Langmuir 2003, 19, 299-305
299
Surface and Aggregation Behavior of Aqueous Solutions of Ru(II) Metallosurfactants: 2. Adsorbed Films of [Ru(bipy)2(bipy′)][Cl]2 Complexes James Bowers,* Mark J. Danks, and Duncan W. Bruce School of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, United Kingdom
John R. P. Webster ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom Received May 20, 2002. In Final Form: October 22, 2002 Neutron reflectometry has been employed to determine the structure of films adsorbed from aqueous solutions of [Ru(bipy)2(bipy′)][Cl]2 complexes where (bipy′) ) 5,5′-dialkyl-2,2′-bipyridine. Surfactants with alkyl chains with carbon numbers of n ) 13 and 19 have been studied for which both alkyl chains on each molecule have the same chain length. Measurements of solutions with concentrations below and in excess of the critical micelle concentration (cmc) have been made. For the complex with n ) 13, an adsorbed film resides at the solution surface and the maximum surface coverage corresponds to an area per molecule of 100 ( 2 Å2 molecule-1, which is comparable with the expected headgroup area of the molecule. A different situation arises for complexes with n ) 19. Here the measured reflectivity data are extremely time-dependent, with the solutions taking up to 12 h to form an equilibrium surface layer. From the equilibrium measurements, the values determined for the area per molecule indicate that for concentrations greater than the cmc there are approximately two headgroups associated with each monolayer adsorption site at the surface, suggesting that the monolayer is highly corrugated or a more complex surface structure exists.
Introduction In the preceding paper in this issue,1 a small-angle neutron scattering (SANS) study of the aggregation behavior of [Ru(bipy)2(bipy′)][Cl]2 complexes where (bipy′) ) 5,5′-dialkyl-2,2′-bipyridine and both alkyl chains are of the same length was reported. Metallosurfactants such as these are of interest because of their applications in the fabrication of heterogeneous catalysts by templating methodologies2,3 and their potential applications in thinfilm devices.4 In addition to studying the fundamental aspects of adsorption and aggregation of metallosurfactants, the work reported here is driven by a desire to understand the surface behavior of the aqueous solutions of these particular complexes. As was discussed in the preceding paper, the critical micelle concentration (cmc) of these surfactants as determined by surface tension measurement appears to increase with increasing alkyl chain length for chains with carbon number n > 15. However, SANS and conductivity measurements reveal that the cmc does not actually exhibit this behavior as a function of increasing alkyl chain length. This suggests that the surfaces of these aqueous solutions may display some interesting structural features. Such aberrant cmc behavior is reportedly characteristic of gemini surfactants,5,6 and this information has led us to make comparisons between the surface behavior of solutions of the * To whom correspondence should be addressed. (1) Bowers, J.; Danks, M. J.; Bruce, D. W.; Heenan, R. K. Langmuir 2002 (preceding paper in this issue). (2) Jervis, H. B.; Raimondi, M. E.; Raja, R.; Maschmeyer, T.; Seddon, J. M.; Bruce, D. W. Chem. Commun. 1999, 2031. (3) Danks, M. J.; Jervis, H. B.; Nowotny, M.; Zhou, W.; Maschmeyer, T. A.; Bruce, D. W. Catal. Lett. 2002, 82, 95. (4) Chu, B. W.-K.; Yam, V. W.-W. Inorg. Chem. 2001, 40, 3324. (5) See Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906 and references therein.
double-chained metallosurfactants we are investigating here and the surface behavior of solutions of gemini and other double-chained, or dimeric, surfactants. In general, aqueous solutions of gemini surfactants possess several unusual attributes. These include the findings that (i) the cmc versus spacer length dependence displays a maximum at moderate spacer length, and (ii) the cmc versus alkyl chain length behavior exhibits a minimum at reasonably long chain lengths. The former behavior has been considered theoretically in terms of the packing behavior of gemini surfactants as a function of spacer length at the air-solution interface by Diamant and Andelman.7,8 The latter behavior has not, as far as we are aware, received extensive theoretical treatment, but mechanisms such as multilayer adsorption or submicelle formation5,6,9,10 have been postulated in the literature. Menger et al.5,9 have reported that the adsorption of gemini surfactants to surfaces is a slow process and surface tension data have been presented that confirm this. However, use of a perturbative method of surface tension measurement, such as the ring method, is not best suited to the study of kinetically limited adsorption processes because the surface area is constantly changing as the measurement progresses. Furthermore, in support of the adsorption rate argument is the theoretical work of Diamant and Andelman,11 which shows that adsorption of ionic surfactants is generally much slower than the adsorption of nonionic surfactants, and the neutron (6) Rosen, M. J.; Mathias, J. H.; Davenport, L. Langmuir 1999, 15, 7340. (7) Diamant, H.; Andelman, D. Langmuir 1994, 10, 2910. (8) Diamant, H.; Andelman, D. Langmuir 1995, 11, 3605. (9) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (10) Mathias, J. H.; Rosen, M. J.; Davenport, L. Langmuir 2001, 17, 6148. (11) Diamant, H.; Andelman, D. J. Phys. Chem. 1996, 100, 13732.
10.1021/la025968i CCC: $25.00 © 2003 American Chemical Society Published on Web 12/20/2002
300
Langmuir, Vol. 19, No. 2, 2003
reflection experiments by Penfold et al.,12 showing that double-chained cationic surfactants take a considerable time to adsorb when in competition to adsorb with a nonionic CnEm surfactant. Time-dependent behavior was not reported from experiments investigating nonionic double-chained surfactants, e.g., ref 13. The number of reports of structural studies of the surfaces of gemini surfactant solutions in the literature is limited. Menger et al.14 have investigated counterion binding using chemical trapping with a view to relating this to cmc behavior and micelle shape. Li et al.15 have studied a range of gemini surfactant solutions with a view to establishing the nature of the prefactor of the Gibbs adsorption equation, as also discussed by Rosen et al.6 and mentioned by Menger et al.14 Very recently, Li et al.16 have used neutron reflection to examine the structure of monolayers of a range of R,ω-bis(N-alkyldimethylammonium) alkyl bromide gemini surfactants with a variety of spacers with a view to relating their findings to the theoretical treatment of Diamant and Andelman.7,8 Further to these, Maiti and Chowdhury17 have reported Monte Carlo simulations of the air-solution interface of gemini surfactants. With the exception of ref 16, there appear to be no reports confirming multilayer adsorption or submicelle formation by surface-specific techniques, although submicelle formation has been reported from experiments with SANS18 and fluorescence methods.10 In this paper we discuss the results of experiments in which we have used neutron reflectometry to investigate the structure at the air-solution interface of these aqueous surfactant solutions with a view to determining the structural origin of the observed surface tension behavior. Further to this, we seek evidence for possible multilayer or surface micelle formation. It transpires that the kinetics of film formation is the major underlying reason for the surface tension behavior. We note that the adsorbed film structure for the longer chain n ) 19 surfactants is not the same at the air-solution surface as at the interface of the micelle with the solution.1 The presented results will contribute to the general understanding of the behavior of gemini and other double-chained surfactants in solutions for which many unanswered questions invite investigation. Experimental Section Neutron Reflection. Neutron reflectometry is a scattering technique used to determine the neutron refractive index profile normal to the interface n(z), where z is the coordinate normal to the interface.19,20 The refractive index is related to the material composition and hence a composition profile can be determined. In an experiment, the reflectivity, R(Q), is measured as a function of momentum transfer normal to the interface, Q ) 4π sin θ /λ, where θ is the grazing angle of incidence of the neutron beam (12) Penfold, J.; Staples, E.; Tucker, I.; Soubiran, L.; Creeth, A.; Hubbard, J. Phys. Chem. Chem. Phys. 2000, 2, 5230 (13) 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, 10298. (14) Menger, F. M.; Keiper, J. S.; Mbadugha, B. N. A.; Caran, K. L.; Romsted, L. S. Langmuir 2000, 16, 9095. (15) Li, Z. X.; Dong, C. C.; Thomas, R. K. Langmuir 1999, 15, 4392. (16) Li, Z. X.; Dong, C. C.; Wang, J. B.; Thomas, R. K.; Penfold, J. Langmuir 2002, 18, 6614. (17) Maiti, P. K.; Chowdhury, D. J. Chem. Phys. 1998, 109, 5126. (18) Hattori, N.; Hirata, H.; Okabayashi, H.; O’Connor, C. J. Colloid Polym. Sci. 1999, 277, 361. (19) Penfold, J.; Thomas R. K. J. Phys.: Condens. Matter 1990, 2, 1369. (20) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899.
Bowers et al. and λ is the neutron wavelength. The neutron refractive index, n, is wavelength-dependent and for a nonabsorbing, isotropic medium is defined by n ≈ 1 - (λ2/2π)Nb. Here, N is the average number density of nuclei and b is the average bound coherent scattering length per nucleus for the medium of interest. The multiple Nb, the scattering length density, is related in a simple manner to the volume fraction composition:
Nb )
∑φ (Nb) i
i
(1)
i
where φi is the volume fraction and (Nb)i is the scattering length density of component i in the medium. From the measured R(Q) data, a scattering length density profile Nb(z) can be extracted by various means. In this work we have fitted the data using simple layer models and calculated the reflectivity using the optical matrix method21 by means of the program MULF.22 The general model is constituted of n layers, each with a scattering length density Nb and thickness d, and interlayer Gaussian roughness zn,n+1, representing a structurally discrete section of the model. However, one-layer models with included layersubphase roughness, z1B, were found to adequately describe the data, and recourse to two-layer models was not required. Different nuclei possess different scattering lengths,23 and this is no less true for isotopes of the same element. This is particularly exemplified for 1H (H) and 2H (D) (bH ) -3.7406 fm and bD ) 6.671 fm, where 1 fm ) 10-15 m), thus encouraging the use of different contrasts of water or surfactant with tailored refractive indices. The work of Thomas and colleagues24 has demonstrated the powerful ability of neutron reflectometry to determine the structure of adsorbed surfactant layers at surfaces by such means. In this work we have not employed extensive isotopic substitution but have used surfactants with deuterated alkyl chains and water with a refractive index matched to that of air (i.e., null-reflecting water with Nb ) 0 Å-2). Such a contrast scheme enables the adsorbed amount to be readily determined but unfortunately does not permit an overly precise elucidation of the surface structure. For these measurements, the average area per molecule, Apm, can be deduced directly from the scattering length density Nb and thickness d of the surface layer:
∑m b
i i
Apm )
i
dNb
(2)
In eq 2, mi is the number of nuclei i in the surfactant molecule j, and bi is the scattering length of nucleus i. It follows from eq 2 that the adsorbed amount, Γ, of species j can be determined from Γ ) 1/NAApm, where NA is Avogadro’s constant. The reflectivity measurements were conducted on the reflectometer SURF at the ISIS Facility, Rutherford Appleton Laboratory, Didcot, U.K.20 The aqueous solutions resided in poly(tetrafluoroethylene) troughs on the standard CRISP/SURF sample auto-changer. The neutrons, incident through air, were reflected from the air-solution interface and detected by a 3He single detector. SURF is a time-of-flight instrument and the neutrons have wavelengths in the range 0.5 < λ < 6.5 Å. The grazing incidence angle was θ ) 1.5°, yielding a utilizable momentum transfer range of 0.0477 < Q < 0.67 Å-1. The angular resolution was δθ/θ ≈ 8%. Materials. D2O was obtained from Aldrich, >99.9 atom % D, and H2O was treated with an Elgastat ultrapure water system. Null-reflecting water was prepared by mass. Deuterated 1-bromoalkanes [1-bromododecane-d25 (98 atom % D) and 1-bromooctadecane-d37 (98 atom % D)] used in the synthesis of the deuterium-labeled surfactants were purchased from C/D/N Isotopes Inc. and used without further purification. The deu(21) See ref 19 for references and details. (22) MULF neutron reflectivity analysis program by J. Penfold. Rutherford Appleton Laboratory, Didcot, U.K. (23) See, e.g., Sears, V. F. Neutron News 1992, 3, 26, for a compilation of neutron scattering lengths and cross sections. (24) See Lu, J. R.; Thomas, R. K. J. Chem. Soc.: Faraday Trans. 1998, 94, 995 and references therein.
Adsorbed Films of [Ru(bipy)2(bipy′)][Cl]2 Complexes
Langmuir, Vol. 19, No. 2, 2003 301
Table 1. Some Parameters of the Complexes and Aqueous Solutions carbon no. n
scattering length Σimibi (fm)
critical micelle concn cmc (mmol dm-3)
area per molecule Apm (Å2)
chain length ln-1a
13 19
671.66 911.52
0.30b 0.03c
110 ( 20b
16.7 24.3
a Calculated from Tanford’s formula, l max ) 1.540 + 1.265n Å. From surface tension measurements and obtaining the average value from equation 3 using p ) 2 and p ) 3. c From SANS and conductivity measurements.
b
terium-labeled analogues of the ruthenium(II) surfactants were synthesized in a manner identical to that reported in the previous paper in this issue1 except that the dialkylated ligands were further purified by flash chromatography. The CHN data and the surface tension data obtained were consistent with the corresponding data for the nondeuterated surfactants. Table 1 shows some useful parameters for the surfactants and the solutions, including scattering lengths per surfactant molecule and the critical micelle concentrations of the aqueous solutions. In the synthesis, the chain length of the deuterated part of the alkyl chain is n - 1, hence the fully extended length of the n 1 chain ln-1 is tabulated since this value is most directly compared with the neutron-reflectivity-derived film thickness. Solutions of the deuterium-labeled surfactants were prepared by mass in null-reflecting water. The concentrations of the solutions studied are given in Table 2. Nomenclature. As in the preceding paper in this issue,1 Ru52 Cn refers to double-chained surfactants as depicted in Figure 1. Both alkyl chains are of equal chain length with carbon number n. Surface Tension Measurements. Surface tension isotherms were measured by the ring method on a KSV automated tensiometer. A 1-h period was allowed between measurements for different concentrations. The cmc for the Ru52 C19 surfactants has been further determined by conductivity measurements and the value obtained is consistent with SANS data.1 The Gibbs adsorption isotherm for the present systems can be written as
Γ)-
dσ 1 pRT d ln c
(3)
where Γ is the relative excess of surfactant with respect to water, σ is the surface tension, c is the surfactant concentration in the aqueous solution, R is the gas constant, and T is the thermodynamic temperature. For all the measurements reported here, T ) 293 ( 2 K. Equation 3 can be applied in the limit c f cmc for c < cmc and the derived Γ can be used to estimate an average area per molecule. For divalent cationic species, such as those under investigation here, Li et al.15 have recently demonstrated that the prefactor is empirically p ) 2 for the majority of divalent surfactant systems with monovalent counterions. If we apply eq 3 with p ) 2 to our surface tension data for Ru52 C13 surfactants, we determine Apm ) 90 Å2 molecule-1. Unfortunately, we have been unable to measure an entirely reproducible surface tension isotherm and subsequently determine an area per molecule for the adsorbed films of the Ru52 C19 surfactants. Measurements with different batches of surfactant do not, however, yield qualitatively different isotherms. Measuring the surface tension after leaving the sample for extended periods of time prior to the measurement does not make a significant difference. This may be because the surface is perturbed by the change in the surface area caused by pulling the ring through the interface. It is most likely that a noninvasive technique such as capillary-rise25 or surface quasi-elastic light scattering is required.
Results and Discussion Data Analysis. For the isotopic labeling used in the experiments, the determined adsorbed amount is insensi(25) Preliminary capillary-rise experiments confirm this to be the case; the cmc is found to be at c ) 0.03 mmol dm-3 for the n ) 19 surfactants.
tive to the precise surface model used. This originates from the coupling between d and Nb in the optical-matrix calculation. Accordingly, we are unable to determine the scattering length density profile normal to the interface as precisely as is possible with neutron reflection without recourse to extensive isotopic substitution and partial structure factor analysis using the kinematic approximation. However, an analysis of the sensitivity of reflectivity data to film thickness variation shows that the thickness can be determined in most cases to within (3 Å, despite the coupling of d and Nb. We therefore base our discussion on the major results, namely, the determined areas per molecule, but we comment also on the structural model when the model parameters are more reliably determined. Adsorption of Ru52 C13 Surfactants. Reflectivity curves and modeled curves are shown in Figure 2. The concentrations studied were in the range 0.01 e c e 1 mmol dm-3, embracing concentrations both above and below the critical micelle concentration. The resulting area per molecule and adsorbed amount values are displayed in Table 2. The reflectivity data shown in Figure 2 can be modeled with a one-layer model. Although we are unable to determine the thickness of the tail-group region with great precision, the range of possible fitted layer thickness (the lower limit is d ) 12 Å) is not incompatible with a tilting of the alkyl chains, which would enable the tail group and headgroup cross-sectional areas presented to the interface to be roughly equivalent. If present, tilting of the chains and kinks and defects in the chains would not invalidate the results presented here. To represent the data in the framework of a reasonable one-layer (monolayer) model, it appears to be necessary to include roughness on the solution side of the interface. The magnitude of this roughness is z1B ≈ 8 Å. If this roughness is not included in the model, then the thickness of the film is ∼22 Å, which is greater than the length of a fully extended dodecyl chain (l12 ) 16.7 Å). This added roughness most probably originates from the contribution to the scattering length density profile from the headgroup region rather than from a moderate degree of disorder in the adsorbed layer or capillary wave contributions. (This is because the data are not overly sensitive to a small degree of roughness at the air-film interface.) The headgroup has a nominal scattering length density of Nb ≈ 1.85 × 10-6 Å-2 and a not-insignificant diameter of 2r ≈ 10 Å, and replacing the roughness z1B with the inclusion of a second layer representing the headgroup also leads to good representation of the data without significantly affecting the choice of thickness and scattering length density of the outermost layer. However, the quality of the fit is not further improved and we thus settle for the one-layer model with roughness, as that model requires fewer parameters. We note here that, given the headgroup size and the contributions to the roughness from structure and capillary waves, the film thickness could be in excess of the fully extended alkyl chain length. The model with added roughness may, in reality, not significantly differ from the one-layer slab model. In our analysis, though, we have used the fully extended length of the dodecyl chain as the upper value for the film thickness. The area per molecule at different concentrations determined from eq 2 is displayed in Figure 3. The data show that the maximum surface coverage corresponds to Apm ) 100 ( 2 Å2 molecule-1, which is comparable with the expected headgroup area of the molecule, Apm,hg ≈ 95 Å2, and the value determined from surface tension measurements, Apm,σ ≈ 110 ( 20 Å2 molecule-1. This Apm is reasonably constant at concentrations close to and above the cmc, c g 0.3 mmol dm-3. At lower concentrations, c
302
Langmuir, Vol. 19, No. 2, 2003
Bowers et al.
Table 2. Model Parameters and Fitting Results for One-Layer Models concn c (mmol dm-3)
thicknessa,b,c d (Å)
scattering length densityb Nb (10-6 Å-2)
area per moleculec Apm (Å2)
adsorbed amount Γ (10-6 mol m-2)
1 0.5 0.3 0.1 0.05 0.03 0.01
15 ( 3 15 15 15 15 15 15
n ) 13 4.26 4.49 4.36 3.58 3.17 2.70 2.38
105 100 ( 2 103 125 141 166 ( 7 188
1.58 1.67 1.62 1.33 1.18 1.00 0.88
1 0.5 0.3 0.1 0.03 0.02 0.01 0.003
23.5 ( 2 25.8 26.7 ( 1 25.8 22.0 20.6 18.5 14.9
n ) 19 5.19 5.84 6.76 6.48 5.22 4.70 4.59 3.79
75 ( 2 60 51 55 79 94 107 ( 3 161
2.22 2.75 3.29 3.05 2.09 1.76 1.55 1.03
a Typical errors are shown for relevant concentrations and these errors apply thereafter. The quoted thicknesses are in the center of a range of acceptable modeled values for the most suitable one-layer models and are given as indicative values. The model used also includes a roughness on the solution side of the layer, such that z01 ) 0 Å and z1B ) 8 Å; see text for details. b The tabulated scattering length densities are determined from the thicknesses given in the second column. c Area per molecule calculated according to eq 2 ignoring roughness z1B.
Figure 1. Molecular structure of a [Ru(bipy)2(bipy′)][Cl]2 complex where (bipy′) ) 5,5′-dialkyl-2,2′-bipyridine. In this work both alkyl chains have equal length such that m ) n.
e 0.03 mmol dm-3, the area per molecule is approximately double Apm,hg. The measured reflectivity curves do not vary with time and indicate that the adsorbed amount and structure at the surface are stable after a very short time (∼ minutes), unlike, as we shall see, for the Ru52 C19 surfactant solutions Adsorption of Ru52 C19 Surfactants. A very different story emerges for the Ru52 C19 surfactant solutions. The reflectivity curves measured are extremely time-dependent, with the solutions taking up to 12 h to form an unchanging adsorbed layer. The time dependence of adsorption may arise from the low monomer concentrations of the surfactant solutions and hence the adsorption may be diffusion-limited. We shall first discuss the equilibrium structure of the adsorbed films of Ru52 C19 surfactants before returning to discuss the time dependence. The reflectivity curves and model fits to a onelayer model with an added roughness of z1B ) 8 Å are displayed in Figure 4 and the parameters used are given in Table 2. Unlike for the case of the Ru52 C13 surfactants, however, one-layer slab models with zero roughness and with a thickness in excess of the octadecyl chain length
Figure 2. Measured (points) and modeled (lines) reflectivity data for aqueous solutions of Ru52 C13 surfactants with concentrations, from top to bottom, 0.01 (1), 0.03 (4), 0.05 (2), 0.1 (O), 0.3 (cmc, b), 0.5 (0), and 1 mmol dm-3 (9). For clarity, the data and model curves have been shifted relative to each other by an appropriate number of orders of magnitude. The 1 mmol dm-3 data are on an absolute scale.
cannot be used to model the data adequately. The inclusion of a second layer in place of the roughened region does not significantly affect the values of the thickness and scattering length density of the top layer; however, the parameters describing the second layer cannot be reliably obtained from the present data. From the equilibrium measurements, the determined area per molecule values reveal that for a wide concentration range with c g cmc ) 0.03 mmol dm-3, there is on average more than one headgroup associated with each adsorption site on the surface. More specifically, in the concentration region 0.3 g c g 0.03 mmol dm-3, there are approximately two headgroups per site. As a consequence of this small area per molecule determined from the monolayer model, a higher scattering length density of the alkylated region of the surface (see Table 2) compared with the density of the film for the Ru52 C13 surfactants arises. In fact, the density of the tailgroup region for the Ru52 C19 surfactants is very close to that of the pure deuterated n-alkane. The
Adsorbed Films of [Ru(bipy)2(bipy′)][Cl]2 Complexes
Figure 3. Area per adsorbed surfactant molecule at the airsolution interface as a function of concentration for aqueous solutions of Ru52 C13 surfactants (b) and Ru52 C19 surfactants (0). The vertical dashed lines indicate the critical micelle concentrations of the solutions.
Figure 4. Measured (points) and modeled (lines) reflectivity data for aqueous solutions of Ru52 C19 surfactants with concentrations, from top to bottom, 0.003 (3), 0.01 (1), 0.02 (4), 0.03 (cmc, 2), 0.1 (O), 0.3 (b), 0.5 (0), and 1 mmol dm-3 (9). For clarity, the data and model curves have been shifted relative to each other by an appropriate number of orders of magnitude. The 1 mmol dm-3 data are on an absolute scale.
result from the area per molecule for c > cmc suggests that the surface is composed of a corrugated monolayer or a more complex structure. The area per molecule vs concentration behavior is shown in Figure 3. The area per molecule remains constant above the cmc until c > 0.3 mmol dm-3. At this concentration a maximum (minimum) is detected in the determined adsorbed amount (area per molecule). It is interesting that this extremum is mirrored in the concentration dependence of the film thickness; the thickness of the adsorbed film reaches a maximum value corresponding to the maximum in the adsorbed amount (see Table 2). It is noticeable, too, that the integrated reflectivity reaches a maximum at this concentration. It is not known what causes this unusual minimum in the area per molecule vs concentration behavior. Another result that we find difficult to explain is that the film thickness (see Table 2) for c > 0.03 mmol dm-3 (the cmc) appears to be larger than the length of a fully extended octadecyl chain (l18 ) 24.3 Å). Aside from being possibly too large, such a thickness does not allow for any tilting of the alkyl chains in the layer. The
Langmuir, Vol. 19, No. 2, 2003 303
Figure 5. Variation of reflectivity from the air-solution interface as a function of time for an aqueous Ru52 C19 surfactant solution with concentration c ) 0.1 mmol dm-3. From bottom to top, the time at the start of the run elapsed since stirring is 5 min (9), 35 min (b), 70 min (O), 105 min (2), 140 min (4), 165 min (1), 200 min (3), 235 min ([), 345 min (]), 480 min (solid triangle pointing left), and 12 h (open triangle pointing left).
Figure 6. Variation in the adsorbed amount of surfactant at the air-solution interface as a function of time for an aqueous Ru52 C19 surfactant solution with concentration c ) 0.1 mmol dm-3.
magnitude of this film thickness cannot be remedied by extension to two-layer models. This and the area per molecule results perhaps hint that the surface structure cannot be described adequately by these one-layer models and monolayer interpretation. Time Dependence of Adsorption of Ru52 C19 Surfactants. Earlier we mentioned the strong time dependence of the adsorption of the Ru52 C19 surfactants and noted that the equilibrium concentration of the adsorbed layer corresponded to approximately two molecules per nominal monomolecular adsorption site at the surface. Reflectivity data and the adsorbed amount as a function of time are displayed in Figures 5 and 6, respectively, for a mixture with c ) 0.1 mmol dm-3. Analysis of the kinetics of the formation of the surface can be performed with simple arguments based on the Langmuir model of adsorption. This approach is somewhat flawed if multilayer adsorption or strong lateral interactions occur. From the available data we adjudge that adsorption occurs in three kinetic regimes. The first regime is the rapid
304
Langmuir, Vol. 19, No. 2, 2003
Bowers et al.
Figure 7. Analysis of the adsorption of an aqueous Ru52 C19 surfactant solution with concentration c ) 0.1 mmol dm-3 by use of a Langmuir model of monolayer adsorption. See text for details.
formation of a monolayer that is not close-packed. The kinetics within the second time regime can be described by analogy with the Langmuir model of monolayer adsorption. For the present purposes, if we neglect the desorption process and the fact that the model is not necessarily valid for systems in which strong lateral interactions are significant, one can write the rate of adsorption, ratea, as
ratea )
dΓ ) kac(Γsat - Γ) dt
(4)
where c is the solution concentration, Γsat is the equilibrium (saturated) adsorbed amount of the surfactant, and ka is the first-order rate constant for adsorption. In eq 4, c is analogous to pressure and Γ/Γsat is analogous to fractional coverage in the Langmuir model. For c ) 0.1 mmol dm-3, the rate constant is ka ) 5.3 × 10-4 mmol-1 dm3 s-1. Our data conform to such first-order kinetics for t < 4 h (see Figure 7), after which deviations occur, perhaps arising from unaccounted lateral interactions. Unfortunately, we have too few data points at longer times to determine the rate law in this third regime. The change in kinetic behavior from the second to third regimes occurs when Apm ≈ 88 Å2 molecule-1, at which point the monolayer may start becoming significantly corrugated. Adsorption of double-chained surfactants and gemini surfactants appears to be an inherently slow process. Penfold et al.12 have investigated the adsorption from an aqueous solution containing a mixture of distearoyloxydimethylammonium chloride (DISDAC) and hexaethylene monododecyl ether (C12E6) by neutron reflectometry. They find that the amount of DISDAC increases with time whereas the amount of C12E6 at the interface decreases with time, and the equilibration time is in excess of several hours. Menger and co-workers have also reported the slow nature of adsorption of gemini surfactants.5,9 However, Li et al.16 do not report any significantly long equilibration times other than an aging of 2-3 h per sample as a consequence, we presume, of the time taken to perform the reflection measurements, and the cmc values for the surfactant solutions studied are in the range 0.28 e cmc e 1.2 mmol dm-3. Menger and Littau9 report the cmcs of aqueous solutions of dialkylated gemini surfactants with stilbene spacers. The cmcs at 23 °C are 2.7, 1.6, and 1.8 mmol dm-3 for, respectively, surfactants with alkyl chains containing 12,
16, and 20 carbons. The solutions have been aged for 24 h prior to surface tension measurement by the ring method. It is notable that the adsorption is time-dependent and that the cmcs display an apparent anomalous behavior as a function of alkyl chain length. The cmcs of the solutions of the metallosurfactants studied here are 0.3 and 0.03 mmol dm-3 for the solutions of the Ru52 C13 and Ru52 C19 surfactants, respectively, which are at least 1 order of magnitude lower than those of the gemini surfactants studied by Menger and Littau. Penfold et al.12 have found that even at relatively high concentrations of DISDAC (c ) 50 mmol dm-3) compared with the concentration of the C12E6 (0.05 e c e 0.5 mmol dm-3; cmc ) 0.08 mmol dm-3) the DISDAC takes a long time to adsorb to the surface. Accordingly, it is unsurprising that the Ru52 C19 metallosurfactant studied here displays a significant time dependence for adsorption. However, the contrasting fact that the Ru52 C13 surfactant adsorbs relatively rapidly together with the results of Penfold et al.12 and Menger and Littau9 and even Li et al.16 demonstrate that there remain many open questions regarding the time dependence of adsorption of double-chained surfactants. A detailed investigation of the time dependence of adsorption as a function of concentration has not been conducted. With the limited data at hand, however, the equilibration time does not conform to a monotonic function of concentration. For c ) 1 mmol dm-3, the equilibration time teq falls within the range 3.5 < teq < 7 h; for 0.1 > c > 0.02 mmol dm-3, teq ∼ 11-12 h; and for c ) 0.01 mmol dm-3, 6 < teq < 10 h. The longer teq values correspond to solutions with concentrations near the minimum in area per molecule vs concentration plot. At concentrations well below and well above the cmc, the equilibration times are shorter, although they are still long in comparison with the almost instantaneous equilibration of Ru52 C13 surfactant solutions. Molecular Arrangement in the Adsorbed Layer. To explain the results of the surface tension measurements of these solutions, one must turn to a surface explanation for the behavior of the n ) 19 surfactants, given that the SANS measurements1 have shown that, as far as one can tell, premicellar aggregation in the bulk solution is not a suitable model for these particular surfactants. Such surface explanations may include the presence of surface micelles and multilayer formation. Owing to the absence of reflectivity data measured by different refractive index contrasts, we can only speculate here about the arrangement of molecules in the surface region. In doing so we mention models that remain consistent with the data and others that are not. Neutron reflection is able to determine an average scattering length density profile normal to the surface and, therefore, we should also consider other possible molecular arrangements involving structural inhomogeneities in the surface plane. We are currently unable to verify or discount either explanation. In light of recent results in which Li et al.16 report the presence of a sublayer of adsorbed surfactant beneath the monolayer, surface micelles are an intriguing alternative explanation for the effective area per molecule we observe. Accumulation of surfactant molecules, either as aggregates or as an additional (multilayer) adsorption layer, beneath the monolayer may be a possible explanation for the small area per headgroup found for the n ) 19 surfactants for c g cmc. As mentioned earlier, two-layer models can be developed to analyze our data according to such a model, but we do not pursue these models here any further because of insufficient data at different refractive index contrasts. Other feasible models that cannot be discounted
Adsorbed Films of [Ru(bipy)2(bipy′)][Cl]2 Complexes
at this juncture include a corrugated monolayer model or, perhaps, a model in which the headgroups form in two tiers. Such models, as well as multilayering or adsorption of aggregates to the monolayer, are nonetheless in qualitative agreement with the findings of Maiti and Chowdhury from their Monte Carlo simulations.17 For hydrophobic spacers, the headgroups form a monomolecular film at the free surface. However, for hydrophilic spacers, it is found that the headgroups are pulled further into solution and pull the alkyl tails with them. The explanation for this is that the free energy gain caused by the large exchange energy for the insertion of a hydrocarbon chain into aqueous solution is compensated for by two factors. These factors are the reduced free energy associated with the maximized contact of the headgroups with water and the increase in conformational entropy of the hydrocarbon chains at the free surface mediated by the increased contact of the headgroup with water. Finally, we compare the structure at the free solution surface with the structure of the micelle shell for the Ru52 C19 surfactants. The structure at the air-solution interface appears to be significantly different from the structure of the shell of the micelles formed in the solution.1 The area per headgroup at the micelle-solution interface determined from the SANS data is not consistent with the area per molecule determined from neutron reflection measurements.
Langmuir, Vol. 19, No. 2, 2003 305
Summary Neutron reflection has been used to investigate the surfaces of aqueous solutions of [Ru(bipy)3][Cl]2-based double-chained surfactants. For shorter chain lengths, represented in this work by chains with n ) 13, the surfactant adsorption conforms to reasonable expectations. For longer-chain molecules with n ) 19, the structural studies have highlighted the strong time dependence of adsorption. For these longer-chain molecules, the adsorbed film possesses more than one headgroup per nominal adsorption site; for certain concentrations at c > cmc there are two headgroups per nominal adsorption site. Detailed studies of how these complexes pack in the monolayer promise to make interesting future investigations. Acknowledgment. We thank ISIS for the allocation of beam time on the SURF instrument, Johnson Matthey for generous loans of ruthenium salts, the Royal Society for funds to purchase the surface tensiometer, and EPSRC for financial support to M.J.D. We acknowledge helpful discussions with Professor A. Laschewsky of the Universite´ Catholique de Louvain, Belgium. LA025968I
