The effect of chain length and salt on phase diagrams of the n-alkyl ammonium halide—water system

NOTES The Effect of Chain Length and Salt on Phase Diagrams of the n-Alkyl Ammonium Halide-Water System The phase diagrams of systems containing octylammonium chloride (OAC1), dodecylammonium chloride (DDACI), and decylammonium bromide (DABr) with water, as well as those of ternary systems containing the corresponding ammonium salt, for which a constant molar ratio of 2.8 between the amphiphile and the salt was maintained, are discussed. The hexagonal phase appears at low concentrations in the chloride binary systems beginning at 3 mole% in DDAC and 9 mole% in OACI, and is absent in that of DABr. The lamellar phase was present in all the phase diagrams and the nematic was present in all except the binary OAC1. The addition of-CH2- groups to the amphiphile molecule and the addition of salt enhance the formation of the nematic and lameilar phases at lower concentrations and allows these mesophases to persist to higher temperatures. © 1988AcademicPress,Inc. INTRODUCTION More than five million tons of amphiphilic compounds are used annually in the United States and Europe, and the rate of use of such compounds is growing at a faster rate than the economy in general (1). Lyotropic liquid crystals, especially in the nemafic phase, present a very useful tool for the study of concentrated amphiphilic solutions. The optical axis in these anisotropic systems can be inferred from the textures observed with a polarizing microscope and phase changes (readily observed microscopically) can be introduced either by varying the temperature or by varying the concentration of one oftbe components of the solution (amphiphile, H20, salt, cosurfactant, etc). Phase diagrams are useful for two reasons: first, they provide basic information about the thermodynamic conditions under which a given mesophase is stable; and second, they enable the researcher to choose the sample composition which results in a specific desired phase in a known temperature range. The system which we choose to study was that of the n-alkyl ammonium chlorides, because much information from a variety of experimental techniques (2--4) is available on the lyomesophases of these substances [especially on decylammonium chloride (DACI)]. Work on this system was begun by some of our group who studied the DAC1/ H20 binary diagram as well as those containing varying amounts of NH,C1 (5). In the present study we include three other amphiphiles: octylammonium chloride (OAC1), dodecylammonium chloride (DDAC1), and decylammonium bromide (DABr). EXPERIMENTAL SECTION OACI, DDACI, and DABr were prepared from their respective amines and purified as described in Refs. (5, 6). The water was deionized and triply distilled. The salts were recrystallized from this same water. The techniques of

sample preparation and homogenization were performed as in Ref. (5). The phases were determined by examining at least three samples in flame sealed microslides on the polarizing microscope. Transition temperatures were normally measured at 2°C/rain (heating), slower rates being used when necessary. Care was taken to avoid leaving the sample (either in the test tube or in the mieroslide) at a temperature corresponding to a two-phase region, due to the rapid development of concentration gradients in these regions. In all of the phase diagrams studied, supercooling takes place (especially when the sample is in a microslide) and therefore the phases in the lower temperature regions of the diagrams presented are supercooled. As very large concentration gradients develop in the coagel, a microslide whose sample has entered this phase must be discarded. This made exact determination of the coagel stability temperature line impossible. We have indicated with a dashed line on the phase diagrams (Fig. 1-3) our estimation of this temperature. Some errors are undoubtably present in our diagrams as a result of water loss on sealing the microslides and as a result of temperature gradients in the glass walls of the microslides; however, comparing transition temperatures in the bulk sample (culture tube) to those in the microslide we do not believe our errors exceed + 1°C or +0.2 wt% amphiphile.

DISCUSSION The phase diagrams for DDAC1, OAC1, and DABr are presented in Figs. 1 to 3; (a) the binary diagrams and (b) the ternary diagrams (containing salt). A phase diagram for the DDACI binary system was done in 1950 (7) and agrees with our data except that no nematic phase was found at that time. When we began this work, no systems of simple hydrocarbon amphiphiles were known to have a nematic phase (8); however, recently

587 Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988

0021-9797/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

588

NOTES b 120

DDACI TERNARY

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FIG. 1. (a) Binary phase diagram of DDAC! with water. (b) Ternary phase diagram of DDACI/NI-LC1/ H20. H, hexagonal; N, Nematic; L, Lamellar; L + I, H + I, and N + I are two-phase regions.

Saupe (2) and others (9) have also found it in systems similar to ours. Auvray et al. (10) have investigated parts of the phase diagram where the hexagonal phases of DACI and DDACI appear and obtained essentially the same resuits as we did for the t~ions studied. In all three binary systems with the chloride counterion (DDAC1, DACI, and OACI) the first mesophase to appear,

as amphiphile concentration is increased, is the hexagonal. This mesophase forms at about 3 mole% in the DDACI system, and at 5 and 9 mole% in the DACI and OACI systems, respectively. The nematic is present below the hexagonal in the DDAC1 (Fig. la) and DACI (5) systems; the region below the hexagonal in OACI (Fig. 2a) being inaccessible due to the formation of the coagel. The hex-

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FIG. 2. (a) Binary phase diagram of OACI with water. (b) Ternary phase diagram of OACI/NI-LCI/H20. Journal of Colloid and Interface Science, VoL 122, No. 2, April 1988

NOTES

589

DABr

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TERNARY

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FIG. 3. (a) Binary phase diagrams of DABr with water. (b) Ternary phase diagram of DABr/NH4Br/H20. agonal is separated from the nematic and lamellar phases by a reentrant isotropic phase which appears to be connetted to the regular isotropic region, and which has a viscosity close to that of the nematic (or regular isotropic) as opposed to the gelatin-like hexagonal phase. We are investigating the nature of this reentrant isotropic phase by X-ray scattering. The DABr binary diagram (Fig. 3a) is differentiated from the C1- diagrams by the lack of a hexagonal phase. Experimentally this system was somewhat more difficult to homogenize (especially at higher amphiphile concentrations) so the microslides had to be filled in a heated ambient

/

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/il

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to avoid formation of the coagel. For this reason this phase diagram was not extended to concentrations (temperature) as high as the others. The ternary diagrams, containing salt, are shown in Figs. l-3b. The molar ratio ofamphiphile to salt is maintained at 2.76 to l in the DDACI and OAC! systems and at 2.80 to 1 in the DABr system. To permit a direct comparison of the mesophases formed by amphiphiles of different length, in Fig. 4 we superimpose the DDACI, DACI (redrawn from Ref. (5) and shown as a dotted line), and OAC1 systems: (a) the binary systems, and (b) those containing salt.

III

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FIG. 4. Superposition of the phase diagrams of DDACI[C~2], DACI[CL0] (shown as a dotted line), and OACI[Cs]. (a) Binary phase diagrams. (b) Ternary phase diagrams. Journal of Colloid and Interface Science, Vol. 122,No. 2, April1988

590

NOTES CONCLUSIONS

The increase in the chain length of the amphiphile and the addition of salt produce similar results, i.e., move the nematic and lamellar phases toward regions of lower amphiphile concentrations and higher temperatures. The addition of salt also destroys the hexagonal, when present in the binary system. The effect of chain length on mesophase formation in these amphiphiles in their anhydrous form is shown in Ref. (6) where the temperature of the onset of mesophase formation is seen to increase about 9°C for each -CH2group. An increase in the amphiphile chain length is also known to increase the hydrophobic properties of the amphiphile, whether measured by a decrease in the CMC or an increase in its surface active properties (11). The salt would be expected to partially screen the charged head groups of the amphiphile from each other resulting in a smaller effective molecular head group area on the micelle (12, 13). Thus even though the increase in chain length affects the interior of the micelle and the salt affects the micellar surface, both enhance the nematic and lamellar phases, in temperature and in concentration. The result is the shitl of these phases toward higher temperatures and lower concentrations observed in all the systems studied. ACKNOWLEDGMENTS We thank Professors Ted Taylor and .~bio V. Pinto for helpful discussions and the CNPq and CAPES for financial support.

3. Gault, J. D., Kavanagh, E., Rodrigues, L. A., and Gallardo, H., J. Phys. Chem. 911(9), 1860 (1986). 4. Stefanov, M., and Saupe, A., Mol. Cryst. Liq. Cryst. 108, 309 (1984). 5. Rizzatti, M., and Gault, J. D., J. Colloid Interface Sci. 110(1), 258 (1986). 6. Gault, J. D., Mfiller, H., and Gallardo, H., Mol. Cryst. Liq. Cryst. 130, 163 (1985). 7. Broome, F. K., Hoerr, C. W., and Harwood, H. J., J. Amer. Chem. Soc. 73, 1350 (1950). 8. Hoffmann, H., Ber. Bunsanges., Phys. Chem. 88, 1078 (1984). 9. Radley, K., and Tracy, L. C., Mol. Cryst. Liq. Cryst. Lett. 1(3, 4), 95 (1985). I0. Auvray, X., Jandaly, J., Anthore, R., and Petipas, C., C. R. Acad. Sci. Paris 299(16) II, 1124 (1984). 11. Tanford, C., "The Hydrophobic Effect: Formation of Micelles and Biological Membranes," 2 ed., p. 68, Wiley, New York. 12. Per Ekwall, in "Advances in Liquid Crystals" (G. Brown, Ed.), Vol. 1, pp. 1-139. Academic Press, New York, 1975. 13. Tracey, A. S., Canad. J. Chem. 62(11), 2161 (1984). JOHN D. G A U L T ~ MARCOS A. LEITE

MARA R. RaZZATTI HUGO GALLARDO

Departamento de Ffsica Universidade Federal de Santa Catarina 88000 Florian6polis, SC, Brazil Received October 3, 1987; accepted April 27, 1987

REFERENCES

1. Chem. Eng. News 63(3), 29 (1985). 2. Saupe, A., Nuovo Cimento 30(1), 16 (1984).

Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988

~To whom correspondence should be addressed.