Polyoxyalkylene block copolymers adsorbed in hydrocarbon and fluorocarbon oil-in-water emulsions

Abstract

Small-angle neutron scattering has been used to investigate the structure of the adsorbed layers formed by three poly(oxyethylene)-poly(oxypropylene) block copolymers (Poloxamer 188, Poloxamer 407 and Poloxamine 908) in two similar oil-in-water emulsions; one with a fluorocarbon oil phase, the other with a hydrocarbon oil phase. We find that the structure of the buoy layer is remarkably similar in the two cases. Since previous work has shown that the fluorocarbon/water interface is impenetrable to these polymers, these new findings also suggest that there cannot be significant penetration of the hydrocarbon/water interface. This is discussed in relation to the findings from other studies. The effects of added electrolyte and temperature on the adsorbed layer structure are also reported.

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Introduction

The arrangement of polymer molecules adsorbed at an interface in a colloidal system is not just a subject of academic interest, but is also central to a diverse range of technological and biomedical applications.¹ Arguably the most widely studied block copolymers, particularly in aqueous systems, are those formed from poly(oxyethylene) (PEO) and poly(oxypropylene) (PPO), and materials with a variety of architectures, molecular weights and HLB values are available commercially from several manufacturers under a number of tradenames.²

In earlier papers we have reported on our work with two PEO-PPO-PEO triblock species (Poloxamer 188 and Poloxamer 407), and a four-arm (each a PEO-PPO diblock) star-like material (Poloxamine 908). The colloidal system being stabilised by the adsorbed polymer was a perfluorocarbon-in-water emulsion.^3,4 This was not only easy to prepare in a reproducible manner and well suited to our principal method of investigation, small-angle neutron scattering (SANS), but also had clinical analogues (such as the “artificial blood” substitutes).

Our interest in this system was driven by the now well established fact that the structure, and not just the chemical nature, of the adsorbed layer can have a profound influence on the in vivo fate of biocolloids.^5,6 Given the extensive literature concerning the use of synthetic colloidal systems as carriers for the targeted delivery of drugs, being able to probe and describe the adsorbed layer is clearly of some importance.

In this paper we report the results of a complementary investigation designed to augment our earlier work. We have repeated our earlier SANS experiments but this time utilising hydrocarbon (as opposed to fluorocarbon) oils, to see if the changes in the Hamaker constant of the oil, the polymer–interface interaction, and perhaps the solubility of the blocks has any effect on the structure of the adsorbed layer.

Experimental

Materials

Decalin™ (decahydronapthalene), C₁₀H₁₈, 98% cis isomer mixture, and perhydrofluorene (dodecahydrofluorene), C₁₃H₂₂, were purchased from Aldrich Chemical Company. Deuterium oxide, D₂O, 99.9 atom% D, was supplied by CDN Isotopes, Canada. Perdeuterodecalin, C₁₀D₁₈, a cis/trans isomer mixture, 98 atom% D, was obtained from Cambridge Isotope Laboratories, MA, USA. All solvents were used as received.

Samples of Polaxamer 188 (Pluronic F68), Poloxamer 407 (Pluronic F127), and Poloxamine 908 (Tetronic 908) were donated by BASF, Parsippany, NJ, USA. The polydispersities (M_w/M_n) of the polymers were typically less than 1.3. Bulk densities have previously been measured by liquid pycnometry in ethanol. The characteristics of the polymers are summarised in Table 1.

Table 1 Characteristics of the block copolymers^a

	Poloxamer 188	Poloxamer 407	Poloxamine 908
a Table includes data from ref. 26. b NSLD = neutron scattering length density.
Empirical formula	PEO76–PPO29–PEO76	PEO101–PPO67–PEO101	(PEO119–PPO17)2–N–C2H4–N–(PPO17-PEO119)2
MPEO/g mol^−1	6695	8900	20970
MPPO/g mol^−1	1685	3890	3950
Fraction of PPO, νPPO	0.16	0.25	0.13
Bulk density, d/g cm^−3	1.160	1.141	0.914
NSLD ^b, ρ/cm^−2	0.59 × 10^10	0.56 × 10^10	0.49 × 10^10

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Preparation of the emulsions

The emulsions were prepared and characterised in a similar manner to that described in our earlier publication.³ This involves homogenising (Microfluidics Inc. M110 microfluidizer operating at 15 000 psi) a 4% (w/v) solution of the polymer in H₂O with 10% (w/v) perdeuterodecalin. The perdeuterodecalin contained 2% (w/v) perhydrofluorene. This completely miscible but higher boiling point oil has been shown to reduce the rate of Ostwald ripening of the emulsions.^27,28 The perdeuterodecalin was also diluted with 19% (w/w) of normal (h18-) decalin in order to reduce the NSLD, ρ, of the oil droplets to a value that could be contrast matched by a mixed D₂O : H₂O bulk phase. The emulsions were then exchanged into a mixture of 93% D₂O : 7% H₂O (w/w) (the exact mixture composition was optimised in situ on the beam line) by repeated cycles of gentle centrifugation (1000g) and redispersal. This procedure, which minimises the amount of free polymer in solution, does not appear to unduly destabilise the emulsion on the time scales of our measurements.

Due to the need to prime the microfluidizer with polymer solution, the disperse phase fractions are subject to small compositional variations and were therefore assayed by specific gravity measurements after redispersal. The final droplet size distributions were measured by dynamic light scattering (DLS) with a Malvern 4700 correlator (using the cumulants method of data treatment and CONTIN for the size distribution analysis). Some of the hydrocarbon emulsions were prepared with sodium chloride added after homogenisation as an electrolyte. The characteristics of the stock emulsions are summarised in Table 2.

Table 2 Characteristics of the stock hydrocarbon emulsions used in the SANS experiments

	Poloxamer 188	Poloxamer 407	Poloxamine 908
a Parameter used to describe the form of a log-normal particle size distribution, see Appendix.
Volume fraction, ϕ	0.009	0.010	0.010
DLS volume mean radius/nm	635.0	659.5	575.0
DLS surface mean radius/nm	533.4	544.4	409.0
DLS polydispersity, P	0.44	0.47	0.63
Polydispersity, β0^a	0.421	0.447	0.578

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Small-angle neutron scattering⁷

SANS measurements were performed on the D22 diffractometer⁸ at the Institute Laue-Langevin, Grenoble, France.⁹ The collimation length was 11.2 m and the sample-detector distance 10.0 m. The 0.96 × 0.96 m ³He ‘area’ detector (pixel size 7.5 × 7.5 mm) was offset horizontally by 0.38 m. The incident beam was collimated to a diameter of 12 mm at the sample position. A 10% velocity selector was employed operating at 18 210 rpm, making the neutron wavelength, λ, 0.7 nm. This instrument configuration resulted in a reciprocal space (or scattering vector) range Q (=(4π/λ) sin(θ/2), where θ is the scattering angle) of 0.04–0.9 nm⁻¹ with a Gaussian resolution ΔQ/Q of ca. 11.2% FWHM. Samples were contained in 2 mm path length stoppered quartz cuvettes and were mounted in the beam on a thermostatted computer-controlled sample changer. Scattering data were collected from the samples and appropriate sample backgrounds at a number of temperatures between 25–50 °C. Data collection times were typically of the order of 30 min. The raw data was corrected for detector efficiency and the individual sample transmissions and backgrounds, and converted to differential cross-section data (∂Σ/∂Ωvs. Q) using instrument-specific software.¹⁰ These data were then placed on an absolute scale using the scattering from a standard sample (a solid blend of hydrogenous and perdeuterated polystyrene) in accordance with established procedures.¹¹

Neutron scattering length densities for the polymers and for the dispersion medium at 25 °C were obtained as appropriately weighted sums of the calculated values^7,12 for PEO (+0.637 × 10¹⁰ cm⁻² for d = 1.127 g cm⁻³) and PPO (+0.343 × 10¹⁰ cm⁻² for d = 1.004 g cm⁻³) and for H₂O (−0.559 × 10¹⁰ cm⁻² for d = 0.997 g cm⁻³) and D₂O (+6.355 × 10¹⁰ cm⁻² for d = 1.105 g cm⁻³), respectively. The scattering length density of the dispersed phase was calculated from the values for decalin (−0.033 × 10¹⁰ cm⁻² for d = 0.896 g cm⁻³) and perdeuterodecalin (+7.300 × 10¹⁰ cm⁻² for d = 1.014 g cm⁻³) in a similar manner.

Data analysis

The flux of neutrons of a given wavelength, scattered through a particular angle, that arrive on a small area of the detector in a unit time may be expressed as (eqn. (1))where I₀ is the incident flux, ΔΩ is the solid angle element defined by the size of a detector pixel, ε is the detector efficiency, ψ is the neutron transmission of the sample, and V_s is the volume of the sample illuminated by the neutron beam. The first three terms of eqn. (1) are clearly instrument-specific whilst the last three terms are sample-dependent. Only the (∂Σ/∂Ω)(Q) term is specific to SANS and it may be replaced by analogous terms for light or X-ray scattering.

Under the conditions that the hydrocarbon oil phase and dispersion medium are at “contrast match”, and ignoring any contribution from fluctuations in the average layer structure, the scattering from the polymer layer can be described by eqn. (2)¹³

where (∂Σ/∂Ω)(Q) has the usual units of cm⁻¹, ρ_p is the (in this case average) scattering length density of the polymer (in cm⁻²) and ρ_s that of the (matched) droplet cores and dispersion medium, ϕ is the volume fraction of droplets, R is the radii of the droplets (in Å, we have used the surface mean value), d is the bulk density of the polymer (in g cm⁻³), and B is a Q-independent background. The leading term between square brackets is thus a constant for a given emulsion. The second two terms describe how the scattering from the layer is modulated. The remaining quantities are then; Γ, the mass of polymer adsorbed per unit area or “adsorbed amount” (in mg m⁻²) and two measures of the extent of the adsorbed layer, 〈z²〉 (in Ų) and 〈z〉 (in Å), along the interface normal. These are related to the “second moment” of the layer – the distance of the centre-of-mass of the adsorbed polymer layer from the interface—which we give the symbol σ, through the relation σ² = 〈z²〉 − 〈z〉².¹⁴Eqn. (2) is expected to be most valid when Qσ < 1.

We have obtained estimates for Γ, √〈z²〉 and σ by non-linear least squares fitting of the scattering data in the region 0.04 < Q (nm⁻¹) < 0.3 to eqn. (2), averaging over both polydispersity in R (log-normal distribution, see Appendix) and instrument resolution (rectangular distribution) at the same time. With the exception of the Q-independent background all other parameters were constrained to their experimentally determined values. Once the fit had started to converge R and ϕ were also allowed to refine. The maximum change in these values was ca. 3%.

An alternative to eqn. (2), though less satisfactory for incorporating polydispersity, is the following profile independent surface-Guinier type function, eqn. (3)¹⁴

Where we have made comparisons with other techniques, it is our experience that these fitting procedures yield values of Γ accurate to ca. 0.1 mg m⁻² or better, whilst σ, √〈z²〉 and other measures of the layer thickness are generally good to ca. 10% or better.

The polymer volume fraction profiles, Φ(z), that describe the average distribution of segments in the interfacial region¹⁵ can be obtained in two ways. One is the Hilbert transformation method of Crowley.^14,16 This yields a model-independent profile consistent with the data, although considerable data manipulation is required prior to inversion and even then the resulting profile may still contain truncation artefacts. A more fundamental problem with this approach, however, is that the transform uses the interface as a phase reference point. It does not matter if the interface is invisible to the neutrons provided that the interface is impenetrable (since then the droplet radius may be used to locate it). We had reason to believe that in this work (and contrary to our experience with the fluorocarbon emulsions) the polymers might penetrate the hydrocarbon oil interface. For this reason we have used an indirect profile-fitting procedure developed by Cosgrove.¹⁷ In this approach a parameterised mathematical form for the volume fraction profile is back-transformed into scattering data which is then compared to the experimental scattering data. Iteration of the parameters then generates the profile of the chosen mathematical form that best describes the data. It must be stressed here that although modern theoretical descriptions of polymer adsorption can be used to guide the selection of profiles, the only reliable methodology is to test several individual functional forms for Φ(z) on each experimental data set. Before adopting the indirect profile-fitting procedure we also considered it prudent to assess its validity. This is discussed later. At the same time we address an oversight in our earlier work.

Given a volume fraction profile it is then obviously possible to derive other estimates of the thickness of the adsorbed layer such as the span, l, the distance from the interface at which Φ(z) ≈ 0 (or, more practically, the distance at which the integral of Φ(z) reaches ca. 95% of the total), and also the average fraction of polymer segments at the droplet interface, 〈p〉, (eqn. (4))

where the integration limit t is usually taken as ca. 1.3 nm.¹⁷

Results and discussion

First, we shall compare the two approaches for obtaining the volume fraction profiles. In our first paper on fluorocarbon emulsions we published profiles (Fig. 4 in ref. 3), obtained using the Crowley inversion method,^14,16 for the same three copolymers that we have used in this work. Although the caption labelled those profiles as volume fraction profiles they are in fact segment density profiles, and to convert them into volume fraction the integral under the profile must be normalised to the adsorbed amount. The result of doing this is shown as the data points in the present Fig. 1. Noting the general form of those profiles, the original (∂Σ/∂Ω)(Q) data was then modelled by exponential profiles using the new profile-fitting procedure.

Fig. 1 Comparison of the calculated volume fraction profiles for the three copolymers. The SANS data from which these are derived is that previously discussed in ref. 3 and is from fluorocarbon emulsions at 25 °C in the absence of electrolyte. The points were generated by the mathematical inversion procedure, and the lines by the indirect profile fitting procedure assuming exponential profiles. (◆) and (——) Poloxamer 188, (■) and (– – – –) Poloxamer 407, (○) and (–·–·–·–) Poloxamine 908.

The resulting new profiles are also displayed in Fig. 1 (as the lines), and in Table 3 we compare the physical parameters derived from both analyses. The correspondence is reassuringly good. The small differences in Γ, σ and l are probably of the same order as the experimental error. The more noticeable discrepancy in 〈p〉 for Poloxamer 188 is probably because the volume fraction profile for this polymer is the least exponential like, particularly at small z.

Table 3 Comparison of the two methods for obtaining volume fraction profiles. The SANS data used are from the fluorocarbon emulsion systems reported in ref. 3 and were obtained at 25 °C in the absence of electrolyte. ϕ ∼ 0.04. No allowance was made for droplet polydispersity as this is negligible in these systems

	Hilbert inversion method^a (profile-independent)				Indirect profile-fitting method (exponential profile)
	Γ ^b/(mg m^−2	σ ^b/nm	l/nm	〈p〉	/mg m^−2	σ/nm	√〈z^2〉/nm	l/nm	〈p〉
a Cf. with data in Tables 4 and 5 in ref. 3. Data presented here is a re-analysis. b From model fits to eqn. (3).
Poloxamer 188	1.8	2.0	6.4	0.55	2.0	2.8	3.9	10.0	0.39
Poloxamer 407	3.3	3.2	15.3	0.25	3.7	4.9	7.0	15.0	0.24
Poloxamine 908	1.5	3.1	17.6	0.27	1.7	5.4	7.6	16.0	0.22

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Now compare the data in Table 3 with the corresponding new data from the hydrocarbon emulsions in Table 4. Once again, exponential profiles were chosen to model the polymer distribution. This choice will be further justified later. Table 4 also contains estimates of Γ, σ and √〈z²〉 derived from fitting the SANS data to eqn. (2). The measures of the layer thickness are reasonable, but this function appears to overestimate the adsorbed amount. Fig. 2 shows a possible explanation for this. Whilst the various measures of the layer thickness are derived from the Q-dependence of the SANS data, it is the absolute magnitude of the data that determines Γ. Eqn. (2) appears to over-estimate the differential cross-section between 0.06 < Q (nm⁻¹) < 0.09.

Fig. 2 The SANS data from the hydrocarbon emulsions with adsorbed copolymer at 25 °C in the absence of electrolyte. The experimental uncertainty in (∂Σ/∂Ω)(Q) ranges from about 7% at very low Q to around 4% out at Q ∼ 0.3 nm⁻¹. Error bars are omitted for clarity. (·········) Fit of eqn. (2) to the Poloxamer 188 data. Other lines represent the best exponential profile fits to the experimental data. (◆) and (——) Poloxamer 188, (■) and (– – – –) Poloxamer 407, (○) and (–·–·–·–) Poloxamine 908.

Table 4 Comparison of parameters derived from the SANS data from the hydrocarbon emulsions using two different methods of analysis. Data were obtained at 25 °C in the absence of electrolyte. The parameters describing the polydispersity function are detailed in Table 2

	Without polydispersity (eqn. (3))			With polydispersity (eqn. (2) or exponential profile fitting)
	χ ^2	Γ/mg m^−2	σ/nm	χ ^2	R̄/nm	Γ/mg m^−2	σ/nm	√〈z^2〉/nm	l/nm	〈p〉
Poloxamer 188
(eqn. (2))				622.	532.8	2.8	5.4	5.6	n/a	n/a
(See header)	2.25	1.8 ± 0.01	3.8 ± 0.3	0.88	537.7	1.9	3.6	5.0	12.0	0.32
Poloxamer 407
(eqn. (2))				5071.	554.5	4.2	4.2	6.1	n/a	n/a
(See header)	1.29	2.7 ± 0.01	3.8 ± 0.1	1.43	541.2	3.0	4.5	6.3	14.0	0.27
Poloxamine 908
(eqn. (2))				657.	421.5	1.6	4.6	6.1	n/a	n/a
(See header)	2.27	1.0 ± 0.01	4.1 ± 0.3	1.36	403.3	1.1	4.5	6.3	14.0	0.27

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The first observation is that the trends in the data are identical for the fluorocarbon and hydrocarbon systems; in the case of the adsorbed amount, Poloxamer 407 > Poloxamer 188 > Poloxamine 908, whilst for measures of the layer thickness, Poloxamine 908 ≥ Poloxamer 407 > Poloxamer 188. Though the actual magnitudes of these parameters are quite similar, there are some interesting variations. For example, whilst Γ for Poloxamer 188 is essentially unchanged, the values for Poloxamer 407 and Poloxamine 908 are somewhat depressed in the hydrocarbon emulsion systems. In addition, whilst the Poloxamer 407 and Poloxamine 908 adsorbed layers are slightly contracted (or essentially unchanged), the Poloxamer 188 adsorbed layer is slightly expanded, compared to the equivalent fluorocarbon emulsion system. In our previous paper³ we have commented at length on how our data compares with that in the literature and so we shall not do so again here. However, there are two literature reports that are particularly relevant to this work.^18,19 These both studied PEO-PPO-PEO block copolymers adsorbed at the hexane–water interface using neutron reflectometry (NR), and concluded that the PEO blocks were stretched beyond their solution radii-of-gyration (a PEO homopolymer with the same molecular weight as Poloxamer 188 would have an R_g ∼ 3.2–3.8 nm ^20,21). Comparison of our SANS data for Poloxamer 188 in Tables 3 and 4 only suggests that the adsorbed polymer has a size comparable to R_g. However, all these values are still significantly smaller than the extended length of an (EO)₇₆ chain, ca. 10 nm.²²

Compared to what is observed in the fluorocarbon emulsion systems, the bound fraction data for Poloxamer 188 appears to exhibit a small decrease, whilst those for Poloxamer 407 and Poloxamine 908 increase slightly. The latter trend is actually not too suprising since, by analogy with the established behaviour of homopolymers, an increase in 〈p〉 should be accompanied by a thinning of the layer (fewer segments available to form the buoy layer) and a reduction in Γ (because those polymer molecules already at the interface occupy more of it¹). In our earlier work we identified a clear correlation between Γ and ν_PPO (see Table 1), but not between 〈p〉 and ν_PPO.³ As a consequence, we were able to explain the observed differences in adsorbed layer structure in terms of the relative proportions of PPO (and even a small number of PEO) segments located at the interface. Here, we observe the same strong correlation with Γ which suggests that, once again, it is the PPO segments that are preferentially located at the interface. Since the 〈p〉 values for Poloxamer 188 and Poloxamine 908 are somewhat higher than their corresponding ν_PPO it is again likely that some PEO segments are also kept in the vicinity of the interface. However, the lower 〈p〉 of Poloxamer 188 in the hydrocarbon emulsion systems would suggest that a slightly smaller fraction of PEO segments are present at this interface. This would in fact be consistent with the observed thickening of the Poloxamer 188 buoy layer. Interestingly, the data suggests that this subtle effect is achieved without any real change in the number of adsorbed polymer molecules. This may be an effect of the lower curvature of the hydrocarbon emulsion droplets.

The best-fit exponential volume fraction profiles for the hydrocarbon emulsions are shown in Fig. 3, and should be compared with those for the fluorocarbon emulsions in Fig. 1.

Fig. 3 Comparison of the calculated volume fraction profiles for all three copolymers in the hydrocarbon emulsion systems at 25 °C in the absence of electrolyte and generated by indirect profile-fitting assuming exponential profiles. (——) Poloxamer 188, (– – – –) Poloxamer 407, (–·–·–·–·) Poloxamine 908.

The NR studies by Clifton et al.¹⁸ and Phipps et al.¹⁹ also concluded that the copolymers PEO₇₆-PPO₃₈-PEO₇₆ and Poloxamer 407, respectively, penetrated the hexane/water interface. Clifton et al. determined the extent of the polymer in the oil phase to be 0.8 ± 0.4 nm and Phipps et al. measured 4 ± 1 nm. As it is logical to assume that it is the more hydrophobic PPO block which would preferentially locate in the oil phase then the (seemingly big) difference in these values may partly be due to the difference in the number of segments in the respective PPO blocks (although ν_PPO is actually very similar).

However, where there is penetration of the interface, the volume fraction profile is more likely to be an asymmetric Gaussian (peaked in the oil phase but close to that side of the interface) and not the exponential form with which we have analysed our present data. However, since the oil phase and the bulk phase are at contrast match in our experiments (that is, they have been prepared such that each has the same ρ value) the neutrons cannot see the interface, only the polymer that may be spread across it. We have tried to assess the effect on the exponential profiles of polymer penetrating the oil/water interface by repeating some fits but with the droplet radii reduced by an arbitrary but physically meaningful amount (2 nm), and ϕ reduced by a corresponding factor (of R³/R₀³). As might be expected there is a small increase in Γ (arising out of the reduction in available surface area), but the starting height of the profiles, σ, √〈z²〉 and l are otherwise essentially unchanged. From this we conclude that our analysis is insensitive to the position of the oil/water interface and that our data does not contain sufficient information for us to distinguish between a Gaussian and an exponential profile. In any case, if a Gaussian is highly asymmetric it will approximate to an exponential. Attempts to fit some (symmetric) Gaussian profiles were not very successful; in the absence of additional a priori information the extra parameters introduced too much variation for our satisfaction. This is why we have adopted the pragmatic approach of only fitting exponential profiles. What we can say with confidence is that block (step-like) or parabolic profiles are much less satisfactory descriptions of the adsorbed polymer distribution than an exponential.

Phipps et al. modelled their NR data with three layers (of which two were in the aqueous phase) and this led to a volume fraction profile that might be described as a ‘stylised Gaussian’ (see Fig. 6 in ref. 19). For Poloxamer 407 they report Γ ∼ 2.9 mg m⁻², σ ∼ 2.4 nm and l ∼ 9 nm. Whilst their adsorbed amount is in excellent agreement with our data, their thickness measures appear somewhat smaller until one adds on the thickness of the polymer in the oil phase mentioned above. The particular advantage of their experiment over ours was that they varied ρ of the oil phase in order to locate the interface and to highlight those polymer segments on the oil side.

Another consideration may be the presence of capillary waves travelling along the surface of these relatively large droplets. A rough calculation (see Appendix) suggests that if present the droplet interface would have a r.m.s. roughness of no more than 0.4 nm. This is in line with the NR data from the hexane/water interface, in the presence of Poloxamer 407, which ascribed to the interface a Gaussian roughness of ca. 1 nm.¹⁸ Both of these roughness values are actually very similar to the error bounds on the σ values derived from fitting the SANS data to eqn. (3).

In our earlier work with the fluorocarbon emulsion systems⁴ we found that increasing the concentration of electrolyte in the bulk phase or increasing the temperature had an effect on the structure of adsorbed layers of Poloxamer 188. Specifically, for sodium chloride concentrations up to about 0.6 M, or for temperatures in the range 25–45 °C, we observed an expansion of the adsorbed layer of 85–300% (dependent on the thickness measure used). At higher electrolyte concentrations, or higher temperatures (up to 55 °C), the adsorbed layer contracted (but did not noticeably diminish below the 0 M salt or 25 °C thickness). Throughout these changes the adsorbed amount remained effectively constant, although there was a steady but small decrease in the bound fraction. This behaviour, which is more noticeable with electrolyte, and which has many parallels in other solution studies of oxyalkylene-based surfactant systems, was attributed to the effect on the oxyethylene segments of changes in the water solvent quality.

We have repeated these studies, this time also including Poloxamer 407 and Poloxamine 908, as part of this work. Some results, for Poloxamer 188 for brevity, are shown in Tables 5 and 6. Additional data may be found in the associated electronic supplementary information.† The same trends identified in the fluorocarbon emulsion systems are just about discernible.

Table 5 Comparison of parameters derived from the SANS data from the hydrocarbon emulsions as a function of electrolyte concentration using the profile-fitting method of analysis with exponential profiles and incorporating droplet size polydispersity. Data were obtained at 25 °C. For full data, see supplementary information¹

	[NaCl]/M	χ ^2	R̄/nm	Γ/mg m^−2	σ/nm	√〈z^2〉/nm	l/nm	〈p〉
Poloxamer 188	0.09	1.12	536.4	1.9	3.4	4.8	11.6	0.34
	0.36	0.94	531.8	1.9	3.6	5.0	12.0	0.32
	0.56	0.79	524.7	1.5	3.7	5.1	12.2	0.32
	0.76	0.84	533.5	1.7	3.3	4.6	11.2	0.35
	1.02	0.78	521.9	1.7	3.3	4.6	11.2	0.35
	1.20	0.96	524.0	2.0	3.3	4.6	11.2	0.35
	1.41	0.67	535.0	1.9	3.1	4.4	10.8	0.36

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Table 6 Comparison of parameters derived from the SANS data from the hydrocarbon emulsions as a function of temperature using the profile-fitting method of analysis with exponential profiles and incorporating droplet size polydispersity. Data were obtained in the absence of electrolyte. For full data, see supplementary information¹

	T/°C	χ ^2	R̄/nm	Γ/mg m^−2	σ/nm	√〈z^2〉/nm	l/nm	〈p〉
Poloxamer 188	30.0	0.90	536.6	1.9	3.6	5.0	12.0	0.32
	35.0	1.36	537.5	2.0	3.4	4.8	11.6	0.34
	40.0	1.07	535.3	2.0	3.5	4.9	11.6	0.33
	45.0	1.16	536.3	2.1	3.5	4.9	11.8	0.33
	48.5	1.12	535.9	2.0	3.4	4.7	11.4	0.34
	50.0	0.96	534.1	2.0	3.5	4.9	11.8	0.33

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Interestingly, the repeat runs on the different electrolyte samples (Table 5) have consistently lower Γ values than the initial measurements, though 〈p〉 and the various measures of the layer thickness remain quite similar. Since there is no clear pattern to the small changes in the droplet size, Ostwald ripening does not seem to be responsible and so we conclude that polymer has desorbed from the interface. In each instance the repeat measurement commenced almost exactly 6 h later. In our previous work with the fluorocarbon emulsions⁴ scattering data was typically accumulated as four 1 h runs but spread over a much longer duration. Those data did not show any comparable change in adsorption. Electrolyte-induced desorption would therefore appear to be a feature of the hydrocarbon emulsion systems. One may speculate that this is related to the differences in segment–interface interaction parameters resulting from the reduction in the polarizability of the C–H bond compared to the C–F bond.

Summary and conclusions

Small-angle neutron scattering has been used to compare and contrast the adsorption of the same three PEO-PPO block copolymers in two related systems; one a fluorocarbon oil-in-water emulsion, the other a hydrocarbon oil-in-water emulsion.

We have previously investigated adsorption in the fluorocarbon emulsion system in some detail^3,4 and concluded that the polymer segments did not penetrate the interface. This was in keeping with both the general insolubility of hydrocarbon polymers in fluorocarbons and our own observations (e.g., attempting to dissolve the polymers in perfluorodecalin). In contrast, there are literature reports^18,19 that suggest these polymers should be capable of penetrating the hydrocarbon/water interface. We have been unable to find any unambiguous evidence of this, indeed the polymers are also insoluble in normal decalin, which suggests that if there is indeed penetration it can at best only be of the order ≤1 nm (given that the resolution of our measurements is ca. 0.1 nm.

Apart from a slight reduction in the amount of polymer adsorbed at the interface, our data suggest that there is otherwise considerable similarity in the structure of the adsorbed layer (and particularly that of the stabilising PEO blocks) between fluorocarbon–water and hydrocarbon–water emulsion systems. In both types of emulsion the PEO buoy blocks appear to extend distances comparable to their solution radii-of-gyration.

As found in the earlier fluorocarbon emulsion study, changes in temperature or electrolyte concentration are able to bring about subtle modifications of the layer structure.

Appendicies

Droplet size polydispersity

It is common practice to describe polydispersity in particle size by a log-normal distribution. The normalised expression for the fraction of radii n(R), having a radius R, is given by eqn. (A.1)where R_m is the modal value of R, related to the mean value R̄, by eqn. (A.2)

ln R̄ = ln R_m + 1·5 β₀² (A.2)

The standard deviation of such a distribution β, is (eqn. (A.3))

β = R_m[exp(4β₀²) − exp(3β₀²)]^1/2 (A.3)

In dynamic light scattering (DLS), size polydispersity P is defined as the relative standard deviation coefficient of variation,²³ that is (eqn. (A.4))where μ is the variance. Rearranging eqn. (A.2) and substituting into eqn. (A.3) then yields eqn. (A.5)Hence, given a value for P from DLS, β₀ can be calculated iteratively from eqn. (A.5). When P is less than 0.13, P ≈ β₀ and the droplets may effectively be considered monodisperse.

Capillary wave phenomena

The mean-square roughness 〈Δ²〉 of a planar interface is approximately given by eqn. (A.6)²⁴where Δ₀ is the intrinsic width of the interface (taken as 0.1 nm), γ is the interfacial tension and λ_min and λ_max are the minimum and maximum capillary wavelengths possible. If we identify these latter two parameters as 0.1 and 3420 nm (the mean circumference of droplets in the Poloxamer 407 emulsion), respectively, then for γ ∼ 50 mN m⁻¹ (a typical alkane/water interfacial tension at room temperature) we obtain √〈Δ²〉 ∼ 0.4 nm. Although the surface activity of the polymer could be expected to reduce γ, in practice considerable local surface tension gradients tend to have a significant damping effect.²⁵ For this reason the value calculated above is almost certainly an over-estimate.

Acknowledgements

SMK and CW would like to thank the Engineering and Physical Sciences Research Council for providing access to the ILL, and Roland May and Isabelle Grillo for their assistance in helping to operate the D22 diffractometer. The SANS data in this paper were obtained during ILL Experiment No. 9-10-403. The authors would also like to thank Prof. Terry Cosgrove (University of Bristol) for allowing us to use his indirect profile-fitting software.

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Footnotes

† Electronic supplementary information (ESI) available: A schematic representation of a copolymer-stabilised emulsion droplet and the corresponding volume-fraction profile (Fig. S1); comparison of parameters derived from the SANS data from the hydrocarbon emulsions as a function of electrolyte concentration or temperature using the profile-fitting method of analysis with exponential profiles and incorporating droplet size polydispersity (Tables S1 and S2). See http://www.rsc.org/suppdata/cp/b4/b414175j/

‡ Present address: AstraZeneca, Macclesfield Works, Hurdsfield Industrial Estate, Macclesfield, Cheshire, UK SK10 2NA.

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