Langmuir 2007, 23, 467-474
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Lateral Phase Separation Gives Multiple Lamellar Phases in a “Binary” Surfactant/Water System: The Phase Behavior of Sodium Alkyl Benzene Sulfonate/Water Mixtures Claire Richards,*,† Gordon J. T. Tiddy,*,† and Siobhan Casey‡ School of Chemical Engineering & Analytical Science, UniVersity of Manchester, PO Box 88, Manchester, M60 1QD, UK, and UnileVer Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral Merseyside, CH63 3JW, UK ReceiVed June 14, 2006. In Final Form: September 15, 2006 We have examined the structure of the lamellar phase (LR) that coexists with a micellar solution (L1) for a commercial sodium alkyl benzene sulfonate (LAS) mixed with water. The surfactant is a mixture containing C10-C13 alkyl chains, having all positional isomers of the benzene sulfonate group present except the 1-isomer. Unusually for ionic surfactants, the difference in compositions between the coexisting L1 and LR phases is large (L1 ) ∼20 wt % LAS; LR ) ∼65 wt %). The main technique employed was X-ray diffraction, supplemented by optical microscopy and differential scanning calorimetry (DSC). At ambient temperatures, the lamellar phase gives a single diffraction pattern with the main reflection (d) at ∼32.5 Å, whatever the composition. However, above 40 °C, the diffraction peak becomes broader and moves to higher d values. At higher temperatures still, several distinct and different diffraction peaks are observed, differing in detail according to composition. The largest d values (∼42-4 Å) are observed for the lowest LAS concentrations, while the largest number of separate reflections (five) occurs for samples with ∼44-50% LAS, both at the highest temperatures. Although there are some differences in the data between heating and cooling cycles, the d values return to the original value at low temperature. There are no observable transitions in DSC, nor is there any heterogeneity in the lamellar phase observable by microscopy. The data clearly indicate that there is some lateral separation of the different LAS isomers within the bilayers, which results in the formation of local lamellar regions having different surfactant compositions. This lateral phase separation may arise from the presence of an (electrostatic) attractive interaction, which gives rise to an upper consolute loop within the lamellar phase region of a pure LAS isomer. Similar mechanisms may occur in biological membranes and could be responsible for the occurrence of membrane lipid patches.
Introduction Sodium alkyl benzene sulfonates are the most commonly used synthetic anionic surfactant class. They are used as detergents, dispersants, emulsifiers, and wetting agents in a very wide range of products. It is known that the surfactants form a lamellar/ micellar phase dispersion over a wide composition range1,2 at room temperature, but what occurs at higher temperatures and high concentrations is not known. Such information is essential to understand the state of the surfactant in products such as powders and tablets because phase separation of ingredients is commonplace under these conditions of low water activity, even when the concentration of surfactant is relatively low. A partial phase diagram has been produced for a single pure surfactant, sodium 5-dodecylbenzene sulfonate,3 where the lamellar phase occupies a very large region. Also, uniquely for anionic surfactants, there is an upper critical loop within the lamellar phase where two lamellar phases having different water concentrations coexist. This behavior indicates the presence of an attractive force between the charged surfactant layers. Commercial linear alkylbenzene sulfonate surfactants contain a number of linear alkyl chains (typically Cn - Cn+4) where the * Corresponding author. E-mail:
[email protected] (C.R.);
[email protected] (G.J.T.T.). † University of Manchester. ‡ Unilever Research Port Sunlight Laboratory. (1) Sein, A.; Engberts, J.; van der Linden, E.; van de Pas, J. C. Langmuir 1996, 12, 2913. (2) Franses, E. I.; Puig, J. E.; Talmon, Y.; Miller, W. G.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1980, 84, 1547. (3) Ockelford, J.; Timimi, B. A.; Narayan, K. S.; Tiddy, G. J. T. J. Phys. Chem. 1993, 97, 6767.
benzene-sulfonate group is located at all carbon positions except C1. Thus, upward of 20 different surfactants are present in significant quantities, most having an asymmetric carbon giving optical isomers. In this article, we describe an investigation of a commercial material containing C10-C13 alkyl chains where we examine in particular the micellar solution (L1)-lamellar phase (LR) coexistence region. We have employed polarizing microscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC) to elucidate the phase behavior, as part of a more general study. Conventional surfactants mix together very well both in micelles and mesophases4 (hexagonal, cubic, lamellar, etc.) because the surfactant aggregates in both have liquidlike molecular mobility. While different mixed-surfactant ratios occur in coexisting phases (e.g., hexagonal + lamellar phase dispersions), when L1 + LR dispersions occur, each phase has a single, well-defined composition. Here we present evidence that multiple compositions are present within the lamellar phase of this C10C13 linear alkylbenzene sulfonate (LAS) when it is dispersed in a continuous micellar phase. Moreover, the phase structure changes with temperature and shows hysteresis. This is extremely unusual. We propose that this behavior is a consequence of the interlayer attractive interaction referred to above. Experimental Section 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 sulfate (4-phenyl 4-phenyl 3-phenyl 2-phenyl total
4.2 3.2 3.1 3.2 13.7
17.0 8.3 7.8 7.4 40.5
18.9 5.1 4.9 4.4 33.2
7.5 2.0 1.8 1.2 12.6
remaining) and organic impurities (
