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Langmuir 1997, 13, 3800-3806
Structure of the Monolayer Formed at an Air-Water Interface by a Novel Nonionic (Vesicle-Forming) Surfactant D. J. Barlow,*,† G. Ma,† J. R. P. Webster,‡ J. Penfold,‡ and M. J. Lawrence† Pharmacy Department, King’s College London, Manresa Road, London SW3 6LX, U.K., and Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, U.K. Received January 28, 1997. In Final Form: April 25, 1997X Neutron specular reflection measurements together with isotopic substitution have been used to determine the molecular architecture of the monolayer formed at the air-water interface by a synthetic nonionic vesicle-forming surfactant, 1,2-di-O-octadecyl-rac-glycerol-3-(ω-methoxydodecakis(ethylene glycol)). The structure of the monolayer was determined with the surface pressure maintained at 28 and 34 mN/m, where the individual surfactant molecules are calculated to occupy areas of 125 ( 1 and 116 ( 2 Å2, respectively. At 28 mN/m, the widths of the Gaussian distributions for the surfactant hydrophobe and head group layers are determined to be 16.5 ( 1 and 20 ( 1 Å, respectively, with the associated solvent modeled with a tanh distribution of thickness 9 ( 1 Å. An extensive overlap is observed between the hydrophobe and head group layers, with the mixed chain region having a thickness of around 12 Å. As the result of the modest increase in surface pressure to 34 mN/m, the surfactant monolayer is made slightly thicker and more ordered as expected, with the widths of the hydrophobe, head group, and solvent layers increased to 17 ( 1, 22 ( 1, and 9.5 ( 1 Å, respectively. The higher pressure, however, also leads to an increased thickness for the mixed chain region (13 Å), and causes a greater proportion of the alkyl chains to be immersed in the solvent (∼58% versus 38%).
Introduction Neutron specular reflection measurements together with isotopic substitution have been used to determine the structure of the adsorbed layer formed at an airwater interface by the novel nonionic (vesicle-forming) surfactant, 1,2-di-O-octadecyl-rac-glyceryl-3-(ω-dodecakis(ethylene glycol) (2C18E12; DODE). This surfactant represents one of a series of nonionic surfactants which we have developed for use in drug delivery.1,2 In previous studies we have analyzed the DODE monolayer structure by means of an optical matrix treatment3 of its neutron reflectivity data and have successfully deduced some of its basic structural features, such as the overall monolayer thickness, surfactant interfacial area, and the level of head group hydration.4-6 In the following report we present a more detailed picture of the DODE monolayer structure, determined at the surface pressures of 28 and 34 mN/m. The greater detail afforded in these analyses has been obtained through a kinematic treatment7 of the neutron reflectivity data, made possible by the preparation and use of the head group and alkyl chain deuterated forms of the surfactant, in addition to the fully protonated and fully deuterated forms. Experimental Details The syntheses of the fully protonated (h2C18hE12), fully deuterated (d2C18dE12), alkyl chain deuterated (d2C18†
King’s College London. Rutherford Appleton Laboratory. X Abstract published in Advance ACS Abstracts, June 15, 1997. ‡
(1) Lawrence, M. J.; Chauhan, S.; Lawrence; S. M.; Barlow, D. J. STP Pharma Sci. 1996, 6, 49. (2) Lawrence, M. J.; Lawrence, S. M.; Chauhan, S.; Barlow, D. J. Chem. Phys. Lipids 1996, 82, 89. (3) Penfold, J. Prog. Colloid Polym. Sci. 1990, 81, 198. (4) Ma, G.; Barlow, D. J.; Penfold, J.; Webster, J.; Lawrence, M. J J. Pharm. Pharmacol. 1994, 41 (suppl.), 6. (5) Ma, G.; Barlow, D. J.; Penfold, J.; Webster, J.; Lawrence, M. J J. Pharm. Pharmacol. 1995, 47, 1071. (6) Barlow, D. J.; Ma, G., Lawrence, M. J.; Penfold, J.; Webster, J. Langmuir 1996, 11, 3737. (7) Penfold, J.; Thomas, R. K. J. Phys.: Condens. Matter 1990, 2, 1369.
S0743-7463(97)00082-6 CCC: $14.00
hE12,) and head group deuterated (h2C18dE12) forms of DODE were performed essentially as described by Lawrence et al.2 In departure from the reported scheme, however, the glycerol (Sigma) was derivatized as an acetophenone acetal, by refluxing for 2 h with dry toluene (Sigma) and acidic amberlite (Fluka), and then for a further 72 h following dropwise addition of acetophenone (Fluka). The oxyethylene head group moieities were then polymerized onto the predistilled derivative, using either ethylene oxide (UCAR, Belgium) or ethylene-d4 oxide (MSD Isotopes), with the alcohol mixed with potassium tert-butoxide (Aldrich, 5% excess). After extraction of the required polyethoxylated intermediate, the terminal hydroxyls of the head groups were methylated by treatment with dimethyl sulfate (Aldrich). The hydroxyls in the 1and 2-positions on the glyceryl moeity were then deprotected by stirring in acidic methanol solution (90:10 methanol/dilute HCl, 2 h, 60 °C), and the final addition of the two alkyl chains was accomplished by means of a Williamson ether reaction using either octadecyl bromide (Aldrich) or octadecyl-d3 bromide (MSD Isotopes). All forms of the DODE were analyzed using a JoyceLoebl Langmuir trough to ensure that they gave the same force-area isotherms. Semiquantitative estimates for the degree of polydispersity and the mean lengths of the surfactant oxyethylene head groups were determined from electrospray mass spectrograms, obtained (courtesy of Dr. S. Howell, National Institute of Medical Research, MRC Laboratories, Mill Hill) using a Micromass Platform quadrupole machine. Neutron reflection experiments were carried out on the CRISP reflectometer at the Rutherford Appleton Laboratory (Didcot, U.K.). The Teflon surfaces of the trough and all the glassware used were scrupulously cleaned using dry chloroform (spectroscopic grade, Sigma) followed by copious washing using high-purity water (Elga Ultrapure). The reflectivity measurements were made at 298 K using a pulsed white neutron source, with the incidence angle fixed at 1.5°, and the intensities calibrated8 with reference (8) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381.
© 1997 American Chemical Society
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Table 1. Scattering Lengths and Dimensions of Constituent Parts of DODE moiety
volume,a Å3
extended length, Å
2C18H37(C3H5O2)b 2C18D37(C3H5O2)b (OC2H4)12OCH3 (OC2D4)12OCH3 H2O D2O
1120 1120 845 845 30 30
26.0 26.0 43.5 43.5
hii(κ) ) |ni(κ)|2
scattering length, 10-5 Å -24.72 744.52 (99.8% D) 50.83 540.80 (98% D) -1.68 19.15
Volumes calculated according to ref 2. b Glycerohydrocarbon moiety.
hij(κ) ) hji(κ)) Re{ni(κ)nj*(κ)}
(4)
with ni(κ) being the one-dimensional Fourier transforms of ni(z), the mean number density profiles of species i along z
ni(κ) )
∫-∞∞ exp(-iκz) ni(z) dz
(5)
a
to D2O (MSD Isotopes). A flat background (determined by extrapolation to high momentum transfer, κ) was subtracted from all profiles. The DODE adsorbed layers were achieved by dropwise addition of chloroform solutions of the surfactant from a hypodermic syringe, and the neutron reflectivity was measured at surface pressures of 28 and 34 mN/m, maintained in a Nima Langmuir trough. The reflectivity data were obtained for d2C18dE12, h2C18dE12, and d2C18hE12, spread on subphases comprising D2O or null reflecting water (nrw; D2O/H2O 9:1), and also for h2C18hE12 on D2O. Each reflectivity profile was recorded for a total reflected current of 400 µA. In order to confirm the integrity of the different deuterated forms of the DODE, the reflectivity profiles obtained for each of these species spread on nrw were first analyzed in terms of a single uniform layer model, using optical matrix methods.3 The fits to the experimental data were obtained by least-squares optimization, refining the adsorbed layer thickness (τ), its scattering length density (F) and the background scattering (bg). Using the data for the alkyl chain deuterated form of DODE spread on D2O, the numbers of waters associated with the E12 head group (nw) were calculated according to the relation:
∑bL + nw∑bs}/{vL + 30nw}
F){
(1)
where vL and ∑bL are respectively the volume and sum of the atomic scattering lengths for the labeled DODE and ∑bs is the sum of the atomic scattering lengths for D2O (see Table 1). More detailed analyses of the DODE reflectivity profiles were performed according to Simister et al.9 using a direct “model-independent” method based on the kinematic approximation.7 With this approach a description of the structure of the adsorbed surfactant layer was obtained in terms of the relative separations and widths of the distributions of the DODE glycerohydrocarbon region (c), E12 head group region (e), and the associated solvent (s). In this manner, the scattering length density of the system (F) is determined as a function of z, the distance along the normal to the interface, expressed as
F(z) ) bcnc(z) + bene(z) + bsns(z)
(2)
where ni are the number density profiles for each species i, and bi are the corresponding sums of their atomic scattering lengths. With the kinematic approximation, the reflectivity of the layer, R(κ) is thus obtained as
R(κ) ) 16π2/κ2 [bc2hcc + be2hee + bs2hss + 2bcbehce + 2bcbshcs + 2bebshes] (3) where hij are the partial structure factors given by (9) Simister, E. A.; Lee, E. M.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1992, 96, 1373.
Since R(κ) is not determined over a sufficiently wide range of κ, the hii for the adsorbed layer cannot be Fourier transformed to give the Patterson function for the number density, and so the system is described merely in terms of the widths of the distributions (σi). For the following studies, the distribution widths of the glycerohydrocarbon and head group regions were modeled using either Gaussian or uniform layer distributions (eqs 6 and 7, respectively).
ni(z) ) ni exp(-4z2/σi2)
(6)
ni(z) ) ni; -σi/2 < z < σi/2 0; all other z
(7)
The associated solvent was modeled assuming either a uniform layer model (eq 8) or a tanh distribution (eq 9).
ns(z) )
{
0; z < -σs/2 ns1; σs/2 < z < σs/2 ns0; z > σs/2
ns(z) ) ns0[1/2 + 1/2 tanh(z/σs)]
(8)
(9)
where ns0 is the number density for bulk water, ns1 is the number density of the water incorporated in the surfactant layer, and σs is the width of the solvent distribution. The relative positions of the labeled components in the adsorbed layer were determined from the cross-terms presented in eq 4, with
hij(κ) ) Re{ni(κ)nj*(κ) exp[-iκδij]}
(10)
where δij represents the separation between the midplanes of the two distributions i and j. By taking nc(z) and ne(z) as exactly even about their centers, and ns(z) as exactly odd, we thus have
hcs(κ) ) ((hcchss)1/2 sin(κδcs) hes(κ) ) ((heehss)1/2 sin(κδes) hce(κ) ) ((hcchee)1/2 cos(κδce)
(11)
with the constraint that δes ) δcs - δce. Finally, we note that since the reflectivities given by eq 3 are only approximate (and would lead to errors in the partial structure factors)10 corrections are applied, as proposed by Crowley11
R - Rk ) [0.5(1 + {1 - κc2/κ2}1/2)]2(Robs - Rf)/(1 - Rf) (12) where R is the kinematic reflectivity used in eq 3, Rf and (10) Lu, J. R.; Simister, E. A.; Lee, E. M.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1992, 8, 1837. (11) Crowley, T. L. Physica 1993, A195, 354.
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Figure 1. Reflectivity profiles obtained for the various deuterated forms of DODE spread as monolayers on the surface of null reflecting water, at the surface pressures of 28 mN/m (a) and 34 mN/m (b): h2C18dE12 (0), d2C18hE12 (+), d2C18dE12 (3). The lines drawn through the experimental points are those obtained using single uniform layer fits with the parameters presented in Table 2. Momentum transfer is presented in units of Å-1.
Rk are the exact and kinematic reflectivities for a perfectly smooth surface between the two bulk phases, and κc is the momentum transfer at which total reflection would occur for the two bulk phases. In the modeling of the DODE adsorbed layer the widths of the distributions of the surfactant components were calculated in two different ways. The most direct method, applicable for σc and σe , involved the use of plots of ln[hii(κ)] vs κ 2, where the slope is -σi2/8 and the intercept is -2 ln A, where A is the interfacial area occupied by each surfactant molecule. Note here that since A must be the same for the surfactant head group and hydrophobe, the plots for hee(κ) and hcc(κ) should have a common intercept, which should be identical to that obtained in a plot of haa(κ), where (a) signifies the whole surfactant. Separate estimates for σi, and also estimates of the distribution separations δij, were obtained by manual refinement of the values of ni, σi, and δij, optimizing the fits of the resulting curves to the experimental data in plots of κ 2hii(κ) vs κ, or (for the separations) κ2hij(κ) vs κ. Results Optical matrix analyses of the reflectivity data obtained for the three deuterated forms of DODE on nrw (Figure 1) give single uniform layer thicknesses in the range 20-
Barlow et al.
30 Å (Table 2). Since the calculated thicknesses at the two surface pressures are consistently less than the fully extended length of DODE (Table 1), it is apparent that the surfactant molecules are not closely packed and have their alkyl and oxyethylene chains relatively disordered. From the data obtained using d2C18hE12 on D2O (Figure 2), the numbers of solvent molecules associated with the surfactant head groups are estimated as ∼1.6H2O per oxyethylene unit at 28 mN/m and ∼1.0H2O per unit at 34 mN/m. These data compare well with our previous estimates of the level of DODE head group hydration,4,5 and are of the same order as the values calculated for the related single chain nonionic surfactants (in the range 1-2H2O per oxyethylene unit).12 The areas per molecule for DODE, around 125 Å2 at 28 and 116 Å2 at 34 mN/m, can be compared with the estimates made for single chain nonionic surfactants at their cmc, viz., 36 Å2 for C12E3, 48 Å2 for C12E4, and 55 Å2 for C12E6 (ref 13). The integrity of the different deuterated forms of the DODE is confirmed by the fact that the areas per molecule calculated on the basis of the single layer fits (and also from the force area curves, Figure 3) are identical within experimental error, and these results also indicate that any isotope effects on the adsorbed layer structure are negligible at this level of treatment. The same conclusions are reached following the kinematic analyses of the reflectivity data, with common intercepts (of -2 ln A) obtained from the semilogarithmic plots of the partial structure factors hcc, hee, and haa against κ2 (Figure 4). From the slopes of these plots (which give -σi2/8) it can also be seen that the thicknesses of the surfactant head group and hydrophobe regions (respectively, σe and σc) are quite different and that their sum is significantly less than the total adsorbed layer thickness (σa). At 28 mN/m, σe and σc are 19.9 ( 0.4 and 16.3 ( 0.2 Å, respectively, while σa is estimated as 22.0 ( 0.1 Å. At 34 mN/m, the semilogarithmic plots show that the hydrophobe layer remains at roughly the same thickness (16.5 ( 0.2 Å) but that the head group layer thickness is increased to 24.0 ( 0.6 Å. The solvent layer thickness (σs) and further estimates for σe and σc are derived through manual curve fitting to the data plotted as κ2 hii(κ) vs κ (Figure 5). The relative separations of the component distributions are obtained in like fashion from the plots of κ2 hij(κ) vs κ (Figure 5). The results obtained (summarized in Table 3) are consistent with those obtained from the semilogarithmic plots and also with those derived in the optical matrix analyses. Note also that the pairwise separations between the three components of the DODE layer (δec, δes, and δcs) are entirely self-consistent. Discussion and Conclusions In order to utilize these data to provide a detailed physical model of the DODE adsorbed layer, it would be necessary to carry out quite elaborate and extensive molecular dynamics simulations. If we accept a few assumptions and approximations, however, the general construction of the monolayer can be deduced in a much more straightforward manner. Using the strategy detailed by Lu et al.,14 the calculated widths of the Gaussian distributions for the DODE alkyl chains (σc) and head groups (σe) can be taken to be a function of the intrinsic dimensions of these fragments and the overall roughness of the monolayer, ω. Specif(12) Tanford, C.; Nozaki, Y.; Rhode, M. F. J. Phys. Chem. 1977, 81, 1555. (13) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Tucker, I.; Penfold, J. J. Phys. Chem. 1993, 97, 8012. (14) Lu, J. R.; Hromodova, M.; Thomas, R. K.; Penfold, J. Langmuir 1993, 9, 2147.
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Table 2. Structural Parameters for DODE on Null Reflecting Water Assuming Single Uniform Layer Modela surfactant h2C18dE12 d2C18dE12 d2C18hE12
surface pressure (mN/m)
monolayer thickness (Å)
scattering density (Å-2 × 106)
area per molecule (Å2)
background scattering (×106)
28 34 28 34 28 34
23.4 ( 0.3 24.3 ( 0.5 25.0 ( 0.4 26.7 ( 0.4 18.3 ( 0.3 20.5 ( 0.4
1.77 ( 0.02 1.81 ( 0.03 4.15 ( 0.04 4.15 ( 0.04 3.51 ( 0.05 3.28 ( 0.05
125 ( 3 118 ( 2 124 ( 3 116 ( 3 124 ( 4 118 ( 4
4.89 ( 0.10 3.90 ( 0.10 6.28 ( 0.15 6.21 ( 0.18 7.30 ( 0.10 6.06 ( 0.10
a Errors quoted are those calculated from the variance-covariance matrix obtained in the uniform layer least-squares fitting procedure, and in all cases, therefore, are underestimates of the true errors.
Figure 3. Pressure-area isotherms for the various forms of DODE spread as monolayers at the air-water interface: solid line, h2C18hE12; dotted line, d2C18dE12; dashed line, d2C18hE12; dot-dashed line, h2C18dE12.
intrinsic dimensions of these two fragments and is approximated as
δec ) [〈lzc2〉1/2 + 〈lze2〉1/2]/2
(14)
We then further assume that the magnitudes of the 〈lzi2〉 are governed by the conformations and orientations of the fragments such that
〈lzi2〉 ) 〈li2〉 〈cos2 θi〉
Figure 2. Reflectivity profiles obtained for the various forms of DODE spread as monolayers on the surface of D2O, at the surface pressures of 28 mN/m (a) and 34 mN/m (b): h2C18dE12 (0), d2C18hE12 (+), d2C18dE12 (3), h2C18hE12 (O). The lines drawn through the experimental points are those obtained using single uniform layer fits. Momentum transfer is presented in units of Å-1.
ically, the approximation is made that these two contributions combine in quadrature15
σe2 ) 〈lze2〉 + ω2 σc2 ) 〈lzc2〉 + ω2
(13)
where are the means of the squared lengths of the surfactant parts measured along the surface normal. The separation between the head group and alkyl chain distributions (δec) is taken to be dependent only on the (15) Pleshanov, N. K.; Pusenkov, V. M.; Schebetov, A. F.; Peskov, B. G.; Schmelez, G.E.; Siber, E. V.; Soroko, Z. N. Physica B 1994, 198, 27.
(15)
where li are the mean lengths of the fragments and θi are their mean angular displacements from the surface normal. Note here that eq 13 will only hold true if the intrinsic dimensions of the surfactant parts and the monolayer roughness contribute independently to the measured Gaussian distributions, and eq 14 is only true if the distribution separation is independent of the surface roughness. With these provisos, the experimentally determined values for σe, σc, and δec can be used to obtain estimates for 〈lzc2〉, 〈lze2〉, and θi, with the one final approximation that the values of 〈lzi2〉 can be calculated using δec2, which is only really the case if we assume that the surfactant fragments all have the same fixed orientation rather than a distribution. Taking all of these assumptions and approximations as acceptable, the data for the DODE monolayer are calculated as shown in Table 4. If we first take the structure of the monolayer at 28 mN/m, we see that the intrinsic lengths of the alkyl chains and head groups are both quite considerably smaller than the extended lengths of these components, which is consistent with the observations made following the optical matrix treatment of the reflectivity data. Taking lc as the
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Figure 4. The determination of surface coverage and layer thicknesses for the DODE alkyl chains (O), head groups (0), and whole molecule (3), at the surface pressures 28 mN/m (a) and 34 mN/m (b). Momentum transfer (κ) is presented in units of Å-1.
extended length of an octadecyl chain (∼24 Å),16 we obtain θc ) 75°, which indicates that the narrowness of the DODE hydrophobe layer can be accounted for by a very pronounced tilt of the octadecyl chains away from the surface normal. Given that we have not taken into account the glyceryl moeity in these calculations, we can say, therefore, that the octadecyl chains lie virtually flat at the airwater interface. Alternatively, it can be proposed that the alkyl chains are on average oriented perpendicular to the interface but are highly disordered (featuring a high incidence of gauche conformers), even to the extent that they may be folded back toward the interface. In the case of the DODE head groups, if we take le ) 12 × 3.5 ) 42 Å (the extended length of an E12 chain),17 θe is calculated as around 72°. On this basis, therefore, it could be argued that the surfactant head groups are tilted with respect to the surface normal in much the same way as their alkyl chains. Such a description of the head group conformation and orientation seems quite unlikely, however, because the rotational isomeric state computer simulations performed by Sarmoria and Blankschtein,18 have shown that for poly(oxyethylene) chains in water, the polymer scaling law (for the square root dependence of the radius of gyration of a chain on its number of (16) Tanford, C. The Hydrophobic Effect; J. Wiley & Sons: New York, 1973. (17) Rosch, M. In Non-ionic surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1967; Chapter 22, p 753. (18) Sarmoria, C.; Blankschtein, D. J. Phys. Chem. 1992, 96, 1978.
Figure 5. Partial structure factors for the DODE surfactant layer maintained at a surface pressure of 28 mN/m: (a) alkyl chains layer (κ2hcc, O), head group layer (κ2hee, 0), and alkyl chains head group separation (κ2hce, 3; (b) alkyl chain-solvent separation (κ2hcs, 0), and head group-solvent separation (κ2hes, O); (c) solvent (κ2hss, O). In (a) and (b), the curves through the experimental points are calculated according to eqs 6 and 11, using the parameters given in Table 3. In (c), the dashed curve gives the data simulated assuming a tanh distribution for the solvent layer (eq 9), while the solid curve shows the same data simulated assuming a uniform layer (eq 8). The error bars in all cases indicate the standard errors calculated by propagation from the reflectivity data and should thus be taken as underestimates of the true errors. Momentum transfer (κ) is presented in units of Å-1.
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Table 3. Structural Parameters for DODE Adsorbed Layers Calculated from Kinematic Analyses of the Reflectivity Dataa parameter
28 mN/m 34 mN/m
glycerohydrocarbon layer thickness (σc), Å (1) (3) (1) oxyethlyene layer thickness (σe), Å (3) (2) solvent layer thickness (σs), Å (3) hydrophobe-head group layer separation (δce), Å hydrophobe-solvent layer separation (δcs), Å head group-solvent layer separation (δes), Å area per molecule, Å2
16.5 ( 1 18.0 ( 1 20 (1 24.5 ( 1 9(1 19 ( 1 9.5 ( 0.5 12 ( 0.5 2.5 ( 0.5 125 ( 1
17 ( 1 19 ( 1 22 ( 1 24 ( 1 9.5 ( 1 21 ( 1 9.0 ( 0.5 12 ( 0.5 3.0 ( 0.5 116 ( 2
a Layer thicknesses calculated assuming Gaussian distributions (1), tanh distributions (2), or single uniform layers (3). Errors quoted are only approximate and are derived following visual inspection of the fits between the calculated and observed structure factor curves.
Table 4. Intrinsic Fragment Lengths and Surface Roughnesses for the DODE Adsorbed Layers Calculated from Kinematic Analyses of the Reflectivity Data le (Å) lc (Å) ω (Å)
28 mN/m
34 mN/m
12.9 6.1 15.3
14.4 3.7 16.6
oxyethylene units) is applicable for anchored chains with lengths >E10. To obtain a more realistic picture of the DODE monolayer, we thus take le as 12 × 1.3 ) 15.6 Å (i.e., the random coil length of an E12 chain).17 In this case we find that there is much less of a tilt of the head groups, with θe now only 35°. As to the roughness of the DODE monolayer at 28 mN/ m, the calculations show that ω ) 15 Å. This roughness, however, includes contributions from capillary waves (that is, thermal fluctuations) as well as the intrinsic (structural) disorder of the DODE monolayer. The capillary wave contribution can be calculated using the roughness of pure water (6.4 Å), scaled according to the square root of the monolayer surface tension.19 For the DODE monolayer at 28 mN/m, this is 6.4 (73/45)1/2 ) 8.2 Å. We see, therefore, that the capillary waves only account for a proportion of the total roughness and that there is an appreciable static disorder of the surfactant molecules at the interface, with the noncapillary wave roughness of the layer amounting to about 13 Å. For comparison we note that the estimates for the noncapillary wave roughnesses for monolayers involving aerosol OT,20 single chain ionic14 and nonionic21 surfactants are of the order of 5-10 Å, while that for a double-chained sugar-based surfactant22 is close to zero. The DODE monolayer roughness is rather higher than that observed for monolayers involving single chain poly(oxyethylene) sufactants and is more similar to that seen with the monolayers involving charged surfactants, presumably as a consequence of the very long length and high degree of conformational variability of the DODE head group. The volume fraction profiles for the DODE monolayer at 28 mN/m (Figure 6) show that there is an appreciable overlap between the E12 head groups and the glycerohydrocarbon moeities and also that there is a significant penetration of the solvent down into the alkyl chains (19) Schwartz, D. K.; Schlossman, M. L.; Kawamoto, E. H.; Kellogg, G. J.; Pershan, P. J.; Ocko, B. M. Phys. Rev. A 1990, 41, 5687. (20) Li, Z. X.; Lu, J. R.; Thomas, R. K.; Penfold, J. Submitted for publication in J. Phys. Chem. (21) Lu, J. R.; Hromodova, M.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1994, 98, 11519. (22) Cooke, D. J.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Pitt, A. R.; Simister, E. A.; Penfold, J. J. Phys. Chem. 1996, 100, 10928.
Figure 6. Volume fraction profiles for the component parts of DODE at a surface pressure of 28 mN/m: (a) assuming Gaussian distributions for the alkyl chains and head group layers and a tanh distribution for the solvent; (b) assuming single uniform layers for the three components of the adsorbed layer. The alkyl chains distributions are shown by solid lines, the head group distributions by dashed lines, and the solvent distributions by dotted lines. The sum of the three-component volume fractions in (a) is indicated by a dot-dashed curve. z represents the distance along the surface normal, measured in Å, with z ) 0.0 arbitrarily taken as the midpoint of the alkyl chains distribution.
region. Using uniform layer models for the three component distributions (Figure 6b), we calculate that the mixed (2C18/E12) chain region has a thickness of ∼12 Å and that ∼38% of the alkyl chains region is interpenetrated by solvent. It must be noted, however, that the overlaps between the various distributions will include contributions from the monolayer roughness. Although the true overlaps will hence be somewhat smaller than those indicated by the above calculations, it is nevertheless clear that there is a significant mixing of the poly(oxyethylene) and alkyl chains of the surfactant molecules. This is not only consistent with the observations recorded for single chain nonionic surfactants,21 but also in agreement with the miscibility of hydrocarbon and poly(oxyethylene) chains which is suggested by various data on CnEm surfactant micelles.23,24 From the volume fraction profiles we also see that the level of hydration of the E12 head groups of the surfactant (23) Elworthy, P. H.; Patel, M. S. J. Pharm. Pharmacol. 1984, 36, 116. (24) Elworthy, P. H.; Patel, M. S. J. Pharm. Pharmacol. 1984, 36, 565.
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molecules is much lower toward their anchored ends (Figure 6a), and from the uniform layer distributions (Figure 6b) we calculate that the number of the “dehydrated” oxyethylene units is about 1 or 2. It is difficult to know whether these observations have any real significance, however, since the modeling of the solvent layer in these studies is far from ideal: the uniform layer model clearly gives a much better fit to the solvent data than does the tanh profile (Figure 5c), and yet it is the latter that is commonly accepted to be the better model. In the DODE adsorbed layer maintained at a surface pressure of 34 mN/m, the intrinsic dimensions of the alkyl chains and head groups become smaller, in both cases by about 2 Å. For the alkyl chains, this translates to a slight increase in the tilt of the fragment, with θc elevated to 81°, but in view of the assumptions and errors involved in these calculations, this difference is thought not to be significant. In the case of the head groups, if we use le ) 42 Å, we obtain θe ) 70°, and if we use le ) 15.6 Å, we get θe ) 22°. Considering the data obtained at the two surface pressures together, therefore, we can say that if we take le ) 15.6 Å, the effect of the pressure increase is to push the E12 chains closer to the surface normal. The roughness of the DODE monolayer at 34 mN/m is calculated as ∼17 Å and so is slightly increased over that calculated for the lower pressure. The corresponding capillary wave roughness is obtained as 6.44(73/39)1/2 ) 8.8 Å, which again is a lot less than the overall roughness, indicating that the DODE molecules exhibit considerable static disorder even at this higher surface pressure. The volume fraction profiles for the monolayer at this higher pressure (not shown) indicate that the mixed chain region has increased in thickness to around 13 Å, with about 58% of the alkyl chains region interpenetrated by water. The E12 head groups of the surfactant molecules are again found to be less heavily hydrated toward their anchored ends compared with their free endssthe solventdepleted zone again amounting to about 1 or 2 oxyethylene units.
Barlow et al.
Such calculations are only approximate, of course, and in some cases may be regarded as highly presumptive. They do, however, provide a picture of the DODE monolayer structure which is gratifyingly consistent with other findings. In addition to the comments already made, we note, for example, that the figures of ∼38% and ∼58% for the degree of immersion of the DODE alkyl chains in solvent, appear entirely reasonable when compared with the mean value of ∼30% calculated for some single chain nonionic surfactants,10 and the values of 30-40% calculated for cationic surfactants in the series CnTAB.25 By extrapolation from these data, we speculate that further increases in surface pressure are likely to lead to more and more pronounced interchain mixing in the DODE monolayer, and that at some critical value (judged from the force-area isotherm to be around 40 mN/m), some of the surfactant molecules will be extruded onto the air-side of the interface, thus giving rise to islands of “bilayer” adrift in the monolayer. Such events would then be consistent with the observation of a pronounced offspecular reflection developed by the more compressed adsorbed layers.5 Further neutron reflectivity experiments will shortly be carried out to confirm whether these speculations are correct. Acknowledgment. The authors are greatly indebted to Dr. Bob Thomas (Laboratory of Physical & Theoretical Chemistry, Oxford) and offer their most exuberant thanks, firstly, for all of his help given in the performance of the syntheses of the DODE surfactants and, secondly, for his advice and helpful discussions held in the course of preparation of this manuscript. The authors are also very grateful to Dr. Steve Howell at the National Institute of Medical Research, MRC Unit, Mill Hill, for the faciltities and expertise provided in the characterization of the surfactants by means of electrospray MS. LA970082D (25) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. Prog. Colloid Polym. Sci. 1993, 93, 92.
