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Soluble Monolayers of n-Decyl Glucopyranoside and n-Decyl Maltopyranoside. Phase Changes in the Gaseous to the Liquid-Expanded Range Atte J. Kumpulainen,*,† C. Marcus Persson,† Jan Christer Eriksson,† Eric C. Tyrode,† and C. Magnus Johnson‡ Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and The Institute for Surface Chemistry, P.O. Box 5607, SE-100 44 Stockholm, Sweden, and Department of Materials Science and Engineering, Division of Corrosion Science, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received September 3, 2004. In Final Form: October 11, 2004 To examine the transition from the gaseous to the liquid-expanded monolayer state, surface tension data were recorded for n-decyl β-D-glucopyranoside (Glu) and n-decyl β-D-maltopyranoside (Mal) solutions at low concentrations and at different temperatures. Comparisons were also made with n-decyl β-Dthiomaltopyranoside (S-Mal) solutions at room temperature. The transitions observed occur at very low concentrations and surface pressures, about 0.5% of the critical micelle concentration (cmc) and between 0.8 and 1 mN/m for Glu and Mal at 22 °C. For S-Mal the transition is recorded for a concentration of 0.5% of the cmc as well, but the surface pressure is lower, about 0.4 mN/m. The gradual change in molecular area about the transition is from about 500 to 200 Å2 and 400 to 150 Å2 for Mal and Glu, respectively, and from about 800 to 250 Å2 for S-Mal. The comparatively large molecular areas after the transitions are incompatible with the notion that a coherent hydrocarbon film would cover the entire surface already at this stage. Standard surface thermodynamics was applied to elucidate the nature of these transitions in combination with two model concepts: The formation of an infinite network of surfactant molecules and, second, the formation of surface micelles. Hard-disk simulation results were employed to quantify the additional surface pressure after the transition attributed to the formation of surface micelles. In conclusion the formation of surface micelles is plausible as the hard-disk model is capable of accounting for the additional surface pressure increase with acceptable accuracy. Further, vibrational sum frequency spectroscopy was used to investigate the transition for Mal. Using the distinct feature of the non-hydrogenbonded OH (“free OH”) at 3700 cm-1 for probing the surface water state, it could be determined that the surface holds a sizable fraction of unperturbed surface water even after the transition from the Henry range. The decrease in the free OH signal was found to correlate with the increase in surface density of surface micelles.
Introduction As previously reported, anomalous adsorption behavior is seen for mixed n-decyl β-glucopyranoside (Glu) and maltopyranoside (Mal) solutions. Initially, in the liquidexpanded regime the larger Mal headgroup is favored, but as the adsorbed amount increases, the fraction of Mal diminishes, and eventually Mal desorbs from the surface.1 The present report is mainly concerned with the smooth transition from the gaseous state in the Henry region to the liquid-expanded state for the pure Mal, Glu, and n-decyl β-D-thiomaltopyranoside (S-Mal) cases. This transition has been recognized earlier as a first-order phase transition resulting from intermolecular attraction,2,3 but a mechanism involving cluster formation of surfactant molecules in the surface4-6 has also been considered. * Corresponding author: telephone number, +46 8 7909922; fax number, +46 8 208998; e-mail,
[email protected],
[email protected]. † Department of Chemistry, Surface Chemistry, Royal Institute of Technology, and The Institute for Surface Chemistry. ‡ Department of Materials Science and Engineering, Division of Corrosion Science, Royal Institute of Technology. (1) Persson, C. M.; Kumpulainen, A. J.; Eriksson, J. C. Langmuir 2003, 19, 6110. (2) Adamson A. W. Physical Chemistry of Surfaces, 4th ed.; Wiley: New York, 1990. (3) Motomura, K.; Iwanaga, S.-I.; Hayami, Y.; Uryu, S.; Matuura, R. J. Colloid Interface Sci. 1981, 80, 32. (4) Stoeckly, B. Phys. Rev. A 1977, 15, 2558. (5) Smith, T. Adv. Colloid Interface Sci. 1972, 3, 161. (6) Israelachvili, J. N. Langmuir 1994, 10, 3774.
Cluster formation was first proposed by Langmuir7 to account for the nonhorizontal behavior of surface pressure-area isotherms often seen for Langmuir monolayers at transitions. The liquid-expanded to the liquid-condensed transition in Langmuir monolayers has been modeled in terms of formation of dense clusters with the hydrocarbon chains oriented normal to the surface.8-10 A similar concept has also been put forward to account for the transition from the gaseous to the liquid-expanded state for soluble surfactants.11,12 Our approach is principally similar, but the detailed model features are rather different. In the present paper we restrict our investigation to the transition from the Henry’s law range to the liquidexpanded state. Materials and Methods Surface tension was measured with a Kru¨ss K12 tensiometer, employing the Wilhelmy plate method. A platinum plate was used, sand-blasted to ensure a contact angle of 0° at the threephase contact line. The surface tension was calculated from the expression
F ) 2(LT + LW)γ cos θ + LTLW∆Fgh
(1)
(7) Langmuir, I. J. Chem. Phys. 1933, 1, 756. (8) Ruckenstein, E.; Bhakta, A. Langmuir 1994, 10, 2694. (9) Ruckenstein, E.; Li, B. Langmuir 1995, 11, 3510. (10) Ruckenstein, E.; Li, B. Langmuir 1996, 12, 2308. (11) Fainermann, V. B.; Miller, R. Langmuir 1996, 12, 6011. (12) Drach, M.; Rudzinski, W.; Warszynski, P.; Narkiewicz-Michlek, J. Phys. Chem. Chem. Phys. 2001, 3, 5035.
10.1021/la047791t CCC: $30.25 © 2005 American Chemical Society Published on Web 12/09/2004
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where F is the force measured, LT and LW are the thickness and width of the plate, respectively, and γ is the surface tension of the interface. θ is the contact angle at the three-phase line, ∆F the density difference between the liquid and the vapor phase, g the gravitational constant, and h the immersion depth of the plate in the liquid. Every surface tension vs concentration isotherm was recorded at least three times. The temperature was controlled to within (0.2 °C. The water used in the experiments was obtained from a Millipore RiOs-8 and Milli-Q PLUS 185 purification system, finally filtered through a 0.2 µm Millipak filter. The total organic carbon content of the outgoing water was controlled with a Millipore A-10 unit and did not exceed 6 ppb during any of the measurements. n-Decyl β-D-glucopyranoside, n-decyl β-D-maltopyranoside, and n-decyl β-D-thiomaltopyranoside were used as received from Sigma (>98% GC), Anatrace (Anagrade) and Anatrace (Anagrade), respectively. Trials were also made with samples purified in a high-performance surfactant purification unit,13 which removes components with higher surface affinity than the pure surfactant. No difference in surface tension could be detected compared to using the samples as received. Our sum frequency (SF) spectrometer setup has been described in detail elsewhere.14 Briefly, it consists of a Nd:YAG laser (Ekspla, PL2143A/20) with a pulse length of 24 ps, a repetition rate of 20 Hz, an output wavelength of 1064 nm, and an output energy of around 40 mJ. An optical parametric generator/optical parametric amplifier (OPG/OPA) from Laservision is used to produce a visible beam at 532 nm and a tunable infrared beam (710-860 nm, 1.4-12 µm). The bandwidth is 7-9 cm-1 for wavelengths shorter than 5 µm and
