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 Diethylene Glycol Monoalkyl Ether Sulfate/AlCl3 Solutions: The Role of the Alkyl Chain Length Hui Xu,† Jeffrey 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 3QZ, 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 3JW, United Kingdom ‡

S Supporting Information *

ABSTRACT: The influence of the alkyl chain length on surface multilayer formation at the air−water interface for the anionic surfactant sodium diethylene glycol monoalkyl ether sulfate, SAE2S, in the presence of Al3+ multivalent counterions, in the form of AlCl3, is described. In the absence of electrolyte, the saturated monolayer adsorption is determined by the headgroup geometry and is independent of the alkyl chain length. In the presence of Al3+ counterions, surface multilayer formation occurs, due to the strong SAE2S/Al3+ binding and complexation. The neutron reflection data show that the alkyl chain length of the surfactant has a significant impact upon the evolution of the surface multilayer structure with surfactant and AlCl3 concentration. Increasing the alkyl chain length from decyl to tetradecyl results in the surface multilayer formation occurring at lower surfactant and AlCl3 concentrations. At the short alkyl chain lengths, decyl and dodecyl, the regions of multilayer formation with a small number of bilayers are increasingly extended with decreasing alkyl chain length. For the alkyl chain lengths of tetradecyl and hexadecyl, the surface behavior is further affected by decreases in the surfactant solubility in the presence of AlCl3, and this ultimately dominates the surface behavior at the longer alkyl chain lengths.

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INTRODUCTION The effect of different counterions and counterion concentrations on the self-assembly and adsorption behavior of ionic surfactants has been extensively studied.1−3 This is particularly relevant for anionic surfactants, which are extensively used in a wide range of detergency based home and personal care products.4,5 The impact of monovalent electrolytes, such as NaCl, on self-assembly and adsorption is well established. The addition of electrolyte generally reduces the critical micelle concentration, cmc,6 promotes micellar growth7 and induces enhanced adsorption.8 The impact of multivalent counterions, such as Al3+ and Ca2+, is more profound, and can rapidly lead to precipitation.9 It is of great importance in the context of water hardness,10 and in environmental applications associated with water treatment and soil remediation.11 The alkyl sulfates, such as sodium dodecyl sulfate, SDS, have the simplest structure of the commonly used anionic surfactants.4,5 It is well established that the binding of the divalent or trivalent counterions, such as Ca2+ and Al3+, to the alkyl sulfate ions will lead rapidly to precipitation.12,13 It is also well-known that the onset of precipitation will increase as the alkyl sulfates become instrinsically less soluble, as the alkyl chain length increases.12 To improve the salt tolerance and other properties of the anionic surfactants, different structures have been developed, © 2013 American Chemical Society

notably the alkyl benzenesulfonates, LAS, and the polyethylene glycol monoalkyl ether sulfates, SAES.4,5 Although the effect of multivalent counterions on the selfassembly of such anionic surfactants is relatively well established,14−17 their impact on the adsorption at interfaces is relatively unexplored. Notably, Alargova et al.18 discussed the variation in surface tension for SAES/AlCl3 mixtures, but were not able to correlate the changes in surface tension with changes in the adsorption or the structure of the adsorbed layer. More recently, Penfold et al.,19 Petkov et al.,20 and Xu et al.21 have used neutron reflectivity to investigate the adsorption of different anionic/nonionic surfactant mixtures and anionic surfactants in the presence of multivalent counterions. Penfold et al.19 showed how the strong complexation of Ca2+ with the anionic surfactant LAS promoted the transition from monolayer adsorption to the formation of multilayer structures at the air−water interface. It was demonstrated that a nonionic cosurfactant with a relatively large headgroup, such as octaethylene glycol monododecyl ether, C12E8, provided a steric hindrance that progressively inhibited the multilayer Received: August 19, 2013 Revised: September 19, 2013 Published: September 20, 2013 12744

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formation at the interface. Petkov et al.20 described a similar pattern of surface behavior for the anionic surfactant sodium diethylene glycol monododecyl ether sulfate, SLE2S, in the presence of AlCl3, and where the nonionic surfactant dodecaethylene glycol monododecyl ether, C12E12, was used to disrupt the complex formation. Furthermore, we have recently reported how changing the size of the polyethylene oxide linker group in the SAES surfactant, SLES, changes the evolution in the structure of the adsorbed layer with surfactant and Al3+ concentrations.21 It was shown that the polyethylene oxide linker group provided an internal steric contribution, broadly similar qualitatively to that achieved using a nonionic cosurfactant. Surface multilayer formation or surface self-assembly from more concentrated surfactant solutions,22−24 in lung surfactants,25,26 layer-by-layer polyelectrolyte deposition27,28 and in polyelectrolyte/surfactant mixtures29 have been extensively reported. In contrast, reports on surface self-assembly and surface multilayer formation from dilute surfactant solutions, in mixtures,30,31 induced by multivalent counterions19−21 or oligoions32,33 are relatively sparse and recent. However their marked and persistant wetting properties, their enhanced adsorption and efficient delivery and retention of different benefit agents (such as perfumes) are potentially attractive for many applications. Hence this study is part of a systematic program to establish the major factors controlling the surface self-assmebly. In particular in this paper we report the effect of changing the alkyl chain length of the SAE2S anionic surfactant for a fixed ethylene oxide chain length of EO2 on the surface adsorption at the air−water interface, in the presence of AlCl3. Neutron reflectivity and surface tension measurements are presented for sodium diethylene glycol monodecyl ether sulfate, SDE2S, sodium diethylene glycol monotetradecyl ether sulfate, STE2S, and sodium diethylene glycol monohexadecyl ether sulfate, SHE2S, in the presence of AlCl3. These results are compared with previously reported measurements for SLES,21 where the impact of variations in the ethylene oxide chain length was investigated.

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The surface tension measurements were made on a Kruss K10 tensiometer, using a Pr/Ir de Nouy ring at 25 °C. Repeated measurements were made until equilibrium was reached, which was taken to be when the surface tension variation was within 0.2 mN m−1. This took typically three separate measurements and a total time scale of ∼10−20 min. The measurements were made by repeated dilution from the most concentrated solution, and calibrated for H2O with a surface tension value of 72 mN m−1. The de Nouy ring was rinsed in water and dried in a flame between sequential measurements. The neutron reflectivity measurements were made at the air−water interface on the SURF and INTER reflectometers at the ISIS neutron source.35 On SURF the angle of incidence, θ, was 1.5° and neutron wavelengths, λ, from 0.5 to 7 Å were used to cover a wave vector transfer, Q, range in the direction normal to the surface of 0.05−0.5 Å−1 (where Q = 4π sin θ/λ). On INTER, a Q range of 0.03−0.5 Å−1 was covered 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, and each sample (at each surfactant and AlCl3 concentration) was measured separately. Each neutron reflectivity profile took ∼20−30 min and sequential measurements were made on a 5−7 position sample changer. Each sequence of measurements was repeated 2−3 times until the reflectivity showed no change with time, and this was typically ∼2− 3 h. Hence, the data presented represent equilibrium structures. In the kinematic approximation,8 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, and ρ(z) is related to the neutron refractive index, n, and n(z) = 1 − λ2ρ(z)/ 2π). Hence, 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 surfactants, mixed surfactants, polymers, and polymer−surfactant mixtures at the air−water and other interfaces.8,29 The ST measurements were made using the hydrogeneous form of the surfactant in H2O, whereas the NR measurements were made using the alkyl chain deuterated form of the surfactants in nrw. It has been extensively established that H/D isotopic substitution has minimal effect of the adsorption properties of most surfactants.8 It has been specifically determined for SLES21 where a range of different isotopic combinations gave a consistent surface structure. It has recently been shown36,37 that NR provides a reliable evaluation of surface adsorption and good correspondence with ST where ST data can be reliably evaluated. The measurements reported here and related measurements20,21,34 show that the surface structures reported have a good degree of reproducibility. As described earlier in this section the time scales to reach a steady state condition are different for the ST and NR measurements. This is frequently observed in such systems and arises because changes in the detailed surface structure (as determined by NR) do not necessarily result in measurable changes in the ST.

EXPERIMENTAL DETAILS

The alkyl chain deuterated and hydrogenous SAE2S surfactants were custom synthesized in Oxford by sulfonation of the corresponding and previously synthesized diethylene glycol monoalkyl ethers, as described in detail elsewhere.34 Four different surfactants, with the general formula CH3(CH2)n−1(OCH2CH2)2SO4Na with decyl, dodecyl, tetradecyl, and hexadecyl (n = 10, 12, 14, and 16) alkyl chain lengths and abbreviated as SDE2S, SLE2S, STE2S, and SHE2S, were synthesized, with the alkyl chains deuterated or hydrogeneous. The surfactants were recrystallized twice from propanol/ethanol mixtures. The composition and purity of the surfactants were evaluated using MS, GC/MS, NMR and surface tension. Some typical MS data, for SDE2S and SLE2S, are shown in Figure 1 in the Supporting Information, and are reported more extensively elsewhere.34 Analytical grade (>99.9% purity) AlCl3 from Sigma Aldrich was used as supplied. The D2O was obtained from Sigma-Aldrich, and high purity (Elga Ultrapure) was used throughout. All the neutron reflectivity measurements were made using alkyl chain deuterated surfactants in null reflecting water, nrw (92/8 mol % ratio mixture of H2O and D2O). The surface tension measurements were made using hydrogenous surfactants in H2O. All glassware and Teflon troughs (used for the neutron reflectivity measurements) were cleaned in alkali detergent (Decon 90) and rinsed in ultrapure water. No adjustment to the solution pH was made on the addition of AlCl3. The pH value decreased from ∼6.8 to ∼4.5, depending upon the AlCl 3 concentration.

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RESULTS AND DISCUSSION i. SAE2S Surfactants in the Absence of AlCl3. The different alkyl chain length SAE2S surfactants were characterized in the absence of AlCl3 by surface tension and neutron reflectivity. The variation in surface tension for SDE2S, SLE2S, STE2S, and SHE2S in water are shown in Figure 1. The surface tension data in Figure 1 shows that the cmc depends strongly on the alkyl chain length, and decreases significantly as the alkyl chain length increases from 10 to 16. The surface tension above the cmc is similar for SDE2S, SLE2S, and STE2S, and is approximately constant above the cmc with a 12745

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Figure 1. Surface tension variation for SDE2S (red), SLE2S (blue), STE2S (brown), and SHE2S (green).

Figure 2. Dependence of the cmc on the alkyl chain length for SAE2S (red) and SAS (blue). The solid lines are least-squares fits to a straight line dependence.

value ∼43 ± 0.2 mN m−1. The surface tension for SHE2S is different, and has a more pronounced slope in the region above the cmc. There is a slight minimum in the surface tension at the cmc, and this is indicative of a relatively low level of impurity in the surfactants. The slope in the surface tension with concentration below the cmc is broadly similar for all four surfactants, and implies qualitatively that the adsorption is similar. However, the adsorption has been measured and evaluated more directly by NR, and is discussed in detail later in this section. The cmc values were determined from the intersection of two straight lines through the approximately linear regions of the ST below and above the cmc. The variation in the cmc with alkyl chain length is summarized in Table 1, and compared with other reported values where applicable. The values of Barry and Wilson38 for SDE2S, SLE2S, and SHE2S are in good agreement with the data presented here. The value from Barry and Wilson38 for STE2S is significantly higher than measured here. The values of Weil et el.39 and Hato et al.40 for SHE2S are low compared to the data from this study. The variation in log cmc with alkyl chain length is shown in Figure 2. The linear dependence implies that the cmc data of Barry and Wilson38 for STE2S and of Weil et al.39 and Hato et al.40 for SHE2S are the values least consistent with this trend. It has been shown41 that the cmc of hydrocarbon based surfactants should decrease exponentially with the increase in the number of methylene groups, such that log cmc = α − βNc. This is shown in Figure 2 for the SAE2S surfactants measured here and compared with some equivalent data for the alkyl sulfates, SAS.42 Some similar cmc data for the alkyl sulfates were more recently reported by Varga et al.44 but are not included in Figure 2. The free energy of micellization, ΔG0, is related to the logarithm of the cmc43 such that,

ΔG0 /RT = ln cmc = (α − βNc)

(1)

where the intercept α and the slope β in Figure 2 relate to the headgroup contribution to free energy of micellization, and the free energy/methylene of the hydrocarbon chain contribution. From Figure 2, β is 0.69 and 0.71 kcal/mol/methylene group for SAE2S and SAS, respectively. This compares with a value ∼0.77 from Tanford43 for the free energy/methylene of transfer of an alkyl chain from solution to the micelle interior. The adsorption of the SAE2S surfactants at the air−water interface was measured directly using neutron reflectivity. As described elsewhere,8 neutron reflectivity provides a direct measure of surfactant adsorption at the air−water interface. For deuterium labeled surfactant in nrw, the reflectivity arises only from the adsorbed layer at the interface. Assuming that the adsorbed layer is a monolayer of uniform composition, then the reflectivity can be modeled using the optical matrix method45 to determine a thickness d and a scattering length density ρ. The adsorbed amount, Γ, or area/molecule, A, is then determined by Γ=

1 1 = NaA Na(Σb/dρ)

(2)

where Na is Avogadro’s number, and the Σb values for the different alkyl chain deuterated SAE2S surfactants are, 2.45 × 10−3, 2.85 × 10−3, 3.25 × 10−3, and 3.65 × 10−3 M, respectively. The typical error in the determination of A is ±2 Å2 at a value of 50 Å2.8 The adsorption isotherms for SDE2S, SLE2S, STE2S, and SHE2S are shown in Figure 3. The key model parameters associated with the data in Figure 3 are summarized in Table 1 in the Supporting Information. The adsorbed layer thickness is ∼20 ± 2 Å, and the adsorption above the cmc is independent of alkyl chain length and is ∼3.7 ± 0.2 × 10−10 mol cm−2 (A ∼ 45

Table 1. cmc Values for SAE2S Surfactants, from Figure 1 surfactant SDE2S SLE2S STE2S SHE2S

cmc (±0.5) M 1.1 2.6 5.5 1.8

× × × ×

Weil et al.39

−2

10 10−3 10−4 10−4

1.3 × 10−4 12746

Barry and Wilson38 1.2 2.8 8.6 2.2

× × × ×

Hato et al.40

−2

10 10−3 10−4 10−4

1.1 × 10−4

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Figure 3. Total surfactant adsorbed amount (Γ × 10−10 mol cm−2) versus surfactant SAE2S concentration (C × mol L−1) for SDE2S (red), SLE2S (blue), STE2S (dark green), and SHE2S (dark pink).

Figure 4. Surface tension data for SDE2 S in different AlCl 3 concentration: 0.00 mM (red), 0.05 mM (blue), 0.10 mM (dark red), 0.20 mM (dark green), 0.40 mM (dark blue), 0.80 mM (dark pink), and 1.60 mM AlCl3 (dark cyan).

± 2 Å2). The adsorption at concentrations above the cmc is also independent of surfactant concentration. The constant ST above the cmc (see Figure 1) for SDE2S, SLE2S, and STE2S is normally associated with nonionic surfactants.36,37 For ionic surfactants, a gradual decrease in the ST is expected (as observed for SHE2S, and SDS37) as the degree of ionization of the micelles and hence the activity changes with increasing concentration. The NR data also show that the adsorption is constant above the cmc for the SAES surfactants (see Figure 3), and this has also been previously reported for SLES.21,34,37 It has also been shown that there is a strong counterion binding for SLES and that SLES micelles have a low degree of ionization compared to other ionic micelles.21,34,37 All these observations imply that the SAES surfactants are only weakly ionic. This has been previously21 attributed to the presence of the polyethylene oxide group modifying the local dielectric constant, and hence the strength of the electrostatic interaction in the headgroup region, resulting in the strong counterion binding. There is an invariance in variation in the adsorption above the cmc with alkyl chain length for the SAES surfactants, and the mean area/molecule is 45 ± 2 Å2. Similar trends have been reported for other surfactants. For example, Varga et al.44 recently also reported an invariance in the saturation adsorption for the closely related alkyl sulfates, SAS, surfactants over a similar alkyl chain length variation. However, this invariance is not always observed, and for the cationic surfactant, alkyl trimethyl ammonium bromide, the area/molecule decreases from 58 to 44 Å2 as the alkyl chain length increases from 10 to 14 and for alkyl chain lengths >14 is constant at ∼44 Å2. Hence, for the SAES and SAS surfactants, the limiting adsorption is dominated by the alkyl chain packing, and the headgroup plays a more secondary role. For the alkyl trimethyl ammonium surfactants this is only the case once the alkyl chain length reached a critical length. ii. SAE2S Adsorption in the Presence of AlCl3. Surface tension and neutron reflectivity measurements were made at fixed SAE2S concentrations for SDE2S, STE2S, and SHE2S over a range of AlCl3 concentrations, in order to determine the impact of the Al3+ ions on the adsorption. The surface tension data for SDE2S in AlCl3 are shown in Figure 4. The addition of AlCl3 results in marked decrease in

the surface tension at low surfactant concentrations, and this is similar to that was observed for SLES.21 There is a pronounced decrease in the cmc, taken as the relatively broad transition in the slope at the low surfactant concentrations. These changes are due to the strong adsorption associated with the complex formation between the surfactant and Al3+ counterions at the interface. Notably the surface tension decreases from ∼43 mN m−1 to a minimum value ∼32 mN m−1. As the surfactant concentration increases the surface tension increases from that minimum value, and approaches the surface tension value in the absence of AlCl3. This trend was discussed for SLES,21 and the variation observed can be considered as a consequence of the measurements being made a low fixed value of the AlCl3 concentration. That is, at the higher surfactant concentrations, the surfactant is in considerable excess of the AlCl3. Hence, the ST will eventually be dominated by the free surfactant and must approach the value for the free surfactant, in the absence of AlCl3. This has broad similarities with that encountered in polymer−surfactant mixtures.29 In that case, the strong polymer−surfactant complexation results in a pronounced reduction in the ST at low surfactant concentrations. When the surfactant is in significant excess of the polymer, the ST tends toward the value encountered for the free surfactant. The surface tension data for STE2S/AlCl3 are shown in Figure 5. At the lowest AlCl3 concentration (Figure 5a), the surface tension data are qualitatively similar to that shown in Figure 4 for SDE2S, and to that previously reported for SLES.21 However, at the higher AlCl3 concentrations (see Figure 5b), the surface tension behavior is different and remains at a relatively low value at the lower surfactant concentrations. The surface tension observed for SHE2S, as shown in Figure 2 in the Supporting Information, is again different at the higher AlCl3 concentrations, and the values are now much closer to those for SHE2S in the absence of electrolyte. Hence, although the surface tension measurements provide some qualitative insights into some aspects of the overall trends, it is difficult to interpret some of the ST data, and to evaluate that data in terms of the surface adsorption properties. For STE2S and SHE2S, the physical appearance of the solutions changes dramatically for AlCl3 concentrations > 0.6 mM, and precipitation is observed (see Tables 2 and 3 in the Supporting Information). For SDE2S 12747

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Figure 5. Surface tension data for STE 2 S in different AlCl 3 concentrations, (a) 0.0 mM (red), 0.1 mM (dark green), and 0.2 mM AlCl3 (blue); (b) 0.0 mM (red), 0.4 mM (dark pink), 0.6 mM (dark yellow), and 0.8 mM AlCl3 (dark cyan).

and SLE2S at the AlCl3 concentrations used and for STE2S and SHE2S at lower AlCl3 concentrations, the solutions are transparent or at differing degrees of turbidity (see the tables of model parameters from the NR data analysis in the paper and in the Supporting Information). Hence, the changes in the ST behavior for STE2S and SHE2S at the higher AlCl3 concentrations coincide with the onset of precipitation, but are not simply explained by depletion associated with the precipitation. More likely there will be a complex interplay between precipitation and wetting or partial wetting phenomena associated with the dense phase that is formed.33 We have used neutron reflectivity to directly determine the adsorption properties in terms of adsorbed amounts and the structure of the surface region. Figure 6 shows the variation in the neutron reflectivity for SDE 2S at fixed surfactant concentrations of 2, 5, and 10 mM, and for a range of AlCl3 concentrations. The neutron reflection data at the three different surfactant concentrations show different patterns of evolution of the surface structure with increasing AlCl3 concentration. For a surfactant concentration of 2 mM (Figure 6a) in the absence of AlCl3 and at low AlCl3 concentrations, the adsorption is in the form of a monolayer, in which the adsorption increases with increasing AlCl3 concentration (see Table 3). At the higher

Figure 6. Neutron reflectivity as a function of wave vector transfer, Q (Å−1), for SDE2S in different AlCl3 concentrations (a) 2.0 mM SDE2S: 0.00 mM (red), 0.75 mM (blue), 1.00 mM (dark red), and 4.00 mM AlCl3 (dark green); (b) 5.0 mM SDE2S: 0.00 mM (red), 0.05 mM (blue), 0.15 mM (dark red), 0.75 mM (dark green), 9.0 mM (dark pink), and 10.00 mM AlCl3 (dark cyan); (c) 10.0 mM SDE2S: 0.00 mM (red), 2.00 mM (blue), 2.50 mM (dark red), 3.25 mM (dark green), and 4.00 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 2−4.

AlCl3 concentrations measured, the reflectivity is characterized by a single broad interference fringe. This is modeled, using the optical matrix method used to characterize the surface adsorption, by a monolayer + bilayer (see Table 2). This 12748

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Table 2. Key Model Parameters for 2.0 mM SDE2S with AlCl3 AlCl3 conc.

physical appearance

d (±1 Å)

ρ (±0.1 × 10‑6 Å‑2)

A (±2 Å2)

Γ (±0.2 × 10‑10 mol cm‑2)

0.00 mM 0.75 mM AlCl3 conc.

transparent slightly turbid physical appearance

24 22 d1 (±1 Å)

1.5 2.9 ρ1 (±0.1 × 10‑6 Å‑2)

68 38

2.4 4.4

d2

ρ2

d3

ρ3

1.00 mM 4.00 mM

slightly turbid turbid

16 18

4.6 4.3

7 7

0.2 0.2

32 30

3.1 3.3

Table 3. Key Model Parameters for 5.0 mM SDE2S with AlCl3 AlCl3 conc.

physical appearance

d (±1 Å)

ρ (±0.1 × 10‑6 Å‑2)

A (±2 Å2)

Γ (±0.2 × 10‑10 mol cm‑2)

0.00 mM 0.05 mM AlCl3 conc.

transparent transparent physical appearance

21 22 d1 (±1 Å)

2.1 2.9 ρ1 (±0.1 × 10‑6 Å‑2)

55 38

3.0 4.4

d2

ρ2

d3

ρ3

0.15 mM AlCl3 conc.

transparent physical appearance

29 d1 (±1 Å)

3.1 ρ1 (±0.1 × 10‑6 Å‑2)

9 d2

0.9 ρ2

19 N

4.0 ΔQ

0.75 mM 9.0 mM 10.00 mM

slightly turbid turbid turbid

16 16 21

3.8 3.9 2.6

22 22 20

1.0 0.6 1.2

3 3 60

0.05 0.05 0.1

Table 4. Key Model Parameters for 10.0 mM SDE2S with AlCl3 AlCl3 conc.

physical appearance

d (±1 Å)

ρ (±0.1 × 10‑6 Å‑2)

A (±2 Å2)

Γ (±0.2 × 10‑10 mol cm‑2)

47 42

3.5 4.0

0.00 mM 2.00 mM AlCl3 conc.

transparent transparent physical appearance

21 20 d1 (±1 Å)

2.5 2.9 ρ1 (±0.1 × 10‑6 Å‑2)

d2

ρ2

d3

ρ3

2.50 mM AlCl3 conc.

transparent physical appearance

19 d1 (±1 Å)

2.7 ρ1 (±0.1 × 10‑6 Å‑2)

14 d2

0.7 ρ2

29 N

1.8 ΔQ

3.25 mM 4.00 mM

slightly turbid slightly turbid

21 21

2.6 2.5

21 21

0.8 0.6

10 40

0.16 0.08

For the neutron reflection data where extensive multilayer formation occurs, the data are quantitatively analyzed using a surface multilayer model based on the kinematic approximation.46,47 The model has been extensively applied to a range of broadly similar systems,19−21 and is described in detail elsewhere.19,20 The key model parameters, as summarized in Tables 3−5, are the bilayer thickness, dt (where dt = d1 + d2, d1

structure is similar to that described in detail for SLES/AlCl3 mixtures,21 and corresponds to an initial monolayer adjacent to the air phase and a single bilayer beneath and adjacent to the solvent phase. The neutron reflectivity data in Figure 6b, for 5 mM SDE2S, shows a more complex evolution in the reflectivity and hence surface structure, with increasing AlCl3 concentration. At low AlCl3 concentrations the surface structure evolves from a monolayer to a single bilayer at the interface. The single bilayer (three layer structure) is characterized by a single broad interference fringe in the neutron reflectivity data. At higher AlCl3 concentrations (0.75 to 9 mM AlCl3) two interference fringes are visible. As discussed in the context of SLES,21 this corresponds to multilayer formation at the interface, with a finite small number of bilayers, N = 3. The interference fringe at the highest Q value is a broad Bragg peak associated with that structure. The interference fringe at the lower Q value arises from the total film thickness. For such multilayer structures the width of the Bragg peak, ΔQ, is proportional to 1/N. At the highest AlCl3 concentration, 10 mM, the neutron reflection data are dominated by a relatively narrow Bragg peak. This corresponds to a more extensive multilayer formation at the interface, and N ∼ 60. The key model parameters for the structures in Figure 6b are summarized in Table 3. The neutron reflectivity data at a surfactant concentration of 10 mM (see Figure 6c and Table 4) show a slightly different structural evolution. With increasing AlCl3 concentration, the surface structure evolves from a monolayer, to a single bilayer, and to multilayers, with N > 10, without a transition to the intermediate structure with a smaller finite number of bilayers.

Table 5. Variation in Bilayer Thickness with Alkyl Chain Length dt (±2 Å) N, no. of bilayers a

1 3

SDE2S

SLE2S

STE2S

SHE2S

58 38

63 43

67 48

48

a

Note that N = 1 really refers to a monolayer plus a single bilayer (see earlier discussion).

is the alkyl chain and d2 the solvated headgroup regions), the scattering length density of the two regions of the bilayer, ρ1, ρ2, and the number of bilayers, N. When N is large, often the only visible feature is the first order Bragg peak, and hence the modeling is only sensitive to N, dt, and Δρ (ρ1 − ρ2). The width of the Bragg peak is broadened by ΔQ, the instrumental resolution. The value of ΔQ consistent with the data is often greater than the expected contribution from the instrumental resolution. This is because the surface multilayer structure can be likened to lamellar crystallites on the surface, and so ΔQ has an additional contribution from the mosaic spread of the lamellar patched on the surface. 12749

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For the SAE2S surfactants with the longer tetradecyl and hexadecyl alkyl chains, the surface behavior is now somewhat different. For STE2S (tetradecyl alkyl chain), the NR data in Figure 3 in the Supporting Information and the key model parameters in Tables 2−4 in the Supporting Information show a different evolution in the surface structure with surfactant and AlCl3 concentration. At zero and low AlCl3 concentrations, monolayer adsorption is observed. With increasing AlCl3 concentration, there is a transition from monolayer adsorption to three narrow regions, in which there is a single bilayer, three bilayers, and a region where N is between 3 and 10. At even higher AlCl3 concentrations, the surface structure reverts back to a surface with just three bilayers. This is very different from what was observed for SDE2S and SLE2S. Furthermore, at the lower STE2S concentrations (0.2, 0.5 mM), the solutions show evidence of precipitation at the higher AlCl3 concentrations. This is not observed for the data at 1.5 mM, and is not observed for SDE2S and SLE2S at any of the concentrations studied. In these systems, the physical appearance of the solutions evolves from a transparent solution to one which is slightly turbid, and ultimately to one which is more turbid, with increasing AlCl3 concentration. Hence, the change in the evolution of the surface structure and the physical appearance of the solutions for STE2S is reflecting the reduced solubility and greater propensity for precipitation for STE2S. For AlCl3 concentrations > 0.6 mM, precipitation is observed in the bulk solutions. For the SAE2S surfactant with the hexadecyl alkyl chain, SHE2S, the effects of the reduced solubility are even more pronounced. The NR measurements were made in the SHE2S concentration range from 0.2 to 0.8 mM, and for a range of AlCl 3 concentrations. At the lowest SHE2 S concentration (0.2 mM), the surface is initially a monolayer, and then a narrow region where multilayers form at the interface exists (see Figure 4 and Table 5 in the Supporting Information). At higher AlCl3 concentrations, the surface reverts to monolayer adsorption. At the higher SHE2S concentrations, only monolayer adsorption is observed, independent of the AlCl3 concentration. Furthermore, at the higher AlCl3 concentrations precipitation in the bulk solutions is increasingly evident. From the NR data, approximate surface phase diagrams can be constructed, and a comparison of the surface phase diagrams for SDE2S and SLE2S is shown in Figure 7. The corresponding surface phase diagram for STE2S is shown in Figure 5 in the Supporting Information. The strong binding of the Al3+ counterions to neighboring SAES headgroup, reducing local curvature, and charge bridging between layers, as discussed elsewhere14,15,18,20,21 and illustrated in the structural schemes presented in ref 21, drive the transition from monolayer adsorption to the spontaneous formation of extended multilayer structures at the interface. In the modeling of the surface structures, the simplest structure consistent with the data is adopted. Although the NR data is less sensitive to the lateral in-plane distribution at the interface, the layered or lamellar structures derived are supported by more extensive data. NR measurements using a range of different surfactant and solvent contrasts21 and the systematic variation in the model parameters with ethylene oxide chain length21 both provide support for the structures reported here. In this study, the systematic variation in the bilayer spacing with alkyl chain length provide further support. The effects of the variation in the alkyl chain length on the detailed structural

Figure 7. Surface phase diagram for (a) SDE2S and (b) SLE2S with AlCl3 (Reproduced with permission from ref 21. Copyright 2013 American Chemical Society). The symbols indicate points where the neutron reflectivity measurements have been made.

parameters for the different multilayer structures are summarized in Table 5. From Table 5, it is clear that from decyl to hexadecyl alkyl chain length the mean bilayer thickness in all three different structural regions scales with the increasing alkyl chain length. The increase corresponds to ∼(1.265 × 4) Å, where 1.265 Å is the increment per methylene group in a fully extended alkyl chain,43 and the factor x4 accounts for a factor x2 for the increment in the number of methylene groups and the further factor x2 is for the bilayer nature of the structure. For SDE2S the addition of AlCl3 results in surface structures and an evolution in the surface structure qualitatively similar to that previously reported for SLES.21 That is, the surface structure is initially a monolayer at low AlCl3 concentrations and evolves with increasing AlCl3 concentrations into a single bilayer, a three bilayer structure, and ultimately a multilayer structure with a large number of bilayers. However, in detail, the progression of surface phases for SDE2S is different. At low surfactant concentrations, the regions of single and three bilayer structures is more extensive. A similar trend was previously reported for SLES with increasing ethylene oxide chain length.21 In that latter case, the region of monolayer adsorption was more extensive, and this is not the case here. It has been 12750

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different SLE2S and AlCl3 concentrations and the measured pH values has been made.34 No strong correlation was observed, and it is concluded that hydrolysis is not a major contributing factor to the surface ordering observed It is evident from the observations reported here on the solution properties, the measured variation in the cmc with alkyl chain length, and the general trends observed in other surfactants, that the solubility of the SAES surfactants decreases with increasing alkyl chain length. A related, but different manifestation of that changing solubility, is the Krafft temperature; and this is known to increase with alkyl chain length.12 It also increases with the addition of electrolyte, and especially multivalent ions; and this is particularly associated with the problem of hard water tolerance. The effects of alkyl chain length and ethylene oxide chain length, and the impact of multivalent ions on the Krafft temperature of the SAES surfactants have been reported.51,52 The Krafft temperatures for the shorter alkyl chain length SAES surfactants are well below room temperature.51,52 However, for the tetradecyl and hexadecyl chain length and with the addition of AlCl3, the Krafft temperature can be comparable to room temperature. Hence, the effects observed for STE2S and SHE2S are directly associated with the decrease in solubility and the proximity of the Krafft temperature.

shown that changing the alkyl chain geometry will affect the packing at the interface, in the presence of electrolyte. For example, the addition of CaCl2 to LAS-6/LAS-4 mixtures promoted surface multilayer formation.48 However, the tendency toward multilayer formation was greater for LAS-4 rich mixtures. This was attributed to the more asymmetrical LAS-4 alkyl chain promoting greater alkyl chain packing when LAS/Ca2+ complexation occurs compared to the essentially dialkyl chain LAS-6.48 The importance of the alkyl chain hydrophobic interaction is also clearly seen in the comparison of the surface behavior between SDE2S and SLE2S. The reduced hydrophobic contribution of the decyl alkyl chain of SDE2S clearly results in a reduced tendency to form the more extended multilayer structures at the interface. For the longer alkyl chain lengths, tetradecyl and hexadecyl, the pattern of adsorption is different (see Figure 4 in the Supporting Information). For STE2S, the evolution is initially qualitatively similar to SDE2S and SLE2S, but there is only a narrow region of multilayer formation with increasing AlCl3 concentration following the formation of a single three bilayer structures, and then at even higher AlCl3 concentrations the structure reverts to a three bilayer structure. For SHE2S, this trend is even more extreme. The initial monolayer adsorption transforms to a narrow region of multilayer adsorption with increasing AlCl3 concentration, and then at higher AlCl3 concentrations the adsorption is in the form of a monolayer again. For both STE2S and SHE2S at the higher AlCl3 concentrations precipitation occurs in the bulk solutions, consistent with the reduced solubility of the surfactants. Hence, the transition back to monolayer adsorption or structures with reduced order are attributed to depletion effects associated with the bulk solution precipitation. For both the STE2S and SHE2S surfactants, in the region of precipitation, the surface is now in equilibrium with a more dilute solution phase, and so the surface reverts to a structure associated with the surfactant concentration associated with that more dilute solution phase The solution self-assembly properties of the SAES surfactants in the presence of AlCl3 will be reported separately. However, the physical appearances of the solutions provide a useful guide to their phase behavior and are reported here. As such they provide a broad indication of the trends in the solution properties. At low AlCl3 concentrations the solutions are clear and correspond to relatively small micelles in dilute solution. With increasing AlCl3 concentration the solutions become increasingly turbid; consistent with micellar growth and ultimately lamellar structures. At the higher surfactant and AlCl3 concentrations for the tetradecyl and hexadecyl chain length surfactants, the effects of the reduced solubility are observed, and partial precipitation and precipitation occurs. It is clear from these observations and previously reported data20,34 that there is no strong direct correlation between the surface and the solution properties, and hence the planar surface plays an important role in determining the surface structural evolution observed. The hydrolysis of AlCl3 and the formation of oligomeric species in aqueous solution is well established.49,50 The nature of the oligomers formed is highly pH dependent. The measured pH values for these SAES/AlCl3 solutions studied here are reported in the Experimental Details. The variation in pH, from 6.8 to 4.5 as the AlCl3 concentration increases, indeed shows that hydrolysis does take place. However, a detailed comparison of the evolution in the surface structures observed for SLE2S at

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SUMMARY The NR and ST measurements on the surface adsorption of a series of SAE2S anionic surfactants in the presence of AlCl3 show that the nature of the adsorption is affected by the alkyl chain length. For a fixed headgroup geometry, diethylene glycol ether sulfate, the adsorption in the absence of electrolyte is determined by the headgroup, and the alkyl chain variation plays only a secondary role. In the presence of AlCl3, the strong surfactant/Al3+ complexation drives the formation of multilayer structures at the interface, as previously reported.19−21 At the shorter alkyl chain lengths, the differences in the evolution in the surface structure are attributable to the decreasing role of the hydrophobic interaction between the alkyl chains. That is, the more extended multilayer structures are less prevalent for SDE2S than for SLE2S. For the longer alkyl chain length surfactants, STE2S and SHE2S, the effects of the reduced surfactant solubility and the greater tendency for precipitation in the presence of Al3+ results in further changes in the surface behavior. This is especially the case for SHE2S, where only a narrow region of multilayer formation exists, and monolayer adsorption is observed at both low and high AlCl 3 concentrations. The results presented here and for SLES21 show how changes in the alkyl chain length and ethylene oxide chain length of the surfactant can be used to control the surface multilayer formation on the addition of Al3+. In particular, they illustrate the role of changing the surfactant solubility and the steric effects of the headgroup and alkyl chain geometry on the surface properties.

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ASSOCIATED CONTENT

S Supporting Information *

Additional surface tension data, neutron reflectivity, and tables of model parameters from the analysis of the neutron reflectivity data, and surface phase diagrams. This material is available free of charge via the Internet at http://pubs.acs.org. 12751

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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 SURF and INTER reflectometers at ISIS is acknowledged. The invaluable scientific and technical assistance of the Instrument Scientists and support staff is gratefully recognized. Funded through an EPSRC grant, EP/G065705/1, and neutron beam time at the ISIS Facility, UK (STFC), ILL, Grenoble.

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