Structure and Thermal Stability of Dichain Sugar Surfactants at the

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Langmuir 1999, 15, 1817-1828

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Structure and Thermal Stability of Dichain Sugar Surfactants at the Solid/Water Interface Studied by Sum-Frequency Vibrational Spectroscopy Adam M. Briggs, Malkiat S. Johal,*,† and Paul B. Davies* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, U.K. CB2 1EW

Deborah J. Cooke‡ Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, U.K. OX1 3QZ Received September 17, 1998. In Final Form: December 4, 1998 IR-visible sum-frequency spectroscopy (SFS) has been used to study adsorbed monolayers of a homologous series of dichain sugar surfactants, denoted di-(Cn-Glu), in situ at the interface between the bulk aqueous phase and a solid hydrophobic substrate. Sum-frequency (SF) spectra of di-(C6-Glu) demonstrate that the effectiveness and efficiency of adsorption is only marginally affected by temperature up to 95 °C. The line shapes of SF resonances in the C-H stretching region were modeled with use of a refined LevenbergMarquardt fitting program to aid interpretation. Spectra of a partially deuterated d30-di-(C6-Glu) monolayer show that methylene resonances arise solely from the tail groups. The methylene mode amplitudes generally increase with di-(Cn-Glu) tail length due to more gauche defects while the methyl mode amplitudes remain nearly constant. This trend is discussed in terms of statistical considerations and types of gauche defect in dialkyl chains. The dependence of monolayer structure on bulk solution concentration and temperature was elucidated, focusing on the di-(C6-Glu) species. Conformational disorder in the tail group increases with decreasing solution concentration below the critical micelle concentration (cmc). However, even with di-(C6-Glu) at 1/1000 cmc, the terminal methyl groups retain some orientational order. With a saturated di-(C6-Glu) monolayer, spectra at 95 °C are indistinguishable from those recorded at room temperature. Even when adsorbed from a 1/1000 cmc solution (at 20 °C), a partial monolayer of di-(C6-Glu) is robust to 65 °C. Di-(C6-Glu) is typical of the di-(Cn-Glu) series in its concentration and temperature behavior. The exceptional thermal stability of di-(Cn-Glu) monolayers on hydrophobic substrates suggests that they may be suitable candidates for forming temperature-insensitive microemulsions with applications in enhanced oil recovery.

1. Introduction Nonionic surfactants play a key role in the pharmaceutical,1-3 food,4 detergent,5,6 and petrochemical7,8 industries, being widely used in mixtures due to their compatibility with other surfactant types.5,9,10 Obtaining a molecular * To whom correspondence may be addressed (http://pbd.ch. cam.ac.uk/pbd.html). † Current address: Los Alamos National Laboratory, CST-6, Mail Stop J585, Los Alamos, NM 87545. ‡ Current address: Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral L63 3JW, U.K. (1) Gu, X. G.; Schmitt, M.; Hiasa, A.; Nagata, Y.; Ikeda, H.; Sasaki, Y.; Akiyoshi, K.; Sunamoto, J.; Nakamura, H.; Kuribayashi, K.; Shiku, H. Cancer Res. 1998, 58, 3385. (2) Akiyoshi, K.; Sunamoto, J. In Organized solutions: surfactants in science and technology; Friberg, S. E., Lindman, B., Eds.; Marcel Dekker: New York, 1992; p 290. (3) Barlow, D. J.; Ma, G.; Lawrence, M. J.; Webster, J. R. P.; Penfold, J. Langmuir 1995, 11, 3737. (4) Dickinson, E.; Iveson, G.; Tanai, S. In Food Colloids and Polymers: Stability and Mechanical properties; Dickinson, E., Walstra, P., Eds.; Royal Society of Chemistry: Cambridge, U.K., 1993; p 312. (5) Schick, M. J. In Nonionic Surfactants: Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 753. (6) Thompson, L. In Surfactants in lipid Chemistry: Recent Synthetic, Physical, and Biodegradative Studies; Tyman, J. H. P., Ed.; Royal Society of Chemistry: Cambridge, U.K., 1992; p 56. (7) Akstinat, M. H. Tenside Deterg. 1985, 22, 77. (8) Muijs, H. M. In Chemicals in the Oil Industry: Developments and Applications; Ogden, P. H., Ed.; Royal Society of Chemistry: Cambridge, U.K., 1991; p 277. (9) Van Buuren, A. R.; Tieleman, D. P.; De Vlieg, J; Berendsen, H. J. C. Langmuir 1996, 12, 2570.

level understanding of surfactants at buried interfaces,5,11 although experimentally challenging, provides important insights into the development of effective formulations for industrial and household use.5 In this work, sumfrequency spectroscopy (SFS) is used as an interface specific in situ probe of novel nonionic sugar surfactants adsorbed from aqueous solution onto hydrophobic solid substrates. The term “sugar surfactant” has been applied to a broad family of species with closed-ring (mono- or polysaccharide) headgroups12-14 and open-ring monosaccharide headgroups.13-16 These sugar surfactants are related to naturally occurring gangliosides17,18 and are currently of interest because they are biodegradable,19,20 biocompatible,1,3 and can be synthesized using renewable (10) Creeth, A.; Staples, E.; Thompson, L.; Tucker, I.; Penfold, J. J. Chem. Soc., Faraday Trans. 1996, 92, 589. (11) Aveyard, R. Chem. Ind. 1987, 474. (12) So¨derberg, I.; Drummond, C. J.; Furlong, D. N.; Godkin, S.; Matthews, B. Colloids Surf., A 1995, 102, 91. (13) Aveyard, R.; Binks, B. P.; Chen, J.; Esquena, J.; Fletcher, P. D. I.; Buscall, R.; Davies, S. Langmuir 1998, 14, 4699. (14) Zhang, T.; Marchant, R. E. J. Colloid Interface Sci. 1996, 177, 419. (15) Tuzov, I.; Cra¨mer, K.; Pfannemu¨ller, B.; Kreutz, W.; Maganov, S. N. Adv. Mater. 1995, 7, 656. (16) Svenson, S.; Koning, J.; Huhrhop, J. J. Phys. Chem. 1994, 98, 8; 1022. (17) Vie, V.; Van Mau, N.; Lesniewska, E.; Goudonnet, J. P.; Heitz, F.; Le Grimellec, C. Langmuir 1998, 14, 4574. (18) Cheng, Q.; Stevens, R. C. Chem. Phys. Lipids 1997, 87, 4. (19) Matsumara, S.; Imai, K.; Yoshikawa, S; Kawada, K., Uchibori, T. J. Am. Oil. Chem Soc. 1990, 67, 996.

10.1021/la981276b CCC: $18.00 © 1999 American Chemical Society Published on Web 01/28/1999

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resources such as glycerol and hexoses derived from fats and plants.21 They are therefore attractive alternatives to conventional nonionic n-alkyl(poly(ethyleneoxide)) CnEm surfactants.20,22 As the effectiveness of biodegradation is strongly influenced by the surfactant molecular architecture,20 characterization of new saccharide-based species is important with a view to optimizing surfactancy and environmentally benign behavior. The technique of IR-visible SFS has been described extensively in the literature.23-25 It makes use of the nonlinear optical phenomenon of sum-frequency generation (SFG)26 to provide vibrational spectra of molecules at interfaces. Essentially, the interface is irradiated with a fixed frequency visible laser and a tunable IR laser with selected linear polarizations. Spatial and temporal overlap of visible and IR pulses gives rise to emission of light at the sum of the two input frequencies. A vibrational spectrum is obtained by tuning the IR laser since SFG is resonantly enhanced by interfacial molecules when the IR frequency coincides with that of a vibrational mode. Molecules in bulk solution do not contribute to the signal since SFG is forbidden within a centrosymmetric environment in the electric dipole approximation.26 In general, the SF signal consists of a nonresonant contribution27 from the substrate which is virtually frequency independent, upon which resonant features from interfacial molecules are superimposed. In this work a hydrophobed gold surface provides the nonresonant background and with the “ppp” (SF, visible, IR) laser polarization combination28 used here, hydrocarbon resonances appear as either dips or peaks depending upon the net orientation of the functional group. For example, a methyl group pointing “down” toward the substrate would give rise to a dip whereas a methyl group pointing “up” toward the water and away from the substrate would give rise to a peak.29 For convenience, this orientation convention of up and down with respect to the hydrophobic substrate will be used hereafter. The amplitude of an SF resonance depends on the IR and Raman transition dipole moments and is related to the number density and average orientation of the relevant functional group.30 For example, the amplitude ratio of the symmetric (r+) and asymmetric (r-) methyl modes depends on the net tilt of the methyl (CH3) groups.28 Furthermore, methylene (d) modes are generally due to gauche defects in alkyl chains since methylene (CH2) groups are locally symmetric in an all-trans hydrocarbon chain and therefore SF inactive.30 Sensitivity of the d mode amplitude to odd numbers of methylene groups in alltrans chains has not been observed as explained by Johal et al.31 SFS can therefore provide information on the conformation of alkyl chains in surfactant tail groups. The majority of SFS studies have focused on single chain (20) Madsen, T.; Petersen, G.; Sieirø, C.; Tørsløv, J. J. Am. Oil Chem. Soc. 1996, 73, 929. (21) Shinoda, K.; Carlsson A.; Lindman, B. Adv. Colloid Interface Sci. 1996, 64, 253. (22) Aveyard, R.; Binks, B. P.; Lawless, T. A.; Mead, J. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2155. (23) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281. (24) Shen, Y. R. Nature 1989, 337, 519. (25) Eisenthal, K. B. Annu. Rev. Phys. Chem. 1992, 43, 627. (26) Shen, Y. R. Principles of Nonlinear Optics; Wiley: New York, 1984. (27) Guyot-Sionnest, P.; Chen, W.; Shen, Y. R. Phys. Rev. B: Condens. Matter 1986, 33, 8254. (28) Johal, M. S.; Usadi, E. W.; Davies, P. B. J. Chem. Soc., Faraday Trans. 1996, 92, 573. (29) Ward, R. N.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1993, 97, 7; 7141. (30) Ward, R. N.; Duffy, D. C.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1994, 98, 8536. (31) Johal, M. S.; Usadi, E. W.; Davies, P. B. Faraday Discuss. 1996, 104, 231.

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tail groups, and the qualitative effect of chain conformation on methyl and methylene mode amplitudes, denoted I(r) and I(d), respectively, is well documented,30-35,37-38 though rigorous treatment is complex.39 I(d) increases with the population of gauche defects, whereas I(r) decreases since the orientational order of the CH3 groups is reduced.30 In many cases these trends are similar for surfactants with multichain tail groups.31,32,36 These trends are consistent with terminal gauche defects23,40 which are common defects in densely packed monolayers.40,41 The amplitude ratio of the methylene to methyl symmetric stretching modes, I(d+)/I(r+), has consequently been used as a semiquantitative gauge of conformational order in organic monolayers.28,30-34 Values of I(d+)/I(r+) generally decrease as monolayers become more compact33 and dispersion interactions increase. This has been observed by increasing the concentration of surfactant dissolved in bulk solution, thereby driving adsorption toward complete surface coverage31-32,34-36 and with increasing chain length in close-packed monolayers of phosphatydicholine (PC),36 n-alcohol,28,37,42,43 and alkylsiloxane.38 In contrast to these SFS studies, the population of gauche defects is expected to increase with chain length in the extreme case of widely separated surfactants. Considering n-alcohol monolayers, while conformational order increases (at the d-ODT/water interface)43 between C8 and C12 as the monolayers become more solid-like, the defect population was observed to increase for short chains (C3-C8) at the neat liquid/vapor interface.44 Statistical considerations dominated in this latter study, and the fraction of molecules in the all-trans conformation decreased with chain length. In general, the d modes become initially more intense but then diminish as the population of gauche conformers increases. The reduction in I(d+) occurs with increasing disorder as the alkyl chain region approaches orientational isotropy, whereupon all modes are rendered SF inactive.30 A homologous series of dialkyl glucamide sugar surfactants, denoted di-(Cn-Glu), with variable tail length and an invariant open-ring sugar headgroup are studied here. The structure of di-(Cn-Glu) monolayers adsorbed from aqueous solution onto hydrophobic substrates is investigated as a function of solution concentration and temperature. These surfactants, in contrast to their singlechain counterparts (also with amide linking groups),15-17 comprise dichain tail groups and dichain glucamide headgroups. This series has been studied previously by Eastoe et al.45,46 at the air/water interface using surface tension and in bulk aqueous solution using small-angle (32) Johal, M. S.; Ward, R. N.; Davies, P. B. J. Phys. Chem. 1996, 100, 274. (33) Bell, G. R.; Bain, C. D.; Ward, R. N. J. Chem. Soc., Faraday Trans. 1996, 92, 515. (34) Conboy, J. C.; Messmer, M. C.; Richmond, G. L. J. Phys. Chem. B 1997, 101, 6724. (35) Bain, C. D.; Davies, P. B.; Ward, R. N. Langmuir 1994, 10, 2060. (36) Walker, R. A.; Gruetzmacher, J. A.; Richmond, G. L. J. Am. Chem. Soc. 1998, 120, 6991. (37) Johal, M. S., Ph.D. Thesis, University of Cambridge, 1996. (38) Huang, J. Y.; Song, K. J.; Lagoutchev, A.; Vang, P. K.; Chuang, T. J. Langmuir 1997, 13, 58. (39) Hirose, C.; Akamatsu, N.; Domen, K. App. Spectrosc. 1992, 46, 1051. (40) Moller, M. A.; Tildesley, D. J.; Kim, K. S. Quirke, N. J. Chem. Phys. 1991, 94, 8390. (41) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (42) Briggs, A. M.; Usadi, E. W.; Davies, P. B. Unpublished results. (43) Ward, R. N., Ph.D. Thesis, University of Cambridge, 1993. (44) Stanners, C. D.; Du, Q.; Chin, R. P.; Cremer, P.; Somorjai, G. A.; Shen, Y. R. Chem. Phys. Lett. 1995, 232, 407 (45) Eastoe, J.; Rogueda, P.; Howe, A. M.; Pitt, A. R.; Heenan, R. K. Langmuir 1996, 12, 2701. (46) Eastoe, J.; Rogueda, P.; Harrison, B. J.; Howe, A. M.; Pitt, A. R. Langmuir 1994, 10, 4429.

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neutron scattering (SANS) and optical microscopy with 2 H NMR to elucidate the phase behavior. The surfactant di-(C6-Glu) was studied most extensively,46 and di-(C6Glu) monolayers have also been investigated at the air/ water interface by Cooke et al.47 using neutron reflection (NR). Information from surface tension at the air/water interface, particularly values of area per molecule, will be used as a guide for this SFS study of di-(Cn-Glu) surfactants adsorbed at the hydrophobic solid/water interface. Significant structural differences are not expected between monolayers at these two interfaces. The hydrophobic substrate described in section 2 provides an optically flat layer of methyl groups. The attractive interaction of the substrate with surfactant tail groups is therefore weak, arising solely from London dispersion forces. For example, the area per molecule and monolayer thickness derived from NR for the nonionic surfactant C12E4 were found to be very similar at the air/water and hydrophobic solid/water interfaces.48 The surface melting temperature of an n-dodecanol monolayer probed by SFS was found to be identical to within 1.5 °C at the two interfaces.28,49 Surface tension measurements at the air/ water interface have been used previously as a guide for SFS at the hydrophobic solid/water interface.30 A direct comparison of di-(Cn-Glu) monolayers at the two interfaces is further justified by the thermal stability and concentration behavior reported here which is consistent with that observed at the air/water interface, particularly in the case of di-(C6-Glu).45,47 This similar behavior does not, however, prove conclusively that the two monolayers have equivalent structures; hemimicelles may form at the hydrophobic solid/water interface though this is most frequently observed for crystalline substrates.50,51 The intense SF methyl modes observed for the di-(Cn-Glu) surfactants confirm that the sugar surfactants adsorb as monolayers rather than hemimicelles as discussed in section 4.1. 2. Materials The sugar surfactants were provided by Kodak Ltd., Harrow, U.K. The structure of these surfactants, systematically named 7,7-bis[(1,2,3,4,5-pentahydroxyhexanamido)methyl]-n-alkane, is represented in Figure 1 using di-(C6-Glu) as an example. The tail group includes the di-(Cn) alkyl chains but also incorporates two CH2 groups adjacent to the amide groups. The di-(Cn) chain length was varied in the series with n ) pentyl, hexyl, heptyl, octyl; the corresponding surfactants are denoted hereafter by di-(C5-Glu), di-(C6-Glu), di-(C7-Glu), and di-(C8-Glu), respectively. With purification described previously,45,52 surface tension measurements showed sharp curve breaks at the critical micelle concentration (cmc) and mass percentages yielded from CHN microanalysis (Exeter Analytical) were within 2% of those reported by Eastoe et al.45 The di-(C6-Glu) species in particular showed excellent agreement ( 18 MΩ cm). The surfactant solutions were allowed to equilibrate for 30 min in the liquid cell before spectra were recorded. A thin (∼1 µm) film of aqueous surfactant solution was trapped between the hydrophobic substrate and a calcium fluoride prism during spectra acquisition.30 The prism allowed entry of the counterpropagating IR and visible beams onto the sample and exit of the SF beam to the PMT detector.30,31 A ppp polarization combination28 was used to generate maximum SF signal from the gold surface.23 This configuration yielded an optimum signal-to-noise ratio and minimized errors in the resonant amplitudes extracted from modeling line profiles.54,55

4. Results and Discussion 4.1. Interpretation and Modeling of Sugar Surfactant Spectra. Because of its high purity and comple(53) Ong, T. H.; Davies, P. B.; Bain, C. D. Langmuir 1993, 9, 1836. (54) Boggis, S. A. SFG-FIT, version 1.1, University of Cambridge, 1997. (55) Bain, C. D.; Davies, P. B.; Ong, T. H.; Ward, R. N.; Brown, M. A. Langmuir 1991, 7, 1563.

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Figure 2. SF spectra in the CH stretching region of di-(C6Glu) monolayers adsorbed at the d-ODT/water interface from 1.5 cmc solutions. (a) Experimental spectrum (data points) of perprotonated di-(C6-Glu) overlaid with the fitted spectrum (solid plain line). Methyl (r) and methylene (d) modes are identified (see text). (b) Experimental spectrum of tail group perdeuterated d30-di-(C6-Glu). The solid bold line is the interpolation of data points (not shown).

mentary information available from other monolayer studies,45,47 di-(C6-Glu) was used as the primary sugar surfactant. Figure 2 shows the SF spectra in the C-H stretching region of di-(C6-Glu) and d30-di-(C6-Glu) monolayers adsorbed from aqueous micellar solution. The resonant features (Figure 2a) arising from di-(C6-Glu) have been previously assigned to methyl (r) and methylene (d) stretching modes.30 The appearance of r modes as dips shows that the surfactant tail groups are pointing down toward the d-ODT substrate as expected. The dips at ∼2874 and ∼2934 cm-1 arise from the symmetric methyl stretch (r+), which is split by a Fermi resonance. The asymmetric methyl stretch (r-) lies at ∼2964 cm-1. The dip at ∼2852 cm-1 and the broad feature, 2900-2934 cm-1, is the Fermi resonance pair of the symmetric methylene stretch (d+), the latter dominating over any minor contribution from the highly dispersive30 asymmetric methylene stretch (d-). Comparison of the SF spectrum in Figure 2a with other SF spectra in which d-ODT/Au substrates have been used shows that di-(C6-Glu) surfactants are not adsorbed as hemimicelles. The resonant signals (dips) from the r+ modes at resonance are ∼30% of the nonresonant signals. This compares well to room-temperature spectra of didodecyldimethylammonium bromide (DDAB)31 and noctanol43 and spectra of n-decanol42 and n-dodecanol28 just above their phase transition temperatures.56 These four monolayers have similar values for I(d+)/I(r+) and alkyl chain density to those of di-(Cn-Glu) species (section 4.2). Neither DDAB nor n-alcohol molecules form curved micellar aggregates in water; DDAB forms a lamellar structure,31 while n-alcohols do not show association colloid behavior in the strictest sense.57 For a hemimicelle, the net downward orientation of the CH3 groups would be much less than that of a simple monolayer (where all molecules have essentially equivalent orientations) since a large proportion of CH3 groups in a hemimicelle lie almost parallel to the interfacial plane. Oil-like behavior in the (56) Braun, R.; Casson, B. D.; Bain, C. D.; Chem. Phys. Lett. 1995, 245, 326. (57) Laughlin, R. G. J. Soc. Cosmet. Chem. 1981, 32, 371.

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alkyl core and the fact that the central region of a micelle can only accommodate a small proportion of chain termini58 would reduce the net orientation of CH3 groups still further. The value of I(r+) arising from hemimicelles would therefore be equivalent to that arising from a simple monolayer at a very much lower coverage. However, the di-(C6-Glu) spectrum (Figure 2a) is comparable to that observed for a complete monolayer, rather than a submonolayer, of DDAB.31 The strong orientational order of the di-(Cn-Glu) species at very low surface coverage (section 4.3) is also inconsistent with an S-shaped adsorption isotherm of hemimicelle formation, proceeding via an SF inactive step in which the alkyl chains lie flat against the substrate.50,51 Attempts to probe hemimicelles of SDS on bare annealed Au(111)50 by SFS showed that the resonances were too weak to be discerned,42 in contrast to a simple monolayer of SDS adsorbed on d-ODT.35 SF spectra of d30-di-(C6-Glu) recorded above the cmc (Figure 2b) and below the cmc (not shown) were essentially featureless. The spectral region between 2928 and 2940 cm-1 lying marginally above the baseline (Figure 2b) could not be attributed to a molecular resonance given the noise level. However, a contribution from the protonated d30di-(C6-Glu) headgroup in this spectral region is feasible since a mode (unassigned) was observed33 at 2927 cm-1 in the ssp spectrum of C12-maltoside with a similar hydrophilic section. Adsorption of d30-di-(C6-Glu) on the d-ODT substrate was confirmed in an SFS control experiment using a 1:1 mixture of d30-di-(C6-Glu) and di(C6-Glu). The mode amplitudes in the C-H stretching region derived from an SF spectrum of this mixture (not shown) were ∼50% of those from a pure perprotonated di-(C6-Glu) solution of equivalent total concentration, showing that isotope effects were insignificant. The featureless d30-di-(C6-Glu) spectrum (Figure 2b) therefore demonstrates that the two CH2 groups near the headgroup termini (and also the CHOH groups) are SF inactive within the limits imposed by the signal-to-noise ratio. These hydrocarbon groups are therefore randomly oriented or alternatively, almost parallel to the interfacial plane.30,31 From this result, it is clear that the d modes observed for (protonated) di-(C6-Glu) arise entirely from the tail group. Although contributions to the spectra might be expected from CH2 groups adjacent to the amide groups (these CH2 groups being part of the tail group), they would probably give rise to peaks considering their orientation. No such peaks were observed, and it is likely that these CH2 groups are nearly parallel to the surface and, at best, only weakly SF active with our ppp polarization combination. Any contribution to the d modes would be canceled by resonances from gauche defects in the di-(Cn) chains which appear as dips. The amplitude ratio I(d+)/I(r+) is, therefore, used here as a qualitative gauge of conformational order in the di-(Cn) region of the monolayer. The SF resonances were modeled using a line fitting program,54 based on the Levenberg-Marquardt method.59 Strictly, the I(d) and I(r) values extracted from modeling are resonant amplitudes,38 rather than intensities (amplitudes squared), but conventional notation28,30-34 will be maintained in this work. Values of I(d) and I(r) used here are normalized to the nonresonant amplitude, also derived from modeling. The resonances were fitted with Voigt profiles and interference with the nonresonant background accounted for by a relative phase,55 thereby (58) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1985; p 251. (59) Press, W. H.; Teukolsky, S. A.; William, T. V.; Flannery, B. P. Numerical Recipes in C: the art of scientific computing, 2nd ed.; Cambridge University Press: Cambridge, U.K., 1992; p 683.

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giving the resonances a common effective second-order susceptibility.39 The simulation approaches the statistical minimum by iteration, starting from estimated input values for resonant wavenumber, amplitude, Gaussian hwhm (σ), relative phase, and nonresonant amplitude, which were all allowed to float while the Lorentzian hwhm (Γ) was fixed53 at 2 cm-1. This procedure follows earlier semiquantitative methods,28,55 but fitting is less subjective and readily converges toward the global statistical minimum, even for input parameters that give poor initial fits. The accuracy of the Voigt function is maintained by the use of two different numerical integration methods for the complex error function55 depending on σ and proximity to the resonant wavenumber. This approach60 allows both the narrow r modes and the broad d modes to be fitted without introducing spurious oscillations in the latter. Spectra were fitted several times with different input parameters to explore the influence of possible resonant wavenumber and Gaussian line width on the mode amplitudes. The spread of amplitude values from the lowest variance fits was then taken to be the modeling error. The output parameters derived from fitting the di-(C6Glu) spectrum in Figure 2 are typical of all the sugar surfactants in terms of the relative phase, line width, and wavenumber of the five assigned resonances. Modeling all the spectra of saturated di-(Cn-Glu) monolayers gives the following values: the relative phase is 90° ( 5°, the d+ mode is centered at 2852.5 ( 2.5 cm-1 with σ ) 8.4 ( 0.7 cm-1, and the r+ wavenumber is 2974 ( 1.5 cm-1 with σ ranging from 2.6 to 4.0 cm-1 and having a mean value of 3.1 cm-1. These resonant line widths and wavenumbers were not found to vary significantly between sugar surfactants in the series, and a common procedure was adopted for baseline correction. The d+FR, r+FR, and rmodes have greater uncertainties in their line widths and wavenumbers and consequently greater error in their resonant amplitudes. The comparison of mode amplitudes extracted from modeling different spectra therefore focuses on the d+ and r+ modes which can be used to gauge conformational order (section 1). 4.2. Spectra as a Function of Tail Length. Figure 3 shows SF spectra in the C-H stretching region of the four sugar surfactants adsorbed as saturated monolayers at the d-ODT/water interface. Spectra of di-(C5-Glu) and di-(C6-Glu) were recorded at a temperature of 22 ( 2 °C, while di-(C7-Glu) and di-(C8-Glu) were recorded at 36 ( 0.2 °C, so as to be above their Krafft points (∼34 °C). Krafft points were not observed for the shorter chain species,45 and therefore, the spectra in Figure 3 are all of monolayers in equilibrium with their micellar solutions. This difference in temperature is in any case unimportant for comparative purposes since the spectra at 20 and 36 °C are indistinguishable for all surfactants. Direct comparison of SF spectra from different surfactants at full monolayer coverage relies upon the headgroup being SF inactive throughout the series. This seems likely since the area per molecule at the air/water interface, as determined from the surface tension measurements of Eastoe et al.,45 was found to be nearly constant with increasing tail length. This suggests that the headgroups, in contact with the aqueous phase, are conformationally identical throughout the di-(Cn-Glu) series. Any trends in the SF mode amplitudes and amplitude ratios for the di-(Cn-Glu) series (Figure 3) must take into account spectral reproducibility and the inherent errors of line shape modeling (Figure 4a,b). Experimentally, differences in I(d+)/I(r+) between spectra of the same surfactant using different solution concentrations (1-5

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Figure 3. SF spectra of saturated monolayers of the homologous series of sugar surfactants, di-(Cn-Glu). Experimental spectra (data points) are overlaid with fitted spectra (lines): (a) di-(C5-Glu); (b) di-(C6-Glu); (c) di-(C7-Glu); (d) di-(C8-Glu).

cmc) were insignificant as expected. Variations in I(d+)/ I(r+) values from spectra using different d-ODT substrates were similar to the modeling errors associated with individual spectra. This is highlighted in Figure 4b with I(d+)/I(r+) values from spectra using different substrates and solution concentrations (all above the cmc). Baseline correction of raw experimental data and subsequent modeling yields errors in the resonant amplitudes which arise mainly through alternative combinations of mode line width and amplitude. The error bars in Figure 4b show the spread of results obtained by repeated modeling of all three spectra for each surfactant. The I(d+)/I(r+) modeling error associated with an individual spectrum is typically (0.1 (∼10%). The principle conclusion drawn from Figure 4b is that di-(C5-Glu) and di-(C6-Glu) have lower I(d+)/I(r+) ratios than either di-(C7-Glu) or di-(C8Glu) and that differences within these two pairs are not significant. Moreover, I(d+) is observed to increase throughout the series while I(r+) is approximately constant (Figure 4a). The interpretation of I(d+)/I(r+) as a function of tail length is nontrivial. First, the density of the alkyl chains in the surfactant monolayers determines the relative influence of dispersion forces and statistical considerations44 (section 1) on the defect population. Second, there are important differences between single chain and multichain tail groups with regard to the relationship between packing density and population of gauche defects. These two points will be considered in turn. I(d+)/I(r+) values (with errors where known) from some SFS studies of surfactants are presented in Table 1 for single-chain (Cn), dichain (di-(Cn)), and trichain (tri-(Cn))

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Briggs et al. Table 2. Experimental Area per Molecule for Saturated Di-(Cn-Glu) Monolayers and Calculated Tail Widths As Illustrated in Figure 5b for di-(C6-Glu)

Figure 4. Amplitude and amplitude ratio of d+ and r+ modes as a function of tail length in di-(Cn-Glu). Error bars show the range of values resulting from fits having variances within 3% of the minimum obtained and the inherent error of modeling: (a) mean average of d+ (0) and r+ (4) mode amplitudes derived from fitting three spectra for each surfactant (see text), interpolated with bold and plain lines respectively; (b) individual I(d+)/I(r+) values for each surfactant spectra. Solid circles (b) correspond to modeled spectra in Figure 2, open circles (O) refer to spectra not presented, and the half-filled circle (Y) corresponds to the modeled spectrum in Figure 2a. Table 1. I(d+)/I(r+) Values from SFS of Monolayers with Increasing Tail Length alkyl chain length (Cn) in surfactant monolayer

Experimental Details

di-(C12) PC (DLPC)36 di-(C14) PC (DMPC)36 di-(C18) PC (DSPC)36 C8 siloxane38 C18 siloxane38 C10 n-alcohol42 C12 n-alcohol28

vapor/H2O vapor/H2O vapor/H2O SiO2/air SiO2/air d-ODT/H2O; C12 for PC monolayers, serve mainly to reduce the g(d) population in the close-packed chains, increasing the orientational order of the CH3 groups still further. The overall change in the defect population with dichain tail length is therefore difficult to predict since greater chain alignment due to more g(a) defects can reduce the g(d) population. The extreme case of the long di-(C18) chain has been simulated for a solid-like monolayer of DODAC (dioctadecyldimethylammonium chloride).61 It is reasonable to assume a near tetrahedral arrangement at the quaternary nitrogen of DODAC (CNC angles 108-112°)65 and also at (62) Stark, J. G.; Wallace, H. G. Chemistry Data Book, SI Edition; John Murray: London, 1971; p 31. (63) Taylor, D. M.; Dong, Y.; Jones, C. C. Thin Solid Films 1996, 285, 130. (64) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800. (65) Okuyama, K.; Soboi, Y.; Iijima, N.; Hirabayashi, K.; Kunitake, T.; Kajiyama, T. Bull. Chem. Soc. Jpn. 1988, 61, 1485.

1824 Langmuir, Vol. 15, No. 5, 1999

Cq (Figure 5b) for di-(Cn-Glu) species,47 and in this sense the tail-groups are comparable except in length. X-ray diffraction shows that stable packing in crystalline dioctadecyldimethylammonium bromide monohydrate is achieved by a (g+g+t g+g+) conformation next to the quaternary nitrogen atom in one of the di-(C18) chains.65 This defect, which results in close packed di-(Cn) chains, oriented parallel to each other, was incorporated into the simulated DODAC monolayer and found to exhibit high temporal stability, even when the packing density was reduced.61 While the (g+g+t g+g+) sequence of DODAC would be relatively short-lived for the di-(Cn-Glu) species with Cn < C9, the (t g( t) defect of Figure 5b3 demonstrates that close packing can be achieved for less complex defects near Cq. In contrast to the case of the DODAC monolayer, the term methyl-aligning defect is used loosely for di-(Cn-Glu) species and is not limited to the case where the chain termini are perfectly parallel. After discussion of the various factors determining conformational order as a function of chain length, it is not surprising that the change in I(d+)/I(r+) is relatively minor for the di-(Cn-Glu) series compared to monolayers of longer, close-packed surfactants or that the I(d+) values increase without a decrease in I(r+). It seems likely that there is a net increase in g(a) defects due to both statistics and the fact that the tail groups become more spatially constrained with chain length. The change in g(d) population should be relatively small because increases due to statistics would be canceled by decreases associated with methyl alignment. The SFS studies of PC and di-(CnGlu) monolayers suggest a “crossover” alkyl chain length in the region di-(C8) to di-(C12), beyond which dichain alignment (i.e. the g(a) population) is largely unchanged and dispersion forces dominate with I(d+)/I(r+) decreasing due to reductions in the g(d) population. In practice, however, poor solubility45 in the di-(Cn-Glu) series beyond di-(C8-Glu) limits surfactant adsorption. More SFS studies are required to provide a complete picture of dichain conformations and determine the influence of various headgroups. Precise information on the nature and location of gauche defects in the di-(Cn-Glu) series is difficult to deduce a priori. SF spectra with selectively deuterated methylene groups would aid interpretation but would not, even then, provide an unequivocal picture of conformational order. For example, denser packing (with invariant CH3 orientation) can be achieved via (g+t g-) kinks, but these sequences are SF inactive due to local centrosymmetry.23 Furthermore, it is likely that quantitative comparisons within the series are complicated by orientational change. The combined results of NR47 and SFS studies for di-(C6Glu) above and below the cmc (section 4.3) suggest that the tail group becomes more tilted with chain length, and this is manifest in the decreasing I(r+)/I(r-) ratio (Figure 3). While the decrease in the symmetric mode amplitudes, d+ and r+, due to tilt should be similar, it is likely that the latter is canceled by an increase in I(r+) due to the alignment of disordered methyl groups. Although the tail-group region of the di-(C6-Glu) monolayer is somewhat below the close-packed state, the di-(C6-Glu) monolayer was found to be remarkably smooth as probed by NR.47 This may be linked to close packing in the headgroup region of the monolayer and hydrogen bonding between glucamide chains. Considering the molecular architecture of the glucamide headgroup (Figure 1) which includes hydroxyl and peptide groups on a di(C8) skeleton, close packing is likely in view of the experimental values of area per molecule45 (Table 2). In parallel to monolayer studies, di-(Cn-Glu) micelles were

Briggs et al.

Figure 6. SF spectra of di-(C6-Glu) as a function of solution concentration. Experimental spectra (data points) have been overlaid with fitted spectra (lines): (a) 2 cmc; (b) 1/10 cmc; (c) 1/ 1 100 cmc; (d) /1000 cmc.

investigated by SANS and found to change with tail length from short rods for di-(C5-Glu) to long cylinders in di(C6-Glu) and di-(C7-Glu) to disks in di-(C8-Glu).45 These changes are not in accordance with the relatively constant area per molecule observed at the air/water interface (Table 2). Monolayer curvature11 is, however, a distinctive factor, and conformational change in the headgroups may play a role in micellar aggregates.45 4.3. SFS as a Function of Solution Concentration. Di-(C6-Glu) monolayers were studied as a function of solution concentration. The SF spectra in Figure 6 were recorded sequentially with increasing concentration (to maintain accuracy) using solutions prepared from a 2 cmc stock solution. However, after discussion of the SF spectra of saturated monolayers above the cmc (section 4.2), the series will now be considered as if the solutions had been repeatedly diluted. The general trend is one in which the resonances gradually weaken as the solution concentration decreases (Figure 6). At 1/10 000 cmc (not shown), any residual resonances were within the noise level (∼4%), indicating negligible adsorption and/or orientational isotropy. If conformational and orientational order are invariant with decreasing surface coverage of an adsorbed species, then I(r+) is proportional to the number density of that adsorbate, as shown for di-(C6-Glu) in the 1:1 di(C6-Glu)/d30-di-(C6-Glu) mixture (section 4.1). Packing density at the hydrophobic solid/water interface increases with solution concentration, and generally monolayer saturation is reached in the vicinity of the cmc.66,67 SFS spectra of SDS, for example, are virtually unchanged with solution concentrations above 0.5 cmc at the water/dODT35 and CCl4/water68 interfaces. The SF resonances (66) Yao, J. H.; Strauss, G. Langmuir 1992, 8, 9, 2274. (67) Greenwood, F. G.; Parfitt, G. D.; Picton, N. H. Wharton, D. G. In Adsorption from Aqueous Solution. Advances in Chemistry Series 79; Weber, W. J., Mtejevic, E., Eds.; American Chemical Society: Washington, DC, 1968; p 135.

Dichain Sugar Surfactants

Figure 7. Amplitude and amplitude ratio of d+ and r+ modes as a function of di-(C6-Glu) solution concentration (log scale). Values are derived from lowest variance fits of Figure 6. Data points (with error bars due to modeling) are interpolated, assuming constant mode amplitudes above the cmc. (a) Amplitude of d+ (0) and r+ (4) modes, interpolated with bold and plain lines, respectively. Extrapolation to 1/10 000 cmc yields values of