Trialkylphosphine and Trialkylphosphine Oxide Organogelators

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Langmuir 2003, 19, 1036-1046

“Latent” Trialkylphosphine and Trialkylphosphine Oxide Organogelators Activated by Brønsted and Lewis Acids# Caihua Wang,† Allen Robertson,‡ and Richard G. Weiss*,† Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227, and Cytec Canada Limited, Niagara Falls, Ontario, Canada L2E 6T4 Received September 30, 2002. In Final Form: December 5, 2002 The weak bases tri-n-alkylphosphines (R3P, where R ) C14H29, C18H37) and phosphine oxides (R3PdO) have been reacted with the strong Lewis acid BF3 to form zwitterions (R3P+-OBF3- or R3P+-BF3-) and with Brønsted acids (pKa < 5) to form salts (R3P+-OH X-, where X- is several anion types, including chloride, p-dodecylbenzenesulfonate, and p-toluenesulfonate). The abilities of the zwitterions and some of the hydroxyphosphonium salts to gel a variety of organic liquids have been investigated and compared with those of the phosphine oxides (R3PdO, R ) C10H21, C14H29, C18H37). The gelating abilities of the zwitterions are similar to those of the phosphine oxides in that only liquids capable of donating protons or promoting ionic interactions could be gelled, and both are less efficient than the corresponding hydroxyphosphonium salts. Alcohols with short alkyl chains (98% of the total sample weight, is held within the gelator network by capillary forces4 or surface tension.1a In addition, direct incorporation of the liquid in gel networks has been suggested recently.5 The formation and properties of some organogels can also be controlled by chemical or physical triggers, such as light,6 pH changes,6c,7 host-guest interactions,8 or a combination of light and a chemical catalyst,9 that are * To whom correspondence should be addressed. Fax: (202)687-6209. Phone: (202)-687-6013. E-mail: [email protected]. † Georgetown University. ‡ Cytec Canada Ltd. # This paper is dedicated to Professor Vaclav Horak on the occasion of his 80th birthday. (1) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (b) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (2) (a) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352. (b) Abdallah, D. J.; Sirchio, S. A.; Weiss, R. G. Langmuir 2000, 16, 7558. (c) Willemen, H. M.; Vermonden, T.; Marcelis, A. T. M.; Sudho¨lter, J. R. Langmuir 2002, 18, 7102. (3) Van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (4) Lin, Y. C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (5) (a) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Am. Chem. Soc. 1994, 116, 9464. (b) Wang, R.; Geiger, C.; Chen, L.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399.

coupled to the self-assembly process.3 Recently, examples of chemically reversible LMOGs that exhibit very different gelation properties when in their charged or uncharged states have been reported. For instance, LMOGs with a phenazine moiety form less stable (translucent) gels and more stable (colored) ones when the phenazine is protonated.10 Also, “latent” gelators (i.e., molecules requiring a second species to gel liquids) consisting of structurally simple, uncharged alkylamines become rather efficient charged LMOGs, alkylammonium alkylcarbamates, when CO2 is added (eq 1).11

Several tetraalkylammonium and phosphonium salts are “irreversibly” charged LMOGs.12 They include tetraalkylphosphonium cations with decyl to octadecyl alkyl (6) (a) Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1991, 1715. (b) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664. (c) Ahmed, S. A.; Sallenave, X.; Fages, F.; Mieden-Gundert, G.; Mu¨ller, U.; Vo¨gtle, F.; Pozzo, J.-L. Langmuir 2002, 18, 7096. (7) Aggeli, A.; Bell, M.; Boden, N.; Keen, J. N.; Knowles, P. F.; McLeish, T. C. B.; Pitkeathly, M.; Radford, S. E. Nature 1997, 386, 259. (8) (a) Jung, J. H.; Ono, Y.; Sgubjau, S. Tetrahedron Lett. 1999, 40, 8395. (b) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999, 284, 785. (c) Ihara, H.; Sakurai, T.; Yamada, T.; Hashimoto, T.; Takafuji, M.; Sagawa, T.; Hachisako, H. Langmuir 2002, 18, 7120. (9) Frkanec, L.; Jokic´, M.; Makarevic´, J.; Wolsperger, K.; Zˇ inic´, M. J. Am. Chem. Soc. 2002, 124, 9716. (10) Pozzo, J.-L.; Claviewm, G. M.; Desvergne, J.-P. J. Mater. Chem. 1998, 8, 2575. (11) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 10393.

10.1021/la026631j CCC: $25.00 © 2003 American Chemical Society Published on Web 01/24/2003

Latent Organogelators Activated by Acids

Langmuir, Vol. 19, No. 4, 2003 1037

groups and bromide, iodide, and perchlorate as anions. Despite the charge separation indicated by their dative bond structures, the corresponding phosphine oxides (R3PdO), such as trioctadecylphosphine oxide and tritetradecylphosphine oxide, either dissolved or precipitated in the liquids, at the 99.7%, EM Science), nonanoic acid (96%, Aldrich), boron trifluoride diethyl etherate (Aldrich), ammonium chloride (granular, reagent, EM), and triethyloxonium fluoroborate (Et3O+ BF4-, 1 M solution in CH2Cl2) were used as received. Trioctadecylphosphine oxide ((C18)3PdO), tritetradecylphosphine oxide ((C14)3PdO), and tridecylphosphine oxide ((C10)3PdO) were prepared according to a literature method.19 Properties of the phosphine oxides and their acid adducts are included in Table 1. Hydroxytrioctadecylphosphonium chloride ((C18)3P+-OH Cl-) and hydroxytritetradecylphosphonium chloride ((C14)3P+-OH Cl-) were prepared as white solids by bubbling hydrogen chloride gas (produced by dropping concentrated sulfuric acid onto granular ammonium chloride20) into the corresponding phosphine oxide that had been heated to its liquid state or dissolved in ethanol. In the latter case, the ethanol was removed subsequently on a rotary evaporator. Hydroxytrioctadecylphosphonium pdodecylbenzenesulfonate (a slightly yellow, pastelike solid; (C18)3P+-OH C12H25C6H4-SO3-) was prepared by refluxing 0.5 g (0.62 mmol) of trioctadecylphosphine oxide and 0.19 mL (0.62 mmol) of p-dodecylbenzenesulfonic acid in 5 mL of chloroform for 1 h followed by removal of solvent (rotary evaporator). The other hydroxyphosphonium salts (R3P+-OH A-) were prepared by analogous procedures; see the Supporting Information. Trioctadecylphosphonium trifluorooxyborate ((C18)3P+-OBF3-), tritetradecylphosphonium trifluorooxyborate ((C14)3P+-OBF3-), and trioctadecylphosphonium trifluoroborate ((C18)3P+-BF3-) were prepared by stirring the appropriate phosphine or phosphine oxide and a 3-5 molar excess of boron trifluoride diethyl etherate in anhydrous chloroform under a nitrogen atmosphere for 4 h at room temperature. Anhydrous diethyl ether was added to precipitate the products as white powders, obtained in ∼80% yields after vacuum filtration. (C18)3P+-OBF3-: 19F NMR (CDCl3) δ -146.36 (d, JP-O-10B-F ) 6.2 Hz), -146.42 (d, JP-O-11B-F ) 6.2 Hz) ppm; 31P NMR (CDCl3) δ 74 ppm (quartet, JP-O-B-F ) 6.1 Hz). (C14)3P+-OBF3-: 19F NMR (CDCl3) δ -146.37 (d, JP-O-10B-F ) 6.2 Hz), -146.42 (d, JP-O-11B-F ) 6.2 Hz) ppm; 31P NMR (CDCl3) δ 74 ppm (quartet, JP-O-B-F ) 6.0 Hz). (C18)3P+-BF3-: 19F NMR (CDCl3) δ -151.16 (s), -151.21 (s) ppm; 31P NMR (CDCl3) δ 13.6 (s) ppm. Melting ranges and 1H NMR spectral data are listed in Table 1. Samples of (C18)3P+-OBF3- and (C14)3P+-OBF3- were also obtained by refluxing mixtures of the corresponding phosphine oxide (1.5 mmol) and triethyloxonium fluoroborate (8 mmol) for 3 h under a nitrogen atmosphere and adding anhydrous diethyl ether to precipitate the products. Their spectral and physical properties were virtually the same as those from samples prepared by the method above. Gelation Experiments. Known amounts of a liquid and gelator were flame-sealed in a glass tube (5 mm i.d.), heated (until all solid dissolved), and cooled to room temperature twice before taking measurements. Gelation was considered successful if no sample flow was observed upon inverting the tube and there was no visual evidence of macroscopic phase separation. Flowing samples were characterized as either solutions, precipitates in liquids, or jellies (viscous solutions/sols). Gel-to-solution/sol transition temperatures were determined by the inverse flow method:21 sealed tubes with gelled samples were inverted and placed next to a thermometer in a water bath at room temper(19) Franks, S.; Hartley, F. R.; McCaffrey, D. J. A. J. Chem. Soc., Perkin Trans. 1 1979, 3029. (20) Vogel, A. I. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman: Essex, U.K., 1989; p 438. (21) Takahashi, A.; Sakai, M.; Kato, T. Polym. J. 1980, 12, 335.

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Wang et al.

Table 1. Properties of Trialkylphosphine Oxides and Their Salts NMR (CDCl3, ppm) compound (C18)3P+-OH

mpa (°C)

IRd (KBr, cm-1)

concn (mg/mL)

999 (νP-O)

30

Cl-

60.1

(C18)3P+-OH C12H25C6H4-SO3-

30.2

37

(C18)3P+-OH CH3C6H4-SO3-

69.0

46

(C18)3P+-OH NO3-

44.6

(C18)3P+-OH CF3CO2-

41.8

(C18)3P+-OBF3-

73.5

(C18)3P+-BF3-

75.6b

(C14)3P+-OH Cl-

44.5

1004 (νP-O)

43

(C14)3P+-OBF3-

53.5

1092 (νP-O)

25

(C18)3PdO (C14)3PdO (C10)3PdO

80-82c 68-69c 43-44c

1153 (νPdO) 1153 (νPdO) 1153 (νPdO)

21 18 20

1028 (νP-O)

46 40

1401 (νB-O), 1085 (νP-O)

25 14

1H

31P

TGA weight loss (%)e

0.88 (t, 9H), 1.2-1.7 (m, 96H), 2.1 (m, 6H), 8.7 (s, -OH) 0.8-1.6 (m, 152H), 2.0 (m, 6H), 7.2 (d, 2H), 7.8 (d, 2H), 9.1 (s, -OH) 0.88 (t, 9H), 1.2-1.6 (m, 96H), 2.0 (m, 6H), 2.2 (s, 3H), 7.2 (d, 2H), 7.8 (d, 2H), 12.7 (s, -OH) 0.88 (t, 9H), 1.2-1.6 (m, 96H), 1.8 (m, 6H), 14.1 (s, -OH) 0.88 (t, 9H), 1.2-1.6 (m, 96H), 1.8 (m, 6H), 12.9 (s, -OH) 0.88 (t, 9H), 1.2-1.5 (m, 90H), 1.6 (m, 6H), 2.0 (m, 6H) 0.88 (t, 9H), 1.2-1.5 (m, 90H), 1.6 (m, 6H), 2.25 (m, 6H) 0.88 (t, 9H), 1.2-1.7 (m, 72H), 2.0 (m, 6H), 11.7 (s, -OH) 0.88 (t, 9H), 1.2-1.7 (m, 72H), 2.0 (m, 6H) 0.88 (t, 9H), 1.2-1.7 (m, 102H) 0.88 (t, 9H), 1.2-1.7 (m, 78H) 0.88 (t, 9H), 1.2-1.7 (m, 54H)

77

4.4 (4.3)

75 73 64

8.1 (7.2)

59

12.1 (12.4)

74 14 77

5.2 (5.4)

74 49 49 49

a Peak onset in the first DSC endotherm unless indicated otherwise. Detailed data from both first and second heating-cooling cycles of DSC measurements are provided in the Supporting Information. b With decomposition. c From optical microscope. Literature (ref 19) mp values are 73-75 °C for (C18)3PdO, 57-58 °C for (C14)3PdO, and 37 °C for (C10)3PdO. d Only peaks of interest are included. Complete IR spectra are provided in the Supporting Information. e Theoretical weight loss assuming 1:1 trialkylphosphine oxide/acid stoichiometry in parentheses; experimental weight losses from 57 to 197 °C for (C18)3P+-OH Cl-, from 46 to 170 °C for (C18)3P+-OH NO3-, from 52 to 209 °C for (C18)3P+-OH CF3CO2-, and from 42 to 145 °C for (C14)3P+-OH Cl-.

ature, and the water was stirred and heated. The temperature range for gelation was taken to be from the moment of initial sample flow to when it fell completely to the bottom of the tube. Four cooling protocols were used initially to form gels from heated solutions/sols in sealed glass tubes. In the most rapid cooling protocol (P-1), the hot tubes were immersed in an ice/ water bath for a few minutes and then left at ambient temperature. The slower protocols consisted of placing the hot tubes under a stream of water at 22-23 °C (P-2) or leaving them in air at ambient temperatures (P-3). The slowest cooling was achieved by keeping sealed tubes in a hot water bath until the water returned to room temperature (P-4). Unless stated otherwise, all gels discussed in the following sections were prepared by protocol P-1. Variable Temperature NMR Experiments. Subambient and superambient range temperatures were calibrated with the chemical shift of the OH peak of methanol and ethylene glycol, respectively.22 Weighed amounts of a gelator and solvent were degassed (2 freeze-pump-thaw cycles) at 2 y) P P P P P P jelly jelly jelly jelly P jelly jelly

gelation temp

appearance

P P

jelly Tr (1 h) Tr (>1 y) Tr (>2 y) P P P P P P P jelly jelly P jelly jelly jelly jelly jelly Tu (∼2 w) Tu (∼1 m) jelly Tu (∼2 m) Tu (∼2 m) Tu (>2 y) jelly Tu (∼3 m) Tu (∼3 m) Tu (>2 y) P S P P P P

P P P

S P P

35-49 51-52 51-52

jelly jelly jelly Tu (1 d) Tu (∼2 w) Tu (∼2 m) Tu (∼1 m) Tu (>2 y) Tu (>2 y) P P P

P P Tr (∼2 w) Tr (>1 y) Tr (>2 y)

26-37 29-43 38-45 38-45 34-45 37-45

55-67 57-68 66-70

P P Tr Tr Tr

gelation temp 25-44 35-44 42-44

29-37 33-40 30-35 33-37 39-41 34-38 35-40 35-39

2 y) P P P P-Pg P Tu (∼1 w) Tu (∼1 w) P Tu (∼1 w) Tu (∼1 m) jelly Tu (>1 y) Tu (>1 y) Tu (>1 y) P Tu (>2 y) Tu (>2 y) jelly Tu (∼2 m) Tu (>2 y) jelly jelly jelly Tu (>1 y) P P P P P P Tu (∼2 w) jelly jelly Tu (∼1 w) P S S S P P P Tr (2 d) Tr (>1 y) Tr (>2 y)

gelation temp

appearance

gelation temp

39-43 43-47 41-44

jelly jelly jelly Tr (>1 y)

42-45

P Pg jelly 33-48 33-46 33-52 42-46 34-40 35-42 39-42

P Tu (>1 y) jelly jelly Tu (1 d)

37-44

jelly Tu (1 d)

32-40

26-32

jelly jelly jelly P P jelly Tu (1 d) jelly P jelly Tu (1 d)

29-33

S P P

29-33

S P P

35-42 36-44 33-47 32-47

58-61 62-64 62-64

jelly Tr (>1 y)

38-45

34-35

33-37

53-60

a Appearance: jelly ) thick solution; Tr ) translucent gel; Tu ) turbid gel; S ) solution; P ) precipitate in solution; Pg ) partial gel; only a part flowed. Gelation temperatures (°C), determined by the inverse flow method (ref 21). RT ) room temperature. Periods of stability in parentheses: h ) hour; d ) day; w ) week; m ) month; y ) year.

spectroscopic data, hydroxytriphenylphosphonium salts are formed upon reaction between triphenylphosphine oxide and either sulfuric acid (pKa -9 to -1031)17 or hydrogen bromide (pKa ) -931).33 However, only 1:1 or 2:1 hydrogen-bonded triphenylphosphine oxide/hydrogen chloride (pKa ) -731) complexes are formed.33 We have found no reports of trialkylhydroxyphosphonium salts, even though the greater basicity of trialkyl(33) Hadzi, D. J. Chem. Soc. 1962, 5128. (34) Chawla, B.; Mehta, S. K. J. Phys. Chem. 1984, 88, 2650. (35) (a) Denney, B. D.; Denny, D. Z.; Wilson, L. A. Tetrahedron Lett. 1968, 85. (b) Burford, N.; Royan, B. W.; Spence, R. J. Chem. Soc., Dalton Trans. 1990, 2111.

phosphine oxides (pKb ∼ 15.527)36,37 should facilitate their uptake of protons. Here, several acids have been added to trioctadecylphosphine oxide and tritetradecylphosphine oxide, and the products have been characterized by infrared spectroscopy, 1H and 31P NMR spectrometries, DSC, and TGA (Table 1 and Table 2S and Figures 1S15S in the Supporting Information). Hydroxyphosphonium salts were obtained with Brønsted acids of pKa e 0.2 (CF3CO2H) but not with acids of pKa g 5 (e.g., CH3CO2H). Due to their susceptibility to reaction with moisture, alcohols, and so forth and their propensity to lose volatile acids, the (36) Hadzi, D.; Kobilarov, N. J. Chem. Soc. A 1966, 439. (37) Hadzi, D.; Klofutar, C.; Oblak, S. J. Chem. Soc. A 1968, 905.

1040


trialkylhydroxyphosphonium salts could not be purified by recrystallization; they were used as made according to the procedures in the Experimental Section.38 A large degree of proton transfer to the phosphoryl oxygen atom in the hydroxytrioctadecylphosphonium chloride salt is indicated by the reproducibility of successive exotherms (DSC) (Table 2S): solidification temperatures remained at ca. 55 °C during several scans; the solidification temperature of trioctadecylphosphine oxide under analogous conditions is 65 °C. The onset of melting occurred at a lower temperature in the second heating scan (57 °C) than in the first (60 °C) but remained constant thereafter. We ascribe this behavior and the wide melting range during the first heating, 60-68 °C, to an inhomogeneous distribution of HCl initially among the phosphine oxide molecules. A small fraction of HCl may be lost on heating near and above the melting temperature because a weight loss corresponding to one molecule of HCl per molecule of phosphine oxide was observed between ca. 59 and ca. 200 °C (Table 1). In addition, moist pH paper placed over the heated sample became pink (indicative of the evolution of acidic vapors) and the remaining solid was characterized as trioctadecylphosphine oxide by IR and NMR spectroscopies39 and its melting temperature. Our results demonstrate that hydroxytrioctadecylphosphonium chloride and hydroxytritetradecylphosphonium chloride remain stable 1:1 salts for several months when stored in closed vessels at room temperature. Parts of the 1H NMR spectra related to the phosphine oxide and 31P NMR spectra of the products from reaction between trioctadecylphosphine oxide and either p-dodecylbenzenesulfonic acid (pKa ) -6.531) or p-toluenesulfonic acid (pKa ) -6.531) resemble that of hydroxytrioctadecylphosphonium chloride. Proton transfer from all of these acids to the phosphine oxide appears to be nearly complete. As expected from the low volatility of the sulfonic acid components, DSC thermograms on their salts were reproducible over several heating and cooling cycles (Table 2S). On the basis of NMR and IR spectra, partial proton transfer occurs from nitric acid (pKa ) -1.431) or trifluoroacetic acid (pKa ) 0.231) to trioctadecylphosphine oxide. Significant transfer of a proton is indicated by the reproducibility of DSC heating and cooling thermograms of the products. Reactions between trioctadecylphosphine oxide and the much weaker acetic acid (pKa ) 4.828) or nonanoic acid (pKa ) 5.028) were not successful. 1H and 31 P NMR spectra (CDCl3) of the materials isolated after allowing the mixtures to sit for protracted periods were the same as those of the phosphine oxide. 1 H NMR spectra of products from reaction of trioctadecylphosphine oxide or tritetradecylphosphine oxide with the Lewis acid, BF3, were similar to those of the analogous hydroxyphosphonium salts (Table 1). The BF3 adducts to the phosphoryl oxygen must induce a large increase of positive charge at the phosphorus atom because the 31P (38) Although the exact physical properties of salts produced in another laboratory are expected to differ slightly from those reported here, the variations should be small and the overall results should be the same. In support of that hypothesis, gelation experiments with different batches of hydroxytrioctadecylphosphonium chloride gave indistinguishable results. LMOG purity is not a critical issue here. (39) The infrared spectrum of trioctadecylphosphine oxide showed the 1153 cm-1 band characteristic of the phosphoryl group, while that of hydroxytrioctadecylphosphonium chloride contained a peak corresponding to a P-O stretch at 999 cm-1. The 1H NMR spectrum of trioctadecylphosphine oxide had a 12H multiplet at 1.5-1.7 ppm, but that of the salt contained two multiplets (6H each) from the R- and β-methylene groups at 1.63 and 2.07 ppm, respectively. 31P resonances of the salt (77 ppm) and the phosphine oxide (49 ppm) were also easily distinguished.

Wang et al.

chemical shifts were near those of the analogous hydroxyphosphonium salts. DSC thermograms of samples cooled from the melt were reproducible; the solidification temperatures were ca. 48 °C ((C14)3P+-OBF3-) and 57 °C ((C18)3P+-OBF3-) in successive scans (Table 2S). The endotherms of (C18)3P+-OBF3- showed peaks from solidsolid transitions in addition to that from the solidisotropic transition; they were not due to decomposition because 1H and 31P NMR spectra of the salt melted once were identical to those before melting. The 31P resonance of the product obtained upon addition of BF3 to trioctadecylphosphine (14 ppm) is between that of the phosphine (-33 ppm) and the hydroxyphosphonium salts (ca. 75 ppm), and its 1H resonances are like those of the Brønsted salts.40 On this basis, we conclude that the product is the expected adduct, (C18)3P+-BF3-. (C18)3P+BF3- can be heated to 90 °C without decomposition (as per 1H, 31P, and 19F NMR spectra) but appeared to react with the aluminum pans during DSC analyses. Influence of the Rate of Cooling of Solutions/Sols on Gel Properties. Protocol P-1 produced the most stable gels, and in some cases it was the only protocol that gelled hot solutions/sols. The sizes of the aggregate units responsible for the gel networks increased as the rate of cooling decreased. For instance, a P-1 gel consisting of 9 wt % (C18)3P+-OH Cl- in benzene contained birefringent domains without distinguishable strand units (Figure 1A). A P-2 gel of 5 wt % (C18)3P+-OH CH3C6H4SO3- in silicone oil was birefringent, with few detectable aggregates (Figure 1B); micrographs of the P-3 and P-4 gels exhibited strandlike aggregates of ca. 3-10 and 1030 µm lengths, respectively (Figure 1C,D). Gelation Studies of (C10)3PdO, (C14)3PdO, and (C14)3P+-OH Cl-. Of the liquids examined, only silicone oil was gelled by these three compounds (See Table 3S in the Supporting Information). In the 3-7 wt % concentration range of the LMOG, other liquids (including benzyl alcohol, methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-nonanol, 1-decanol, benzene, and toluene) gave either precipitates or solutions. Trioctadecylphosphine oxide and its hydroxyphosphonium chloride salt are much better LMOGs (see below) than tritetradecylphosphine oxide and its hydrochloride salt, and tridecylphosphine oxide is worse yet. The progression of gelation abilities follows the polarities of the gelators and the degree to which they can stabilize intermolecular interactions through London dispersion forces (N.B., the length of the alkyl chains).41 Gelation Studies of (C18)3PdO and Its Hydroxytrioctadecylphosphonium Salts. The gelating abilities of trioctadecylphosphine oxide and its salts from hydrogen chloride, p-dodecylbenzenesulfonic acid, and p-toluenesulfonic acid were examined with a wide range of liquids (Table 2). Relatively high (g3 wt %) concentrations are needed for successful gelation in most cases, but some gels are stable for >2 years when kept in sealed vessels at room temperature. In general, gel longevity increased with gelator concentration. Aprotic polar solvents such as DMSO and acetonitrile and very low polarity ones like alkanes were not gelled. Benzene and toluene were gelled by hydroxytrioctadecylphosphonium chloride, but only at g7 wt % concentrations. (40) Additional small peaks from protons attached to unsaturated carbons (Figure 7S in the Supporting Information) from other unidentified materials could not be removed by several attempted purification procedures. The overall intensity of the impurity, based on the integrated proton peak intensities, is less than 1%. (41) Abdallah, D. J.; Bachman, R. E.; Perlstein, J.; Weiss, R. G. J. Phys. Chem. B 1999, 103, 9269.


Langmuir, Vol. 19, No. 4, 2003 1041 Table 3. Gelation of (C18)3PdO and (C18)P+-OH Cl- in CHCl3/CH3CN Mixturesa CHCl3/ CH3CNb (C18)P+-OH Cl(dielectric (C18)3PdO wt % appearance constant)c wt % appearance 2.5:1 (13.0) 2.0:1 (14.5) 1.5:1 (16.7) 1:1 (19.1)

5 9 5 9 5 9 5 9

jelly gel (∼2 w) gel (∼2 w) gel (∼2 w) P gel (3 d) P P

5 9 5 9 5 9 5 9

S gel (2 d) P gel (2 d) P gel (>1 m) P gel (>1 m)

a See Table 2 for abbreviations. b Volume ratio. c Relative permittivity.

Figure 1. Polarizing optical micrographs (with a full wave plate) of a gel consisting of 9 wt % (C18)3P+-OH Cl- in benzene formed by cooling protocol P-1 (A) and a gel consisting of 5 wt % (C18)3P+-OH CH3C6H4-SO3- in silicone oil formed by cooling protocols P-2 (B), P-3 (C), and P-4 (D).

Alcohols were gelled most commonly, but the smallest and most polar one, methanol, was not. Ethanol was partially gelled by 7 wt % hydroxytrioctadecylphosphonium chloride, and alcohols with 3-5 carbon atom chains were gelled best by the chloride and p-toluenesulfonate salts; their gels with 1-propanol at 7 wt % were stable for 6 days and >1 year, respectively. As the alkyl chains of the alcohols were elongated further and the liquids became less polar, the trends changed somewhat. For instance, 1-decanol was not gelled by 1 month were obtained in the presence of 5 wt % of the p-dodecylbenzenesulfonate salt or trioctadecylphosphine oxide. These results suggest that the liquid participates at the moment of network formation by interacting with the gelator molecules by processes in addition to van der Waals interactions; more than liquid polarity and solubility are at play here. However, liquid molecules are not incorporated within gel fibers, at least not in those gels whose XRD patterns are the same as that of the neat powder (vide infra). Although identification of these (temporal) interactions must await further studies, one liquid-related factor may involve promotion of dissociation of the hydroxytrioctadecylphosphonium salts into their acid and phosphine oxide components in polar liquids at room temperature or even in less polar liquids at elevated temperatures. Since heating is necessary to dissolve the gelators to make solutions/sols and allow gelation on cooling, the amount of dissociated salt may be large, and network formation may be complicated by competing phosphine oxide aggregation, salt reformation, and salt aggregation. Even at room temperature, weak acids such

as chloroform, water, and alcohols are known to form hydrogen-bonded complexes with tertiary phosphine oxides.16 As a result of their very large concentrations, liquid molecules can be effective competitors of the acids for complexation/reaction with phosphine oxides. A different manifestation of salt dissociation was found during attempts to recrystallize hydroxytrioctadecylphosphonium p-dodecylbenzenesulfonate from acetone or ethyl acetate, weakly basic solvents that can compete with the phosphine oxide as H-bond acceptors. Only the phosphine oxide was recovered. These factors will be examined later in greater detail. Gelation Studies of (C18)3PdO and (C18)3P+-OH Cl- in CHCl3/CH3CN Mixtures. Neither acetonitrile nor chloroform, alone, was gelled by hydroxytrioctadecylphosphonium chloride. At 5-9 wt %, the salt precipitated when hot solutions in acetonitrile were cooled, and even 10 wt % remained soluble in chloroform. The natures of samples of 5 or 9 wt % salt in 2.5:1 to 1:1 (v/v) CHCl3/CH3CN were compared with those of trioctadecylphosphine oxide (Table 3). When the volume fraction of chloroform was 2 m) P P P P P P Pg Tu (>2 m) jelly P Tu (2 h) Tu (∼1 m) S jelly Tu (1 h) Tu (∼1 m) S jelly jelly Tu (>2 m) Tu (∼1 m) Tu (>2 m) P P jelly jelly jelly P P P P P jelly Tu (∼1 m) Tu (>2 m)

33-40 38-44 43-45

30-33 39-43 25-30 40-43

38-43 30-33 30-33

(C14)3P+-OBF3appearance

(C18)3P+-BF3appearance gelation temp

P jelly jelly P P S P P S P P P P P P S P P P

jelly

jelly jelly jelly P jelly jelly

Tu (>1 m) Tu (>1 m) Tu (>1 m) Tu (1 d) Tu (1 d) Tu (>1 m)

S S P

Pg

jelly Tu (1 d)

jelly Pg Tu (1 d) jelly jelly Tu (>1 m)

35-40

jelly

37-40 42-44 45-47 45-47

Tu (2 h)

47-53 51-52

P P Tu (1 h)

Tu (>1 m) Tu (>1 m) Tu (>1 m)

43-44 52-55

a Jelly ) thick solution; Tr ) translucent gel; Tu ) turbid gel; S ) solution; P ) precipitate in solution; Pg ) partial gel. b In parentheses: h ) hour; d ) day; m ) month. c Determined by the inverse flow method (ref 21) d Alcoholic liquids react with the gelators in this table; the structure of the actual gelator has not been identified; see text for details.

abilities of (C18)3P+-OBF3- and (C18)3P+-BF3- are similar to that of (C18)3PdO in aprotic liquids. Charge separation, as represented by the importance of the mesomeric zwitterionic forms of the BF3-containing salts, is probably like that of the trialkylphosphine oxides. However, it is not possible to compare the gelating abilities of the zwitterionic gelators with those of (C18)3PdO or the Brønsted salts because both 31P NMR and X-ray diffraction measurements indicate that the BF3-containing salts react with alcoholic solvents. For instance, 31P NMR spectra of the alcohol gel aliquots dissolved in CDCl3 contain a peak at a position similar to that of the phosphine oxide as well as one from the zwitterion (Figure 17S in the Supporting Information), and diffractograms of several gel samples recorded over a 1-month interval indicate that the reaction rate is slow (vide infra). While the zwitterions are the apparent LMOGs in Table 4, the structure of the actual LMOG in these systems has not been pursued. Concentration-Dependent 1H and 31P NMR Spectroscopy of (C18)3P+-OH Cl- in CDCl3. As the concentration of (C18)3P+-OH Cl- in CDCl3 and, thereby, intermolecular interactions were increased, the chemical shifts of the hydroxyl proton and phosphorus singlets moved progressively to lower field (Figure 2). The dependence of the position of the phosphorus resonance on

Figure 2. Concentration dependence of chemical shifts for (C18)3P+-OH Cl- in CDCl3 solutions at room temperature.

salt concentration is largest below 10 mg/mL (0.7 wt %, 0.012 M), and the 31P resonance reached a plateau value at ca. 60 mg/mL (ca. 3.8 wt %, 0.071 M). Due to the presence


Figure 3. Temperature dependence of 1H NMR spectra for a gel consisting of 9 wt % (C18)3P+-OH Cl- in benzene-d6. The arrows mark the hydroxyl proton resonances. The 7.16 ppm peak is from residual protiated solvent.

of adventitious water molecules from the salt and solvent (as indicated by the decrease in the relative area of the hydroxyl peak in Figure 16S with increasing salt concentration), the concentration dependence of the shift of the hydroxyl proton resonance is smaller than that of phosphorus and does not plateau until ca. 120 mg/mL (ca. 7.4 wt %, 0.14 M). The chemical shifts plotted in Figure 2 were extrapolated to 0 wt %, and the differences between the 0 wt % and plateau values, indicative of the magnetic environments of the nuclei at minimum and maximum association, are 9.6 ppm (1H) and 8.9 ppm (31P). Temperature Dependence of NMR Spectra of Sols and Gels of (C18)3P+-OH Cl- in Benzene-d6. A hot sol of 0.11 M (9 wt %) (C18)3P+-OH Cl- in benzene-d6 in a flame-sealed NMR tube was heated and cooled according to protocol P-1. 1H (Figure 3) and 31P (Figure 4) NMR spectra were recorded as the temperature was raised incrementally from ambient (i.e., where the sample is gelled). At higher temperatures, more salt is unassociated or in the form of sols or other “dissolved” aggregates, and both the resolution and signal-to-noise (when an equivalent number of FIDs were accumulated) of the spectra increased: only salt molecules not part of the solid gel network can be detected in these experiments; the relaxation times of nuclei within the gelator networks are very long. At >33.8 °C, where the gel is completely melted (Tgel-sol ) 29-33 °C by the inversion flow method), the half-width of the broad singlet peak at 0.9 ppm in Figure 4 (from terminal methyl groups on the three alkyl chains of the salt molecules) decreased and split into a triplet, a new peak at 1.6 ppm (attributed to the β-methylene groups) became evident, and the broad peaks at 1.9 ppm (due to the R-methylene groups) and 1.4 ppm (from C3C17 methylene units along the alkyl chains) split into multiplets. Analogous changes in signal/noise, peak position, and peak narrowing also occurred at higher temperatures for the 31P singlet (Figure 4), but in none of the spectra was more than one phosphorus resonance observed. After the sample had been heated and recooled very slowly to room temperature, it separated into a bulk solid and liquid. For this reason, only first heating data are presented. As temperature was increased, the hydroxyl proton resonance shifted dramatically downfield from 5.9 ppm

Langmuir, Vol. 19, No. 4, 2003 1043

Figure 4. Temperature dependence of 31P NMR spectra for a gel consisting of 9 wt % (C18)3P+-OH Cl- in benzene-d6.

at 23 °C to 14.8 ppm at 48 °C while the phosphorus peak moved upfield from 72 ppm at 25 °C to 68 ppm at 48 °C. On the basis of the concentration-dependent NMR spectra of (C18)3P+-OH Cl- in CDCl3 and the reasonable expectation that concentrations of “dissolved” salt increase at higher temperatures, the direction of the shift of the hydroxyl resonance in Figure 3 was expected. However, the direction of the phosphorus shifts upon increasing temperature was not. In addition, the hydroxyl proton resonance shifted beyond the 11.3 ppm plateau value found at high salt concentrations in CDCl3 at room temperature. We attribute these changes to another consequence of raising temperature: increased salt dissociation to trioctadecylphosphine oxide and hydrogen chloride. Exchange between the lower field resonance of the proton in hydrogen chloride and the higher field hydroxyl proton in the salt accounts for their overall chemical shift exceeding the room-temperature aggregate limit at T > 33 °C. The 31P shift data indicate that protons exchange rapidly between isolated molecules of the salt and its aggregates, as well as between trioctadecylphosphine oxide and hydrogen chloride. Temperature Dependence of NMR Spectra of Sols and Gels of 5 wt % (C18)3P+-OH Cl- in n-Butanol-d10. Due to increased mobility of molecules of (C18)3P+-OH Cl- in a 5 wt % gel in n-butanol-d10 above 34 °C (but more visibly above 44 °C, where the gel was completely melted; Tgel-sol ) 37-44 °C by the inversion flow method), several signals of the 1H NMR spectra split sequentially into clear multiplets (Figure 5). The first at 0.9 ppm (terminal methyl groups) was followed by the 2.0 ppm (R-methylene groups) and 1.6 ppm (β-methylene groups) signals and then by the 1.5 ppm (γ-methylene groups) peaks. The sharp singlet that moves from 4.7 to 5.5 ppm with increasing temperature is from rapidly exchanging protons in hydroxyl groups of the salt and the solvent. It also shifts to higher field at higher temperatures in neat n-butanol-d10. We hypothesize that in the gel state at room temperature and up to ca. 33 °C, atoms of molecules in the gelator network and in sols interact most strongly near their phosphorus headgroups. As temperature is increased further, these interactions weaken, more gelator molecules

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Wang et al.

Figure 5. Temperature dependence of 1H NMR spectra for a gel consisting of 5 wt % (C18)3P+-OH Cl- in n-butanol-d10. The small singlet at 3.5 ppm is due to residual C1-H protons in the solvent.

Figure 6. Dependence of 31P NMR spectra for a gel consisting of 5 wt % (C18)3P+-OH Cl- in n-butanol-d10.

dissociate from the aggregates yielding trioctadecylphosphine oxide (vide infra) and hydrogen chloride, and the molecules remaining in the aggregates experience greater mobility. The 78 ppm 31P NMR resonance at room temperature (Figure 6) indicates that the vast majority of the phosphorus-containing species that are detected (i.e., dissolved individually and in sols or other aggregates) remain in their protonated (hydroxyl) form. The hydroxyl groups in the liquid molecules can act as H-bond donors (albeit weaker ones than HCl) for trioctadecylphosphine oxide, also, were it to dissociate to lose HCl. Consistent with the results in the benzene-d6 sample, the 31P resonance of (C18)3P+-OH Cl- in n-butanol-d10 also moved to higher field with increasing temperature. However, unlike the spectra in benzene-d6, the phosphorus peak became

broader at higher temperatures due to a complex set of proton-exchange processes between the salt, the phosphine oxide, and solvent molecules. X-ray Diffraction Measurements. X-ray diffraction patterns of neat (C18)3PdO, (C18)3P+-OH Cl-, (C18)3P+OH CH3C6H4-SO3-, (C18)3P+-OH C12H25C6H4-SO3-, (C18)3P+-BF3-, and (C18)3P+-OBF3- have been compared with those of the networks in some of their gels. The latter are obtained as the difference between the normalized diffraction patterns of a gel and its neat liquid component.44 If the sharp peaks of the difference pattern and of the neat gelator are the same, so are their molecular packing arrangements. This is the case for gels of (C18)3PdO with silicone oil, 1-nonanol, and benzyl alcohol as the liquids (Figure 7). However, the trialkylhydroxyphosphonium salts are not always the species constituting the gel networks. For instance, the diffractogram of the (C18)3P+-OH CH3C6H4SO3-/benzyl alcohol gel is that of trioctadecylphosphine oxide (Figure 8). The salt dissociates into its acid and phosphine oxide forms when heated in benzyl alcohol, and network formation by the oxide occurs more rapidly than it reassociates with p-toluenesulfonic acid! By contrast, the networks of (C18)3P+-OH CH3C6H4-SO3- in 1-nonanol, 1-decanol, and silicone oil gels do consist of the undissociated salt. The dielectric constant of benzyl alcohol is higher than that of the other three liquids (Table 1S), and it is the weakest acid (pKa ) 26.9 extrapolated from measurements in DMSO;45 pKa ∼ 16 for primary alcohols31). Both factors make it the liquid in which salt dissociation is easiest. XRD patterns of gel networks of (C18)3P+-OH Cl- in silicone oil, 1-nonanol, benzyl alcohol, 1-butanol, and benzene and of (C18)3P+-OH C12H25C6H4-SO3- in 1(44) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1324. (45) Bordwell, F. G.; Liu, W.-Z. J. Am. Chem. Soc. 1996, 118, 8777.


Figure 7. X-ray diffractograms of (C18)3PdO: (a) neat solid; (b) 5 wt % gel in silicone oil; (c) 5 wt % gel in 1-nonanol; (d) 5 wt % gel in benzyl alcohol.

Figure 8. X-ray diffractograms of (C18)3P+-OH CH3C6H4SO3-: (a) neat solid; (b) 9 wt % gel in 1-nonanol; (c) 9 wt % gel in 1-decanol; (d) 5 wt % gel in silicone oil; (e) 9 wt % gel in benzyl alcohol.

nonanol and benzyl alcohol are of trioctadecylphosphine oxide, also. The chloride and p-dodecylbenzenesulfonate salts are more prone to dissociation than is the ptoluenesulfonate salt, at least in these liquids. Diffraction patterns of the network component of (C18)3P+-BF3- gels in silicone oil, 1-decanol, 1-nonanol, benzyl alcohol, and 1-butanol are all different from that of the neat (C18)3P+-BF3- powder (Figure 9). 31P NMR spectra of gels dissolved in CDCl3 (Figure 17S in the Supporting Information) indicate that the differences are not always a consequence of morphology; as mentioned, although (C18)3P+-BF3- is stable in nonpolar, aprotic liquids such as silicone oil, benzene, and toluene, it reacts slowly at room temperature with alcohols. When diffraction patterns of freshly prepared (Figures 9 and 10) and 1 month old (not shown) gel samples consisting of 9 wt % (C18)3P+-BF3- in 1-nonanol, 7 wt % (C18)3P+-BF3- in 1-decanol, and 9 wt % (C18)3P+-OBF3- in 1-nonanol were compared, small but real differences that could not be attributed to aging were discernible. Diffractograms of (C18)3P+-OBF3- gels in all liquids examined differ from that of the neat LMOG, also (Figure 10). Like (C18)3P+-BF3-, (C18)3P+-OBF3- reacted with alcoholic liquids but not with aprotic, low-polarity ones such as silicone oil. The diffractogram of a 7 wt % (C18)3P+OBF3-/silicone oil gel sample, prepared by dipping a capillary into its hot solution/sol, lacked a low-angle peak

Langmuir, Vol. 19, No. 4, 2003 1045

Figure 9. X-ray diffractograms of (C18)3P+-BF3-: (a) neat solid; (b) 7 wt % gel in silicone oil; (c) 7 wt % gel in 1-decanol; (d) 9 wt % gel in 1-butanol; (e) 9 wt % gel in 1-nonanol; (f) 9 wt % gel in benzyl alcohol.

Figure 10. X-ray diffractograms of (C18)3P+-OBF3-: (a) neat solid; (b) 9 wt % gel in 1-butanol; (c) 9 wt % gel in 1-nonanol; (d) 7 wt % gel in benzyl alcohol; (e) 7 wt % gel in silicone oil measured with the gel inside a capillary; (f) 7 wt % gel in silicone oil measured with the gel on the outside tip of a capillary.

(Figure 10e). However, when the sample was prepared by placing a piece of preformed gel onto the exterior tip of a capillary, low-angle peaks were observed (Figure 10f). The gelator fibrils appear to have been oriented by flow in the former sample.46 The diffractogram of a 9 wt % (C18)3P+OBF3-/1-nonanol gel (Figure 10c) contained diffractions ascribable to at least two morphs. One of them is in nonfibrillar crystallites, observable by optical microscopy, that are not associated with the gel network. The diffractogram of a 7 wt % (C18)3P+-OBF3-/benzyl alcohol gel (Figure 10d) is similar to that of the 7 wt % (C18)3P+OBF3-/silicone oil gel (Figure 10e,f). The very high pKa of benzyl alcohol45 appears to retard the rate of its reaction with the BF3 zwitterions. Unfortunately, our efforts to grow single crystals of these gelators have not been successful. On the basis of information available for other phosphonium salts with long alkyl chains47 and analyses of their XRD patterns, we have developed models for molecular arrangements in their gel networks. Diffraction patterns of all of the hydroxytrioctadecylphosphonium salt and trioctadecyl(46) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Heuze, K.; Fages, F.; Pozzo, J.-L. Langmuir 2002, 18, 7151. (47) Abdallah, D. J.; Robertson, A.; Hsu, H.-F.; Weiss, R. G. J. Am. Chem. Soc. 2000, 122, 3053.

1046


Figure 11. Possible packing arrangements for LMOGs in their neat solid and gel states.

phosphine oxide based gels contain one or more low-angle (2θ < 3°) peaks, indicative of lamellar organizations, that match those of the neat gelator (where comparisons are available). Then, from Bragg’s law, the lamellar thicknesses are 43.4 Å for (C18)3PdO, 42.8 Å for (C18)3P+-OH Cl-, 38.2 Å for (C18)3P+-BF3-, and 36.9 Å for (C18)3P+OBF3- powders. The lamellar thicknesses of (C18)3P+OH CH3C6H4-SO3- and (C18)3P+-OH C12H25C6H4-SO3are 69.5 and 63.1 Å, respectively. The distance between the phosphorus nucleus and the outmost hydrogen atom on the terminal carbon of a fully extended octadecyl chain is calculated to be 25.6 Å.13 These data suggest a packing model in which (C18)3P+-OH Cl-, (C18)3P+-BF3-, and (C18)3P+-OBF3- molecules are “paired” through hydrogenbonding (where possible), electrostatic, and dipolar interactions between the phosphorus headgroups so that two phosphonium ions (and their anions) determine the thickness of a lamella (Figure 11a). Alternatively, the data can be accommodated by an arrangement like that shown in Figure 11c, in which the headgroups are associated within a plane that separates alkyl chains from one molecule. The lamellar thickness of (C18)3PdO is also proposed to consist of two molecules, but with headgroup interactions that are primarily dipolar. Since the lamellar thicknesses of (C18)3P+-OH CH3C6H4-SO3- and (C18)3P+-OH+ C12H25C6H4-SO3- are slightly shorter than 3 times the length of an extended octadecyl chain, we speculate that lamellae of the sulfonates consist of interdigitated bilayers in which two chains of each (C18)3P+-OH cation are paired intramolecularly on one side of the ionic plane while the third one is projected on the other side and paired intermolecularly (Figure 11b).47 Conclusions On the basis of NMR and DSC measurements and attempted recrystallization results, trialkylhydroxyphosphonium salts from reaction between a trialkylphosphine oxide and even strong Brønsted acids dissociate somewhat when heated, especially in more polar liquids such as alcohols. As a result, the nature of the species included within the fibrillar networks depends on the relative rates of (1) nucleation/precipitation of the phosphine oxide, (2) reassociation of the phosphine oxide and Brønsted acid when they are dissociated, and (3) nucleation/precipitation of the salt. When reassociation of a dissociated salt or its nucleation is slower than nucleation/precipitation of the phosphine oxide, the latter will be the LMOG. When dissociation is partial or reassociation is rapid and nucleation of the salt is also rapid, the salt will be the LMOG. In this regard, it is interesting to note that even when the phosphine oxide is the LMOG, the acid component must have an influence on the formation of the fibrillar network because phosphine oxide gels were formed in several liquids from salts when the same liquids

Wang et al.

were not gelled by the phosphine oxide alone. The catalytic nature of the acid is not known at this time, but it probably involves the attenuation (or acceleration) of growth on some faces of embryonic crystallites, leading to elongated objects that can join to form the eventual fibrillar network. The zwitterionic salts are also LMOGs of some aprotic liquids, but the nature of their gels with protic liquids such as alcohols is unclear. The zwitterions react slowly with many alcohols as well as gelling them. Future work will investigate formation of zwitterionic salts by combination of trialkylphosphines and Lewis acids that are stable in protic media. The addition of a Brønsted acid clearly aids gelation in some cases. It also inhibits gelation in others as witnessed by the dependence of LMOG efficiency on the pKa of the acid and the lipophilicity/hydrophobicity and size and shape of its conjugate base. A key factor seems to be the degree of charge separation induced in the phosphoryl group of the phosphine oxide: increased polarity of the trialkylhydroxyphosphonium salts promotes gel formation and stability. Hence, it should be possible to tune the gelating ability of the phosphine oxides by a judicious choice of the acid. Recently, ionic liquids that are also strong Brønsted acids have been utilized as catalysts and “green solvents” for several acid-promoted organic reactions, including alcohol dehydrodimerization.48 It may be possible to effect the same type of reactions by warming gels of trialkylhydroxyphosphonium salts to dissociate a strong acid catalyst and then cooling them to reform the gels and deactivate the acid once the reaction is completed. Regardless of their eventual applications, we have demonstrated that salts formed by combination of two non-LMOG (or poor LMOG) molecules, one a phosphine oxide (or phosphine) and the other an acid, are gelators of a variety of organic liquids, albeit at concentrations somewhat larger than necessary in a number of other systems. These salts are another example of twocomponent LMOGs whose gelating abilities are modulated by electronic charge.10,11 Acknowledgment. We thank the National Science Foundation and the Petroleum Research Fund (administered by the American Chemical Society) for their support of this research. We are grateful to Professor C. L. Khetrapal for useful discussions concerning the NMR experiments. The assistance of Dr. Date Chynwat during the temperature-dependent NMR experiments is greatly appreciated. Supporting Information Available: Syntheses and characterizations of some hydroxytrioctadecylphosphonium salts; transition temperatures and enthalpies from DSC thermograms of all salts prepared; gelation of (Cn)3PdO (n ) 10, 14) and (C14)3P+-OH Cl-; 1H, 31P, and 19F (in the case of boron trifluoride complexes) NMR and IR spectra of the salts; concentration dependence of 1H NMR spectra of (C18)3P+-OH Cl- in CDCl3 solutions at room temperature; and 31P NMR spectra of some gels made from (C18)3P+-BF3- and dissolved in CDCl3. This material is available free of charge via the Internet at http://pubs.acs.org. LA026631J (48) Cole, A. C.; Jensen, J. L.; Ntai, I.; Tran, K. L. T.; Weaver, K. J.; Forbes, D. C.; Davis, J. H., Jr. J. Am. Chem. Soc. 2002, 124, 5962.

Trialkylphosphine and Trialkylphosphine Oxide Organogelators

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