Impact of AlCl3 on the Self-Assembly of the Anionic Surfactant Sodium

Oct 4, 2013 - Rutherford Appleton Laboratory, Science & Technology Facilities Council (STFC), Chilton, Didcot OX11 0QX, Oxon, United. Kingdom. §...
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Impact of AlCl3 on the Self-Assembly of the Anionic Surfactant Sodium Polyethylene Glycol Monoalkyl Ether Sulfate in Aqueous Solution Hui Xu,† Jeffrey Penfold,*,†,‡ Robert K. Thomas,† Jordan T. Petkov,§ Ian Tucker,§ I. Grillo,∥ and A. Terry‡ †

Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, Oxon, United Kingdom Rutherford Appleton Laboratory, Science & Technology Facilities Council (STFC), Chilton, Didcot OX11 0QX, Oxon, United Kingdom § Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral CH63 3JW, Merseyside, United Kingdom ∥ Institut Laue Langevin, 6 Rue Jules Horowitz, F-38042 Grenoble Cedex 09, France ‡

S Supporting Information *

ABSTRACT: Small-angle neutron scattering has been used to study the self-assembly of the anionic surfactant sodium polyethylene glycol monoalkyl ether sulfate in aqueous solution and in the presence of Al3+ multivalent counterions in the form of AlCl3. The addition of the Al3+ ions promotes significant micellar growth of the initially globular micelles into highly elongated structures until ultimately lamellar structures form. Increasing the size of the polyethylene oxide, EO, group progressively suppresses micellar growth before lamellar formation. Reducing the alkyl chain length has a similar effect on the structural evolution. Both trends are associated with increased solubility with increasing EO group size and decreasing alkyl chain length. Both the size of the EO group and the length of the alkyl chain affect sodium diethylene glycol monododecyl ether sulfate/Al3+ complex formation and drive lamellar formation to progressively higher AlCl3 concentrations.

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INTRODUCTION Anionic surfactants are a major component in most detergencybased home and personal care products.1 To optimize their response and performance in different environments, a wide range of different molecular structures have been developed and used.2 In such applications both surface adsorption and self-assembly in solution are important features. The role of electrolytes in modifying those properties is central to their response to different operational environments. The impact of simple monovalent electrolytes, such as NaCl, on self-assembly and adsorption is well established, and many aspects have been extensively studied. The addition of an electrolyte reduces the critical micelle concentration, cmc,3 promotes micellar growth,4 and induces enhanced adsorption at interfaces.5 The effects of the addition of multivalent counterions, such as Ca2+ or Al3+, are much more substantial. For alkyl sulfates such as sodium dodecyl sulfate, SDS, which © 2013 American Chemical Society

has the simplest structure of the commonly used anionic surfactants,1,2 the addition of multivalent counterions leads rapidly to precipitation.6,7 Precipitation induced by multivalent counterions is a familiar problem associated with water hardness and is also related to many environmental applications, such as the removal of toxic metals in wastewater treatment and in soil remediation.8,9 It is well established that the onset of precipitation is enhanced for alkyl sulfates with larger alkyl chain lengths, as the alkyl sulfate becomes intrinsically less soluble.6 However, different anionic surfactant structures infer greater tolerance toward precipitation,1,2 as demonstrated for the alkyl benzenesulfonates6,10 Received: August 28, 2013 Revised: September 30, 2013 Published: October 4, 2013 13359

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ethanol mixtures. The purity of the surfactants was verified by MS, GC/MS, NMR, and surface tension. Analytical grade (>99.9% purity) AlCl3 from Sigma-Aldrich was used throughout. D2O was obtained from Sigma-Aldrich. All glassware and quartz cells were cleaned in alkali detergent (Decon 90) and rinsed in ultrapure water (Elga Ultrapure). No adjustments to the solution pH were made on the addition of AlCl3, and the measured value in the presence of AlCl3 was ∼5.0. Analysis of Micelle Scattering Data. The scattering from interacting globular surfactant micelles in aqueous solution is described by the decoupling approximation21 such that

and the oligoethylene glycol monoalkyl ether sulfates, SAESs.11−14 Recently it has been shown how the strong complexation between the alkylbenzene sulfonate, LAS-6, and Ca2+ 15 and the SAES, sodium diethylene glycol monododecyl ether sulfate, SLES, and Al3+ 16 results in the transition from monolayer to multilayer adsorption at the air−water interface. In both cases the addition of a nonionic surfactant and the relative electrolyte and surfactant concentrations determine the nature of the surface structure. The SAES surfactants are particularly interesting, and in more recent studies17,18 it has been shown how the surface properties can be manipulated by changing the ethylene oxide and alkyl chain lengths. Although the solution properties of the SAES surfactants in the presence of multivalent counterions have been investigated,11,12,16,19,20 the scope of these studies has been fairly limited. In particular, there has been no systematic investigation of the role of changing the ethylene oxide and alkyl chain lengths. Hence, the focus of this paper is to report the use of SANS to investigate the evolution in the solution self-assembly of a range of different SAES surfactants in the presence of Al3+ at relatively low surfactant concentrations. In this study both the ethylene oxide and alkyl chain lengths are varied, and the development of the micelle structure is quantified for the different surfactant structures, up to a surfactant concentration of 20 mM.

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dσ /dΩ = N[S(Q )|⟨F(Q )⟩Q |2 + ⟨|F(Q )|2 ⟩Q − |⟨F(Q )⟩Q |2 ] (1) where Q denotes averaging over particle sizes and orientations, N is the micelle number density, S(Q) is the intermicellar structure factor, and F(Q) is the micelle form factor. The decoupling approximation assumes that for dilute interacting micelles there are no correlations in position, size, and orientation. The structure factor, S(Q), quantifies the intermicellar interactions and correlations and is included using the rescaled mean spherical approximation calculation27 for a repulsive screened Coulombic intermicellar interaction potential. It is characterized by the surface charge on the micelles, z, the micelle diameter, the Debye−Huckel inverse screening length, κ (defined in the usual way), and the micelle number density. The micelle structure, F(Q), is modeled using a standard “core and shell” model for globular micelles21 where F(Q) is

F(Q ) = V1(ρ1 − ρ2 ) F0(Qr1) + V2(ρ2 − ρs ) F0(Qr2)

EXPERIMENTAL DETAILS

(2)

r1 and r2 are the core and shell radii, Vi = 4πri3/3

SANS measurements were made on a range of custom-synthesized SAES anionic surfactants with the general formula CH3(CH2)n(OCH2CH2)mSO4Na (with n = 10, 12, 14, and 16 and m = 1, 2, and 3), namely, sodium ethylene glycol monododecyl ether sulfate, SLE1S, sodium diethylene glycol monododecyl ether sulfate, SLE2S, sodium triethylene glycol monododecyl ether sulfate, SLE3S, sodium diethylene glycol monodecyl ether sulfate, SDE2S, sodium diethylene glycol monotetradecyl ether sulfate, STE2S, and sodium diethylene glycol monohexadecyl ether sulfate, SHE2S. The measurements were made in dilute solutions from 5 to 20 mM and with the addition of AlCl3. The form of the SANS scattering patterns (Q dependence, where Q is the scattering vector defined as Q = (4π/ λ)(sin θ), with 2θ the scattering angle and λ the neutron wavelength) was used to identify the regions of micellar, miceller/lamellar coexistence, and lamellar structures. In the purely micellar regions the scattering data were analyzed quantitatively using a standard model of interacting micellar solutions.21 The SANS measurements were made on two different diffractometers, SANS2D22 at the ISIS pulsed neutron source and D1123 at the reactor source at the Institute Laue Langevin in France. The measurements on SANS2D were made using the white beam time-offlight method, with a neutron wavelength range from 2 to 16.5 Å and a sample to detector distance of 4.0 m, to cover a Q range of 0.006−0.8 Å−1. The measurements on D11 were made using a neutron wavelength of 6 Å (Δλ/λ ≈ 10%) and two different sample to detector distances, 2.0 and 5.0 m, to cover a Q range of 0.006−0.25 Å−1. The solutions were contained in 1 mm path length quartz spectrophotometer cells. All the measurements were made in D2O at 25 °C using hydrogeneous surfactants to maximize the scattering. The scattering from the empty cell and solvent was subtracted from the data. The data were normalized for the detector response, spectral distribution of the incident beam, and solid angle to establish the scattering intensity I(Q) on an absolute scattering cross-section (cm−1) using standard procedures.24,25 Each individual measurement took ∼10−30 min. The SAES surfactants were custom synthesized in Oxford by sulfonation of the corresponding and previously synthesized oligoethylene glycol monoalkyl ethers and diethylene glycol monododecyl ethers.26 The surfactants were recrystallized twice from propanol/

F0(Qri) = 3j1 (Qri)/(Qr ) = 3[sin(Qr ) − Qr cos(Qr )]/(Qr )3 (3) and ρ1, ρ2, and ρs are the scattering length densities of the micelle core and shell and of the solvent (ρi = Nibi, where bi is the scattering length of the ith component). j1(Qri) in eq 3 is a first-order spherical Bessel function. The micelle core−shell model describes the micelle geometry and includes some molecular constraints. The inner core has a radius r1 and contains the alkyl chains in a volume constrained to have a maximum spherical dimension of the fully extended alkyl chain length, r1 = lc(ext). ext is a parameter which allows for some variation in the packing constraint and has values typically between 0.8 and 1.2. For micelle aggregation numbers, ν, greater than that which can be accommodated in a spherical volume defined by lc(ext), the micelles are assumed to be prolate ellipsoids with core dimensions of r1 and r1(ee), where ee is the ellipticity. The outer shell, with radius r2, is then either spherical or elliptical and is constrained to contain the surfactant headgroups and associated hydration. From the known molecular volumes, neutron scattering lengths, and solution concentration, eqs 1−3 are used to calculate the scattering on an absolute scale. The key refinable parameters are then ν, z, and ext. The degree of ionization of the micelles, δ, is defined as δ = z/ν. The model is compared to the experimental data and evaluated by least squares. For the model to be acceptable, it must reproduce the functional form of the data and predict the absolute scattering to within ±20%.

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RESULTS AND DISCUSSION SANS measurements were made for SLE1S, SLE2S, and SLE3S in the absence of electrolyte. The impact of AlCl3 on selfassembly was assessed by SANS for sodium oligoethylene glycol monoalkyl ether sulfate with different ethylene oxide chain lengths and for sodium diethylene glycol monoalkyl ether sulfates with different alkyl chain lengths. In the Absence of AlCl3. At the relatively low surfactant concentrations studied, 5−20 mM, the SAES surfactants in 13360

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increasingly less visible. The change in the scattering is due to a change in the form factor as micellar growth occurs. This is also illustrated in the key model parameters in Table 1. With increasing AlCl3 concentration, significant micellar growth occurs. The micelle aggregation number increases from ∼75 to ∼1300, and correspondingly, the ellipticity increases from 1.2 to ∼14.0. At higher AlCl3 concentrations the physical appearance of the solutions changes from clear to slightly turbid, consistent with significant micellar growth. At an AlCl3 concentration of 4.0 mM, the solutions become turbid and the form of the scattering changes dramatically. This is illustrated in Figure 1, where the appearance of a pronounced Bragg peak is a result of lamellar phase formation. Broadly similar scattering data were obtained for SLE2S at surfactant concentrations of 5 and 20 mM, and the key model parameters are summarized in Tables S4 and S5 in the Supporting Information. Changing the ethylene oxide chain length has a significant impact on the effect of AlCl3 on the micellar structure. Reducing the ethylene oxide chain length to SLE1S changes the response to one closer to that observed for SDS. The key model parameters from the analysis of the corresponding SANS data for SLE1S are summarized in Tables S6−S8 in the Supporting Information. In this case the micellar growth is more modest, as the onset of lamellar formation and ultimately precipitation occurs at relatively lower AlCl3 concentrations. The corresponding data are shown in Figures S2−S4 in the Supporting Information. Increasing the ethylene oxide chain length of the surfactant results in a more gradual and less substantial change in the scattering data. This is illustrated in Figure S5 in the Supporting Information for 10 mM SLE3S in AlCl3. The increase in the scattering at low Q values with increasing AlCl3 concentration is less pronounced compared to that observed for SLE2S in Figure 1. This implies that the micellar growth is now much more gradual, and this is confirmed by the key model parameters summarized in Table S9 in the Supporting Information. The micelle aggregation number increases only by a factor of 4 and the corresponding change in the ellipticity is similarly modest in the AlCl3 concentration range studied. The data for 10 mM AlCl3 (see Figure S5 in the Supporting Information) are different, and a Bragg peak is only just visible in the data at high Q. The data arise here from a mixed micellar/lamellar phase (L1/Lα). This is qualitatively different from what was observed for SLE2S, where the transition was directly from micellar to lamellar, without any coexistence region evident in the data. The variation in micelle size with AlCl3 concentration for SLE1S, SLE2S, and SLE3S at the three different SLES concentrations measured are summarized in Figure 2. The variations in aggregation number are plotted up to the point at which the onset of lamellar formation occurs. For all three SLES surfactants the micellar growth at surfactant concentrations of 5 and 20 mM are relatively modest before lamellar formation occurs. At 10 mM significant growth occurs for SLE1S and SLE2S, whereas for SLE3S only modest growth occurs. This is illustrated more directly in Figure 3, where the variation in aggregation number with AlCl3 concentration is plotted for SLE1S, SLE2S, and SLE3S at a surfactant concentration of 10 mM. Hence, the micellar growth, before lamellar formation, depends on the ethylene oxide chain length and surfactant concentration, but notably for SLE3S micellar growth is not substantial at any of the surfactant concentrations studied. The results show the competition between the impact of the

aqueous solutions form weakly interacting globular micelles. Typical SANS data are shown in Figure S1 in the Supporting Information for SLE1S at 5, 10, and 20 mM. The data are well described by the core−shell model of interacting globular micelles described earlier and encapsulated by eqs 1−3. The key model parameters are summarized in Table S1 in the Supporting Information. At a surfactant concentration of 5 mM the micelles are dilute and the scattering is dominated by the micelle form factor, F(Q). With increasing surfactant concentration, the contribution from intermicellar interactions described by S(Q), is increasingly evident. In the surfactant concentration range studied there is just a modest increase in the aggregation number from ∼70 to ∼85. This is accompanied by an increase in the ellipticity from 1.2 to ∼1.4. The core and shell radii are correspondingly ∼17 and 20 Å, respectively, and ext is 1.0 ± 0.05. The micelles are only weakly ionized, and the degree of ionization, δ, varies from 0.07 to 0.15. This compares with a typical value of ∼0.3 for the anionic surfactant SDS and other ionic surfactants.21,28 This is consistent with the adsorption data for SAES surfactants,17 which exhibit a nonionic surfactant-like behavior with a plateau in the adsorption above the cmc. In the absence of electrolyte similar micellar structures are obtained for SLE2S and SLE3S, as indicated by the key model parameters summarized in Tables S2 and S3 in the Supporting Information. In the tables of the model parameter the physical appearance of the micellar solution is also recorded. Consistent with the description of the scattering data, the solutions are transparent and of low viscosity. SLES/AlCl3. With the addition of AlCl3 the scattering data change significantly due to micellar growth. This was previously reported for SLES/nonionic surfactants11−13,16 and for other anionic surfactants.14 This is illustrated in Figure 1 for SLE2S/ D2O, where the AlCl3 concentration is increased from 0.0 to 4.0 mM. The data in Figure 1 show that with increasing AlCl3 concentration the scattered intensity at low scattering vector, Q, values increases and the suppression at low Q due to S(Q) is

Figure 1. Scattered intensity, I(Q), versus wave vector transfer, Q, for 10 mM SLE2S in D2O/AlCl3 for (red) 0.0 mM, (blue) 1.5 mM, (green) 3.0 mM, and (cyan) 3.5 mM AlCl3, from the lower to upper curve. The solid lines for 0.0−3.5 mM AlCl3 are model calculations for a core−shell model and the parameters summarized in Table 1. The solid line in the lowest curve (gray) for 4.0 mM AlCl3 is just a line joining the data points for greater clarity. 13361

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Table 1. Key Model Parameters from Core−Shell Model Fits for 10 mM SLE2S/D2O SANS Data in the Presence of AlCl3 AlCl3 concn (mM)

physical appearance

0.0 1.5 2.5 3.0 3.5

clear clear clear clear slightly turbid

ν 77 139 169 428 1297

± ± ± ± ±

2 5 7 10 20

z (±1)

r1 (±1 Å)

r2 (±1 Å)

ext (±0.05)

ee (±0.1)

10 7 1 1 1

15 18 18 18 20

19 22 22 22 24

0.92 1.10 1.10 1.10 1.20

1.7 1.8 2.1 5.4 13.7

surfactants studied. More specifically, changing the ethylene oxide chain length from EO1 to EO3 makes little difference in the micelle aggregation number. A notable feature of the SAES micelles in the absence of electrolyte is the relatively low value of the degree of ionization of the micelles. It is much lower than that normally observed for anionic surfactants.28 It implies that the sodium counterions are strongly bound to the anion, and this was also inferred from the nature of the adsorption isotherm for SLES.17,18 In that case it was attributed to the presence of the ethylene oxide groups modifying the local dielectric constant and hence the intra-headgroup interactions, resulting in stronger counterion binding. Caponetti et al.29−31 reported similar values for the aggregation number for SLE2S, as reported here. Furthermore they reported a tolerance to the addition of NaCl, manifest as a lack of change in the micelle structure, which increased as the ethylene oxide chain length increased. At low surfactant and AlCl3 concentrations SAES selfassembly is in the form of globular (elliptical) micelles. At higher AlCl3 concentrations there is a transition to lamellar structures or lamellar/micellar coexistence. Wennerstrom et al.32 discussed the phase behavior of ionic surfactants with divalent counterions, and the data presented here are broadly consistent with their main observations. In the purely micellar regions the strong complexation between the sulfate headgroups and Al3+ counterions promotes micellar growth at relatively low AlCl3 concentrations. The strong complex formation reduces the mean area per molecule and the preferred curvature more effectively than the equivalent monovalent electrolyte, such as NaCl. This was previously demonstrated by Alargova et al.,11−13 Penfold et al.,15 and Petkov et al.16 in related systems. This is further supported by the theoretical calculations of Srinivasan and Blankschtein20 and the simulations of Sammalkorpi et al.33,34 To induce micellar growth with NaCl equivalent to that observed here for AlCl3 would require NaCl concentrations ∼20−50 times larger. The phase behaviors of SLE1S and SLE2S are broadly similar, and the behavior is shown for SLE1S in Figure 7i. The phase behavior for SLE3S is different (see Figure 7ii), and the purely lamellar phase occurs only at higher AlCl3 and surfactant concentrations. Correspondingly, the micellar/lamellar coexistence region is more extensive. Increasing the ethylene oxide chain length has a significant impact on the phase behavior. The increased steric hindrance or packing constraint introduced by the larger ethylene oxide group and greater solubility partially suppresses the lamellar phase formation. This is similar to what was observed in the surface adsorption,17 where the increased ethylene oxide chain length suppresses the formation of surface multilayer structures that occur with the addition of AlCl3. The variation in the micelle aggregation number for SLE1S, SLE2S, and SLE3S with AlCl3, summarized in Figures 2 and 3, illustrate another important aspect of the internal steric hindrance. It was previously shown how the addition of a nonionic cosurfactant with a relatively large headgroup suppressed the micellar

ethylene oxide chain length and the surfactant concentration on the effect of the Al3+ complexation with the surfactant headgroup, and this will be discussed in more detail later in the Discussion. SAE2S/AlCl3. Similar SANS measurements were made for different SAES surfactants with different alkyl chain lengths, decyl, tetradecyl, and hexadecyl, for sodium diethylene glycol monoalkyl ether sulfate. The measurements were made at surfactant concentrations of 10 and 20 mM and for a range of AlCl3 concentrations. The data in Figure 4 are for 20 mM SDE2S in D2O and with increasing amounts of AlCl3. The data are consistent with relatively small globular micelles, and there is modest micellar growth as the AlCl3 concentration increases from 0.8 to 8.0 mM. The key model parameters are summarized in Table 11 in the Supporting Information. Broadly similar data are obtained for 10 mM SDE2S and are shown in Figure S6 and Table S10 in the Supporting Information. Increasing the alkyl chain length to tetradecyl, STE2S, results in an evolution in the scattering and hence self-assembly with AlCl3 broadly similar to that observed for 10 mM SLE2S (see Figure 1). Similar patterns of scattering with increasing AlCl3 were obtained for STE2S at surfactant concentrations of both 10 and 20 mM. The scattering data for 10 mM STE2S are shown in Figure S7 in the Supporting Information, and the key model parameters are summarized in Table S12 in the Supporting Information. The data for 20 mM STE2S are summarized in Figure S8 and Table S13 in the Supporting Information. The increased alkyl chain length promotes greater micellar growth compared to the shorter alkyl chain length. However, this is partially masked by the lower intrinsic solubility associated with the longer alkyl chain length. Hence, the onset of lamellar formation and precipitation starts to occur at lower AlCl3 concentrations. This is even more pronounced for the hexadecyl alkyl chain length SAES surfactant, SHE2S, as illustrated in Figure 5. For SHE2S, apart from at the very lowest AlCl3 concentrations, less than 2 mM, lamellar formation and precipitation quickly dominate the scattering as the AlCl3 concentration is increased. The impact of the alkyl chain length on the micellar growth in the presence of AlCl3 is summarized in Figure 6. In broadest terms, at a fixed ethylene oxide chain length micellar growth increases with increasing alkyl chain length in the presence of AlCl3. Discussion. For the SAES surfactants studied the general phase behavior with surfactant and AlCl3 concentration is broadly similar. Approximate phase diagrams, derived from the SANS data, are illustrated for SLE1S and SLE3S in Figure 7. At these relatively low surfactant concentrations and in the absence of AlCl3, all the different SAESs form relatively small globular micelles. The micelles are slightly elliptical, with an axial ratio of ∼1.2−1.4. The micelle size varies little with the surfactant concentration and is similar for the different 13362

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Figure 3. Variation in micelle aggregation number with AlCl 3 concentration for (red) SLE1S, (blue) SLE2S, and (green) SLE3S, from the upper to lower curve, at a surfactant concentration of 10 mM. The solid lines are lines through the data points only.

Figure 4. Scattered intensity, I(Q) (cm−1), versus wave vector transfer, Q (Å−1), for 20.0 mM SDE2S in different AlCl3 concentrations, 0.8 mM (red), 2.0 mM (blue), 5.0 mM (brown), and 8.0 mM (green), from the lower to upper curve. The solid lines are model calculations as described in the text and using the parameters in Table S11 in the Supporting Information.

micellar growth. It is also shown that increasing the ethylene oxide length makes the internal steric hindrance increasingly more effective. The variation in micelle aggregation number depends on both the AlCl3 and surfactant concentrations and also the ethylene oxide chain length. Most significantly, for SLE3S, the variation in aggregation number with AlCl3 concentration at each of the surfactant concentrations studied is more gradual compared to that observed for SLE1S and SLE2S. This is a further clear indication of the effect of the larger ethylene oxide group in disrupting the effects of the sulfate/Al3+ complexation on micellar growth. The variations in aggregation number for SLE1S and SLE2S show a more complex interplay between the effect of AlCl3 and surfactant concentrations. Increasing the AlCl3 concentration promotes micellar growth, and the extent of the growth is most significant at a surfactant concentration of 10 mM. At the lower surfactant

Figure 2. Variation in micelle aggregation number with AlCl3 concentration for (i) SLE1S, (ii) SLE2S, and (iii) SLE3S at (red) 5 mM, (blue) 10 mM, and (green) 20 mM, from the upper to lower curve. The solid lines are lines through the data points only.

growth of SLES in the presence of Al3+ counterions.13,16 This was attributed to the steric hindrance provided by the nonionic headgroup disrupting the sulfate/Al3+ complex formation. The role of the ethylene oxide linker group in the SAES surfactants provides an analogous role. The ethylene oxide group now acts as an internal steric barrier to complex formation and the resulting reduction in the packing and curvature that drives 13363

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Figure 5. Scattered intensity, I(Q) (cm−1), versus wave vector transfer, Q (Å−1), for 20.0 mM SHE2S in different AlCl3 concentrations, 5.0 mM (red), 10.0 mM (blue), 15.0 mM (brown), and 20.0 mM (green).

Figure 7. Approximate solution phase diagram for (i) SLE1S and (ii) SLE3S, derived from SANS data. Each dot represents a point at which a SANS measurement was made.

directly the role of the ethylene oxide chain length, where significant micellar growth occurs for SLE1S and SLE2S before lamellar formation. In contrast, lamellar formation for SLE3S is suppressed up to relatively high AlCl3 concentrations, and before that only modest micellar growth is observed. Alternatively, the changes observed can be related to changes in the intrinsic solubility of the surfactant; and as the ethylene oxide chain length increases the surfactant solubility increases. It is well established, for example, that for SDS the addition of multivalent counterions rapidly leads to precipitation8−10 and that the addition of the ethylene oxide group in the SAES surfactants provides a route to greater tolerance to precipitation by multivalent counterions.1,2,34−37 At a fixed ethylene oxide chain length of dioxyethylene, the effect of changing the alkyl chain length has been characterized (see Figures 5 and 6; also see Figure S7 in the Supporting Information). Changing the alkyl chain length also changes the solubility of the surfactant.6 Increasing the alkyl chain length reduces solubility and promotes micellization, whereas reducing the alkyl chain length suppresses the onset of micellization. Increasing the alkyl chain length promotes a greater tendency toward micellar growth and lamellar formation and ultimately precipitation. Furthermore, the associated decrease in solubility results in a greater tendency for precipitation. This is evident in

Figure 6. Variation in micelle aggregation number, ν, with AlCl3 concentration at (i) 10 mM and (ii) 20 mM for (red) SDE2S, (blue) SLE2S, and (green) STE2S, in order from the upper to lower curve.

concentration of 5 mM lamellar formation occurs before significant micellar growth occurs. The higher surfactant concentration, 20 mM, also promotes lamellar formation before significant growth takes place. Figure 3 illustrates more 13364

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the scattering data as the alkyl chain length increases from dodecyl to tetradecyl and to hexadecyl. The onset of precipitation is especially pronounced (see Figure 5) for the hexadecyl alkyl chain length. This greater tendency toward precipitation for the hexadecyl alkyl chain also had a significant impact on the surface adsorption properties.18 At the other extreme, the increased solubility of SAES with a decyl alkyl chain results in a much more reduced micellar growth and a significantly reduced tendency toward lamellar structures.

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SUMMARY

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ASSOCIATED CONTENT

S Supporting Information *

Additional SANS data and tables of model parameters from the analysis of the neutron scattering data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: jeff[email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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The strong complexation between the SAES sulfate headgroup and the Al3+ counterions promotes a reduction in the area per molecule and the curvature and hence promotes micellar growth. Ultimately there is a transition from an elongated micellar structure to a more planar lamellar structure. The extent of micellar growth depends on the surfactant and Al3+ concentrations. Micellar growth and the transition to lamellar structures are also controlled by the size of the ethylene oxide linker group. The ethylene oxide group provides an internal steric hindrance to the complexation, and increasing the ethylene oxide chain length reduces the micellar growth. This also has important consequences for the appearance of the overall phase behavior, and increasing the ethylene oxide size to trioxyethylene results in an extended micellar/lamellar coexistence region. Increasing the alkyl chain length for a given constant ethylene oxide chain length results in an increase in the micellar growth. Ultimately, for tetradecyl and hexadecyl alkyl chain lengths, this leads to a greater tendency toward lamellar formation and precipitation. The results have shown how the SAES surfactant architecture, by changing the ethylene oxide or alkyl chain length, can be used to manipulate solution self-assembly and the response to the addition of multivalent counterions. Furthermore, it provides an important complement to the previously reported surface adsorption studies.16−18

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ACKNOWLEDGMENTS

This work was funded through Engineering and Physical Sciences Research Council (EPSRC) Grant EP/G065705/1. The provision of neutron beam time on the SANS2D instrument at the ISIS facility (STFC), Didcot, U.K., and on the D11 instrument at the Institut Laue Langevin, Grenoble, France, is acknowledged. The invaluable scientific and technical assistance of the instrument scientists and support staff is gratefully recognized. 13365

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