The Formation of Surface Multilayers at the Air–Water Interface from

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The Formation of Surface Multilayers at the Air−Water Interface from Sodium Polyethylene Glycol Monoalkyl Ether Sulfate/AlCl3 Solutions: The Role of the Size of the Polyethylene Oxide Group Hui Xu,† Jeff Penfold,*,†,‡ Robert K. Thomas,† Jordan T. Petkov,§ Ian Tucker,§ and John P. R. Webster‡ †

Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 2JD, United Kingdom STFC, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom § Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral CH63 2JW, United Kingdom ‡

S Supporting Information *

ABSTRACT: Neutron reflectivity, NR, and surface tension, ST, have been used to study the surface adsorption properties at the air−water interface of the anionic surfactant sodium polyethylene glycol monododecyl ether sulfate (sodium lauryl ether sulfate, SLES) in the presence of Al3+ multivalent counterions, by the addition of AlCl3. In the absence of AlCl3 and at low AlCl3 concentrations monolayer adsorption is observed. With increasing AlCl3 concentration, surface multilayer formation is observed, driven by SLES/Al3+ complex formation. The onset of multilayer formation occurs initially as a single bilayer or a multilayer structure with a limited number of bilayers, N, ≤3, and ultimately at higher AlCl3 concentrations N is large, >20. The evolution in the surface structure is determined by the surfactant and AlCl3 concentrations, and the size of the polyethylene oxide group in the different SLES surfactants studied. From the NR data, approximate surface phase diagrams are constructed, and the evolution of the surface structure with surfactant and electrolyte concentration is shown to be dependent on the size of the polyethylene oxide group. As the polyethylene oxide group increases in size the multilayer formation requires increasingly higher surfactant and AlCl3 concentrations to promote the formation. This is attributed to the increased steric hindrance of the polyethylene oxide group disrupting SLES/Al3+ complex formation.

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stimulated a wide range of experimental studies,4,8,11−19 and the development of associated theoretical treatments.20,21 In applications where the precipitation effects are a disadvantage, a number of different strategies to minimize the effects of multivalent counterions have been developed and adopted.1,4,8 This can involve the introduction of a nonionic cosurfactant,8 or sequestering agents or water softeners, such as ethylenediaminetetraacetic acid, EDTA.4 Alternatively, modifying the alkyl sulfate structure by introducing an EO chain to form the polyethylene glycol monododecyl ether sulfate, SLES, improves solubility and tolerance to the addition of multivalent counterions.4,15−18 Until recently, the majority of the studies in this area relate to solution properties and how the self-assembly and solubility are affected by the addition of multivalent counterions. In their pioneering study, Alargova et al.17 reported the variation in surface tension for SLES/Al3+ mixtures, but were not able to correlate the decrease in surface tension with increasing Al3+ concentration with any structural or compositional changes in the surface layer. More recently, Penfold et al.22 and Petkov et

INTRODUCTION Anionic surfactants are the basis of the formulations associated with most detergency based home and personal care products, and a wide range of different molecular structures have been developed and used.1 In these formulations, the anionic surfactants are commonly used as mixtures, predominantly with nonionic surfactants. This provides synergistic enhancements in many aspects of performance, and greater flexibility in formulation and processing.2,3 The anionic surfactants, sodium dodecyl sulfate, SDS, sodium dodecyl benzene sulfonate, LAS, and sodium polyethylene glycol monododecyl ether sulfate, SLES, are the most commonly used and investigated.4 The effects of the addition of monovalent electrolytes, such as NaCl, to anionic surfactants are well established and extensively studied. The addition of electrolyte in general reduces critical micellar concentration values,5 promotes micellar growth,6 and enhanced adsorption.7 The impact of multivalent counterions, such as Ca2+ or Al3+, is more profound; and the addition of multivalent counterions to SDS, for example, leads rapidly to precipitation.8 This is the problem associated with the familiar effects of water hardness. Furthermore, it is related to environmental applications, such as the removal of toxic metals in wastewater treatment and in soil remediation.9,10 The importance of these applications has © 2013 American Chemical Society

Received: August 6, 2013 Revised: August 22, 2013 Published: August 22, 2013 11656

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normal to the surface. Q is defined as Q = 4π sin/λ, θ is the grazing angle of incidence, and λ is the neutron wavelength. The measurements were made on the SURF and INTER reflectometers at the ISIS pulsed neutron source in the United Kingdom32 and on the FIGARO reflectometer at the ILL in France.33 On all three reflectometers, the measurements were made at a fixed angle of incidence using the white beam time-of-flight method. On SURF the angle of incidence was 1.5°, and neutron wavelengths from 0.5 to 7 Å were used to cover a Q range of 0.05−0.5 Å−1. On INTER an angle of incidence of 2.3° and a wavelength range of 0.5−15 Å were used to give a Q range of 0.03−0.5 Å−1. On SURF and INTER, the measurements were made using a single detector, and the data were corrected using established procedures.34 On FIGARO, the data were recorded on a twodimensional area detector at grazing incidence of 3.8° and a wavelength range of 2.4−24 Å to provide a Q range of 0.035−0.3 Å−1. On all three reflectometers the data were calibrated with respect to the direct beam and the reflectivity from a D2O surface. The measurements were made at 25 °C in sealed Teflon troughs with sample volumes ∼25 mL. Each individual neutron reflectivity profile took ∼20−30 min, and sequential measurements were made on a 5−7 position sample changer. The sequence of measurements was repeated 2−3 times until the reflectivity showed no changes with time. Hence the data presented represent equilibrium structures and adsorption. For monolayer adsorption the kinetics of the adsorption was instantaneous compared to the time scales of an individual measurement. For the more complex surface structures equilibrium was reached after ∼2−3 h. In the kinematic approximation, the reflectivity is related to the square of the Fourier transform of the scattering length density profile, ρ(z), normal to the surface,7 such that

al.23 have used NR to investigate the surface adsorption properties of anionic/nonionic surfactant mixtures in the presence of multivalent counterions. Penfold et al.22 studied the adsorption at the air−water interface of LAS/nonionic mixtures in the presence of Ca2+ ions, and Petkov et al.23 made a similar study on the impact of Al3+ ions on SLES/nonionic surfactant adsorption. In both cases the strong surface complex formation between the multivalent counterions and neighboring surfactant headgroups gave rise to a transition from a monolayer to a multilayer structure at the interface as the counterion concentration was increased. The evolution of the surface structure was determined by the surfactant and multivalent counterions concentrations and the composition of the anionic/nonionic mixture. The nonionic surfactant with a bulky headgroup (C12E8 or C12E12) introduces a steric hindrance which disrupts the complex formation, as originally proposed by Alargova et al.17 Although surface multilayer structures have been reported previously in concentrated surfactant systems,24−26 in lung surfactants,27,28 and in polyelectrolyte/surfactant mixtures,29 there are relatively few observations reported for relatively dilute systems.22,23,30,31 The surface multilayer structures induced by polyelectrolytes29 and multivalent counterions22,23 exhibit extreme and persistent wetting properties on hydrophobic surfaces and offer great potential for applications requiring enhanced adsorption, delivery and retention of benefit agents to interfaces, and in soft or biolubrication. The focus of this paper is on the role of multivalent counterions in inducing surface multilayer formation in dilute surfactant solutions. It is important to understand the factors which determine the multilayer formation and how to control and manipulate their formation. Here the specific role of the surfactant structure of the sodium polyethylene glycol monododecyl ether sulfate, SLES, anionic surfactant is investigated. In particular, the effect of changing the size of the ethylene oxide linker group will be explored. Neutron reflectivity and surface tension will be used to investigate the adsorption and the structure of the adsorbed layer at the air− water interface for a range of different SLES anionic surfactants in the presence of Al3+ counterions.

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R(Q ) =

16π 2 | Q2

∫ ρ(z) e−iQz dz|2

(1)

ρ(z) = Σini(z)bi, ni(z) is the number density of the ith component, and bi its scattering length. In neutron reflectivity ρ(z), can be manipulated using principally H/D isotopic substitution, as H and D have different scattering lengths (−3.7 × 10−5 Å for H and 6.67 × 10−5 Å for D). The measurements made here are for deuterium labeled surfactants in null reflecting water, NRW, where NRW is a 92/8 mol ratio mixture of H2O and D2O with a scattering length density of zero. In such circumstances, the reflectivity arises only from the deuterated material adsorbed at the interface. The neutron reflectivity data can equally well be described using an adaptation of the familiar optical matrix methods.35 For a simple monolayer, adsorption of the deuterated surfactant at the air-NRW interface the reflectivity can often be described by a single layer of uniform composition7 to give an area/ molecule, A,

EXPERIMENTAL DETAILS

Neutron reflectivity and surface tension measurements were made for different sodium polyethylene glycol monododecyl ether sulfate (SLES) anionic surfactants in the presence of AlCl3 at the air−water interface. Three different SLES surfactants, with different ethylene oxide groups, were investigated; sodium ethylene glycol monododecyl ether sulfate, SLE1S, sodium diethylene glycol monododecyl ether sulfate, SLE2S, and sodium triethylene glycol monododecyl ether sulfate, SLE3S. The surface tension measurements were made at different fixed AlCl3 concentrations and for a range of surfactant concentrations. The neutron reflectivity measurements were made at a range of different surfactant and AlCl3 concentrations. (i). Surface Tension. The surface tension measurements were made on a Krüss K10 tensiometer using a Pt/Ir de Nouy ring. The measurements were made at 25 °C. They were repeated until equilibrium was reached, as indicated by three repeated measurements being within 0.2 mN m−1. The measurements were made by repeated dilutions from the most concentrated solution. The tensiometer was calibrated using ultrapure water for a value of 72 mN m−1. The ring was washed and dried in a Bunsen burner flame between measurements. (ii). Neutron Reflectivity. The neutron reflectivity measurements were made at the air−water interface, where the reflectivity R(Q) is measured as a function of the wave vector transfer, Q, in a direction

A=

∑ bi /dρ i

(2)

where d and ρ are the monolayer thickness and scattering length density, respectively, the adsorbed amount is then Γ = 1/NaA, and Na is Avogadro’s number. The Σb values for alkyl chain deuterium labeled SLE1S, SLE2S, and SLE3S are 2.80 × 10−3, 2.85 × 10−3, and 2.89 × 10−3 Å, respectively. The typical error in the determination of A is ±2 Å2 at a value of ∼50 Å2 For the neutron reflectivity data where extensive surface multilayer formation occurs at the interface the data were analyzed using a surface multilayer model based on the kinematic approximation,36,37 and which has been extensively used for broadly similar systems.22,23 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 densities of the two bilayer regions, ρ1, ρ2, and the number of bilayers N. As frequently only the first order Bragg peak from the multilayer structure is visible, the modeling is most sensitive to N, dt, and Δρ (ρ1 − ρ2). (iii). Materials. The alkyl chain deuterium labeled SLES surfactants and the protonated SLES surfactants, SLE1S, SLE2S, and SLE3S, were 11657

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custom synthesized in Oxford by sulfonation of the corresponding and previously synthesized polyethylene glycol monododecyl ethers, as described in detail elsewhere.38 The surfactants were recrystallized twice in propanol/ethanol mixtures. The purity of the surfactants were assessed using mass spectrometry, MS, GC/MS, and NMR spectroscopy, and from the surface tension and neutron reflectivity measurements. Analytical grade (>99.9% purity) AlCl3 from Sigma Aldrich was used. The D2O was obtained from Sigma Aldrich, and high purity (Elga Ultrapure) water was used throughout. All glassware and sample troughs were cleaned in alkali detergent (Decon 90), followed by washing in ultrapure water. The surface tension measurements were made using hydrogenous surfactants in H2O, and the neutron reflectivity measurements using the deuterium labeled surfactants in NRW. With the addition of the AlCl3, no adjustments to the pH of the solutions were made. In general, the pH of the SLES solution in the presence of AlCl3 is ∼5.

SLE1S to SLE3S. The increase in solubility would normally correlate with an increase in the CMC; however, the opposite is observed here. This implies that increasing the ethylene oxide chain length results in a greater tendency for micelle formation. It is assumed that the presence of the polyethylene oxide group reduces the intramicellar headgroup repulsion and the electrostatic contribution to the free energy of micellization. This supposition is supported by some related experimental and theoretical studies on SLES micelles42,43 and on mixed anionic/ nonioic micelles.44 The surface adsorption was measured directly using neutron reflectivity. The measurements were made using the alkyl chain deuterium labeled SLES surfactants, dC12hE1S, dC12hE2S, and dC12hE3S. The neutron reflectivity data are consistent with a thin monolayer, ∼17 ± 2 Å, of uniform composition. The reflectivity data for SLE1S, SLE2S, and SLE3S are then modeled as a monolayer of uniform composition to give d and ρ, as summarized in Table 1 in the Supporting Information. For the deuterium labeled surfactant in NRW the product dρ is directly related to the area/molecule via eq 2. The corresponding adsorption isotherms are shown in Figure 2.

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RESULTS AND DISCUSSION (i). SLES Adsorption. The surface adsorption properties of the three different SLES surfactants were initially characterized at the air−water interface in the absence of electrolyte. The surface tension data for fully protonated SLE1S (hC12hE1S), SLE2S (hC12hE2S), and SLE3S (hC12hE3S) are shown in Figure 1.

Figure 2. Adsorbed amount (Γ × 10−10 mol cm−2) versus surfactant concentration for (red) SLE1S, (blue) SLE2S, and (green) SLE3S.

Figure 1. Surface tension versus surfactant concentration for (red) hC12hE1S, (blue) hC12hE2S, and (green) hC12hE3S.

The adsorption isotherm was measured from relatively low surfactant concentrations to concentrations in excess of the CMC. The mean adsorption above the CMC increases as the polyethylene oxide group decreases in size. The area/molecule above the CMC varies from 40 ± 2 Å2 for SLE1S, 46 ± 2 Å2 for SLE2S, to 52 ± 2 Å2 for SLE3S; and the corresponding adsorbed amounts are 4.2, 3.6, and 3.2 ± 0.2 × 10−10 mol cm−2. For all three surfactants, the adsorption at concentrations greater than the CMC is constant. This is different from that normally observed in ionic surfactants, where the activity increases and the associated adsorption increases with increasing concentration above the CMC. This has been previously demonstrated in cationic and anionic surfactants.7 In contrast, the adsorption of nonionic surfactants is constant at concentrations greater than the CMC.7 The data in Figure 2 indicates that the SLES surfactants are behaving more like a nonionic surfactant. This implies that the activity is varying only slightly above the CMC. It further implies that the degree of ionization of the SLES molecules is low and slowly varying compared to normal ionic surfactant, as was observed in small angle neutron scattering, SANS, studies on SLES micelles.38 This could be as a direct result of the polyethylene oxide group

For all three surfactants, there is a realtively weak minimum at the critical micelle concentration (CMC), and this is indicative of a relatively low level of impurity, ≤ 0.01%. The surface tension value of the plateau region above the CMC increases as the ethylene oxide chain length increases. It is 39 ± 0.5, 41 ± 0.5, and 43 ± 0.5 mN m−1 respectively for SLE1S, SLE2S, and SLE3S. This indicates that the surfactant becomes less surface active as the ethylene oxide chain length increases. However, the CMC values show an opposite trend. The CMC is 3.7, 2.6, and 2.2 ± 0.2 mM for SLE1S, SLE2S, and SLE3S respectively. These values are systematically lower than those previously reported in the literature.39−41 The values for SLE1S range from 4.2−4.6 mM.39−41 For SLE2S, a value of 2.8 mM was reported by Barry and Wilson.41 For SLE3S values of 2.8 and 3.1 mM were reported.39,40 Introducing the polyethylene oxide group between the alkyl chain and the ionic headgroup reduces the susceptibility to precipitation in the presence of multivalent electrolyte. This is associated with an increased solubility, which increases as the size of the polyethylene oxide group increases (ethylene oxide chain length increases) from 11658

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Figure 3. Surface tension data for SLE2S in AlCl3; see legend for details.

electrolyte. The impact of the trivalent electrolyte, AlCl3, on the surface tension of the SLES surfactants is qualitatively different. The effects observed are broadly independent of AlCl3 concentration, in the concentration range shown. However the addition of AlCl3 results in a substantial decrease in the surface tension values over almost the entire SLES concentration range. There is a substantial decrease in the CMC with the addition of AlCl3, and this is broadly independent of AlCl3 concentration. The slope of the surface tension with SLES concentration below the CMC is more gradual in the presence of AlCl3. Above the CMC, the surface tension is lower than in the absence of AlCl3, but increases toward the value in the absence of AlCl3 as the SLES concentration increases. Compared to the behavior in the presence of 0.1 M NaCl, the strong SLES/AlCl3 complexation results in the AlCl3 having a significant effect on the surface tension at much lower electrolyte concentrations. Significant changes in the surface tension are observed for concentrations < 0.3 mM, whereas much larger NaCl concentrations ∼ 100 mM, are required to achieve equivalent effects. Even when adjusting for ionic strength, which would make 0.3 mM AlCl3 1.2 mM, the difference is still large. The form of the surface tension curve for SLES/AlCl3 with increasing surfactant concentration is a direct consequence of the measurements being made at the fixed low electrolyte concentrations of the AlCl3 . As the SLES concentration increases it is increasingly in excess of the AlCl3 concentration, and this drives the surface tension toward a value similar to that in the absence of electrolyte at the higher SLES concentrations. The surface tension trends in the presence of AlCl3 are broadly similar for all three SLES surfactants. The effect of increasing the fixed AlCl3 concentration eventually reduces the surface tension increase as the SLES concentration increases above the CMC. From the surface tension behavior, it is difficult to infer anything significant about the adsorption behavior in the presence of AlCl3. However, the evolution in the adsorption and in the structure of the adsorbed layer have been directly determined by NR. NR measurements were made at the air− water interface predominantly for the alkyl chain deuterated

modifying the local dielectric constant, and hence the intraheadgroup interactions and the degree of counterion binding. Below the CMC, the slope of the adsorption curve with surfactant concentration increases as the polyethylene oxide group size decreases. This can be interpreted qualitatively as an increase in the cooperativity in the adsorption as the ethylene oxide chain length decreases. This is also an indication of the increased steric hindrance provided by the increasingly larger polyethylene oxide group on the packing at the interface. This is also evident in the mean area/molecule above the CMC, which increases from 40 to 52 Å2 as the polyethylene oxide group increases from EO1 to EO3. The neutron reflectivity measurements also provide a good indication of the purity of the surfactants and the absence of any significant isotope effect. Comparison of the neutron reflectivity data for 1 mM alkyl chain deuterated SLE1S, SLE2S and SLE3S and an equi-molar mixture of alkyl chain deuterated SLES (dC12hEnS)/fully protonated SLES (hC12hEnS), gives adsorption values which are within error identical. For SLE1S this gives 3.1 ± 0.2 × 10−10 and 3.4 ± 0.2 × 10−10 mol cm−2, respectively, 3.3 ± 0.2 × 10−10 mol cm−2 for both solutions for SLE2S, and 2.8 ± 0.2 × 10−10 and 3.1 ± 0.2 × 10−10 mol cm−2 for SLE3S. A full set of parameters are listed in Table 2 in the Supporting Information. (ii). SLES/AlCl3 Adsorption. Surface tension measurements were made for SLE1S, SLE2S, and SLE3S in the presence of fixed amounts of AlCl3. The surface tension data for SLE2S in the presence of 0.0−0.3 mM AlCl3 are shown in Figure 3. Similar surface tension data for SLE1S and SLE3S are shown in Figures 1 and 2 in the Supporting Information. The variation in the surface tension with surfactant concentration at different fixed AlCl3 concentrations is compared with that for the same pure SLES surfactants. In Figure 1 in the Supporting Information, the surface tension for SLE1S in the presence of 0.1 M NaCl is also plotted. The surface tension in 0.1 M NaCl shows the expected trend. That is, the CMC has decreased significantly and the surface tension above the CMC is at a much lower value compared to that observed in the absence of 11659

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Figure 4. Neutron reflectivity as a function of wave vector transfer Q (Å−1) for (a) 1 mM SLE1S, 0.0 mM (red), 0.02 mM (blue), 0.05 mM (dark red), 0.1 mM (dark green), and 0.2 mM AlCl3 (dark cyan); (b) 2 mM SLE2S, 0.0 mM (red), 0.4 mM (blue), 0.5 mM (dark red), and 0.6 mM AlCl3 (dark green); (c) 0.5 mM SLE3S, 0.0 mM (red), 0.05 mM (blue), 0.15 mM (dark red), 0.5 mM AlCl3 (dark green), and 0.8 mM AlCl3 (dark cyan); (d) 4 mM SLE3S, 0.0 mM (red), 1.5 mM (blue), 1.6 mM (dark red), and 1.8 mM AlCl3 (dark green). 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 Table 1 and in Tables 3−5 in the Supporting Information.

Table 1. Key Model Parameters for 1.0 mM SLE1S with AlCl3 AlCl3 conc.

physical appearance

d1 (±1 Å)

ρ1 (±0.2 × 10‑6 Å‑2)

A (±2 Å2)

Γ (±0.2 × 10‑10 mol cm‑2)

0.00 mM 0.02 mM AlCl3 conc.

transparent transparent physical appearance

18 22 d1 (±1 Å)

2.9 3.6 ρ1 (±0.2 × 10‑6 Å‑2)

54 35

3.1 4.7

d2

ρ2

d3

ρ3

0.05 mM AlCl3 conc.

transparent physical appearance

24 d1 (±1 Å)

3.4 ρ1 (±0.2 × 10‑6 Å‑2)

9 d2

1.5 ρ2

25 N

4.4 ΔQ

0.10 mM 0.20 mM

transparent turbid

19 21

5.1 3.2

19 21

2.7 1.5

3 10

0.16 0.20

decreases as the SLES ethylene oxide chain length increases (as illustrated in Figure 2). The addition of AlCl3 results in an increase in the adsorbed amount; see Table 1, and Tables 3−5 in the Supporting Information. The adsorbed amount has increased to ∼4.4 × 10−10 mol cm−2 for SLE1S, ∼4.2 × 10−10 mol cm−2 for SLE2S, and ∼4.1 × 10−10 mol cm−2 for SLE3S. With increasing AlCl3 concentration, the form of the reflectivity changes, and in Figure 4a−c a single interference fringe is visible (for 1 mM SLE1S/0.05 mM AlCl3, 2 mM SLE2S/0.15 mM AlCl3, and 0.5 mM SLE3S/0.15, 0.5, and 0.8 mM AlCl3). These data are modeled as a monolayer + bilayer structure (see Table 1, and Tables 3 and 4 in the Supporting

SLE1S, SLE2S, and SLE3S in NRW in the presence of AlCl3. For each SLES surfactant, the measurements were made at surfactant concentrations of 0.5, 1.0, 2.0, and 4.0 mM and in a range of AlCl3 concentrations up to 2 mM. The NR data in Figure 4 shows the range of data observed for the different SLES surfactants at different surfactant concentrations and a range of AlCl3 concentrations. These data summarize the range of behavior encountered. The NR data in Figure 1 in the absence of AlCl3 and at low AlCl3 concentrations are consistent with a thin monolayer of surfactant at the interface, with a thickness ∼ 20 Å. In the absence of electrolyte and in that SLES concentration range, the adsorbed amount varies with SLES concentration and 11660

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Information) and represent the formation of a monolayer and a single bilayer at the interface. At higher AlCl3 concentrations (see Figure 4a, b, and d), the reflectivity is dominated by a single pronounced Bragg peak at a Q value of ∼0.12 Å−1. This is consistent with the formation of a multiple bilayer, multilayer, structure at the interface, with a bilayer thickness of ∼48 Å. With just a single Bragg peak visible and generally no total film thickness fringes visible at Q values below the Bragg peak, the reflectivity is most sensitive to dt, Δρ, and N. The width of the Bragg peak is proportional to 1/N and is convovled with wth instrumental resolution, ΔQ. The visibility of the Bragg peak is related to N and Δρ. Hence, for example, the Bragg peaks in Figure 4b and d arise from a greater number of bilayers, N, than the Bragg peak in Figure 4a (see table 1, and Tables 3 and 5 in the Supporting Information). Comparing the data in Figure 4a with that in Figure 4b and d, N has increased from 10 to ∼40. In Table 1 and Tables 3 and 5 in the Supporting Information, ΔQ is generally larger than the known instrumental resolution (∼0.05). This is because at partial coverage the multilayers are in a form analogous to lamellar crystallites at the surface, and so ΔQ is then broadened by the mosaic spread of the lamellar patches on the surface. In this case, ΔQ is then inversely related to Δρ (which in part reflects the coverage), and this is consistent with the model parameters in Table 1 and Tables 3 and 5 in the Supporting Information. The NR data in Figure 4a at an AlCl3 concentration of 0.1 mM has another and different functional form, which is intermediate between the single interference fringe at 0.05 mM AlCl3 and the Bragg peak at 0.2 mM AlCl3. The NR data is in the form of two interference fringes in the Q range measured, and it is associated with a thicker surface layer than is present with the single interference fringe observed at an AlCl3 concentration of 0.05 mM. A detailed quantitative analysis shows that it is consistent with the adsorption of 3 bilayers at the interface (see Table 1). The interference feature at Q ∼ 0.12 Å−1 is hence a broad Bragg peak associated with a multilayer structure in which N is small. The interference fringe at the lower Q value is then an interference fringe arising from the total film thickness. Throughout, the simplest model consistent with the data has been adopted, and this provides a good description of all the data. Some slight deviations from the model are evident in some limited examples (see, for example, in Figure 4a). This most likely arises from slight variations over the lateral surface area. That is, the surface is not completely homogeneous laterally. (iii). Surface Phase Diagrams. From the NR measurements for SLES concentrations from 0.5 to 4 mM and AlCl3 concentrations up to 2 mM, the structure of the adsorbed layer for SLE1S, SLE2S, and SLE3S varies from a simple monolayer, to a single bilayer, and eventually to multilayers where N is ∼3 and N > 10. From this data, it is possible to construct approximate surface phase diagrams, and these are shown in Figure 5. With the addition of AlCl3, the strong complexation between SLES and Al3+ drives the transition from monolayer adsorption to the formation of surface multilayer structures. At low AlCl3 concentrations, the surface layer is a monolayer. With increasing AlCl3 concentration, there is a transition from monolayer to multilayer adsorption. At the higher AlCl3 concentrations, the multilayers have N > 10 bilayers. At AlCl3 concentrations intermediate between the monolayer and

Figure 5. Approximate surface phase diagrams for SLES/AlCl3 (a) SLE1S, (b) SLE2S, and (c) SLE3S. The symbols (+) indicate points at which the NR measurements have been made. The lines represent only approximate boundaries and act as a guide to the eye. Further details are shown in the figures.

multilayer formation there are two distinct regions where the surface adsorbed layer exists as a single bilayer or as a multilayer with 3 bilayers. As the size of the polyethylene oxide group increases from EO1 to EO3, the region of monolayer only 11661

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Figure 6. Neutron reflectivity as a function of wave vector transfer, Q (Å−1), for 0.5 mM SLES in the presence of AlCl3: 0.5 mM dC12hE1S with 0.15 mM AlCl3 (red), 0.5 mM dC12hE2S with 0.075 mM AlCl3 (blue), and 0.5 mM dC12hE3S with 0.15 mM AlCl3 (dark green). 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 Table 2.

Table 2. Key Model Parameters for 0.5 mM SLES with AlCl3 SLES

physical appearance

d1 (±1 Å)

ρ1 (±0.2 × 10‑6 Å‑2)

d2

ρ2

d3

ρ3

dC12hE1S dC12hE2S dC12hE3S

transparent transparent transparent

22 24 25

4.7 4.1 3.7

7 10 12

0.6 0.9 0.3

25 26 31

4.3 3.3 2.7

The initial layer, d1, is an alkyl chain layer adjacent to the airphase. d2 is a layer containing surfactant headgroups and solvent. d3 represents an interdigitated alkyl chain layer which with the headgroup layer forms the bilayer structure. It also contains the final headgroup layer. The structure is shown in the schematic diagram in Figure 7. Although referred to as a single bilayer, it comprises in reality as an initial monolayer with a single bilayer underneath, adjacent to the solution phase. From the parameters in Table 2, d1, d2, and d3 all increase as the ethylene oxide chain length increases. However, the most significant change is in d2, the headgroup region. Assuming that the ethylene oxide chain is fully extended and is ∼3.5 Å/ ethylene oxide, and allowing ∼2 Å for the sulfate group,45 the extent of the headgroup can be estimated as 5.0, 9.0, and 12.5 Å

formation increases. That is, an increasingly higher AlCl3 concentration is required to induce surface multilayer formation. The other notable feature is that at the lower surfactant concentrations,