Ion Specific Effects in Trivalent Counterion Induced Surface and

Apr 7, 2014 - Robert K. Thomas,. †. Jordan T. Petkov, ... Institut Laue Langevin, 6 Rue Jules Horowitz, F-38042, Grenoble, Cedex 09, France. •S Su...
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Ion Specific Effects in Trivalent Counterion Induced Surface and Solution Self-Assembly of the Anionic Surfactant Sodium Polyethylene Glycol Monododecyl Ether Sulfate Hui Xu,† Jeffrey Penfold,*,†,‡ Robert K. Thomas,† Jordan T. Petkov,§ Ian Tucker,§ John R. P. Webster,‡ I. Grillo,∥ and A. Terry‡ †

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

S Supporting Information *

ABSTRACT: The effect of different trivalent counterions, Al3+, Cr3+, Sc3+, Gd3+, and La3+, on the surface adsorption and Al3+, Cr3+, and Sc3+ for solution self-assembly of the anionic surfactant sodium polyethylene glycol monododecyl ether sulfate has been studied by neutron reflectivity and small angle neutron scattering. The strong binding and complexation between the trivalent counterions and the anionic surfactant result in significant micellar growth and the formation of surface multilayer structures at the air−water interface at relatively low counterion concentrations. Broadly similar surface and solution behaviors are observed for the different trivalent counterions. The evolution in the surface and solution structures in detail depends upon the nature of the counterion, its hydrated radius and its strength of binding. Exceptionally the addition of Cr3+ counterions have a less pronounced effect. This is attributed to a greater reluctance for exchange within the primary hydration shell for Cr3+ ions, which results in a shielding of the electrostatic interactions and a reduced surfactant−counterion binding

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INTRODUCTION The impact of the addition of electrolytes to the surface adsorption and self-assembly of anionic surfactants is relatively well established for simple monovalent electrolytes, such as NaCl, and other electrolytes.1−3 The addition of electrolyte screens the headgroup electrostatic interactions and reduces the critical micellar concentration, cmc,4 promotes micellar growth,5 and enhances adsorption at interfaces.6 In this context, anionic surfactants have been extensively studied because of their abundant role in most detergency home and personal care products.7 The impact of multivalent counterions, such as Ca2+ or Al3+, is more significant; and the strong binding and complexation rapidly leads to precipitation for the simplest of the alkyl sulfate structures, such as sodium dodecyl sulfate, SDS.8−10 This is the familiar problem of tolerance to hard water in surfactant based formulations,11 and for the alkyl sulfates is greatly enhanced as the alkyl chain length increases.12 To © 2014 American Chemical Society

minimize the onset of precipitation different alkyl sulfate geometries, and other strategies have been developed.13 For example, the alkyl benzene sulfonate 14 and oligoethylene glycol monoalkyl sulfate15−18 surfactants have a greater tolerance to precipitation. The solution properties of anionic surfactants in the presence of multivalent ions have been more extensively studied because of the wider environmental implications, associated with the removal of toxic metallic contaminants in wastewater treatment and in soil remediation.19−21 However, the impact of multivalent ions on the surface or interfacial properties of anionic surfactants has been relatively unexplored. Penfold et al.22 have previously investigated, using neutron reflectivity, NR, and small angle neutron scattering, SANS, the Received: March 10, 2014 Revised: April 6, 2014 Published: April 7, 2014 4694

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impact of the addition of Ca2+ counterions to the surface and solution properties of anionic solutions of sodium dodecyl benzene sulfonate, LAS, and LAS/nonionic surfactant mixtures. Significant micellar growth and the transition to lamellar structures were observed in solution, and the formation of surface multilayer structures at the air−water interface occurred. The evolution in the surface and solution properties depended upon the relative surfactant and counterion concentrations, and could also be controlled by the addition of a nonionic cosurfactant. Broadly similar trends in surface and solution behavior were reported by Petkov et al.23 for the anionic surfactant sodium polyethylene glycol monododecyl ether sulfate, SLES, and SLES/nonionic surfactant mixtures in the presence of Al3+ ions. Xu and co-workers24−26 showed how the surface and solution properties could be manipulated by altering the SLES structure, by changes in the ethylene oxide and alkyl chain lengths. A notable aspect of the surface properties in the presence of multivalent ions for the SLES surfactant was that surface multilayer structures were induced by the addition of Al3+ ions but not Ca2+ ions.24 This latter point raises the prospect of ion specific effects such as the strong ion binding, the degree of hydration and changes in the entropic contribution to the counterion binding energy. Different aspects of this have been explored and reported for di- and trivalent metal ions. Duan and Gregory27 reported on the variations in coagulation of colloidal particles by Al and Fe salts, and on the effects of varying the solution pH. Pereira et al.28,29 have investigated the effects of metal ion hydration on the interaction with SDS and sodium carboxylates, for a range of different trivalent metal ions. PatonMorales and Talens-Alesson9 and Talens-Alesson30 investigated the effects of Al3+, Zn2+, and Na+, and ion mixtures on the flocculation of SDS. Ion specific effects are the focus of this paper, and we report on the effects of the addition of a range of trivalent ions, Cr3+, Sc3+, Gd3+, and La3+, on the surface adsorption and for Cr3+ and Sc3+ on the solution self-assembly properties of the anionic surfactant SLES, and compare them with previously reported results for the addition of Al3+.24−26 The evolution in the adsorption at the air−water interface and of the structure of the surface layer are determined using NR for two different SLES surfactants, sodium diethylene glycol monododecyl ether sulfate, SLE2S, and sodium triethylene glycol monododecyl ether sulfate, SLE3S, for the counterions listed above. The variation in the solution self-assembly, the micelle structure and size, was evaluated using SANS for SLE2S and SLE3S, for the addition of Cr3+ and Sc3+ counterions.

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upon the isotopic content of the solutions. The SANS measurements were made in D2O, using hydrogeneous surfactants, at surfactant concentrations of 5.0, 10.0, and 20.0 mM. The SANS measurements were made for the addition of ScCl3 and CrCl3 only, and compared with SANS data for the addition of AlCl3 from ref 26. Electrolyte concentrations of up to 20.0 mM were used. The D2O was obtained from SIGMA and high purity water (Elga Ultrapure) was used throughout. All glassware, Teflon troughs (used for the NR measurements), and Quartz spectrophotometer cell (used for the SANS measurements) were cleaned in alkali detergent (Decon 90), and rinsed thoroughly in high purity water. No adjustment was made to the pH of the solutions, which typically decreased from ∼7 in the absence of electrolyte, to ∼4.5 depending on the electrolyte and the electrolyte concentration. The neutron reflectivity measurements were made at the air−water interface on the INTER reflectometer at the ISIS neutron source.32 The reflectivity R(Q) was measured over a Q range of 0.03−0.5 Å−1 using an angle of incidence of 2.3° and neutron wavelengths from 0.5 to 15 Å. The reflectivity, R(Q), was calibrated with respect to the direct beam intensity and the reflection from a D2O surface. The measurements were made in sealed Teflon troughs at 25 °C with sample volumes of ∼25 mL. The data acquisition for each neutron reflectivity profile took ∼20−30 min. Repeated measurements were made until the reflectivity showed no change with time, and this was typically ∼2−3 h. Hence, the data presented represent steady state or equilibrium structures. In the kinematic approximation,3 the reflectivity is related to the square of the Fourier transform of the scattering length density profile, ρ(z), normal to the surface (ρ(z) = Σini(z)bi, where ni(z) and bi are the number density and neutron scattering length of the ith component, respectively, and ρ(z) is related to the neutron refractive index, n(z), and n(z) = 1 − λ2ρ(z)/2π). By manipulation of ρ(z) through deuterium labeling (H, D have different scattering lengths, −3.7 × 10−6 Å for H and 6.67 × 10−5 Å for D), the neutron reflectivity profile can directly provide information about the amount adsorbed at the interface, and the structure of the adsorbed layer. This has been extensively demonstrated and exploited for a range of surfactant systems.6 The SANS measurements were made on two different diffractometers, SANS2D33 at the ISIS pulsed neutron source and D1134 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, in order to maximize the scattering. The scattering from the empty cell and solvent were 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 (in cm−1), using standard procedures.35,36 Each individual measurement took ∼10−30 min. The form of the SANS scattering patterns (Q dependence, where Q is the scattering vector defined as Q = 4π/λ sin θ, 2θ is the scattering angle, and λ is the neutron wavelength) was used to identify the regions of micellar, micellar/ lamellar coexistence, and lamellar structures. In the purely micellar regions, the scattering data were analyzed quantitatively using a standard model of interacting micellar solutions.37

EXPERIMENTAL DETAILS

The NR and SANS measurements were made using two anionic surfactants, sodium diethylene glycol monododecyl ether sulfate, SLE2S, and sodium triethylene glycol monododecyl ether sulfate, SLE3S. The synthesis, purification, and characterization of the surfactants have been described in detail elsewhere.24,31 The NR measurements were made at the air−water interface at surfactant concentrations of 0.5 and 1.0 mM, for the alkyl chain deuterated surfactants in null reflecting water, nrw (92/8 mol ratio mixture of H2O and D2O). The measurements were made in the absence of electrolyte and for the addition of CrCl3, ScCl3, GdCl3, and LaCl3, at concentrations up to 2.0 mM. The data for the addition of AlCl3 are reproduced from ref 20. The CrCl3, ScCl3, GdCl3, and LaCl3 were supplied as analytical grade (>99.9% purity) by SIGMA. Because of their poor solubility, the hydrated forms of CrCl3, CrCl3·6H2O, and LaCl3, LaCl3·7H2O, were used, and the hydration has little impact

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RESULTS AND DISCUSSION i. Surface Adsorption. NR measurements were made for 0.5 and 1.0 mM SLE2S with the addition of CrCl3, ScCl3, GdCl3, and LaCl3 and for 1.0 mM SLES3S with the addition of CrCl3, ScCl3, GdCl3, and LaCl3 at the air−water interface. The NR data in Figure 1a shows the variation in the reflectivity for 1.0 mM SLE2S and the addition of CrCl3 up to a concentration 4695

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presented and discussed in some detail in ref 24. The data for 1.0 mM SLE2S in the presence of AlCl3 are reproduced from ref 24 in Figure 1c for comparison. The data for 1.0 mM SLE2S with GdCl3 and LaCl3, for 0.5 mM SLE2S in CrCl2, ScCl3, GdCl3, and LaCl3, and for 1.0 mM SLE3S with CrCl2, ScCl3, GdCl3, and LaCl3 show broadly similar trends, and are summarized in the Supporting Information, Figures S1−S9 and Tables S1−S9. The main features observed are summarized in Figure 1c for SLE2S/AlCl3. In the absence of electrolyte and at the lowest electrolyte concentrations, the NR data correspond to a thin monolayer, typically ∼20 Å (shown in Figure 1c for 0.0 and 0.02 mM AlCl3). At the higher AlCl3 concentrations, the NR data in Figure 1c has three distinctly different forms. At 0.05 mM AlCl3, a pronounced interference fringe with a minimum at Q ∼ 0.08 Å−1 is present in the data. As described in detail in refs 23 and 24, this is modeled as three layers at the interface, and corresponds to a single monolayer at the air−water interface, and a surfactant bilayer immediately below. The NR data for 0.15 and 0.3 mM AlCl3 correspond to multiple bilayer formation (multilayer) at the air−water interface, and this is characterized by a single Bragg peak at Q ∼ 0.12 Å−1 (corresponding to a bilayer thickness of ∼48 Å). The difference in the data for 0.15 and 0.3 mM AlCl3 is the number of bilayers. The width of the Bragg peak, ΔQ, is directly related to 1/N (where N is the number of bilayers). For the addition of 0.15 mM AlCl3, the detailed modeling showed that there were three bilayers and the fringe at the lower Q arises from the total film thickness. In contrast, at 0.3 mM AlCl3, the number of bilayers is ∼20, and the total thickness of the film at the interface is now too large for interference fringes from the film to be visible within the measured Q range. The NR data for 1.0 mM SLE2S with the addition of CrCl3 (see Figure 1a) show some of the characteristics shown in Figure 1c for SLE2S/AlCl3. In the absence of electrolyte and for the addition of 0.1 mM CrCl3, the data are in the form of a monolayer, with a thickness of ∼20 Å (see Table 1). The data are analyzed using the optical matrix method adapted from thin film optics,38 and the adsorbed layer is treated as a thin layer of uniform composition at the interface. This is characterized by a thickness d and a scattering length density ρ; and the values obtained are summarized in Table 1. As described elsewhere,6 this provides a direct evaluation of the adsorbed amount, Γ, or area/molecule, A, at the interface; where Γ = 1/NaA, and A = Σb/dρ, with Na being Avogadro’s number and Σb being the sum of scattering lengths for the alkyl chain deuterated surfactant molecule at the interface (the Σb values for SLE2S and SLE3S are 2.85 × 10−3 and 2.89 × 10−3 Å, respectively). In the absence of electrolyte, the area/molecule is ∼50 Å2, consistent with the previous measurements.24 The addition of 0.1 mM CrCl3 results in an increase in the adsorption as the intraheadgroup repulsive interactions are suppressed, and the area/molecule decreases to ∼40 Å2. Similar changes in the adsorption were reported in the presence of similar concentrations of AlCl3.23,24 For CrCl3 concentrations of 0.15−1.2 mM (see Figure 1a) there is a pronounced interference fringe in the NR data. As described earlier for SLE2S/AlCl3 mixtures, the data are modeled as two to three layers using the optical matrix approach; and the key model parameters are summarized in Table 1. This corresponds to an initial monolayer and an adjacent bilayer beneath, as discussed and described in detail elsewhere;23−25 where in this case d1 refers to the initial monolayer and d2 and d3 are the dimensions

Figure 1. Neutron reflectivity as a function of wave vector transfer, Q (Å−1), for 1.0 mM SLE2S in (a) different CrCl3 concentrations: 0.00 mM (red), 0.10 mM (blue), 0.15 mM (brown), 0.50 mM (green), 1.20 mM (yellow), 1.50 mM (purple), and 2.00 mM CrCl3 (cyan), (b) different ScCl3 concentrations: 0.00 mM (red), 0.10 mM (blue), 0.15 mM (brown), 0.25 mM (green), and 0.50 mM ScCl3 (purple), and (c) different AlCl3 concentrations: 0.0 mM (red), 0.02 mM (blue), 0.05 mM (dark red), 0.15 mM (dark green), and 0.3 mM AlCl3 (dark cyan). The different data are shifted vertically for clarity. The solid lines are model fits to the data as described in the text and for the parameters in Tables 1 and 2, and from ref 24 for the Al3+ data.

of 2.0 mM. A similar range of measurements for SLE2S and SLE3S in the presence of AlCl3 were made; and these are 4696

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Table 1. Key Model Parameters from Analysis of the NR Data for 1.0 mM SLE2S with CrCl3. CrCl3 conc (mM) 0.0 0.1 CrCl3 conc (mM)

d1 (±1 Å)

ρ1 (±0.1 × 10‑6 Å‑2)

A (±2 Å2)

Γ (±0.2 × 10−10 mol cm−2)

20 23

2.8 3.2

51 39

3.3 4.3

d1

ρ1

d2

ρ2

d3

ρ3

0.15 0.5 1.2 CrCl3 conc (mM)

49 17 22 d1

1.2 1.8 3.6 ρ1

18 28 26 d2

4.4 0.8 0.3 ρ2

20 19 N

3.3 2.7 ΔQ

1.5 2.0

26 26

2.8 4.0

25 25

0.8 1.0

20 40

0.1 0.07

Table 2. Key Model Parameters from Analysis of the NR Data for 1.0 mM SLE2S with ScCl3 ScCl3 conc (mM) 0.0 0.1 ScCl3 conc (mM) 0.15 0.25 0.5

ρ1 (±0.1 × 10‑6 Å‑2)

d1 (±1 Å) 20 20

A (±2 Å2)

Γ (±0.2 × 10−10 mol cm−2)

51 38

3.3 4.4

d1

2.8 2.7 ρ1

d2

ρ2

N

ΔQ

24 25 25

1.9 2.3 2.6

24 25 25

1.0 1.0 1.0

30 40 80

0.09 0.08 0.07

of the adjacent bilayer (d2 is headgroup region and d3 the interdigitated alkyl chain region). The approach adopted is to analyze the data using the simplest model consistent with the data; and for the data in 0.15 mM CrCl2 where the interference fringe is less well defined, two layers suffice. The key model parameters (see Table 1) are similar to those reported in the presence of AlCl3.23−25 At the higher CrCl3 concentrations (1.5, 2.0 mM), the reflectivity is dominated by a pronounced and relatively sharp Bragg peak at a Q value of ∼0.12 Å−1. The multilayer (multiple bilayer) data were analyzed using a surface multilayer model based on the kinematic approximation.39,40 In this approach, the key model parameters are the bilayer thickness, dt, where the thickness of the alkyl chain region is d1 and of the headgroup region d2, such that dt = d1 + d2, the scattering length density of the two regions of the bilayer ρ1 and ρ2, and the number of bilayers, N. As only the first order Bragg peak is usually visible the modeling is most sensitive to N, dt, and Δρ (ρ1 − ρ2). In Figure 1a, the width of the Bragg peak decreases as the CrCl3 concentration increases from 1.5 to 2.0 mM, and the number of bilayers increases from ∼20 to ∼40. The key model parameters are summarized in Table 1. The final column in Table 1 is the parameter ΔQ, which is a convolution of the instrumental resolution (∼0.05) and an additional contribution such that ΔQ is ∼0.07−0.1. At partial surface coverage of the surface multilayer structure, the multilayers are in a form analogous to lamellar crystallites, and so ΔQ is broadened by the mosaic spread of the lamellar patches at the surface. The NR data for 1.0 mM SLE2S in the presence of ScCl3 are different from that in CrCl3 and AlCl3. The data are initially in the form of a monolayer at low ScCl3 concentrations, and transform immediately to a multilayer structure, with the number of bilayers being >40. This occurs without the appearance of the intermediate structures with a smaller number of bilayers. The key model parameters are summarized in Table 2. Similar data are observed for the addition of GdCl3 and LaCl3, and at the lower surfactant concentration of 0.5 mM for the addition of ScCl3, GdCl3, and LaCl3. Broadly similar data were measured for 1.0 mM SLE3S in ScCl3, GdCl3, and LaCl3

(see Figures S1−S9 and Tables S−S9 in the Supporting Information). For the addition of CrCl3, in the concentration range up to 1.5 mM, only monolayer adsorption was observed (see Figure S6 and Table S6 in the Supporting Information). The NR data can be summarized as an approximate surface phase diagram, as shown in Figure 2 for 1.0 mM SLE2S and SLE3S. The variation in the surface structure with counterion concentration for the addition of Cr3+, Sc3+, Gd3+ and La3+ are presented and compared with the previously published data for Al3+.24 Comparing the evolution of the surface structure for 1.0 mM SLE2S with multivalent counterion concentration for Sc3+, Gd3+, and La3+ with Al3+ (see Figure 2a), there is a direct transformation from monolayer to multilayer (N > 20) formation for Sc3+, Gd3+, and La3+. The intermediate structures, formed with a lower number of bilayers, observed with the addition of Al3+, are not observed. The surface multilayer formation arises from the strong binding and complexation of the trivalent ions with neighboring surfactant headgroups, as proposed by Alargova et al 42 and demonstrated more directly by Xu and co-workers.24−26 The changes in the structural evolution demonstrated here correlate with a decrease in the hydrated radius of the ions and the increase in binding28,41 for Sc3+, Gd3+, and La3+ and compared to Al3+. Previous observations that Ca2+ binding with LAS promotes surface ordering,22 but not with SLES 24 where the stronger Al3+ binding is required reinforces the importance of the nature of the counterion and its binding. The evolution in the surface structure with Cr3+ concentration is quite different to that observed for Al3+ and the other multivalent ions. Although the purely monolayer region occurs over a similar concentration range, there is an extended range of Cr3+ concentrations over which only a single bilayer is observed. Furthermore, the Cr3+ concentration at which multilayer formation with a large N forms is much higher. It is known that the hydration of Cr3+ ions is less labile, and so exchange of water molecules in the hydration shell is much slower.28,29 It was reported by Pereira et al.28 that this resulted in a shielding of the electrostatic interaction between Cr3+ ions and dodecyl sulfate, and hence a 4697

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for SLE3S. This implies that the steric contribution from the larger ethylene oxide group is now dominating the interaction. ii. Solution Self-Assembly. To complement the surface measurements, the solution microstructure has been evaluated using SANS. Measurements were made at surfactant concentrations of 5.0, 10.0, and 20.0 mM for SLE2S with the addition CrCl3 and ScCl3. Similar measurements were made for 20.0 mM SLE3S. These are compared with the data previously reported for SLE2S and SLE3S in the presence of AlCl3.26 Figure 3a shows the variation in the scattered intensity for 5.0 mM SLE2S in the presence of CrCl3, from 0.0 to 2.0 mM, and the key model parameters are summarized in Table 3a.

Figure 2. Surface phase diagram for (a) 1.0 mM SLE2S and (b) 1.0 mM SLE3S, in the presence of five different trivalent counterions: Al3+, Cr3+, Sc3+, Gd3+, and La3+ (see legend for details).

weaker interaction. The weaker binding is consistent with the shift in the onset of multilayer formation at the surface (for large N) to higher electrolyte concentrations reported here. In Figure 2b, the evolution in the surface structure with multivalent ion concentration is plotted for 1.0 mM SLE3S. For the larger ethylene oxide group, as previously reported for the addition of AlCl3,23 the onset of multilayer formation occurs at higher multivalent ion concentrations. This is observed here for Sc3+, Gd3+, and La3+, as well as for Al3+. This was previously23 attributed to an increase in the steric hindrance of the larger ethylene oxide group disrupting the SLES/Al3+ complex formation that drives the surface multilayer formation. The situation is quite different for the addition of Cr3+, and over the Cr3+ concentration studied only monolayer formation occurs. Hence, the weaker Cr3+ interaction, due to the hydration effects discussed earlier, and the contribution from the steric hindrance of the larger ethylene oxide group, results in a much weaker interaction overall. For Sc3+ and La3+, the transition to multilayer formation (for N large) still occurs directly from monolayer adsorption, as was observed for 1.0 mM SLE2S. For SLE3S, the response to the addition of Gd3+ is now slightly different and there is a region of intermediate structures between the regions of monolayer and multilayer (N large) formation. However, the impact of the changes in the binding and hydration, from Al3+ to Sc3+ and La3+, is less pronounced

Figure 3. SANS scattering intensity, I(Q) (cm−1), versus wave vector transfer, Q (Å−1 ), for (a) 5.0 mM SLE2S in different CrCl3 concentrations: 0.0 mM (red), 0.4 mM (blue), 0.8 mM (brown), 1.2 mM (green), 1.8 mM (yellow), and 2.0 mM CrCl3 (purple); (b) 5.0 mM SLE2S in different ScCl3 concentrations: 0.0 mM (red), 0.4 mM (blue), 0.8 mM (brown), 1.2 mM (green), 1.5 mM (yellow), and 1.8 mM ScCl3 (purple). The solid lines are model calculations as described in the text and for the parameters in Table 3.

The data in Figure 3a are consistent with the formation of small globular micelles, and there is very little evidence of intermicellar interactions, due to the dilute concentration studied. With increasing CrCl3 concentration the scattered intensity at low Q values increases; consistent with micellar growth. The data are analyzed quantitatively using a wellestablished micelle model,37 where the scattering is described by the decoupling approximation as 4698

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micellar growth is more significant compared to that observed for CrCl3. The aggregation number increases from ∼70 to ∼800, and the ellipticity from ∼1.2 to ∼14.0. The data at the highest ScCl3 concentration measured, 1.8 mM, is accompanied by precipitation and the formation of a solid lamellar phase. Similar variations and transitions to lamellar phase separation were also reported for SLES/AlCl3 mixtures.26 Similar data were obtained for 10.0 and 20.0 mM SLE2S with the addition of CrCl3 and ScCl3, and for 20.0 mM SLE3S in the presence of CrCl3 and ScCl3, and are summarized in Figures S10−S15 and Tables S10−S15 in the Supporting Information. Equivalent data for SLE2S and SLE3S in the presence of AlCl3 are reported in full in refs 26 and 31. The variation in the micelle aggregation number with multivalent ion concentration for 5.0 mM SLE2S, 20.0 mM SLE2S, and 5.0 mM SLE3S with the addition of Al3+, Cr3+, and Sc3+ ions is summarized in Figure 4. The varaitions in the micelle aggregation number presented in Figure 4 illustrate the main features encountered. In Figure 4a, the variation in the micelle growth for 5.0 mM SLE2S is similar for all three counterions, up to a counterion concentration of ∼1.0 mM. At higher multivalent ion concentrations, the addition of Al3+ ions results almost immediately in lamellar formation. Arising from the stronger Sc3+ interaction, more substantial micellar growth occurs before lamellar formation. In contrast, the addition of Cr3+ results in more modest micellar growth compared to that obsered for Sc3+, but over the same multivalent ion concentration the onset of lamellar phase separation does not occur. This is consistent with the surface behavior and the implication that there is a weaker interaction between the sulfate headgroup and the Cr3+ ion. In Figure 4b, at the higher SLE2S concentration of 20.0 mM, the varaition in the micelle aggregation number is similar for all three counterions up to a concentration of 6.0 mM. The onset of lamellar formation and phase separation occurs at similar multivalent ion concentrations for Al3+ and Sc3+, and occurs at a much higher concentration for Cr3+. Furthermore, the micellear growth is more significant with the addition of Cr3+ before lamellar formation than was observed at the lower surfactant concentration. Notably, at the lower multivalent ion concentrations, the micellar growth for the addition of AlCl3 and ScCl3 is broadly similar over the surfactant concentration range from 5 to 20 mM. The variation in the micelle aggregation number with multivalent ion concentration in Figure 4c is for 5.0 mM SLE3S, with the addition of Al3+, Sc3+, and Cr3+, and the micellar growth is much less pronounced. It is more significant for Sc3+ before the onset of lamellar phase separation, and lamellar phase separation occurs for the addition of both AlCl3 and ScCl3. However, very modest micellar growth is observed for the addition of Cr3+, and no lamellar formation occurs over the multivalent ion concentration range measured. These differences are more pronounced at a surfactant concentration of 20.0 mM (see Figure S16 in the Supporting Information). The mechanism, as outlined by Alargova et al.42 and reinforced by the studies of Xu et al.,24−26 responsible for both the surface and solution properties in the presence of multivalent electrolyte arises from the strong binding and bridging of the multivalent ions between adjacent surfactant headgroups. This is responsible for the pronounced micellar growth at relatively low electrolyte concentrations compared to that required with 1:1 electrolytes. For example, the relative ionic strength of NaCl required to promote similar micellar

Table 3. Key Model Parameters from Analysis of SANS Data for 5.0 mM SLE2S. (a) with CrCl3 CrCl3 conc (mM)

ν (±5)

0.0 0.4 0.8 1.2 1.8 2.0

70 103 128 149 318 496

ScCl3 conc (mM)

ν (±5)

z (±1.0)

0.0 0.4 0.8 1.2 1.5

70 113 137 168 828

1 5 5 5 1

z (±1.0)

R1 (±1 Å) R2 (±1 Å) ee (±0.1)

1 17 3 15 1 17 1 17 1 17 1 17 (b) with ScCl3

20 19 20 20 20 20

R1 (±1 Å) R2 (±1 Å) 17 17 17 17 17

20 20 20 20 20

1.2 2.3 2.2 2.5 5.3 8.3 ee (±0.1) 1.2 1.9 2.3 2.8 13.9

I(Q ) = Np[S(Q )|⟨F(Q )⟩Q |2 + ⟨|F(Q )|2 ⟩Q − |⟨F(Q )⟩Q |2 ] (1)

S(Q) is the intermicelle structure factor, calculated using the rescaled mean spherical approximation for a repulsive screened Coulombic intermicellar potential,43 and is characterized by the micelle number density, Np, the surface charge, z, the micelle diameter, σ, and the inverse screening length, κ. F(Q) is the micelle form factor, which describes the micelle shape and size. Here it is modeled as a core−shell particle with an inner core of alkyl chains with a radius R1, and an outer shell of headgroups and associated hydration with a radius R2 (σ = 2R2), as described in detail elsewhere.37 Molecular constraints are included and the micelle inner radius is limited to the fully extend alkyl; chain length, lc, such that R1 = lc for spherical micelles. For aggregation numbers, ν, larger than can be packed into a spherical geometry, it is assumed that the micelles are prolate ellipses with core dimensions of R1 and R1·ee (ee is the elliptical ratio). From the known molecular volumes, neutron scattering lengths, and solution concentration, the scattering can be calculated using eq 1 on an absolute scale, and compared with the data. The key refinable parameters are then ν and z (the degree of ionization of the micelle, δ, is z/ν). Model fits are taken as acceptable when the functional form of the data is represented and the absolute intensity is predicted to within ± 20%; and the comparison is evaluated using a least-squares criterion. From Figure 3a and Table 3a, it is evident that there is micellar growth with increasing CrCl3 concentration, resulting in an increase in the aggregation number and ellipticity. The micelle radius, R2, is ∼20 Å, and the ellipticity varies from ∼1.2 to ∼ 8.3, as the aggregation number increases from ∼70 to ∼500. In the absence of electrolyte, the charge on the SLES micelles is relatively low; the degree of ionization, δ, is ∼0.1− 0.15. Typically, for anionic surfactant,s it is ∼0.3,37 in agreement with the “dressed micelle model”.44 The low values for δ for the SLES surfactants are attributed to stronger counterion binding due to a modification of the dielectric constant in the headgroup region arising from the presence of the ethylene oxide groups.26 In the presence of the multivalent electrolyte, the ion binding is close to complete and δ is ∼0.0. Equivalent data for the addition of ScCl3 are shown in Figure 3b, and the key model parameters are summarized in Table 3b. In the ScCl3 concentration range from 0.0 to 1.5 mM, the 4699

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Sammalkorpi co-workers3,45 have performed molecular dynamic simulations on SDS and sodium hexyl sulfate in the presence of NaCl and CaCl2, and showed that CaCl2 is more effective in promoting micellar growth. In particular, they demonstrated that the addition of Ca2+ produced more compact densely packed micelles which were stabilized by salt bridges between neighboring surfactant headgroups. For 1:1 electrolytes, Hayter44 produced a self-consistent theory of dressed ionic micelles, based on a solution of the nonlinear Poisson−Boltzmann equation, which describes the ionic distribution about the micelle. The predicted degree of ionization, δ, values are in good agreement with experimental measurements for a range of ionic surfactants, in the absence and in the presence of 1:1 electrolyte. It was shown previously that, for the SLES surfactants,26 δ is small compared to other ionic surfactants: typically, it is ∼0.1−0.2 compared with ∼0.3 for most other ionic surfactants.44 This was also consistent with the nonionic nature of the measured adsorption isotherms for SLES,25 and was attributed to a tighter counterion binding associated with a modification of the local dielectric constant due to the ethylene oxide groups and hence to a stronger intraheadgroup interaction. This argument is further reinforced by the observation that the addition of Ca2+ promotes surface self-assembly for LAS33 but not for SLES where the stronger binding of the trivalent ions is required. Hence, the strength of binding of the multivalent counterion is an important factor, as demonstrated here. In the presence of multivalent electrolyte the degree of inoisation of the SLES micelles is remarkably low, and implies that there is a strong almost complete binding of the counterions to the micelles surface. Scrinivasan and Blankschtein46 have used the molecular thermodynamic approach to predict the micellar properties of the SLES surfactant in the presence of NaCl/CaCl2 and NaCl/ AlCl3 mixtures. The theoretical treatment makes assumptions that account for the electrostatic interactions with the headgroup and the changes in the counterion valency and hydrated ion size. The calculations of Scrinivasan and Blankschtein are broadly consistent qualitatively with the trends reported here. However, further detailed consideration is required into the way in which the ion binding and binding strengths are altered by the change in the headgroup environment and electrostatic interactions due to the presence of the ethylene oxide groups.

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SUMMARY The NR and SANS results presented show that for the addition of multivalent counterions to dilute solutions of the SLES anionic surfactants, the onset of surface multilayer structures and the micellar growth in solution depend upon the nature of the counterion and its hydration. A range of different counterions were studies, Cr3+, Sc3+, Gd3+, and La3+, and compared with previously reported data for Al3+.23−26 For the different counterions studied, the ion binding varies with ion size in the order La3+ > Gd3+ > Sc3+ > Cr3+ > Al3+ > Ca2+ > Na+, and the hydrated radius decrease from Al3+ to La3+, from 4.75 to 4.52 Å.28,41 In solution, the SANS results confirm the earlier results for the addition of Al3+. That is, the addition of the trivalent ions at relatively low concentrations results in significant micellar growth.15−17,23−26 This is also observed for the addition of Sc3+, and the onset of the growth coincides with the counterion charge being in excess.15−17,23,47 However, compared to Al3+, the growth due to the addition of Sc3+ is more pronounced

Figure 4. Variation in micelle aggregation number, v, with trivalent counterion M3+ concentration for (a) 5.0 mM SLE2S, (b) 20.0 mM SLE2S, and (c) 5.0 mM SLE3S, in the presence of different trivalent counterions: Al3+ (red), Cr3+ (blue), and Sc3+ (green). The solid lines are guides to the eye only. The concentration at which lamellar phase separation occurs is indicated.

growth compared to AlCl3 is ∼20−50 times larger.26 In the absence of electrolyte the Debye−Huckel inverse screening length, κ−1, is typically ≥30 Å. The addition of millimolar concentrations of the trivalent electrolyte hardly changes that value. In contrast, 100.0 mM NaCl reduces κ−1 to ≤10 Å; and in that case, the charge screening effect is self-evident. 4700

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before lamellar formation and precipitation occurs. This is attributed to the increased binding for Sc3+ compared to Al3+, and the associated decrease in the hydrated radius and hydration. For Cr3+, the micellar growth before lamellar phase separation is less pronounced, especially for SLE3S. This is attributed to a weaker headgroup−Cr3+ interaction, which arises from a shielding of the electrostatic interaction by the less labile hydration shell for Cr3+.28,29 At the surface, the NR results show that due to the strong binding of the multivalent ions to neighboring SLES molecules surface multilayer structures are formed. Compared to the previous results for the addition of Al3+,24,25 the Sc3+, La3+, and Gd3+ ions also strongly promote surface multilayer formation. The stronger binding and reduced hydration radius results in a more immediate transition from monolayer to multilayer formation with a large number of bilayers (N > 20). For SLE2S, the extended region of counterion concentrations over which the multilayer structures have a finitely small number of bilayers (N < 3) observed with Al3+ is not observed with Sc3+, La3+, and Gd3+ ions. For SLE3S, the differences in the evolution of the surface structure with counterion concentration, when comparing Al3+ with Sc3+, La3+, and Gd3+, are not so pronounced. This implies that the steric contribution from the larger ethylene oxide group has a more significant effect in determining the way in which the surface structure evolves. However, as was observed in the evolution of the solution microstructure, the addition of Cr3+ ions has quite a different effect on the way the surface structure evolves with counterion concentration. For SLE2S, the region of concentrations over which a single bilayer is formed is extended to higher counterion concentrations, and the onset of multilayer formation (with N large) is shifted to higher concentrations. For SLE3S, over the concentration range measured for Cr3+, only monolayer adsorption is observed. Both these observations are consistent with the weaker interaction between the sulfate and Cr3+ ions, as discussed earlier in the context of the solution microstructure. Hence, it has been demonstrated, using NR and SANS, how the nature of the trivalent ion and its associated hydration affect the evolution in the surface and solution structures with counterion concentration. This is important in the context of manipulating these structures, and in particular in the applications associated with wastewater treatment and soil remediation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: jeff[email protected]. Author Contributions

All the authors have given their approval of the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The provision of beam time on the INTER and SANS2D instruments at ISIS and on the D11 instrument at the Institute Laue Langevin, Grenoble is acknowledged. The invaluable scientific and technical assistance of the Instrument Scientists, and support staff is gratefully recognized. Funded through EPSRC grant, EP/G065705/1 and neutron beam time at the ISIS Facility, UK (STFC) and the ILL, Grenoble, France.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing neutron reflectivity as a function of wave vector transfer for different concentrations of SLE2S and SLE3S in different concentrations of CrCl3, ScCl3, GdCl3, and LaCl3; tables showing key model parameters for SLE2S and SLE3S with CrCl3, ScCl3, GdCl3, and LaCl3; figures showing SANS scattering intensity versus wave vector transfer for different concentrations of SLE2S and SLE3S in different CrCl3 and ScCl3 concentrations; figure showing variation in micelle aggretation number with trivalent counterion concentration for 20 mM SLE3S; tables showing key model parameters for different concentrations of SLE2S and SLE3S with CrCl3 and ScCl3. This material is available free of charge via the Internet at http://pubs.acs.org. 4701

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