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Langmuir 1999, 15, 5232-5236
Swelling Behavior of Lamellar Phases with Calcium Dodecyl Sulfate, Heptanol or Octanol, and Water U. Hornfeck, R. Hammel, and G. Platz* Physikalische Chemie I, University of Bayreuth, 95440 Bayreuth, Germany Received October 16, 1998. In Final Form: April 26, 1999 The swelling behavior of calcium dodecyl sulfate (CDS)/water with heptanol and octanol was investigated at 25 °C. Both systems form a condensed lamellar phase at a high content of surfactant and cosurfactant and a highly swollen lamellar phase at a low surfactant content. There is no single-phase channel between both lamellar regions. A dilute L3 phase exists in the heptanol system as well as in the octanol system. This L3 phase is water-like transparent and shows no streaming birefringence. On dilution of samples with an appropriate composition of water, iridescent phases can be obtained which display bright blue to green colors. In contrast to CDS with pentanol or hexanol, the lamellar region is broken into a dilute phase and a condensed phase because of decreasing undulation forces of systems containing longer chain alcohols.
1. Introduction The Krafft point of calcium dodecyl sulfate (CDS) is about 50 °C.1 Addition of short- and medium-chain alcohols decreases the solubilization temperature below room temperature, and CDS forms highly swollen lamellar phases together with pentanol and hexanol.2 Decanol and CDS exhibit a condensed lamellar phase at 50 °C and high concentrations of alcohol and surfactant.3 No swelling was observed as in nearly all ionic systems with purely bivalent counterions,4,5 but a mixed system with calcium and sodium counterions is reported which forms a swollen lamellar phase and a condensed one.6 It is assumed that attraction forces induced by bivalent counterions prevent swelling to higher extents. SDS, water, and alcohols from pentanol to octanol display swollen lamellar phases. It is surprising that the swelling capability of SDS/water/ pentanol or hexanol is lower than that of the CDS systems. However, it is known that addition of salt, for example, NaCl, increases the stability region of the SDS/water/ pentanol or hexanol phases to higher dilution.7 It seems that increasing the ion strengths increases also the swelling capabilities. Moreover, in the system calcium dodecyl(monooxyethylene) sulfate, decanol, and water, a condensed lamellar phase coexisting with a swollen lamellar phase is detected.8 To obtain more insight into the influence of the alcohols on the different swelling behavior of CDS with mediumand long-chain alcohols, we investigated CDS/water with heptanol and octanol. 2. Experimental Section Calcium dodecyl sulfate (CDS) was prepared as described in previous papers.2,9 The alcohols were purchased from Merck, Darmstadt. The samples were homogenized by heating to about (1) Moroi, Y.; Oyama, T.; Matuura, R. Colloid, J. Interface Sci. 1977, 60 (1), 103. (2) Hornfeck, U.; Gradzielski, M.; Thunig, C.; Mortensen, K.; Platz, G. Langmuir 1998, 14 (11), 2958. (3) Maciejewska, D.; Khan, A.; Lindman, B. Colloid Polym. Sci. 1986, 264, 909. (4) Sein, A.; Engberts, J. B. F. N.; E. v.d. Linden, J. C. v.d. Pas. Langmuir 1996, 12, 2913-2923. (5) Khan, A.; Fontell, K.; Lindblom, G.; Lindman, B. J. Phys. Chem. 1982, 86, 4266. (6) Khan, A.; Jo¨nsson, B.; Wennerstro¨m, H. J. Phys. Chem. 1985, 89, 5180. (7) Guerin, G.; Bellocq, A. M. J. Phys. Chem. 1988, 92, 2550-2557. (8) Khan, A.; Lindman, B.; Shinoda, K. J. Colloid Interface Sci. 1989, 128 (2), 396.
50-60 °C and shaking in 10-mL reagent tubes. The phases were characterized after a storage time of 2 weeks at 25 °C. Birefringence was observed with the sample tubes using crossed polarizers and denoted as described in previous papers.10,11 Except of this, the following method was used in order to recognize singlephase areas in low viscous regions. Alcohol or water was slowly added (ca. 0.01 mL per 1-2 min) under computer control during continuous magnetic stirring to samples with selected compositions (vessel volume 50 mL). A T90-20 dispenser (Schott) was used. Conductivity (LF3000, WTW, Weilheim) and turbidity (DP550, Mettler-Toledo, Darmstadt) were detected simultaneously. In the concentrated region above 20 wt % CDS in water, phase boundaries were scanned in 10 wt % intervals of CDS/ water and typically 20 steps of different alcohol concentrations. The dilute region was scanned in 1-2 wt % CDS intervals. The boundaries below 2 wt % CDS in water were estimated in probes that were prepared by dilution of different 2 wt % samples in 10 steps with water. Dynamic light scattering was measured with a Brookhaven Instrument BI 9000. Rheological data were collected from a Bohlin Rheometer type CS and a HewlettPackard OCR-D instrument. The Spectra Physics UV/vis spectroscopic equipment was from Perkin-Elmer. All measurements were carried out at 25 °C.
3. Results 3.1. Phase Behavior. The partial phase diagrams in Figures 1-3 explain that there are at least six different single-phase regions in the systems CDS-water-heptanol or octanol at 25 °C. The octanol phase diagram looks quite similar to that of the heptanol system (Figure 2). (1) The L1 phase consists of nearly pure water and is not markedly expanded into the phase triangle (see Figure 8). The phase boundaries are given by the solubilities of heptanol (0.174 ( 0.005 wt %12) or octanol (0.054 ( 0.005 wt %12) and CDS (0.026 wt %13,14) in water at 25 °C. (2) The water-clear isotropic L2 phase expands from the alcohol corner far into the middle of the phase diagram. (9) Moroi, Y.; Motomura, Y.; Matuura, R. Bull. Chem. Soc. Jpn. 1971, 44, 2078. (10) Platz, G.; Thunig, C.; Hoffmann, H. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 667. (11) Hoffmann, H.; Munkert, U.; Thunig, C.; Valiente, M. J. Colloid Interface Sci. 1994, 163, 217-228. (12) Alcohols with Water; Solubility Data Series; Barton, F. M., Kertes, S., Eds.; Pergamon Press: New York, 1984; Vol. 15. (13) Lee, R. S.; Robb, I. D.; J. Chem. Soc., Faraday Trans. 1 1979, 75, 22116. (14) Kallay, N.; Pastuovic, M.; Matijevic, E. J. Colloid Interface Sci. 1985, 106 (2), 452.
10.1021/la981447c CCC: $18.00 © 1999 American Chemical Society Published on Web 07/02/1999
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Figure 4. Phase volume intersection of 1:1 heptanol/water at 25 °C.
Figure 1. Partial phase diagram of CDS/heptanol/water at 25 °C in weight fractions. LR denotes the swollen and the condensed lamellar phase. L2 is the heptanol-rich clear isotropic phase. L3 is the clear isotropic phase above the lamellar phase.
Figure 5. Phase volume intersection of 1:1 octanol/water at 25 °C.
Figure 6. Phase volume intersection between 33 wt % octanol in water and 33 wt % CDS in water, which explains that there is no coexistence between both lamellar phases.
Figure 2. Dilute lamellar (lower phase) and L3 phase (upper phase) of the heptanol system in weight fractions at 25 °C.
Figure 3. Dilute lamellar (lower phase) and L3 phase (upper phase) of the octanol system in weight fractions at 25 °C.
The solubility of water in heptanol is 5.4 ( 0.4 wt % and in octanol 4.6 ( 0.1 wt %.12 (3) There is a region of crystalline CDS. (4) A condensed lamellar phase (LR) is found in about the same concentration region as the lamellar phase of CDS/water/decanol at 50 °C.3 (5) It was very surprising that a second lamellar phase (LR) exists at very high dilution. There is no single-phase connection to the condensed lamellar phase. (6) The highly diluted lamellar phase is accompanied at somewhat higher alcohol concentrations by an isotropic
phase of low viscosity. Because of its location at somewhat higher alcohol concentrations above the dilute lamellar phase, it follows that a L3 phase is present. Highly diluted L3 phases are usually somewhat opaque and display flow birefringence.11 The L3 phase of the calcium systems is not turbid and shows no streaming birefringence because of its much lower extension into the dilute region. The properties and microscopic structure of this phase will be presented in a forthcoming paper.15 To elucidate the essential phase relations, phase volume intersections are given in Figures 4 and 5. A mixture of CDS and water 1:1 per weight is a completely intransparent white dispersion that becomes more and more stiff on adding alcohol. The viscosity becomes lower as soon as the dispersion becomes somewhat transparent and birefringent until the condensed lamellar phase is formed. This phase is in coexistence with the L2 phase at somewhat higher alcohol concentrations. More than 90% of the alcohol cloudy dispersions, probably of crystalline CDS, which were not further investigated were formed. The L2 phase and the condensed lamellar phase are not connected (at least at 25 °C) in a single-phase channel with their dilute relatives. This important fact is elucidated exemplary in Figure 6 where no single-phase region is found. Moreover, it is also found that there is no coexistence between the swollen phase and the dilute lamellar phase in the heptanol and the octanol systems. From this, it follows from the appropriate phase diagram intersections that there is no coexistence between the swollen phase and the dilute lamellar phases because of the formation of at least one three-phase region like dilute lamellar phase, L3 phase, and crystalline CDS. The experimental check of this assumption is still lacking because there are no macroscopic phase separations in the regions with the (15) Platz, G.; Hornfeck, U.; Hoffmann, H. To be submitted for publication.
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Figure 7. Phase volumes of CDS with heptanol and octanol at a molar ratio of heptanol/CDS of 5.6 (weight ratio 1.14) and octanol/CDS of 4.3 (weight ratio 0.98) at 25 °C. LRl is the lamellar region that displays schlieren in the reagent tube between crossed polarizers. LRh is the lamellar region with bright birefringence and oily streaks which occurs at higher alcohol content, LRl-h is usually located between LRl and LRh and displays properties of LRl and LRh. L3 is the isotropic water-clear phase above the lamellar phase.
Figure 8. Conductivity and turbidity (1 - I/Io) of 6/6.8 wt % CDS/heptanol (molar ratio of heptanol/CDS ) 5.6) as a function of dilution with water at 25 °C. The measurements were carried out under continuous stirring. The L3 phase which occurs above 3 wt % CDS is characterized by its remarkable high conductivity and its low turbidity.
condensed lamellar phase and crystalline CDS. Further investigations are in progress. The interesting behavior in the highly dilute region is displayed exemplary in Figure 7, which shows the phase volumes when a L3 phase with a molar ratio of heptanol/ CDS ) 5.6 (weight ratio ) 1.14) is diluted with water. The L3 phase coexists with a lower lamellar phase with a LRh texture in a very narrow two-phase region. In the sample tube, the lamellar LRh region is very clear and shows bright birefringence with oily streaks. No macroscopic orientation order can be observed in the reagent tubes with inner diameters of 10 mm because of the narrow expansion of the wall orientation. This statement is supported by the fact that a homeotropic orientation is obtained with LRh phases between the microscope slide and cover glass. The LRl region, which occurs at higher dilution, is characterized by its low birefringence in the volume phase, its wall orientation induced birefringence after shear (never homeotropic) with an optical axis parallel to the reagent glass, and its shear-sensitive schlieren texture. LRl-h seems to be a transition between LRl and LRh.10,11 In contrast to LRh and L3, LRl becomes more and more turbid under dilution (Figure 8). The most surprising effect occurs in this turbid LRl region: Between 1.2 and 0.52 wt % CDS, blue and green iridescent colors are formed within several minutes. This iridescence is clearly visible, but it may be somewhat hidden by the turbidity of the solutions. On further dilution, clouds of a markedly more turbid emulsion without iridescence are formed. The amount of the clouds
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Figure 9. Extinction normalized by multiplying with λ4 as a function of the wavelength: CDS/heptanol/water with a molar ratio of heptanol/CDS ) 5.6; concentrations of CDS in wt %.
increases until the emulsion fills the whole probe tube. Crystalline dispersions of CDS start to precipitate below 0.45 wt % CDS down to 0.03 wt % CDS, where the clear isotropic water phase is finally formed. Figure 8 shows that the electrical conductivities are in accordance with the expected microscopic state of the phases. The L3 phase displays a high conductivity as expected for a bicontinuous sponge structure.16 The regions of the lamellar phases are characterized by their low electrical conductivities which arise from the high electrical resistances across the hydrocarbon part of the bilayers,17 which are partially ordered parallel to the electrodes of the conductivity cell. From the turbidity curve, it can be concluded that the clear L1 phase exists below 0.25 wt % CDS. This is exactly the solubility of CDS in pure water. 3.2. Iridescent Phases. Iridescent phases are found for CDS/heptanol as well as for CDS/octanol. Solutions containing 2 wt % CDS and different amounts of heptanol or octanol, mostly dilute lamellar phases, were mixed with pure water in weight ratios of 1:9 until 9:1 and stored at rest. Iridescent phases were found within the following CDS/alcohol weight ratios and weight concentration regimes: heptanol/CDS ) 1.38-1.5; blue, 0.6-0.8 wt % CDS; green, 0.4-0.5 wt % CDS; octanol/CDS ) 1.25-1.5; blue, 0.5-0.8 wt % CDS; green, 0.3-0.5 wt % CDS. When intermediate CDS/alcohol ratios and intermediate CDS concentrations are chosen, bright blue but also clear green iridescence without turbidity effects can be obtained after several days. The lamellar repetition distances (D) in the iridescent region were obtained from extinction measurements as in earlier investigations10 (Figure 9). The proportionality of D as a function of 1/Φ (Φ ) volume fraction of CDS + alcohol) proves the highly swollen state of the lamellar phase (Figure 10). From the slope of the curve, a lamellar thickness d ) 2.4 nm is obtained using the relation D ) d/Φ. For CDS/hexanol/water, the same value (d ) 2.4 nm) was obtained by SANS measurements.2 From the interlamellar distances of the octanol system (98 nm at 1 wt % CDS and 1.375 wt % octanol) which were also measured by UV-vis, it follows that the lamellar thickness is 2.33 ( 0.1 nm. It can be proven by polarizers that the swollen lamellar phases with blue colors and those with higher concentra(16) Gazeau, D.; Belloq, A. M.; Roux, D.; Zemb, T. Europhys. Lett. 1989, 9, 447-452. (17) Miyamoto, V. K.; Thompson, T. E. J. Colloid Interface Sci. 1967, 25 (1), 16-25.
Swelling Behavior of Lamellar Phases
Figure 10. Interlamellar distances (D) as a function of the reciprocal volume fraction (1/Φ): (b) hexanol/CDS measured by SANS;2 (0) heptanol/CDS from Figure 4; molar ratios of alcohol/CDS ) 5.6, T ) 25 °C.
tions are strongly ordered parallel to the walls of the reagent glass and the cuvettes of the UV-vis apparatus. In these cases, nearly the whole scattering occurs in the back direction of the incident light beam, this means at a single q value which is given by 4πn/λ0, and from this, it follows that D ) λ0/2n (n and λ0 are the refractive index (1.333) and wavelength of the incident light). This fact leads to peaks that are essentially sharper than expected for a randomly ordered lamellar system. In randomly ordered systems, only the onset of the peak coming from long to short wavelengths is “sharp”. So in any case, UVvis measurements (λ0 g 200 nm) are for investigating interlamellar distances (D g 80 nm) of iridescent and highly swollen lamellar phases because of their easy use and cheap accessibility. 3.3. Investigation of Vesicular Dispersions. Vesicular dispersions are reported to exist in the system SDS/octanol/water.18 Therefore, it was of special interest to investigate the behavior of the calcium system in the two-phase region lamellar-L1, which is located at concentrations below the maximal swelling capacity of the swollen lamellar phase. The location of this boundary depends on the ratio of CDS and alcohol. Octanol/water in a weight ratio of 1:1 forms no iridescent phase on dilution, and the phase volumes as a function of dilution in Figure 7 elucidate the transition from LRl to a vesicular dispersion phase at 1.3 wt % octanol and CDS. This is nearly the same concentration where the iridescent phases occur at higher octanol/CDS ratios. Neglecting the small influence of somewhat different CDS/alcohol ratios, the interlamellar distance at the transition concentration can be extrapolated to be 180 nm. On further dilution, the repulsion forces are not high enough to stabilize the larger distances, and a two-phase region lamellar/water phase which contains vesicles is formed. For example, when 5 mL of water is added to 1 mL of the swollen lamellar phase with 2 wt % CDS and 2 wt % octanol, a highly viscoelastic and slightly turbid solution is formed after shaking and mixing. The elastic properties disappear and the viscosity decreases after more rapid shaking. One could expect that the system forms vesicles with radii in the order of the interlamellar distances of the highest swollen lamellar phase. To prove this assumption, dynamic light scattering was carried out at a (18) Auguste, F.; Douliez, J. P.; Belloq, A. M.; Dufourc, E. F.; GulikKrzywichi, T. Langmuir 1997, 13, 666.
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Figure 11. Hydrodynamic radii from dynamic light scattering assuming water viscosity; 25 °C: (0) heptanol, ([) octanol. Alcohol/CDS, see Figure 2.
scattering angle of 90°. Mean relaxation time constants with a typical polydispersity of about 30% were estimated from the correlation curves. The results could be essentially influenced by shape fluctuations of nonspherical vesicles.19,20 However, for rather small vesicles, as expected in this system, the translation diffusion should be the leading term in the correlation curves. It can be assumed that the short-range Brownian motion of the vesicles which is detected by dynamical light scattering at 90° proceeds in a water-viscous local surrounding. Therefore, hydrodynamic radii (rH) were calculated using the water viscosity (0.089 MPa‚s at 25 °C) for the volume phase: rH ) 2.47e-20m/Dc. To a good approximation, the radii of the vesicles (rv) should be given by these hydrodynamic radii. In the vesicular region above 0.2 wt % CDS/octanol, radii on the order of 180 nm were found (Figure 11). The vesicles that exist just below the iridescent phase of the CDS/ heptanol system undergo coagulation on diluting below 0.7 wt % CDS together with heptanol. Assuming a lamellar thickness (d) of 2.4 nm, the vesicle volume fraction (ΦV) of the amphiphilic material in the vesicles can be calculated as ΦV ) 3d/rv ) 0.04. The volume fraction of the vesicle system is by a factor of r/3d larger than the volume fraction of the amphiphilic material in the solution. From these data, it is clear that sterical interaction of the vesicular system may occur even at concentrations below 1 wt % CDS and alcohol. Furthermore, one must take into account that also different forms of lamellar dispersions such as networks of lamellar tubes with diameters similar to those of the vesicles could exist together with free vesicles. The viscosity data support this assumption. η increases exponentially with the concentration, which indicates that the dispersed lamellar aggregates come more and more in contact (Figure 12). Free vesicles of the heptanol system should coexist with the coagulated ones at concentrations below 0.6 wt % CDS together with heptanol. The lamellar order which arises in the swollen lamellar phase leads to a strong reduction of the slope of the viscosity curve as a function of the concentration. It is of interest that for SDS/ alcohol/water, an exponentially growth of the viscosity was also found in the isotropic L1 phase.21 In that case, a transition from spheres to rodlike or disklike micelles was assumed. (19) Milnerand, S. T.; Safran, S. A. Phys. Rev. A 1987, 36, 4371. (20) Joannic, R.; Auvray, L.; Lasic, D. D. Phys. Rev. Lett. 1997, 78, 3402. (21) Backlund, S.; Bakken, J.; Blokhus, A. M.; Hoiland, H.; Vikholm, I. Acta Chem. Scand. A 1986, 40, 241-246.
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Figure 12. Viscosites of dilute regions at 25 °C: (0) heptanol, lamellar phase; ([) octanol, vesicular dispersion. Alcohol/CDS, see Figure 2.
It can be stated that the behavior of the CDS systems in the concentration regimes below the maximal swelling capacities deserves further investigation. Knowledge of the multiphase connections would be necessary for a better understanding. 4. Discussion Two coexistent stable lamellar phases in surfactant systems with calcium counterions are described for calcium dodecyl(monooxyethlene) sulfate8 and a mixed sodium/ calcium counterion system.6 The situation in the CDSheptanol or octanol-water systems is somewhat more complicated but not essentially different because there are no coexisting lamellar phases but there are coexisting regions such as dilute lamellar-L3 and L3-condensed lamellar. The purely repulsive overall forces with a nonmonotonic course as a function of the layer distance are sufficient for the stability of two coexistent lamellar phases.22 The situation is identical to the case of a real gas with coexisting liquid and gas phases. It can be extrapolated from Figure 5 that the condensed lamellar phases of the heptanol system and of the octanol system are characterized by interlamellar distances on the order of 38 Å. This is because the thickness of the water layer is only about 15 Å. The Debye length of a 50% solution of CDS in water (0.43 mol/L) is near 2.5 Å. No electrostatic repulsion stabilization has to be taken into account, therefore. However, the strongly hydrated calcium counterions may lead to sterically induced repulsion forces at low distances. Undulations are the only repulsive interactions that work at all distances.23 Now it is of interest to understand the attraction forces that prevent the condensed phase from swelling. Monte Carlo simulations indicate that counterion attraction is (22) Wennerstro¨m, H. In Physics of Amphiphilic Layers; Meunier, J., Langevin, D., Boccara, N., Eds.; Springer-Verlag: New York, 1987; p 171. (23) Helfrich, W. Z. Naturforsch. 1987, 33a, 305-315.
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present in systems with bivalent counterions at low interlamellar distances.24 One can understand this behavior under the assumption that the entropic repulsion is strongly reduced at counterion/counterion distances below their Bjerrum length. The Bjerrum length (lB) is an essential parameter in electrostatic considerations. It is defined as the distance of two ions at which the electrostatic repulsion is as high as the thermal energy, kBT (see, for example, ref 25 or ref 26). lB is 28.5 Å for calcium ions in water at room temperature. The range of the counterion attraction should be essentially smaller for the systems with sodium dodecyl sulfate because the Bjerrum length of Na+ is only about 7 Å. This could explain why the lamellar phase with SDS and octanol displays continuous swelling.18 The undulations increase with the alcohol concentration but decrease with chain length of the alcohol. Due to the decreasing fluctuations with increasing chain length of the alcohol, long-chain alcohols are not expected to build any swollen lamellar phase. Indeed no swelling is observed for decanol.3 CDS and heptanol or octanol are optimally suited for planar lamellar layers. The fluctuations are high enough for an extremely diluted lamellar and at a somewhat higher CDS/alcohol ratio, but no such phases in the mediate dilute region are stable. Hexanol and pentanol fit less well into a planar lamellar layer. However, strong undulations stabilize a single lamellar phase, which reaches from the condensed to the highly dilute region. L3 phases are known to exist in ionic as well as nonionic systems. However, they are typically not present in ternary systems of ionic surfactant, medium-chain alcohol, and water without added electrolyte. It is known that the sponge phase disappears in nonionic surfactant-alcoholwater systems above a certain but small content of ionic surfactant.11 In this context, it is of interest that a L3 phase is found in the system SDS, octanol, and brine27 but not with pure water.18 L3 phases are stabilized by such extended fluctuations that the dynamic behavior of the sponge structure can be well described under the assumption of an overlapping system of disklike micelles.28 It is stated that the L3 phases disappear on adding ionic surfactant because of the increased stiffness of the charged lamellar systems. From this point, the occurrence of L3 and iridescent phases indicates that CDS behaves not like a typical ionic surfactant. This fact can be understood under the assumption that the calcium ion induces electrostatic headgroup-headgroup attraction, which leads to a reduced headgroup area and to the typical behavior of a nonionic two-chain surfactant. LA981447C (24) Guldbrand, L.; Jo¨nsson, B.; Wennersto¨m, H.; Linse, P. J. Chem. Phys. 1984, 80, 2221. (25) Ramanathan, G. V. J. Chem. Phys. 1988, 88 (6), 3887-3892. (26) Friedmann, H. L. J. Solution Chem. 1980, 9 (6), 371-379. (27) Herve, P.; Roux, D.; Belloq, A. M.; Nallet, F.; Gulick-Krzywicki, T. J. Phys. II 1993, 3, 1255. (28) Miller, C. A.; Gradzielski, M.; Kra¨mer, U.; Thunig, C. Prog. Colloid Polym. Sci. 1991, 84, 243-249.
