Alkanoic Acid Self-Assembled Monolayers - American Chemical Society

Chapter 9

n-Alkanoic Acid Self-Assembled Monolayers Adsorption Kinetics S. H. Chen and C. W. Frank

Downloaded by PURDUE UNIV on May 22, 2016 | http://pubs.acs.org Publication Date: December 1, 1990 | doi: 10.1021/bk-1990-0447.ch009

Department of Chemical Engineering, Stanford University, Stanford, CA 94305-5025

FTIR methods were used to study the kinetics of formation of self-assembled monolayers ofn-alkanoicacids by adsorption from solutions. With reference to Langmuir-Blodgett deposited monolayers, the adsorption of n-alkanoic acids from hexadecane solutions onto glass and aluminum substrates was shown to lead to monolayer formation. Stearic acid adsorption, as monitored by FTIR, was found to follow a transient Langmuir adsorption model. Additionally, fluorescence measurements using a pyrene-tagged n-alkanoic acid probe co-adsorbed into the monolayers were used to investigate the dependence on chain length. An increase in the negative free energy of adsorption with increasing acid chain length was shown. Organized organic molecular monolayers formed by spontaneous adsorption from solution, known as self-assembled (SA) films, have attracted a great deal of research interest in recent years(7-5) because of their potential technological and scientific applications(6,7). The formation of compact, well-organized structures that closely resemble Langmuir-Blodgett monolayers has been evidenced by studies employing a number of physical measurements. In the formation of ordered structures from amphiphilic molecules, e.g., in the Langmuir-Blodgett transfer process(7,5) as well as in the self-assembling of amphiphilic molecules(/-4), the head group-substrate interaction and the tail group-tail group interaction are the major contributing factors. Surfactants solvated in hydrophobic, nonpolar solvents adsorb onto a hydrophilic polar surface such that the hydrophilic head groups attach to the surface while the hydrophobic tails line up with one another, forming a hydrophobic exterior surface that is analogous to the solution phase. In such systems the van der Waals attractive interaction among the hydrophobic portions of the adsorbate molecules contributes to their alignment. The ability of these molecules to form organized monolayers increases with increasing length of the aliphatic chain(5,S). Clearly, both the substrate-head group binding and the van der Waals attraction among the hydrocarbon chains will control the adsorption kinetics. Allara, Nuzzo, Whitesides and coworkers(3,4) and Sagiv and coworkers(/,2), in their series of extensive studies using a number of characterization techniques, have demonstrated that SA films of structure similar to that of the Langmuir-Blodgett films can be produced under appropriate conditions. They also showed that the kinetics of adsorption is important in the formation of these organized two-dimensional films. In 0097-6156/91/0447-0160$06.00/0 © 1991 American Chemical Society Scheuing; Fourier Transform Infrared Spectroscopy in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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9. CHEN AND FRANK

general, the well-known Langmuir isotherm is applicable for monolayer adsorption(9), but very little kinetic data have been reported for these types of systems. Fourier-transformed infrared spectroscopy (FTIR), either in the transmission mode(70), the grazing incidence reflection (GI) mode(7,5) or the attenuated total reflection (ATR) mode(7,2), has been the most widely used experimental tool for the characterization and structure determination of SA monolayers. GI-IR is especially useful in determining the molecular orientation in the film structures because it senses only the vibrational component perpendicular to the substrate surface(7,J). Polarized ATR-IR can also be used to study molecular orientation(7,77). McKeigue and Gulari(72) have used ATR-IR to quantitatively study the adsorption of the surfactant Aerosol-OT. Besides FTIR methods, other spectroscopic techniques such as fluorescence and UV spectroscopy may be considered as complementary experimental tools to FTIR. Fluorescence spectroscopy of systems containing small fractions of covalently bound fluorescent probes within the compound of interest can provide molecular-level information. The usefulness of the fluorescence probe approach has been demonstrated for Langmuir-Blodgett films(73). In this work we utilized FTIR methods to examine the SA monolayers on flat, polar solid surfaces prepared from nonpolar solutions. We used ATR and GI FTIR measurements to characterize the material and bonding of the SA monolayers, and used transmission and ATR FTIR to monitor the dynamics of the SA adsorption process. With reference to measurements on standard Langmuir-Blodgett monolayer samples, we were able to quantify the SA kinetic results. We also used fluorescence spectroscopy of incorporated pyrene probes in SA mixed monolayer films as a simple method for the determination of the relative adsorption and thermodynamic constants. Experimental Sample Preparation. The homologous series of the even n-alkanoic acids, abbreviated as C through were used as the adsorbates. Hexadecane (HD), which is nonpolar and has a rather high boiling point, was used as the solvent. Microscope glass slides and evaporated aluminum (on silicon wafers) were used as the substrates. Pyrene end-tagged hexadecanoic acid (Py-C ) was used as the fluorescence probe. SA monolayers were prepared by immersing the substrates in the w-alkanoic acid/HD solutions for a predetermined period of time. After removal from the solutions, any remaining liquid droplets on the surface or the edges of the substrates were blown off with a nitrogen jet. For the equilibrium monolayer characterization, relatively high solution concentration (>10"M) were used. For time-dependent kinetic studies, stearic acid (C ) solutions of concentrations between 10" and 10"M were used. Immersion times from a few seconds up to 24 hours were used. The reference samples of Langmuir-Blodgett monolayers of C^-Qg monolayers were prepared using a Joyce-Loebl Langmuir trough. Film materials were deposited using solutions in CHC1 . For the cadmium salt monolayers, a subphase of 2.5x10"^ CdCl solution was used. All monolayers were deposited at 30 dynes/cm and 15 C. Pyrene-labeled SA mixed monolayers were prepared by adsorption from solutions of the desired concentration of a particular fatty acid, along with a small fraction (1-5%) of the probe Py-C . All solutions used were of total acid concentration of 5xl0" M. 1 0

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Sample Characterization. Infrared spectra of the adsorbed films were obtained with a Perkin-Elmer 1710 FTIR Spectrometer, equipped with a DTGS detector and a nitrogen-purged sample chamber. The transmission IR spectra (high frequency range; >2000 cm") of the adsorbed species were directly measured on the glass slides. The 1

Scheuing; Fourier Transform Infrared Spectroscopy in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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FTIR SPECTROSCOPY IN COLLOID A N D INTERFACE SCIENCE

ATR-IR spectra were taken on the same spectrometer, using a multiple-internalreflection attachment obtained from Harrick Scientific Inc, using an ATR crystal (one-pass parallelepiped KRS-5; 45% 50x10x3mm) also from Harrick. The ATR-IR spectra of adsorbed species were obtained by pressing two film-covered substrates against the internal reflection surfaces of the ATR crystal. All spectra were obtained with 4 cm" resolution. In practice, it was usually necessary to average 1000-2000 scans in order to obtain spectra of acceptable signal-to-noise ratio. The reference (background) spectra were taken with clean substrates prior to adsorption. The auxiliary film characterization tools used include contact angle goniometry and ellipsometry. A Ramé-Hart goniometer was used to measure the advancing contact angles of HD and water on substrates before and after adsorption of the fatty acids. A Gaertner ellipsometer with a He-Ne laser source was used to measure the film thickness. For the fluorescent probe studies,fluorescenceemission spectra of the pyrene-doped monolayers were obtained with a Spex Fluorolog 212 spectrofluorometer. The excitation wavelength was set at 343 nm and the spectra were taken in the front-face mode.


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Results Characterization of Monolayers. The change in surface wettability of the substrate was used as a qualitative indicator for monolayer adsorption. Clean glass and aluminum substrates were wetted by both water and HD. The contact angles were small (< 10 ). After adsorption of the amphiphilic fatty acid molecules, the surface wettability was drastically reduced, as indicated by the sheeting-off of the solutions. The advancing water contact angle on equilibrated fatty acid/Al samples obtained was 105±2 , and the hexadecane contact angle was 47±2 . For fatty acid/glass samples, the hexadecane contact angle was close to that of fatty acid/Al. These values were similar to those previously reported(l,3). To confirm the formation of monolayers, the thickness of equilibrated fatty acid films Q4-C22/AI, prepared from relatively concentrated (0.01M) solutions, were measured by ellipsometry. Theresults show that thefilmthickness correspond to the extended zig-zag molecular lengths of thefilm-formingmolecules(J), suggesting that they are indeed monolayers with the chains organized approximately perpendicular to the substrates. FTIR measurements were used to characterize the adsorbed monolayers. Transmission-IR spectra were obtained for the IR transparent range of glass (wavenumber^OOOcm*), which contains the C H and C H stretch peaks of the hydrocarbon tails of the fatty acid molecules. A typical transmission-IR spectrum of two stearic acid monolayers (one monolayer on each side of a glass slide) adsorbed from 0.01 M solution is shown in Figure 1(a). The symmetric and asymmetric C H stretches (2850cm and 2920cm" respectively) and the symmetric and asymmetric CH stretches (2890cm and 2962cm", respectively) were observed as expected. ATR-IR measurements were used to obtain the full-range IR spectra of the adsorbed species. Two substrates with adsorbedfilmswere pressed against the two ATR crystal surfaces so that the adsorbate was sampled by the internally reflected IR beam. A moderate pressure was necessary in order to "push" the substrate surface into the penetration range of the IR beam, which is on the order of several micrometers. Shown in Figures 1(b) and 1(c) are typical ATR-IR spectra of adsorbed stearic acid adsorbed at equilibrium on glass and Al substrates, respectively, the most significant difference between these two spectra is that only the 1420/1470cm and 1580cm" peak, assigned to the symmetric and asymmetric COO" stretches were observed for C /A1, while the 1730cm' peak, assigned to the free acid C=0 stretch, was seen in addition to the carboxylate stretches for C /glass. This implies that on the Al substrate the anchoring acid head groups were totally deprotonated into salt (COO")(5), while on glass only a portion of them were deprotonated on adsorption, the remainder remaining as free acid. The glass slides contain several metal oxides such as Na 0, CaO, MgO, and A1 0 , e

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9. CHEN AND FRANK

n-Alkanoic Acid Self-Assembled Monolayers163

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Figure 1.

FΉR spectra of C monolayers: (a) High-frequency range transmission-IR spectrum of two C /glass monolayers (one monolayer on each side), (b) ATR-IR spectra of C /glass, and (c) C /aluminum (with oxide layer on top); prepared from 0.01M solution in HD, immersion time=30min. 18

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FTIR SPECTROSCOPY IN COLLOID AND INTERFACE SCIENCE

along with the major component Si0 . Apparently these metal atoms present on the surface enhance the anchoring of the acid head groups, perhaps by chemical bonding, to the metal sites (salt formation), while only physisorption (polar interaction with free acid) occurs on the silicon sites. GI-FTIR spectra for have also obtained for C /A1 monolayers. The results were very similar to those of Allara and Nuzzo(4). The spectra show enhanced symmetric and asymmetric CH stretches and diminished symmetric and asymmetric CH stretches, as expected for ordered hydrocarbon chains approximately perpendicular to the substrate surface. Similar to the ATR-IR result, carboxylate peak was also observed in the GI-IR spectra. With these results, the formation of ordered monolayer structure is also supported. 2

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Quantification of FTIR Measurements. The transmission-IR and ATR-IR absorbance can be used to quantitatively determine the amount of adsorbate present. To serve as a standard reference, Langmuir-Blodgett deposited C^-C^ Cd salt monolayers were used. The C H stretch peak absorbances were correlated with the absolute surface concentration of CH groups, based on measured deposition ratio and molecular area, together with the number of CH units in the molecule. With monolayers deposited at 30 dynes/cm, the deposition ratios were generally close to 1 (1.0-1.2), except for the very lone (0.6-0.7). At this surface pressure, the molecular area of the acids were 18-20 A /molecule, decreasing with increasing chain length. Figure 2 shows such a correlation between the transmission FTIR 2920 cm* peak absorbance and N ^ , the surface concentration of CH groups. The Absorban2

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ce/CH thus obtained can then be used to quantify the similar measurements for the SA monolayers. The values obtained for transmission-IR and ATR-IR are listed in Table I. 2

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Table I. Reference Values of 2920 cm Absorbance/CH

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Absorbance!A? CH group

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Transient Adsorption Behavior. The adsorption of C on glass or Al at various times of immersion from solutions of various adsorbate concentrations was followed by transmission-IR and ATR-IR. Contact angle measurement was also used to follow the surface wettability change for these samples. Both the IR absorption peak intensities and the contact angles increased initially with increasing time of immersion as well as increasing adsorbate concentration, and asymptotically reached plateau values at long immersion times and high solution concentrations. The time required to reach the plateau maxima increased with decreasing adsorbate concentration. Figures 3(a) and 3(b) show the 2920cm" transmission IR peak intensity and the HD contact angle variation for C /glass, and Figures 4(a) and 4(b) the 2920cm" ATR-IR peak intensity and the water and HD contact angle variation for C /A1, respectively. Using the reference Absorbance/CH Â values in Table I, the surface concentrations of adsorbed C were also calculated and are shown in theright-handscales of Figures 4(a) and 5(a). The plateau value of approximately 8.5xl0" mol/cm (corresponding to 19.6Â /molecule) agrees quite well with the well-known packing densities in L-B monolayers of simple amphiphilic compounds. The intensity of the peaks other than the one at 2920cm also increased proportionately. The plateau intensities of the high 18

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n-Alkanoic Acid Self-Assembled Monolayers


CHEN AND FRANK

Figure 2.

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Correlation of 2920cm", 2850cm*, and 2962cm transmission-IR peak absorbances with surface concentration of -CH - units of L-B monolayers of C -C28. 2

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Alkanoic Acid Self-Assembled Monolayers - American Chemical Society

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