CD3SOCD3): δ 4.02 (t, J = 6.5, 2H, CH2O), 3.56 (s,H, CH2CO2CH2), 3.46 (s, 6H, 3CH2CO2H), 2.76 (s, 4H,CH2CH2N), 1.57 (m, 2H, CH2CH2O), 1.26 (br s, 10H,

CH2)5), 0.87 (t, J = 6.4, 3H, CH3); 13C NMR (CD3SOCD3): δ

72.80, 171.35, 64.18, 55.02, 54.82, 54.73, 51.81, 51.57, 31.55,8.94, 28.45, 25.72, 22.43, 14.32. Anal. calcd for C18H32N2O8:, 53.45; H, 7.97. Found: C, 53.53; H, 8.02. ES MS (positive

on mode) calcd for C18H33N2O8 (M + H+) 405.2, found 405.1.

.5. Synthesis of ethylenediaminetetraacetic acid monoecyl ester (8b)

With the procedure used for the preparation of 8c, 3.80 g13.9 mmol) of 12 and 2.17 g (13.7 mmol) of 1-decanol wereonverted into 2.80 g (47%) of 8b: mp 193–194 ◦C (dec);H NMR (CD3SOCD3): δ 4.01 (t, J = 6.6, 2H, CH2O), 3.54s, 2H, CH2CO2CH2), 3.45 (s, 4H, 2CH2CO2H), 3.44 (s,H, CH2CO2H), 2.75 (s, 4H, NCH2CH2N), 1.55 (m, 2H,H2CH2O), 1.24 (br s, 14H, (CH2)7), 0.85 (t, J = 6.8, 3H,H3); 13C NMR (CD3SOCD3): δ 172.76, 171.33, 64.18, 55.01,4.82, 54.73, 51.82, 51.56, 31.64, 29.29, 29.04, 28.98, 28.45,5.72, 22.46, 14.32. Anal. calcd for C20H36N2O8: C, 55.54; H,.38. Found: C, 55.63; H, 8.45. ES MS (negative ion mode)alcd for C20H35N2O8 (M − H+) 431.2, found 431.2; calcd for40H71N4O16 (2M − H+) 863.5, found 862.1.

.6. Synthesis of ethylenediaminetetraacetic acid monoodecyl ester (8c) [24]

A modified literature procedure was used [24]. A mixturef 2.3 g (8.4 mmol) of monoanhydride 12, 1.56 g (8.37 mmol)f 1-dodecanol, and 86 mL of dry DMF (227056) was stirredt 100 ◦C under N2 for 24 h, cooled to 23 ◦C, and pourednto 500 mL of an ice–H2O mixture. The resultant precipitateas filtered, washed with H2O (0 ◦C), recrystallized from 5:22O–EtOH, and dried (8 h, 23 ◦C, 0.05 mmHg) to give 1.35 g

35%) of 8c: mp 187–188 ◦C (dec); 1H NMR (CD3SOCD3):4.01 (t, J = 6.6, 2H, CH2O), (s, 2H, CH2CO2CH2), 3.45

s, 4H, 2CH2CO2H), 3.44 (s, 2H, CH2CO2H), 2.75 (s, 4H,CH2CH2N), 1.55 (m, 2H, CH2CH2O), 1.24 (br s, 18H,

CH2)9), 0.85 (t, J = 6.6, 3H, CH3); 13C NMR (CD3SOCD3):172.77, 171.33, 64.18, 55.01, 54.81, 54.72, 51.81, 51.55,

1.66, 29.38, 29.33, 29.29, 29.08, 28.99, 28.45, 25.72, 22.46,4.33. Anal. calcd for C22H40N2O8: C, 57.37; H, 8.75.ound: C, 57.62; H, 8.85. ES MS (negative ion mode) calcdor C22H39N2O8 (M − H+) 459.3, found 459.4; calcd for44H79N4O16 (2M − H+) 919.5, found 919.2.

.7. Synthesis of surfactant cobalt(III) chelate 4a

With the procedure used for the preparation of 4c, 2.5 g6.2 mmol) of 8a and 2.49 g (6.16 mmol) of sodium hexa-itrocobaltate(III) gave 0.59 g (18%) of 4a: mp 258–260 ◦C
dec); 1H NMR (CD3SOCD3): δ 4.02 (t, J = 6.6, 2H, CH2O),.85–3.93 (m, 12H, 3CH2CO, NCH2CH2N, CH2CO2CH2),.55 (m, 2H, CH2CH2O), 1.24 (br s, 10H, (CH2)5), 0.84 (t,= 6.7, 3H, CH3); 13C NMR (CD3SOCD3): δ 180.06, 179.09,
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sicochem. Eng. Aspects 302 (2007) 186–196

78.10, 166.60, 66.22, 65.14, 64.28, 63.90, 62.38, 60.43, 60.20,1.57, 28.95, 28.91, 28.24, 25.69, 22.43, 14.32; IR (KBr):736 cm−1 (s) (C O), 1673 cm−1 (vs) (C O), 1639 cm−1

s) (C O), 1422 cm−1 (m) (NO2), 1334 cm−1 (s) (NO2),33 cm−1 (s) (NO2), 655 cm−1 (m) (NO2); UV–vis (H2O): λmax50 nm (log εmax 4.31), 345 (3.52), 500 (2.36). Anal. calcd for18H29N3O10CoNa: C, 40.84; H, 5.52. Found: C, 40.80; H,.51. ES MS (negative ion mode) calcd for C18H29N3O10Cosurfactant anion) 506.1, found 506.1; calcd for C18H29N2O8Cosurfactant anion − NO2) 460.1, found 460.4.

.8. Synthesis of surfactant cobalt(III) chelate 4b

With the procedure used for the preparation of 4c, 0.112 g0.259 mmol) of 8b and 0.104 g (0.257 mmol) of sodium hex-nitrocobaltate(III) gave 0.053 g (37%) of 4b: mp 289–292 ◦Cdec); 1H NMR (CD3SOCD3): δ 4.04 (t, J = 6.6, 2H, CH2O),.86–3.96 (m, 12H, 3CH2CO, NCH2CH2N, CH2CO2CH2),.56 (m, 2H, CH2CH2O), 1.24 (br s, 14H, (CH2)7), 0.85 (t,= 6.7, 3H, CH3); 13C NMR (CD3SOCD3): δ 180.05, 179.08,78.09, 166.60, 66.22, 65.14, 64.29, 63.90, 62.38, 60.43, 60.20,1.64, 29.31, 29.27, 29.04, 29.00, 28.24, 25.85, 25.69, 22.46,4.33; IR (KBr): 1735 cm−1 (s) (C O), 1672 cm−1 (vs) (C O),639 cm−1 (s) (C O), 1422 cm−1 (m) (NO2), 1334 cm−1 (s)NO2), 832 cm−1 (m) (NO2), 654 cm−1 (m) (NO2); UV–visH2O): λmax 250 nm (log εmax 4.29), 345 (3.50), 500 (2.38).nal. calcd for C20H33N3O10CoNa: C, 43.10; H, 5.97. Found:, 43.33; H, 6.00. ES MS (negative ion mode) calcd for20H33N3O10Co (surfactant anion) 534.1, found 534.1; calcd

or C20H33N2O8Co (surfactant anion − NO2) 488.2, found88.4.

.9. Synthesis of surfactant cobalt(III) chelate 4c

A mixture of 0.980 g (2.13 mmol) of 8c, 1.05 g (12.8 mmol)f sodium acetate, 0.870 g (2.15 mmol) of sodium hexanitro-obaltate(III), and 8.0 mL of H2O was stirred at 23 ◦C formin, and then it was heated to 50 ◦C over 10 min. After0 min at 50 ◦C, the temperature was increased to 75 ◦C dur-ng 15 min. After 30 min at 75 ◦C, the reaction mixture wasooled to 23 ◦C, and the resultant precipitate was collected byltration, washed with 1:1 EtOH–H2O, and air-dried. Then itas recrystallized from 1:1 Me2CHOH–H2O and dried (6 h,3 ◦C, 0.05 mmHg) to give 0.548 g (44%) of 4c: mp 329–331 ◦Cdec); 1H NMR (CD3SOCD3): δ 4.03 (t, J = 6.5, 2H, CH2O),.86–3.95 (m, 12H, 3CH2CO, NCH2CH2N, CH2CO2CH2),.56 (m, 2H, CH2CH2O), 1.23 (br s, 18H, (CH2)9), 0.85t, J = 6.5, 3H, CH3); 13C NMR (CD3SOCD3): δ 180.06,79.09, 178.11, 166.60, 66.22, 65.14, 64.28, 63.90, 62.38, 60.43,0.19, 31.65, 29.36, 29.26, 29.07, 29.00, 28.24, 25.68, 22.46,4.33; IR (KBr): 1734 cm−1 (s) (C O), 1672 cm−1 (vs) (C O),638 cm−1 (s) (C O), 1422 cm−1 (m) (NO2), 1333 cm−1 (s)NO2), 832 cm−1 (m) (NO2), 653 cm−1 (s) (NO2); UV–vis
H2O): λmax 250 nm (log εmax 4.22), 345 (3.38), 500 (2.30).nal. calcd for C22H37N3O10CoNa: C, 45.13; H, 6.37. Found:, 44.89; H, 6.40. ES MS (negative ion mode) calcd for22H37N3O10Co (surfactant anion) 562.2, found 562.1; calcd

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or C22H37N2O8Co (surfactant anion − NO2) 516.2, found16.2.

.10. Synthesis of N-alkylethylenediamine ligands 14 [25]

Ligands 14 were prepared by a literature procedure [25] fromlkyl bromides and ethylenediamine (13).

.11. Synthesis of N-octylethylenediaminetriacetic acid9a) [26]

A modified literature procedure [27] was used. A solu-ion (4 ◦C) of 2.93 g (52.2 mmol) of KOH (85%) in 3.3 mLf H2O was added dropwise over 30 min to a solution of.94 g (52.2 mmol) of monochloroacetic acid (Fisher) in 5.0 mLf H2O, keeping its temperature at <20 ◦C. Then 1.00 g5.80 mmol) of 14a was added, followed by an additional solu-ion (0 ◦C) of 2.93 g (52.2 mmol) of KOH in 3.3 mL of H2O.he resultant reaction mixture was stirred under N2 for 7ays at 23 ◦C and then cooled in an ice bath, followed byhe addition of 1.4 mL of 9 M sulfuric acid, to give a pH of. Thereafter the mixture was held at 4 ◦C for 10 h to yieldprecipitate. After the addition of 50 mL of H2O (4 ◦C), theixture was stirred for 30 min, and the solid precipitate was

ollected by filtration, washed with H2O and then EtOH, andecrystallized (0 ◦C) from 2:1 H2O–EtOH to give 1.24 g (62%)f 9a: mp 142–144 ◦C (lit. [26], mp 147–148 ◦C); 1H NMRCD3OD): δ 3.69 (s, 2H, NCH2CO2), 3.51 (s, 4H, 2NCH2CO2),.23 (m, 4H, NCH2CH2N), 3.05 (t, J = 5.4, 2H, CH2N), 1.68m, 2H, CH2CH2N), 1.17–1.39 (m, 10H, (CH2)5), 0.82 (t,= 6.8, 3H, CH3); 13C NMR (CD3OD): δ 175.48, 170.35, 56.91,6.66, 56.43, 54.88, 50.79, 33.07, 30.40, 27.82, 25.43, 23.84,4.57.

.12. Synthesis of N-decylethylenediaminetriacetic acid9b)

With the procedure used for the preparation of 9a, 0.875 g4.37 mmol) of 14b was converted into 1.03 g (63%) of 9b: mp31–133 ◦C; 1H NMR (CD3OD): δ 3.71 (s, 2H, NCH2CO2),.53 (s, 4H, 2NCH2CO2), 3.25 (m, 4H, NCH2CH2N), 3.07 (t,= 5.4, 2H, CH2N), 1.71 (m, 2H, CH2CH2N), 1.19–1.38 (m,4H, (CH2)7), 0.85 (t, J = 6.7, 3H, CH3); 13C NMR (CD3OD):175.48, 170.36, 56.92, 56.67, 56.47, 54.89, 50.80, 33.22,

0.80, 30.74, 30.60, 30.46, 27.83, 25.44, 23.90, 14.60. Anal.alcd for C18H34N2O6: C, 57.73; H, 9.15. Found: C, 57.61; H,.25.

.13. Synthesis of N-dodecylethylenediaminetriacetic acid9c) [26]

With the procedure used for the preparation of 9a, 0.770 g3.37 mmol) of 14c gave a solid precipitate that was collected
y filtration, washed with H2O and then EtOH, and recrystal-ized four times from H2O (0 ◦C) to give 0.425 g (31%) of 9c:p 140–144 ◦C (lit. [26], mp 143–145 ◦C); 1H NMR (CD3OD):3.71 (s, 2H, NCH2CO2), 3.54 (s, 4H, 2NCH2CO2), 3.25 (m,
2

(


H, NCH2CH2N), 3.07 (t, J = 5.2, 2H, CH2N), 1.71 (m, 2H,H2CH2N), 1.18–1.38 (m, 18H, (CH2)9), 0.85 (t, J = 6.8, 3H,H3); 13C NMR (CD3OD): δ 175.49, 170.36, 56.93, 56.65,6.47, 54.90, 50.79, 33.24, 30.92, 30.84, 30.73, 30.65, 30.47,7.83, 25.43, 23.91, 14.61.

.14. Synthesis of surfactant cobalt(III) chelate 5a

Combined literature procedures [12,28] for related com-lexes were used. To a solution of 0.900 g (2.60 mmol) of 9and 1.92 g (23.4 mmol) of sodium acetate in 8.0 mL of H2O23 ◦C), 1.05 g (2.60 mmol) of sodium hexanitrocobaltate(III)as added. The mixture was stirred under N2 and heated to0 ◦C over 40 min. After 20 min at 50 ◦C, the temperature wasncreased to 90 ◦C over 1.0 h, where it was held for 7 h. The resul-ant precipitate was collected by filtration at 23 ◦C, washed withix 3 mL portions of EtOH (4 ◦C), dried (23 ◦C, 0.75 mmHg),nd recrystallized (0 ◦C) from 1:1 MeOH–H2O to give 0.810 g66%) of 5a: mp 255–258 ◦C; 1H NMR (CD3SOCD3): δ

.26–3.87 (m, 12H, 3CH2CO2, NCH2CH2N, CH2N), 1.53 (m,H, CH2CH2N), 1.05–1.32 (m, 10H, (CH2)5), 0.85 (t, J = 6.5,H, CH3); 13C NMR (CD3SOCD3): δ 179.79, 178.69, 177.76,5.99, 64.05, 62.63, 61.53, 59.88, 59.53, 31.19, 28.69, 28.51,6.78, 22.08, 21.54, 13.97; IR (KBr): 1686 cm−1 (s) (C O),654 cm−1 (s) (C O), 1622 cm−1 (s) (C O), 1424 cm−1 (m)NO2), 1328 cm−1 (s) (NO2), 831 cm−1 (m) (NO2), 661 cm−1

w) (NO2); UV–vis (H2O): λmax 250 nm (log εmax 4.31), 3403.52), 492 (2.41). Anal. calcd for C16H27N3O8CoNa·H2O: C,9.27; H, 5.97; N, 8.59. Found: C, 39.33; H, 5.87; N, 8.37. ESS (negative ion mode) calcd for C16H27N3O8Co (surfactant

nion) 448.1, found 447.9; calcd for C16H27N2O6Co (surfactantnion − NO2) 402.1, found 402.2.

.15. Synthesis of surfactant cobalt(III) chelate 5b

With the procedure used for the preparation of 5a, 1.00 g2.67 mmol) of 9b was converted into crude product that wasecrystallized (0 ◦C) from 1:1 EtOH–H2O to yield 0.850 g64%) of 5b: mp 246–249 ◦C; 1H NMR (CD3SOCD3): δ

.26–3.85 (m, 12H, 3CH2CO2, NCH2CH2N, CH2N), 1.53m, 2H, CH2CH2N), 1.06–1.35 (m, 14H, (CH2)7), 0.85 (t,= 6.0, 3H, CH3); 13C NMR (CD3SOCD3): δ 179.78, 178.68,77.75, 66.00, 64.06, 62.63, 61.52, 59.88, 59.53, 31.29, 28.93,8.88, 28.75, 28.71, 26.77, 22.12, 21.55, 13.99; IR (KBr):673 cm−1 (s) (C O), 1631 cm−1 (s) (C O), 1431 cm−1 (m)NO2), 1332 cm−1 (s) (NO2), 830 cm−1 (m) (NO2), 656 cm−1

w) (NO2); UV–vis (H2O): λmax 250 nm (log εmax 4.35), 3403.57), 491 (2.47). Anal. calcd for C18H31N3O8CoNa: C, 43.29;, 6.26; N, 8.41. Found: C, 43.02; H, 6.40; N, 8.35. ESS (negative ion mode) calcd for C18H31N3O8Co (surfactant

nion) 476.1, found 475.7; calcd for C18H31N2O6Co (surfactantnion − NO2) 430.2, found 430.0.

.16. Synthesis of surfactant cobalt(III) chelate 5c

With the procedure used for the preparation of 5a, 0.400 g0.994 mmol) of 9c was converted into 0.352 g (70%) of 5c:

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oh


p 252–254 ◦C; 1H NMR (CD3SOCD3): δ 2.27–3.85 (m, 12H,CH2CO2, NCH2CH2N, CH2N), 1.52 (m, 2H, CH2CH2N),.04–1.40 (m, 18H, (CH2)9), 0.85 (t, J = 6.6, 3H, CH3);3C NMR (CD3SOCD3): δ 179.76, 178.66, 177.74, 65.98,4.03, 62.61, 61.49, 59.84, 59.50, 31.28, 29.03, 28.99, 28.95,8.84, 28.71, 26.76, 22.09, 21.52, 13.95; IR (KBr): 1676 cm−1

s) (C O), 1636 cm−1 (s) (C O), 1424 cm−1 (m) (NO2),334 cm−1 (m) (NO2), 833 cm−1 (m) (NO2), 661 cm−1 (w)NO2); UV–vis (diffuse reflectance; 18.0 mg of 5c dilutedith 1.20 g of KBr): λmax 253, 343, 495 nm. Anal. calcd for20H35N3O8CoNa·H2O: C, 44.04; H, 6.84; N, 7.70. Found: C,4.14; H, 6.68; N, 7.72. ES MS (negative ion mode) calcd for20H35N3O8Co (surfactant anion) 504.2, found 504.0; calcu-

ated for C20H35N2O6Co (surfactant anion − NO2) 458.2, found58.4.

.17. Synthesis of 12-hydroxydodecanenitrile (16) [29]

A mixture of 1.00 g (3.98 mmol) of 11-bromoundecanol (15)Aldrich), 0.381 g (7.78 mmol) of NaCN, and 15.0 mL of DMSOas stirred for 24 h at 90 ◦C. Then the reaction mixture was

dded to 30 mL of H2O and extracted four times with 25 mL por-ions of CH2Cl2. The combined extracts were washed six timesith H2O, dried, and rotary evaporated to give 0.742 g (94%) of6: mp 33–35 ◦C (lit. [29], mp 37 ◦C); 1H NMR (CDCl3): δ 3.64t, J = 6.6, 2H, CH2O), 2.34 (t, J = 7.1, 2H, CH2CN), 1.61–1.70m, 3H, CH2CH2CN, OH), 1.57 (m, 2H, CH2CH2O), 1.44 (m,H, CH2), 1.29 (br s, 12H, (CH2)6).

.18. Synthesis of 12-hydroxydodecanamine (17) [29]

A mixture of 3.80 g (19.3 mmol) of 16, ca. 5 g of Raney NiAldrich, 22,167-8), 80 mL of 95% EtOH, and 50 mL of con-entrated ammonium hydroxide was stirred under H2 (50 psi)or 20 h at 23 ◦C. Then the reaction mixture was filtered throughpad of Celite, which was washed with 50 mL of 95% EtOH.he combined filtrates were rotary-evaporated to give 3.9 g ofaterial, which was chromatographed on a 50 cm × 2.5 cm (i.d.)

olumn of silica gel (ICN 02776, 60 A, 32–63 �m) packed inH2Cl2 and eluted with 7:3 CH2Cl2–MeOH to remove unre-cted 16, followed by 7:3:0.3 CH2Cl2–MeOH–15 M aqueousH3 to yield 2.2 g (57%) of 17 (Rf = 0.57): mp 78–79 ◦C (lit.

29], mp 80 ◦C); 1H NMR (CDCl3): δ 3.65 (t, J = 6.6, 2H,H2O), 2.69 (t, J = 7.0, 2H, CH2N), 1.57 (m, 2H, CH2CH2O),.42 (m, 2H, CH2CH2N), 1.22–1.39 (br s, 19H, OH, NH2,CH2)8); 13C NMR (CDCl3): δ62.89, 41.80, 32.94, 32.73, 29.45,9.33, 26.77, 25.65.

.19. Synthesis of ethylenediaminetetraacetic acid mono-(12-hydroxy)dodecylamide (10)

A mixture of 0.568 g (2.82 mmol) of 17, 0.786 g (2.87 mmol)
f 12, and 25 mL of dry DMF was stirred at 80 ◦C for 9 h under2, cooled to 23 ◦C, and poured into 300 mL of ice–H2O. The
esultant precipitate was filtered, washed with cold H2O, andir-dried to give crude product. A mixture of this material and

Atw1


0 mL of concentrated ammonium hydroxide was filtered toemove undissolved solid, and the filtrate was acidified to pH.5 with 10% hydrochloric acid. The resultant precipitate wasollected by filtration, washed with H2O, recrystallized from2O (75 ◦C), and dried (23 ◦C, 0.05 mmHg) to give 0.834 g

63%) of 10: mp 240–243 ◦C (dec); 1H NMR (CD3SOCD3):8.01 (t, J = 5.6, 1H, NH), 3.44 (s, 4H, 2CH2CO), 3.37 (s, 2H,H2CO), 3.36 (t, J = 6.5, 2H, CH2O), 3.20 (s, 2H, CH2CO),.06 (apparent q, J = 6.7, 2H, CH2NH), 2.68–2.78 (m, 4H,CH2CH2N), 1.34–1.44 (m, 4H, CH2CH2O, CH2CH2NH),.24 (br s, 16H, (CH2)8); 13C NMR (CD3SOCD3): δ 172.98,72.86, 170.60, 61.18, 58.02, 55.50, 55.01, 52.52, 52.08, 38.73,3.01, 29.64, 29.59, 29.51, 29.44, 29.24, 26.87, 25.98. Anal.alcd for C22H41N3O8: C, 55.56; H, 8.69. Found C, 55.36;, 8.76. ES MS (positive ion mode) calcd for C22H42N3O8

M + H+) 476.3, found 476.3.

.20. Synthesis of surfactant cobalt(III) chelate 6

A mixture of 0.500 g (1.05 mmol) of 10, 0.250 g (3.05 mmol)f sodium acetate, 0.420 g (1.04 mmol) of sodium hexanitro-obaltate(III), and 32 mL of H2O was stirred at 23 ◦C for 5 min,nd then it was heated to 50 ◦C over 10 min. After 10 mint 50 ◦C, the temperature was raised to 75 ◦C over 15 min.fter 30 min at 75 ◦C, the reaction mixture was cooled to3 ◦C, and the resultant precipitate was collected by filtra-ion, washed with 50 mL of 1:1 EtOH–H2O and air-dried toive crude product. This material was recrystallized from 3:1e2CHOH–H2O (23 ◦C), washed with Me2CHOH and then

t2O to remove Me2CHOH, and dried (23 ◦C, 0.05 mmHg)o give 0.308 g (50%) of 6: mp 226–230 ◦C (dec); 1H NMRCD3SOCD3): δ 8.25 (br t, 1H, NH), 4.32 (t, J = 5.1, 1H, OH),.78–3.92 (m, 16H, 3CH2CO, NCH2CH2N, CH2CONHCH2,H2O), 1.34 (m, 4H, CH2CH2O, NHCH2CH2), 1.23 (br s,6H, (CH2)8); 13C NMR (CD3SOCD3): δ 180.40, 179.15,78.13, 165.50, 66.22, 64.52, 64.30, 62.86, 61.70, 61.08,1.11, 38.75, 32.89, 29.44, 29.36, 29.15, 29.04, 26.72, 25.86;R (KBr): 3304 cm−1 (s, br) (OH, NH), 1657 cm−1 (vs, br)C O), 1552 cm−1 (m) (amide II), 1445 cm−1 (m) (NO2),333 cm−1 (s) (NO2), 830 cm−1 (m) (NO2), 656 cm−1 (m)NO2); UV–vis (H2O): λmax 255 nm (log εmax 4.27), 345 (3.51),00 (2.41). Anal. calcd for C22H38N4O10CoNa·H2O: C, 42.72;, 6.52. Found: C, 42.78; H, 6.64. ES MS (negative ion mode)

alcd for C22H38N4O10Co (surfactant anion) 577.2, found77.1; C22H38N3O8Co (surfactant anion − NO2) 531.2, found31.3.

.21. Synthesis of 12-bromo-1-dodecanaminium bromide18) [30]

A modified literature procedure was used [30]. A mixturef 2.11 g (10.5 mmol) of amino alcohol 17 and 6.0 mL of 48%ydrobromic acid was refluxed for 15 h and rotary evaporated.

solution of the residue in 10 mL of H2O was extracted fourimes with 30 mL portions of CH2Cl2. The combined extractsere dried and rotary evaporated to give 2.97 g (82%) of 18: mp40–142 ◦C (lit. [30], mp 140 ◦C); 1H NMR (CDCl3): δ 8.03 (br

Physi

sC(δ

2

2b

Mreuogδ

J(42

2N

oNtawatwtot2HMraca13C311121Ctf1

2

oc51orMtco(aho(8C(N623(8λ

fH(

3

3

starting with the reaction of alcohol ROH (R = C8H17, C10H21,C12H25) with 12, the monoanhydride of EDTA, to give 8 (Eq.(1)). Surfactants 4 were then obtained by the reaction of 8 withsodium hexanitrocobaltate(III) (Eq. (2)).


, 3H, NH3+), 3.42 (t, J = 6.9, 2H, CH2Br), 3.04 (t, J = 7.6, 2H,

H2N), 1.77–1.91 (m, 4H, CH2CH2Br, CH2CH2N), 1.37–1.47m, 4H, 2CH2), 1.29 (br m, 12H, (CH2)6); 13C NMR (CDCl3):40.07, 34.02, 32.76, 29.38, 29.25, 28.83, 28.69, 28.10, 27.43,6.43.

.22. Synthesis of (12-aminododecyl)trimethylammoniumromide (19)

A mixture of 3.96 g (11.5 mmol) of 18 and 200 mL of 4.2 Me3N in EtOH (0.84 mol) was stirred at 23 ◦C for 4 days,

efluxed for 20 h under a dry ice–Me2CO condenser, and rotaryvaporated. A solution of the residue in 5.0 mL of H2O was sat-rated with K2CO3 and extracted six times with 20 mL portionsf CH2Cl2. The combined extracts were rotary evaporated toive 3.16 g (85%) of 19: mp 201–203 ◦C; 1H NMR (CDCl3):3.58 (m, 2H, CH2N+(CH3)3), 3.48 (s, 9H, N(CH3)3), 2.69 (t,= 7.0, CH2NH2), 1.74 (m, 2H, CH2CH2N+(CH3)3), 1.21–1.51

m, 20H, NH2, (CH2)9); 13C NMR (CDCl3): δ 66.87, 53.27,2.00, 33.38, 29.41, 29.33, 29.27, 29.21, 29.09, 26.75, 26.05,3.20.

.23. Synthesis of ethylenediaminetetraacetic acid mono-(12-trimethylammonio)dodecylamide bromide (11)

A mixture of 0.595 g (1.84 mmol) of 19, 0.505 g (1.84 mmol)f 12, and 25 mL of dry DMF was stirred at 85 ◦C for 5 h under2. DMF was removed under vacuum (23 ◦C, 0.05 mmHg), and

he residue was dissolved in 10 mL of MeOH, followed by theddition of 200 mL of Me2CHOH. The resultant cloudy mixtureas heated until it became clear, and then it was allowed to sit

t 23 ◦C for 3 weeks, open to the atmosphere, to allow MeOHo evaporate. The precipitated solid was collected by filtration,ashed with Et2O, and air-dried to give 0.659 g of crude product

hat was purified by a literature method [31]. A mixture of 0.42 gf crude product, 0.40 g of NaBr, and 0.50 mL of H2O was addedo a 15 cm × 1 cm (i.d.) column of charcoal (Darco G-60, Aldrich4,227-6) packed dry. The column was eluted with 100 mL of2O, to remove NaBr, and then with 100 mL of MeOH. TheeOH fraction was rotary evaporated, and a solution of the

esidue in 60 mL of 1:20 MeOH–Me2CHOH was allowed to sitt 23 ◦C for 3 weeks, open to the atmosphere. The resultant pre-ipitate was collected by filtration, washed with 5 mL of Et2O,nd dried (23 ◦C, 0.05 mmHg) to yield 0.110 g (15%) of 11: mp41–143 ◦C; 1H NMR (CD3SOCD3): δ 8.02 (t, J = 5.8, 1H, NH),.43 (s, 4H, 2CH2CO2H), 3.36 (s, 2H, CH2CO), 3.24 (m, 2H,H2N+(CH3)3), 3.19 (s, 2H, CH2CO), 3.00–3.10 (m with s at.03, 11H, CH2NH, N(CH3)3), 2.68–2.78 (m, 4H, NCH2CH2N),.64 (m, 2H, CH2CH2N+(CH3)3) 1.39 (m, 2H, CH2CH2NH),.25 (br s, 16H, (CH2)8); 13C NMR (CD3SOCD3): δ 172.94,72.79, 170.57, 65.64, 57.95, 55.37, 54.04, 52.48, 52.00, 38.58,9.52, 29.29, 29.13, 29.07, 28.84, 26.72, 26.09, 22.37; IR (KBr):

−1
694 cm (s) (C O). Anal. calcd for C25H49N4O7Br·H2O:, 48.78; H, 8.35. Found: C, 48.60; H, 8.29. ES MS (posi-
ive ion mode) calcd for C25H49N4O7 (surfactant cation) 517.4,ound 517.2; calcd for C50H97N8O14 (2 surfactant cations − H+)033.7, found 1032.9.


.24. Synthesis of surfactant cobalt(III) chelate 7

A mixture of 0.603 g (1.01 mmol) of 11, 0.280 g (3.41 mmol)f sodium acetate, 0.472 g (1.17 mmol) of sodium hexanitro-obaltate(III), and 36 mL of H2O was stirred at 23 ◦C formin, and then it was heated to 50 ◦C over 10 min. After0 min at 50 ◦C, the reaction mixture was heated to 75 ◦Cver 15 min, held at 75 ◦C for 30 min, cooled to 23 ◦C, andotary evaporated. The residue was recrystallized from 9:1:1

e2CHOH–MeOH–H2O (23 ◦C), washed with Me2CHOH andhen Et2O, and dried (23 ◦C, 0.05 mmHg) to give 0.405 g ofrude product. A total of 2.20 g of crude product from this andther preparations was chromatographed on a 50 cm × 2.5 cmi.d.) column of neutral alumina (J.T. Baker 0537-05) packed drynd eluted with 3:1:1 MeCN–EtOH–concentrated ammoniumydroxide to give 1.58 g of product that was chromatographedn an identical column eluted with 95% EtOH to yield 1.2 g35%) of 7: mp 226–228 ◦C (dec); 1H NMR (CD3SOCD3): δ

.24 (t, J = 5.4, 1H, NH), 2.78–3.92 (m with s at 3.02, 25H,H2N+(CH3)3, 3CH2CO, NCH2CH2N, CH2CONHCH2), 1.66

m, 2H, CH2CH2N+(CH3)3), 1.18–1.41 (m, 18H, (CH2)9); 13CMR (CD3SOCD3): δ 180.32, 179.13, 178.11, 165.55, 66.22,5.66, 64.46, 64.28, 62.93, 61.72, 60.14, 52.50, 38.63, 29.22,9.12, 29.02, 28.87, 28.79, 26.54, 26.10, 22.33; IR (KBr):300 cm−1 (s, br) (NH), 1653 cm−1 (vs, br) (C O), 1542 cm−1

m) (amide II), 1447 cm−1 (m) (NO2), 1324 cm−1 (s) (NO2),24 cm−1 (m) (NO2), 654 cm−1 (m) (NO2); UV–vis (H2O):max 255 nm (log εmax 4.28), 345 (3.51), 500 (2.36). Anal. calcdor C25H46N5Co·2H2O: C, 45.80; H, 7.69. Found: C, 45.91;, 7.69. ES MS (positive ion mode) calcd for C25H46N4O7Co

surfactant − NO2−) 573.3, found 573.4.

. Results and discussion

.1. Syntheses

Surfactants 4 were synthesized as illustrated in Scheme 1,

Scheme 1.

192 D.A. Jaeger et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 186–196

saNcb(

ma((s

AfgTs

3

5s1

f

acrnaCwfac

oadtaabcw

Scheme 2.

Surfactants 5 were synthesized as illustrated in Scheme 2,tarting with the monoalkylation of ethylenediamine (13) withlkyl bromide RBr (R = C8H17, C10H21, C12H25) to give-alkylethylenediamine 14, followed by its alkylation withhloroacetic acid gave 9 (Eq. (3)). Surfactants 5 were obtainedy the reaction of 9 with sodium hexanitrocobaltate(III) (Eq.4)).

Surfactant 6 was synthesized as illustrated in Scheme 3. Com-ercially available bromo alcohol 15 was converted into nitrilo

lcohol 16, which was reduced to give amino alcohol 17 (Eq.5)). Then the reaction of monoanhydride 12 with 17 gave 10Eq. (6)). Surfactant 6 was obtained by the reaction of 10 withodium hexanitrocobaltate(III) (Eq. (7)).

Surfactant 7 was synthesized as illustrated in Scheme 4.mino alcohol 17 was converted into bromo ammonium salt 18,

ollowed by SN2 displacement of bromide by trimethylamine toive 19 (Eq. (8)). The reaction of 12 with 19 gave 11 (Eq. (9)).hen the reaction of 11 with sodium hexanitrocobaltate(III) gaveurfactant 7 (Eq. (10)).

.2. Surfactant composition and structure

The molecular compositions of surfactant Co(III) chelates 4,
, 6, and 7 were established by combustion analyses and electro-pray mass spectrometry. Their structures were established byH and 13C NMR, IR, and UV–vis spectroscopy, as was doneor related surfactant 1 [12].
dU

e

Scheme 4

Scheme 3.

For each surfactant, the 13C NMR spectrum was consider-bly more informative than the 1H NMR spectrum, due to theomplex nature of the latter, resulting from the diastereotopicelationship between the two hydrogens of each of the fiveonequivalent methylene groups of the bridging ethylene groupnd the coordinated carboxylatomethyl groups within theo(III)-based headgroup. In each 13C NMR spectrum, signalsere observed for the nonequivalent carbonyl carbons (four each

or 4, 6, and 7, and three for 5), and for the five nonequiv-lent methylene groups of the Co(III)-based headgroup, withhemical shifts consistent with those for surfactant 1 [12].

In the IR spectra of 4–7, strong absorption bands werebserved for their coordinated carboxylate groups and ester andmide groups. Comparable bands were observed for 1’s coor-inated carboxylate groups and amide group [12]. It is clearhat the NO2 units of 4–7 are bonded to Co(III) by nitrogens nitro ligands ( NO2) as illustrated, instead of by oxygens nitrito ligands ( ONO); each IR spectrum contained severalands consistent with nitro coordination [32,33]. Also, note thathelates 4–7 were prepared from sodium hexanitrocobaltate(III),hich itself contains nitro ligands. The UV–vis spectra of 4–7
isplayed absorption bands that are comparable to those in 1’sV–vis spectrum [12].There are four possible geometric isomers (not counting
nantiomers) for each of the octahedral Co(III) complexes

.

Physicochem. Eng. Aspects 302 (2007) 186–196 193

d8EtaabrtiidttaN

2miTdhoi

Table 1Values of cac, γcac, and Tk for surfactants in watera

Surfactant cac (×103 M) γcac (mN/m) Tk (◦C)

1 0.53 ± 0.03 40 542-Na3 0.44 ± 0.02 43 ≤234a 2.8 ± 0.2 53 554b 844c b

5a 37 ± 1 32 285b 9.1 ± 0.2 33 405c 806 7.5 ± 0.2 46 ≤237 7.5 ± 0.3 50 ≤238a-Na3 19.5 ± 0.5 40 ≤238b-Na3 6.4 ± 0.1 28 ≤238c-Na3 2.7 ± 0.2 21 ≤239a-Na3 25 ± 1 28 ≤239b-Na3 6.3 ± 0.1 28 ≤239c-Na3 3.7 ± 0.2 29 ≤2310-Na3 9.9 ± 0.1 40 ≤2311 4.7 ± 0.2 56 ≤23

b

3

K(NeH

idAottfcomparison.

With respect to series 5, the corresponding Tk values of series4 are higher, and that of individual surfactant 1 (R = C12H25)is lower. Note that in addition to R groups, surfactants 1 and


erived from sodium hexanitrocobaltate(III) and the trianions of–11, as there are for complexes of Co(III) and the tetraanion ofDTA wherein the latter functions as a pentadentate ligand, with

he sixth coordination site occupied by a unidentate ligand suchs NO2

−, Cl−, Br−, or H2O [34]. For each system, two of thesere A and B, and the other two (not shown) can be discounted,ecause, by literature analogy [34] they probably involve moreing strain than contained in A and B. Furthermore, it is knownhat the complexation of Co(III) by EDTA and NO2

− gives 20,n which the nitro ligand is equatorial, and not 21, in which its axial [34–36]. By analogy, structure B can be provisionallyiscounted for surfactants 4–7, leaving structure A. The forma-ion of only one isomer in each system is fully consistent withhe number of signals observed in 4–7’s 13C NMR spectra (seebove), and the chemical shifts are similar to those in 20’s 13CMR spectrum [34].

Structure A is also consistent with the fact that within2, Co(III)’s EDTA chelate, the equatorial carboxylate–etal–nitrogen rings are more strained than the correspond-

ng axial rings, as determined by X-ray crystallography [37].herefore, as the five coordination sites of the trianions of 8–11isplace five of the six nitro ligands from Co(III) within theexanitrocobaltate(III) anion (see Schemes 1–4), the numberf equatorial carboxylate–metal–nitrogen rings should be min-mized, resulting in complex A, and not its isomer B.

Fc

a The cac values are averages, with average deviations, of ≥2 determinationsy surface tensiometry at 23 ◦C.b Not detected up to 94 ◦C.

.3. Surfactant characterization

Surfactants were characterized by measurement of theirrafft temperatures (Tk) and critical aggregation concentrations

cac) in water. Aggregated surfactants were characterized by 1HMR spectroscopy in D2O, and 5a in water was studied by cryo-

tch high resolution scanning electron microscopy (cryo-etchRSEM).The solubility of an ionic surfactant in water generally

ncreases with increasing temperature, but it typically increasesramatically at a point known as the Krafft temperature [1].ggregation of an ionic surfactant into assemblies can occurnly above its Tk and cac values. The Tk values of surfac-ants 4–7 and 11, and those of 8-Na3, 9-Na3, and 10-Na3 (therisodium salts of 8–10) are listed in Table 1; the values of sur-actants 1 and 2-Na3 (the trisodium salt of 2) are included for

ig. 1. Plots of surface tension for surfactants 4a (open circles) and 5a (closedircles) in water.

194 D.A. Jaeger et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 186–196

Fc

4ta5Cttrip�lo

taTacTmhiF

F(

aud[gaw�

ectdssthd

ig. 2. Plots of surface tension for surfactants 6 (open circles) and 7 (closedircles) in water.

also contain amide and ester groups, respectively, as part ofheir substituents on one of the two nitrogens of the ethylenedi-mine unit within the chelate. In particular, 1 and 4 differ fromby CH2CONH and CH2CO2 units, respectively. By itself, theH2 group should increase Tk values on going from series 5

o surfactant 1 and series 4 [38]. However, it is apparent thathe CONH group of 1 imparts a net decrease to its Tk value,elative to 5c’s value, whereas the CO2 group of series 4 likelymparts an increase to its Tk values, relative to 5’s values. Theolar �-hydroxy substituent of surfactant 6 and the charged-trimethylammonio substituent of 7 are responsible for their

ower Tk values, compared to parent surfactant 1. The Tk valuesf 8-Na3, 9-Na3, 10-Na3, and 11 are ≤23 ◦C.

The cac values of surfactants were determined by surfaceensiometry, using plots of surface tension versus log[surfactant]t 23 ◦C. The cac values of surfactants 4–7 and 11 are listed inable 1, as well as those of the trisodium salts 8-Na3, 9-Na3,nd 10-Na3. For 4a, 5a, and 5b, whose Tk values are ≥23 ◦C,lear supersaturated solutions, prepared above their respectivek values, were employed in measurement of their cac values;
easurements were not made for 4b, 4c, and 5c, due to their
igh/undetected Tk values. The cac values of 1 and 2-Na3 arencluded for comparison. Representative cac plots are shown inigs. 1–3.

wres

Fig. 4. Cryo-etch HRSEM micrographs of surfactant 5a in water, etch

ig. 3. Plots of surface tension for surfactants 8a-Na3 (closed circles), 9a-Na3

open circles), 10-Na3 (open squares), and 11 (closed triangles) in water.

Note that the cac value of 4a is lower than that of 5a byfactor of about 15; the structural difference is the CH2CO2

nit within the former. By itself the CH2 group is expected toecrease 4a’s cac value by a factor of two relative to 5a’s value38]; the remainder of the decrease can be attributed to the CO2roup. The cac values of surfactant Co(III) chelates 6 and 7 arebout 14 times greater than that of surfactant Co(III) chelate 1,hich can be attributed solely to their polar �-hydroxy and ionic-trimethylammonio groups.

Even though surfactant Co(III) chelates 4a and 5a have decid-dly different cac values, related series 8-Na3 and 9-Na3 haveomparable cac values. The difference in response of cac valueso the nature of the substituent on a nitrogen of the ethylene-iamine unit (CH2CO2R versus R) may be due to the fact thaturfactants 4a and 5a have monoanionic headgroups, whereasurfactants 8-Na3 and 9-Na3 have trianionic headgroups. Onhe other hand, the cac value of 2-Na3, which has a trianioniceadgroup and a CH2CONHC12H25 substituent on nitrogen, isecidedly less than those of 8c-Na3 and 9c-Na3.

1H NMR spectra of surfactant Co(III) chelates 4–7 in D2O◦
ere recorded at 23 C at concentrations of 1.4–2.5 times their
espective cac values. Each spectrum contained slightly broad-ned signals, consistent with the presence of small aggregatesuch as micelles or small vesicles [39]. This behavior is in

ed at −105 ◦C for 5 min; scale bar: (a) 500 nm and (b) 125 nm.

Physi

mfiabCt�vvlcaw

8Dtm

HpT1−tintottci

o2ubPsiassp

4

dtbaaKs

ttosewa

A

f

R

[[[[

[[

[

[

[

[[

[

[[

[[[[[[[[[


arked contrast to the formation of vesicles and rods by sur-actant Co(III) chelate 1, as reported earlier [12]. The differencen aggregate morphology may be associated with 1’s secondarymide group, which can participate in intermolecular hydrogenonding, a potential organizational feature. Although surfactanto(III) chelates 6 and 7 also contain secondary amide groups,

heir N-alkyl substituents contain polar �-hydroxy and ionic-trimethylammonio groups, respectively, which may precludeesicle formation. As noted previously [12], the formation ofesicles and rods by surfactant 1 is interesting, because with aarge headgroup [the chelated Co(III) unit] and a single hydro-arbon chain, it is predicted to form micelles as does 2-Na3 [12],nd not vesicles, based on correlations of aggregate morphologyith surfactant structure [40].The 1H NMR spectra of trisodium tricarboxylate surfactants

-Na3 and 10-Na3 and quaternary ammonium surfactant 11 in2O were also recorded at 23 ◦C at concentrations of two times

heir respective cac values. Each spectrum contained slightly tooderately broadened signals.Aqueous 0.17 M (7.3 wt%) 5a was studied by cryo-etch

RSEM [20,21]. In this method an aqueous sample of a com-ound at 23 ◦C is plunge-frozen into liquid ethane at −183 ◦C.he sample is then fractured to expose a fresh surface, and at0−7 Torr its temperature is increased from −183 ◦C, and held at105 ◦C to effect the etching process (5 min for 5a). Thereafter,

he temperature is decreased to ca. −180 ◦C, and the samples coated with a 2 nm layer of Cr and observed with a scan-ing electron microscope. The goal of the etching process iso sublime away ice corresponding to bulk water and to mostf the compound’s loosely bound water of hydration, leavinghe compound and its tightly bound water of hydration. Thushe morphology of a cryo-etched sample is representative of theompound’s hydrated state. The plunge-freezing of pure watertself gives featureless vitreous solid water [41].

Fig. 4 contains cryo-etch HRSEM micrographs of aque-us surfactant 5a. Figs. 4a and b, taken at magnifications of0 000 and 80 000 times, respectively, show fibrous, partic-late networks. The origin of the morphologies, which haveeen observed previously for other surfactants [42] is uncertain.erhaps they represent submicroscopic networks of aggregatedurfactant 5a that exist before plunge-freezing. However, thiss unlikely, given the results of a cryo-etch HRSEM study ofqueous sodium chloride and other inorganic salts [21]. Con-equently, the morphologies most likely reflect characteristicegregation patterns formed by the surfactant during the freezingrocess and/or the cryo-etch process.

. Summary

Surfactant Co(III) chelates 4–7 were prepared from EDTAerivatives 8–11, respectively, and sodium hexanitrocobal-ate(III). The molecular compositions of 4–7 were establishedy combustion analyses and electrospray mass spectrometry,
nd their structures were determined by 1H and 13C NMR, IR,nd UV–vis spectroscopy. Surfactants 4–7 were characterized byrafft temperature and critical aggregation concentration mea-
urements in water. The Tk values of 4 and 5 are >23 ◦C, and

[[

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hose of the former are greater than the corresponding values ofhe latter. The Tk values of 6 and 7 are ≤23 ◦C. The appearancef the 1H NMR spectra of 4–7 in D2O suggested that they formmall aggregates such as micelles or small vesicles. In a cryo-tch HRSEM study of 5a, characteristic segregation patternsere observed, which were likely formed during the freezing

nd/or cryo-etch steps of sample preparation.

cknowledgment

We thank the National Science Foundation (CHE-0092560)or the support of this research.

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Surfactant transition metal chelates

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