Langmuir 1996,11, 3630-3632
3630
Cholestanyl Substituted Quaternary Ammonium Salts as Gelators of Organic Liquids Liangde Lu and Richard G. Weiss* Department of Chemistry, Georgetown University, Washington, D.C. 20057 Received J u n e 12, 1995. I n Final Form: August 18, 1995@ Low concentrations ( < 5%, wt/vol) of some structurally simple, low molecular weight, quaternary ammonium salts containing a cholestanyl group have been found to gel a variety of organic liquids in a thermally reversible fashion. Factors related to gel stability, including the structures of the substituents on nitrogen, the type of counterion, and the nature of the liquid component, are investigated.
Introduction Ionic surfactadsin aqueous media are known to form many types of aggregates, including lyotropic liquid crystals, vesicles, and mice1les.l Recently, combinations of the anionic surfactant sodium bis(2-ethylhexy1)sulfosuccinate and one of several phenols2 were shown to form very stable gels in benzene and other nonaqueous solvents. We have been investigating a novel class of gelators, designated ALS (containing a n aromatic, a linking, and a steroidal group), that form thermally reversible gels with a wide spectrum of organic l i q ~ i d s . ~The - ~ gelator molecules aggregate into strands with exceedingly high aspect ratios; the strands are connected in highly branched giant colloids that immobilize the liquid component primarily by surface tension. During the course of further investigations, we discovered that low concentrations of some completely saturated trialkylcholestanylammonium salts gel a variety of nonpolar and polar liquids in a thermally reversible fashion. The salts also represent a n interesting variant to the uncharged azahomosteroid gelators NH and Here, we present some observations
Table 1. Transition Temperatures for Neat Phasesaibof Molecules in Chart 1
gelator
transition temperatures ("C)
K
p-1'
K, K,,
a-2
K
p-2c
K,
p-2'
P-3 P-4 p-5'
1
-
ca. 225 (decamp)
91
K
.
192-196 (decompP
a-1
. 1
181-185 (decompP
*i
K,, 138K,,,
ca. 172 (decomp)
.i
118-124 (decompP
*i
68-69
Ki 60-6211 Ki 88 102 K, K,, S
- -K,- K,, -S -i 79
87
139
,
1
134
From optical microscopy. K = crystal; S = enantiotropic smectic; i = isotropic. Temperatures at heat flow maxima in differential scanning calorimetry heating scans. [I
R
NR NO
R=H R=O*
on the properties of principally those gels with alcohols and alkanes as the liquid component and explore the principal factors responsible for their gelation.
Experimental Section Gelator salts were synthesized by standard techniques, starting usually from the corresponding tertiary cholestanylamines; quaternization was effected under s N 2 type conditions in the presence of an excess of the appropriate alkyl halide. Mixtures of a and p diastereomers were separated by column @
Abstract Dublished inAdvanceACSAbstracts. October 1.1995.
(1)Ringsdld, H.; Schlarb, B.; Venzmer, J. Angeh. Chem., Int. Ed. Engl. 1988,27, 114. (2) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J . Phys. Chem. 1994,98, 3809. (3)Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989,111, 5542. (4) Furman, I.; Weiss, R. G. Langmuir 1993, 9, 2084. (5) Mukkamala, R.; Weiss, R. G. J. Chem. Soc., Chem. Commun. 1995, 375. (6) (a)Terech, P.; Furman, I.; Weiss, R. G . J . Phys. Chem. 1995,99, 9558. (b) Terech, P.; Furman, I.; Bouas-Laurent, H.; Devergne, J. P.; Ramasseul, R.; Weiss, R. G. Faraday Discussions submitted for publication.
0743-7463/95/2411-3630$09.00/0
chromatography (silica gel). Phase transition temperatures of the the salts are presented in Table 1. The test of gelation consisted of heating aknown small amount of potential gelator and a liquid in a sealed glass tube until a solution was obtained. After being cooled to room temperature in air or to ca 0 "C in an ice bath, the tube was inverted. Gelation was considered successful if no sample flow occurred. In many cases, the gels appeared to be hazy or took on a bluish hue due to the Tyndall effect. Two criteria of gel stability,the temperature at which gelation occurs (T,)according to the "inverse flow" method7 and the time required to detect macroscopic phase separation of gels incubated at room temperature, have been employed.
Results and Discussion Relationship between Gelator Structure and Gelation Efficiency. Although gels form when solutions of a-1 or p-1 and n-alkanes (like heptane and dodecane) are cooled, those with the p isomer are stable for longer periods at room temperature. In a n extreme case, 5.0 wt %/3-1, the quaternary salt with one long alkyl chain (see Chart 11, slowly formed stable translucent gels with 1-pentanol or 1-nonanol,but solutions containing 5 wt % a-1did not. Simple geometric considerations indicate that steroids with p linkages to substituents at C3 can adopt ( 7 ) Takahashi, A,; Sakai, M.; Kato, T. Polym. J . 1980, 12, 335.
0 1995 American Chemical Society
Letters x
x
I*
X X
f
X
x
x.1 x=cI
p-2
P-1
0-2'
X
30 20 0
10
20
30
40
50
60
v% of H20
Figure 1. Gelation temperatures for gels composed of 0.93 w t % of a-2 ( x ) or 8-2 (*) and water/l-propanol mixtures versus the volume percent of water.
11-5
I!.
$'
x=I x = CI
a-2
Table 2. Gelation Properties of 8-2 with Selected Liquids ~
liauid component methanol 1-propanol lIl(v/v)l-propanol/H20 1-octanol dodecane hexadecane tetrahydrofuran Dow-Corning silicone oil 704
wt % of gelator
phase formation
0.8 1.0 1.0 0.9 3.4 1.0 2.5
a a a a b
1.0
C
2.5 1.0 2.0 2.0
approximate stable periode 6h 1 day 12 days 9 months 20 days
C
b b
> 12 months
> 12 months
C
b d
> 4 months >4 months
a Transparent gels a t room temperature. Translucent gels at room temperature. Solution upon heating, precipitate upon cooling to room temperature. A clear gel obtained after the cosolvent (CH2C12) was evaporated and the solution was cooled to room temperature. e Periods with lower limits indicate that the gel has remained stable since its date of preparation.
more extended conformations than those of the corresponding a epimer^.^!^ The quaternary salts with two long alkyl chains, a-2 andp-2, are very good gelators of n-alkanes (from heptane to hexadecane), 1-alkanols (from methanol to 1-octanol), and other solvents. A few formulations employing 8-2 are included in Table 2. Consistent with our hypothesis that gelator strands consisting of molecular salts with the more extended p configuration a t C3 of the steroidal group pack more efficiently than those with the a configuration, the Tgof gels withp-2 are higher than those of a-2. The gelation temperatures of a-2 andp-2 increase with greater water content initially and reach plateaus above ca. 35 vol % (Figure 1). Our current hypothesis is that hydration of the cationic group and its counteriong provides additional intermolecular stability through multiple hydrogen-bonding interactions.1° However, the (8) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. A m . Chem. SOC.1994, 116,6664. (9)Underwood, A.; h a c k e r , E. J . Colloid Interface Sci. 1987,117, 242. (10)Vogtle, F. Supramolecular Chemistry; Wiley: New York, 1991; pp 283-289.
higher polarity of the water-rich liquid mixtures must also force a large fraction of the hydrophobic gelator molecules to remain in the strand superstructure that immobilizes the liquid. We believe that more efficient molecular packing in the strand networks of gelator m01ecules~~~Jl contributes significantly to the greater gelling ability of the ,8 isomers. By contrast, the analogous neutral tertiary amine, /3-3, does not gel the same liquids even a t relatively high concentrations. Although benzyl alcohol can be gelled by > 5 wt % of the related amide, p-4, the alkanes and 1-alkanols in Table 2 are not. Clearly, the interplay between the molecular s t r u ~ t u r e ~ ,and ~ J ' polarity of the liquid component and the solubility of the gelator influences gelation in a complex way.3,4J2 These results indicate that gelation can be induced2 and gel stability can be enhanced13 by the electrostatic interactions available among ionic surfactant molecules; all of the tetraalkylammonium salts in Chart 1function as efficient gelators of some liquids. However, large electrostatic interactions are not a sufficient condition for a potential gelator to function efficiently. In spite of its structural resemblance to p-2, /3-5 did not gel any of the liquids tested. It is interesting to note that /3-5 and 8-5' form thermotropic smectic phased4 but p-2 and 8-2' do not. Previously, we have found that only those A I S molecules (of a family consisting of steroidal, anthraquinonyl, and linking groups) that form liquidcrystalline phases are also gelators of alkanes and a l k a n ~ l s .However, ~ some ALS molecules with anthracenyl groups that are not thermotropic liquid crystals do function as efficient gel at or^.^ It is known that cationic surfactants with chloride counterions aggregate in aqueous media at higher concentrations than those with bromide or iodide counteri ~ n s . ~The J ~ opposite trend should apply in nonpolar solvents. The influence of the counterions on gelatorsp-2 and/?-2 (Table 3) is clearly present, but its attribution is not easily discerned with the available data. (11)Wade, R. H.; Terech, P.; Hewat, E. A.; Ramasseul, R.; Volino, F. J. Colloid Interface Sci. 1986,114,442. (12)Brotin, T.;Utermohlen, R.; Fages, F.; Bouas-Laurent, H.; Desvergne, J. P. J. Chem. Soc., Chem. Commun. 1991,410. (13)Fuhrhop, J.-H.; Svenson, S.; Boettcher, C.; Rossler, E.; Vieth, H.-M. J . Am. Chem. SOC.1990,112,4307. (14)Demus, D.; Richter, L. Textures o f Liquid Crystals; Verlag Chemie: New York, 1978;pp 166-211. (15) (a) Zana, P. In Cation Surfactants; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1990;pp 41-85. (b) Ingram, B. T.; Ottewill, R. H. In Cation Surfactants; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1990;pp 87-93.
Letters
3632 Langmuir, Vol. 11, No. 10, 1995 Table 3. Attempted Gelation of 8-2 (Iodide) and 8-2’ (Chloride)with Selected Liquids liquid component
wt % of gelator
1-propanol 1-octanol dodecane Dow-Corning silicone oil 704
1.0 3.4 1.4 2.0
8-2
8.2’
(iodide)
(chloride)
G”
PPtb PPt
G G G
G G
0 G = gel a t room temperature. ppt = precipitate from hot solution when cooled.
In previous work, we have shown that truncation ofthe alkyl chain on the D-ring (C17) of a steroidal group significantly decreases a molecule’s ability to gel organic liquid^.^ Shortening of alkyl chains also leads to a loss of gelling ability of molecules that lack a steroidal group.12 Consistent with these observations, lengthening alkyl chains on molecules that form “micellar fibers” in aqueous media lowers the surfactant solubility and eventually leads to lower critical micelle concentrations (cmcLg Liquid Structure and Polarity. The influence of the liquid component on gelation and gel stability is very complex. We have identified bulk polarity, molecular structure, and gelator solubility within the liquid as three key contributing factor^.^^^ In some cases, hydrogenbonding and other molecular interactions may also play important roles. In general, gels of 8-2 had higher Tg values in nonpolar solvents like n-alkanes than in polar solvents like 1-alkanols. The greater solubility ofp-2 in the latter facilitates dissolution of the gelator strand networks at lower temperature^.^^^ With less polar gelators, we have found that it is the less polar liquids that form gels with lower Tgvalues. A very polar liquid like water, in which gelators like a-2and 8-2 have very limited solubility, again leads to higher Tgvalues (Figure 1).
Conclusions. We have identified several new factors responsible for the formation of thermally reversible gels comprising a n organic liquid and a low molecular weight gelator. The new gelators reported here, simple quaternary ammonium salts containing a steroidal group, open new vistas for immobilizing organic liquids. Principal factors responsible for successful gel formation, including gelator packing efficiency within the strands that immobilize the liquid component (N.B. the configuration a t C3 ofthe steroidal group), the number oflong alkyl chains on the gelator, the charge on the gelator, the nature of the counterion, and liquid polarity (N.B. gelator solubility), have been examined. The results suggest avenues for the construction of even simpler gelators that contain none of the normal key components of ALS molecule^.^-^ Preliminary experiments with the very simple salts 6 show that they gel organic liquids such as dodecane, 1-pentanol, phenyl compounds, and silicone oil very efficiently. Thus,
+ (‘1EH37)
3N-CnH2w1
I’ 6
the rigidity associated with a steroidal (or fused aromatic) group is not necessary for a molecule to be successful in gelation! It remains to be discoveredwhether even simpler molecules can serve as gelators of thermally reversible organogels. The physical properties of these and related gels will be reported in a full account.
Acknowledgment. We thank the National Science Foundation (Grants CHE-9213622 and CHE-9422560)for its support of this research. LA950458S
