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Liquid Crystal Phase Behavior of Branched Poly(oxyethylene) Surfactants John M. Walsh† and Gordon J. T. Tiddy*,‡ Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral Merseyside CH63 3JW, United Kingdom, and Molecular Materials Centre & Department of Chemical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, United Kingdom Received December 16, 2002. In Final Form: March 31, 2003 The phase behavior of several novel surfactants having hydrophobic and poly(oxyethylene) (EOm) polar groups linked by a glyceryl (G) moiety has been studied using the optical microscope penetration scan. Use of the G linker gives surfactants with two EO groups attached to one hydrophobic chain, or two hydrophobic chains attached to one EO group, so that both the hydrophobic and polar groups can be considered to be “branched”. Several surfactants having the general formula X-O-CH2-CH(OY)-CH2O-X, where X ) alkyl chain and Y ) EOm or vice versa, have been examined, mostly with oleyl chains (C18d) as the hydrophobic group(s). In addition, we also examined two conventional surfactants with oleyl hydrophobic groups to allow a proper comparison of the novel surfactants with the available literature. Generally, we find that the mesophases observed are similar to those found for conventional surfactants, with the “packing constraint” model providing a reasonable qualitative description of behavior. Also, the data suggest that the G linker is part of the hydrophobic region of the surfactant aggregates. Remarkably, one of the compounds, C18dG-di-EO6, exhibits four different regions for the small-micelle cubic phases (I1). A second surfactant, di-C18dG-EO12, shows very different mesophase sequences on heating and cooling, indicating the existence of several phases of similar stability with slow equilibration times. This slow phase equilibration is unusual for poly(oxyethylene) surfactants.
Introduction The establishment of structure/activity relationships linking surfactant chemical structure to phase behavior and surface/interfacial properties is an important objective for formulation engineering. One of the major areas of current interest is to elucidate the differences between linear and branched-chain surfactants. The latter usually have much lower Krafft temperatures and hence potentially have more widespread applications. With linear surfactants, the relationship between liquid crystal type and surfactant chemical structure is well established, at least for the water-continuous phases [small-micelle cubic (I1), hexagonal (H1), bicontinuous cubic (V1), various intermediate phases (Int)] and the lamellar phase (LR).1 It involves a simple model to relate the various micelle shapes to surfactant hydrophobic group structure {“packing constraints”} and a consideration of the maximum volume fraction of micelles with a particular shape that can pack into a given volume.4 The intermediate phases still present problems, but their general pattern of occurrence, in that they replace V1 for long-chain surfactants, is also known.1 When reversed (oil-continuous) mesophases are considered, the picture is less clear.2,3 The usually expected pattern of phases with increasing surfactant concentration is LR, reversed bicontinuous cubic (V2), reversed hexagonal (H2), and reversed small-micelle cubic (I2), with a liquid surfactant phase (L2) occurring at the highest concentrations (in some cases only above the melting point of thermotropic mesophases). But in practice, †
Unilever Research Port Sunlight Laboratory. UMIST. * Corresponding author.
‡
(1) Hassan, S.; Rowe, W.; Tiddy, G. J. T. Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; J. Wiley and Sons Ltd.: Chichester, U.K., 2002; Vol. 1, Chapter 21, p 465. (2) Seddon, J. M. Biochim. Biophys. Acta 1990, 1031, 1. (3) Seddon, J. M.; Templer, R. H Structure and Dynamics of Membranes: from Cells to Vesicles; Lipowsky, R., Sackmann, E., Eds.; Elsevier Science B.V.: Amsterdam, 1995; Chapter 3.
the sequences V2/LR or H2/V2 often occur on increasing surfactant concentration.3 Concepts based on curvature energies are employed to describe this behavior, but to date these have only a crude link to surfactant chemical structure and apply only to systems with small intermicellar interactions. In elucidating the link between surfactant chemical structure and mesophase behavior for the water-continuous liquid crystals, studies on poly(oxyethylene) (EOm) surfactants have played a major role because the sizes of both polar and nonpolar groups can be varied in a systematic way.4 We have undertaken a similar systematic study of branched chain poly(oxyethylene) surfactants (the ones most likely to form reversed phases) to provide more data for a general understanding of this area. In our first paper on the subject, we described the mesophase behavior of midchain EO-substituted dodecyl (C12) surfactants, along with surface chemical data.5 This demonstrated that the packing constraints concept was successful in rationalizing the observed mesophase behavior, but most of the systems examined gave only water-continuous mesophases. Here we report on the phases formed by several surfactants where a glycerol moiety has been inserted between the alkyl chain and EO groups with the general formula X-O-CH2-CH(OY)-O-CH2-O-X; X ) alkyl chain and Y ) EOm or vice versa. This results in surfactants with two alkyl chains (a type of “branching”) and one headgroup, or vice versa. The longest alkyl chains employed (C18) allow us to obtain reversed phases. Similar surfactants have been studied previously by Kratzat, Finkelmann, and co-workers.6-10 They report full (4) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1983, 79, 975. (5) Thompson, L.; Walsh, J. M.; Tiddy, G. J. T. Colloids Surf., A 1996, 106, 223. (6) Kratzat, K.; Finkelmann, H. J. Colloid Interface Sci. 1996, 181, 542. (7) Kratzat, K.; Finkelmann, H. Colloid Polym. Sci. 1994, 272, 400.
10.1021/la020967b CCC: $25.00 © 2003 American Chemical Society Published on Web 06/05/2003
Phase Behavior of Poly(oxyethylene) Surfactants
phase diagrams showing mainly water-continuous mesophase structures that are consistent with the expected pattern, and some reversed phases. This represents a valuable body of data, but there are two difficulties in relating these results to the behavior of conventional surfactants. First, the synthesized compounds have the EO group terminated by OMe rather than OH. As we have demonstrated for conventional surfactants,11 this can lead to a marked lowering of mesophase melting temperatures, particularly for short EO derivatives. But it should not change the mesophase sequences at lower temperature. Second, the glycerol moiety used as a spacer introduces some uncertainty about exactly where the nonpolar group/aqueous region interface is located. The location of the interface is important in the application of packing constraint concepts to mesophase formation, an essential step in the rationalization of the water-continuous mesophase behavior.4 Glycerol linker groups contain ether-oxygens, which might be expected to reside in the nonpolar region (water is not highly soluble in dialkyl ethers). But the EO polar groups are mainly comprised of numerous ether-oxygens, all of which are normally assumed to reside in the aqueous region; hence the uncertainty. To resolve these issues, we have examined surfactants with longer alkyl chains than those employed by Kratzat, Finkelmann, and co-workers6-10 because these generally form mesophases over a wider temperature and composition range. We have also carried out measurements on some conventional surfactants with long chains because there are only limited data available in the literature on long-chain derivatives. This study has employed optical polarizing microscopy to elucidate the surfactant phase behavior, mainly using the Lawrence water penetration composition/temperature scan.4,12 In this, the surfactant is placed in contact with water and the interdiffusion region is examined under crossed polars. All the various mesophases can be distinguished by their characteristic optical textures, and other phase transitions such as partial miscibility (“clouding”) can be observed in normal light. We have not determined the mesophase composition regions because of the considerable effort required13 and because these composition regions can be estimated by the examination of surfactant phase diagrams showing similar phase sequences already available in the literature. We start with a report of two conventional surfactants, cis-octadec-9,10-enyl (oleyl)-tetraoxyethylene ether (C18d EO4) and oleyl nonaoxyethylene ether (C18dEO9). The phase behavior of oleyl poly(oxyethylene) surfactants has been reported14,15 by Kunieda, Shigeta et al., who demonstrate that for very long EO groups (m > 30) the cloud point decreases with the growth of EO number as for EO polymers,15 rather than increasing as normally observed. In their study, the EO groups of the surfactants employed were polydisperse mixtures rather than a single EO number. In addition, the studies cover only a limited range of compositions as a function of temperature and the (8) Kratzat, K.; Schmidt, C.; Finkelmann, H. J. Colloid Interface Sci. 1994, 163, 190. (9) Kratzat, K.; Stubenrauch, C.; Finkelmann, H. Colloid Polym. Sci. 1995, 273, 257. (10) Kratzat, K.; Finkelmann, H. Liq. Cryst. 1993, 13, 691. (11) Conroy, J. P.; Hall, C.; Leng, C. A.; Rendall, K.; Tiddy, G. J. T.; Walsh, J.; Lindblom, G. Prog. Colloid. Polym. Sci. 1990, 82, 253. (12) Lawrence, A. S. C.; Bingham, A.; Capper, C. B.; Hulme, K. J. Phys. Chem. 1964, 68, 3470. (13) Laughlin, R. G. The Aqueous Phase Behaviour of Surfactants; Academic Press: London, 1994. (14) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. 1997, 101, 7952. (15) Shigeta, K.; Kunieda, H. Langmuir 2001, 17, 4717.
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behavior of derivatives with short EO groups (m < 5) was inferred from mixtures of the polydisperse C18dEO5 with oleyl alcohol. We do not expect large deviations from their reported pattern of phases, particularly for the longer EO derivatives, but do require data on pure, single EO chain derivatives as a basis for considering the mesophase behavior of the glycerol-based surfactants. We examined several glycerol-based surfactants, most being oleyl derivatives. The compounds are abbreviated as follows (G ) glyceryl): oleylG-di-EOm for the structure X-O-CH2-CH(OY)-O-CH2-O-X, where X ) EOm and Y ) oleyl; dialkylG-EOm for the structure X-O-CH2CH(OY)-O-CH2-O-X, where X ) alkyl (usually oleyl) and Y ) EOm. The full list of surfactants examined is C18dEO4, C18dEO9, C18dG-di-EO4, C18dG-di-EO6, di-C7GEO9, di-C18dG-EO12, di-C18dG-EO12, and di-C18dG-EO22.3, where EO22.3 represents poly(ethylene glycol) 1000, a polydisperse mixture of average molar mass 1000. Before describing the detailed phase behavior of the surfactants, we consider the changes in packing constraints expected from the introduction of a glycerol moiety as a linker between the headgroup and alkyl chain regions. Packing Constraints for Surfactants with a Glycerol Spacer between the Alkyl Chains and the Polar Groups A general description of how water-continuous mesophase structure depends on alkyl chain packing constraints has been reviewed recently;1 hence only a brief description is given here. Micellar solutions (L1) only exist up to a certain concentration of surfactant because the micelles become close-packed to form mesophases at higher concentrations. Micelles are assumed to be smooth spheres, rods, or disks (bilayers), with a sharp boundary between the aqueous (headgroups plus water) and hydrophobic (all alkyl chains) regions. The radii (r) of spherical and rod micelles cannot be larger than the alltrans alkyl chain (lt). Hence, as the micelle surface area of the headgroup (a) decreases, the sequence of micelle shapes is sphere f rod f disk. The transitions occur when a ) 3v/lt (sphere f rod) and a ) 2v/lt (rod f disk, where v ) volume of hydrophobic group). It is usual to employ the “packing parameter” (alt/v) to describe these changes (spheres, alt/v > 3; rods, 3 > alt/v > 2; disks or reversed phases, 2 > alt/v). For spherical micelles, the mesophase sequence with increasing concentration is small-micelle cubic (I1) f hexagonal (H1) f bicontinuous cubic (V1) [or various intermediate phases (Int)] f lamellar phase (LR). With rod micelles, it is hexagonal (H1) f bicontinuous cubic (V1) [or various intermediate phases (Int)] f lamellar phase (LR), while only lamellar phase (LR) is seen for disk micelles. Thus, for novel surfactants, the mesophase sequence can be predicted if the area of the headgroup is known. Branched chain surfactants differ from linear ones only in that the volume of the hydrophobic group is larger; hence the headgroup area where the micelle shapes change from sphere f rod or rod f disk is larger. To relate surfactant structure to observed phase behavior, we need to calculate the a values at which shape transitions occur for the different hydrophobic groups. Values of a for the various EO groups can be obtained from surface tension or mesophase X-ray data.4,5 Note that here we have assumed identical values for a, whatever the shape of the micelles. In fact, it is likely that a will be somewhat smaller for aggregates with curved surfaces than for flat surfaces.5 Table 1 shows the expected micelle shapes and phase sequences for the surfactants employed in this study. Two possible positions of the nonpolar group/aqueous region
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Table 1. Calculations of Packing Parameter, Expected Micelle Shapes, and Mesophase Sequences for the Surfactants Studied Herea hydrophobic group C18d C18d C18d C18d C18dG C18dG di-C7 di-C7G di-C18d di-C18dG di-C18d di-C18dG
v (Å3)
lt (Å)
502 502 502 502 576 576 432.4
23.9 23.9 23.9 23.9 27.7 27.7 9.9
506 1004 1078 1004 1078
13.7 23.9 27.7 23.9 27.7
polar group a (Å2, 25 °C) EO4 42.6 EO9 51.8 di-EO4 63.4 di-EO6 71.6 di-EO4 85.2 di-EO6 96.2 EO9 sphere 27.0 rod 37.4 EO9 51.8 EO12 sphere 41-48 rod 47-56 EO12 55-65 EO22.3 sphere 77-89 rod 89-104 EO22.3 103-120
micelle shape (alt/v)
predicted mesophases
rod ?(2.0) rod (2.5) sphere (3.0) sphere (3.41) sphere (4.1) sphere (4.6)
H1 f V1/Int f LR H1 f V1/Int f LR I1 f H1 f V1/Int f LR I1 f H1 f V1/Int f LR I1 f H1 f V1/Int f LR I1 f H1 f V1/Int f LR
reversed phases (0.61) reversed phases (0.86) disks(1.4)
reversed phases reversed phases LR
disk/reversed phases (1.0-1.1) disk (1.1-1.3) disk (1.4-1.7)
LR, reversed phases LR LR
disk (1.8-2.1) rod (2.1-2.5) rods (2.6-3.1)
LR H1 f V1/Int f LR H1 f V1/Int f LR
Most alkyl group volumes and bond lengths are taken from ref 1; v(CH2) ) 27.0 Å3, v(CH2) ) 54.2 Å3, lt(C-C-C) ) 2.54 Å, lt(CH3) ) 2.3 Å; the CdC and C-O bonds are assumed to be the same length as C-C bonds; fragment volumes for the CHdCH and glycerol CH2‚CH(O)‚CH2 moieties of 42.8 and 74 Å3 were used. Values of a are taken from ref 5 where available; otherwise, see the text. a
interface for the G-containing surfactants are considered: the glyceryl moiety is located either in the polar region or with the hydrophobic groups. Where the hydrophobic group is shown as Cn or di-Cn, the glyceryl moiety is assumed to be located in the polar zone. For these surfactants, the polar/nonpolar interface is located at the CH2 group(s) of the Cn (or di-Cn) chain(s) attached to the first ether oxygen. Where the glyceryl moiety is included in the nonpolar region, the polar/nonpolar interface is placed at the glyceryl CH (or CH2) group(s) attached to the EO group(s). The EOm groups determine the surface area as their cross section is larger than that of the glyceryl group. With the Cn and di-Cn surfactants in spherical or rod micelles, the area (a) at the interface will be smaller than that of the EOm group because the polar/nonpolar interface is closer to the alkyl chains. This reduction is proportional to the lt values of the Cn and CnG groups, being reduced by lt(Cn)/lt(CnG) for rod micelles and by [lt(Cn)/lt(CnG)]2 for spherical micelles. Where alt/v > 3, if spherical micelles form for surfactants where the hydrophobic group is Cn or di-Cn, they will also occur if the hydrophobic group is CnG or di-CnG. Hence the packing parameter is not listed twice for these surfactants. However, when spherical micelles are disallowed for CnG or di-CnG surfactants, we need a separate calculation for rod micelles; hence two possibilities are shown in Table 1. The molecular fragment volumes and lengths employed are based on those already widely used or are estimates from closely related structures. The packing constraint approach assumes that the micelle surface is smooth, that the hydrophobic group density is that of normal paraffins, that micelle shapes are limited to perfect spheres, circular rods, and disks, and that there is no contribution from alkyl chain interactions to curvature. The latter assumption is already known to break down for long Cn surfactants1 (n > 16), while the other assumptions are certainly not completely valid. Small errors in these estimates will not cause additional major problems. Area (a) values are taken from ref 5, except for EO12 and EO22.3 which were not included in that study. Here we can only estimate a range of values. For the lower limit, we have extrapolated from the values in ref 5, assuming that area is proportional to the EO number. The upper limit assumes that an a value of 65 Å is required for the occurrence of a small-
micelle cubic phase (several of which are observed16 for C12EO12). In fact, the micelles in the I1 phase may deviate slightly from a spherical shape.16 Calculations of the a values from X-ray data16 for I1 cubic phases of C12EO12 gave 54-58 Å, much closer to the lower limit. In Table 2, we have made the same packing constraint calculations for branched surfactant systems with published phase behavior, mostly taken from the work of Kratzat, Finkelmann, and co-workers.6-10 These provide further data for comparison with the packing constraints model. Note that many of these surfactants have short Cn chains (n < 10), which, for monoalkyl derivatives, results in very limited mesophase formation.4 Other authors prefer to use the hydrophilic-lipophilic balance (HLB) concept14,15,17 to describe the phase behavior and emulsification properties of poly(oxyethylene) surfactants. Here the HLB number (NHLB) is given by
NHLB ) 20[FEO/FS][mVEO/VS] where F and V indicate density and molar volume, respectively, and the subscripts EO and S refer to the EO moiety and total surfactant. This does allow some predictions of cloud temperatures for conventional (linear) surfactants but is less successful in describing mesophase properties. It is not able to distinguish between linear and branched derivatives and hence is less successful for multiple headgroups or multiple/branched alkyl chains. Experimental Section The surfactants employed were synthesized at Unilever Research Port Sunlight Laboratories and used without further treatment. Purity to >98% was established by NMR and chromatography. Mesophase sequences were investigated using a Nikon Optiphot polarizing microscope with attached Linkam hot-stage. Transition temperatures were generally reproducible to (0.1 °C, with heating/cooling rates of ca. 1-5 °C/min being employed.
Results Conventional Oleyl Surfactants. First we consider the conventional surfactants C18dEO4 and C18dEO9. Sche(16) Sakya, P.; Seddon, J. M.; Templer, R. H.; Mirkin, R. J.; Tiddy, G. J. T. Langmuir 1997, 13, 3706. (17) Griffin, W. C. J. Soc. Cosmet. Chem. 1954, 5, 249.
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Table 2. Calculations of Packing Parameter, Expected Micelle Shapes, and Mesophase Sequences Predicted and Observed for the Surfactants Studied Elsewherea hydrophobic group
v (Å3)
lt (Å)
a (Å2, 25 °C)
micelle shape (alt/v)
predicted mesophases
mesophases observed (ref)
di-C6 di-C6G di-C7 di-C7G di-C7 di-C7G di-C8 di-C8G C16 C16G C14 C14G C14
378.4 452 432.4 506 432.4 506 486.4 560 459.2 533 405.2 479 405.2
8.7 12.5 10.0 13.8 10.0 13.8 11.2 14.0 21.4 24.2 18.9 21.7 18.9
EO8Me 24.5 EO8Me 50.5 EO8Me 24.5 EO8Me 50.5 EO10Me 27.9 EO10Me 53.1 EO8Me 32.3 EO8Me 50.5 di-EO4Me 66.6 di-EO4Me 85.2 di-EO4Me 66.6 di-EO4Me 85.2 di-EO5Me 68.8
reversed phases (0.56) disks (1.4) reversed phases (0.56) disks (1.4) reversed phases (0.65) disks (1.45) reversed phases (0.74) disks (1.3) sphere (3.1) sphere (3.9) sphere (3.1) sphere (3.9) sphere (3.2)
reversed phases LR reversed phases LR reversed phases LR reversed phases LR I1 f H1f V1/Int f LR I1 f H1 f V1/Int f LR I1 f H1 f V1/Int f LR I1 f H1 f V1/Int f LR I1 f H1 f V1/Int f LR
C14G
479
21.7
di-EO5Me 90.7
sphere (4.1)
I1 f H1 f V1/Int f LR
di-C10CHCH2 C7(C5)-
621.2 351.4
16.3 9.9
EO10 53.1 EO10 53.1
disks (1.4) disks (1.5)
LR LR
L1 (LR below 20 °C) (7) L1/LR (7) L1/LR (7) L1/LR (7) L1/LR (H1 below 18.5 °C) (7) L1/LR (H1 below 18.5 °C) (7) LR (no L1) (7) LR (no L1) (7) L1/I1 (below 22 °C)/H1 (10) L1/I1 (below 22 °C)/H1 (10) L1/I1 (below 19 °C)/H1 (10) L1/I1 (below 19 °C)/H1 (10) L1/I1 (below 21 °C)/H1 (below 18 °C) (10) L1/I1 (below 21 °C)/H1 (below 18 °C) (10) LR (no L1) (4) L1/LR (H1 below 10 °C) (5)
a
References 4, 5, 7, and 10. Alkyl group volumes, bond lengths, and a values were calculated as for Table 1.
Figure 1. Schematic phase diagrams for (a) the C18dEO4/water system and (b) the C18dEO9/water system determined by polarizing microscopy. [Surfactant concentration increasing left to right; W ) dilute aqueous solution; L1 ) micellar solution; L2 ) surfactant liquid, L3 ) third liquid region; LR ) lamellar phase; NR ) micellar nematic phase of rod micelles; H1 ) hexagonal phase; I1 ) small-micelle cubic phase; V1 ) bicontinuous cubic phase; V21 and V22 ) reversed bicontinuous cubic phases; S ) solid surfactant; * ) multiphase region.]
matic phase diagrams for these, showing the mesophase sequences and the temperature ranges, are given in Figure 1. The major features of C18dEO4/water mesophase behavior are the extensive lamellar phase (100 °C; the H1 phase melts at the higher temperature of 87 °C, while the (20) Clerc, M. J. Phys. II 1996, 6, 961.
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Table 3. Comparison of Selected Phase Transition Temperatures (°C) of Synthesized Surfactants with Literature Data for Surfactants Having Similar Behaviora transition surfactant
V2 limits
L3 range
LR limit
cloud point
ref
C18dEO4 C16EO4
66, 66 78, 78
43.5-67 58-76
54.8 74
100
64 63 69.7
this work 4, 19 this work
transition surfactant
I1 limits
H1 limit
cloud point
ref
C18dG-di-EO4 C18dG-di-EO6 C12EO12
32 21, 63, 63 6, 48
72 87 76
92.4 >100 98
this work this work 4, 16
transition
a
surfactant
H1 limit
V1 limit
L3 range
LR limit
cloud point
ref
di-C7G-EO9 C7(C5)-EO10
8 10
8 8
66-76 69-79
75 74
34 46
this work 5
Either the upper limiting temperature (limit) or full temperature range (range) of the phase is listed.
Figure 2. Schematic phase diagrams for (a) the C18dG-diEO4/water system and (b) the C18dG-di-EO6/water system. [Symbols as for Figure 1. I11, I12, I13, I14 ) small-micelle cubic phase regions separated by refractive index discontinuities; see the text.]
neat surfactant remains a liquid to below 0 °C. However, quite remarkably, there are now four distinct cubic phases. We have labeled these with superscripts in the order in which they appear on increasing concentration and then temperature. These are clearly visible from the penetration scan, as shown in Figure 3. On heating from 0 °C, the initial sequence of phases is L1/I11/I12/I13/H1. Note that the I11 phase is seen as a very narrow band (Figure 3a). The I11 phase disappears at 21 °C so that the sequence observed is L1/I12/I13/H1. Then, at 57.4 °C, I14 is formed at the I12/I13 boundary (Figure 3b-d) and it completely replaces I13 by 58.5 °C (Figure 3e). At a slightly higher temperature, I12 is also replaced by I14 (Figure 3e-g); finally the I14 phase melts to an L1 phase (Figure 3h), but the H1 phase remains to 87 °C. The changes are reversible with temperature. The possible structures of these phases are discussed below. DialkylG-EOm Surfactants. Schematic phase diagrams for these surfactants are shown in Figures 4 and 5. All show the expected large region of lamellar phase, but with significant differences between each surfactant. For di-C7G-EO9, there is a micellar solution region with a cloud point and an L3 region as well as the H1 and V1
mesophases (Figure 4a). The latter occur only at low temperature. The behavior very closely resembles that of the midchain EO-substituted dodecyl (C12) surfactant CH3(CH2)5C(C5H11)H-EO10 [C7(C5)-EO10] as shown in Table 3. Di-C18dG-EO22.3 has a phase behavior (Figure 4b) that is at first sight rather conventional, with only hexagonal and lamellar mesophases and a cloud point of 69.7 °C. It is grouped in Table 3 with C18dEO9 and C16EO8 because their cloud points and H1 upper temperature limits are very similar. But there are no nematic or V1 phases for di-C18dG-EO22.3, nor was there any indication of intermediate phases. In the reports of oleyl surfactant behavior by Kunieda, Shigeta et al.,14,15 the range of V1 occurrence is much more limited than that of the H1 phase, while they did not observe nematic or intermediate phases at all. Looking at the pattern of behavior,14,15 it appears probable that a V1 phase could occur for di-C18dG-EOm surfactants with m ∼ 30. The absence of these phases is almost certainly due to the presence of the polydisperse EO group, since many other commercial nonionic surfactants frequently do not form V1 phases. The reason for the lower stability of the V1 phase in systems with shorter (polydisperse) EO groups is not known but may be associated with the additional freedom permitted by the phase rule for surfactants with polydisperse EO groups. While it is commonplace to treat commercial EO surfactants as a single component in phase rule terms (see for example ref 21), this is strictly incorrect. Each different EO size (and different Cn) represents a different chemical species. Since a commercial EO22.3 will typically contain an EO10-EO40 distribution with the most common species at ca. EO16-EO25, the system contains at least 30 components. Hence it is possible for coexisting hexagonal and lamellar phases to have different EO compositions and for the surfactant compositions in both phases to vary across the two-phase region. Unfortunately, the authors are unaware of any experimental measurements on this matter. (21) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1998, 14, 2627.
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Figure 3. Optical micrographs recorded during a heating penetration scan of C18dG-di-EO6. Magnification, ca. ×100. (a) No polarizers; (b-h) partially crossed polarizers.
Figure 5 gives two schematic phase diagrams for diC18dG-EO12, one observed on heating from 0 °C and the second on cooling from 98 °C. On heating, the system exhibits three mesophases in the sequence W f LR f V21 f H2 f L2 up to 79.7 °C when an L3 region appears. This is transformed to a second cubic phase (V22) at 87 °C, where the lamellar phase is transformed into V21. The V2
and H2 phases are stable to >100 °C. On cooling (Figure 5b), the lamellar phase reappears at 80.5 °C, within the V21 region. The V22 phase can be cooled to 20.3 °C when L3 reforms. This, along with the water-rich arm of the V21, reverts to LR at ∼5 °C. On reheating, the sequence in Figure 5a is repeated. If the heating cycle is reversed when the L3 phase is present (at ca. 83 °C), then a V2 phase
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Figure 4. Schematic phase diagrams for (a) the di-C7G-EO9/ water system and (b) the di-C18dG-EO22.3/water system. [Symbols as for Figure 1.]
Figure 5. Schematic phase diagrams for (a) the di-C18dGEO12/water system, heating run from 0 °C; (b) the di-C18dGEO12/water system, cooling run from ca. 99 °C. [Symbols as for Figure 1; H2 ) reversed hexagonal phase.]
forms (or V2 phases) at ca. 70 °C. Some of the sequences observed are shown in the optical micrographs of Figure 6. Figure 6a shows the lamellar, cubic (V21), reversed hexagonal, and L2 phases while Figure 6b shows the myelin formation at the W/LR boundary, both on heating at 31.5 °C. Figure 6c shows the W/L3/LR phases, again on heating but at 85.0 °C. For the cooling cycle, Figure 6d clearly shows the two bicontinuous cubic phases and the lamellar phase at 75.6 °C, which persist to much lower temperatures (Figure 6e, 33.3 °C). Figure 6f shows the eventual reappearance of the lamellar phase at 2.5 °C; this now requires reheating to 83.4 °C for the L3 phase to reappear (Figure 6g). Given the complex nature of these phase sequences and the clear existence of several metastable states, from the limited experiments carried out here we are unable to decide on which of Figure 5a or Figure 5b is the closer to equilibrium. Note that the H2 phase is stable over a very wide range of temperatures. This contrasts with the report of Kunieda, Shigeta et al.14,15 who found only a very limited H2 range. In their system, the H2 phase occurred only for derivatives with small EO groups, which lose their water solubility very rapidly on increasing temperature. The increased stability of the H2 phase arises simply because the EO12 headgroup maintains its water solubility to a much higher temperature. Discussion Is the G linker group part of the hydrophobic group and do packing constraints allow prediction of mesophase structures? Before considering the utility of packing constraints in assessing mesophase behavior, we need to estimate the magnitude of changes induced by
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the presence of an oleyl group. The conventional surfactants C18dEO4 and C18dEO9 can be compared with the results for various C16 derivatives already published1,4,13,16,19,22 (we are not aware of a significant body of data on single-component C18 surfactants). For C18dEO9, the behavior is exactly as expected. However, with C18d EO4 we have a large area of lamellar phase rather than even just a small area of hexagonal phase as might be expected from the packing constraints calculations. However, these show the H1 phase at the borderline of stability. Given that increasing alkyl chain length is already known to decrease the stability of water-continuous EO surfactant mesophases,1 the absence of H1 is not surprising. Recall that the phase does not occur at room temperature for CnEO5 surfactants. Since there is only a small lowering of the upper temperature limit for watercontinuous mesophases (a few degrees at most) for C18d EO9, the packing constraints treatment should be generally valid for oleyl surfactants with the limiting area values slightly larger than those given by the simple model (say, where alt/v > 3.3 for spheres and 3.2 > alt/v > 2.1 for rods). In considering whether the G linker is part of the hydrophobic group, we have phase data on various branched surfactants available from the literature4,5,7,10 as listed in Table 2, together with packing constraint data. Unfortunately, most of the data are for surfactants with a G spacer having a headgroup terminated by OMe rather than OH. However, we see that good agreement with the packing constraints predictions is obtained if the G spacer is considered as part of the hydrophobic core. The other branched surfactants also show good agreement between observed phase behavior and the packing constraints predictions. Considering the data for the oleyl di-EOn derivatives, there is also good agreement with the packing constraints. Both form I1 and H1 phases, particularly if the G spacer is included in the hydrophobic core. The treatment does not describe what happens at low water content (where V1 and LR phases are expected but not observed) or at high temperature where the EO groups become dehydrated. [This reservation also holds for the surfactants listed in Table 2.] No other theory is able to predict this behavior. The dialkyl surfactants also show behavior that is in agreement with the packing constraints predictions, if we consider only the di-C18dEO12 phases formed on heating. Again, the best agreement is if the G linker is assumed to be hydrophobic, although only the di-C7 compound allows a proper test. There is a major weakness of the packing constraints predictions that is clearly illustrated by comparing the data of the di-C7G-EO9 derivative with that of di-C18dEO12. The former has the smallest headgroup but shows phases indicating a more positive micellar curvature than the latter. This clearly demonstrates that increasing chain length introduces another complication, that of negative curvature due to hydrophobic chain-chain interactions, which leads to larger limiting values for the various micelle shapes (as indicated above). The hysteresis in LR/V2/L3 phase transitions is remarkable. It is fairly commonplace to find long equilibration times for L3/LR and LR/V2 boundaries, but this is the first example of such marked hysteresis in EO surfactants that we are aware of. It may well arise from the particular difficulty of exchange between different molecular con(22) Kato, T.; Taguchi, N.; Terao, T.; Seimiya, T. Langmuir 1995, 11, 4661.
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Figure 6. Optical micrographs recorded for the di-C18dG-EO12 penetration scan. Heating cycle: (a,b) 31.5 °C; (c) 85.0 °C. Cooling cycle: (d) 75.6 °C; (e) 33.3 °C; (f) 2.5 °C. Reheating cycle: (g) 83.4 °C.
formations involving the G linker. There are likely to be differences in the range of local configurations for the G linker moieties between the three phases. Changes in these states could take a long time if correlated movement of the two long hydrophobic chains is also required. The existence of the hysteresis does suggest that all three phases have a similar free energy.
What is the structure of the four cubic phases? The observation of four I1 phases for oleyl-G-di-EO6 is remarkable. We note that four different I1 structures, all of which contain small globular micelles, have already been reported but not in a single surfactant system. Three of the structures are comprised of (almost) spherical micelles, these being a body-centered cubic (C12EO12, space
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group Im3m),16 a face-centered cubic (C12EO12, Fm3m),16 and hexagonally packed spheres (C12EO8, p6(3)/mmc).20 Note that no optical birefringence was observed for this hexagonal phase.20 The fourth structure comprises two spherical micelles and six disk micelles per unit cell (C12EO12 and C12EO8, Pm3n).16,20 Strictly speaking, the phase of hexagonally packed spheres should not be labeled a cubic structure, because it is not! However, we still employ the I1 symbol to emphasize the structural similarity of this phase to the other phases comprised of small micelles. The phases comprising hexagonally packed spheres and mixed disk/spherical micelles were observed for C12EO8, in composition regions similar to those of I12 and I13 shown for oleyl di-EO4 (but below 15 °C).20 All three cubic structures were observed for C12EO12 in regions similar to those for I11, I12, and I13 as shown for oleyl di-EO6 (but the region occupied by I14 remains as I12). Unfortunately, there are no temperatures listed for the data on the phase containing hexagonally packed spheres,20 but measurements do not seem to have been made over a temperature range outside that of 10-15 °C. For the Im3m phase of C12EO12, the measurements were made over a much wider temperature range (>30 °C). Thus it may be that the phase containing hexagonally packed spheres exists only over a limited temperature range. Thus, considering the location of the reported structures on the respective phase diagrams16,20 we tentatively assign the phases as follows: I11, Fm3m; I12, Im3m; I13, Pm3n; and I14, P6(3)/mmc. Obviously, we require X-ray studies to support this hypothesis. Cloud Point Mechanism: Packing Constraints or HLB? In a previous paper, we have suggested that while the cloud point (Tc) of EO surfactants arises from the dehydration of EO groups with increasing temperature, there are two distinct mechanisms.11 One, where mesophase dispersions do not occur for dilute solutions above Tc, involves attractive interactions between small micelles (examples are C12EO8 and C12EO12). The second, where LR + W dispersions are observed above Tc (examples are C12EO4 and C16EO6), arises from the stronger attractions between disk micelles (note that these micelles could be long rods with flattened surfaces, rather than circular disks). Here Tc cannot occur below the H1 upper temperature limit. The HLB approach considers only the volume fraction of EO groups for the surfactant (mVEO/VS). For
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the surfactants studied here, oleylG-di-EO4 and di-EO6 would be expected to have a similar HLB to the linear materials oleyl EO8-10 and oleyl EO12-14. [The range of EO values recognizes that there are two OH groups in the di-EO materials; hence they are more hydrophilic than a simple EO surfactant.] Clearly the cloud point of oleyl di-EO4 (92.4 °C) is much higher than that of oleyl EO9 (64 °C). Since the H1 phase of oleyl di-EO4 melts at 72 °C and there is no lamellar phase, a Tc value of >85 °C might be expected. Thus the observation is in better agreement with packing constraints than with HLB. Similarly, the Tc of oleyl di-EO6 (Tc > 100 °C) is above that of C16EO12 (Tc ) 94 °C),4 with an H1 melting temperature of 87 °C. Without a measurement of Tc, both approaches could appear equally valid. Looking at the dialkyl surfactants, the HLB comparisons would be di-C7G-EO9 (Tc ) 33.9 °C) with C7-8EO4-5 (C8, allowing for the hydrophobic G linker, Tc ) 40.4 °C for C8EO4),11 dioleyl G-EO22.3 (Tc ) 69.7 °C) with C18-19EO11-12 (Tc ) 94 °C for C16EO12), and dioleyl EO12 (Tc < 0 °C) with C18-19EO6 (Tc ) ca. 30 °C, based on interpolation between the values for C16EO6 and C22d EO6).11 The HLB approach gives values that are too high, particularly for the long-chain surfactants. However, it is not obvious that the packing constraints model is that much better, although it does predict a very low Tc value for dioleyl EO12. Summary/Conclusions We report the mesophase behavior of several novel poly(oxyethylene) surfactants containing a glyceryl (G) linker to attach a single alkyl chain to two EO groups or vice versa. Together with data from the literature, the results show that the packing constraints concepts allow a much better correlation of mesophase behavior with chemical structure than the HLB concept. However, for long alkyl chain surfactants (Cn, n > ca. 16) the negative curvature arising from chain-chain repulsions (neglected in packing constraints) also plays an important role. The G linker appears to reside in the hydrophobic region of the micelles. Several unusual features are demonstrated, including the occurrence of four different I1 cubic phases for oleylGdi-EO6 and significantly different mesophases between heating and cooling scans for di-C18dEO12. LA020967B
