Mechanism of Coadsorption of Long-Chain Alkylamines and Alcohols

Coadsorption of long-chain primary amines and alcohols on silicates (quartz and ... Manoj V. Warrier, Miguel A. Garcia-Garibay, and Harold G. Monbouqu...
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Langmuir 2001, 17, 2711-2719

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Mechanism of Coadsorption of Long-Chain Alkylamines and Alcohols on Silicates. Fourier Transform Spectroscopy and X-ray Photoelectron Spectroscopy Studies I. V. Chernyshova* and K. Hanumantha Rao† Division of Mineral Processing, Department of Chemical and Metallurgical Engineering, Lulea˚ University of Technology, SE-971 87 Lulea˚ , Sweden Received August 21, 2000. In Final Form: January 29, 2001 Coadsorption of long-chain primary amines and alcohols on silicates (quartz and albite) at pH 6-7 was studied using Fourier transform (DRIFTS and IRRAS) and X-ray photoelectron spectroscopy. The ionization state of the amino headgroups, the molecular orientation, and packing in the adsorbed mixed monolayers were determined. The results were interpreted in terms of the modified model of 2D-3D precipitation, where the elementary adsorbing species from the solution is the amine-alcohol-water association.

Introduction The understanding of the mechanisms of surfactant adsorption can provide guidance to manipulation of the surface properties.1 In recent years, a strong interest has appeared in the behavior of mixed surfactant systems, whose physicochemical properties (wettability, frictional property, chemical reactivity, biocompatibility, permeability, etc.) are synergetic or negatively synergetic.2-4 Up to now, theoretical and experimental efforts (see reviews5,6) to getting insight onto the synergetic adsorption of surfactants have been concentrated mainly onto systems at temperatures above the Krafft point, where aggregation at the solid/liquid interface leads to formation of mixed hemimicelles or mixed admicelles (bilayers). To interpret the experimental data, the regular solution theory extended for mixed micelles7,8 has been frequently used.9 At the same time, very little is known about the structure of the adsorbed films and the mechanism of coadsorption at temperatures below the Krafft point. The possible variants are mixed or demixed precipitation via associations of the mixed surfactants or via competitive adsorption of their monomers. Long-chain alkylamines (a weak-electrolyte type surfactant) have to some extent been neglected in colloidal chemistry,10 despite their wide application in technology *To whom correspondence should be addressed. Permanent address: St. Petersburg State Technical University, Polytechnicheskaya 29, 195251 St. Petersburg, Russia. E-mail: [email protected]. Fax: +7 (812) 428-5712. † E-mail: [email protected]. (1) Riviere, J. C.; Mihra, S., Eds. Handbook of Surface and Interface Analysis; Marcel Dekker: New York, 1998. (2) Sharma, R., Ed. Surfactant Adsorption and Surface Solubilization; ACS Symposium Series 615; American Chemical Society: Washington, DC, 1995. (3) Holland,P. H., Rubingh,D. N., Eds. Mixed Surfactant Systems; ACS Symposium Series 501, American Chemical Society: Washington, DC, 1992. (4) Scamehorn, J. F., Ed. Phenomena in Mixed Surfactant Systems; ACS Symposium Series 311, American Chemical Society: Washington, DC, 1986. (5) P. H. Holland and D. N. Rubingh in ref 3, pp 2-30. (6) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143. (7) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p.337. (8) Rosen, M. J.; Hua, X. Y. J. Colloid Interface Sci. 1982, 86, 164. (9) Huang, L.; Maltesh, C. Somasundaran, P., ref 2, pp 241-254 (10) Backlund, S.; Friman, R.; Karlsson, S. Colloids Surf. 1997, 123124, 125.

as collectors in flotation of silicates,11 templates for synthesis of aluminosilicate mesoporous molecular sieves,12 components of biological sensors,13,14 and microemulsifying cosurfactants in the “water/anionic surfactant/oil” system.15 Synergy in coadsorption of alkylamines is a very promising direction of the research activity since it permits achievement of the same surface property at a lower consumption of the reagents. The increase in the adsorbed monolayer hydrophobicity has been reported for the mica surface conditioned in the binary solutions of C12-amine/ C8-alcohol16 and C12-amine/C12-alcohol17 and interpreted in terms of coadsorption of the alcohol and amine. An alternative explanation18 consists of formation of ionmolecular pairs in the solution and subsequent adsorption at the silicate surface. However, neither of these hypotheses has been verified by using a direct (spectroscopic) method. In our previous work,19,20 it has been demonstrated that adsorption of 1-alkylamines with a chain of more than 10 carbons in length on silicates at pH 6-7 is consistent with the model of successive two-dimensional (2D) and threedimensional (3D) precipitation rather than with the hemimicelle model.21-24 Under these conditions, the silicates are negatively charged, while the amines are practically totally protonated (for C12-ammonium acetate (11) Smith, R. W. Reagents in Mineral Technology; Somasundaran, P., Moudgil, B. M., Eds.; Marcel Dekker: New York, 1988; pp 219-256. (12) Mokaya, R.; Jones, W. J. Mater. Chem. 1998, 8, 2819. (13) Mader, C.; Kupcu, S.; Sleytr, U. B.; Sara, M. Biochim. Biophys. Acta 2000, 1463, 142. (14) Hianik, T.; Kupcu, S.; Sleytr, U. B.; Rybar, P.; Krivanek, R.; Kaatze, U. Colloids Surf., A 1999, 147, 331. (15) Wormuth, K. R.; Kaler, E. W. J. Phys. Chem. 1987, 91, 611. (16) Yoon, R.-H.; Ravishankar, S. A. J. Colloid Interface Sci. 1994, 166, 215. (17) Yoon, R.-H.; Ravishankar, S. A J. Colloid Interface Sci. 1996, 179, 391. (18) Somasundaran, P.; Ananthapadmanabhan, K. P. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 2, p 777. (19) Chernyshova, I. V.; Rao, K. Hanumantha; Vidyadhar, A.; Shchukarev, A. V. Langmuir 2000, 16, 8071. (20) Chernyshova, I. V.; Rao, K. Hanumantha; Vidyadhar, A.; Shchukarev, A. V. Langmuir 2001, 17, 775. (21) Gaudin, A. M.; Fuerstenau, D. W. Trans. Soc. Min. Eng. AIME. 1955, 202, 958. (22) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90. (23) Novich, B. E.; Ring, T. A. Langmuir 1985, 1, 701. (24) Fuerstenau, D. W.; Jang, H. M. Langmuir 1991, 7, 3138.

10.1021/la001201j CCC: $20.00 © 2001 American Chemical Society Published on Web 04/05/2001

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Figure 1. XPS N1s spectra of a fracture quartz surface conditioned in a solution (pH 6.5) of a 1:1 binary solution of C16-amine and C16-alcohol with total concentrations of (1) 5 × 10-5 and (2) 1 × 10-4 M.

pKa,aq ≈10.6, Ksol ) 2 × 10-5 M, Kraft point is 26 °C25). Since the solubility of the amines is rather low, the neutral amine precipitates at the interface first in the 2D space, enhancing the adsorption and increasing the density of the monolayer by screening electrostatic repulsion between the headgroups. It is interesting that the first monolayer (the 2D precipitate) was found to be composed of both ionized and neutral amine, which are in the equilibrium:

Since long-chain alcohols, similar to neutral amines, can act as a hydrogen acceptor in H-bonding with the ammonium headgroup and the hindrance due to entropic factors is unlikely, one can expect that in the presence of the alkyl alcohol, long-chain alkylamines will 2D-3D precipitate at a lower total solution concentration. The goal of the present work was to study the effect of long-chain alcohols on the mechanism of adsorption of primary long-chain amines at pH 6-7 and the structure of the adsorbed film by using IR spectroscopy in the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and infrared reflection absorption spectroscopy (IRRAS) modes and X-ray photoelectron spectroscopy (XPS). Experimental Section Reagents. C8-amine and C12-amine were purchased from Fluka, and C16-amine was supplied by Akzo Nobel. The C8-, C12-, and C16-alcohols were purchased from Fluka. All reagents were (25) Smith, R. W. In Reagents in Mineral Technology; Somasundaran, P., Moudgil, B. M., Eds.; Marcel Dekker: New York, 1988; pp 219-256.

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Figure 2. XPS N1s spectra of a fracture albite surface conditioned in a solution (pH 6.5) of 1:1 binary solution of C16amine and C16-alcohol with total concentrations of (1) 5 × 10-5 and (2) 1 × 10-4 M. 99% pure and were used as received. The solutions of C12- and C16-amines were prepared from their ammonium acetate salts, which were synthesized by the standard procedure.19,20 The XPS analysis (Table 1) reveals that nevertheless the acetate salts contain a molecular amine form as an impurity. Solutions of the amines were prepared by dissolving the necessary weight in water and the pH was adjusted to be within the 6-7 range by the addition of small quantities of NaOH in the case of C12- and C16-amines or HCl in the case of the C8-amine. Because the C12and C16-alcohols are practically insoluble in water, these reagents were first dissolved in ethanol. The stock aqueous solution contained 5% ethanol. All the binary solutions of the amines and alcohols were equimolar. Deionized water of specific conductance of 0.4-0.7 µΩ-1 cm-1 was used in all experiments. The bulk precipitate from the mixture of C16-amine and C16-alcohol was prepared by dissolving equimolar quantities of these compounds in acetone, adding water, and air-drying the flakes that precipitated. Materials. Clear natural quartz crystals and polycrystalline (1-2-mm size crystals) albite of gray-white color, supplied by the Mevior S.S., Greece, were handpicked from their “Dasaki” feldspar deposit near Thessaloniki. A quartz plate with dimensions of about 20 × 20 mm2 was cut from a single crystal along the (002) crystallographic plane, which was checked by X-ray diffraction (XRD) analysis. The identity of the albite was also verified by XRD analysis. XPS data on the fractured silicate surfaces are shown in Table 1. The samples used in the present study were free from nitrogen impurity. The working surface of the quartz plate used in the IRRAS studies was prepared by successive polishing with SiC papers down to 0.25 µm size and afterward thoroughly washing with deionized water. To prepare powder, the crystals were crushed, ground, and wet sieved. The -5 µm size was obtained by microseiving in an ultrasonic bath. Spectroscopic Characterization. The FTIR spectra were obtained with a Perkin-Elmer 2000 spectrometer at 4 cm-1 resolution with a narrow band liquid N2-cooled MCT detector. The IRRAS spectra were collected ex situ using an IRRAS accessory (Harrick, Inc.), by coadding 1000-1500 scans, in both s- and p-polarized radiation. To obtain the selected polarization,

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Table 1. XPS Characterization of Solid Amines and the Initial Fracture Surfaces of Albite and Quartza element atom % (BE, eV) N

C

O

solid C12-amine

sample

4.65 (399.5)

2.58 (531.2)

solid C12-amine acetate

1.46 (399.4) 3.78 (401.1)

fresh fracture of natural albite

0

fresh fracture of natural quartz albite, C16-amine acetate, 1 × 10-4 M albite, C16-amine acetate/ C16-alc., 1 × 10-4 M

0 0.37 (399.6) 0.67 (401.5) 0.60 (399.5) 0.57 (401.6)

89.36 (285.0) 0.07 (286) 0.02 (287.7) 76.19 (285) 5.66 (286.9) 3.94 (288.4) 8.6 (285.0) 0.93 (286.4) 12.77 (285.0) 29.06 (285.0)

albite, C16-amine acetate/ C16-alc., 5 × 10-5 M quartz, C16-amine acetate, 1 × 10-4 M quartz, C16-amine acetate/ C16-alc., 1 × 10-4 M quartz, C16-amine acetate/ C16-alc., 5 × 10-5 M

0.2 (399.2) 0.2 (400.2) 0.47 (401.7) 0.39 (399.3) 0.32 (401.5) 0.28 (399.5) 0.18 (401.6) 0.29 (399.7) 0.12 (402.2)

quartz, C16-amine acetate/ C16-alc., 5 × 10-5 Mc

0.33 (399.7) 0.06 (402.1)

quartz powder, C12-amine acetate/ C16-alc., 1 × 10-5 M quartz powder, C16-amine acetate/ C12-alc., 1 × 10-5 M

0.24 (400.1) 0.10 (402.2) 0.19 (400.1) 0.12 (402.0)

28.48 (285.0) 2.77 (286.2) 1.08 (287.9)b 22.54 (285.0) 2.93 (286.3) 0.54 (288.3)b 23.09 (285.0) 1.95 (286.0) 23.37 (285.0) 1.58 (286.4) 21.44 (285.0) 1.59 (286.4) 0.22 (286.6) 24.76 (285.0) 1.76 (286.3) 0.33 (288.2) 11.53 (285.0) 1.64 (286.6) 6.56 (285.0) 1.18 (286.6) 0.47 (288.3)b

Si

Al

Na

7.99 (531.2) 0.99 (533) 58.37 (532.2)

19.57 (103.0)

6.85 (74.7)

5.80 (1072.1)

57.29 (532.8) 39.08 (531.7)

29.94 (103.6) 19.6 (102.6)

6.92 (74.2)

1.94 (1071.7)

38.27 (531.8)

19.49 (102.7)

6.96 (74.4)

1.78 (1071.7)

43.72 (531.9)

19.73 (102.8)

6.99 (74.4)

2.67 (1071.8)

47.5 (532.5)

26.75 (103.3)

44.85 (532.8)

29.38 (103.5)

48.64 (532.8)

27.71 (103.6)

45.07 (532.8)

27.68 (103.5)

56.38 (532.8)

30.11 (103.5)

60.41 (532.7)

31.07 (103.5)

a These surfaces conditioned with pure solutions of C16-amine and C16-alcohol and the 1:1 mixture, and the quartz powder conditioned in the C12-amine/C16-alcohol and C16-amine/C12-alcohol solutions. The standard error in the BEs is (0.1 eV, the standard error in the surface atom concentrations is 10-15%. b Carboxyl of acetate or contamination. c Duplicate experiment demonstrating reproducibility.

a wire-grid polarizer placed after the sample was used. The measurements were conducted immediately after 5 min of conditioning the quartz plate and with the solution. The excess solution, if any, was removed carefully from the surface with filter paper. The DRIFT spectra were measured on the -5 µm powder which was conditioned for 10 min with solution, filtered, and air-dried overnight. The untreated (initial) mineral powder was used as reference. The samples were not mixed with KBr. A DRIFTS Perkin-Elmer accessory was utilized. Each spectrum is an average of 500 scans. The XPS spectra were recorded with an AXIS Ultra (Kratos) electron spectrometer under Al monoirradiation with sample cooling. The vacuum in the sample analysis chamber during measurements was 10-8 Torr. The value of 285.0 eV was adopted as the standard C1s binding energy. The powdered samples were measured 2 days later after obtaining the corresponding DRIFT spectra. The measurements on the fracture surfaces were performed just after immersing for 10-15 min in the solution and shaking of the drops. The sample was set into the precooled holder. The results in Table 1 are presented as fractions of the 100% base since the mineral matrix silicon fraction is the same within the standard error in determining the relative atomic concentration (10%), which permits comparing between the data for quartz and albite.

Results I. XPS. To determine the ionization state of the amino groups in the mixed film, the XPS measurements were carried out at the fracture surface of quartz and albite and the powder quartz conditioned with binary solutions of the long-chain amines and alcohols. The results are shown in Figures 1 and 2 and Table 1. For comparison, Table 1 also lists the XPS data that were obtained earlier19,20 for the solid amines and the pure C12-amine

adsorbed on quartz and albite. The C1s spectrum of the mixed film contains a peak at 286.0-286.8 eV assigned to carbon in the C-N and C-O bonds,26 in addition to the main peak at 285 eV originating from alkyl chains. Surprisingly, as in the case of adsorption of the pure amine,19,20 the N1s spectra of the mixed films are split into two components. The peak with the lower binding energy (BE) (399.5-399.7 eV for the fracture quartz) is due to the molecular amine and the peak with the higher BE (401.5-401.7 eV for the fracture quartz) is due to the protonated amine in equilibrium 1. It follows that precipitation of molecular amine takes place also in the case of coadsorption. Moreover, the relative content of the neutral amine in the mixed film is significantly higher than that in the “pure amine” film, the total concentration of the solution being the same, which means that the longchain alcohol promotes precipitation of the amines. Comparing the N1s spectra of the mixed film adsorbed at the same concentration on quartz and albite, one can see that, as in the case of adsorption of the pure amine, the surface concentration of the adsorbed amine and, as a consequence, the surface concentration of neutral form on albite is higher, which was explained previously20 by a higher proton affinity of the oxygen at the nonstoichiometric albite surface to the ammonium group, as compared to that at the quartz surface. The higher surface concentration of the protonated species on albite than on quartz can be explained by adsorption of the neutral amine onto the proton sites arose on albite in aqueous media due to substitution of sodium cations. (26) Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Co., Physical Electronics Division: Eden Praire, MN, 1995.

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Figure 3. XPS N1s spectra of a quartz powder conditioned 10 min in 1:1 binary solution (pH 6.5) of (1) C16-alcohol/C12-amine and (2) C16-alcohol/C12-amine with a total concentration of 1 × 10-5 M.

To further clarify the mechanism of coadsorption, we measured the XPS spectra of the powdered quartz conditioned 10 min in the binary solutions of the surfactants with the reversal chain lengths (C12-amine/C16alcohol and of C16-amine/C12-alcohol) of the same total concentration (Table 1). It is interesting that the N1s spectra occurred to be almost the same within the experimental error in terms of both the total atomic concentration of nitrogen and the relative content of the ionic and molecular forms (Figure 3). This observation can be understood if one assumes that the precipitating species are the 1:1 associates of the amine and the alcohol, formed in the solution. In this case, neglecting in the first approximation the dependence of the association constant on the different electron-donating effect of the hydrogencarbon chains of 12 and 16 carbon length, the solubility product of the mixed phase is controlled by the sum number of the hydrocarbon groups in a such associate. As a consequence, the quantities precipitating from the binary solutions of the reversal combinations of the surfactant chain are expected to be the same. II. IR Spectroscopic Data. To provide data on the arrangement of the alcohol and amine headgroups in the mixed monolayers, the IR spectroscopic measurements were performed using the DRIFTS and IRRAS methods. Figure 4a, curve 0 shows the DRIFT spectrum of the initial -5 µm quartz powder, which was used as reference while recording the spectra of the conditioned powder. The narrow band at 3745 cm-1 is assigned27,28 to the stretching (27) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterisation and Chemical Modification of the Silica Surface; Elsevier: Amsterdam, 1995.

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ν(OH) vibrations of surface isolated silanol groups. The complex absorption band in the 3000-3700 cm-1 spectral region is due to adsorbed H-bonded hydroxyls and water.28 These observations are in agreement with the known fact that at pH 2-7 the quartz surface is covered mostly by hydroxyls, which bear the surface negative charge.27 Curves 1 and 2 of Figure 4 show the DRIFT spectra of the quartz conditioned for 10 min with the mixed solutions of the 1-alkylamine and alkyl alcohol of different chain lengths of the same total concentration of 1 × 10-5 M. All display the typical bands νas(CH2) (∼2920 cm-1), νs(CH2) (∼2851 cm-1), and νas(CH3) (∼2955-2965 cm-1) of alkyl chains due to the adsorbed surfactants. A broad band in the 3700-2500-cm-1 region is attributed to the H-bonded ν(N+-H), ν(O-H), and ν(N-H) stretching vibrations of the adsorbed amine, alcohol, surface silanol, and water. Although it is problematic to split this band into the components, in the case of coadsorption a weak band at 3630 cm-1, which is marked by asterisks in Figure 4, can be distinguished. Taking into account that the monomer of C12-alcohol in CCl4 absorbs at 3640 cm-1 (the spectrum is not shown) and in the case of albite this band is at 3698 cm-1 (the spectrum is not shown), this band can be assigned to the weakly H-perturbed ν(OH) vibrations of the surface silanols or coadsorbed water29 in the aminealcohol-silanol system. The presence of the extra band at 3630 cm-1 in the spectrum of the mixed films means that some cooperative effect takes place. The intensities of the ν(CH) bands are the same within the experimental error for the amine and alcohol pairs with reverse chain lengths, increasing with increasing the sum chain length (compare curves 1 and 2 in parts a and b of Figure 4). As follows from comparison of the ν(CH) band intensities of the adsorbed pure surfactants (Figure 4c curves 3 and 4), the adsorption of the pure amine is by a factor of ca. 3 larger than that of alcohol, the chain length being the same. Therefore, taking into account the general tendency of increasing adsorption with increasing the surfactant chain length (Traube’s rule), one can expect that in the case of competitive adsorption of monomers from the binary solutions of amine and alcohol with the reverse chain lengths, the surface coverage will be larger in the case of the mixture with the longer amine homologue. At the same time, if the adsorbed species are the 1:1 solvated associates, the surface coverages should be similar, as observed. It is most probable that these solvated associates are the ion-molecular pairs, in which the alcohol hydroxyl molecule acts as base while the ammonium headgroup acts as acid. Such dimers have been suggested to be formed between the protonated and molecular long-chain amine species in the solution at pH > 730 and between the protonated amine and alcohol.18 As the narrow band at 3330 cm-1 of the bulk long-chain molecular amine (Figure 5, curve 2) is absent in the spectra of the mixed films, the phase-separated molecular amine is not adsorbed under the given conditions. The bulk mixed precipitate is not adsorbed either since the satellite band at 3285 cm-1 characteristic of the mixed precipitate (Figure 5, curve 4) is also absent in the spectra shown in Figure 4. (This satellite band can be assigned to the ν(NH) mode of the amine acting as hydrogen acceptor in H-bonding in (28) Coretsky, C. M.; Sverjensky, D. A.; Salisbury, J. W.; D’Aria, D. M. Geochim. Cosmochim. Acta 1997, 61, 2193. (29) Futamata, M. Surf. Sci. 1999, 427-428, 179. (30) Laskowski, J. S. In Challenges in Mineral Processing; Sastry, K. V. S., Fuerstenau, M. C., Eds.; Society of Mineral Engineers: Littleton, 1989; pp 15-34. (31) Park, S. Y.; Franses, E. Langmuir 1995, 11, 2187 and references therein.

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Figure 4. DRIFT spectra of (a, curve 0) initial quartz and the quartz conditioned 10 min in the 1:1 binary solutions (pH 6-7) of alkylammonium acetate and alkyl alcohol of different chain lengths at a total concentration of 1 × 10-5 M. The component chain lengths are (a) (1) amine-16/alcohol-12 and (2) amine-12/alcohol-16; (b) (1) amine-16/alcohol-8 and (2) amine-8/alcohol-16; (c) (1) amine-16/alcohol-16 and (2) amine-12/alcohol-12 and, for comparison, the quartz conditioned in the solutions of the pure (3) C16amine acetate and (4) C16-alcohol. The absorbance scale is the same for all the graphs. For convenience, the spectra are split in the vertical direction. Asterisk indicates the 3630-cm-1 band.

Figure 5. (a) Absorption spectra of (1) C12-ammonium acetate, (2) C16-amine, (3) C16-alcohol, and (4) precipitate of C16-amine and C16-alcohol. Spectra 2 and 3 were obtained in the transmission mode (the substance was deposited as a thick polycrystalline film on KBr plate from the solutions in ethanol). Spectra 1 and 4 were obtained by the DRIFTS method (the substance was mixed with KBr, a spectrum of KBr was used as reference). (b) Enlarged region of the δ(CH2) vibrations.

the mixed phase of the long-chain amine and alcohol, which indicates a thermodynamic possibility of formation of such a mixed phase. We failed to confirm this conclusion by the X-ray diffraction spectra (not shown), since the long-chain amines and alcohols are known to show polymorphism.31) Since solid alcohols have no characteristic narrow bands in the 4000-2500-cm-1 range (Figure 5, curve 3), it is hard to deduce about the absence or presence of the phase-separated alcohol at the surface on the basis of the IR spectra. However, one can conclude that the mixed film does not contain a detectable amount of domains of alcohol, since the ν(CH) pattern of adsorbed pure alcohol

differs significantly from that of the adsorbed pure amine and the mixed film (Figure 4c). The negative asymmetric band at 3745 cm-1 due to free surface silanols, being rather intensive in the spectrum of the quartz conditioned with the solution of the pure amine (Figure, 4c, curve 3), is significantly weaker in the case of the coadsorption, vanishing for pairs amine12/alcohol-16 and amine-8/alcohol-16. This band is also very weak for adsorbed single alcohol (Figure 1c, curve 4). Since the negative intensity of the silanol band results from the interaction of silanols (probably via H-bonding in the case under question) with the adsorbate,19,20 it

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Figure 6. IRRAS spectra of a quartz surface after 5 min conditioning in the 1:1 solution of molecular C12-amine and alkyl alcohol with the chain length of (1) 8, (2) 12, and (3) 16 carbons of the total concentration 2 × 10-5 M at naturally established pH.

follows that free silanols are less involved in the coadsorption as well as in the adsorption of the alcohol, as compared to the adsorption of the pure amines. In the case of the C8-amine/C16-alcohol surfactant system (Figure 4b curve 2), the DRIFT spectrum exhibits the broadest widths of the ν(CH) bands, which implies the least order of the hydrocarbon chains among the cases under examination. Moreover, the “center of gravity” of the complex broad “H-bonded” band is at the highest frequency (at ca. 3330 cm-1), and the intensity of this band is highest as compared to the corresponding characteristics of the “H-bonded” bands in the other spectra shown in Figure 4, which can be assigned to the largest amount of the coadsorbed molecular water. In principle, the frequencies of the CH2 stretching bands of hydrocarbon chains can be used for comparing the conformational ordering of the chains in the adsorbed layers.19,20 When the chains are highly ordered (all-trans zigzag conformation), the narrow absorption bands appear at 2918 (νas(CH2)) and 2848 cm-1 (νs(CH2)) in the IR spectrum of the layer. If a conformational disorder is included in the chain organization these bands shift upward to 2926 and 2856 cm-1, depending upon the content of gauche-conformers in the ensemble. In the DRIFT spectra shown in Figure 4 the band positions are almost the same, probably due to high heterogeneous

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composition of the adsorbed film on the fine particles. To distinguish the dependence of the band position on the relationship between the lengths of the hydrocarbon, chains the IRRAS spectra were measured, which occurred to be more sensitive in terms of their frequencies to the film composition. These spectra were collected with unpolarized radiation at the angle of incidence of 10°, which implies that only the modes parallel to the surface are active. Figure 6 shows the ν(CH) bands in the IRRAS spectra of the quartz surface conditioned with the binary solutions of C12-amine and alcohols of different chain lengths. In this case the solution was prepared by dissolving the molecular amine (not the acetate salt) and pH was naturally established (ca. 7.5). The negative intensity of the absorption bands is due to the well-known optical effect.32 As seen from Figure 6, when the chain lengths of the coadsorbed amine and the alcohol are the same, the frequencies of the νas(CH2) and νs(CH2) bands are the lowest and the bandwidths are the narrowest; therefore, the order and packing of the chains is the highest. This result agrees with the surface force data of Yoon and Ravishankar,16,17 who found that coadsorption of C12-amine with C12-alcohol at mica yields a much higher hydrophobic effect (much stronger hydrophobic force) as compared to the combination of C12-amine with C8-alcohol. To quantify the orientation and packing of the hydrocarbon chains in the mixed film on quartz, the s- and p-polarized IRRAS spectra were measured at the angle of incidence of 73°, which was found33 to be optimal for the orientation measurements on quartz. The spectra were collected after the 5-min conditioning of the quartz surface in the binary solutions of the C12- and C16-amines and alcohols. Qualitatively, the spectra of the mixed films (Figure 7) are similar to the spectra of adsorbed pure amine.19,20 An exception is that the δ(CH2) band at ca. 1466 cm-1 in the s-polarized spectra of the film adsorbed from the mixture solution of C16-amine and C16-alcohol is broader and more likely to be singlet than doublet (Figure 8). Therefore, the chains in the mixed film are likely to be arranged in a hexagonal cell. The positions of the νas(CH2) and νs(CH2) bands (Table 2) show that the hydrocarbon chains are packed rather densely when C16amine and C16-alcohol are coadsorbed at the polished quartz surface at a coverage less than a theoretical monolayer (see below for details of the calculation), however, not better than the assembled monolayers of

Figure 7. IRRAS spectra of a quartz surface conditioned with 1:1 solution (pH 6-7) of aliphatic alcohol and amine with the same chain lengths (Cn). The total concentration is (1) 1 × 10-4 M (C12), (2) 2 × 10-4 (C12) M, (3) 5 × 10-5 M (C16), and (4) 1 × 10-4 M (C16). s-polarized spectra are shown on the left and p-polarized spectra are shown on the right.

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Figure 8. Same as in Figure 7 but with enlagred regions of the CH stretching and bending vibrations and spectra 1 and 2 are multiplied by 2. Table 2. Characteristics of the Layers Adsorded on Quartz, Taken from the IRRAS Spectra Reported in References 19 and 20 and Shown in Figures 7 and 8

1 2 3 4 5 6 a

solution

νasym(CH2)s,a cm-1

νsym(CH2)s, cm-1

DRasym

DRsym

C16-amine acetate, 1 × 10-4 M C16-alcohol, 1 × 10-4 M C16-amine acetate/C16-alc., 1 × 10-4 M C16-amine acetate/C16-alc., 5 × 10-5 M C12-amine acetate/C12-alc., 1 × 10-4 M C12-amine acetate/C12-alc., 2 × 10-4 M

2918.3 2918.3 2919.1 2918.5 2921.5 2921.5

2851.0 2850.8 2850.8 2850.8 2853.8 2852.7

-0.62 -0.64 -0.76 -0.73 -1.5? -1.5?

-0.72 -0.62 -0.77 -0.78

γ and φ, calcd from DRs, deg

no. of monolayers

30, 52 25 37 37 iso iso

1.16 0.8 1.30 0.6 0.2 0.4

The band position in the s-polarized spectrum.

the amine and alcohol adsorbed separately. In the case of the C12/C12-surfactant mixture, the surface is only sparsely covered and the chains are highly disordered. These observations can be understood if one accepts that in the case of the C16/C16 pair the concentrations at which the IRRAS spectra were obtained are above the critical concentration of aggregation, while in the case of the C12/ C12 pair this critical concentration is not reached. Notice that self-assembly of the C12/ C12 pair at the low total solution concentration (Figure 6, curve 2) was observed at the higher pH, where the proportion of the neutral amine in the solution is higher.11 The procedure and background of the molecular orientation (MO) measurements have been described in detail elsewhere.33 First, the dichroic ratio (DR) was measured from the experimental IRRAS spectra. The DR is defined as the ratio of the peak intensity of the absorption band in the s-polarized spectrum, As, to that in the p-polarized spectrum, Ap:

DR )

As Ay ) Ap Ax + Az

(2)

The MO is measured by fitting this DR value with the theoretical one calculated using the linear approximation formulas for anisotropic film at an isotropic transparent (32) Horn, A. Spectroscopy for Surface Science; Clark, R. J. H., Hester, R. E., Eds.; John Wiley and Sons: New York, 1998; Vol. 26, pp 273339. (33) Chernyshova, I. V.; Rao, K. Hanumantha J. Phys. Chem. B 2001, 105, 810. (34) Mielczarski, J. A.; Yoon, R. H. J. Chem. Phys. 1989, 93, 2034.

substrate, represented by Mielczarski.34 In doing so, one excludes the film thickness from the MO measurements and reduces the error produced by the a priori uncertainty in the anisotropic absorption coefficients of the film. Since the birefringence of quartz is small in the 2900-2800cm-1 spectral range (at 2857 cm-1 no ) 1.4845 and ne ) 1.49235), we took the value of 1.49 for the refractive index of quartz. The optical parameters n2x ) n2y ) 1.49, n2z ) 1.55, and k2max ) 1.04 reported for the νas(CH2) band of an 11-monolayer CdAr Langmuir-Blodgett film on glass36 were adopted for estimation of the thickness of the mixed film. The monolayer thickness was assumed to be equal to the fully extended length of a C16-amine molecule, 21.8 Å.37 The results of the MO and thickness measurements are shown in Table 2. At coverages below and slightly above one monolayer of the C16/C16 mixture, the tilt angle was found to be the same of 37° (Table 2). Since the DRs for the νas(CH2) and νs(CH2) bands for these films occurred to be the same within the experimental error, the symmetry of these films was assumed to be uniaxial (which agrees with the above conclusion about hexagonal packing of the chains, drawn from the qualitative analysis of the δ(CH2) bands). Since this value is higher than that for the assembled films of pure C16-amine and alcohol (28 ( 2°), the mixed film is not composed of separate amine and alcohol domains, but rather alcohol and amine are mixed in a single phase. (35) Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: New York, 1998; Vol. III. (36) Braudez, D.; Buffetau, T.; Desbat, B.; Fournier, P.; Ritcey, A.; Pezolet, M. J. Phys. Chem. B 1998, 102, 99. (37) Gidalevitz, D.; Huang, Z.; Rice, S. A. Biophys. J. 1999, 76, 2797.

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Chernyshova and Rao

The ν(CH) bands of the C12/C12 films are significantly less intensive in the p-polarized spectra than in s-polarized ones and are characterized by a very poor signal-to-noise ratio (Figure 8). Therefore, the MO can be estimated with a high degree of uncertainty. Taking into account that for an isotropic film at quartz for the angle of incidence of 73° DR ) 1.5, we inferred that the chains in the mixed C12/C12 films adsorbed at the total solution concentration of 1 × 10-4 and 5 × 10-5 M are likely to be totally disordered and the surface coverages are of 0.4 and 0.2 monolayers, respectively. Discussion The most probable explanation of the appearance of the neutral amine in the monolayer adsorbed onto silicates conditioned by the binary solutions of the long-chain amine and alcohol is the 2D precipitation, as in the case of adsorption of the pure amine.19,20 This mechanism is also consistent with the observed higher surface concentration of the total adsorbed and neutral amine on albite (see section XPS in Results). Since the amount of the 2Dprecipitated amine is significantly larger in the presence of the alcohol in the solution of the same total concentration, it follows that the alcohol induces this process. This effect is easily understood if one assumes that the precipitating species is a soluble associate of the protonated amine and alcohol. This species is expected to have a lower solubility as compared to the separated amine, due to a greater number of the hydrocarbon groups at the same hydrophilicity of the headgroup, since hydrophilicity of the attached alcohol hydroxyls is negligible as compared to that of the ammonium groups. The coadsorption via the associates could explain (1) the intermixing of the surfactants in the adsorbed monolayer, which follows from the observed difference in the tilt angles of the hydrocarbon chains in the mixed and separated monolayers of the amine and alcohol and (2) the practical independence of the amount of the adsorbed hydrocarbon groups and the precipitated amine from the fact that how the given two hydrocarbon chains are distributed between the amine and alcohol. What remains still unclear is the structure of the solvated associate and the mixed film. As a matter of fact, the diameter of a well-packed hydrocarbon chain (4.34.5 Å38) is larger than the diameter of the ammonium headgroup (3.7 Å11) and the diameter of the amine headgroup (3.38 Å39) and much larger than that of the alcohol hydroxyl (ca. 1 Å40). It follows that if the chains associate by hydrophobic interaction in the soluble aminealcohol complex, the headgroups are 4.3-4.5 Å apart (Figure 9a), which prevents their coordination directly to each other (for comparison, the distance between the terminal atoms in the strong O‚‚‚H-O H-bonds is ca. 2.7 Å41). It follows that such a complex is rather weak. However, steric factors allow formation of a strong complex in which both the H-bonding and hydrophobic interaction contribute if there will be a water molecule between the ammonium and alcohol headgroups, as depicted in Figure 9b. The assumption that the elementary adsorbing species has this or a similar structure allows attributing the extra band at 3630-3698 cm-1 in the DRIFT spectra of the mixed film to the ν(OH) mode of this incorporated water. (38) Schwartz, D. K. Surf. Sci. Rep. 1997, 27, 241. (39) Leja, J. Surface Chemistry of Froth Flotation; Plenum Press: New York, 1982; p 541. (40) Natal-Santiago, M. A.; Dumesic, J. A. J. Catal. 1998, 175, 252. (41) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman: San Francisco, 1960.

Figure 9. Schemes of the long-chain amine/alcohol complexes.

The assumption that the elementary adsorbing species is “ammonium-water-alcohol” allows understanding why the average tilt angle of the hydrocarbon chains in the mixed films is larger than that in the pure films of the amine and alcohol. The tilt angle of 37° implies the distance between the hydroxyl oxygen and the amine nitrogen to be 5.3-5.6 Å, which is realistic for such a complex (Figure 9b). The following 2D precipitation consisting in the splitting off a proton of the ammonium group can yield the “amine-water-alcohol” species depicted in structures c and d of Figure 9. However, in the species shown in Figure 9c the lateral H-bonds are rather strong so that the distance between the terminal atoms in the N‚‚‚H-O and O‚‚‚H-O H-bonds is on the order of 2.7-3.0 Å, (the origin of the H-bonded groups and cooperative enhancement of the H-bond strength due to the mediating water molecule justify this assumption).41 As a result, the distance between the hydroxyl oxygen and the amine nitrogen is on the order of 4.9 Å. Moreover, the functioning of the alcohol oxygen as proton acceptor in the H-bond with the incorporated water prevents repeating this unit at the surface. At the same time, the species shown in Figure 9d permits the headgroups to accommodate apart from each other at the distance on the order of 5.4-5.6 Å and the feasibility of the repeating this motif along the surface. Thus, we suggest that at the silicate surface the 2D precipitation consists of transition from species b to species d shown in Figure 9. The involvement of the surfactant headgroups in the lateral H-bonds reduces interaction between them and the surface silanols, which explains the much smaller number of the interacting free surface silanols, observed in the DRIFTS spectra. Therefore, it seems that the lateral bonding contributes to the synergetic behavior of these surfactants in flotation.42 (42) Vidyadhar, A.; Chernyshova, I. V.; Rao, K. Hanumantha; Forssberg, K. S. E.; Pradip, Int. J. Mineral Process., in press. (43) Zisman, W. A. Ind. Eng. Chem. 1963, 55, 19.

Coadsorption of Surfactants on Silicates

From the above positions, the maximum in the chain order observed when the chain lengths of the cosurfactants are the same (Figure 6) can be explained in the following way. When the chains are of different lengths, the terminal part of the longer chain extends outward from the wellpacked 2D precipitate. This part can freely form the gauche-conformers. Since the maximum hydrophobicity is observed when CH3 groups rather than CH2 groups are directed toward the aqueous phase,43 one can expect that in the case of different chain lengths, the mixed monolayer will be less hydrophobic, which agrees with the surface force data.16,17 Conclusions The regularities in the FTIR and XPS spectra of mixed monolayers of coadsorbed long-chain amines and alcohols

Langmuir, Vol. 17, No. 9, 2001 2719

on quartz and albite may be understood in terms of deprotonating of ammonium group in the adsorbed ”alkylammonium-water-alcohol” complex (Figure 9b-d). This process, which can be regarded at the first step as 2D precipitation of molecular amine, explains the synergetic adsorption of these surfactants. Acknowledgment. The authors wish to thank Dr. A. V. Shchukarev, Department of Inorganic Chemistry, Umea˚ University, Sweden, for the measurements of the XPS spectra. I.V.C. gratefully acknowledges the financial support of The Swedish Institute and Division of Mineral Processing, Luleå University of Technology, Sweden. LA001201J