Adsorption of Model Perfumes at the Air–Solution Interface by

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Adsorption of Model Perfumes at the Air−Solution Interface by Coadsorption with an Anionic Surfactant Robert Bradbury,† Jeffrey Penfold,*,†,‡ Robert K. Thomas,† Ian M. Tucker,§ Jordan T. Petkov,§ and Craig Jones§ †

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

S Supporting Information *

ABSTRACT: The adsorption of the model perfumes phenyl ethanol, PE, and linalool, LL, at the air−solution interface by coadsorption with the anionic surfactant sodium dodecyl 6-benezene sulfonate, LAS-6, has been studied primarily by neutron reflectivity, NR. The variation in the mixed surface adsorption with solution composition is highly nonideal, and the more hydrophobic LL is more surface active. At a LAS-6 concentration of 0.5 mM the adsorption of PE and LL is broadly similar but with the LL systematically more surface active, and at 2 mM the LL completes more effectively for the surface than the PE. The variation in surface composition with solution composition and concentration reflect the greater hydrophobicity and hence surface activity of LL, and the greater solubility of PE in aqueous solution. Changing the geometry of the LAS isomer, from the symmetrical LAS-6 geometry to the more asymmetrical LAS-4, results in the LL competing more effectively for the surface due to changes in the packing constraints associated with the hydrophobic region. The results provide insights into the factors that affect coadsorption that can be more broadly applied to the surface delivery of a wide range of molecules other than perfumes.

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INTRODUCTION The solubilization and coadsorption at interfaces of a wide range of small molecules by surfactants are important aspects of many of the applications of surfactants.1,2 This embraces applications as broad as detergents, cosmetics, foods, pesticides, and drugs. It encompasses a wide range of coadsorbates, from simple alcohols, perfumes, flavors, dyes, drug molecules, antibacterial and antimicrobial agents, and more broadly hydrotopes.3 The particular focus of this study is perfumes, which are important ingredients in a wide range of surfactant based home and personal care products.4−6 Surface delivery and retention, evaporation into the vapor phase, and the impact upon surfactant self-assembly are the key elements of perfume performance. Although there have been numerous studies on their solubilization into micellar phases and their impact upon surfactant self-assembly5−15 and on evaporation into the vapor phase,16−18 there is relatively little on their adsorption at interfaces.19 Although fragrances are complex blends of different components,4 these studies have focused on individual model components with differing degrees of hydophobicity/ hydrophilicity, and this approach provides important insights. A wide variety of different perfume components, which are alcohols or phenols with differing degrees of hydrophobicity and solubility, have been studied and include phenyl ethanol, linalool, limonene, eugenol, and geraniol. The degree of © 2013 American Chemical Society

hydrophobicity is usually classified by the octanol/water partition coefficient, logP (defined as log ([solute concentration in octanol]/[solute concentration in water])), where the more hydrophilic perfumes such as phenyl ethanol have a log P of less than 2 (log P for phenyl ethanol is 1.4 to 1.54,7), and the more hydrophobic perfumes have a log P greater than 3 (linalool has a log P of 3.31). The solubility limit (∼150 mM for PE, and ∼10 mM for LL) and the Hildebrand solubility parameter, SP,20 are related measurements of the relative hydrophobicity of PE and LL, and the SP values for PE and LL are 12.0 and 9.6, respectively. Friberg et al.18 measured the vapor pressure of different fragrant components, including PE, during controlled evaporation by analysis of the associated headspace. Behan et al.17 investigated the nature of perfume-SDS interactions in solution through the measurements of the vapor phase concentrations, and demonstrated that the headspace concentrations were directly related to the solution phase volumes, which in turn could be rationalized in terms of their log P values. In the numerous studies which probed the impact of the addition of perfume on surfactant phase behavior5−15 a key factor is the Received: January 10, 2013 Revised: February 13, 2013 Published: February 13, 2013 3361

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water before being thoroughly dried in a vacuum oven. The surface tension data included were measured at equilibrium, with values for surface tension being an average over four readings that were within 0.5 mN m−1. All samples were prepared in ultra high quality (UHQ) water with each isotherm being measured using the 2-fold dilution method, starting at the highest concentration. (ii). Neutron Reflectivity. The neutron reflectivity measurements were made at the air−water interface on the SURF reflectometer at the ISIS pulsed neutron source in the UK21 and on the FIGARO reflectometer at the Institute Laue Langevin in France.22 The reflectivity, R(Q) was measured as a function of the wave vector transfer, Q, in the direction normal to the surface (where Q is defined as Q = 4π sin θ/λ, θ is the grazing angle of incidence and λ is the neutron wavelength). On SURF the neutron beam was incident at a θ of 1.5°, and neutron wavelengths from 1 to 7 Å were used to cover a Q range of 0.048−0.5 Å−1. The samples were aligned and the data corrected and normalized using well-established procedures.23 On FIGARO a fixed angle of 3.8° in combination with neutron wavelengths in the range 2−30 Å provided a Q range of 0.03−0.4 Å−1. On FIGARO the samples were aligned and corrected in a similar way to SURF, but an area detector was used to subtract the specular signal from background scattering. The measurements were made at a constant temperature of 25 °C and the samples (with a total volume ∼25 mL) were contained in stainless steel troughs. The surfactant adsorption measurements are usually made using Teflon troughs.23 Measurements preliminary to this study showed that for the more hydrophobic perfumes very low adsorption at the air−water interface was observed using the Teflon troughs. The adsorption of the more hydrophilic surfactants was consistent with previous measurements.19 It was concluded that for the more hydrophobic perfumes substantial adsorption occurred on the Teflon surface, which was sufficient to deplete the solution and the adsorption at the air−water interface. Using stainless steel troughs overcame this problem and did not affect the adsorption on the more hydrophilic perfumes. The measurements were made initially for the pure surfactant, and the perfume was added progressively using a micropipet. Each individual NR measurement took ∼30 to 60 min, and some repeated measurements were made to ensure that there were no time effects on the time scale of the measurements. The NR measurements were made for isotopic combinations of deuterated surfactant/hydrogeneous perfume, and hydrogeneous surfactant/deuterated perfume in null reflecting water, nrw (92 mol % H2O to 8 mol % D2O has a scattering length of zero, the same as air). In such cases the reflectivity arises only from the adsorbed layer of deuterated material at the interface and is the basis of extensive measurements of surfactant and mixed surfactant adsorption reported in the recent literature.24 In such circumstances the NR data can be modeled by assuming that it is a single layer of homogeneous composition. Using the optical matrix approach for thin film reflectivity,25 adapted for neutrons,26 the adsorbed layer can be described by a thickness τ and a neutron scattering length density ρ (where ρ = ∑b/V, and ∑b is the sum of scattering lengths of the adsorbed molecule and V its molecular volume, and the values for ∑b and v for the components used in this study are summarized in Table 1). It has been shown21 that the product τρ is related to the adsorbed amount, such that for a binary mixture

location of the perfume molecule within the surfactant structure. Fischer et al.7 used pulsed field gradient NMR to provide a clear correlation between partitioning/solubilization and the log P value of the perfume, and postulated on the location of the perfume within the surfactant aggregate. For hydrophilic fragrances, log P < 2, the perfume molecules were found to be distributed equally between the aqueous phase and the micellar aggregates, whereas for hydrophobic fragrances, log P > 3.5, the perfume molecule was fully solubilized into micellar aggregates. A number of related studies13,14,16,19 have reported on the impact of perfume addition on a variety of different surfactant self-assembled systems, where the changes in structural evolution are related to the location of the perfume molecules within the aggregates. The general observation was that the more hydrophobic molecules are located in the more hydrophobic regions of the aggregates and promote structures with lower curvature, whereas the more hydrophilic components are located at the hydrophobic/hydrophilic interface and support structures with higher curvature. Similar observations were made on a variety of different ionic and nonionic surfactant systems with a wide range of different perfume molecules.8−12 In surface structural measurements using NR Penfold et al.19 compared the location of PE and Benzyl alcohol, BA, within a CTAB monolayer at the air−water interface. The measurements showed that the alcohol was located at the hydrophobic/hydrophilic interface, that the more hydrophobic PE was located slightly further from the CTAB headgroup than the BA, and that the PE had a greater impact upon the conformation of the CTAB alkyl chain. In contrast there are few reported studies which directly probe perfume or mixed perfume/surfactant adsorption at interfaces, and the NR studies reported by Penfold et al.19 on PE and BA, coadsorption with the cationic surfactant CTAB are exceptions. Hence to address this paucity of information we have studied and report here NR measurements on the coadsorption of two model perfume molecules with different structures and different degrees of hydrophobicity, PE and LL, with the anionic surfactants, LAS-6 and LAS-4, at the air−water interface. The sodium dodecyl benzene sulfonate was chosen as it represents one of the most extensively used anionic surfactants in formulation. The study is in essence a paradigm for a broad range of coadsorbates, such as those described earlier in the Introduction. An experimental approach that can be more broadly applied to such systems is described. The results provide insight into how factors such as molecular structure and relative hydrophobicity/hydrophilicity affect the coadsorption of small molecules, and which can be more broadly applied.

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EXPERIMENTAL DETAILS

The experimental data for the adsorption of perfume/surfactant mixtures were obtained predominantly using NR and supplemented by some surface tension measurements. Measurements were made for LAS-6/LL, LAS-6/PE, and LAS-4/LL mixtures at the air−water interface. Measurements were made at both fixed surfactant concentrations and variable solution compositions and at fixed solution compositions and variable surfactant concentrations. (i). Surface Tension. The surface tension measurements were made using a Kruss K10 tensiometer at a room temperature of 25 °C, and the measurements were made using a Pt/Ir de Nouy ring. The tensiometer was calibrated using ultrapure water before making the measurements. The Pt/Ir ring was washed with ultrapure water and acetone between each measurement before being dried in a bunsen burner flame. Each solution vessel was washed copiously in ultrapure

ρτ =

∑ b1/A1 ∑ b2/A 2

(1)

Hence from the two complementary NR measurements with the different isotopic combinations the adsorbed amount of each component (surfactant and perfume) can be established. (iii). Materials Used. The LAS, sodium para-dodecyl benzene sulfonate, was custom synthesized at Oxford and Unilever R&D27,28 in two different isotopic forms, with and without the dodecyl alkyl chain and phenyl ring deuterium labeled, d-LAS and h-LAS, and in two different isomeric forms as described earlier. For the two different structural isomers of LAS used the only difference in the dodecylbenzene sulphonates is the position along the hydrocarbon chain comprising the surfactant tail to which the sulphonated aromatic 3362

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Table 1. Neutron Scattering Length, Molecular Weight, and Molecular Volumes for the Different Surfactant and Perfume Components Used in This Study component

molecular weight

molecular volume (Å3)

∑b (×10‑3 Å)

h-LAS-6 d-LAS-6 h-PE d5-PE h-LL d11-LL d17-LL

348 378 122 127 154 165 171

567

0.35 3.37 0.22 0.74 0.05 1.20 1.92

a

198 340

LAS-4 has the same ∑b and molecular volume as LAS-6.

ring is joined. For the LAS-6 isomer the phenyl ring is joined at the “C6” position of the alkyl chain, and the more asymmetric isomer is functionalized at the “C4” position and is referred to as LAS-4. The purity of both the LAS-6 and LAS-4 was verified from surface tension and neutron reflectivity, and the cmc of both isomers was ∼2 mM. Two different isotopic forms of PE were used, h-PE and d5-PE. The hPE was obtained from Sigma Aldrich and the d5-PE from CND Isotopes, both with a purity of >98%; and were used as supplied. Similarly different isotopic forms of LL were used. The h-LL was obtained from Sigma-Aldrich with a purity of 97% and used as supplied. The deuterated LL was synthesized at two levels of deuteration, d11-LL and d17-LL at Unilever R&D.28 The molecular structures of the surfactants and perfumes used in the measurements are shown in figure 1. The phenylethanol enantiomer used was 2-

Figure 2. Variation in LAS-6/LL surface tension with solution concentration for different solution compositions (mole ratio LAS-6/ LL) as indicated in the legend.

Figure 1. Molecular structures of (a) 2-phenylethanol, (b) linalool, and (c) LAS-6 and LAS-4.

phenylethanol, and a single enatomer of linalool was used, as shown in figure 1. UHQ (Elga Ultrapure) water and D2O obtained from SigmaAldrich was used throughout. The stainless steel troughs and all associated glassware were cleaned in Decon 90 and rinsed in UHQ thoroughly.

Figure 3. Variation in cmc for LAS-6/LL and LAS-6/PE with solution concentration for increasing perfume composition. Calculated curves for ideal mixing for LAS/PE (···) and LAS-LL (−) are shown. Nonideal calculations for an interaction parameter, β, of 1.0 for LAS/ LL (− −) and −4.0 for LAS/PE (-···-) and shown, and described in the text.

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RESULTS AND DISCUSSION (i). Surface Tension. Surface tension measurements were made for the LAS-6/LL and LAS-6/PE mixtures at different LAS-6/perfume compositions (from 100:0 to 20:80 mol ratio), and the surface tension data for LAS-6/LL are shown in Figure 2. Similar data for LAS-6/PE are shown in Figure S1 in the Supporting Information. The cmc (2.4 mM) and area/molecule at the cmc (64 Å2) for the LAS-6 are broadly consistent with the previous reported measurements.27,29 For both the LAS-6/LL and LAS-6/PE mixtures the broad trends with increasing amounts of added perfume are similar. The general form of the surface tension variation with solution concentration is similar, and the cmc is shifted systematically to higher solution concentrations as shown in Figure 3 (key parameters are summarized in Table 1 in the Supporting Information). The simplest treatment of the surfactant/perfume mixture is as an ideal mixture of two surface active components as follows. The cmc variation for ideal mixing is then given by30

α (1 − α1) 1 = 1 + C1 C2 C*

(2)

where C*, C1, and C2 are the mixed cmc and cmc values of component 1 and 2 and α1 is the mole fraction of component 1 in solution. Taking the effective cmc of the perfume component as the solubility limit gives rise to the solid lines in Figure 3. This gives a reasonable representation of the cmc variation for the LAS-6/LL mixture where the solubility limit of LL is ∼10 mM. But for the LAS-6/PE mixture the much higher solubility of PE (∼150 mM) results in a gross overestimation of the mixed cmc. Assuming nonideal mixing, and applying the Regular Solution approach31 to the variation in cmc with solution composition, where the departure from ideal mixing is described by a single interaction parameter, β. The variation in the mixed cmc, C*, in then defined as31 3363

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α (1 − α1) 1 = 1 + C* f1 C1 f2 C2

decrease in cmc in surfactant/ethanol mixtures, followed by an increase for more ethanol rich mixtures. This was described by Huang et al. as due to two factors. The initial decrease in the cmc was ascribed to the initial coadsorption of ethanol into the micelle polar region reducing the micelle charge density and hence the electrostatic repulsion within the micelles. As the ethanol concentration increased the solvent dielectric constant decreases and the electrostatic repulsion increases. Furthermore the hydrophobic interaction between the alkyl chains decreases.34 Both of these factors then contribute in the subsequent increase in the cmc. Modifying the Gibbs equation to include the perfume component leads to

(3)

and the activity coefficients are f1 = exp β(1−x) , f 2 = exp βx2, and x is the mole fraction of component 1 in mixed micelles. This gives rise to the dashed lines in figure 2. This corresponds to a β ≈ +1.0 for LAS-6/LL, and a β ≈ −4.0 for LAS-6/PE. This would imply that the LAS-6/LL mixing is close to ideal. An interaction parameter, β, >2 would imply demixing. The LAS-6/PE mixing would appear to be highly nonideal and more synergistic. However, the cmc data can be considered in an alternative way. If the cmc is obtained as a function of the surfactant concentration at the different surfactant/perfume compositions, then the cmc decreases modestly with increasing perfume content, as shown in Figure 4. The variation in cmc is broadly similar for both LL and PE. 2

−

dγ = 2ΓLASd ln[LAS] + ΓPerf d ln[Perf] RT

(4)

and assuming [Perf]= α[LAS], and Γperf = ςΓLAS eq 4 can be written as −

dγ 1 = (2 − ζ )ΓLAS RT d ln[LAS]

(5)

From the data plotted in Figure 2, Figure S1 in the Supporting Information, and summarized in Table S1 in the Supporting Information, the factor ς can be extracted. The variation in ς between the LAS-6/LL and LAS-6/PE data gives a measure of the relative surface activities of the LL and PE components compared to the LAS-6. A value of zero implies that the LAS adsorption is unaffected by the addition of the perfume to the solution. A positive value would imply an increasing impact on and competition with the LAS adsorption by the perfume. For the LAS-6/LL mixture ς is positive and ∼0.4, whereas for the LAS-6/PE mixture it has positive and negative values, and is on average zero. The surface tension data described provides some insights into the interaction between LAS-6 and the model perfumes, LL and PE. However, it is difficult to interpret the data in terms of the relative adsorption of the LAS and perfume, and to establish whether the trends described are reflected in the adsorption. (ii). Neutron Reflectivity. It is difficult to extract directly adsorbed amounts from the ST data. Here NR has been used to directly measure the amounts adsorbed at the air−water interface for both LAS-6/LL and LAS-6/PE mixtures. By making measurements in nrw for the isotopic combinations, d-

Figure 4. Variation in cmc (from surface tension variation with solution concentration, but adjusted for surfactant concentration only) with solution composition for LAS-6/LL and LAS-6/PE.

It is generally accepted that the cmc of ionic and nonionic surfactants increases in less polar solvents, although Penfold et al.32 reported a decrease in the cmc in nonionic surfactant/ sorbitol/water mixtures. Huang et al.33 observed an initial

Figure 5. NR data for 2 mM LAS-6/LL, (a) d-LAS-6/h-LL/nrw, (b) h-LAS-6/d-LL/nrw. The different LAS-6/LL compositions are indicated in the figure legend. The solid lines are model calculations for a single layer as described in the text, and for the parameters summarized in Table 2. 3364

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Table 2. Key Model Parameters for 2 mM LAS-6/LL Data in Figure 5 LAS-6/LL composition (mole fraction LL)

contrast

τ (Å)

ρ (×10‑6 Å‑2)

A (Å2)

Γ (×10‑10 mol cm‑2)

0.2 0.3 0.4 0.5 0.6 0.2 0.3 0.4 0.5 0.6

dh dh dh dh dh hd hd hd hd hd

20 ± 1 20 20 20 22 17 ± 3 22 29 33 28

3.1 ± 0.1 3.0 2.8 2.7 2.4 0.6 ± 0.2 0.5 0.5 0.6 0.8

55 ± 2 59 63 65 67 590 ± 20 380 ± 20 220 ± 10 140 ± 7 104 ± 5

3.02 ± 0.15 2.81 2.65 2.59 2.48 0.28 ± 0.1 0.44 0.76 1.18 1.55

Figure 6. Variation in LAS-6, LL, and total adsorption with solution composition (mole fraction LL) at (a) 0.5 mM LAS-6 and (b) 2.0 mM LAS-6. The different components are as shown in the figure legend.

Figure 7. Variation in LAS-6, LL, and total adsorption with solution concentration for LAS-6/LL solution compositions of (a) 70:30 and (b) 40:60 mol ratio. The different components are as shown in the figure legend.

density, ρ, and the key model parameters for the data in Figure 5 are listed in Table 2. From the measurements with the two different isotopic combinations (d-LAS-6/h-LL and h-LAS-6/dLL) and using eq 1 with the known scattering lengths (see Table 1) of the different isotopically labeled components, the adsorbed amount of each component can be directly obtained (and these values for the data in Figure 5 are also summarized in Table 2). The variation in adsorbed amount s for LAS-6 and LL and the total adsorption with solution composition at LAS-6 concentrations of 0.5 and 2.0 mM are shown in Figure 6a,b, and the associated parameters are summarized in Table S2 in the Supporting Information.

surfactant/h-perfume and h-surfactant/d-perfume, and using eq 1, the adsorbed amounts of each component can be evaluated. Measurements were made at two different surfactant concentrations, 0.5 and 2.0 mM, at a range of different solution compositions, and at two fixed solution compositions (40:60 and 70:30 mol ratio surfactant/perfume) and a range of surfactant concentrations. For the LAS/LL mixture an additional set of measurements were made at a solution concentration of 1 mM, using a different LAS isomer, LAS-4. (a). LAS-6/LL. Figure 5 shows the NR data for 2 mM LAS-6/ LL at solution compositions from 0.2 to 0.6 mol fraction LL The NR data are well described by a thin monolayer (∼20 Å) of homogeneous composition, Analysis of the data using a single layer model gives a thickness, τ, and a scattering length 3365

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The variations in the adsorbed amounts of the LAS-4, LL, and the total adsorption for the 1.0 mM LAS-4/LL mixture are similar to that observed for LAS-6/LL at a surfactant concentration of 2 mM. However, although the amount of LAS adsorbed is similar, the amount of LL adsorbed and the total adsorption are systematically higher. This implies that there is a greater synergy between the LAS-4 and LL at the interface compared to LAS-6/LL mixture. This may be associated with more favorable packing constraints associated with the different alkyl chain geometry of LAS-4 compared to the LAS-6. It is postulated that the greater asymmetry in the alkyl chains relative to the phenyl ring for LAS-4 provides greater freedom to accommodate the linalool that is possible with LAS-6. The LAS-6 more closely resembles a symmetrical dichain, and so the alkyl chain packing will be more optimal. (c). LAS-6/PE. NR measurements were made for LAS-6/PE mixtures at fixed LAS-6 concentrations of 0.5 and 2 mM, for a range of LAS-6/PE compositions from 10:90 to 90:10 mol ratio LAS-6/PE, and for the isotopic combinations d-LAS-6/h-PE and h-LAS-6/d-PE. The adsorbed amounts were evaluated as described earlier for the LAS-6/LL mixtures and provide a direct comparison with the LAS-6/LL adsorption behavior. The variations in LAS-6, PE, and total adsorption with solution composition are shown in Figure 9a,b for LAS-6 concentrations of 0.5 and 2.0 mM, and the associated parameters are summarized in Table S5 in the Supporting Information. At the LAS-6 concentration of 0.5 mM and over most of the composition range studied the amount of PE adsorbed at the interface is relatively low and constant. At the higher PE compositions, >70 mol % PE, the amount of PE at the interface increases more significantly. This is reflected in a corresponding decrease in the LAS-6 adsorption such that the total adsorption remains relatively constant over the entire composition range. At the higher LAS-6 concentration of 2 mM the adsorption trends are broadly similar. Except for the solutions richer in PE, the amount of PE at the interface is lower than was observed at the lower LAS-6 concentration. Compared to the equivalent data for the LAS-6/LL mixture the lower PE adsorption reflects the greater hydrophilicity, lower surface activity, and greater solubility of PE compared to that for LL. This results in the PE competing less favorably for the interface in the presence of the LAS-6 than is observed for LL. The adsorption data shown in Figure 10a,b show the adsorbed amounts of LAS-6, PE, and the total adsorption as a function of surfactant concentration at two fixed solution compositions, 70:30 and 40:60 mol ratio LAS-6/PE. The associated parameters are summarized in Table S6 in the Supporting Information. At both solution compositions (70:30 and 40:60), the variation in PE, LAS-6, and total adsorption with increasing solution concentration (form 0.2 to 5 mM) is essentially constant. A similar behavior was observed for the LAS-6/LL mixture at the surfactant richer composition of 70:30. At the LL richer composition (40:60), the amounts of LAS-6 and LL at the interface were no longer constant with composition. The lack of variation with solution composition for the LAS-6/PE mixture for the PE richer composition (40:60) merely reflects the greater solubility of the PE and its greater preference for the solution than the surface. (iii). General Discussion. The variation in surface composition for the LAS-6/LL mixture at LAS-6 concen-

At both LAS-6 concentrations broadly similar patterns of adsorption are observed. That is, as the solution becomes richer in LL the amount of LL adsorbed increases, the amount of LAS-6 adsorbed decreases, and the total adsorption increases. Although for solutions rich in LAS-6 the LL adsorption is slightly higher at the lower LAS-6 concentration, for LL rich compositions the LL adsorption and variation with increasing LL composition is greater at the higher LAS-6 concentration. At the lower LAS-6 concentration of 0.5 mM the decrease in LAS-6 adsorption with increasing LL composition is almost compensated by the increase in the LL adsorption, such that the total adsorption is almost constant. However, at the LAS-6 concentration of 2 mM the increase in LL adsorption is more pronounced and the total adsorption rises more sharply; consistent with a surface synergy between LL and LAS-6. Extrapolating the adsorption data in Figure 6 to 100% LAS-6 implies that the LAS-6 adsorption is lower at 0.5 mM than at 2.0 mM, as expected. Hence at the lower LAS-6/LL composition, the LL can compete more effectively for the surface than at the higher LAS concentration where the LAS-6 adsorption is more pronounced. For solutions richer in LL the more pronounced increase in the LL adsorption at the higher LAS-6 concentration merely reflects the greater LL solution concentration. NR measurements were also made at two fixed LAS-6/LL compositions (70:30 and 40:60) and at different LAS-6 concentrations up to 5 mM, and the variations in adsorbed amounts are shown in Figure 7 and summarized in Table S3 in the Supporting Information. For the LAS-6 richer composition (70:30) the variation in adsorption is, within error, broadly constant. For the LL richer composition (40:60) the amount of LL at the surface increases, the amount of LAS-6 decreases, and the total adsorption is almost constant with increasing solution concentration. (b). LAS-4/LL. At a surfactant concentration of 1 mM the adsorption of LL in the presence of a different LAS isomer (LAS-4) was measured, and the adsorbed amounts of LAS-4, LL, and the total adsorption are presented in Figure 8 and the adsorption parameters are summarized in Table S4 in the Supporting Information.

Figure 8. Variation in LAS-4, LL, and total adsorption with solution composition (mole fraction LL) for 1 mM LAS-4/LL. The different components are as shown in the figure legend. 3366

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Figure 9. Variation in LAS-6, PE, and total adsorption with solution composition (mole fraction PE) at (a) 0.5 mM LAS-6 and (b) 2.0 mM LAS-6. The different components are as shown in the figure legend.

Figure 10. Variation in LAS-6, PE, and total adsorption with solution concentration for LAS-6/PE solution compositions of (a) 70:30 and (b) 40:60 mol ratio. The different components are as shown in the figure legend.

approach31 can be used to estimate the surface composition, such that

trations of 0.5 and 2.0 mM and for the LAS-6/PE mixture at a LAS-6 concentration of 0.5 mM are shown in Figure 11. The variation in surface composition with solution composition for both the LAS-6/LL and LAS-6/PE mixtures is highly nonideal. Within the RST framework, Holland’s

π=

f xi RT ln i + πimax Ai fsi xsi

(6)

where π is the total surface pressure, Ai is the area/molecule of component i at the interface, πimax the surface pressure of component I above the cmc, xi and xis are the micelle and surface mole fractions, and f i and fsi are the corresponding activity coefficients. From the values obtained from the earlier ST measurements the dashed curves in figure 11 are RST calculations for LAS-6/LL at 2 mM for interaction parameters of −3 and −1. Although the calculations are qualitatively similar to the measured variation in the surface composition, the detailed variation is not well described. This contrasts with the measured variation in cmc, as described earlier. In that case the LAS-6/LL mixture was close to ideal mixing and for the LAS-6/ PE was described by a β ≈ −4.0. In both approaches it was assumed that an effective cmc for the perfume could be ascribed to the solubility limit of the perfume component. Although it provides a reasonable description of the cmc data it does not necessarily imply that this assumption is valid. The surface composition data imply that other factors can contribute to the surface mixed adsorption that are not taken into account in the RST approach to surface mixing, such as packing constraints. However, although the solubility limit of LL is comparable to many surfactant cmc values, the solubility

Figure 11. Variation in surface composition with solution composition for LAS-6/perfume, (o) LAS-6/LL at 0.5 mM LAS-6, (●) LAS-6/LL at 2.0 mM LAS-6, and (Δ) LAS-6/PE at 0.5 mM LAS-6. The solid line equates to a surface composition identical to the solution composition. 3367

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limit for PE is much higher in comparison, and this is clearly having an impact on the competitive adsorption of the PE. The comparison between the relative adsorption of LL and PE with LAS-6 shows that LL can compete more effectively for the surface than PE. This is clearly reflecting the greater hydrophobicity and surface activity of LL compared to PE, where the x15 greater solubility limit of PE compared to LL is a measure of the greater hydrophilicity and lower surface activity of PE. The increase in total adsorption for the LAS-6/perfume mixtures as the solution becomes richer in perfume, especially for the LAS-6/LL mixtures, is an indication of a synergistic interaction at the interface. The comparison between the adsorption data of LL with the two different LAS isomers, LAS-6 and LAS-4, provides some insights into the role of packing at the interface. Although the adsorption behavior is broadly similar, the LL and total adsorption are systematically higher for the LAS-4 than the LAS-6 isomer. The main difference in the two isomers is that the LAS-6 is essentially a dichain surfactant with two C6 chains attached to the phenyl ring, whereas the LAS-4 isomer is more asymmetrical with what is essentially C4 and C8 chains attached to the phenyl ring. This change in geometry and the changes in the adsorption imply that the LL can pack more favorably with the more asymmetrical chain distribution.

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 reflectometer at ISIS and on the FIGARO reflectometer at the Institute Laue Langevin, Grenoble is acknowledged. The invaluable scientific and technical assistance of the Instrument Scientists and support staff is gratefully recognized. Funded through an EPSRC CASE award with Unilever, and neutron beam time at the ISIS Facility, UK (STFC) and the ILL, Grenoble, France.

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REFERENCES

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SUMMARY We have shown for the first time how NR, in combination with isotopic substitution, can be used to quantify the adsorption of model perfumes at the interface in the presence of surfactant. The results show that the more hydrophobic LL competes more effectively for the surface than the more soluble and hydrophilic PE. The variations in the surface composition with solution composition and with solution concentration show that the surface mixing behavior is nonideal. The variation in mixed cmc with solution composition can be described by RST, assuming that the perfume component has an effective cmc determined by the solubility limit. The surface adsorption is nonideal but is not well described by RST using the same approximation. Comparing the NR results and the ST data suggests that the ST interpretation is not consistent with the adsorption data. Measurements with two different LAS isomers, LAS-6 and LAS-4, for LAS/LL mixtures show that the alkyl chain geometry, and hence the associated packing constraints, have an impact upon the surface adsorption of the LAS/LL mixture. This approach illustrates the potential of such measurements in developing an understanding of surfactant/ fragrance adsorption, such that optimized delivery of fragrance components to interfaces can be developed, and this will be the focus of subsequent papers in this area. Finally, the implications of this work are much broader. In particular they provide insight into the factors which could affect the coadsorption of a wide range of other active benefit agents, such as flavors and antibacterial and antimicrobial agents, at interfaces. It also highlights a deficiency in current theoretical treatments of such mixtures.

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Article

ASSOCIATED CONTENT

S Supporting Information *

Some additional surface tension data and tables of key model parameters from the neutron reflectivity analysis. This material is available free of charge via the Internet at http://pubs.acs.org. 3368

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