Phase Behavior and Characterization of the System Acetic Acid

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Phase Behavior and Characterization of the System Acetic Acid-Dodecylamine-Water S. Karlsson,*,† R. Friman,† M. Bjo¨rkqvist,‡ B. Lindstro¨m,§ and S. Backlund† Department of Physical Chemistry, A° bo Akademi University, Porthaninkatu 3-5, FI-20500 Turku, Finland, Laboratory of Industrial Physics, Department of Physics, University of Turku, FI-20014 Turku, Finland, and Department of Chemistry and Process Technology, Mid Sweden University, SE-85170, Sundsvall, Sweden Received November 15, 2000. In Final Form: February 23, 2001 The phase behavior of carboxylic acid/alkylamine mixtures in water is largely dictated by a proton transfer from the acid to the amine, which will lead to charged species. If the distribution of carbon atoms is unequal between the acid and the amine, the result will be an ionic surfactant with an organic counterion. In this work the phase diagram for the ternary system acetic acid-dodecylamine-water at 298.2 K has been determined. Dodecylamine forms a lamellar phase with water, but when acetic acid is added up to an equimolecular ratio between acid and amine, three new phases appear. These phases are an isotropic solution phase, a hexagonal liquid crystalline phase, and a cubic liquid crystalline phase. These three phases are not able to incorporate any excess amine. The solution phase shows the existence of micelles, which are spherical at high dilution, but show an elongation close to the phase border to the hexagonal phase.

Introduction Already in 1942 Ralston et al.1 determined the binary phase diagram for dodecylamine-water, showing a general trend that medium- and long-chained alkylamines form a lamellar liquid crystalline phase in contact with water. Since then, dodecylamine has been studied in different fields, among them flotation2,3 and recently as a molecular template in the synthesis of mesoporous silica material.4 If an amine is combined with a carboxylic acid, a proton transfer from the acid to the amine with a subsequent formation of a complex in an equimolecular ratio will take place.5-12 For medium chain length alkylamines and carboxylic acids with equal numbers of carbon atoms in the alkyl chain, the formed equimolecular complex shows surfactant properties and forms a lamellar liquid crystalline phase in contact with water.6-8,11,12 In a ternary A° bo Akademi University. University of Turku. § Mid Sweden University. * Corresponding author. Phone +358 2 215 4298; fax +358 2 215 4706; e-mail [email protected]. † ‡

(1) Ralston, A. W.; Hoerr, C. W.; Hoffman, E. J. J. Am. Chem. Soc. 1942, 64, 1516. (2) Laskowski, J. S. In Advances in Flotation Technology; Parekh, B. K., Miller J. D., Eds.; Society for Mining, Metallurgy, and Exploration: Littleton, CO, 1999; p 59. (3) Pugh, R. J. Colloids Surf. 1986, 18, 19. (4) Tanev, P. T.; Pinnavia, T. J. Science 1995, 267, 865. (5) Huyskens, P.; Felix, N.; Janssens, A.; Van den Broeck, F.; Kapuku, F. J. Phys. Chem. 1980, 84, 1387. (6) Friberg, S.; So¨derlund, G. Kolloid Z. Z. Polym. 1971, 243, 56. (7) Sarthz-Lincoln, B.; Friberg, S. 6th International Congress on Surface Active Substances; Carl Hanser Verlag: Mu¨nchen, 1973; Vol. II. (8) Friberg, S. E.; Sun, W. M.; Yang, Y.; Ward, A. J. I. J. Colloid Interface Sci. 1990, 139, 160. (9) Sœten, J. O.; Sjo¨blom, J.; Gestblom, B. J. Phys. Chem. 1991, 95, 1449. (10) Backlund, S.; Karlsson, S.; Sjo¨blom, J. J. Disp. Sci. Technol. 1994, 15, 561. (11) Backlund, S.; Friman, R.; Karlsson, S. Colloids Surf. A: Physicochem. Eng. Aspects 1997, 123-124, 125. (12) Karlsson, S.; Backlund, S.; Friman, R. Colloid Polym. Sci. 2000, 278, 8.

phase diagram at 298.2 K for a system carboxylic acid (Cn-1COOH)-alkylamine (CnNH2)-water, where n g 7, the diagram is dominated by a large lamellar liquid crystalline phase, which extends from the amine-water binary axis up to an equimolecular ratio between the acid and the amine. At excess acid, however, the lamellar phase is destabilized. The lamellar phase is in equilibrium with almost pure water through a large two-phase region. If, however, the carbon atom balance between the acid and the amine is altered, other liquid crystalline phases will also be present.10 Alkylammonium surfactants have been studied extensively.13-20 In a work by Broome et al.13 they investigated the binary phase behavior for dodecylammonium chloride-water and found an isotropic solution region and a hexagonal and a lamellar liquid crystalline phase. Compared to a trimethylammonium surfactant, the absence of isotropic cubic liquid crystalline phases is striking.21 Another feature is that, by adding methyl groups to the polar headgroup, the Krafft point of the surfactant was lowered. The Krafft point and solubility of the dodecylammonium chloride surfactant has a strong pH dependence as pointed out by Dai and Laskowski.14 Both the Krafft point and the solubility were lowered at higher pH, and above pH 10 no micelles were formed, regardless of the temperature. A replacement of the halide counterions by organic counterions will influence the self-assembly of alkylammonium surfactants.15-20 If the chloride ion in dodecylammonium chloride is replaced by a perfluoroacetate ion, (13) Broome, F. K.; Hoerr, C. W.; Harwood: H. J. J. Am. Chem. Soc. 1951, 73, 3350. (14) Dai, Q.; Laskowski, J. S. Langmuir 1991, 7, 1361. (15) Furuya, H.; Moroi, Y.; Sugihara, G. Langmuir 1995, 11, 774. (16) Sugihara, G.; Arakawa, Y.; Tanaka, K.; Lee, S.; Moroi, Y. J. Colloid Interface Sci. 1995, 170, 399. (17) Jansson, M.; Stilbs, P. J. Phys. Chem. 1985, 89, 4868. (18) Jansson, M.; Stilbs, P. J. Phys. Chem. 1987, 91, 113. (19) Jansson, M.; Li, P.; Henriksson, U.; Stilbs, P. J. Phys. Chem. 1989, 93, 1448. (20) Jansson, M.; Jo¨nsson, B. J. Phys. Chem. 1989, 93, 1451. (21) Balmbra, R. R.; Clunie, J. S.; Goodman, J. F. Nature 1969, 222, 1159.

10.1021/la001594m CCC: $20.00 © 2001 American Chemical Society Published on Web 05/11/2001

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there will be a small increase in the Krafft point and a decrease in the critical micellar concentration (cmc).15 On the other hand, if the ion is changed to methylsulfonate, the cmc is almost unchanged.16 If the alkyl chains of the organic counterions are lengthened, i.e., the hydrophobicity of the counterions is increased, there will be both a reduction of the cmc and an increase in the fraction of bound counterions to the micelle.15-20 This behavior has theoretically been explained with an increased cosolubilization of the organic counterion as the number of carbon atoms in the alkyl chain of the counterion increases.20 According to this model, the fraction of acetate ions in the micelle at cmc for the decylammonium acetate surfactant is 0.30. On the other hand, this has not been observed for the dodecyltrimethylammonium surfactants, where the cmc is higher and the counterion binding lower for acetate as a counterion than for bromide.22 This was explained in terms of a stronger hydration of the acetate ion than of the bromide ion. In an earlier work, a partial determination of the ternary phase diagram for the system acetic acid-dodecylaminewater was made.10 It was considered relevant to determine the complete phase diagram for this system. Furthermore, a characterization of the different phases present in the system was considered important, especially the waterrich region of the isotropic solution region, to verify whether these systems form micelles or not. Experimental Section Materials. Acetic acid (>99.5%) and dodecylamine (g98%) were purchased from Fluka, Switzerland. Heavy water, with an isotopic purity of 99.9%, was purchased from Larodan Chemicals, Sweden. Hexamethyldisiloxane (98%) was purchased from Merck-Schuchardt. The chemicals were used without further purification. Water was distilled and deionized immediately before use. Phase Diagram. The samples for the phase diagram were weighed into glass ampules, which were immediately flamesealed. The samples were heated and centrifuged back and forth several times before equilibrated in a water bath at 298.2 K. The samples were allowed to equilibrate for at least 6 months before being examined visually between crossed polarizers. Liquid crystalline samples were also examined in an optical microscope between crossed polarizers. SAXS. The small-angle X-ray scattering (SAXS) measurements were carried out at 298.2 ( 0.5 K with an integrated camera system equipped with Kratky slit collimation and a linear, position-sensitive detector. Sample and optics were contained in a vacuum bed. An anode producing Cu KR X-rays with a wavelength of 0.154 nm was used. A Ni filter was employed in order to remove undesired Cu Kβ rays. A sample cell designed for pastes was used for the cubic phase. A cooking foil (Terinex Ltd, England) with a thickness of 12 µm was used as windows in the sample cell. Disposable capillaries with an outer diameter of 2 mm and a wall thickness of 0.01 mm were used for the lamellar and hexagonal phases. Pulsed Field Gradient Spin-Echo (PFG SE) NMR. The PFG SE technique is convenient for simultaneous determination of the self-diffusion coefficients of all components in a multicomponent solution.23 The self-diffusion coefficients reflect the microstructure of a solution and can be used to analyze micellar solutions.24 The pulse sequence used is known as the StejskalTanner experiment and consists of two radio-frequency (rf) pulses, a 90° pulse applied at time t ) 0 and a 180° pulse at time t ) τ, which gives rise to an echo signal at t ) 2τ because of a refocusing of the transverse magnetization. In addition, two field-gradient pulses are applied with time duration δ and separation ∆ between (22) Brady, J. E.; Evans, D. F.; Warr, C. G.; Grieser, F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 1853. (23) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1. (24) Lindman, B.; Puyal, M.-C.; Kamenka, N.; Rymde´n, R.; Stilbs, P. J. Phys. Chem. 1984, 88, 5048.

Karlsson et al. their leading edges, placed on either side of the 180° rf pulse. The echo intensities are then fitted to the following equation:

I ) I0 exp[-γ2G2δ2(∆ - δ/3)D]

(1)

where γ is the magnetogyric ratio, G is the gradient strength, and I0 is the intensity in the absence of gradient pulses. In the experiment G and ∆ were kept constant while G was varied. The measurements were done on a Bruker spectrometer operating at a 1H frequency of 250 MHz. In the experiment, peaks from the dodecylamine and acetic acid could be resolved, giving the self-diffusion coefficients for the acid and the amine separately. To monitor the micelle diffusion, a hydrophobic probe, HMDS, was added. Because of its hydrophobicity, HMDS is solubilized inside the micelles. The diffusion coefficient of HMDS will then equal the diffusion coefficients of the micelles. Self-diffusion coefficients for HMDSfree solutions were also measured in order to check that the probe did not affect the diffusion coefficients of the other components. No changes could be observed. Viscosity and Conductivity. The viscosities were measured with Ostwald or U ¨ bbelohde viscometers. The conductivities were determined using a Wayne-Kerr bridge with automatic recording of the resistance. All measurements were carried out at 298.2 K.

Results and Discussion Phase Diagram. The phase behavior of acid-aminewater systems is dictated by a proton transfer from the acid to the amine, giving new properties to the molecules. A similar approach has also been used by others.25-27 The phase diagram for the system acetic acid-dodecylaminewater at 298.2 K is shown in Figure 1a. It consists of four one-phase regions, one isotropic solution region (L), and three liquid crystalline regions, namely a lamellar (D), a hexagonal (E), and a cubic (cub.). In fact, the phase diagram can be divided into two subregions according to behavior. The dividing line between the two subregions starts at the water corner and has a binary acid mass fraction of around wacid,bin ) 0.1. As was shown already by Ralston et al.,1 dodecylamine forms a lamellar liquid crystalline phase with water, but the phase boundaries in the binary water-dodecylamine system determined in this work are quite different. The lamellar phase extends between mass fractions of amine, wamine ) 0.74 and 0.77; i.e., the lamellar region is substantially smaller than that of Ralston et al.1 On addition of small amounts of acetic acid there is a marked swelling of the lamellar phase toward the water corner. However, the region is very narrow, making it difficult to determine the exact phase boundaries of this phase. This swelling is a consequence of a proton transfer from the acid to the amine leading to charged lamellae. On further addition of acetic acid the lamellar phase is destabilized. When an equimolecular ratio between the acid and the amine (wacid,bin ) 0.245 in the binary acidamine system) is reached, three new phases are formed. These are a solution phase (L), a hexagonal liquid crystalline phase (E), and a cubic liquid crystalline phase (cub.). These phases form close to the equimolecular ratio between the acid and the amine and are relatively stable at excess acid. In fact, the solution region covers more than half of the phase diagram due to the mutual solubility of acetic acid and water. Lamellar Phase, D. The lamellar phase is formed in the binary system water-dodecylamine. The diffracto(25) Bergmeier, M.; Hoffmann, H.; Thuning, C. J. Phys. Chem. B 1997, 101, 5767. (26) Beck, R.; Gradzielski, M.; Horbaschek, K.; Sakhawat Shah, S.; Hoffmann, H.; Strunz, P. J. Colloid Interface Sci. 2000, 221, 200. (27) Fukuda, K.; Kawasaki, M.; Kato, T. Maeda, H. Langmuir 2000, 16, 2495.

Acetic Acid-Dodecylamine-Water

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damine ) φamined aamine ) 2

Figure 1. (a) Ternary phase diagram at 298.2 K for the system acetic acid-dodecylamine-water. (b) Part of the phase diagram at 298.2 K for the system acetic acid-dodecylamine-water. The two lines drawn in the phase diagram indicate two constant binary acid mass fractions. Table 1. Length per Amine Molecule (lamine) and Area per Amine Molecule (aamine) for Some Samples in the Lamellar (D) and Hexagonal (E) Phasesa phase

wacid,bin

wwater

l [nm]

a [nm2]

D D E E E E E E E E E E E

0 0.01 0.245 0.245 0.245 0.245 0.245 0.245 0.30 0.30 0.30 0.30 0.30

0.239 0.238 0.50 0.54 0.58 0.62 0.66 0.70 0.50 0.53 0.56 0.59 0.62

1.69 1.68 1.51 1.48 1.48 1.48 1.46 1.45 1.36 1.38 1.36 1.38 1.37

0.19 0.20 0.435 0.441 0.443 0.444 0.448 0.453 0.480 0.473 0.482 0.476 0.480

a

grams measured with SAXS from samples in this phase show the characteristic features of a lamellar phase, i.e., diffraction peaks with relative positions 1:2:3. At the phase border on the water-poor side of the lamellar phase, the number of water molecules per amine molecule is three. This number is close to the hydration number of the amine group.28 The length (lamine) and the area (aamine) per amine molecule for two samples in the lamellar phase can be seen in Table 1. The parameters have been calculated using the following equations based on simple geometry (28) Laughlin, R. L. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1978; Vol. 3, p 41.

(3)

where d is the measured d spacing of the sample, damine is the thickness of the bilayer, φamine is the volume fraction of amine, and vamine is the volume per amine molecule. The length of the amine molecule is then calculated as half of the bilayer thickness. The volume of the amine molecule is calculated from the density of the pure compound. As can be seen from Table 1, the amine molecule is in a rather extended and tightly packed state. The length of a fully extended alkyl chain according to Tanford29 with 12 carbon atoms is 1.675 nm. Assuming all acetic acid reacts with the amine, a fraction of about 0.04-0.05 of the amine in the lamellae is charged at the maximum acid uptake. This charge is responsible for the swelling of the lamellar phase toward the water corner. The lamellar phase is destabilized for rather small additions of acetic acid. This is due to the change in optimum area per surfactant molecule on introducing charges in the lamellae. An increase in the optimum area per headgroup will favor the formation of phases where the curvature of the interface between the hydrophobic and hydrophilic regions is positive. Solution Phase, L. Dodecylamine itself will not form micelles in contact with water. The fact that the behavior of acid-amine systems is governed by a proton transfer from the acid to the amine, as showed before,12 will lead to the formation of dodecylammonium acetate (DaAc). This compound can be considered a cationic surfactant with an organic counterion, which would indicate an ability to form micelles. The PFG SE-NMR method is convenient for verifying the existence of micelles.23 The self-diffusion coefficients were measured in the solution phase L along a line with an equimolecular ratio between the acid and the amine as seen in Figure 1b. The diffusion coefficients for the different components in samples along this line together with the diffusion coefficient for the hydrophobic probe HMDS as a function of the mass fraction of DaAc, wDaAc, are shown in Figure 2. As can be seen, the diffusion of water is high and comparable to the diffusion of neat water while the diffusion of the dodecylammonium ion (Da+) is about 1 order of magnitude less. This fact indicates the existence of discrete micelles in this system. Already a signal from HMDS in the NMR measurements supports the existence of micelles since in the absence of micelles the HMDS would phase separate and give no signal.17 The observed diffusion coefficient for one component is a weighed mean value of contributions of the component in different sites according to eq 4

The parameters have been calculated using eqs 2, 3, 14, and

15.

vamine φamined

(2)

Dobs )

∑i PiDi

(4)

where Pi is the fraction of the component in site i. In micellar systems a two-site model is often used and eq 4 can be written

Dobs ) PmicelleDmicelle + (1 - Pmicelle)Dfree

(5)

where Pmicelle is the fraction of the component associated with the micelle and Dmicelle and Dfree are the diffusion (29) Tanford, C. In The Hydrophobic Effect. Formation of Micelles and Biological Membranes, 2nd ed.; John Wiley & Sons: New York, 1980.

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Figure 2. Diffusion coefficients, D, at 298.2 K as a function of the mass fraction of DaAc, wDaAc, for the different components in the system water-DaAc.

Figure 3. Relative diffusion coefficient for water, Dwater/D0,water, at 298.2 K as a function of the volume fraction of the micelles, φmicelle, in the system water-DaAc. The solid line is not a result of a mathematical model but drawn as an aid to the eye.

coefficients of the micelle and of the freely diffusing fraction.30,31 The measured diffusion coefficient of Da+ thus has a contribution both from the freely diffusing Da+ and from the fraction associated in the micelles. At low DaAc contents the fraction of the freely diffusing Da+ is significant, and the observed diffusion coefficient for Da+ is larger than the one for the micelle (HMDS). When the DaAc content is increased, the fraction of free Da+ becomes negligible, and the surfactant and the micelle diffusion coefficients are (almost) equal. The Ac- has a diffusion coefficient between those for water and Da+. This reflects the fact that a fraction of the Ac- kinetically is a part of the micelle while a fraction is free, as will be shown below. With increasing DaAc content one can see a decrease in both the micellar diffusion and the water diffusion. The decrease in micellar diffusion with increasing volume fraction of micelles is largely due to repulsion between the micelles.32 The decrease in water diffusion has two main contributions. First of all, there is an obstruction effect caused by a hinderence in diffusion of water by the increased volume fraction of the micelles. This effect has theoretically been described by Jo¨nsson et al.33 for different micellar shapes. Second, the hydration of the surfactant headgroups and counterions will cause a decrease in the water diffusion, an effect becoming stronger as the water/ surfactant ratio becomes smaller. In Figure 3 the relative diffusion coefficient for water, Dwater/D0,water where D0,water is the diffusion coefficient for neat water, is shown. As can be seen, the relative diffusion coefficient decreases with increasing volume fraction of micelles (φmicelle) to a value of about 0.65, which is close to the theoretical obstruction by oblate-shaped micelles in the absence of hydration.33 However, in this case also hydration of both the ammonium group and the acetate ion will reduce the water diffusion, and no conclusion regarding the shape of the micelles can be drawn. To analyze the micellar diffusion, the following equations have been used:34,35

Dsphere ) D0sphere(1 - k1φsphere)

(30) Lindman, B.; So¨derman, O.; Wennerstro¨m, H. In Surfactant Sience Series; Zana, R., Ed.; Marcel Dekker: New York, 1987; Vol. 22, p 295. (31) Lindman, B.; Olsson, U.; So¨derman, O. In Handbook of Microemulsion Science and Technology; Kumar, P., Mittal, K. L., Eds.; Marcel Dekker: New York, Basel, 1999; p 309. (32) So¨derman, O.; Hansson, E.; Monduzzi, M. J. Colloid Interface Sci. 1991, 141, 512. (33) Jo¨nsson, B.; Wennerstro¨m, H.; Nilsson, P. G.; Linse, P. Colloid Polym. Sci. 1986, 264, 77. (34) Nyde´n, M.; So¨derman, O. Langmuir 1995, 11, 1537. (35) Nilsson, F. Thesis, Lund University, 1998.

(

0 1 - k2 Dprolate ) Dprolate

D0sphere )

φprolate A3B

( ))

+ k3

kBT 6πηR

0 Dprolate ) D0sphereA

(6)

φprolate A3B

2

(7)

(8) (9)

where φi is the volume fraction, B is the ratio between the major and minor axis in a prolate, kB is the Boltzmann constant, T is the temperature, η is the viscosity of the solvent, R is the minor hydrodynamic radius (for a sphere R is the hydrodynamic radius), k1, k2, and k3 are constants, and A is defined as

A)

ln(B + xB2 - 1)

xB2 - 1

(10)

Using 2 nm for R, 2.2, 2.9, and 2.1 for k1, k2, and k3 respectively, and 1.2 and 3 for B, a comparison between experimental and theoretical micellar diffusion coefficients is given in Table 2. The choice of values for k1, k2, and k3 has been “arbitrary” in the sense that to the authors’ knowledge no values for these constants have been reported in the literature for the system under consideration. This is, however, not so critical since the results are rather insensitive to these values and do not affect the general conclusions drawn in this paper. As can be seen, the micellar diffusion is high at low DaAc contents, indicative of spherical micelles. When the DaAc content is increased, the model for spherical micelles can no longer explain the results. Instead, the prolate model describes the results better. This also supports the idea that the decrease in water diffusion is largely due to hydration. In Figure 4 the relative viscosity, ηrel ) ηsample/ηwater, and conductivity along the equimolecular line is seen. The viscosity is sensitive to changes in micellar size. An abrupt increase in viscosity is often interpreted as a micellar growth.36,37 In this system the viscosity increases steeply at a mass fraction of DaAc of about 0.175-0.20. Also, the conductivity shows a change in behavior at similar mass (36) Kabir-ud-Din; Bansal, D.; Kumar, S. Langmuir 1997, 13, 5071. (37) Kabir-ud-Din; Kumar, S.; Kirti; Goyal, P. S. Langmuir 1996, 12, 1490.

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Table 2. Measured and Calculated Micellar Diffusion Coefficients in the Solution Phase (L) along the Equimolecular Linea wacid,bin wDaAc Dmicelle [m2 s-1] 0.245 0.245 0.245 0.245 0.245 0.245 0.245

0.05 0.075 0.10 0.125 0.15 0.175 0.20

2.10E-10b 1.68E-10 1.06E-10 6.89E-11 5.67E-11 1.30E-11 1.46E-11

Dtheor,sphere Dtheor,prolate1 Dtheor,prolate2 [m2 s-1] [m2 s-1] [m2 s-1] 1.37E-10 1.31E-10 1.24E-10 1.18E-10 1.12E-10 1.05E-10 9.92E-11

8.33E-11 7.41E-11 6.54E-11 5.73E-11 4.98E-11 4.28E-11 3.64E-11

5.49E-11 4.89E-11 4.32E-11 3.78E-11 3.29E-11 2.82E-11 2.40E-11

aThe calculated values are obtained for a sphere and for prolates with B ) 1.2 (prolate 1) and B ) 3 (prolate 2). b Read as 2.10 × 10-10.

Figure 5. Fraction of bound acetate, Pb,acetate, at 298.2 K as a function of the mass fraction of DaAc, wDaAc, in the system water-DaAc.

Figure 4. Relative viscosity, ηrel, and conductivity, κ, at 298.2 K as a function of the mass fraction of DaAc, wDaAc, in the system water-DaAc. The solid lines are not a result of a mathematical model but drawn as an aid to the eye.

fractions. This can then be regarded as a formation of rodlike micelles.38,39 This is also well in agreement with the phase behavior of the system since the narrow twophase region between the solution phase and the hexagonal liquid crystalline phase indicates a resemblance between the aggregate shapes in both phases.32 Finally, an estimation of the fraction of bound counterions is made, based on the diffusion measurements. The fraction of bound acetate can be written as

Pacetate )

cb,acetate cb,acetate ) cb,amine ctot,amine - cfree,amine

(11)

where cb,i is the concentration of the component bound to the micelle. To obtain Pacetate, one would have to know the cmc of dodecylammonium acetate. However, this has not been determined in this work. Using the two-site model, the fraction of bound acetate can be written

Pb,acetate )

Dfree,acetate - Dobs,acetate Dfree,acetate - Dmicelle

(12)

The value obtained with eq 12 corresponds to

Pb,acetate )

cb,acetate cb,acetate ) ctot,acetate ctot,amine

(13)

The last equality follows from the fact that the samples (38) Backlund, S.; Høiland, H.; Kvammen, O. J.; Ljosland, E. Acta Chem. Scand. 1982, A36, 698. (39) Backlund, S., unpublished results.

are along the equimolecular line. For large concentrations (ctot,amine . cfree,amine) eqs 11 and 13 can be considered equal. In Figure 5 the fraction of bound acetate calculated using eq 12 is shown. As can be seen, Pb is more or less constant for higher content of DaAc, much in agreement with earlier work on both inorganic24 and organic counterions.17 The value of around 0.76 also seems reasonable compared to work done on decylammonium acetate at 311.2 K.17,18 It is thus conluded that along the equimolecular line the micelles formed by dodecylammonium acetate are spherical in shape over a large concentration region with a change to rodlike micelles at higher mass fractions of the equimolecular compound. The fact that there is no increase in the diffusion of HMDS at high mass fractions of the equimolecular compound indicates that there is no exchange of HMDS between the micelles. The Hexagonal Phase, E. The hexagonal phase appears for an equimolecular ratio between the acid and the amine. For this ratio, the phase is stable for water mass fractions, wwater, between 0.43 and 0.74. The diffractograms of samples in this phase show the relative positions of the peaks of 1:31/2:2, indicative of a hexagonal phase. For a hexagonal phase the following equations based on simple geometric considerations apply

damine ) r

(

)

(14)

vamine r

(15)

31/2π 2φamine

aamine ) 2

1/2

where r is the radius of the hexagonally packed tubes. From eq 14 one can see that the logarithm of the d spacing (log d) should vary linearly with the logarithm of the volume fraction of amine (log φamine) and that the slope of the line should ideally be -1/2.40 If a swelling experiment is performed along a line with constant acid:amine ratio and the water content is varied, then log d should vary linearly with log φamine. In Figure 6 the results from such an experiment is seen for two different acid:amine ratios (A, wacid,bin ) 0.245 and B, wacid,bin ) 0.30; see also Figure 1b). The slopes of the fitted lines to the data points are -0.43 for A and -0.50 for B. The fact that the slopes are this close to “ideal” indicates that the swelling takes place (40) Hyde, S. T. Colloid Surf. A: Physicochem. Eng. Aspects 1995, 103, 227.

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and compares it with the “ideal phase sequence”42 for a binary surfactant-water system as suggested by Fontell, one could argue that the structure of the cubic phase has to be a bicontinuous one. Conclusion

Figure 6. Logarithm of the Bragg spacing d in the hexagonal phase, log d, at 298.2 K as a function of the logarithm of the volume fraction of dodecylamine, log φamine, for two different binary acetic acid mass fractions in the system acetic aciddodecylamine-water.

without change in the area per polar headgroup, which is also seen from Table 1, where the length () r) and the area per amine molecule are presented. It is, however, not proof of a defect-free hexagonal phase.41 This analysis is based on the approximation of a constant molar volume of amine. The Cubic Phase. As was already mentioned, the cubic phase in the system is formed for an equimolecular ratio between acid and amine and a water mass fraction of about 0.4. The phase is stable for a small excess of acid, but the extension of the phase is small in all directions. The samples in this phase are very viscous and optically isotropic when viewed between crossed polarizers. This is in accordance with samples of cubic phases.42,43 However, the number of reflections in the SAXS diffractogram is too low for an unambigious determination of the space group for this phase. If one considers the position of this phase in the phase diagram, on the water-poor side of the hexagonal phase, (41) Hyde, S. T.; Fogden, A. Prog. Colloid Polym. Sci. 1998, 108, 139. (42) Fontell, K. Colloid Polym. Sci. 1990, 268, 264. (43) Fontell, K. Adv. Colloid Interface Sci. 1992, 41, 127.

In this work the ternary phase diagram for the system acetic acid-dodecylamine-water has been determined and characterized. The phase diagram can be thought of as consisting of two parts: one is the binary system dodecylamine-water with small additions of acetic acid, and the other is the equimolecular ratio acid:aminewater-excess acid. These two subsystems do not mix. The binary amine-water system consists of only one one-phase region, a lamellar liquid crystalline phase which will swell on addition of small amounts of acetic acid. This swelling is a result of a charging up of the lamellae due to a proton transfer from the acid to the amine. For an equimolecular ratio between the acid and the amine, three new phases appear: a micellar solution, a normal hexagonal phase, and a cubic liquid crystalline phase. These phases are characteristic for an ionic surfactant with one alkyl chain. The acid:amine equimolecular mixture can be considered a cationic surfactant with an organic counterion after the proton transfer. The micelles formed for DaAc in water are spherical up to mass fractions of DaAc of about 0.1, followed by a small elongation which seems to increase very close to the phase border to the hexagonal phase. However, the methods employed in this work do not give a clear answer in this matter. In the hexagonal phase the swelling of the phase takes place without a change of the area per surfactant. The point group of the cubic phase could not be determined, but it is believed that the phase has a bicontinuous structure. Acknowledgment. Gerd Persson is acknowledged for help with the NMR measurements. S.K. acknowledges The Nordic Academy for Advanced Study (NorFa), and M.B. acknowledges The Ministry of Education (Finland) (GSMR) for financial support. LA001594M