Langmuir 1988,4, 569-572 by several ionic diameter^.'^
Conclusions The analysis of the electrical double-layer equilibria, as presented in this study (the theoretical part is published separately'), introduced the evaluation of the equilibrium constant for association of surface charged groups with counterions. The surface association equilibrium constant was obtained on the basis of a statistical distribution of counterions in the vicinity of central charged groups on the surface. This procedure is limited to 1:l electrolytes and systems of relatively low surface charge density. The specificity of counterions in adsorption and coag-
569
ulation phenomena is described by means of the ionic size parameter, i.e., the distance of closest approach between surface charged groups and associated counterion. In the pH region around pzc, the low value of $+, does not permit association. Consequently,no specific effect of counterions in this region is expected. The presented results indicate possible applications; the interpretation of adsorption equilibria, colloid stability, and electrokinetic properties.
Acknowledgment. We are indebted to Mr. Darko BabiE for useful discussions and his help with computations.
Characterization of Vacuum-Deposited Perfluorocarboxylic Acid Monomolecular Film by Penning Ionization Electron Spectroscopy Munehisa Mitsuya* Advanced Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan
Hiroyuki Ozaki and Yoshiya Harada Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Meguro, Komaba, Tokyo 153, Japan
Kazuhiko Sekif and Hiroo Inokuchi Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Received June 12, 1987. In Final Form: November 16, 1987 The molecular arrangement of perfluorocarboxylic acid monomolecular film, vacuum-depositedon a dehydrated SiOzsubstrate, is studied through cyclic thermal treatment using Penning ionization electron spectroscopy. The film was ascertained to cover the substrate surface up to 60 O C . Reversible spectral change due to the thermal fluctuation of fluorocarbon tails is observed in the temperature range 80-100 "C,while irreversible disordering of the molecules occurs above 150 "C. The film prepared on an undehydrated substrate easily desorbs in ultrahigh vacuum even at room temperature. These results suggest that the chemical bond between the molecules and the substrate dominates the stability of the ordered film.
Introduction There has been considerable interest in the use of molecular orient6d films where designed molecules are artificially arranged in a preferable direction. Such film is known to be traditionally prepared by the LangmuirBlodgett (LB) method.' Another method for preparing oriented films is vacuum deposition, which has been developed mainly from the viewpoint of epitaxial In a sequence of studies of molecular arrangements and the related properties of such films, much attention has begun to be devoted to the structure studied in more detail from a microscopic point of view. Tredgold and Winter postulated that the electrical conduction in monolayers is via defects and presented experimental evidence to support the view.' Peterson et al. showed that molecules are reorganized both on the subphase and on the substrateas Matsuzaki et al. investigated the growth mechanism of stearic acid films from the vapor phase and determined Present address: Department of Materials Science, Faculty of Science, Hiroshima University, Higashisenda-machi, Hiroshima 730, f
Japan.
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the optimum condition for preparing homogenous films.g Such studies on the packing of constituent molecules will become more important in both scientific and practical fields. This paper reports on the Penning ionization electron spectroscopy (PIES)of perfluorocarboxylic acid monomolecular film prepared by vacuum deposition. Because of the small polarizability of constituent fluorine atoms, the first layer deposited on a hydrophilic surface with its polar groups is supposed to hinder further deposition, and (1) Blodgett, K. B. J. Am. Chem. Soc. 1935, 57, 1007. (2) Buchholz, J. C.; Somorjai, G. A. J. Chem. Phys. 1977, 66, 573. Firment, L. E.; Somorjai, G. A. J. Chem. Phys. 1978, 69, 3940. (3) Ueda, Y.;Ashida, M. J. Electron Microsc. 1980,29, 38. (4) Kobayashi, T.; Fujiyoshi, Y.;Iwatau, F.; Uyeda, N. Acta Crystallogr., Sect. A Found. Crystallogr. 1981,37,692. Kobayashi, T.; Fijiyoshi, Y.;Uyeda, N. Acta Crystallogr., Sect. A: Found. Crystallogr. 1982,38, 3.M.
(5)Debe, M. K. J. Vac. Sci. Technol. 1982,21, 74. ( 6 ) Harada, Y.;Ozaki, H.; Ohno, K. Phys. Rev. Lett. 1984,52, 2269. (7)Tredgold, R. H.; Winter, C. S. J.Phys. D 1981,14, L185. ( 8 ) Peterson, I. R.; Russell, G. J.; Roberta, G. G. Thin Solid Films 1983,109, 371. (9) Matsuzaki, F.; Inaoka, K.; Okada, M.; Sato, K. J. Cryst. Growth 1984, 69, 231.
0 1988 American Chemical Society
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successive sublimation results in a monomolecular filmslo However, it was not known whether the substrate surface is thoroughly covered with molecules. PIES seems to be one of the most appropriate methods available to clarify this problem. In Penning ionization, electrons are ejected by collisions between targets T and metastable atoms A*: T+A*-T++A+eThe energy distribution curves of the ejected electrons, PIE spectra, probe the electron distribution exposed to the outermost surface layer selectively, since metastable atoms do not penetrate into the inner layers.l' This is a contrast to the photoemission process: T + hv T+ + ewhere photons used for excitation penetrate into the inner layers and electrons within the escape depth of the solid are observed. From a comparison of the PIE spectrum of the deposited film with that of the substrate, it was indicated that no detectable defect such as a pinhole exists in the as-deposited film. Furthermore, a two-step disordering process was observed through a cyclic thermal treatment: reversible fluctuation of fluorocarbon tails around 80-100 OC and irreversible disordering of molecules beyond 150 O C . The second disordering temperature is higher than the bulk melting point (88 "C). This suggests that direct interaction of molecules with the substrate modifies thermal stability of the molecular assembled film.
-
Experimental Section Perfluorodecanoic acid, having a normal-chain structure, CF3(CF2)&OOH,was commercially obtained and utilized after recrystallizationfrom its benzene solution. Its melting point was 88-90 "C, in agreement with the reported data.12 Surface-oxidized silicon(lll),having a SiOzlayer thickness of approximately 1nm, was used as the substrate. This substrate satisfies the following three requirements. The first is a presence of polar groups at the surface for obtaining good molecular orientation. The second is a reasonable conductivity, indispensable for electron-emission measurements. The third is that the peak energy of the emission from the substrate is different from that for perfluorocarboxylic acid, as will be shown later. This enablesdetection of the exposed substrate part by the emission inherent in the substrate. The monomolecular film was prepared by vacuum deposition Pa.'O in the sample vapor of about The PIE spectra and ultraviolet photoelectron (UP) spectra were measured in an ultra-high-vacuum (about lo4 Pa) electron ~pectr0meter.l~ He* (2%, 19.82 eV) metastable atoms produced by electron impact and the He I resonance line (21.22 eV) obtained by dc discharge were used as the excitation sources. The film deposited on the substrate was fixed at the top of the sample probe of the spectrometer and was rotated from outside of the vacuum chamber, so that its surface could be exposed successively to the metastable atoms and the light beam. Electron spectra were obtained at an ejection angle of 90° relative to the excitation source,using a 180" hemispherical analyzer of 5-cm mean radius. The details of the spectrometer have been described e1~ewhere.l~ Results and Discussion In a preliminary experiment, perfluorodecanoic acid was vacuum deposited on a substrate cleaned first with isopropyl alcohol and then with distilled water. Contact angle measurements with a series of n-alkanes revealed that the (10) Mitsuya, M.; Taniguchi, Y. J. Colloid Interface Sci. 1985, 107,
287.
(11) Harada, Y.Surf. Sci. 1985,158,455 and references therein. Ozaki, H.; Harada, Y. J. Am. Chem. SOC.1987,109,949. Ozaki, H.; Harada, Y.; Nishiyama, K.; Fujihira, M. J. Am. Chem. SOC.1987, 109,950. (12) Hare, E. F.; Shafrin, E. G.; Zisman, W. A. J . Phy. Chem. 1954,58, 236. (13) Harada, Y.; Ozaki, H. Jpn. J . Appl. Phys. 1987, 26, 1201.
Mitsuya et al.
L
0.70 o.6
50
100
150
TEMPERATURE / " C
Figure 1. Change in n-hexadecane contact angle (e) as a function of postheatingtemperature: (a) without preheating;(b) preheated
at 120 "C; (c) preheated at 220 "C.
critical surface tension of the film was 11dyn/cm. This value indicates that the major component of the film surface is the trifluoromethyl group of perfluorodecanoic acid.1° The contact angle was also ascertained to be independent of the time (up to 5 h) during which the film was kept under vacuum after deposition. This result rules out the desorption of molecules under the pressure of Pa. However, the film is unstable in ultrahigh vacuum. The PIE spectrum of the vacuum-depositedfilm gradually changed at room temperature. The rise in the substrate temperature to 50 OC accelerated this spectral change and made the substrate emission clearly observable. This means that deposited molecules are desorbed from the substrate under ultrahigh vacuum. In order to make clear the desorption mechanism, the effect of the thermal treatment of the substrate was investigated. Washed substrates were heated to a prescribed temperature in the deposition chamber (preheating), and perfluorodecanoic acid was deposited after the substrate was cooled to room temperature. Then the substrate with the film on it was again heated under vacuum (postheating). The preheating and postheating times were 1h and 30 min, respectively. The contact angle (e) of n-hexadecane on the film is shown in Figure 1 as a function of the postheating temperature. For film prepared on the substrate without preheating, the contact angle readily decreased on postheating (a in Figure 1). This suggests an easy desorption of molecules. With the increase in preheating temperature, the decrease in the contact angle becomes smaller. When the preheating temperature was raised to 120 "C, the contact angle remained unchanged through postheating up to 100 "C (b in Figure 1). Preheating at higher temperatures did not lead to further changes in the contact angle curve; curve c for 220 OC and curve b for 120 OC are almost the same (Figure 1). In summary, an easy desorption of perfluorodecanoic acid molecules can be prevented by preheating the substrate above 120 "C. This result most certainly indicates the removal of water molecules adsorbed on the hydrophilic SiOz surface, which promotes the desorption of sample molecules. Such a water layer is stable even under vacuum at room temperature, while it is easily desorbed at high temperatures. Thus monomolecular films tightly bonded to the substrate surface could be obtained. The PIE and UP spectra of such a monomolecular film deposited on the dehydrated substrate (preheated at 170 "C) were next considered, together with the change in the spectra through a cyclic thermal treatment. Figure 2 shows the PIE spectra of the film (a-j) compared to that of the substrate (k). The abscissa indicates the kinetic energy of electrons, Ek. In order to exclude the least possibility
PIES PerfluorocarboxylicAcid Characterization
Langmuir, Vol. 4, No. 3, 1988 571
15
10
5
0
ELECTRON ENERGY/eV
Figure 3. Ultraviolet photoelectron spectra of the film (a, f, h, j) and the substrate (k). Sample temperatures are (a) 0 "C,(f) 100 "C, (h) 150 "C, and (i) 200 "C.
of desorption, the substrate was at first cooled to 0 "C during the evacuation of the spectrometer. The first spectrum, a in Figure 2, was recorded 90 min after starting evacuation under a vacuum of about lo-' Pa. The film was then brought to room temperature and allowed to stand for 2 days before measuring the second spectrum (Figure 2b). Afterwards,the subsequent spectra (Figure 2 e j ) were recorded during repeated heat cycles in which the sample was heated to higher temperatures and cooled spontaneously to room temperature overnight. In the temperature range 0-60 "C, the spectrum does not change significantly (Figure 2a-d). The feature with electron energy Ek around 4 eV is assigned to the Fzp lone-pair orbital.14 On the other hand, two assignments are possible for the emission with Ekaround 9 eV. One is the valence band formed by the FZporbitals on the terminal carbon atom, from analogy with poly(tetrafl~oroethylene).'~The other is the C-C u orbital since the frontier orbital of hexafluoroethane has a C-C ~haracter.'~ At present, the more resonable assignment cannot be determined. In the spectra a-d of Figure 2, the emission inherent in the substrate, with Ekaround 6.5 eV, is scarcely observed. This means that the film surface has no detectable defect such as pinholes nor any desorption of molecules during the measurements. In the course of the film preparation using the vacuum deposition method, the organization of molecules proceeds directly on the substrate surface. Consequently, even if the surface contains an irregular part, such as a difference in levels, molecules are supposed to be deposited over the geometrical irregularities. This
behavior seems to result in a sufficient covering of the substrate surface even for the monomolecular film. In the temperature range 80-100 "C, spectra e and f of Figure 2 show the two following types of change. The first is the appearance of the emission from the substrate (indicated by arrows in the figure) as a shoulder. The second is an obscuring of the emission around 9 eV. This suggests the structural disorder due to thermal agitation,16J7which shields the F, orbital from the attack of metastable atoms, or the bending of the trifluoromethyl group resulting in the concealment of the part of the C-C u orbital exposed outside the surface. These two spectral changes are apparently reversible; when the film was cooled to room temperature (Figure 2g), the emission from the substrate disappeared and the higher energy peak recovered. Consequently, both of these changes can be ascribed to the thermal fluctuation of fluorocarbon tails anchored by polar groups to the substrate surface. When the substrate was heated above 150 "C, emission from the substrate is enhanced (h and j of Figure 2). This may be caused by the irreversible collapse of molecular array and/or the desorption of carboxylic acid molecules, since the spectrum does not recover upon cooling to room temperature (i). Figure 3 shows representative UP spectra of the film (a-j) and the substrate (k). Symbols in this figure correspond to those in Figure 2. In contrast to a drastic change in the corresponding PIE spectrum, UP spectrum is little affected by heating. The intensity of the FZplone-pair band alone is decreased. PIE and UP spectroscopies are generally complementary. In multilayer films, from the analyses of these two spectra, the electronic states of the outer and inner layers were probed separately.6 In the present work, the subtle structural change in the monomolecular film was clearly detected from the PIE spectra, as was shown from the comparison of Figures 2 and 3. The PIES measurements mentioned above have shown that the monomolecular film of perfluorocarboxylic acid exhibits a two-step disordering process upon heating. The first step is a reversible disordering of fluorocarbon tails with ordered heads (80-100 "C) and the second is an irreversible disordering of molecules (150 "C). Such two-step behavior resembles that of the cadmium arachidate (CdA) LB film investigated by infrared absorption spectra.'*
(14)Pireaux, J. J.; Riga, J.; Caudano, R.; Verbist, J. J.; Delhalle, J.; Delhalle, S.; Andre, J. M.; Gobillon, Y.Phy. Scr. 1977,16,329.Delhalle, J.; Delhalle, S.; Andre, J. M.; Pireaux, J. J.; Riga, J.; Caudano, R.; Verbist, J. J. J. Electron Spectrosc. Relat. Phenom. 1977,12,293. (15)Sauvageau, P.;Doucet, J.; Gilbert, R.; Sandorfy, C. J. Chem. Phys. 1974,61,391.
(16).Mott, N. F.; Davis, E. A. Electronic Processes in Non-Crystalline Matenals, 2nd ed.; Clarendon: Oxford, 1979. (17)Seki, K.;Ueno, N.; Sakamoto, K.; Hashimoto, S.; Sugita, K.; Inokuchi, H. Rep. Prog. Polym. Phys. Jpn. 1982,15,581. (18)Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1986,82, 2136.
I , , ,
15
,
I , , , , I , I
10
, , I
5
0
ELECTRON ENERGY /eV
Figure 2. Penning ionization electron spectra of the film (a-j) and the substrate (k). Sample temperatures are (a)0 "C, (b) room temperature, (c) 40 "C, (d) 60 "C,(e) 80 "C, (f) 100 "C, (9) room temperature &r cooling case f, (h) 150 "C, (i) room temperature after cooling case h, and (i) 200 "C. Arrows indicate substrate emission.
Langmuir 1988,4, 572-578
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What is worth noticing is the difference in the transition temperature, although care should be taken in comparing the transition temperature obtained by different methods of characterization. In the case of CdA monomolecular film, the disordering of hydrocarbon tails begins at room temperature, and a breakdown of the cadmium lattice occurs at the melting point of bulk CdA (110 "C). The stability of the fluorocarbon chain up to 60 OC can be ascribed to the large van der Waals radius of the fluorine atom. The rigidity of the fluorocarbon chain due to steric hindrance should suppress the thermal motion of the chain. A more striking difference, compared to the CdA film, is that the second disordering of perfluorodecanoic acid occurs a t a temperature fairly higher than its bulk melting point. This can be attributed to the strong chemical bond formed between the carboxylic group and the substrate. Thermal characteristics of solids, such as the melting point and enthalpy and entropy of fusion, are determined by intra- and intermolecular factors of constituent molecules. In the film, however, the interaction between molecules and the substrate also affects these characteristics. This interaction should be strong when a chemical bond is formed between molecules and the substrate, as in the present case. The fluorine substitution for the hydrogen atoms in the alkyl group should not be a main factor concerning the increase in the interaction. This is
supported by the fact that perfluorodecanoic acid deposited on the substrate without preheating desorbs easily. Consequently,strong interaction between the f i i and the substrate must be derived from the dehydrated, clean substrate and not from the fluorinated compound itself. It is generally thought that a substance can be endowed with novel characteristics when in an ultra-thin-film state. In this article, we have presented a typical example of this potential, a rise in disordering temperature.
Conclusion The order-disorder transition of a vacuum-deposited film was examined by using Penning ionization electron spectroscopy. A monomolecular f i of perfluorocarboxylic acid deposited on a dehydrated SiOz substrate was shown to cover the substrate surface. Cyclic thermal treatment of the film revealed that the molecular arrangement of the film is maintained even at an elevated temperature as long as the head group of the molecule is directly attached to the substrate. The cleanliess of the substrate surface was ascertained to play an important role in the stability of the molecules assembled on it.
Acknowledgment. We are grateful to Dr. Shigeru Masuda for his valuable discussion. Registry No. SOz, 7631-86-9;CF3(CF2)&02H, 335-76-2; CH3(CH2)14CHS,544-76-3.
Effect of Thermal Pretreatment on the Surface Reactivity of Amorphous Silica C. H. Lochmuller* and M. T. Kersey Paul M. Gross Chemical Laboratory, Duke University, Durham, North Carolina 27706 Received July 27, 1987. In Final Form: November 13, 1987 The effect of thermal pretreatment of an amorphous silica on the distribution and condensation of surface reactive groups has been examined by using elemental carbon analysis in conjunction with steady-state and time-dependent fluorescence spectroscopy. It is found that the condensation of surface silanols is a reversible process up to heat pretreatment temperatures of 800 OC and that above 800 O C the condensation reaction is no longer reversible. Steady-state and time-dependent results indicate that the distribution of a covalently bound probe, approximating a chlorcdimethyloctadecylsilaneligand, is relatively unaffected by thermal surface modification. Finally, the present work suggests that isolated surface silanols are more reactive toward chlorotrimethylsilane than paired silanols.
Introduction Silica gel is the most commonly used stationary phase support in reversed-phase high-performance liquid chromatography (RPHPLC). Its use has prompted a large number of investigations into the effect of thermal pretreatment on the bulk and surface properties of silica.' Heat pretreatment of silica prior to chemical modification has also been examined to discover what effects, if any, are seen in the ultimate chemical separations.2 For example, the separation of basic solutes appears to be de(1) (a) Scott, R. P. w.;Traiman, S. J. Chromatogr. 1980,196,193. (b) Unger, K. K. Porous Silica; Elsevier: Amsterdam, 1979. (c) Belyakova, L. D.; Kiselev, A. V.; Kivaleva, N. V. Anal. Chem. 1964,36, 1617. (d) HalHsz, I.; Martin, K . Angew. Chem. 1978,90,954. (e) Scott, R. P. W. J. Chromatogr. Sci. 1980, 18, 297. (0Lark, K. D.; Unger, K. K. J . Chromutogr. 1986,352, 199. (9) Scott, R. P. W.; Kucera, P. J. Chromatogr. Sci. 1976, 13, 337. (h) Davydov, V. Ya; Kiselev, A. V.; Zhuravlev, L. T. Trans. Faraday SOC.1964,60, 2254. (2) Mauss, M.; Engelhardt, H. J . Chromatogr. 1986, 371, 235.
pendent, in part, on the distribution of surface-bound silanes and on the activity of the underlying, residual surface silanols. Thermal pretreatment of amorphous silica has been used to deactivate these irregular, heterogeneous surfaces. Scott'g suggested that silica surfaces have three strongly adsorbed surface water layers. Thermogravimetricanalysis of silica indicated a continuous loss of surface adsorbed water and the possible condensation of surface silanols at temperatures exceeding 450 OC. Further examination of amorphous silica by Mauss and Engelhardt2 showed a general decrease in the surface area and pore volumes of silica with increasing pretreatment temperature. Their results suggested that loss of surface area and pore volume upon dehydration of silica surfaces is reversible up to 800
"C. There is general agreement in the literature that two types of surface silanols are accessible for surface modification. Figure 1 depicts the two silanol types: isolated
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