The Structures of Salicylate Surfactants with Long Alkyl Chains in Non

14 Nov 2013 - non-aqueous media, as a function of added water. Surfactant ... United Kingdom, and on the NG7 30 m SANS instrument at the. National ...
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The Structures of Salicylate Surfactants with Long Alkyl Chains in Non-Aqueous Media Chern Leing Lee,†,∥ Peter J. Dowding,‡ Allyson R. Doyle,‡ Katrina M. Bakker,‡ Su Shiung Lam,†,⊥ Sarah E. Rogers,§ and Alexander F. Routh*,† †

Department of Chemical Engineering & Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge, CB2 3RA, United Kingdom ‡ Infineum UK Limited, Milton Hill, Abingdon, Oxfordshire, OX13 6BB, United Kingdom § Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom S Supporting Information *

ABSTRACT: The self-assembled structures formed by alkyl salicylate surfactants, as a function of metal headgroup counterion, in dodecane and toluene have been investigated. Results from optical microscopy are combined with small angle neutron scattering to show that moisture in the organic phase can have a dramatic effect on the observed structures. A simple acidic cation produces a cluster of surfactant chains irrespective of the oil type or presence of water. However, systems with an alkali metal counterion (potassium or sodium) result in cylindrical micelles in dry dodecane changing to lamellar structures in the wet case and fuzzy spheres in dry toluene changing to bidisperse emulsions with the presence of water. However, if magnesium or calcium counterions are used, this leads to different structures, depending on the oil type and the presence of moisture.

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INTRODUCTION Overbased engine oil additives are sterically stabilized calcium carbonate particles. They are added to engine oil formulations to react with acid, which is produced during the combustion process. If left untreated, this acid will cause corrosion to the metal surfaces in the engine. The calcium carbonate particles are typically less than 10 nm in diameter, so as to not affect the oil rheology. The particles are colloidally stabilized with a layer of surfactant. This steric layer is usually either a benzyl sulfonate or salicylate headgroup with an alkyl chain of average length C16. Previously, we investigated the effect of water on the colloidal stability of overbased engine oil additives. A 3.9 wt % dispersion of particles stabilized with the sulfonate surfactant was made up in dry cyclohexane and shaken with water. Small angle neutron scattering showed that the water formed a monolayer surrounding the particle.1 It was also demonstrated that the bond between the sulfonate group and the particle surface increased by about 5 Å, and this could be detected using rheometry.1 The effect of electrolyte type and concentration on the size of the water layer has also been investigated with many specific ion effects demonstrated.2 The effect of water on the salicylate surfactant stabilized particles is more complex, with the dispersions displaying some aggregation. This is possibly due to some of the surfactant being desorbed from the surface of the particle and forming © 2013 American Chemical Society

self-assembled structures with water. We therefore wish to examine such structures formed by free salicylate surfactant in non-aqueous media, as a function of added water. Surfactant solutions display many different microstructures, and there are reports of spherical, cylindrical, bicontinuous, and lamellar type structures. Prediction of the observed structure is a difficult problem.3 For example, it is reported that changing the surfactant concentration, chain length, or solvent type can change the observed structures.3 One explanation concerns the relative size of the surfactant headgroup in determining the interface curvature that the surfactant can support. This is turn leads into the macroscopic structures that can be created.4,5 The effect, of changing counterion, on microemulsion structure has been extensively studied for the surfactant bis-2-ethylhexylsulfosuccinate (Aerosol-OT). Dunn et al.6 used dynamic light scattering to show changes in micelle diffusion coefficient when the counterion was changed from sodium to nickel to magnesium. They also demonstrated that the amount of water added to the system had an effect on the micelle diffusion. Small angle neutron scattering has been used to examine the same AOT system with a range of counterions. At low water concentrations, in cyclohexane, a change from spherical Received: April 4, 2013 Revised: November 6, 2013 Published: November 14, 2013 14763

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The Correlation Length Model. This model is usually used to fit SANS data of polymer solutions.14

micelles to rod shaped aggregates was seen when various counterions were used.7 Later work8−10 suggested that cylindrical shaped micelles were seen for larger cations. Similar results are reported by Bardez et al.11 using aluminum as the counterion for AOT in isooctane. An example of the macroscopic effect of the surfactant microstructure is given by Schurtenberger et al.12 A solution of lecithin in isooctane has a small amount of water added. The lecithin formed cylindrical micelles, filling space and dramatically increasing the dispersion viscosity. With the discussion above, it should not be surprising that dispersions of alkyl salicylate surfactant in oil will display a range of structures upon addition of water. We wish to investigate these, and this article reports on the experimental investigation of the effect of surfactant counterion, surfactant concentration, oil type, and time.

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P(Q ) =

P(Q ) = (ρ2 − ρ0 )2 R 2 6Ps(Q , R 2) + (ρ1 − ρ0 )2 R16Ps(Q , R1) − (ρ2 − ρ0 )2 R16Ps(Q , R1)

⎡ 3(sin QR − QR cos QR ) ⎤2 Ps(Q , R ) = ⎢ ⎥ ⎣ ⎦ (QR )3

(5)

Equating ρ1 = ρ2 in eq 4 gives the scattered intensity of a sphere with radius R2. The form factor for a fuzzy sphere Pfs which does not have a uniform density has been discussed in detail by Stieger et al.16,17 2 ⎛ 3(sin QR − QR cos QR ) ⎡ (σ Q )2 ⎤⎞ s ⎟ ⎢ ⎥ Pfs(Q , R ) = ⎜⎜ exp − ⎟ 2 ⎦⎠ (QR )3 ⎣ ⎝

(6)

where σs denotes the width of the smeared particle surface. Form Factor for a Cylinder. Similarly to the case of concentric spheres, the scattering intensity for concentric cylinders is calculated by adding the scattering from the separate components. For a concentric cylinder of length L, inner radius R1, and outer radius R2, the result is given by18 I(Q ) = αNp

∫0

π /2

sin β[(ρ1 − ρ0 )2 V12Pc(Q , β , R1)

+ (ρ2 − ρ0 )2 V2 2Pc(Q , μ , R 2) − (ρ2 − ρ0 )2 V12Pc(Q , β , R1)]2 dβ

(7)

where V1 = πR12L is the volume of the inner cylinder and V2 = πR22L is the volume of the entire entity. The integral accounts for random alingment of the cylinders in the sample, and the form factor for a cylinder inclined at an angle β is given by

⎛ sin(QL cos β /2) J (QR sin β) ⎞2 1 ⎟ Pc(Q , β , R ) = ⎜2 ⎝ (QL cos β /2) (QR sin β) ⎠

(8)

where J1(x) is a first order Bessel function. The thickness of the cylindrical shell can be computed by hs = R2 − R1. For a simple cylinder, without a shell, one simply equates the scattering length densities, ρ2 = ρ1. Polydispersity. To account for any polydispersity, a Schultz distribution was used, along with the form factors, which were weighted by their relevant occurrence.19

(1)

The constant of proportionality is dependent on the instrument, Np is the number density of scattering entities of volume Vp, and Δρ is the difference in scattering length density between the scattering entities and solvent. The scattering vector Q is determined by the incident wavelength λ and the angle of scatter θ by

⎛θ⎞ 4π sin⎜ ⎟ ⎝2⎠ λ

(4)

The function Ps(Q, R) is the form factor for a sphere of radius R and is given by

Materials. All the salicylate surfactants were provided by Infineum UK Limited. The surfactants consist of a salicylate headgroup with the acidic proton replaced by various metals. The chain is around C16 in length, and the industrial surfactants display the chain at various positions on the aromatic ring. Dodecane and toluene (ReagentPlus grade) were purchased from Sigma-Aldrich Co Ltd. Deuterated dodecane (d-dodecane), toluene (d-toluene), and deuterium oxide were purchased from Goss-Scientific Instruments Ltd. and used as received. Samples were made by mixing the as provided surfactant in solvent that had been dried using molecular sieves (Sigma Aldrich). To make wet samples, water was placed into the dry mixture using a micropipet, and the sample was then vigorously shaken by hand. All samples were stored in sealed vials so as to prevent any evaporation. Methods. Confocal Microscopy. To enable fluorescent imaging of each phase, Nile red was added to the organic phase and fluorescein to the aqueous phase. The dyes were each added at a concentration of 50 μM, and each solution was then passed through a 0.2 μm PTFE filter. The aqueous phase was then added to the organic one as described above. The confocal microscope was a Leica TCS AP5 instrument operating with an argon laser. Optical Microscopy. Images were recorded using a Leica DME microscope fitted with a digital camera. SANS Experiments. The SANS data were collected on the SANS 2D instrument at the Rutherford Appleton Laboratory in Oxfordshire, United Kingdom, and on the NG7 30 m SANS instrument at the National Institute of Standards and Technology based in Washington, DC, USA. The supplied salicylate surfactants were diluted to typically 5 wt % in either d-dodecane or d-toluene. The solutions were then transferred to cells with a path length of either 1 or 5 mm. All the experimental SANS data were analyzed by a software package developed by Steve Kline.13 The number of neutrons observed at a given scattering vector Q is depicted as I(Q) and given by

Q=

(3)

The first term on the right-hand side of eq 3 accounts for any large aggregates. The second term is a functional form used to describe the behavior of polymer dissolving in a solvent. ξ and m are the correlation length and the Lorentzian exponent, respectively. Form Factor for a Concentric Sphere. The overall scattering for a core−shell particle of inner radius R1 and scattering length density ρ1 and outer radius R2 with shell scattering length density ρ2 is determined by adding the scattering from the inner sphere to the scattering from the shell of thickness R2 − R1. The scattering is then given by15

MATERIALS AND METHODS

I(Q ) ∝ NpVp2(Δρ)2 P(Q )S(Q )

A C +B n + Q 1 + (Qξ)m

I(Q ) ∝ Np(Δρ)2 S(Q )

∫0

∞

f (R )Vp2P(R , Q ) dR

(9)

where f(R) is the particle size distribution and Vp is the particle volume. Schulz Distribution. The Schultz distribution fs(R) is given by20

(2)

The form factor P(Q) accounts for scattering from within entities, and S(Q) is the structure factor accounting for arrangement of scattering entities in the sample. A discussion of the various models used for the form factor is given below.

⎛ R ⎞ z exp[− (z + 1)(R /R̅ )] fs (R ) = (z + 1)z + 1⎜ ⎟ ⎝ R̅ ⎠ R̅ Γ(z + 1) 14764

(10)

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where R̅ is the average radius of the particle and z = 1/p2 − 1

A dandelion morphology is seen in Figure 1b which shows a 1 wt % solution of potassium salicylate surfactant in dry dodecane. Both of these solutions are 1 day old, but the dry solutions are not observed to show much change with age. The presence of a trace amount of water changes the observed morphologies. Parts c−g of Figure 1 all show 5 wt % solutions of surfactant in dodecane, where 50 μL of water has been added to 3 g of solution. The mixtures have all been left for different amounts of time, as described in the figure caption. The acid surfactant, shown in Figure 1c, forms a simple emulsion drop. The sodium salicylate surfactant forms multiple emulsions, as shown in Figure 1d, and these slowly evolve over time to give a white precipitate, as shown in Figure 1e. The potassium salicylate surfactant forms needle-like structures in wet dodecane, and these evolve slowly into more compact blobs, as shown in Figure 1f. For calcium salicylate surfactant, no structures were observed in dry dodecane. However, addition of water results in a bimodal emulsion mixture with large 100 μm droplets seemingly stabilized by smaller micrometer sized drops, as shown in Figure 1g. Results from confocal microscopy studies on the acid salicylate surfactant are shown in Figure 2. In these images,

(11) 20

The parameter p is the dimensionless standard deviation. It is possible to combine two Schulz distributions to describe systems with two distinct sizes. This distribution is simply a combination of two single distributions.13,20,21 Structure Factor. The Percus−Yevick approximation22 has been used to describe the hard sphere structure factor. A square well potential has been included to describe any short-range attraction which is present.23,24

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RESULTS Optical microscopy demonstrates that a range of morphologies are observed in dodecane. The morphology is dependent on the surfactant cation, the presence or lack of water, the surfactant concentration, and the age of the dispersion. Figure 1 shows a selection of the morphologies we have observed. Figure 1a shows a solution of 5 wt % alkyl salicylic acid surfactant in dry dodecane. It can be seen that the surfactant forms a cluster.

Figure 2. Confocal microscopy images of emulsions formed by 5 wt % alkyl acid salicylate surfactant in 3 g of dodecane with 40 μL of water. The yellow region indicates the presence of fluorescein, and the red region indicates the presence of Nile red.

the aqueous phase is tagged with fluorescein and appears yellow, while the organic phase is tagged with Nile red and appears red. Unsurprisingly, Figure 2 shows the aqueous phase to be forming the core of the emulsion droplets. However, within the aqueous phase, some dark regions appear, indicating that some organic material also exists within the larger emulsion droplets. This is direct evidence for multiple emulsions. Small angle neutron scattering shows the structure on smaller length scales. The scattering pattern for the salicylic acid surfactant in dry d-dodecane is shown in Figure 3. It can be seen that the structure only evolves a small amount over a period of 8 h. Changing the surfactant concentration from 5 to 10 wt % does not change the scattering pattern, implying that

Figure 1. Structures formed by different salicylate surfactants in dodecane. (a) 5 wt % acid salicylate surfactant in dry dodecane 1 day after mixing; (b) 1 wt % potassium salicylate surfactant in dry dodecane 1 day after mixing; (c) 5 wt % acid salicylate surfactant in dodecane with 50 μL of water added to 3 g of solution after 1 day; (d) 5 wt % sodium salicylate surfactant in dodecane with 50 μL of water added to 3 g of solution after 1 day; (e) 5 wt % sodium salicylate surfactant in dodecane with 50 μL of water added to 3 g of solution after 7 days; (f) 5 wt % potassium salicylate surfactant in dodecane with 50 μL of water added to 3 g of solution after 7 days; (g) 5 wt % calcium salicylate surfactant in dodecane with 50 μL of water added to 3 g of solution after 1 day. 14765

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Figure 3. SANS from a mixture of alkyl salicylic acid in deuterated dodecane: (a) 5 wt %; (b) 10 wt %. The circle and rectangular data points indicate the experiment data that were collected at 1 and 8 h after the solution was mixed. The solid lines show the curve fit by using the correlation length model.

salts look similar and are well fit with a cylindrical micelle model with the small upturn at low Q captured using a weak square well potential. The magnesium salt is anomalous, and this is fit with a core−shell cylinder model. The addition of a trace amount of water results in changes to some of the scattering patterns. As shown in Figure 5, for alkyl salicylic acid in d-dodecane, the structure remains consistent with the correlation length model and the pattern does not significantly change when the concentration changes from 5 to 10 wt %. The scattering intensity is seen to increase by a factor of around 20 over a period of 10 days, although the shape of the scattering pattern remains fixed. The other salicylate surfactants demonstrate considerable changes in scattering patterns, as shown in Figure 6. The alkyl magnesium salicylate forms fuzzy spheres with a radius of around 20 Å. The calcium salicylate forms a core−shell cylinder. The parameters used to obtain the model fits are shown in Supporting Information. The monovalent cations were run in two different contrasts, using either H2O or D2O as the wetting agent. Both alkali salt surfactants formed repeated lamellar sheets. For the case of potassium, when D2O is used, the aqueous layer can be determined to be 123.5 Å, separated by 7 Å layers of hydrogenated surfactant. When H2O is used as the wetting agent, it is not possible to distinguish between the water core and the hydrogenated shell; hence, the repeat unit is about 130 Å in length. It is noticeable how the thickness of the water layer is almost 17 times thicker than the layer of surfactant. It seems that many water molecules are sandwiched between a single layer of surfactant molecules. It is entirely reasonable to question the long-time stability of such structures. However, we stress that the good fit, for the same structure in both contrasts, provides confidence in the proposed structures. The parameters used for the fits are shown in the Supporting Information. The situation with sodium is more complex. In order to provide a reasonable fit to the SANS data, it was necessary to assume that the system formed a mixture of lamellar sheets and spheres. The lamellar sheets either appeared as 20 Å water layers surrounded by 6 Å surfactant layers or as a single layer of around 30 Å thickness. The spheres consisted of water cores

the upturn at low Q is due to the structure formed and not a result of attractions between objects (i.e., not due to the presence of an attractive S(Q) contribution). The scattering pattern, for the salicylic acid surfactant, is fit well with a correlation length model. The scattering patterns for the alkyl salicylate surfactant with other cations in dry d-dodecane are shown in Figure 4. The difference from the acid form is immediately evident. The scattering patterns from the sodium, potassium, and calcium

Figure 4. SANS profile of 5 wt % solutions of salicylate surfactants in deuterated dodecane. The open symbols are the SANS data, while the solid lines represent the fitting models of their respective models. The data and model fits of sodium and potassium salicylate were multiplied by 10 for clarity. The scattering was obtained within a few hours of the samples being mixed. 14766

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Figure 5. (a) SANS profile from 3 g of 5 wt % alkyl salicylic acid in deuterated dodecane with 50 μL of water. (b) SANS profile of 10 wt % alkyl salicylate acid in deuterated dodecane with 50 μL of water. The open symbols show the experimental data, and the solid lines represent the model fit to its respective data.

Figure 6. SANS profile of (a) 5 wt % sodium salicylate and magnesium salicylate in 3 g of deuterated dodecane with 50 μL of water. (b) 5 wt % potassium salicylate and calcium salicylate in 3 g of deuterated dodecane with 50 μL of water. The open symbols show the experimental data, and the solid lines represent the model fits. The data was obtained within a few hours of mixing.

with a shell comprising surfactant chains. The size of the spherical micelles suggests that they must be separate entities and not contained within the lamellar structure. A summary of the structures fit to the SANS data for the various salicylate surfactants in dry and wet d-dodecane is shown in Figure 7. The influence of oil structure/polarity was investigated by repeating the experiment using d-toluene as the oil phase. The results for the dry system are shown in Figure 8. The salicylic acid surfactant maintains a correlation length type structure with the size being slightly smaller than that in d-dodecane. The monovalent potassium and sodium salicylate surfactants both form fuzzy spheres of around 20 Å in radius. This should be compared with the cylindrical micelles formed in dry dodecane. The magnesium salicylate headgroup still forms core−shell cylindrical micelles. The size of the micelle in toluene is slightly

smaller than that in d-dodecane. However, the calcium salicylate surfactant forms a core−shell micelle, rather than the cylindrical micelle seen in d-dodecane. The results for the wet system are shown in Figure 9. Again, the addition of water completely changes the observed structures. The scattering from the salicylic acid headgroup remains well fitted by the correlation length model, and it is noticeable that this is the structure observed for the acid form in all our experiments. The calcium salicylate surfactant remains consistent with a core− shell micelle, although the size increases slightly, presumably as the core is swollen by water. The sodium, potassium, and magnesium salicylate headgroups all display bidisperse emulsion sizes. The scattering patterns in Figure 9 show 14767

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Figure 7. A schematic diagram showing the structures formed by various salicylate surfactants in dry and wet deuterated dodecane.

Figure 8. SANS profile of 5 wt % salicylate surfactants in dry deuterated toluene. The open symbols show the experimental data, and the solid lines represent the model fit to its respective data. The data was obtained within a few hours of mixing.

Figure 9. SANS profile of 5 wt % salicylate surfactants in wet deuterated toluene. All the samples have a total weight of 3 g and were mixed with 50 μL of water. The open symbols show the experimental data, and the solid lines represent the model fit to its respective data. The data was obtained within a few hours of mixing.

relaxations in the intensity at two well-defined values of scattering vector Q. For example, the potassium data shows relaxations at Q values of 0.01 and 0.1 A−1. For these counterions, the data is well fitted by the bidisperse model. It is not possible from the SANS data to determine whether the small emulsion droplets are within the larger droplets or

alongside, although the confocal and optical microscope images tended to show multiple emulsions with the smaller droplets contained within the larger ones. 14768

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Figure 10. A schematic diagram showing the structures formed by various salicylate surfactants in dry and wet deuterated toluene.

A summary of the fitted structures in dry and wet d-toluene is sketched in Figure 10.

micrometers in size. The size range accessible by the SANS instruments used in this study is too small to be able to characterize any of the emulsion droplets. The core−shell cylinder structures proposed from SANS results are likely to be stabilizing the smaller emulsion droplets which are in turn stabilizing the larger droplets. The magnesium and calcium cations behave very differently. There are no optical microscopy images for the solutions of the magnesium cation surfactant. This is because they were found to not produce precipitate in dodecane and toluene in both dry and wet states. This means that the structures observed in the SANS experiments are colloidally stable and remain as discrete entities. The SANS results for the acid salicylate surfactant indicate that the number density of scattering entities increases with time but that the structure of later forming clusters is similar to the early forming ones. The microscopy studies indicate that a slow evolution of structure may well be occurring, and the confocal images (Figure 2) indicate a ring-like emulsion structure. This is consistent with the lamellar structure seen for the sodium and potassium surfactants in the wet d-dodecane system, and it is at least conceivable that the multiple emulsions observed in the wet d-toluene case are the route to forming the onion-like structure. Irrespective of the fits to the neutron data, it is unambiguous that the counterion is affecting the microstructure. Merely looking at Figure 6 shows that different counterions lead to very different scattering patterns. The simplest explanation for the effect of surfactant counterion concerns the charge density and, most importantly, the size of the counterion. This then determines the amount of curvature to the oil−water interface that can be supported by the surfactant. This is a similar argument to that put forward by Eastoe et al.8 They reported that for AOT in cyclohexane, with various counterions, the

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DISCUSSION The self-assembled structures formed by salicylate surfactants in non-aqueous media are clearly dependent upon the nature of the surfactant counterion, the oil polarity, and the water content. The structure will be determined by a delicate balance of the energies in the system. Surfactants with a protonated salicylic acid headgroup form clusters of surfactant chains irrespective of the oil type or the presence of water. The optical microscopy images in the dry state, Figure 1a, display a red precipitate that is consistent with the clustering observed in the SANS experiment. In the wet state, the clusters are found to persist and the optical image (Figure 1c) shows the formation of emulsion droplets with the surfactant clusters presumably at the interface. The confocal images, in Figure 2, show that the emulsion droplets will order into a multiple emulsion but the surfactant itself remains as a cluster, as shown in the SANS results. According to the SANS results, the monovalent sodium and potassium surfactants are similar to each other for all the cases examined, as may be expected due to the similarities between the cations. The macroscopic picture is however more complex with the potassium surfactant forming needle-like dandelion structures for all surfactant concentrations examined (Figure 1b), while the sodium surfactant seems to form multiple emulsions (Figure 1d). The SANS results show that on length scales of tens of nanometers the two systems have similar structures but these micellar entities are packing together in very different ways to appear different under the microscope. The calcium salicylate surfactant is found to form core−shell cylindrical micelles in wet dodecane, yet the optical microscope shows large 100 μm droplets stabilized by other droplets a few 14769

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Small angle neutron scattering experiments at the Rutherford Laboratory were supported by beamtime allocations from the Science and Technology Facilities Council.

surfactant was observed to form spherical micelles if the counterion had a hydrated radius of less than 3 Å, and cylindrical micelles for larger hydrated counterions. Sodium and potassium have hydrated ion radii of 1.6 and 1.1 Å, respectively. Calcium has a hydrated ion radius of 2.7 Å, and magnesium, 3.1 Å. The system here is, of course, a different surfactant and solvent, but it is noticeable that for sodium and potassium, the smallest hydrated counterions, we observe spherical micelles. Magnesium and calcium have a larger hydrated ion size, and these systems seem to be close to the energetic boundary where a crossover between structures is observed. The hydrated ion size argument seems to work well in dry dodecane, where magnesium, the largest hydrated counterion, is the anomalous case. In wet dodecane, both magnesium and calcium differ from the cases with sodium and potassium. For the situation with dry toluene as the solvent, magnesium and calcium, the larger counterions, differ from the smaller potassium and sodium. For wet toluene, the size argument struggles in that magnesium, the largest counterion, displays similar properties to the smallest ions and calcium is the anomalous case. The effect of oil type on micelle shape is well-known and discussed by Kabalnov and Wennerstrom.5 The energetics of the system is altered by the interaction between oil and surfactant chains, so the observation of different structures in toluene compared to dodecane is not a surprise.

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CONCLUSIONS The structures formed by alkyl salicylate surfactants in organic media depend on the oil type, surfactant concentration, the presence or lack of water, and the chemistry of the surfactant counterion. This work has investigated dodecane and toluene in both wet and dry situations. An acidic cation gives a cluster of surfactant chains irrespective of the oil type or moisture content. Sodium and potassium cations behave similarly, forming cylinders in dry dodecane and lamellar structures in the presence of water and fuzzy spheres in dry toluene transforming to a bidisperse emulsion in the presence of moisture. Calcium and magnesium cations behave differently from each other, with alkyl magnesium salicylate surfactants seemingly producing colloidally stable surfactant structures.

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ASSOCIATED CONTENT

S Supporting Information *

The parameters used to obtain the fits for the SANS data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ∥

Technip Malaysia, second Floor Wisma Technip, 241 Jalan Tun Razak, 50400 Kuala Lumpur, Malaysia. ⊥ Department of Engineering Science, Faculty of Science and Technology, University Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are very grateful to Infineum for a Case award and to the Dorothy Hodgkins Trust for a studentship for CLL. 14770

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(22) Percus, J. K.; Yevick, G. J. Analysis of Classical Statistical Mechanics by Means of Collective Coordinates. Phys. Rev. 1958, 110, 1−13. (23) Lebowitz, J. L.; Percus, J. K. Mean Spherical Model for Lattice Gases with Extended Hard Cores and Continuum Fluids. Phys. Rev. 1966, 144, 251−258. (24) Sharma, R. V.; Sharma, K. C. The Structure Factor and the Transport Properties of Dense Fluids Having Molecules with Square Well Potential, a Possible Generalization. Phys. A (Amsterdam, Neth.) 1977, 89A, 213−218.

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