L a n g m u i r 1996,11, 1835-1839
1835
Notes Phase Behavior of Dodecyltrimethylammonium Bromidemater Mixtures
2. Experimental Section
K. M. McGrath* Department of Applied Mathematics, Research School of Physical Sciences a n d Engineering, The Australian National University, Canberra, ACT 0200, Australia Received November 29, 1994. I n F i n a l Form: February 6, 1995
1. Introduction
To fully understand the behavior of surfactants in water, it is necessary to determine the phase diagrams for series of surfactants which differ from each other in one characteristic only. In this way it is possible to determine the effect that a single change to the nature of the surfactant under certain conditions induces. To this end the phase behavior of the binary dodecyltrimethylammonium bromide (DTABYwater system has been investigated. Determination of this phase diagram completes the series of four surfactants DTAB, dodecyltrimethylammonium chloride (DTAC),1-9hexadecyltrimethylammonium bromide (CTAB),6,9-14and hexadecyltrimethylammonium chloride (CTAC),'J5 which differ only by the length of the hydrocarbon chain or the nature of the counterion. These latter three surfactants have been extensively studied for all concentrations and temperatures up to 100 "C. The DTAB/water system in contrast while having been widely studied in the low concentration regime5J6-23has had little work performed at higher concentrations and temperature^.^,^^-^^ Hence the phase
* Present address: Laboratoire de Minbralogie-Cristallographie, Universites de Paris-VI (Pierre-et-Marie-Curie) et de Paris-VII, Tour 16-4, Place Jussieu-case 115, 75252 Paris Cedex 05, France. (1)Broome, F. K.; Hoerr, C. W.; Harwood, H. J. J. Am. Chem. SOC. 1951, 73, 3350-3352. (2)Weiner, N. D.; Zografi, G. J. Pharm. Sci. 1965,54, 436-442. (3) Emerson, M. F.; Holtzer,A. J. Phys. Chem. 1967,71,1898-1907. (4) Balmbra, R. R.; Clunie, J. S.; Goodman, J. F. Nature 1969,222, 1159-1160. ( 5 ) Lee, K. H.; de Mayo, P. Photochem. Photobiol. 1980,31,311-314. (6) Blackmore, E. S.; Tiddy, G. J. T. J . Chem. Soc., Faraday Trans. 2 1988,84, 1115-1127. (7) Blackmore, E. S.; Tiddy, G. J. T. Liq. Cryst. 1990, 8 , 131-151. (8) Kang, C. Liq. Cryst. 1992, 12, 71-81. (9) Laughlin, R. G. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker, Inc.: New York and Basel, 1991; Vol. 2; pp 1-40. (10)Scott,A. B.;Tartar, H.V. J.Am. Chem. SOC. 1943,65,692-698. (11)Zana, R. J. Colloid Interface Sci. 1980, 78, 330-337. (12) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1981,84,100-107. (13) Evans, D. F.; Allen, M.; Ninham, B. W.; Fouda, A. J. Solution Chem. 1984,13,87-101. (14)Berr, S. S. J . Phys. Chem. 1987, 91, 4760-4765. (15) Henriksson. U.: Blackmore. E. S.: Tiddv. G. J. T.: Soderman. 0. J.Phys. Chem. 1992, 96, 3894-3902. (16)Klevens, H. B. J. Phys. Chem 1948,52, 130-147. (17)Debye, P. J. Phys. Colloid Chem. 1949, 53, 1-8. (18)Voeks, J. F.;Tartar, H.V. J. Phys. Chem. 1955,59,1190-1192. (19)Bruning, W.; Holtzer, A. J. Am. Chem. SOC.1961, 83, 48654866. (20)Schick, M. J. J. Phys. Chem. 1963, 67, 1796-1799. (21)Birdi, K. S. Acta Chem. Scand. A 1986,40, 319-321. (22) Brady, J. E.; Evans, D. F.; Warr, G. G.; Grieser, F.; Ninham, B. W. J. Phys. Chem. 1986,90, 1853-1859. (23) Zielinski, R.; Ikeda, S.;Nomura, H.; &to, S. J. CoZZoidInterfuce Sci. 1988,125,497-507. (24) Luzzati, V.; Reiss-Husson,F. Nature 1966, 210, 1351-1352. I
behavior of DTAB in water for all concentrations and temperatures between 20 and 100 "C is reported.
"
I
0743-7463/95/2411-1835$09.00/0
DTAB (99% pure) was purchased from Kodak Eastman Fine Chemicals and was further purified by washing in hot ethyl acetate followed by hot filtration to remove any organic contaminants. The surfactant was then recrystallized several times from absolute ethanol and dried under vacuum over phosphorus pentoxide yielding a white powder. Anal. Calcd for C 1 ~ H 3 a B r : C,58.43;H, 11.12;N,4.54;Br,25.91. Found: C,58.29;H, 10.88; N, 4.44; Br, 26.39. Decomposition temperature -135 "C. Electrical conductivity measurements were obtained using a TPS Conductivity Meter Model 2102A (Auto-Ranging, Auto Cell, K factor) which was calibrated using standard potassium chloride Sacm-l and 1 x solutions (0.1 M KC1, conductivity 1.290 x M KC1, conductivity 1.471 x Sscm-l). All samples were equilibrated to 25 f 0.1 "C. Surface tension measurements were performed using a du Nouy tensiometer where a platinum ring was employed. The surface tension was determined from the maximum force exerted on the ring without detachment ofthe meniscus.271~8All samples were equilibrated to 25 f 0.1 "C. All phase behavior was investigated initially by polarizing optical microscopy using the isothermal concentration gradient method.29 A small amount of the powder or melt was placed on a slide with a free cover slip; water was allowed to penetrate completely, at constant temperature. The phases were observed as bands of variable birefringence as the water evaporated. An Olympus BH-2 polarizing optical microscope with a Mettler FP82HT hot stage attached to a Mettler FPBOHT Central Processor capable of temperature control to f O . 1 "C was used. For each given temperature a new concentration gradient was performed. Bulk samples were prepared by adding weighed amounts ofthe surfactant and purified water into a glass ampule which was flame sealed. The samples were homogenized by continual heating (at 90 "C) and centrifugation over a period of several weeks. Samples of a given composition were also viewed under crossed polarizing filters (where all samples were sealed using Eccobond 286, a general purpose epoxy adhesive purchased from W. R. Grace and Co.) and t h e temperature was varied between 20 and 100 "C. The structure of the liquid crystalline phases was checked by small-angle X-ray scattering (SAXS) measurements using a Kiessig camera,3Owith point collimation. All diffraction patterns were recorded on X-ray sensitive film (CEA Reflex 25, double coated high speed film for direct X-ray exposure purchased from CEA AB Sweden). The camera is capable of recording simultaneously both small- (sample to film distance of 200 or 400 mm) and wide-angle (film placed in the range of 40 to 70 mm) scattering detection. The camera is attached to Philips generator 1120/ OOwithaCu(Ka1inewithA= 1.544 39A)finefocus highintensity source PW2213/20 using a nickel filter. Samples used for X-ray analysis were loaded into thin walled Lindemann X-ray transparent capillaries of internal diameter 0.7 or 1.0 mm which were sealed. Bulk samples were also sealed between two mica windows, resulting in an approximate sample thickness of 1mm. Powder diffraction patterns obtained using each of the three (25) Luzzati, V.; Tardieu, A.; Gulik-Krzywicki,T.; Rivas, E.; ReissHusson, F. Nature 1968,220, 485-488. (26) Luzzati, V. In Biological Membranes: Physical Fact and Function; Chapman, D., Ed.; Academic Press: London, 1968; pp 71123. (27) du Noiiy, P. L. J. Gen. Physiol. 1919, I, 521-524. (28) Furlong, D. N.; Freeman, P. A.; Metcalfe, I. M.; White, L. R. J. Chem. SOC.,Faraday Trans. 1 1983, 79, 1701-1719. (29)Rendall, K.; Tiddy, G. J. T.; Trevethan,A. J. Chem. SOC.,Faraday Trans. 1 1983, 79, 637-649. (30) Kiessig, V. H. Kolloid Z. 1942, 98, 213-221.
0 1995 American Chemical Society
Notes
1836 Langmuir, Vol. 11, No. 5, 1995 65
1.6
1.5
n
IE
n
E
;55
E
P
€>
v)
z
Lu
k
E3
W
0
3K
1.4
v
0
z
45
8
a
u)
-2.75
-2.25
-1.75
-1.25
Log[DTAB]
1.3
1.2
0.010
0.012
0.014
0.016
0.018
0.020
0 22
W A B I (M)
Figure 1. Surface tension of aqueous DTAB measured at 25 "C using the du Noiiy ring method. Errors are indicated by the size of the data points.
Figure 2. Specific conductivity as a function of DTAB concentration at 25 "C (errors in the measurements are of the order of the data points).
methods to house the sample were identical(apartfrom reflexions due to the mica windows).
gives the percentage dissociation as 34.3%. Hence, approximately 34% of all bromide counterions dissociate upon micellization. The cmc of DTAB as determined here ((1.5 f 0.1) x M) is comparable to values obtained by a number of other groups using a variety of techniques.5~10~11~13~16~zo~zz~ 3.2. DTAB Self-Assembly.Previous results for the phase progression in the DTAB/water system are scarce with the only two reported studies being performed by in the early 1960s and later by Blackmore Luzzatiet aZ.24325 and Tiddy.6 The results reported by Luzzati et a1.24,25 were for calculated structural parameters determined within the cubic phase (70 "C and 82 wt % DTAB). The phase was assigned as the bicontinuous cubic phase QZ3O (Ia3d, type I) having unit cell length of 76.9A. It was also stated that both hexagonal and lamellar phases were also formed. Later in 1988 Blackmore and Tiddy6 performed concentration gradients at various temperatures and determined that a hexagonal phase forms below 15 "C. Above 33 "C a cubic phase forms, which precedes a lamellar phase (2' =- 59 "C), which confirm the results of Luzzati et aZ.24,25 A more complete determination of the DTAB/water phase behavior was therefore required. ?he results obtained here for this binary system are shown schematically in Figure 3. Three liquid crystalline phases only form in the temperature range of 20-100 "C. At 20 "C a hexagonal phase (Ha) forms between 56.0 and 73.3 wt % which precedes a region of coexistence between the hexagonal phase and hydrated crystals of DTAB. No other liquid crystalline phases were observed to form until an elevation in temperature occurred, with a cubic phase (Qa)formingat 36.5 "C. At still higher temperatures (65.4 "C) and concentrations (89.7 wt %) a lamellar phase (La) is formed. Results from concentration gradients performed at various temperatures indicate that no other phases are formed in this temperature range and that hydrated DTAl3 crystals are present at all temperatures. All optical textures produced from both concentration gradients and bulk samples were c l a ~ s i c a l . ~ ~ - ~ l
3. Results 3.1. Critical Micelle Concentration.Surface tension measurements of DTAB for concentrations between 2 x and 0.05 M at 25 "C are shown in Figure 1. A polynomial of degree two was fitted to the data points prior to surfactant aggregation having a correlation coefficient of 0.999, (Note that it has recently been ~ h o w n ~that l , ~the ~ use of a linear fit to the data prior to micellization causes an underestimation of the surface excess concentration.) By use ofthis fit, the critical micelle M. concentration (cmc) was determined to be 1.35 x The surface excess concentration of DTAB can be calculated from the gradient of the fitting curve in the limit as the concentration tends toward the cmc ((3.4 f 0.1) x lop3 mol*cm-2) and from this the surface area per surfactant molecule (A) a t the interface (49 f 1Az). Figure 2 shows the corresponding electrical conductivity measurements. The cmc is determined to be 1.61 x M DTAB using linear fits for the data points both before and after the onset of surfactant aggregation. By use of a monodisperse mass action model and assuming that for each concentration there is exclusively an average micelle, the conductivity of the micellar system may be treated as a solution of mixed electrolyte^.^^-^^ Employment of this model in conjunction with a known value of the micellar aggregation number enables the number of bound countenons ( m )to be determined and from this the percentage dissociation. Evans et al. have measured the micellar aggregation number for DTAB to be 55 using the fluorescence probe technique.13 Hence, the number of countenons which remain bound upon micellization is found to be 36, or on average 34.5%of all bromide ions dissociate during micelle formation. The more simplistic method of taking a direct ratio of the slopes above and below the cmc (31)Simister, E. A.; Thomas, R. K.; Penfold, J.; Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I.; Lu, J. R.; Sokolowski, A. J. Phys. Chem. 1992,96,1383-1388. (32) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1993,97,6024-6033. (33) Hutchinson, E. J. Colloid Sci. 1954,9,191-196. (34)Evans, H.C. J. Chem. SOC.1958,579-586. (35) Kamrath, R. F.; Franses, E. I. J . Phys. Chem. 1984,88,1642-
1648. (36) McGrath, K. M. Polymerisation of Surfactant Lyotropic Liquid Crystalline Phases. Thesis, The Australian National University, 1994.
(37) Rosevear, F. B. J. Am. Oil Chem. SOC.1954,31,628-639. (38)Rosevear, F. B. J. SOC.Cosmet. Chem. 1968,19, 581-594. (39) Kleman, M.; Colliex,C.;Veyssie, M. InLyotropicLiquid Crystals and the StructureofBiomembranes; Friberg, S.,Ed.; American Chemical Society: Washington, DC, 1976; pp 71-84. (40) Allain, M.; Kleman, M. J . Phys. (Paris) 1987,48, 1799-1807.
Langmuir, Vol. 11,No. 5, 1995 1837
Notes
H
20
L 0
20
40
60
st1
loo
phase being observed a t a similar sample composition and temperature. The DTAB lamellar phase produces a diffraction pattern at small angles comprised of three sharp rings in the ratios of 1:2:3 characteristic of a layered structure. This phase is also found to have molten paraffinic chains. From the locations of the Bragg peaks the unit cell dimensions can be calculated assuming that the symmetry of the aggregates is higher than the symmetry of the array and complete segregation of paraffin and polar regions.32 Hence, the aggregates of the hexagonal phase can be described as circular cylinders, and the bilayers of the lamellar phase are indefinite and flat. Table 1gives the structural parameters obtained using these assumptions. All equations were calculated for T = 27 "C unless stated otherwise. In the case of DTAB where the specific volume has not been determined, calculations were based on the specific volume of the paraffinic chains, which were deduced from measured values of the methylene and methyl g r o ~ p s . ~ O - ~ ~
DTAB Figure 3. Partial phase diagram of the binary DTABIwater system: L1,micellarsolution;Ha, normal hexagonalphase; &a, bicontinuous cubic phase, type Q230 (Iu3d Gyroid IPMS25*43-47); La, lamellar phase; crystals, hydrated DTAB crystals. The 4. Discussion horizontally shaded areas (showingtie lines) indicate a region where two liquid crystalline phases coexist and the diagonally Comparison of the cmc of DTAB (ca. 1.5 x M) with shaded areas indicate coexistence between a liquid crystalline that of the CISanalogue (CTAB, cmc ca. 9 x M)lo-l4 phase and hydrated crystals.The experimentalaccuracy of this demonstrates that increasing the length of the hydrophase diagram is of the order of fl "Cin the temperature axis carbon chain has the tendency of lowering the concentraand f0.05 wt % in the composition axis. wt%
tion at which aggregation is initiated. This is a general trend for saturated paraffinic chain surfactants17 and it The regions of the phase diagram initially assigned by has also been shown that increasing the length of the optical microscopy were established by determining the hydrocarbon chain increases the average micellar agstructures using SAXS. Diffraction patterns were obgregation number and shifts the Kraf'ft discontinuity to tained for powdered bulk samples in each region of the higher temperature^.^^ In contrast, comparison with DTABIwater system and were comprised of Debyeindicates that a change DTAC (cmc = ca. 2 x Scherrer rings which were produced by all domains in the from bromide to chloride counterion shifts the cmc only irradiated volume. slightly to higher concentrations. The DTAB micellar phase produced diffraction patterns From both the optical microscopy and X-ray analysis of at both small and wide angles comprised of one diffuse the DTABIwater systemit was observed that DTAB follows ring only, indicative of randomly oriented surfactant the traditional phase progression predicted for a singleaggregate^.^^^^^?^^ chain surfactant of intermediate chain length and head The hexagonal phase pattern was characterized by five group area. That is, with increase in concentration the sharp rings at small angles with spacings in the ratios of phase progression is one of micellar, hexagonal, bicon1:43:44:47:49, which is expected for parallel cylinders tinuous cubic, and lamellar phases followed by hydrated packed in a two-dimensional hexagonal array (Table 1). ~ r y s t a l s . ~How, ~ , ~ though, ~ is this phase behavior exAs for the micellar phase, the hydrophobic interior of the plained in relation to that observed for a change in hexagonal phase is in a liquid-like state, as evidenced by hydrocarbonchain length (CTAB)or counterion (DTAC)? the observance of a diffuse ring in the wide-angle regime The phase diagram obtained by Balmbragfor the CTABI at Q = 2nI4.5 A-1. water system shows that a hexagonal phase is formed a t The cubic phase formed in the DTABIwater system has -32 "C and 25 w t %, a cubic phase a t -40 "C and 76 w t been found to produce a diffraction attern with Debye% and a lamellar phase a t -50 "C and 87.5 wt %. Blackmore and Tiddy6 (who performed concentration Scherrer rings in the ratios of ,/6:98:,/1&/16 (Table 1). gradients only on this system), have observed in addition The diffraction pattern may be uniquely assigned as to the hexagonal, cubic, and lamellar phases an interarising from thela3d space group (i.e., bicontinuous cubic ~ * ~ ~ - ~mediate ~ phase which lies between the hexagonal and cubic phase Q230 (type I) in the notation of L ~ z z a t i ) . ~This phases. Here the phase progression is such that the cubic phase has also been shown to be related to the Gyroid hexagonal phase forms a t 29 "C and is followed by the infinite periodic minimal s ~ r f a c e . ~ * Here * ~ ~the hydroformation of an intermediate phase which exists between carbon chains were also found to be in a fluid-like state. 48 and 52 "C. A cubic phase a t 50 "C then precedes a Note that the unit cell length calculated here compares with the lamellar phase which is observed to form at 53 "C. The favorably with that obtained by Luzzati et very small range over which the intermediate phase is M)2*375
aZ.24925
(41) Boltenhagen,P.; Lavrentovich,0.;Klbman,M. J. Phys. ZZ 1991, 1,1233- 1252. (42) Muller, A. Proc. R. SOC.London 1930,A127,417-430. (43) Luzzati, V.; Spegt, P. A. Nature 1967,215, 701-704. (44) Fontell, K. Mol. Cryst. Liq. Cryst. 1981,63, 59-82. (45) Ranqon, Y.; Chawolin, J. J. Phys. (Paris) 1987,48,1067-1073. (46) Clerc, M.; Dubois-Violette, E. J. Phys. ZZ 1994,4, 275-286. (47) Luzzati, V.; Vargas, R.; Paolo, M.; Gulik, A.; Delacroix, H. J. Mol. Biol. 1993,229, 540-551. (48) Scriven, L. E. Nature 1976,263, 123-125. (49) Scriven, L. E. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2; pp 877-893.
(50)Luzzati, V.; Mustacchi, H.; Skoulios, A.; Husson, F. Acta Crystallogr. 1960, 13, 660-667. (51) Husson, F.; Mustacchi, H.; Luzzati, V. Acta Crystallogr. 1960, 13,668-677. (52) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964,68,35043510. (53) Tardieu, A.; Luzzati, V.; Reman, F. C. J. Mol. Biol. 1973, 75, 711-733. (54) Small, D. M. The Physical Chemistry of Lipids: From Alkanes to Phospholipids. Handbook of Lipid Research; Plenum Press: New York, 1986; Vol. 4, pp 285-343. (55) Fontell, K. Adv. Colloid Interface Sci. 1992,41, 127-147.
Notes
1838 Langmuir, Vol. 11, No. 5, 1995 Table 1. Structural Parameters for DTAB/Water Mesophases at 27 "C DTAB %(w/w)
phase
observed Q(A-l)
unit cell length ( a )(A)
volume fraction @
paraffin chain thickness (d,) (A)
water and head group thickness (dw) (A)
mean area per polar head (A) (AZ)
hkl
~~~
35.8 43.3
L1
56.0
Ha
59.3
Ha
L1
+
65.8
70.7
Ha
73.0
Ha
83.1"
Qa
(Iu3d) Qa
(Ia3d) La
90.9c a
La
0.23 0.28
0.147 0.158 0.168 0.289 0.335 0.441 0.501 0.170 0.297 0.340 0.451 0.511 0.178 0.307 0.352 0.468 0.183 0.320 0.370 0.483 0.186 0.322 0.368 0.491 0.193 0.224 0.297 0.317 0.198 0.229 0.198 0.400 0.593 0.213 0.420
43.3
0.36
27.1
16.2
51.8
42.6
0.38
27.4
15.2
51.2
40.8
0.41
27.5
13.3
51.0
39.7
0.44
27.7
12.0
50.7
39.1
0.45
27.7
11.4
50.7
79.7
0.51
77.7
0.53
31.7
0.55
17.5
14.2
32.8
29.5
0.57
16.7
12.8
34.8
1010 1120 2020 2130 3030 lOl0 1120 2020 2130 3030 lOi0 1120 2020 2130 1040 1120 2020 2130 1010 1120 2020 2130 211 220 321 400 211 220 001 002 003 001 002
Temperature 41 "C. Temperature 65 "C. Temperature 92 "C.
formed may account for it being missed in the phase diagram obtained by Balmbra.g Both of these studies make no mention of the formation of the gel phase observed by Vincent and S k o ~ l i o s The .~~ phase was observed at low temperatures, between -5 and 20 "C (a temperature region which is often not studied in liquid crystal work) and for water contents below 20%. Combination of these three studies gives a phase progression which has been shown to be typical for surfactants having hydrocarbon chains of this length.6 By comparison of the DTAB and CTAB results it would appear that a change in the hydrocarbon chain length from a Clzto a c16 significantly alters the self-assembling behavior ofthe surfactant, which is expected based purely on an argument of the surfactant ~arameter.~'-~O A CIS single-chain surfactant has inherently a different disorder/ order transition, as a function of volume fraction and temperature, which affects both the length and volume of the chain differently as compared to a Clz single-chain surfactant. Since the nature of the paraffinic chain is a key determinant in surfactant self-assembly, this manifests itself in the observed self-assembly of these surfactants. Hence, it may be concluded that lengthening the hydrocarbon chain promotes the formation of intermediate (56)Vincent, J. M.; Skoulios, A. Acta Crystallogr. 1966, 20, 441447. (57) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525-1568. (58) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Biochim. Biophys. Acta 1977,470, 185-201. (59) Mitchell,D. J.;Ninham, B. W. J.Chem. SOC.Faraday Trans. 2 1981, 77, 601-629. (60) Hyde, S. T. Pure Appl. Chem. 1992, 64, 1617-1622.
phases and also induces freezing of the paraffinic chains at higher surfactant concentrations. This conclusion was first drawn by Tiddy et aL6 who contended that there is a definitive variation in the phase progression with change of hydrocarbon chain length. Note that the formation of intermediate phases and the occurrence of freezing of the paraffinic chains for a c16 cationic surfactant have also been observed in the CTAC/water system. In this system, two intermediate phases and a lamellar gel phase are observed (-68 wt % CTAC and 30 "C, -75 wt % CTAC and 35 "C, and -77 wt % CTAC and below 40 "C, re~pectively),~J~ this will be discussed further below in relation to a change in counterion. The Krafft temperature, and corresponding curve for the CTAB/water system, is also observed to be displaced to higher temperatures in comparison to the DTAB/water system following the lower solubility found in general for longer chained surfactants. The trend of increasing insolubility with increasing paraffinic chain length has also been observed for saturated fatty acid soaps.54 What effects then does a change in counterion induce? The DTAC/water system is also found to differ from that of the DTAB/water system by the formation of a discrete cubic phase prior to the hexagonal phase. The formation of this additional phase can only be attributed to the change in counterion from a bromide to a chloride. The existence of a discrete cubic phase formed prior to formation of the hexagonal phase may be explained by the different effect that a bromide versus chloride countenon has on the growth of the micellar aggregates. The
Notes
chloride and bromide ions have different sizes in solution61 and the average number of bound water molecules for the bromide ion is 1.5 in comparison with 2.0 for the chloride ion.62 Therefore, the hydrated chloride ion is larger than the hydrated bromide ion and as such the chloride ion is not as closely associated with the cationic head group of the surfactant and will not be as effective as the bromide counterion at neutralizing the head group charge. This will lead to a greater electrostatic repulsion between the head groups of the surfactants not only within the micellar aggregates but also between the surfactant aggregates themselves. Hence, the surfactant parameter will be greater for the bromide counterion surfactant than for the correspondingchloride at the same composition. This difference in the extent of counterion binding for the two ions has an effect on the self-assembly of the surfactant molecules in the form of micellar el0ngation,6~,~~ with the bromide counterion promoting elongation. As the concentration is increased in the micellar region of the phase diagram, the number of micellar aggregates remains approximately constant and there is an increase in the average aggregation number. The chloride counterion does not induce such an elongation and instead the size of the micellar aggregatesremains approximately constant with the number of aggregates increasing with increasing surfactant concentration. This increase in the number of surfactant aggregates induces order upon the system. Hence, the formation of a discrete cubic phase prior to the hexagonal phase in the DTAC/water system is favored. Note that, the discrete cubic phase (Pm3n)consists of two types of micellar aggregates, neither of which are spherical in shape.47 Hence some distortion of the micellar shape does occur for the chloride counterion with increase of surfactant concentration but not to the same extent as seen for the bromide counterion. The effect on the self-assemblyof the surfactant induced by the introduction of the chloride counterion is not (61) Nicoli,D. F.; Dorshow, R. B. In Physics ofAmphiphiles: Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M., Eds.; Elsevier Science Pub. Co.: Amsterdam, New York, and North Holland, 1983;pp 429-447. (62) Ketelaar, J. A. A. Chemical Constitution; New York, 1958. (63) Porte, G.; Appell, J. InPhysics ofAmphiphiles: Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M., Eds.; Elsevier Science Pub. Co.: Amsterdam, New York, and North Holland, 1983; pp 461468. (64) Berr, S.;Jones, R. R. M.; Johnson, J . S., Jr. J.Phys. Chem. 1992, 96, 5611-5614. (65) Torrie, G.M.; Patey, G. N. Electrochim. Acta 1991,36,16771684.
Langmuir, Vol. 11, No. 5, 1995 1839
restricted to the low concentration regime of the phase diagram. Comparison of the CTAB and CTAC phase diagrams shows that the extent and longevity of the gel phase in the CTAC/water system are much larger than those observed in the CTABIwater system. It has been shown that the more strongly hydrated the counterion, the greater the stiffness of the chains and the stronger the repulsive interactions between the head groups of the surfactant (this effect is particularly enhanced for doublechain surfactants).22 That is, the freezing or melting of the paraffinic chains (the disordedorder transition) is related in a complex way to the energy required to hydrate or dehydrate the polar head group and associated counterions which in turn is related to the electrostatics of the ions (i.e.,the counterions interaction with the surrounding water medium). Hence, in the case of anionic counterions the effect is enhanced as the degree of hydration is increased. This change in hydrated ion size has also been studied for cationic ions (Na+and K+)for simple 1:lelectrolytes.66 This study has shown that there is a considerable change in the density profile/surface charge for the ion near the "neutral" surface with a change in size of the ion and that this influences the structuring near the surface. Hence, assuming that the same behavior will be observed for anionic ions, this can then be used to explain the increased stability of the gel and intermediate regions found in the CTAC/water system compared with the CTAB/water system.
5. Conclusions The DTAB/water phase behavior presented here completes the series of four quaternary ammonium surfactants: DTAB, DTAC, CTAB, and CTAC. From these results it appears that changes in the nature of the surfactant (such as a change in counterion or chain length) have a severe effect on the subsequent self-assembly in water. Such that increasing the length ofthe hydrocarbon chain promotes the formation of intermediate phases and freezing of the hydrocarbon chains, whereas a change from a bromide to a chloride counterion alters the progression of the phase formation.
Acknowledgment. I thank Ms Nicole Moriarty for performing the surface tension experiments and Calum Drummond and Patrick Kbkicheff for useful discussions. LA940945S
