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Surfactant-Specific Electrode Measurements of Mesophases. Electrode and X-ray Measurements of Hexadecyltrimethylammonium Bromide/Hexadecanol Gel (Lβ) Dispersions Show Large, Nonequilibrium Dissolution Effects D. Bloor,† J. Gray,‡ J. Hughes,† and G. J. T. Tiddy*,‡ University of Salford, School of Sciences, Cockcroft Building, Salford, M5 4WT, U.K., and Department of Chemical Engineering, UMIST, PO Box 88, Manchester M60 1QD, U.K. Received April 19, 2001. In Final Form: July 2, 2001 A commercially available surfactant-specific electrode has been employed to investigate the dilution behavior of hexadecyltrimethylammonium bromide/hexadecanol gel phase (Lβ) dispersions. Initial measurements on known surfactants demonstrated that the electrode gave reliable data. The structure of the gel phase and the extent to which it swells in water were established by optical microscopy and X-ray diffraction. Surprisingly large nonequilibrium concentrations of monomeric hexadecyltrimethylammonium bromide occur in gel phase dispersions immediately after dilution, which can reach the critical micelle concentration of the pure surfactant. Much lower levels occur after some time (hours to days). These time-dependent changes are likely to be important in the practical applications of gel phase surfactants as emulsion stabilizers or in conditioning products.
Introduction Surfactants employed in formulated products are used in a noncrystalline state, which is frequently a micellar solution, but can also be a liquid crystal (mesophase), particularly for products dissolved, dispersed or diluted in use. In fact, for both single and mixed surfactant systems, mesophases occur far, far more often than micelles but are mostly unrecognized.1,2 While there are numerous experimental phase diagrams for surfactant/ cosurfactant/additive/water systems,3,4 few of these consider the detailed composition equilibria between the mesophases (hexagonal, lamellar, cubic, and gel being the most common) and the dilute (monomeric) surfactant solution. There are no studies of the way that surfactant monomer concentrations (activities) vary within mesophases because methods to obtain these data have not been available to date. Indeed, most authors do not consider the monomers at all. This is a severe limitation, given the role that monomers are likely to play in many applications, such as the kinetics of dissolution and emulsification/adsorption. Surfactant-specific electrodes offer a promising technique to obtain this information. More importantly, the monomer concentration is related to the surfactant chemical potential. In a phase of aggregates with a range of aggregation numbers at equilibrium, the chemical potential (µn) of all identical molecules is the same, whatever the aggregation number, i.e. † ‡
University of Salford. UMIST.
(1) Fairhurst, C.; Fuller, s.; Gray, J.; Holmes, M. C.; Tiddy, G. J. T. Lyotropic Surfactant Liquid Crystals. In Handbook of Liquid Crystals; Wiley VCH: Weinheim, 1998; Vol. 3, pp 341-392. (2) Hassan, S.; Rowe, W.; Tiddy, G. J. T. Handbook of Surfactants; Wiley: New York, 2001, in press. (3) Laughlin, R. G. The Aqueous Phase Behaviour of Surfactants; Academic Press: London, San Diego, 1994; pp 257-327, 368-416, and 480-508. (4) Ekwall, P. Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1975; p 1.
µn ) µn° +
Xn kT log n n
n ) 1, 2, 3, ... where µn is the mean chemical potential of a molecule in aggregates of aggregation number n, µn° is the standard part of the chemical potential, and Xn is the concentration of molecules for the n-aggregates. For multiple phases coexisting at (local) equilibrium, the chemical potential of each component is the same in every phase present. Thus measurements of surfactant chemical potentials give essential thermodynamic data that are vital for understanding and controlling the behavior of formulated products. The thermodynamics of nonideal solutions has been a major activity in physical chemistry over the past century. Similar studies of soft matter phases have been restricted by the limited techniques available to measure thermodynamic parameters; usually only vapor pressures of solvents (e.g., water) or small organic solutes (e.g., perfumes) can be accessed. Validation of surfactantspecific electrodes for measurements in these phases represents a significant new method that could lead to a quantitative description of the detailed compositions of multiple phases present during product processing and use. Surfactant-Specific Electrodes Surfactant electrodes give a response that is directly proportional to the monomer concentration. Early surfactant-specific electrodes employed a poly(vinyl chloride) (PVC) membrane containing an active ionic species (ionophore) of opposite charge to the surfactant together with a suitable plasticizer.5,6 After “conditioning” with the surfactant under investigation, the electrode could be used, (5) Gharibi, H.; Takisawa, N.; Brown, P.; Thomason, M. A.; Painter, D. M.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1991, 87, 707-710.
10.1021/la010577+ CCC: $20.00 © 2001 American Chemical Society Published on Web 09/08/2001
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but this design was found to have poor long-term stability because the ionophore/surfactant complex is slowly solubilized in the solution under investigation. To overcome this problem, the Salford strategy has been to use an active ionic species covalently bonded to the PVC, which results in a membrane with an active complex that is completely immobile.7 Such membranes, which can be made with cationic or anionic surfactants, give a good Nernstian response even after several months of continuous use.8-14 Recently, a new method at Salford employs a PVC coated wire (CW) electrode15 for cationic or anionic surfactants. This considerably simplifies the mechanical construction. The electrodes perform well in low (10-4 mol dm-3) and high (10-1mol dm-3) ionic strength media, with excellent long-term stability. Quite remarkably, they show a response down to a surfactant concentration less than 5 × 10-7 mol dm-3 within 10 s. During the time that these electrodes have been under development, commercial surfactant-specific electrodes have become available. While their detailed structure is not published, they do provide an easily available, convenient technique for routine measurements of surfactant solution properties. Moreover, their use does not require extensive experience of specific-active electrodes. They have not been employed previously for measurements of mesophase systems. In this work we have examined the potential of a commercial surfactant electrode [Orion 93-42BN] for the measurement of surfactant activities in the gel (Lβ) phase dispersions of the hexadecyltrimethylammonium bromide (CTAB)/hexadecanol (cetyl alcohol, COH) system. These were carried out together with polarizing optical microscopy and low-angle X-ray diffraction studies and form part of a wider investigation of gel phases from both natural and synthetic surfactants. The semicrystalline Lβ phase was selected rather than the more common lamellar (LR) phase because it was thought less likely to form curved “defect regions” (with a slightly higher free energy) than the flat lamellae.1,2 For example, vesicles or liposomes do not form spontaneously; hence the gel phase was considered likely to have more well-defined phase equlibria. One of the objectives of the present work is to measure the electrode response in the Lβ phase/aqueous phase coexistence region, to establish whether it is constantsas expected from the phase rule. Dilute dispersions of gel phases comprising ionic surfactants are of particular interest because their equilibrium swelling behavior can be employed to study weak, long-range, colloidal forces. These remain a matter of controversy.16 In the absence of specific effects, one expects charged surfaces having univalent counterions to swell indefinitely because theory predicts that the electrostatic repulsions are always larger than attractive interactions from the surfactant layers. These typically are ca. 40 Å in thickness, while their separation is >400 Å at 10% (6) Schefer, U.; Ammann, D.; Pretsch, E.; Oesch, U.; Simon, W. Anal. Chem. 1986, 58, 2282. (7) Davidson, C. J. Ph.D. Thesis, University of Aberdeen, U.K., 1983. (8) Bloor, D. M.; Wan-Yunus, Wmz; Wan-Badhi, W. A.; Li, Y.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 3395. (9) Bloor, D. M.; Li, Y.; Wyn-Jones, E. Langmuir 1995, 11, 3778; (10) Bloor, D. M.; Mwakibete, H. K. O.; Wyn-Jones, E. J. Colloid Interface Scii. 1996, 178, 334. (11) Li, Y.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1996, 12, 4476. (12) Fox, G. J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1998, 14, 1026. (13) Ghoreishi, S. M.; Li, Y.; Bloor, D. M.; Warr, J.; Wyn-Jones, E. Langmuir 1999, 15, 4380. (14) Missel, P. J. Langmuir 1999, 15, 7122. (15) Xu, R.; Bloor, D. M. Langmuir 2000, 16, 9555. (16) Spalla, O. Curr. Opin. Colloid Interface Sci. 2000, 5, 5-12.
Bloor et al.
concentration (by wt). However, long-range attractions in water have been an attractive topic in recent years. X-ray diffraction has been employed here to determine the layer separation of the dilute gel dispersions employed in this work to establish whether a change in composition does occur in the separated phases. Our initial measurements were of micellar solutions simply to verify that the electrode gave critical micelle concentrations (cmc’s) in agreement with known data. Then we examined mixtures of CTAB/COH to investigate the onset of the solution (L1)/gel two-phase dispersion. Finally we separated a gel dispersion into the separate phases and remixed them in varying proportions to determine if the electrode gave a constant response (as required by the phase rule). The results show that nonequilibrium effects play a major role. Experimental Section Materials. All surfactants and electrolytes were the best grades commercially available and were used as received. Electrode Measurements. A commercially available surfactant-specific electrode obtained from Orion Research Inc., Boston, MA (model 93-42BN), was used to determine the electromotive force (emf) of a range of ionic surfactant solutions. The surfactant electrode has a recommended measurement range of 1.0 × 10-5 to 5.0 × 10-2 mol dm-3. We employed it to measure the emf of surfactant samples to approximately 10-5 mol dm-3 concentration, but at lower concentrations it was not always possible to obtain a stable reading. The emf measurements were made using a sodium reference electrode (KENT-EIL glass sodium electrode, model Corning solid-state ISE 30-25-00) together with a Corning ion analyzer ((0.1 mV precision). All samples contained 10-4 mol dm-3 added sodium bromide as the reference electrolyte. Following the manufacturer’s instructions, the surfactant electrode was stored in a dry state. Prior to use it was soaked in 10-4 mol dm-3 sodium dodecyl sulfate for 10 min, followed by thorough washing in distilled water. The experiments were performed in a jacketed glass vessel kept at a constant temperature of 25.0 ( 0.1 °C by a circulating water bath. The surfactant sample was continually stirred by use of a magnetic follower controlled by an air-driven stirrer. To reduce any background interference, the jacketed vessel and stirrer were housed in a Faraday cage. A suitable volume (either 20 or 25 mL) of the sodium bromide solution was placed in the jacketed vessel and allowed to equilibrate to 25 °C before small aliquots of surfactant solution were injected into it. The surfactant solution was then allowed to equilibrate until a constant emf reading ((0.2 mV) was observed (2-5 min). Small-Angle X-ray Diffraction. Small-angle X-ray diffraction was performed at the CLRC Daresbury Laboratory (Station 2.1) on samples containing CTAB/hexadecanol 1:2 mole ratio with added 10-4 mol dm-3 sodium bromide solution varying from 98 to 80 wt %. The samples were placed in sealed 1 mm Lindemann tubes and the temperature during X-ray analysis was kept constant at 25 °C (better than (0.5 °C) by use of a modified Linkam THM60 hot stage. Calibration of the equipment was made using the standard supplied (rat-tail collagen).
Results Our initial measurements were made on micellar solutions of readily available cationic surfactants: tetradecyl trimethylammonium bromide (TTAB), hexadecyltrimethylammonium chloride (CTAC), CTAB, and didodecyldimethylammonium bromide (DDAB). For most of these the counterion is the same as that of the reference electrolyte. The response of the electrode to an increase in concentration is given by the Nernst equation
emf ) Eo + S log as where Eo is a constant and as is the surfactant activitys here exactly equal to the monomer concentration below
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the cmc and very close to it above the cmc. Below the cmc the surfactant electrode performs under Nernstian conditions in the sense that the slope (S) is acceptable at 59.1 mV/decade change in surfactant concentration (mol dm-3). The cmc is detected by a sharp decrease in the slope of emf vs concentration (above the cmc the emf is expected to be constant or to decrease slightlyssee refs 7-10 for details). As well as the actual value of the cmc determined, the dependence of the emf on surfactant concentration below the cmc is a good test of electrode behavior. For CTAB there is an excellent agreement between the observed behavior (Figure 1) and theoretical expectations. The response is Nernstian, while the cmc of 0.000 69 mol dm-3 compares reasonably well with the value of 0.000 80 mol dm-3 reported in the literature.17 We note that the presence of the sodium bromide will reduce the cmc slightly, so this may account for the slightly low value. Naturally, as we did not purify the surfactant, a trace of surface-active impurities could also be present. As a comparison, the chloride salt CTAC was also studied. Here the cmc was observed at 0.0012 mol dm-3, similar to that previously reported (at 30 °C). Again, the slightly lower value may be due to the presence of the added sodium bromide. Here the dependence of the emf on concentration was slightly curved, with the largest slope at low concentrations. This almost certainly does result from the presence of surface-active impurities. A better response for the C14 salt, TTAB, was also observed. Here the cmc was calculated to be 0.0032 mol dm-3, which is consistent with the 0.0035 mol dm-3 previously, reported.17 Once again the slope of the emf response is close to the theoretical value. In the measurements here, DDAB gave a very poor response with the electrode. It was selected as a surfactant known to form a lamellar phase rather than micelles, at least above concentrations of a few percent. A cmc of ∼0.0004 mol dm-3 was calculated from the data instead of the value previously reported 0.0144 mol dm-3. It is also clear the emf response is sub-Nernstian with the slope of only 53 mV instead of 59 mV. The measurements from which the cmc is inferred are all close to the lower limit of the recommended concentration range for the electrode, with only a limited range of data being available. In fact,
from a consideration of the general literature and counting the number of “hydrophobic” CH2 groups, we expect DDAB to have a “cmc” equivalent to about that of a C22 monoalkyl derivative.1,2 Given that the cmc is lowered by a factor of 4 for a chain length increase of (CH2)2, and taking the cmc for a typical C12 ionic surfactant as ca. 10-2 mol dm-3, the estimate is 1 × 10-5 mol dm-3, which is consistent with the electrode data. Our general conclusion from measurements on the single surfactant systems is that the Orion electrode can be employed for reliable emf studies on these surfactant solutions, provided that proper attention is paid to the recommended operating procedures and precautions. Gel Phase Dispersions. It is already generally known that mixtures of long-chain alcohols with monoalkyl ionic surfactants form lamellar (LR) phases rather than micelles at the “cmc”.1-4 If the surfactant and alcohol are of similar chain lengths (Cn), with Cn > 10, then a gel (Lβ) phase occurs instead of the LR phase. For CTAB/COH mixtures the Lβ phase is known, although the detailed boundaries between the dilute aqueous region and the gel phase have not been determined. Part of the reason for this is that the solubilities of CTAB and hexadecanol in the isotropic aqueous region that coexists with the gel are very low. Optical microscopy of samples with an overall concentration of 20 wt % showed that a single gel phase was present over the range >1:1 to 1:1 to
