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Liquid crystal and solution phases of sodium dodecyl-p-benzene sulfonate (LAS) and hexa-oxyethylene glycol dodecyl ether (C12E6); 1:1 mixtures in water
Colloids and Surfaces A: Physicochem. Eng. Aspects 288 (2006) 103–112
Liquid crystal and solution phases of sodium dodecyl-p-benzene sulfonate (LAS) and hexa-oxyethylene glycol dodecyl ether (C12E6); 1:1 mixtures in water Claire Richards a,∗ , Gordon J.T. Tiddy a , Siobhan Casey b a
School of Chemical Engineering & Analytical Science, University of Manchester, PO Box 88, Manchester M60 1QD, UK b Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral Merseyside CH63 3JW, UK Received 2 December 2005; received in revised form 6 April 2006; accepted 7 April 2006 Available online 28 April 2006
Abstract The liquid crystals and other phases formed when the mixed surfactant system sodium dodecyl-p-benzene sulfonate (LAS) and hexa-oxyethylene glycol dodecyl ether (C12 E6 , 1:1 mixtures by weight) is dispersed in water have been investigated using optical microscopy, X-ray diffraction and differential scanning calorimetry. Despite the fact that neat LAS is a multi-phase solid and C12 E6 , is a low melting crystalline solid, the mixed surfactants (no water) form a liquid phase. On addition of water there is a mixed mesophase coexistence region, followed by a lamellar phase and a micellar solution that extends up to ca. 50 wt.% surfactant. In the mixed mesophase region (ca. 88–95% surfactant) there appear to be “intermediate” and lamellar phases present. After heating, the system takes several days to relax to the original state. Based on simple “packing constraints” concepts hexagonal, cubic and lamellar phases are expected. The absence of the more viscous phases points to an additional contribution from the alkyl chains to the surfactant bilayer curvature. © 2006 Elsevier B.V. All rights reserved. Keywords: Surfactant; Liquid crystal; Dissolution; Mixed anionic nonionic
1. Introduction Mixed surfactants are widely used in products such as detergents, agrochemicals, pesticides and personal care creams. Frequently they are present at low water levels where liquid crystal and solid phases would be expected [1]. There exist very few studies of the phases that occur at high surfactant concentrations in these systems, despite their possible influence on product properties such as powder morphology and dissolution rates. One common combination is that of the anionic surfactant sodium linear alkyl p-benzene sulfonate and a non-ionic surfactant of the poly-oxyethylene glycol alkyl ethers type. Together these can give much improved product behaviour than either alone. From a fundamental scientific viewpoint these mixtures are of considerable interest because of the complex phase behaviour of each component alone with water. The oxyethylene surfactants
∗
Corresponding author. Tel.: +44 161 3068867; fax: +44 1613064399. E-mail address: [email protected] (C. Richards).
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form cubic (I1 , V1 ), hexagonal (H1 ) and lamellar (L␣ ) phases, depending on the size of the oxyethylene group [2]. Commercial materials have a very similar behaviour to pure surfactants. However, commercial alkyl p-benzene sulfonate surfactants comprise a mixture of alkyl chain lengths and all (except the 1-) positional isomers for the benzene sulfonate group (including optical isomers). The neat LAS has been reported to form a “C” (=crystalline) or “HC” (=hydrated crystalline) phase, depending on whether or not water is present. [3] However, we find that it exists as multiple solid and liquid crystal phases at room temperature rather than a single crystalline solid [4]. On addition of water it forms a lamellar phase which coexists with a micellar solution over a wide composition region. The overall pattern of surfactant liquid crystal structures and their formation mechanisms are well established, based on packing constraints [5,6] and maximum volume fractions of ideal spheres and rods [1,2]. However, there are many unexplained features. Why does the neat non-ionic surfactant form a liquid phase rather than liquid crystals (which appear to require the presence of water for their formation [2]), unlike most ionic surfactants and the non-ionic sugar surfactants? Is LAS soluble in the liquid non-
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Table 1 Alkyl chain and isomer distribution of the linear-alkylbenzene sulfonate, sodium salt (LAS) Isomer
Alkyl chain distribution (wt.% of sample) C10
>4-Phenyl 4-Phenyl 3-Phenyl 2-Phenyl Total
C11
C12
C13
4.2 3.2 3.1 3.2
17.0 8.3 7.8 7.4
18.9 5.1 4.9 4.4
7.5 2.0 1.8 1.2
13.7
40.5
33.2
12.6
Isomers are described according to the position of the phenyl group on the alkyl chain (so 4-phenyl would refer to an isomer with the benzene ring on the fourth carbon in the alkyl chain).
ionic, and does it induce mesophase formation? How does the presence of an upper consolute loop within the lamellar phase region that is found [7] for pure LAS isomers (coexistence below the critical temperature of two lamellar phases with different water contents) influence the behaviour of mixed surfactants? How does the presence of mixed surfactants influence the formation of non-cubic “intermediate” phases between the H1 and L␣ phases? Most importantly, can the simple model based on packing constraints [2,5,6] that describes mesophase formation for single surfactant/water systems be applied to mixed surfactants? We have undertaken a systematic examination of 1:1 (by weight)1 mixtures of sodium dodecyl-p-benzene sulfonate (LAS) with several non-ionic and ionic surfactants. In this paper we describe the results for the LAS:hexaethylene glycol dodecyl ether [C12 H25 (OCH2 CH2 )6 OH, C12 EO6 ] system. The surfactant/water mixtures have been examined using optical microscopy, DSC, and X-ray diffraction. Remarkably, the data show that the mixed surfactant system exhibits simple phase behaviour at medium concentrations, but the system becomes very complex at high surfactant levels. 2. Materials and methods 2.1. Materials The sodium dodecyl-p-benzene sulfonate (LAS) was a commercial material obtained from Unilever. It had been freezedried following the removal of excess sodium sulphate (<0.6% remaining) and organic impurities (<0.1% remaining). The material was stored in a sealed container in a dessicator. A full analysis obtained at Unilever by mass spectroscopy (isomer and chain length distribution) is given in Table 1. C12 EO6 was pure material (>99.7% as determined by gas chromatography) obtained from Sigma Aldrich and used as received. 2.2. Methods All optical microscopy experiments were carried out using a Carl Zeiss Axioplan-2 polarising optical microscope; photographs were taken using a JVC-TK 1280E CCD camera and 1
Corresponding to a molar ratio of 1:1.3 (C12 E6 :LAS).
analysed using image analysis software attached to the microscope. Where required, temperature control was obtained using a Linkam hot-stage attachment for the optical microscope with an accuracy of ±1 ◦ C. Cooling rates were controlled using a stream of nitrogen gas from a reservoir of liquid nitrogen, thus allowing rapid cooling rates to be obtained. Small angle X-ray diffraction was carried out on station 16.1 at the synchrotron radiation source (SRS), Daresbury Laboratories, Warrington with X-rays ˚ The detector used was a Quantum 4 ADS. of wavelength 1.4 A. Samples were contained in heat sealed Lindemann tubes to prevent water loss and heating and cooling was carried out using a Linkam hot stage made specifically to fit capillaries. Temperatures were obtained with an accuracy of ±0.5 ◦ C (calibrated by Linkam and confirmed by measuring melting temperatures for pure solids whose heating and cooling behaviour was well documented). All DSC measurements were carried out on a Thermal Analysis DSC Q100. The samples were placed into hermetically sealed aluminium sample pans to prevent water loss. They were weighed before and after the experiment to ensure no water had been lost. The instrument was calibrated using a known amount of indium which has a very sharp melting temperature of 156.6 ◦ C. All traces were measured at a rate of 5 ◦ C/min following heat-cool-reheat cycles between the temperatures of 0 and 100 ◦ C. 2.3. Predictions from packing constraints [1,5,6] From the “packing constraints” concepts one can obtain an overall picture of surfactant phase behaviour. A brief description is given here. We assume that there are three idealised micelle shapes: spheres, rods and discs. Micellar solutions (L1 ) exist only up to a certain concentration of surfactant because the micelles become ordered, forming mesophases at higher concentrations. The micelles are assumed to be smooth, with a sharp boundary between the aqueous (head groups plus water) and hydrophobic (all alkyl chains) regions. The radii (r) of spherical and rod micelles cannot be larger than the all-trans alkyl chain (lt ). Hence as the micelle surface area of the head group (a) decreases, the sequence of micelle shapes is sphere → rod → disc. The transitions occur when a = 3v/ lt (sphere → rod) and a = 2v/ lt (rod → disc, where v volume of hydrophobic group). With conventional surfactants having a linear alkyl chain ˚ 2 (sphere/rod) and the shape transitions occur at a = ca. 68 A 2 ˚ (rod/disc). For spherical micelles the mesophase a = ca. 46 A sequence with increasing concentration is: small-micelle cubic (I1 ) → hexagonal (H1 ) → bicontinuous cubic (V1 ) [or various intermediate phases (Int)] → lamellar phase (L␣ ). With rod micelles it is: hexagonal (H1 ) → bicontinuous cubic (V1 ) [or various intermediate phases (Int)] → lamellar phase (L␣ ), while only lamellar phase (L␣ ) is seen for disc micelles. For the surfactants studied here, the phase behaviour is given in Fig. 1. The non-ionic surfactant C12 EO6 forms H1 , V1 and L␣ phases below ca. 38 ◦ C, changing to L␣ mesophase only above this, whilst LAS in water forms only L␣ phase with a large L1 /L␣ coexistence region over a wide temperature range. The
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detailed behaviour of the lamellar phase is very unusual; several different lamellar phases coexist with a micellar solution in the L1 + L␣ region. At higher concentrations, a complicated multiphase region occurs also including further multiple L␣ phases. In considering the application of packing constraints we need estimates of the parameters v, a and lt for the surfactant mixture. Within the mixed micelles the value of lt required is that of the component(s) with the longest chains. For C12 EO6 we take ˚ 2 and lt = 16.8 A ˚ at 25 ◦ C; for LAS we the values [8] a = 48.1 A 2 ˚ take a = 60 A [2] but the value of lt is more difficult to establish. Because of the mixture of chain lengths and isomers, lt varies ˚ [taking the benzene ring contribution as 2.8 A. ˚ from 10 to 20 A Thus the lt value for the mixed micelles is some average of the longest LAS components. Whilst only one surfactant chain has to be in the all-trans configuration to give the maximum micelle radius, this can not always be the same molecule. Hence we (arbitrarily) assume that at least 20% of the micelle members must be capable of the maximum lt value. Thus the value required is that of the longest 40% LAS isomers. Using the chain length and isomer distributions from Table 1, the value estimated is ˚ Thus the shape transitions occur with a = ca. 72 A ˚2 lt = 17.6 A. 2 ˚ (sphere/rod) and a = ca. 48 A (rod/disc). Thus if the a values of the surfactants in mixed aggregates are the same as in the single surfactant systems, we expect H1 , V1 and L␣ phases. 3. Results 3.1. LAS/C12 EO6 surfactant mixture (1:1)
Fig. 1. (a, top) Phase diagram of the C12 EO6 /water system from reference 2. [The phases are: dilute surfactant solution (W), micellar solution (L1 ), hexagonal (H1 ), cubic (V1 ) and lamellar (L␣ ) liquid crystalline phases; solid surfactant is present in area S]. (b, lower) Phase diagram of the LAS/water system from ref. [4]. [A micellar solution is observed at low concentrations (L1 ). Next is a multiphase region where two or more L␣ phases coexist with a micellar solution. In this region L␣ is used to denote the appearance of two or more L␣ phases above the temperatures indicated by grey bars (observed using X-ray diffraction). L∗␣ indicates a situation whereby multiple L␣ phases coexist with water. Finally in the concentrated region, solid and multiple liquid crystalline phases coexist].
Equal amounts of LAS and C12 E6 were placed into a sealed tube and mixed at 80 ◦ C (above the melting temperature [2] of the C12 E6 −27 ◦ C). Most of the LAS dissolved in the C12 E6 to form a slightly cloudy, non-viscous liquid that appeared isotropic under crossed polarisers. The liquid is stable once formed; there is no obvious separation of the components. There is a small amount of undissolved material (Fig. 2) that appears to be a minor LAS component (possibly with the sodium sulphate as well, but the irregular, soft crystals are not commonly observed for inorganic solids). We estimate the volume fraction of this undissolved material as ca. 5% or less. Further measurements were also made using DSC and WAXS. No transitions were detected on heating/cooling DSC scans over the temperature range 0–100 ◦ C, see Table 2 below.
Fig. 2. Optical textures observed for the residual solid within the LAS/C12 EO6 (1:1) mixture (no water) at 25 ◦ C 3 weeks after initial heating to mix. Photographs taken with (a) partially crossed polarisers and (b) crossed polarisers. Width of both photographs is 1210 m.
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Table 2 Compositions of surfactant mixture for various regions in capillary scan (25 ◦ C). Description
˚ d spacing (A)
Mass fraction (±1%)
100%/L␣ boundary L␣ L␣ /cloudy boundary Cloudy Cloudy 2 Cloudy/L␣ boundary L␣ L␣ /homeotropic boundary Homeotropic Maltese crosses in homeotropic L1 /homeotropic
33.2 33.6 34.5 37.0 37.9 40.7 44.2 (d), 22.0 (d/2) 46.6 (d), 23.3 (d/2) No line 48.0 53.0
0.93 0.87 0.79 0.71 0.69 0.64 0.60 0.58 – 0.56 0.49
Fig. 3. Wide angle X-ray scattering data obtained for 100% LAS/C12 EO6 (1:1) mixture (no water) during a variable temperature scan whereby the sample was heated at 5 ◦ C/min from 0 to 100 ◦ C and cooled back to 0 ◦ C. Scans here are 0 ◦ C before heating and at 100 ◦ C.
The 3-week old sample was heated in a Lindemann tube from 0 to 140 ◦ C with X-ray scans taken every 10 ◦ C. At 0 ◦ C, we see some weak sharp peaks in the low angle pattern (see Fig. 3) that probably come from a small amount of solid LAS present (because ˚ supersodium sulphate does not have reflections at ca. 25–38 A) imposed on a asymmetric broad peak. In the high angle pattern, the same asymmetric broad peak occurs, probably indicative of polydisperse aggregates (completely different to LAS alone at 0 ◦ C where a number of reflections from the solid are seenmore than observed in our experiment suggesting that while in our mixture, most of the LAS is mixed with the C12 E6 , there is a small amount of solid LAS present [4]). There is no major change on heating to 140 ◦ C (Fig. 3) except for a very slight shift ˚ in d spacing for the main peaks. (+1 A)
3.2. LAS/C12 EO6 surfactant mixture (1:1) with water: water penetration scan The optical microscopy penetration scan is a well-established method for assessing surfactant mesophase behaviour [9,10], although it is usually employed for single surfactant systems rather than mixtures. The surfactant is sandwiched between the microscope slide and cover slip, enabling a concentration gradient to be established on contacting the surfactant with water. Alternatively, the surfactant is placed in a rectangular capillary and water is added. Both result in the formation of the various liquid crystals as a sequence of distinct bands having characteristic optical textures. With mixed surfactants it is important to bear in mind that there may be deviations of the local surfactant composition from the initial state, both because of different rates of diffusion and because phases with different surfactant compositions occur when multi-phase regions are present. On contacting the surfactant mixture with water at 25 ◦ C a large L␣ phase region formed very rapidly (Fig. 4a). As the L␣ phase grew in width, three distinct zones within the single phase became apparent. These can clearly be seen at 40 ◦ C (Fig. 4b). A zone that is easily sheared occurs on the water-rich side, followed by a cloudy section in the middle and a more viscous region on the surfactantrich side. There are no distinct phase boundaries between the regions when polarisers are uncrossed, possibly indicating that all have the same continuous phase. Further heating to 75 ◦ C does not change the textures observed and only seems to increase the width of the liquid crystalline regions. On heating to 90 ◦ C, the L␣ phase becomes homogenous throughout, clearly identifiable by the distinct oily streak and Maltese cross textures (Fig. 4c). On cooling, the three regions within the L␣ phase again become apparent at 84 ◦ C, even more so on cooling to room temperature (23.8 ◦ C, Fig. 4d). The results obtained in the phase penetration scan experiment are summarised in Table 3. 3.3. LAS/C12 EO6 surfactant mixture (1:1) with water: microscopy of samples with known compositions The samples for this study were stored at ambient temperature for 3 weeks before measurements, to allow some degree of equilibration. Each sample was sandwiched between a microscope slide and a cover-slip. All phase data were taken by focussing on the centre of a large sample with short experiment times to minimize effects of water loss. The samples were heated from ca. 3 ◦ C to 100 ◦ C at a rate of 5 ◦ C/min (rate reduced around phase transitions). The results are displayed in Table 4.
Table 3 Phase behaviour observed during temperature controlled phase penetration scan on LAS/C12 EO6 (1:1)/water system Temperature (◦ C)
Sequence of phases with increasing surfactant concentration
25–90 90–100 100–84 84–23.8
L1 L1 L1 L1
Less viscous L␣ region
Less viscous L␣ region
Cloudy L␣ region L␣ L␣ Cloudy L␣ region
More viscous L␣ region
More viscous L␣ region
L2 + undissolved LAS(a) L2 + undissolved LAS(a) L2 + undissolved LAS(a) L2 + undissolved LAS(a)
Sample was initially mixed separately by heating and cooling to ensure uniformity throughout before placing on the microscope slide for this experiment. This state is represented by (a) in the table.
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Fig. 4. Optical textures observed on initial contact of LAS/C12 EO6 (1:1) mixture with water at 25 ◦ C. (a) Immediate formation of large L␣ region (taken with crossed polarisers). (b) Three distinct zones within L␣ phase at 40 ◦ C (taken with partially crossed polarisers). (c) L␣ phase textures at 90 ◦ C. (d) Reappearance of three regions on cooling (23.8 ◦ C, taken with crossed polarisers) (width is 1210 m. S and W represent the surfactant-rich and water-rich sides, respectively).
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Table 4 Phase data from optical microscopy experiments on samples of known composition [LAS/C12 E6 (1:1) + water] during heating and cooling Sample composition (wt.% LAS:C12 E6 mixture in water) 10 20 30 40 50 60 70 80 85 90 95 100
Temperature (◦ C) 3 (H)
10 (H)
20 (H)
Two phase (49 ◦ C) Two phase (30 ◦ C)
30 (H)
40 (H)
50 (H)
60–100 (H)
L1 L1 L1 L1 L␣ L␣ L␣ L␣ L␣ L␣ L␣ L2 + S
100–3 (C) L1 L1 L1 L1 L␣ L␣ L␣ L␣ L␣ L␣ L␣ L2 + S
Any transition temperatures are given in parentheses, otherwise it should be presumed that the phase described was observed over the whole temperature range. ‘H’ and ‘C’ next to the given temperatures indicate whether the phases were observed on the heating or cooling runs respectively.
Up to 40% surfactant a micellar solution is formed. Between 40 and 85% only a L␣ phase is observed. Interestingly, at higher concentrations (90 and 95%), at least two L␣ phases are present at lower temperatures as indicated by initial cloudiness in the samples. The optical textures are those of the L␣ phase. That this cloudiness does not reappear on cooling highlights some hysteresis in the system. We did not examine in detail how long was required for the “cloudy” state to reappear, but our estimate is that hours-days are necessary. Similarly, on heating, the transition to a single L␣ phase must occur over a range of temperatures that, again, we did not study in detail. 3.4. LAS/C12 EO6 surfactant mixture (1:1) with water: X-ray diffraction The first measurements were made using a sample prepared for the phase penetration scan experiment as described
above. X-ray diffraction was applied to various points along the rectangular capillary identified by simultaneous microscopy observations. The results are displayed in Fig. 5. For the most concentrated region the broad peak is similar to that observed for the neat surfactant at high angle (Fig. 3 above). The “cloudy” L␣ phase region gives overlapping peaks, indicative of multiple phases, whilst only a sharp reflection is seen at the onset of the “clear” L␣ phase. Note that reflections within the homeotropic L␣ phase are very weak because the mesophase axis is along the X-ray beam. At the highest water concentration a very broad peak is observed again, indicating a micellar solution. Following these measurements, a series of samples of known composition were examined at 25 ◦ C after three weeks equilibration at room temperature. The X-ray data are displayed in Fig. 6 and tabulated in Table 5. Note that the sample within the “cloudy” region gives two sharp reflections, indicating the coexistence of two phases. Thus, the broad peaks observed in Fig. 5 reflect local compo-
Fig. 5. SAXS data taken at various positions in phase penetration scan of 50:50 LAS/C12 E6 at room temperature. Top left hand corner contains schematic diagram of phases observed in the capillary and the corresponding photograph taken during the experiment. Two dimension images are displayed at the right hand side of plot with borders colour coded to data series. Integrated plots give d spacings for each point on the photograph. Size of X-ray beam is 0.52 mm (approximately the size of the coloured spots on the microscopy image).
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Fig. 6. Small-angle X-ray scattering data for samples of known composition of LAS/C12 EO6 (1:1) at 25 ◦ C. Insets to main plot are expanded areas where intensity was low. Table 5 Alkyl chain and water layer dimensions (dalk , dw , resp.) along with calculated areas per molecule for selected samples of known composition [LAS:C12 E6 1:1 + water] at 25 ◦ C Sample composition (wt.% LAS:C12 E6 mixture in water)
˚ do spacing (A)
˚ dalk (A)
˚ dw (A)
Area/molecule ˚ 2) (A
49.9 59.7 79.9
53.4 44.5 33.9
16.3 16.0 16.0
37.1 28.5 17.9
53.0 53.8 54.0
sition variations due to the size of the X-ray beam. From the X-ray results, alkyl chain and water layer dimensions could be calculated along with values for area/molecule (Table 5) [11]. As expected, the d-spacings increase with water content. A calibration plot (Fig. 7) was compiled using data obtained for samples of known composition at 25 ◦ C. This was employed
Fig. 7. Calibration plot – d spacings vs. mass fraction for LAS:C12 EO6 (1:1)/water system.
to estimate the approximate compositions for the positions in the capillary experiment where X-ray diffraction was carried out (Table 6) and was subsequently used in studies of diffusion kinetics to be described in further papers [12].
Table 6 Microscope observations and corresponding d spacings for samples of known composition LAS:C12 EO6 (1:1) mixture with water at 25 ◦ C Sample compostion (wt.% LAS:C12 E6 (1:1) mixture in water)
˚ do (A)
˚ d/2 (A)
˚ d/3 (A)
Microscope observations
Phase
100
32.8
–
–
Non-birefringent and low viscosity isotropic liquid with some LAS solid present Cloudy with some oily streaks and Maltese crosses Cloudy with some oily streaks and Maltese crosses
LAS dissolved in C12 E6 (small amount un-dissolved LAS present) Multiple phases: L␣ + Intermediate? Two phase: L␣ + L␣
Oily streaks and maltese crosses Oily streaks and maltese crosses Lamellar streaks (dilute) Isotropic Isotropic Isotropic Isotropic
L␣ L␣ L␣ L1 L1 L1 L1
95.0 88.6 85.0 79.9 59.7 49.9 40.0 29.9 20.1 10.2
86.0, 33.9 16.53 31.8 33.3 16.5 31.95 33.9 16.6 44.5 21.8 53.4 26.5 56.93 (broad spacing) 60.98 (broad spacing) Broad diffuse spacing Broad diffuse spacing
10.8
– –
110
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Fig. 8. Small angle X-ray scattering data obtained for 95% LAS:C12 EO6 (1:1) during a variable temperature cycle; the scans depicted here are at 0, 5 and 10 ◦ C on the heating cycle.
X-ray measurements over a range of temperatures were also carried out for the three surfactant/water samples with the highest surfactant concentrations. The samples were cooled to 0 ◦ then heated at 5 ◦ /min to 95 or 120 ◦ before cooling back down to 0 ◦ . A scan was taken every 5 ◦ . Recall that a two-phase region was observed for the 90 and 95% LAS/C12 EO6 samples during the optical microscopy experiments. For the 95% sample the scans are given in Figs. 8 and 9. Clearly, there are multiple spacings present over 0–25 ◦ . The patterns at 10, 15 and 25 ◦ resemble those reported by Holmes et al. for intermediate phases ˚ This is [13–16], with a characteristic broad reflection at ca. 86 A. discussed further below. On further heating to 30 ◦ , the sample becomes one phase (L␣ ) and remains this way on subsequent heating to 120 ◦ (Fig. 10). There is no subsequent change on cooling to 0 ◦ suggesting that there is some hysteresis, however, the substance does eventually return to the original state in a matter of hours. Selected scans for the 90 and 85% samples are displayed in Figs. 10 and 11, respectively. For the 90% sample there appear to be multiple phases at 5 ◦ , which gradually reduce in number until, at 50 ◦ and above, a single lamellar phase occurs. This single phase remains on cooling to 10 ◦ . This behaviour is consistent with optical microscopy observations above. At 85% the number of reflections is very much smaller and the reflection ˚ is broader than usual. No sharp reflections occur at ca. 32 A ˚ reflection reappears at 60 ◦ on coolabove ca. 80 ◦ , but the 32 A
Fig. 9. Small angle X-ray scattering data obtained for 95% LAS:C12 EO6 (1:1) as for Fig. 7; the scans depicted here are at 15, 20, 25, 30 and 120 ◦ C on the heating cycle and 10 ◦ C on the cooling cycle.
Fig. 10. Small angle X-ray scattering data obtained for 90% LAS/C12 EO6 (1:1) during a variable temperature cycle. The scans depicted here are at 5, 10, 15, 20, 30, 35, 45, 50 and 95 ◦ C on the heating cycle and 10 ◦ C on the cooling cycle.
ing. Note that the sample gives a lamellar optical texture at all temperatures. It is clear that at low temperatures, a number of phases occur in the samples stored at room temperature for three weeks, and that these do not reappear immediately on cooling. The fairly ˚ are much larger than the broad diffraction lines at ca. 70–90 A ˚ Without a detailed investigation layer spacings of ca. 30–34 A. we are unable to draw firm conclusions about the phase structures. However, Holmes and co-workers have determined the structures of various “intermediate” mesophases that, for binary surfactant systems, occur between the lamellar and hexagonal phases, when a bicontinuous cubic phase (V1 ) does not form. These phases have a surface curvature intermediate between those of lamellar and hexagonal phases. Their structures closely resemble the lamellar phase, but the bilayers contain water-filled ˚ diameter. Their complex diffraction holes of roughly 50–100 A ˚ The head patterns do have reflections in the range 70–120 A. group area within these structures is typically that at the rod/disc shape transition. Table 5 shows that in the mixed lamellar phase, this is precisely where the values fall for the present system. Hence we suspect that the phase having diffraction lines at ca. ˚ is of this type. But, there are also other phases present. 70–90 A Immediately after heating and cooling only a single lamellar phase is present.
Fig. 11. Small angle X-ray scattering data obtained for 85% LAS/C12 EO6 (1:1) during a variable temperature scan whereby the sample was heated at 5 ◦ C/min from 0 to 120 ◦ C and cooled back to 0 ◦ C. Depicted here are the scans at 5, 50, 55, 60, 65, 70, 75, 80, 85 and 120 ◦ C on the heating cycle and 0 ◦ C on the cooling cycle.
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Table 7 Phase transition temperatures and enthalpy changes for differential scanning calorimetry experiments on C12 E6 /LAS/water samples Sample composition (Wt% LAS/C12 E6 in water) (%)
Cycle
No. of transitions
Onset temperature (◦ C)
Peak temperature (◦ C)
Enthalpy change (J/g of sample)
100 95
Heat–cool–heat Heat1 Cool 1 Heat 2
0 1 1 1
– 2.3 ca. 5 2.3
– 5.9 Not measurable 5.9
– 1.1 Not measurable 1.1
90
Heat 1 Cool Heat 2
1 1 1
28.0 37.2 28.0
35.2 30 35.2
1.3 1.5 1.3
85
Heat 1 Cool Heat 2
1 0 1
59.2 64.2 47.4
60.0 52.2 59.2
2.2 1.2 1.2
80
Heat 1 Cool Heat 2
1 0 1
78.5 – Not measurable
79.0 – ca. 80
1.2 – Not measurable
60 50 40 30
Heat–cool–Heat Heat–cool–Heat Heat–cool–Heat Heat–cool–Heat
0 0 0 0
– – – –
– – – –
– – – –
3.5. LAS/C12 EO6 surfactant mixture (1:1) with water: DSC measurements In order to obtain further insight into the temperature/history dependence of the phase behaviour, DSC measurements were carried out. Compositions in the range 30–100% LAS/C12 EO6 (1:1) were examined during heating/cooling/heating cycles. The results are displayed in Table 7. Measurable transitions were obtained only for the concentrated samples. Typical traces are illustrated in Fig. 12. The peaks are small and broad. They show hysteresis. No transitions were observed for the 100% mixture or for mixtures with <80% water, which is consistent with optical microscopy experiments. All the peaks observed are small and broad, with transition enthalpies much smaller than those expected for melting of a crystalline surfactant (ca. 200–250 J/g). They are of the magnitude expected for transitions between lyotropic phases [17–20]. For the 95% sample the peak at ca. 5 ◦ appears to be reversible (almost) but unfortunately it lies almost outside of the range of the cooling data. On heating it appears to correspond to the change in X-ray diffraction patterns over 0–10 ◦ . But there is no second transition over 20–30 ◦ corre˚ suggesting sponding to the disappearance of the line at ca. 86 A, that this transition involves a minimal structural reorganisation, consistent with the assignment of an intermediate-type structure. With both the 90 and 85% samples there is no obvious change in the X-ray data at the DSC transition temperatures. Either there is a relatively subtle change in the structure of the major lamellar phase, or the transition arises from a minor component. Note that the initial transitions are not completely reproduced on the cooling or the second heating scans. Again, this shows hysteresis in the phase behaviour. Further studies are required to resolve these issues.
Fig. 12. Differential scanning calorimetry curves for 95% LAS/C12 E6 (top) and 90% LAS/C12 E6 (lower) in water. Samples cooled to 0 ◦ C for 10 min after which heat–cool–heat cycles were carried out over 0–100–0–100 ◦ C at a rate of 5 ◦ C/min.
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tants, giving lower interfacial tensions and improved cleaning properties. The elimination of the viscous phases is likely to lead to an enhanced rate of dispersion/dissolution, again giving improved product performance. Acknowledgements Thanks go to Unilever and EPSRC for financial support and CCLRC Daresbury for X-ray facilities. References
Fig. 13. Phase diagram of LAS:C12 E6 (50:50)/water system.
4. Summary and phase diagram Fig. 13 summarises our best estimate of the equilibrium phase states for the LAS/C12 EO6 (1:1) system. There are large micellar and lamellar regions that occupy most of the diagram. On cooling samples with the highest surfactant concentrations (>85%) there is a slow (days) relaxation of the lamellar phase formed at high temperatures to a multi-mesophase mixture. One of the most interesting observations is the formation of a liquid phase above ca. 95% surfactant. This liquid must be regarded as resembling a molten electrolyte. The structure of this liquid phase is debatable. We know that LAS alone forms extensive aggregates because it is a mesophase. The structure of liquid non-ionics is unknown however. The C12 E6 alone has low viscosity when heated above its melting temperature (27 ◦ ) so if aggregates are present in the liquid then they must be small. When mixed with LAS, the viscosity of the liquid appears to be higher than for the non-ionic alone therefore the aggregate size is bigger, but not big enough to be a liquid crystal phase (as would be expected for the 100% volume fraction1 ). Thus, in the mixture the LAS exists in small aggregates rather than well-defined reversed micelles. Clearly, the addition of only 5% water (mol. ratio LAS/water = ca. 1:2) causes the aggregates to grow sufficiently to form the mesophase. Another important observation is that the viscous H1 and V1 phases formed by the C12 E6 alone are eliminated in the mixed system; only an L␣ phase is formed. The area ˚ 2 ) is just per molecule calculated from the X-ray data (a = 53–4 A ˚2 large enough for the H1 and V1 phases to form [a = ca. 48 A (rod/disc transition)]. Hence there must be a significant contribution to the phase behaviour from “negative curvature” within the surfactant film. This probably arises from the presence of the more branched LAS isomers (C4 and higher branches). It is likely to enhance the adsorption of the mixed surfactant system at flat interfaces (e.g. oil/water) over that for the single surfac-
[1] S. Hassan, W. Rowe, G.J.T. Tiddy, Surfactant liquid crystals, in: K. Holmberg (Ed.), Applied Surface and Colloid Chemistry, 1, John Wiley & Sons Ltd., Chichester, 2001, pp. 465–508. [2] T. Bostock, D.J. Mitchell, G.J.T. Tiddy, L. Waring, Phase-behavior of polyoxyethylene surfactants with water—mesophase structures and partial miscibility (cloud points), J. Chem. Soc., Faraday Trans. I 79 (1983) 975–1000. [3] A. Sein, J.B.F.N. Engberts, E. van der Linden, J.C. van der Pas, Lyotropic phases of dodecylbenzenesulfonates with different counterions in water, Langmuir 12 (1996) 2913–2923. [4] S. Casey, C. Richards, G.J.T. Tiddy, Unpublished results, 2005. [5] J.N. Israelachvili, D.J. Mitchell, B.W. Ninham, Theory of self assembly of hydrocarbon amphiphiles into micelles and bilayers, J. Chem Soc., Faraday Trans. II 72 (1976) 1525–1568. [6] J.N. Israelachvili, Intermolecular and Surface Forces, First ed., Academic Press, London, 1985. [7] J. Ockelford, K.S. Narayan, B.A. Timimi, G.J.T. Tiddy, An upper critical point in a lamellar liquid crystalline phase, J. Phys. Chem. 97 (2003) 1487–1491. [8] G.J.T. Tiddy, B.A. Timimi, K. Metcalfe, M. Walker, Unilever Internal Report, Port Sunlight, 1987. [9] A. Lawrence, Polar interaction in detergency, in: K. Durham (Ed.), Surface Activity and Detergency, Macmillan, London, 1961, pp. 159–192. [10] E.S. Blackmore, G.J.T. Tiddy, J. Chem. Soc., Faraday Trans. II 84 (1984) 1115–1127. [11] V. Luzzati, Chapter three, in: D. Chapman (Ed.), Biological Membranes, Academic Press, New York, pp. 71–123. [12] S. Casey, C. Richards, G.J.T. Tiddy, Unpublished results 2, 2005. [13] M.C. Holmes, Intermediate phases of surfactant–water mixtures, Curr. Opin. Colloids Interface Sci. 3 (1998) 485–492. [14] M.C. Holmes, G.J.T. Tiddy, Intermediate lyotropic liquid crystalline phases in the non-ionic surfactant C16 E6 /water system, J. Phys. Chem. 98 (1994) 3015–3023. [15] C.E. Fairhurst, M.C. Holmes, M.S. Leaver, Structure and morphology of the intermediate phase region in the non-ionic surfactant C16 E6 /water system, Langmuir 13 (1997) 4964–4975. [16] C.E. Fairhurst, M.C. Holmes, M.S. Leaver, Shear alignment of a rhombohedral mesh phase in an aqueous mixture of a long chain non-ionic surfactant, Langmuir 12 (1996) 6336–6340. [17] G.G. Chernik, A differential scanning calorimetry study of a binary system. I. Interpretation of DSC curves for isothermal and nonisothermal phase transitions in binary systems, J. Colloids Interface Sci. 141 (1991) 400–408. [18] G.G. Chernik, E.P. Sokolova, A differential scanning calorimetry study of a binary system. II. Phase diagram and transition enthalpies for the lyotropic liquid crystal dimethyldecylphosphine oxide-water system, J. Colloids Interface Sci. 141 (1991) 409–414. [19] G.G. Chernik, V.K. Filippov, A differential scanning calorimetry study of a binary system. III. Heat capacity of the lyotropic liquid crystal dimethyldecylphosphine oxide-water system, J. Colloids Interface Sci. 141 (1991) 415–425. [20] G.G. Chernik, Differential enthalpy and entropy effects of dissolution of phases for a nonionic surfactant-water system, Mol. Crys. Liq. Crys. 193 (1990) 93–97.
Liquid crystal and solution phases of sodium dodecyl-p-benzene sulfonate (LAS) and hexa-oxyethylene glycol dodecyl ether (C12E6); 1:1 mixtures in water Claire Richards a,∗ , Gordon J.T. Tiddy a , Siobhan Casey b a
School of Chemical Engineering & Analytical Science, University of Manchester, PO Box 88, Manchester M60 1QD, UK b Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral Merseyside CH63 3JW, UK Received 2 December 2005; received in revised form 6 April 2006; accepted 7 April 2006 Available online 28 April 2006
Abstract The liquid crystals and other phases formed when the mixed surfactant system sodium dodecyl-p-benzene sulfonate (LAS) and hexa-oxyethylene glycol dodecyl ether (C12 E6 , 1:1 mixtures by weight) is dispersed in water have been investigated using optical microscopy, X-ray diffraction and differential scanning calorimetry. Despite the fact that neat LAS is a multi-phase solid and C12 E6 , is a low melting crystalline solid, the mixed surfactants (no water) form a liquid phase. On addition of water there is a mixed mesophase coexistence region, followed by a lamellar phase and a micellar solution that extends up to ca. 50 wt.% surfactant. In the mixed mesophase region (ca. 88–95% surfactant) there appear to be “intermediate” and lamellar phases present. After heating, the system takes several days to relax to the original state. Based on simple “packing constraints” concepts hexagonal, cubic and lamellar phases are expected. The absence of the more viscous phases points to an additional contribution from the alkyl chains to the surfactant bilayer curvature. © 2006 Elsevier B.V. All rights reserved. Keywords: Surfactant; Liquid crystal; Dissolution; Mixed anionic nonionic
1. Introduction Mixed surfactants are widely used in products such as detergents, agrochemicals, pesticides and personal care creams. Frequently they are present at low water levels where liquid crystal and solid phases would be expected [1]. There exist very few studies of the phases that occur at high surfactant concentrations in these systems, despite their possible influence on product properties such as powder morphology and dissolution rates. One common combination is that of the anionic surfactant sodium linear alkyl p-benzene sulfonate and a non-ionic surfactant of the poly-oxyethylene glycol alkyl ethers type. Together these can give much improved product behaviour than either alone. From a fundamental scientific viewpoint these mixtures are of considerable interest because of the complex phase behaviour of each component alone with water. The oxyethylene surfactants
∗
Corresponding author. Tel.: +44 161 3068867; fax: +44 1613064399. E-mail address: [email protected] (C. Richards).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.04.037
form cubic (I1 , V1 ), hexagonal (H1 ) and lamellar (L␣ ) phases, depending on the size of the oxyethylene group [2]. Commercial materials have a very similar behaviour to pure surfactants. However, commercial alkyl p-benzene sulfonate surfactants comprise a mixture of alkyl chain lengths and all (except the 1-) positional isomers for the benzene sulfonate group (including optical isomers). The neat LAS has been reported to form a “C” (=crystalline) or “HC” (=hydrated crystalline) phase, depending on whether or not water is present. [3] However, we find that it exists as multiple solid and liquid crystal phases at room temperature rather than a single crystalline solid [4]. On addition of water it forms a lamellar phase which coexists with a micellar solution over a wide composition region. The overall pattern of surfactant liquid crystal structures and their formation mechanisms are well established, based on packing constraints [5,6] and maximum volume fractions of ideal spheres and rods [1,2]. However, there are many unexplained features. Why does the neat non-ionic surfactant form a liquid phase rather than liquid crystals (which appear to require the presence of water for their formation [2]), unlike most ionic surfactants and the non-ionic sugar surfactants? Is LAS soluble in the liquid non-
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Table 1 Alkyl chain and isomer distribution of the linear-alkylbenzene sulfonate, sodium salt (LAS) Isomer
Alkyl chain distribution (wt.% of sample) C10
>4-Phenyl 4-Phenyl 3-Phenyl 2-Phenyl Total
C11
C12
C13
4.2 3.2 3.1 3.2
17.0 8.3 7.8 7.4
18.9 5.1 4.9 4.4
7.5 2.0 1.8 1.2
13.7
40.5
33.2
12.6
Isomers are described according to the position of the phenyl group on the alkyl chain (so 4-phenyl would refer to an isomer with the benzene ring on the fourth carbon in the alkyl chain).
ionic, and does it induce mesophase formation? How does the presence of an upper consolute loop within the lamellar phase region that is found [7] for pure LAS isomers (coexistence below the critical temperature of two lamellar phases with different water contents) influence the behaviour of mixed surfactants? How does the presence of mixed surfactants influence the formation of non-cubic “intermediate” phases between the H1 and L␣ phases? Most importantly, can the simple model based on packing constraints [2,5,6] that describes mesophase formation for single surfactant/water systems be applied to mixed surfactants? We have undertaken a systematic examination of 1:1 (by weight)1 mixtures of sodium dodecyl-p-benzene sulfonate (LAS) with several non-ionic and ionic surfactants. In this paper we describe the results for the LAS:hexaethylene glycol dodecyl ether [C12 H25 (OCH2 CH2 )6 OH, C12 EO6 ] system. The surfactant/water mixtures have been examined using optical microscopy, DSC, and X-ray diffraction. Remarkably, the data show that the mixed surfactant system exhibits simple phase behaviour at medium concentrations, but the system becomes very complex at high surfactant levels. 2. Materials and methods 2.1. Materials The sodium dodecyl-p-benzene sulfonate (LAS) was a commercial material obtained from Unilever. It had been freezedried following the removal of excess sodium sulphate (<0.6% remaining) and organic impurities (<0.1% remaining). The material was stored in a sealed container in a dessicator. A full analysis obtained at Unilever by mass spectroscopy (isomer and chain length distribution) is given in Table 1. C12 EO6 was pure material (>99.7% as determined by gas chromatography) obtained from Sigma Aldrich and used as received. 2.2. Methods All optical microscopy experiments were carried out using a Carl Zeiss Axioplan-2 polarising optical microscope; photographs were taken using a JVC-TK 1280E CCD camera and 1
Corresponding to a molar ratio of 1:1.3 (C12 E6 :LAS).
analysed using image analysis software attached to the microscope. Where required, temperature control was obtained using a Linkam hot-stage attachment for the optical microscope with an accuracy of ±1 ◦ C. Cooling rates were controlled using a stream of nitrogen gas from a reservoir of liquid nitrogen, thus allowing rapid cooling rates to be obtained. Small angle X-ray diffraction was carried out on station 16.1 at the synchrotron radiation source (SRS), Daresbury Laboratories, Warrington with X-rays ˚ The detector used was a Quantum 4 ADS. of wavelength 1.4 A. Samples were contained in heat sealed Lindemann tubes to prevent water loss and heating and cooling was carried out using a Linkam hot stage made specifically to fit capillaries. Temperatures were obtained with an accuracy of ±0.5 ◦ C (calibrated by Linkam and confirmed by measuring melting temperatures for pure solids whose heating and cooling behaviour was well documented). All DSC measurements were carried out on a Thermal Analysis DSC Q100. The samples were placed into hermetically sealed aluminium sample pans to prevent water loss. They were weighed before and after the experiment to ensure no water had been lost. The instrument was calibrated using a known amount of indium which has a very sharp melting temperature of 156.6 ◦ C. All traces were measured at a rate of 5 ◦ C/min following heat-cool-reheat cycles between the temperatures of 0 and 100 ◦ C. 2.3. Predictions from packing constraints [1,5,6] From the “packing constraints” concepts one can obtain an overall picture of surfactant phase behaviour. A brief description is given here. We assume that there are three idealised micelle shapes: spheres, rods and discs. Micellar solutions (L1 ) exist only up to a certain concentration of surfactant because the micelles become ordered, forming mesophases at higher concentrations. The micelles are assumed to be smooth, with a sharp boundary between the aqueous (head groups plus water) and hydrophobic (all alkyl chains) regions. The radii (r) of spherical and rod micelles cannot be larger than the all-trans alkyl chain (lt ). Hence as the micelle surface area of the head group (a) decreases, the sequence of micelle shapes is sphere → rod → disc. The transitions occur when a = 3v/ lt (sphere → rod) and a = 2v/ lt (rod → disc, where v volume of hydrophobic group). With conventional surfactants having a linear alkyl chain ˚ 2 (sphere/rod) and the shape transitions occur at a = ca. 68 A 2 ˚ (rod/disc). For spherical micelles the mesophase a = ca. 46 A sequence with increasing concentration is: small-micelle cubic (I1 ) → hexagonal (H1 ) → bicontinuous cubic (V1 ) [or various intermediate phases (Int)] → lamellar phase (L␣ ). With rod micelles it is: hexagonal (H1 ) → bicontinuous cubic (V1 ) [or various intermediate phases (Int)] → lamellar phase (L␣ ), while only lamellar phase (L␣ ) is seen for disc micelles. For the surfactants studied here, the phase behaviour is given in Fig. 1. The non-ionic surfactant C12 EO6 forms H1 , V1 and L␣ phases below ca. 38 ◦ C, changing to L␣ mesophase only above this, whilst LAS in water forms only L␣ phase with a large L1 /L␣ coexistence region over a wide temperature range. The
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detailed behaviour of the lamellar phase is very unusual; several different lamellar phases coexist with a micellar solution in the L1 + L␣ region. At higher concentrations, a complicated multiphase region occurs also including further multiple L␣ phases. In considering the application of packing constraints we need estimates of the parameters v, a and lt for the surfactant mixture. Within the mixed micelles the value of lt required is that of the component(s) with the longest chains. For C12 EO6 we take ˚ 2 and lt = 16.8 A ˚ at 25 ◦ C; for LAS we the values [8] a = 48.1 A 2 ˚ take a = 60 A [2] but the value of lt is more difficult to establish. Because of the mixture of chain lengths and isomers, lt varies ˚ [taking the benzene ring contribution as 2.8 A. ˚ from 10 to 20 A Thus the lt value for the mixed micelles is some average of the longest LAS components. Whilst only one surfactant chain has to be in the all-trans configuration to give the maximum micelle radius, this can not always be the same molecule. Hence we (arbitrarily) assume that at least 20% of the micelle members must be capable of the maximum lt value. Thus the value required is that of the longest 40% LAS isomers. Using the chain length and isomer distributions from Table 1, the value estimated is ˚ Thus the shape transitions occur with a = ca. 72 A ˚2 lt = 17.6 A. 2 ˚ (sphere/rod) and a = ca. 48 A (rod/disc). Thus if the a values of the surfactants in mixed aggregates are the same as in the single surfactant systems, we expect H1 , V1 and L␣ phases. 3. Results 3.1. LAS/C12 EO6 surfactant mixture (1:1)
Fig. 1. (a, top) Phase diagram of the C12 EO6 /water system from reference 2. [The phases are: dilute surfactant solution (W), micellar solution (L1 ), hexagonal (H1 ), cubic (V1 ) and lamellar (L␣ ) liquid crystalline phases; solid surfactant is present in area S]. (b, lower) Phase diagram of the LAS/water system from ref. [4]. [A micellar solution is observed at low concentrations (L1 ). Next is a multiphase region where two or more L␣ phases coexist with a micellar solution. In this region L␣ is used to denote the appearance of two or more L␣ phases above the temperatures indicated by grey bars (observed using X-ray diffraction). L∗␣ indicates a situation whereby multiple L␣ phases coexist with water. Finally in the concentrated region, solid and multiple liquid crystalline phases coexist].
Equal amounts of LAS and C12 E6 were placed into a sealed tube and mixed at 80 ◦ C (above the melting temperature [2] of the C12 E6 −27 ◦ C). Most of the LAS dissolved in the C12 E6 to form a slightly cloudy, non-viscous liquid that appeared isotropic under crossed polarisers. The liquid is stable once formed; there is no obvious separation of the components. There is a small amount of undissolved material (Fig. 2) that appears to be a minor LAS component (possibly with the sodium sulphate as well, but the irregular, soft crystals are not commonly observed for inorganic solids). We estimate the volume fraction of this undissolved material as ca. 5% or less. Further measurements were also made using DSC and WAXS. No transitions were detected on heating/cooling DSC scans over the temperature range 0–100 ◦ C, see Table 2 below.
Fig. 2. Optical textures observed for the residual solid within the LAS/C12 EO6 (1:1) mixture (no water) at 25 ◦ C 3 weeks after initial heating to mix. Photographs taken with (a) partially crossed polarisers and (b) crossed polarisers. Width of both photographs is 1210 m.
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Table 2 Compositions of surfactant mixture for various regions in capillary scan (25 ◦ C). Description
˚ d spacing (A)
Mass fraction (±1%)
100%/L␣ boundary L␣ L␣ /cloudy boundary Cloudy Cloudy 2 Cloudy/L␣ boundary L␣ L␣ /homeotropic boundary Homeotropic Maltese crosses in homeotropic L1 /homeotropic
33.2 33.6 34.5 37.0 37.9 40.7 44.2 (d), 22.0 (d/2) 46.6 (d), 23.3 (d/2) No line 48.0 53.0
0.93 0.87 0.79 0.71 0.69 0.64 0.60 0.58 – 0.56 0.49
Fig. 3. Wide angle X-ray scattering data obtained for 100% LAS/C12 EO6 (1:1) mixture (no water) during a variable temperature scan whereby the sample was heated at 5 ◦ C/min from 0 to 100 ◦ C and cooled back to 0 ◦ C. Scans here are 0 ◦ C before heating and at 100 ◦ C.
The 3-week old sample was heated in a Lindemann tube from 0 to 140 ◦ C with X-ray scans taken every 10 ◦ C. At 0 ◦ C, we see some weak sharp peaks in the low angle pattern (see Fig. 3) that probably come from a small amount of solid LAS present (because ˚ supersodium sulphate does not have reflections at ca. 25–38 A) imposed on a asymmetric broad peak. In the high angle pattern, the same asymmetric broad peak occurs, probably indicative of polydisperse aggregates (completely different to LAS alone at 0 ◦ C where a number of reflections from the solid are seenmore than observed in our experiment suggesting that while in our mixture, most of the LAS is mixed with the C12 E6 , there is a small amount of solid LAS present [4]). There is no major change on heating to 140 ◦ C (Fig. 3) except for a very slight shift ˚ in d spacing for the main peaks. (+1 A)
3.2. LAS/C12 EO6 surfactant mixture (1:1) with water: water penetration scan The optical microscopy penetration scan is a well-established method for assessing surfactant mesophase behaviour [9,10], although it is usually employed for single surfactant systems rather than mixtures. The surfactant is sandwiched between the microscope slide and cover slip, enabling a concentration gradient to be established on contacting the surfactant with water. Alternatively, the surfactant is placed in a rectangular capillary and water is added. Both result in the formation of the various liquid crystals as a sequence of distinct bands having characteristic optical textures. With mixed surfactants it is important to bear in mind that there may be deviations of the local surfactant composition from the initial state, both because of different rates of diffusion and because phases with different surfactant compositions occur when multi-phase regions are present. On contacting the surfactant mixture with water at 25 ◦ C a large L␣ phase region formed very rapidly (Fig. 4a). As the L␣ phase grew in width, three distinct zones within the single phase became apparent. These can clearly be seen at 40 ◦ C (Fig. 4b). A zone that is easily sheared occurs on the water-rich side, followed by a cloudy section in the middle and a more viscous region on the surfactantrich side. There are no distinct phase boundaries between the regions when polarisers are uncrossed, possibly indicating that all have the same continuous phase. Further heating to 75 ◦ C does not change the textures observed and only seems to increase the width of the liquid crystalline regions. On heating to 90 ◦ C, the L␣ phase becomes homogenous throughout, clearly identifiable by the distinct oily streak and Maltese cross textures (Fig. 4c). On cooling, the three regions within the L␣ phase again become apparent at 84 ◦ C, even more so on cooling to room temperature (23.8 ◦ C, Fig. 4d). The results obtained in the phase penetration scan experiment are summarised in Table 3. 3.3. LAS/C12 EO6 surfactant mixture (1:1) with water: microscopy of samples with known compositions The samples for this study were stored at ambient temperature for 3 weeks before measurements, to allow some degree of equilibration. Each sample was sandwiched between a microscope slide and a cover-slip. All phase data were taken by focussing on the centre of a large sample with short experiment times to minimize effects of water loss. The samples were heated from ca. 3 ◦ C to 100 ◦ C at a rate of 5 ◦ C/min (rate reduced around phase transitions). The results are displayed in Table 4.
Table 3 Phase behaviour observed during temperature controlled phase penetration scan on LAS/C12 EO6 (1:1)/water system Temperature (◦ C)
Sequence of phases with increasing surfactant concentration
25–90 90–100 100–84 84–23.8
L1 L1 L1 L1
Less viscous L␣ region
Less viscous L␣ region
Cloudy L␣ region L␣ L␣ Cloudy L␣ region
More viscous L␣ region
More viscous L␣ region
L2 + undissolved LAS(a) L2 + undissolved LAS(a) L2 + undissolved LAS(a) L2 + undissolved LAS(a)
Sample was initially mixed separately by heating and cooling to ensure uniformity throughout before placing on the microscope slide for this experiment. This state is represented by (a) in the table.
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Fig. 4. Optical textures observed on initial contact of LAS/C12 EO6 (1:1) mixture with water at 25 ◦ C. (a) Immediate formation of large L␣ region (taken with crossed polarisers). (b) Three distinct zones within L␣ phase at 40 ◦ C (taken with partially crossed polarisers). (c) L␣ phase textures at 90 ◦ C. (d) Reappearance of three regions on cooling (23.8 ◦ C, taken with crossed polarisers) (width is 1210 m. S and W represent the surfactant-rich and water-rich sides, respectively).
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Table 4 Phase data from optical microscopy experiments on samples of known composition [LAS/C12 E6 (1:1) + water] during heating and cooling Sample composition (wt.% LAS:C12 E6 mixture in water) 10 20 30 40 50 60 70 80 85 90 95 100
Temperature (◦ C) 3 (H)
10 (H)
20 (H)
Two phase (49 ◦ C) Two phase (30 ◦ C)
30 (H)
40 (H)
50 (H)
60–100 (H)
L1 L1 L1 L1 L␣ L␣ L␣ L␣ L␣ L␣ L␣ L2 + S
100–3 (C) L1 L1 L1 L1 L␣ L␣ L␣ L␣ L␣ L␣ L␣ L2 + S
Any transition temperatures are given in parentheses, otherwise it should be presumed that the phase described was observed over the whole temperature range. ‘H’ and ‘C’ next to the given temperatures indicate whether the phases were observed on the heating or cooling runs respectively.
Up to 40% surfactant a micellar solution is formed. Between 40 and 85% only a L␣ phase is observed. Interestingly, at higher concentrations (90 and 95%), at least two L␣ phases are present at lower temperatures as indicated by initial cloudiness in the samples. The optical textures are those of the L␣ phase. That this cloudiness does not reappear on cooling highlights some hysteresis in the system. We did not examine in detail how long was required for the “cloudy” state to reappear, but our estimate is that hours-days are necessary. Similarly, on heating, the transition to a single L␣ phase must occur over a range of temperatures that, again, we did not study in detail. 3.4. LAS/C12 EO6 surfactant mixture (1:1) with water: X-ray diffraction The first measurements were made using a sample prepared for the phase penetration scan experiment as described
above. X-ray diffraction was applied to various points along the rectangular capillary identified by simultaneous microscopy observations. The results are displayed in Fig. 5. For the most concentrated region the broad peak is similar to that observed for the neat surfactant at high angle (Fig. 3 above). The “cloudy” L␣ phase region gives overlapping peaks, indicative of multiple phases, whilst only a sharp reflection is seen at the onset of the “clear” L␣ phase. Note that reflections within the homeotropic L␣ phase are very weak because the mesophase axis is along the X-ray beam. At the highest water concentration a very broad peak is observed again, indicating a micellar solution. Following these measurements, a series of samples of known composition were examined at 25 ◦ C after three weeks equilibration at room temperature. The X-ray data are displayed in Fig. 6 and tabulated in Table 5. Note that the sample within the “cloudy” region gives two sharp reflections, indicating the coexistence of two phases. Thus, the broad peaks observed in Fig. 5 reflect local compo-
Fig. 5. SAXS data taken at various positions in phase penetration scan of 50:50 LAS/C12 E6 at room temperature. Top left hand corner contains schematic diagram of phases observed in the capillary and the corresponding photograph taken during the experiment. Two dimension images are displayed at the right hand side of plot with borders colour coded to data series. Integrated plots give d spacings for each point on the photograph. Size of X-ray beam is 0.52 mm (approximately the size of the coloured spots on the microscopy image).
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Fig. 6. Small-angle X-ray scattering data for samples of known composition of LAS/C12 EO6 (1:1) at 25 ◦ C. Insets to main plot are expanded areas where intensity was low. Table 5 Alkyl chain and water layer dimensions (dalk , dw , resp.) along with calculated areas per molecule for selected samples of known composition [LAS:C12 E6 1:1 + water] at 25 ◦ C Sample composition (wt.% LAS:C12 E6 mixture in water)
˚ do spacing (A)
˚ dalk (A)
˚ dw (A)
Area/molecule ˚ 2) (A
49.9 59.7 79.9
53.4 44.5 33.9
16.3 16.0 16.0
37.1 28.5 17.9
53.0 53.8 54.0
sition variations due to the size of the X-ray beam. From the X-ray results, alkyl chain and water layer dimensions could be calculated along with values for area/molecule (Table 5) [11]. As expected, the d-spacings increase with water content. A calibration plot (Fig. 7) was compiled using data obtained for samples of known composition at 25 ◦ C. This was employed
Fig. 7. Calibration plot – d spacings vs. mass fraction for LAS:C12 EO6 (1:1)/water system.
to estimate the approximate compositions for the positions in the capillary experiment where X-ray diffraction was carried out (Table 6) and was subsequently used in studies of diffusion kinetics to be described in further papers [12].
Table 6 Microscope observations and corresponding d spacings for samples of known composition LAS:C12 EO6 (1:1) mixture with water at 25 ◦ C Sample compostion (wt.% LAS:C12 E6 (1:1) mixture in water)
˚ do (A)
˚ d/2 (A)
˚ d/3 (A)
Microscope observations
Phase
100
32.8
–
–
Non-birefringent and low viscosity isotropic liquid with some LAS solid present Cloudy with some oily streaks and Maltese crosses Cloudy with some oily streaks and Maltese crosses
LAS dissolved in C12 E6 (small amount un-dissolved LAS present) Multiple phases: L␣ + Intermediate? Two phase: L␣ + L␣
Oily streaks and maltese crosses Oily streaks and maltese crosses Lamellar streaks (dilute) Isotropic Isotropic Isotropic Isotropic
L␣ L␣ L␣ L1 L1 L1 L1
95.0 88.6 85.0 79.9 59.7 49.9 40.0 29.9 20.1 10.2
86.0, 33.9 16.53 31.8 33.3 16.5 31.95 33.9 16.6 44.5 21.8 53.4 26.5 56.93 (broad spacing) 60.98 (broad spacing) Broad diffuse spacing Broad diffuse spacing
10.8
– –
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Fig. 8. Small angle X-ray scattering data obtained for 95% LAS:C12 EO6 (1:1) during a variable temperature cycle; the scans depicted here are at 0, 5 and 10 ◦ C on the heating cycle.
X-ray measurements over a range of temperatures were also carried out for the three surfactant/water samples with the highest surfactant concentrations. The samples were cooled to 0 ◦ then heated at 5 ◦ /min to 95 or 120 ◦ before cooling back down to 0 ◦ . A scan was taken every 5 ◦ . Recall that a two-phase region was observed for the 90 and 95% LAS/C12 EO6 samples during the optical microscopy experiments. For the 95% sample the scans are given in Figs. 8 and 9. Clearly, there are multiple spacings present over 0–25 ◦ . The patterns at 10, 15 and 25 ◦ resemble those reported by Holmes et al. for intermediate phases ˚ This is [13–16], with a characteristic broad reflection at ca. 86 A. discussed further below. On further heating to 30 ◦ , the sample becomes one phase (L␣ ) and remains this way on subsequent heating to 120 ◦ (Fig. 10). There is no subsequent change on cooling to 0 ◦ suggesting that there is some hysteresis, however, the substance does eventually return to the original state in a matter of hours. Selected scans for the 90 and 85% samples are displayed in Figs. 10 and 11, respectively. For the 90% sample there appear to be multiple phases at 5 ◦ , which gradually reduce in number until, at 50 ◦ and above, a single lamellar phase occurs. This single phase remains on cooling to 10 ◦ . This behaviour is consistent with optical microscopy observations above. At 85% the number of reflections is very much smaller and the reflection ˚ is broader than usual. No sharp reflections occur at ca. 32 A ˚ reflection reappears at 60 ◦ on coolabove ca. 80 ◦ , but the 32 A
Fig. 9. Small angle X-ray scattering data obtained for 95% LAS:C12 EO6 (1:1) as for Fig. 7; the scans depicted here are at 15, 20, 25, 30 and 120 ◦ C on the heating cycle and 10 ◦ C on the cooling cycle.
Fig. 10. Small angle X-ray scattering data obtained for 90% LAS/C12 EO6 (1:1) during a variable temperature cycle. The scans depicted here are at 5, 10, 15, 20, 30, 35, 45, 50 and 95 ◦ C on the heating cycle and 10 ◦ C on the cooling cycle.
ing. Note that the sample gives a lamellar optical texture at all temperatures. It is clear that at low temperatures, a number of phases occur in the samples stored at room temperature for three weeks, and that these do not reappear immediately on cooling. The fairly ˚ are much larger than the broad diffraction lines at ca. 70–90 A ˚ Without a detailed investigation layer spacings of ca. 30–34 A. we are unable to draw firm conclusions about the phase structures. However, Holmes and co-workers have determined the structures of various “intermediate” mesophases that, for binary surfactant systems, occur between the lamellar and hexagonal phases, when a bicontinuous cubic phase (V1 ) does not form. These phases have a surface curvature intermediate between those of lamellar and hexagonal phases. Their structures closely resemble the lamellar phase, but the bilayers contain water-filled ˚ diameter. Their complex diffraction holes of roughly 50–100 A ˚ The head patterns do have reflections in the range 70–120 A. group area within these structures is typically that at the rod/disc shape transition. Table 5 shows that in the mixed lamellar phase, this is precisely where the values fall for the present system. Hence we suspect that the phase having diffraction lines at ca. ˚ is of this type. But, there are also other phases present. 70–90 A Immediately after heating and cooling only a single lamellar phase is present.
Fig. 11. Small angle X-ray scattering data obtained for 85% LAS/C12 EO6 (1:1) during a variable temperature scan whereby the sample was heated at 5 ◦ C/min from 0 to 120 ◦ C and cooled back to 0 ◦ C. Depicted here are the scans at 5, 50, 55, 60, 65, 70, 75, 80, 85 and 120 ◦ C on the heating cycle and 0 ◦ C on the cooling cycle.
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Table 7 Phase transition temperatures and enthalpy changes for differential scanning calorimetry experiments on C12 E6 /LAS/water samples Sample composition (Wt% LAS/C12 E6 in water) (%)
Cycle
No. of transitions
Onset temperature (◦ C)
Peak temperature (◦ C)
Enthalpy change (J/g of sample)
100 95
Heat–cool–heat Heat1 Cool 1 Heat 2
0 1 1 1
– 2.3 ca. 5 2.3
– 5.9 Not measurable 5.9
– 1.1 Not measurable 1.1
90
Heat 1 Cool Heat 2
1 1 1
28.0 37.2 28.0
35.2 30 35.2
1.3 1.5 1.3
85
Heat 1 Cool Heat 2
1 0 1
59.2 64.2 47.4
60.0 52.2 59.2
2.2 1.2 1.2
80
Heat 1 Cool Heat 2
1 0 1
78.5 – Not measurable
79.0 – ca. 80
1.2 – Not measurable
60 50 40 30
Heat–cool–Heat Heat–cool–Heat Heat–cool–Heat Heat–cool–Heat
0 0 0 0
– – – –
– – – –
– – – –
3.5. LAS/C12 EO6 surfactant mixture (1:1) with water: DSC measurements In order to obtain further insight into the temperature/history dependence of the phase behaviour, DSC measurements were carried out. Compositions in the range 30–100% LAS/C12 EO6 (1:1) were examined during heating/cooling/heating cycles. The results are displayed in Table 7. Measurable transitions were obtained only for the concentrated samples. Typical traces are illustrated in Fig. 12. The peaks are small and broad. They show hysteresis. No transitions were observed for the 100% mixture or for mixtures with <80% water, which is consistent with optical microscopy experiments. All the peaks observed are small and broad, with transition enthalpies much smaller than those expected for melting of a crystalline surfactant (ca. 200–250 J/g). They are of the magnitude expected for transitions between lyotropic phases [17–20]. For the 95% sample the peak at ca. 5 ◦ appears to be reversible (almost) but unfortunately it lies almost outside of the range of the cooling data. On heating it appears to correspond to the change in X-ray diffraction patterns over 0–10 ◦ . But there is no second transition over 20–30 ◦ corre˚ suggesting sponding to the disappearance of the line at ca. 86 A, that this transition involves a minimal structural reorganisation, consistent with the assignment of an intermediate-type structure. With both the 90 and 85% samples there is no obvious change in the X-ray data at the DSC transition temperatures. Either there is a relatively subtle change in the structure of the major lamellar phase, or the transition arises from a minor component. Note that the initial transitions are not completely reproduced on the cooling or the second heating scans. Again, this shows hysteresis in the phase behaviour. Further studies are required to resolve these issues.
Fig. 12. Differential scanning calorimetry curves for 95% LAS/C12 E6 (top) and 90% LAS/C12 E6 (lower) in water. Samples cooled to 0 ◦ C for 10 min after which heat–cool–heat cycles were carried out over 0–100–0–100 ◦ C at a rate of 5 ◦ C/min.
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tants, giving lower interfacial tensions and improved cleaning properties. The elimination of the viscous phases is likely to lead to an enhanced rate of dispersion/dissolution, again giving improved product performance. Acknowledgements Thanks go to Unilever and EPSRC for financial support and CCLRC Daresbury for X-ray facilities. References
Fig. 13. Phase diagram of LAS:C12 E6 (50:50)/water system.
4. Summary and phase diagram Fig. 13 summarises our best estimate of the equilibrium phase states for the LAS/C12 EO6 (1:1) system. There are large micellar and lamellar regions that occupy most of the diagram. On cooling samples with the highest surfactant concentrations (>85%) there is a slow (days) relaxation of the lamellar phase formed at high temperatures to a multi-mesophase mixture. One of the most interesting observations is the formation of a liquid phase above ca. 95% surfactant. This liquid must be regarded as resembling a molten electrolyte. The structure of this liquid phase is debatable. We know that LAS alone forms extensive aggregates because it is a mesophase. The structure of liquid non-ionics is unknown however. The C12 E6 alone has low viscosity when heated above its melting temperature (27 ◦ ) so if aggregates are present in the liquid then they must be small. When mixed with LAS, the viscosity of the liquid appears to be higher than for the non-ionic alone therefore the aggregate size is bigger, but not big enough to be a liquid crystal phase (as would be expected for the 100% volume fraction1 ). Thus, in the mixture the LAS exists in small aggregates rather than well-defined reversed micelles. Clearly, the addition of only 5% water (mol. ratio LAS/water = ca. 1:2) causes the aggregates to grow sufficiently to form the mesophase. Another important observation is that the viscous H1 and V1 phases formed by the C12 E6 alone are eliminated in the mixed system; only an L␣ phase is formed. The area ˚ 2 ) is just per molecule calculated from the X-ray data (a = 53–4 A ˚2 large enough for the H1 and V1 phases to form [a = ca. 48 A (rod/disc transition)]. Hence there must be a significant contribution to the phase behaviour from “negative curvature” within the surfactant film. This probably arises from the presence of the more branched LAS isomers (C4 and higher branches). It is likely to enhance the adsorption of the mixed surfactant system at flat interfaces (e.g. oil/water) over that for the single surfac-
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