Langmuir 1994,10, 2025-2027
2025
Notes Surfactant Aggregation in Formamide Consuelo Gamboa,' Hernb Rios, and Victor Sanchez Departamento de Qulmica, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Casilla 325, Santiago, Chile Received February 1, 1993. In Final Form: February 23, 1994
Introduction Molecular aggregationof amphiphiles in organicsolvents is a subject of increasing interest, particularly in comparison with aggregation patterns in water.ld Evans et alm6 proposed that hydrogen bonding must play a fundamental role in amphiphile self-assembly. Others' claim that normalmicelles seems to be absent in pure formamide. However, their optional temperature was below the Krafft point of the corresponding surfactants in pure formamide. Rico et al.' determined the critical micelle concentrations (cmc) of cetyltrimethylammonium bromide (CTABr) and sodium dodecyl sulfate (SDS)in pure formamide at 60 "C. They showed that cmc's and Krafft temperatures of these surfactants were higher in formamide than in water. Only a few studies have been published on specific counterion effects on micellar properties in organic solvents. The aggregation properties of alkyltrimethylammonium bromides and alkylpyridinium bromides in ethylene glycol were investigated using membrane selective electrodes.* In formamide, the micellar aggregates of cetylpyridinium chloride are much smaller than in water.O A 2H NMR relaxation study of 2H-labeled SDS'O show that above the Krafft point, SDS micelles grow with increasing concentration. Solubility and electrical conductivity measurements in ethylene glycol, formic acid, and formamide have been used to estimate an operational critical micelle concentration (cmcop)of several cationic surfactants.5 Evans and Evans1' measured the forces between dihexadecyldimethylammoniumbilayers on mica in formamide solutions containing bromide and acetate counterions. They reported that the unusual counterion effects observed for amphiphiles in aqueous solutions do not result from the unique structural properties of water. Here we explore the solvent and specific counterion effect on aggregation of cetyltrimethylammonium chloride (CTACl),CTABr, and cetyltrimethylammonium tosylate (CTAOTOS) in formamide above their Krafft points. (1)Almgren, M.; Swarup, S.; Lofroth, J. E. J. Phys. Chem. 1986,89, 4621. (2)Sjoberg, M.; Henribson, U.; Warnheim, T. Langmuir 1990,6(7), 1205.
(3)Singh, H. N.;Saleem, S. M.; Singh, R. P.; Birdi, K. 5.J. Phys. Chem. 1980,84,2191. (4)Warnheim, T.;Jonsson, A.; Sjoberg, - M. Prog. Colloid Polym. Sci. 1990,82,271. (5)Bmana-Limbele, W.; Zana, R. Colloid Polym. Sci. 1989,267,440. (6)Bwley, A. H.; Evans, D. F.; Laughlii, R. G.J. Phys. Chem. 1988, 92,791. (7) Rico, I.; Lattes, A. J. Phys. Chem. 1986,90,5870. (8)Gharibi, H.;Palepu, R.; Bloor, D. M.; Hall,D. G.;Wyn-Jones, E. Langmuir 1992,8,782. (9)Thomason, M. A.;Bloor, D. M.; Wyn-Jones, E. Langmuir 1992,8, 2107. (10)Ceglie, A.;Colafemmina,G.;Della Monica, M.; Olsson, U.; Jonsson, B. Langmuir 1993,9,1449. (11)Evans, J. B.; Evans, D. F. J. Phys. Chem. 1987,9I,3828.
These Surfactants were selected because their physicochemical properties in water are ~ell-known.~2J3 Experimental Section CTABr and CTAOTOS were purchased from Sigma. CTACl was obtained by passing an aqueous solution of CTABr through a Dowex 1x8-200 ion exchange resin (Aldrich Chemical Co.) pretreated with HC1. The elutant was then dried and recrystallized. CTACl purity was checked by surface tension. No minimum appears in the surface tension-log [CTACD plot and thecmcobtained (1.3 X 1VM)isingoodaccordwiththereported value.l2 CTABr and CTAOTOSwere crystalbd in ethylacetate/ ethanol. This method for preparing CTACl has been employed previously.14 All the surfactantswere dried under vacuum before used. Krafft temperatures were determined in the following way: several test tubes containing 10 mL of formamide-surfactant mixtures were slowly heated with constant agitation; the temperature at which all surfactant completely dissolved was recorded. The solutionsunder agitation were slowly cooled and the temperature at which the surfactant precipitated from the solution was recorded. Precipitation was detected using a HeNe laser and observing the onset of light scattering of the fist formed crystal at a 90' angle with respect to incident light.' Conductivity K and surfacetensions T were measured by using a Radiometer CDM 83 conductometer and in a K r ~ D-8 s tensiometer, respectively. cmc's were obtained from breaks in the electricalconductivityand in the surfacetension plots. The surfacetension of pure formamide at 60 O C was 57.0 mN/m, very close to the reported value of 58.2 mN/m at 20 OC.16 Surface excess r and polar head area A0 of the surfactant at the formamide-air or water-air interfaces were calculated from the Gibbs treatment of the surface tension r data below the cmc.16 Densities were measured with a 50-mLLipkin-likedensimeter with an accuracy of f0.00005 g/mL. From density data of the surfactant solutions, the apparent molar volumes (&) were obtained as before,12by using the following relationship = (l/d)M2- (1OOO/c)(d- do)/do
(1)
where Mz is the molecular weight of the solute, d the density of the solutionof molar concentration c, and do is the solventdensity. The partial molar volumes VZat differentc values were calculated from V , = 4:
+3~~.~/2(d~Jdc~.~)
(2)
The &values were plotted against cossto obtain the slope and values from eq 2. &o is the apparent molar volume extrapolated to zero concentration. &o
Results and Discussion Figure 1shows the specific conductivity versus surfactant concentration plot for CTABr and CTAOTOS in formamide at 60 OC; Figure 2 shows the corresponding plots for surface tension data. The change in the slope has been interpreted as a cmc? but there is some controversy about the real meaning of this concept in (12)Gamboa, C.; Rios, H.;Sepulveda, L. J. Phys. Chem. 1989,93, 5540. (13)Gamboa,C.; Sepulveda, L. J. Colloid Interface Sci. 1986,113, 566. (14)Olea, A.;Gamboa,C. Langmuir 1993,9,2066. (15)Handbook of Chemistry and Physics, "1st ed.; Lide, David R., Ed.;CRC Press: Boca +tan, FL, 1.990. (16)Adamson, A. Physical Chemietry of Surfaces;Wiley: New York, 1990. (17)Olofsson, G.J . Chem. SOC.,Faraday Trans. 1991,87,3037.
0743-7463/94/2410-2025$04.50/00 1994 American Chemical Society
Notes
2026 Langmuir, Vol. 10, No. 6, 1994
Table 1. Kraft Temperature and cmc for CTACl, CTABr, and CTAOTOS in (A) F o d d e and (B) Water
A surfactant CTACl CTABr CTAOTOS
2,
.-e .->
y 2.0 -
c
Krafft temp, O C 22.0 43.0 50.0
cmc at 60 O C , M 0.085,4 0.0~6)0.095~ 0.065,O 0.070) 0.07oC
B
P C
surfactant Krafft temp, O C cmc at 25 O C , d M CTACl
