Langmuir 1997, 13, 6985-6994
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Temperature Independent Adsorbate Structure of 4-Octyl-, 4-Decyl-, and 4-Dodecylbenzenesulfonates at the Al2O3/ Water Interface R. P. Sperline* and Y. Song Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received June 24, 1997. In Final Form: October 8, 1997X In situ Fourier transform infrared (FTIR)-attenuated total reflection (ATR) spectroscopy and linear dichroism (LD) analysis were used to characterize the Gibbs’ surface excess (Γ) and molecular orientation (Θ, mean tilt from the interface normal) of sodium 4-n-octylbenzenesulfonate (SOBS), sodium 4-ndecylbenzenesulfonate (SDeBS), and purified sodium 4-n-dodecylbenzenesulfonate (SDoBS) adsorbed at the Al2O3/water interface. The Γ values were confirmed by the solution depletion method. The Θ values were determined separately for the phenyl rings, alkyl chains, and SO3- head groups. Thin films (150220 nm) of Al2O3 on the surface of ZnSe IR internal reflection elements allowed in situ IR ATR adsorption measurements on a model hydrophilic solid from the near IR to 950 cm-1. Both Γ and Θ for SOBS and SDeBS were insensitive to ionic strength, but were slightly sensitive to pH and temperature. The crystal structure of SOBS‚1/2H2O was determined by single-crystal X-ray diffraction and compared with polarized single-crystal transmission and IR ATR spectroscopy to assist in assignment of IR bands at 1010, 1036, and 1125 cm-1 to modes with transition moments parallel to the 1,4-axis of the phenyl ring (“axial” bands). Temperature-dependent transmission IR spectra of slurries of SOBS and SDeBS were obtained; crystalline hemihydrates of SOBS, SDeBS, and SDoBS were identified, along with monohydrates of SOBS and SDeBS. The Γ, Θ, and band positions in temperature-dependent IR ATR showed no phase changes associated with the onset of alkyl chain motion at temperatures as much as 40 and 45 °C below the Krafft temperature, in the case of SDeBs and SDoBs, respectively; the IR spectra indicated that the environments of both the polar and nonpolar moieties of the surfactants closely resembled those in micelles at all temperatures examined. The IR bands associated with the crystalline surfactants were not observed in adsorbed layers of these surfactants. Absorptivities of the νas(SO3-) bands and ν(CH2) bands were shown to be reliable for the determination of Γ in in situ IR ATR, but the absorptivities of the axial bands decreased upon adsorption, making them unreliable for this purpose.
Introduction Our previous papers began an IR spectral examination of surfactant aggregation at lower pH,1 including a study of adsorbed sodium dodecyl sulfate (SDS) at the Al2O3/ aqueous solution interface. Adsorbed SDS presented IR spectra more closely resembling those of micelles than of either liquid crystals or crystals.2 Given the low Krafft temperature (TK) of SDS (15 °C), it was perhaps not possible to lower the temperature (T) sufficiently to cause the “freezing” of alkyl chain conformational motion. Longer chain surfactants would be required to raise the TK and allow the possibility of chain freezing above 0 °C. A change from crystalline order to micelle-like molecular motions should be observed above some critical T at which the chains “melt”. This T should be roughly correlated with TK,3,4 or the critical micellization temperature (CMT). Several groups have examined the IR spectra of n-dodecyl chains in SDS phases and solutions at various values of T.5-9 In transmission IR spectra of SDS, changes across the CMT (∼9-15 °C) indicate that a transition from * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 15, 1997. (1) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1992, 8, 21832191. (2) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1997, 13, 37273732. (3) Lindman, B.; Wennerstrom, H. In Solution Behavior of Surfactants; Mittal, K. L.; Fendler, E. J., Eds.; Plenum: New York, 1982; Vol. 2, pp 3-26. (4) Johansson, A.; Lindman, B. In Liquid Crystals and Plastic Crystals; Ellis Horwood: Chichester, England, 1974; Vol. 2, p 192. (5) Mantsch, H. H.; Kartha, V. B.; Cameron, D. G. FT-IR Studies of Aqueous Surfactants: The Temperature Induced Micelle Formation. In Surfactants in Solution; Mittal, K. L.; Lindman, B., Eds.; Plenum: New York, 1983; Vol. 1, pp 673-690.
S0743-7463(97)00667-7 CCC: $14.00
crystalline structure to micelle structure is accompanied by loss of ordering in the alkyl chains.5,7,8 Free rotation about the director does occur in the alkyl of adsorbed SDS, and despite this motion a solid-like tilt (Θ) of the alkyl chains persists in situ, at least at 28 °C.1,10 The Θ is the angle between the average extended all-trans alkyl chain and the surface normal. This behavior is reminiscent of that of liquid crystals. Chain motion around the long axis (molecular “director”) of an all-trans alkyl chain occurs in adsorbed commercial-grade laurylbenzenesulfonate (SLaBS).10 Octyl-, decyl-, and dodecylbenzenesulfonate surfactants with a wide range of TK were studied, and the influences of pH, T, and ionic strength on the mean orientations (Θ, mean tilt angle to the normal of the plane adsorption) of the alkyl chains and benzene rings and on the Gibbs’ surface excesses (Γ) were examined by in situ IR ATR. The question was addressed of whether the presence of a benzene ring increases the likelihood of chain freezing between TK and 0 °C, leading to an observable phase change in adsorbed surfactant. In addition, the structure of SOBS‚1/2H2O was determined by X-ray diffraction for comparison with the structures of crystalline SDS and with the structures of adsorbed SDS and the alkylbenzenesulfonates. (6) Kartha, V. B.; Leitch, L. C.; Mantsch, H. H. Can. J. Chem. 1984, 62, 128-132. (7) Cameron, D. G.; Umemura, J.; Wong, P. T. T.; Mantsch, H. H. Colloids Surf. 1982, 4, 131-145. (8) Weers, J. G.; Scheuing, D. R. In FTIR Spectroscopy in Colloid and Interface Science; Scheuing, D. R., Ed.; American Chemical Society: Washington, D.C., 1990; Vol. ACS Symp. Ser. 447, pp 87-122. (9) Cross, W. M.; Kellar, J. J.; Miller, J. D. Preprints, XVII Int. Miner Process. Cong.; Dresden, Sept 23-28, 1991; 1991, 319-338. (10) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1994, 10, 37-44.
© 1997 American Chemical Society
6986 Langmuir, Vol. 13, No. 26, 1997
Sperline and Song
Experimental Section Materials. SOBS. Sodium 4-n-octylbenzenesulfonate [SOBS, Aldrich Chemical Company, #28,748-2; CAS Registry No. 614903-7 (supplied by author).] was recrystallized twice from hot water by cooling. Thin tabular crystals of SOBS hemihydrate, precipitated by evaporation from solution at 24 °C, were separated by suction filtration and were dried in air. Crystals grown by evaporation at 5 °C had the same habit. Crystals with the same IR spectrum were also obtained by cooling a 20-25 wt % solution in water from 30 °C to room temperature (RT). Distilled water was used throughout; other reagents were reagent grade. Solutions were prepared by previously published methods.1,10,11 SDeBS. Pure sodium p-(n-decyl)benzenesulfonate (SDeBS) was synthesized by sulfonation of n-decylbenzene [1-phenyldecane, Aldrich, 11,321-2; CAS Registry No. 104-72-3 (supplied by author)] and recrystallization from hot water.12-14 SDeBS was sufficiently soluble in water to allow ATR studies of adsorption, but not sufficiently soluble to allow direct determination of molar absorptivities. SDeBS was not sufficiently soluble in 0.15 M NaCl for IR ATR analyses, but it was assumed that SDeBS would be as insensitive to ionic strength as was SOBS. Clear dense crystals with a needle habit were formed by cooling a 2.5-20 wt % solution in water from 60 °C to RT. These crystals were collected by filtration and briefly dried in ambient air. Another form, with a lamellar habit, was formed by rapidly cooling a 2.5 wt % solution to -20 °C, followed by ageing of the suspension at RT. SDoBS. The purity of p-(n-dodecyl)benzenesulfonate (SDoBS) became a critical issue. The spectra of commercial-grade SLaBS had always exhibited small bands of variable intensity in the ATR experiments. These bands were interpreted as due to impurities because they did not appear in ATR spectra of recrystallized SOBS. In ion-pairing reversed-phase chromatography of SDoBS (tetramethylammonium chloride in water/ isopropanol on ODS silica), at least four pairs of peaks were observed, most probably due to para- and ortho-substitutional isomerism on the benzene ring and to variety of alkyl chain lengths. Mass spectroscopy confirmed the presence C10, C11, C12, C13, etc., alkyl chains with more C11 than C12. The distribution of chain-branching isomers was unknown. To obtain a compound of known chain length, pure SDoBS was synthesized by sulfonation of 1-phenyl-n-dodecane [Aldrich, #11,323-9, 97%; CAS Registry No. 123-01-3, (supplied by author)] and recrystallization from hot water.12-14 The starting material had only unbranched C12 alkyl chains as determined by proton nuclear magnetic resonance (1H NMR), and the product had only para-substituted SO3- groups. Needle-habit crystals were prepared in the same fashion as were the SDeBS crystals, from 2 wt % solutions. The ATR spectra did not display the small bands previously attributed to impurities. Pure SOBS, SDeBS, and SDoBS gave only clear crystals of needle or platelet habit. None of the pure surfactants showed formation of suspended or curd-like precipitates or gels. The previously studied, unpurified SLaBS gave only powder-like precipitates. Determination of TK. Solutions of known concentration were created by dissolution above 55 °C. These solutions were cooled to below the anticipated crystallization point, then heated with agitation at 0.1 °C/min to dissolve the crystals. The presence of crystals was readily determined in a bright, narrow light beam by reflections from the crystal faces. For SDoBS, below TK, ultraviolet-visible (UV-vis) spectrophotometric measurements of the decanted supernatant were used to determine solution concentration. Solution Depletion Method. Aqueous surfactant solutions were analyzed by UV-vis spectrophotometry (224 nm, max ) 1140 m2 mol-1) both before and after 3 days of contact with γ-Al2O3 (Johnson Matthey Electronics #12867, surface area 175. m2 g-1, pellets ground to powder). Slurries of 0.15 g in 15 mL were (11) Sperline, R. P.; Song, Y.; Freiser, H. Colloids Surf. A. 1994, 93, 111-126. (12) Cerfontain, H.; Sixma, F. L. J.; Vollbracht, L. Recueil Trav. Chim. 1964, 83, 226-232. (13) Cerfontain, H. Mechanistic Aspects in Aromatic Sulfonation and Desulfonation; Editor, Ed.; Interscience: New York, 1968; Chapter 3. (14) Paquette, R. G.; Lingafelter, E. C.; Tartar, H. V. J. Amer. Chem. Soc. 1943, 65, 686-692.
Figure 1. Krafft temperature (TK) determination for SOBS, SDeBS, and SDoBS at 22, 46, and 72 °C, respectively. Solubilities below TK for SDeBS and SDoBS, respectively, were 1.1 × 10-3 and 8.47 × 10-5 M at 4 °C, and 1.75 × 10-3 and 1.85 × 10-4 M at 23 °C. shaken hourly for 6 h, allowed to settle for 3 days, then decanted. Low T samples were kept continuously at 5 °C. Data Collection. The spectrometer and its operating parameters, internal reflection element (IRE) preparation, cells, and polarizer have been previously described,1 as have the ATR and transmission cell calibration methods.1,2,15 Optical properties of the sputtered Al2O3 films were assumed to be the same as those for crystalline R-Al2O3. The optical constants at each wavelength were found by interpolation using purely empirical equations10 fitted to the optical constant data available for Al2O3,16 ZnSe,17,18 and H2O.19,20 A used Al2O3/IRE film was subjected to atomic force microscopy (AFM, NanoScope III, Digital Instruments) in the contact mode. The conical tip had sucessfully been used for atomic imaging. Results for surface regions exposed to surfactant solutions and unexposed regions were indistinguishable. The cold RF-sputtered Al2O3 films were assumed to be amorphous; no evidence of crystallinity was observed. A 30-µm square had a roughness figure of 1.6 nm root mean square (rms), avoiding the largest handling scratches. A 200-nm square of surface consisted primarily of disc shaped, fused 35-nm diameter nodules separated by narrow boundaries; the roughness was 0.3-0.5 nm rms. This figure increased to 0.9 nm rms when some obvious scratches were included. The 0.3-0.5 nm roughness figure was smaller than the extended molecular length. This result, combined with grain geometry, suggested that only a small fraction of any packed, adsorbed surfactant molecules could have been held in pores and been severely tilted to the mean plane of the film. The low solubilities of SDoBS (
