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Adsorption at the Air-water interface in Biosurfactant-Surfactant mixtures: Quantitative Analysis of Adsorption in a Five Component Mixture Jessica R Liley, Robert K. Thomas, Jeffrey Penfold, Ian M. Tucker, Jordan T. Petkov, Paul S Stevenson, Ibrahim M. Banat, Roger Marchant, Michelle Rudden, and John Robert Peter Webster Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03187 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 22, 2017
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Adsorption at the Air-water interface in Biosurfactant–Surfactant mixtures: Quantitative Analysis of Adsorption in a Five Component Mixture Jessica R. Liley,†,k Robert K.Thomas,∗,† Jeffrey Penfold,‡,⊥ Ian M. Tucker,¶ Jordan T. Petkov,¶,# Paul S. Stevenson,¶ Ibrahim M. Banat,§ Roger Marchant,§ M Rudden,§ and John R. P. Webster‡ †Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, United Kingdom ‡STFC, Rutherford-Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom ¶Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral CH63 3JW, United Kingdom §School of Biomedical Sciences, University of Ulster, Coleraine, Northern Ireland kCurrent address: LGC, Queens Road, Teddington, Middlesex, TW11 0LY, United Kingdom ⊥Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, United Kingdom #Current address: Lonza UK, GB-Blackley, Manchester, Lancs., M9 8ES, United Kingdom E-mail:
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Abstract The composition of the air–water adsorbed layer of a quinary mixture consisting of three conventional surfactants, octa–ethylene glycol monododecyl ether (C12 E8 ), dodecane–6–p– sodium benzene sulfonate (LAS6), and diethylene glycol monododecyl ether sodium sulfate (SLE2 S) mixed with two biosurfactants, the rhamnolipids L–rhamnosyl–L–rhamnosyl–β– hydroxydecanoyl–β–hydroxydecanoyl, R2, and L–rhamnosyl–β–hydroxydecanoyl–β–hydroxy decanoyl, R1, has been measured over a range of compositions above the mixed critical micelle concentration (CMC). Additional measurements on some of the subsets of ternary and binary mixtures have also been measured by NR. The results have been analysed using the pseudophase approximation in conjunction with an excess free energy, GE , that depends on quadratic and cubic terms in the composition. The compositions of the binary, ternary and quinary mixtures could all be fitted to two sets of interaction parameters between pairs of surfactants, one for micelles and one for adsorption. No ternary interactions or ternary corrections were required. Since the system contains two strongly anionic surfactants, this indicates that the pseudophase approximation can be extended in practice to ionic surfactants, contrary to the prevailing view. The values of the interaction parameters show that the quinary mixture, SLE2 S–LAS6–C12 E8 –R1–R2, which is known to be a highly effective surfactant system, is characterized by a sequence of strong surface but weak micellar interactions. About half of the minima in GE for the strong surface interactions occur well away from the regular solution value of 0.5.
Introduction There is an increasing interest in the properties of biosurfactants as the development of biosustainable and biodegradable surfactant based products becomes more important. 1 A wide range of different biosurfactants is produced by many different organisms, of which one of the more commonly studied and most promising categories is the glycolipids, e.g. rhamnolipids
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and sophorolipids. Glycolipids are disaccharides acetylated by long chain fatty acids and their production and basic characterisation have been studied by several authors. 2,3 Apart from their biosustainability and biodegradability, these biosurfactants exhibit a high tolerance to pH, temperature and salinity, have anti-microbial and anti-bacterial properties, and can be synthesised from a variety of non-petrochemical sources. These attractive properties have already led to a diverse range of applications which include enhanced oil recovery, 4 bioremediation of heavy metals, 5 and some specialised applications in health care, cosmetics and food processing. 6 The rhamnolipids are one of the most extensively studied biosurfactants, 7 but their potential application has been hindered by the challenges of cheap large scale production and purification, 8 as a result of which the most promising route to a more widespread incorporation in formulation is by blending them with different conventional surfactants. This requires a detailed understanding of their mixing behaviour in multi-component mixtures with a range of different conventional surfactants, and this is the focus of this paper. Rhamnolipids are produced by a variety of different strains of Pseudomonas aeruginosa and generally exist with a structure which has one or two rhamnose molecules linked to one or two molecules of β–hydroxydecanoic acid. The two most common forms, L–rhamnosyl–L– rhamnosyl–β–hydroxydecanoyl–β–hydroxydecanoyl, Rha2 C10 C10 or R2, and L–rhamnosyl– β–hydroxydecanoyl–β–hydroxydecanoyl, RhaC10 C10 or R1, are the ones studied here. The surface adsorption 9–14 and self-assembly 13,15–18 of the rhamnolipids have been studied mainly by surface tension, although there are a small number of x-ray, light and neutron scattering studies. Chen et al. 19,20 used neutron reflectivity (NR) to study the adsorption of the rhamnolipids R1 and R2, and their mixtures with each other and with dodecane–6–p– sodium benzene sulfonate (LAS6) at the air–water interface. R1 is more surface active than R2 and in mixed R1–R2 adsorption R1 preferentially partitions at the interface, R2 competing less favourably due to the steric or packing constraints associated with the bulkier R2 headgroup. The mixing behaviour is non-ideal and is not consistent with the straightforward
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application of the regular solution approximation. 21 Mixtures of R1 or R2 with the anionic surfactant LAS6 are weakly non-ideal, and only R1 competes strongly with LAS6 for the interface. In ternary R1–R2–LAS6 mixtures at pH 9 in buffer, the relative surface activities were found to be in the order LAS6 >R1 >R2. An unusual feature of the ternary mixtures is that there is a pronounced synergy in the total adsorption at a LAS6–rhamnolipid composition of 1:1, a synergy that is not observed in any of the associated binary mixtures. Surfactant mixing has been extensively studied and thermodynamic treatments based on the pseudo phase approximation (PPA) are well established. 21–25 The two main problems with the PPA are that it is generally thought to apply only to nonionic surfactants and that it is mainly used only with the regular solution approximation, which restricts the excess free energy of mixing, GE , for a binary mixture to be Bx1 x2 , where B is an interaction parameter and xi the mole fraction. On the more positive side, the PPA provides an intermediate stage between the direct experimental data, from surface tension or neutron reflectometry (NR) measurements, and the interaction parameters. The latter are highly suitable for comparison with theoretical models. At present, theoretical models either use the PPA to link molecular level calculations with measurements on multi-component systems or they bypass it altogether, e.g. 26,27 The ability of neutron reflectometry (NR) to determine directly the adsorbed amounts of each component in multi-component mixtures at the air-water interface both below and above the critical micelle concentration (CMC), allows a greater range of types of GE to be fitted to experiment, e.g., 28–31 and this in turn allows some assessment of the range of validity of the PPA. Here we use NR to probe the surface mixing properties of a quinary (5-component) surfactant mixture, in which the rhamnolipids R1 and R2 are combined with the ternary mixture of conventional surfactants, octa–ethylene glycol monododecyl ether (C12 E8 ), diethylene glycol monododecyl ether sodium sulfonate (SLE2 S) and LAS6. The measurements were all done above the CMC. The ternary C12 E8 –LAS6–SLE2 S surfactant mixture is a good model representation of surfactant based formulations typical of many home and personal care products. Recently
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Liley et al. 30,31 have used NR to make a detailed study of the surface mixing at the airwater interface of the C12 E8 –LAS6–SLE2 S mixture in the absence of electrolyte and in the presence of NaCl and CaCl2 . At surfactant concentrations in excess of the mixed CMC the surface mixing is highly non-ideal and the surface composition is still significantly different from the solution composition. The relative surface activities are in the order C12 E8 >LAS6 >SLE2 S with the result that the surface is significantly depleted of the anionic SLE2 S, even at surfactant concentrations CMC. The non-ideality is consistent with the PPA when terms of higher order than the quadratic B terms (see above) are included in the expression for GE . Addition of electrolyte drives the surface composition much closer to the solution composition for concentrations CMC, and there is a shift in the relative surface activities in the order LAS6 >SLE2 S >C12 E8 . The focus of this study is to understand how replacing part of the C12 E8 –LAS6–SLE2 S ternary mixture with rhamnolipids affects the adsorption and the balance in the relative surface mole fractions.
Experimental Details The samples of h-C12 E8 , d-C12 E8 , h-LAS6, d-LAS6, h-SLE2 and d-SLE2 , where d and h indicate perdeuterated (98%, unless stated otherwise) and protonated hydrophobic units respectively, were the same ones as used by Liley et al., who described in detail their synthesis and characterization. 30 The hydrogeneous rhamnolipids were obtained from Jeneil Biosurfactant Co. and separated into the pure R1 and R2 components as described by Chen et al. 19 The deuterium labelled rhamnolipids, d–R1 and d–R2 were grown in a Pseudomonas aeruginosa culture fed with D2 O and glycerol–d7 . The initial extraction of the surface active components and subsequent purification have been described elsewhere. 8 The pure d–R1 and d–R2 components were separated and characterised using the same procedure as used for the hydrogeneous surfactants and were approximately 90 % deuterium labelled. The purity of the surfactants was assessed by surface tension below and up to the CMC and by NR measurements at concentrations above the CMC. D2 O (99.9 %) was obtained from Fluorochem. 5 ACS Paragon Plus Environment
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High purity water (Elga Ultrapure) was used throughout. All glassware for solution preparation and Teflon troughs for the NR experiments were cleaned in dilute Decon90 solution and rinsed in ultrapure water, ethanol, and acetone, and dried under a nitrogen gas flow. The NR measurements were made at a surfactant concentration of 2 mM, well above the mixed CMC for all the mixtures studied. Measurements were made for the binary mixtures R1–C12 E8 , R2–C12 E8 , R1–SLE2 S, and R2–SLE2 S, and for the ternary mixtures R1–R2– C12 E8 and R1–R2–SLE2 S. Data for the equivalent R1, R2 and LAS6 mixtures were taken from Chen et al. 19,20 The measurements for the ternary mixtures were made at a fixed R1– R2 mole ratio of 1:1, and variable R (R1 + R2)–C for each of C = C12 E8 , LAS6 or SLE2 S mixtures. A series of measurements were made for the 5-component mixture of R1–R2– C12 E8 –LAS6–SLE2 S at a fixed mole ratio of overall R:overall C of 30 : 70, i.e. 30 mole % of the C12 E8 –LAS6–SLE2 S ternary mixture was replaced by rhamnolipid. The measurements were made for two rhamnolipid R1–R2 mole ratios of 1:1 and 1:2, shown in two triangular diagrams as Figure S1 in the Supporting Information. At the fixed R:C and R1:R2 mole ratios the measurements were made over a range of C12 E8 –LAS6–SLE2 S compositions. All the solutions in the present set of measurements were adjusted by the addition of small amounts of NaOH solution to give a final pH of approximately 6.5. For experiments in null reflecting water (NRW) the neutron reflectivity R is given by 64π 2 R = 4 ρ2 sin2 κ
κd 2
(1)
where d and ρ are the thickness and scattering length density of the adsorbed layer, κ is the wave vector transfer in the direction perpendicular to the surface, z, which is given by κ = 4π(sin θ)/λ, where θ is the grazing angle of incidence and λ is the neutron wavelength. P The scattering length density ρ equals i bi niz where ni is the number density of species i and bi its neutron scattering length. NRW is a 92 : 8 mole % H2 O:D2 O mixture with a scattering length density of zero, and hence a refractive index the same as air. For a
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monolayer of deuterium labelled surfactant adsorbed at the interface the reflectivity then arises only from the adsorbed layer. For a single surfactant species at the interface the area P per molecule A is then related to the product d × ρ and the i bij value of the surfactant by P
i bij
A=
(2)
dρ
where i are the component nuclei of surfactant j. The surface excess, Γ, is given by Γ = 1/Na A. For a ternary mixture Eqn (2) becomes P dρ =
i bi1
A1
P
i bi2
+
A2
P +
i bi3
A3
(3)
and this is similarly extended to a quinary mixture. For mixtures, a series of NR measurements with each of the components deuterium labelled in turn leads to a set of simultaneous equations, which can be solved to determine the adsorbed amounts of each component. The values of the sum of scattering lengths for the different surfactants studied here have been given in previous papers. 19,30 For the binary, ternary and quinary mixtures measurements were made for the respective isotopic combinations of dd, dh, and hd, ddd, dhh, hdh, and hhd, and and ddddd, dhhhh, hdhhh, hhdhh, hhhdh, and hhhhd in NRW, where d and h refer to the deuterium labelled and hydrogeneous hydrophobic chains of the surfactants. In all cases the system is over determined and the sets of 3, 4 or 6 simultaneous equations based on Eqns (2) and (3) and their extension to five components were solved as described previously. 30 The NR measurements were made on the SURF and INTER reflectometers at ISIS 32 and on FIGARO at the ILL. 33 The measurements were made at a fixed glancing angle of incidence θ using a range of wavelengths sorted by time of flight, to measure R(κ). The reflected intensity was normalised to the direct beam and the absolute reflectivity values were calibrated by reference to the reflectivity from the surface of D2 O. The samples were contained in sealed Teflon troughs held at 298K and containing a sample volume of ≈ 25 7 ACS Paragon Plus Environment
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mL.
Results Analysis Procedure The compositions of the adsorbed layers above the CMC were analysed using the PPA and an excess free energy, GE , containing both quadratic and cubic terms, as described by Liley et al. 30 Each binary pair is characterized by four parameters, the quadratic and cubic terms in GE , i.e. B µ and C µ for micellization, and B σ and C σ for surface adsorption. The primary aim of the experiments is to test whether parameters obtained for the binary mixtures can be used to describe quantitatively the adsorption behaviour in the ternary and quinary mixtures, i.e. we attempt to fit the ternary and quinary data without any additional parameters. However, for a two parameter GE the sensitivity of the data to each of the four parameters varies with the range of surface compositions being probed, which may also vary significantly between binary, ternary and quinary mixtures. It is therefore more even-handed to follow the approach of Liley et al. which was to attempt to fit the whole set of data to a single set of parameters for micellization and adsorption. The excess free energy of mixing, GE , can be expressed as a series consisting of quadratic (regular solution) and cubic terms in the mole fraction xi where i is numbered from 1 to 5 34
GE =
X j
