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Solution Interface
Journal of Colloid and Interface Science 209, 25–30 (1999) Article ID jcis.1998.5869, available online at http://www.idealibrary.com on
Time Dependent Anchoring of Adsorbed Cationic Surfactant Molecules at Mica/Solution Interface Bangyin Li,1 Masatoshi Fujii, Kazuhiro Fukada, Tadashi Kato, and Tsutomu Seimiya2 Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, 1-1 Minami-Oosawa, Hachioji, Tokyo 192-03, Japan Received March 20, 1998; accepted September 17, 1998
hydrophobicized surface hydrophilic. This behavior is are directly reflected in the stability of suspended particles; cationic surfactants are well known (7) for showing both stabilizing and destabilizing properties of the suspension which occur above and below the particular concentration of surfactant. The purpose of the present study is to elucidate the nature of the interaction of cationic surfactant with the adsorption sites on mica surfaces. It is also the purpose of the present study to determine the standard conditions for obtaining well-defined reproducible hydrophobic surfaces suited to our AFM surface force studies. Similar studies have also been carried out by Chen et al. (8). Their fine analyses by XPS, contact angle, and surface force measurements also revealed the kinetic behavior of cationic surfactant at mica/solution interface as well as that of equilibrium. They too have found that the adsorption of cationic surfactant on mica surface is an extremely slow process. The principal difference of the present studies from that of Chen is that the preparative method of the “adsorbedspecimen” which leads to different conclusions. In Chen’s studies, the mica specimen was withdrawn from the adsorption bath at 25°C and was subjected to XPS and contact angle analyses after drying without rinsing, while the mica surface in the present studies was adsorbed at 5°C rinsed with purified water after withdrawn from adsorption bath to remove mobile molecules prior to the XPS and contact angle analyses. The combination of the information obtained by both methods should help to resolve the complex behavior of surfactant molecules at solid/solution interfaces.
The nature of adsorbed cationic amphiphiles at the mica/solution interface was studied by XPS and contact angle measurements. The elemental analyses of freshly cleaved mica surfaces by XPS showed that the potassium atoms on the surface lattice of mica are not necessarily distributed equally to each surface on cleavage. The adsorbed cationic amphiphile molecules remaining on mica surfaces after rinsing with distilled water were found to be anchored to the surface by ion-exchange, replacing surface potassium and/or other cations. The ratio of adsorbed cationic amphiphile molecules with single alkyl chains to the maximum potassium ions on mica surface was estimated to be twice as large as that of amphiphiles having two alkyl chains. The contact angle of water drops placed on the adsorbed surface showed a gradual decrease with the elapse of time due to the dissolution of adsorbed surfactant into the water drop; however, the decrease was not observed for those mica surfaces when aged for more than 3 days in the adsorption bath. The anchoring of adsorbed molecules by ion-exchange was found to occur extremely slowly, however; the anchored molecules may not easily be desorbed when rinsed with deionized water. The time dependent anchoring of adsorbed molecules was studied in terms of adsorption time, alkyl chain length, and concentration of cationic surfactant. © 1999 Academic Press Key Words: adsorption; cationic surfactant; potassium ion; cesium ion; ion-exchange capacity of mica; hydrophobicity; contact angle; wettability; XPS; ESCA.
INTRODUCTION
The adsorption of anionic (1, 2) or cationic surfactants (3– 8) on solid surfaces positively or negatively charged in water were known to form two- or three-dimensional molecular aggregates (1– 8) at the interface above and below the cmc of corresponding surfactants. The adsorbed molecules which are in direct contact with a mica surface are expected to be bound by ion-exchange and modify the solid surface to be hydrophobic while the molecules in the outermost part of the surface aggregates are held by hydrophobic interaction and render the
EXPERIMENTAL
Materials, Methods, and Procedures The octadecyltrimethylammonium bromide and other homologues and analogues of alkylammonium cations used in the present studies are abbreviated as 1Cn TAB for alkyltrimethylammonium bromide or 2Cn DAB for dialkyldimethylammonium bromide hereafter. These compounds, having purities of 98%, were purchased from Tokyo Kasei Kogyo Co., Japan, and were used without purifying further. The adsorption was carried out at 5°C throughout these experiments as an optimum
1 The paper fulfills in part the requirement of Ph.D. degree of Bangyin Li at Tokyo Metropolitan University in February 1997. 2 To whom correspondence should be addressed.
25
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
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FIG. 1. Relative intensity of the XPS signal of potassium atoms for “Mica”, “H1-Mica”, and “K1-Mica”.
condition to ensure the reproducible array of adsorbed molecules as judged through AFM observation. The wettability of mica surface covered with adsorbed surfactant is estimated by measuring advancing contact angle of water droplets of 2–5 ml placed on the mica surface in a humidified glass box. The crystal of Muscovite mica (from India) was cleaved in aqueous surfactant solution to avoid any contamination from ambient atmosphere and to allow surfactant molecules to adsorb on the newly exposed surfaces. The specimens of mica were withdrawn from the aqueous surfactant solution at appropriate times, rinsed for 2 min under a stream of deionized water of about 1 liter to remove surfactant solution together with weakly bound surfactant molecules and
leaving the molecules chemically anchored. The water used was purified by a Milli-Q system of Millipore Corporation, U.S.A. Hydrochloric acid (ca. 35%), potassium chloride (99.9%), and cesium chloride (99.0%) used were all purchased from Wako Pure Chemical Industries, Japan. Both XPS and contact angle measurements were made for dried specimen of mica. The contact angle of water drops was determined goniometrically in the field of optical microscope as an average of eight drops randomly placed on mica surface of 10 3 20 mm. The contact angle measurements were carried out in atmospheres of both open laboratory air and a clean humidified environment in a glass chamber. No significant difference was noted for these environmental changes. The desorption problems inevitably encountered during the rinsing process will be discussed later in detail. The X-ray photoelectron spectrometer used was ESCA-750 of Shimadzu Co., Japan. The X-ray of MgK a was used and the intensity of photoelectron was measured at a take-off angle of 90°. The XPS spectrum elucidated the change of atomic composition of the surface: the atomic densities of Si, Al, K, Cs, N, Cl, and Br were determined before and after the surface modification of mica. RESULTS AND DISCUSSION
Anchoring of Adsorbed Molecules by Ion-Exchange Figure 1 shows the XPS spectrum indicating the change of potassium densities of mica surfaces with and without the modification by other cations. The surface density of K atoms is expressed in Fig. 1 as I1K, indicating the intensity of photoelectron emitted by K atoms located at the surface zone of mica specimen. Those filled circles plotted above the mark “Mica” of the horizontal axis represent the K atom densities observed for each surface of mica freshly cleaved
FIG. 2. XPS spectra of nitrogen (a) and cesium (b) atoms for the surfaces “Mica”, “Cs1-Mica”, and “1C18TAB-Mica”.
TIME DEPENDENT ANCHORING OF CATIONIC SURFACTANT ON MICA
27
FIG. 3. XPS spectra of bromine (a) and chlorine (b) atoms for the mica surfaces modified with various hydrophobic cations.
in air. Large fluctuation shows that the K atoms are not necessarily shared equally between the two surfaces on cleavage. The filled triangles above the mark “H1-Mica” indicate the decrease of surface K atoms when the mica surface was ion-exchanged with aqueous hydrochloric acid (pH 3) for 6 h to wash out all K ions located at the outer lattice sites of mica accessible to H1 ions in the solution (8 –11). The fluctuation of the signal I9K observed for “Mica” is minimized by removing those ion-exchangeable potassium atoms unevenly shared on cleavage and leaving those nonexchangeable atoms located at the inner lattice sites of the mica although it accompanied the decrease of I1K. By treating “H1-Mica” with aqueous KCl solution of 5.0 3 1022 M for 6 h at room temperature, the fluctuation of I1K is largely minimized while the intensity I1K itself is maximized as shown by the filled diamonds for “K1-Mica”. Figure 2a and b represent the XPS spectra of IN1s and ICs3d observed for modified mica surfaces with various cations to show that the first layer of adsorbed cationic surfactant is bound to mica surface by ion-exchange. The abbreviations of “Cs1Mica”, “2C18DAB-Mica”, “1C18TAB-Mica”, and “4C1ACMica” indicated on each curve represent the type of modi-
fication made to the original “Mica” surface, namely, the symbol 2C18DAB represents that the mica surface was modified with adsorbed dioctadecyldimethylammonium bromide, and 4C1AC with tetramethylammonium chloride, respectively. Figure 2b shows that the spectrum of cesium atoms ICs3d observed for “Cs1-Mica” disappeared for “2C18DAB-Mica”, “1C18TAB-Mica”, and “4C1AC-Mica” adsorbed surfaces, indicating that the cesium cation was replaced with adsorbed surfactant. The increase of I1K for the process from “H1-Mica” to “K1-Mica” should represent the maximum number of adsorption sites accessible to the cationic surfactant molecules by ion-exchange and is indicated in Table 1 as DI1K. Expressed together with DI1K in Table 1, is the value I1Cs indicating the surface density of cesium atoms determined by XPS for “Cs1-Mica” which was obtained by treating “H1-Mica” with the aqueous cesium chloride solution of 5.0 3 1022 M. It is worth noting here that the value of DI1K is good compared with I1Cs emphasizing that the surfactant cations are adsorbed to the same ionexchange sites as Cs1 cations. The amount of adsorption was also calculated from the IN1s data of XPS spectra for those hydrophobic cations which were listed in Table 2.
FIG. 4. Aging of the contact angle of water drop placed on modified surfaces of 1C18TAB-Mica (a) and 2C18DAB-Mica (b). Each curve was obtained by immersing the mica specimen in 1C18TAB or 1C18TAB 0.1 mM aqueous solution for different lengths of time at 5°C.
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FIG. 5. Equilibrium contact angle of water drop placed on mica surfaces modified with two kinds of surfactant 1Cn TAB-Mica (a) and 2Cn DAB-Mica (b) plotted against adsorption time.
These values are consistent with molecular area data analyzed from AFM images taken for the same samples. The observed AFM images, not shown here suggest that the adsorbed surfactant molecules are aligned epitaxially to the surface lattice of cleaved mica. Details of the AFM studies will be published in a separate paper. Figure 3a and b shows the XPS spectra of Br3p and Cl2p atoms for various adsorbed surfaces. No bromine nor chlorine atoms were detected for those mica surfaces to which alkylammonium cations were adsorbed. This means that these organic cations did not accompany their bromide or chloride counter anions when adsorbed. Quantitative calculations of the amount of adsorption, Table 2, indicate that the amount of single-chain cationic surfactant adsorbed is twice as large as that expected from DI1K as estimated in Table 1. The fact that the counter anion was not detected in the present analysis may be due to the possibility that one half of the 1C18TAB molecules were exchanged with cat-
ions other than potassium. It is not certain at this moment whether the hydronium ion is taking the place of the potassium ion. The physical adsorption of surfactant molecules forming a bimolecular layer which accompanies the counter bromide anions may be possible, although these may be lost during the rinsing process during sample preparation. Figure 4a and b shows that the contact angle of water drops on the adsorbed surface is not stable, and changes with time, due probably to the desorption of adsorbed molecules into the water drop. The decrease of contact angle should begin at the moment of placing water drops on the adsorbed surface. The contact angle measured decays with time over the range of 2 to 3 h. It is interesting to note that this decay behavior is very much dependent on the length of immersion time of mica specimen in adsorption bath. For those adsorbed surfaces obtained in adsorption times less than 2 min, the observed contact angle is unstable, decaying rapidly from 76 to 56° for 1C18TAB-Mica (a) and from 83
FIG. 6. Schematic illustration of the time dependent anchoring process of adsorbed molecules as viewed from contact angle change.
FIG. 7. Equilibrium contact angle of adsorbed mica surface obtained for various single chained homologues of cationic surfactant at varied concentrations.
29
TIME DEPENDENT ANCHORING OF CATIONIC SURFACTANT ON MICA
FIG. 8. Effect of chain length of adsorbed cationic surfactant 1Cn TAB (a) and 2Cn DAB (b) on the maximum contact angle of water drop placed on adsorbed mica surface at 5°C.
to 72° for 2C18DAB-Mica (b) within 60 min, while for those surfaces of adsorption time longer than 96 h, the contact angle became stable over prolonged time of 2–3 h. The change of contact angle was from 82 to 79° for 1C18TABMica (a) and from 88 to 87° for 2C18DAB-Mica (b). The equilibrium contact angle values for 1C18TAB-Mica and 2C18DAB-Mica were plotted in Fig. 5 (a and b) as a function of adsorption time. The time dependent anchoring behavior of adsorbed surfactant in the present studies suggests that the ion exchange adsorption of cationic surfactant on cleaved mica surface proceeds very slowly and requires more than 90 h to be anchored in 0.1 mM solution at 5°C and to show a stable contact angle after rinsing. Such slow anchoring may involve many complex rate-determining processes, e.g., slow diffusion of molecules at the interface, hindered penetration and/or overturn of surfactant molecules in the adsorbed film, as depicted schematically in Fig. 6. Figure 7 shows the change of equilibrium contact angle with the concentration of adsorbed surfactant for various single chained homologues of cationic surfactants. The adsorption time was 72 h and the temperature was chosen at 5°C as the optimum conditions to give reproducible results. The maximum contact angle increases with alkyl chain length and shows the tendency to saturate for all surfactant homologues tested. The maximum contact angles attained in Fig. 7 are plotted in Fig. 8 as a function of alkyl chain length for both single chained 1CnTAB (a) and double chained 2CnDAB (b) cationic surfactants. Both of these figures show
that these surfactants do not endow the mica surface with the hydrophobicity of solid paraffin wax, which is as high as 105–106° (12). In Fig. 8b, filled circles represent the mica surface cleaved in air before immersion in surfactant solution, while the open circles are for the mica surfaces cleaved in surfactant solution. The agreement of both results suggests the absence of any serious airborne contamination of the surface when the sample was cleaved in air. CONCLUSION
The surface density of potassium ions on cleaved mica surfaces was unevenly shared between two surfaces newly formed by cleavage; however, the density became equal when the surface was treated with aq. KCl solution. Anchoring of cationic surfactant on cleaved mica surfaces by ion-exchange was found to be an extremely slow process. It takes more than 70 h to saturate the exchange capacity of a mica surface at 5°C at a surfactant concentration of 0.1 mM. The ion-exchange of cationic surfactant with potassium, cesium, and the other interfacial cations was confirmed by XPS. When the adsorbed mica surface is withdrawn from aqueous surfactant solution soon after immersion and rinsed with water, the adsorbed molecules are easily washed away and the surface remains hydrophilic, while surfaces equilibrated for a period of more than 70 to 90 h in surfactant solution at 5°C showed stable hydrophobicity. The adsorbed molecules do not desorb into the water drops placed on the surface for contact angle measure-
TABLE 1 Relative Intensities of the XPS Signal for Potassium and Cesium Atoms on “Mica”, “H1-Mica”, “K1-Mica”, and “Cs1-Mica” Surfaces Surfaces analyzed I1K XPS intensities DI1K I1Cs
“Mica”
“H1-Mica”
0.181 6 0.008 0.164 6 0.003 DI1K 5 [I1K] K12Mica 2 [I1K] H12Mica 5 0.023
“K1-Mica”
“Cs1-Mica”
0.187 6 0.002 0.0224 6 0.003
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TABLE 2 Relative XPS Intensities of Nitrogen and Potassium Atoms for Adsorbed Mica Surfaces
Cations adsorbed
I1N
I1N/DI1K
Density of adsorbed cation (molecules/nm2)
2C18DAB 1C18TAB 4C1AC
0.0252 0.0438 0.0393
1.1 1.9 1.7
2.3 4.1 3.7
DI1K 5 0.023
ment. The maximum hydrophobicity given to the mica surface by the adsorbed cationic surfactant is 80 –90°, compared with that of paraffin wax which is 110°. The present paper offers evidence of slow interfacial processes in agreement with Chen et al. (8), and should help to elucidate the complex mechanism of slow anchoring processes of organic cations at solid/solution interfaces.
REFERENCES 1. Wanless, E. J., and Ducker, W. A., J. Phys. Chem. 100, 3207 (1996). 2. Wanless, E. J., and Ducker, W. A., Langmuir 13, 1463 (1997). 3. Mann, S., Cleveland, J. P., Gaub, H. E., Stucky, G. D., and Hansma, P. K., Langmuir 10, 4409 (1994). 4. Mann, S., and Gaub, H. E., Science 270, 1480 (1995). 5. Aksay, I. A., Trau, M., Manne, S., Honma, I., Yao, N., Zhou, L., Fenter, P., Eisenberger, P. M., and Gruner, S. M., Science 273, 892 (1996). 6. Jaschke, M., Butt, H. J., Gaub, H. E., and Mann, S., Langmuir 13, 1381 (1997). 7. Meguro, K., and Kondo, T., J. Chem. Soc. Jpn. 76, 642 (1955). 8. Chen, Y. L., Chen, S., Frank, C., and Israelachvili, J., J. Colloid Interface Sci. 153, 244 (1992). 9. (a) Gaines, G. L., Jr., J. Phys. Chem. 61, 1408 (1957). (b) Claesson, P. M., Herder, P., Stenium, P., Eriksson, J. C., and Pashley, R. M., J. Colloid Interface Sci. 109, 31 (1986). 10. Christensson, H. K., J. Phys. Chem. 97, 12034 (1993). 11. Wan, K. T., Smith, D. T., and Lawn, B. R., J. Am. Ceram. Soc. 75, 667 (1992). 12. (a) Dann, J. R., J. Colloid Interface Sci. 32, 302 (1970). (b) Adamson, A. W., “Physical Chemistry of Surfaces,” 6th ed., p. 365. Wiley, New York, 1997.
Time Dependent Anchoring of Adsorbed Cationic Surfactant Molecules at Mica/Solution Interface Bangyin Li,1 Masatoshi Fujii, Kazuhiro Fukada, Tadashi Kato, and Tsutomu Seimiya2 Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, 1-1 Minami-Oosawa, Hachioji, Tokyo 192-03, Japan Received March 20, 1998; accepted September 17, 1998
hydrophobicized surface hydrophilic. This behavior is are directly reflected in the stability of suspended particles; cationic surfactants are well known (7) for showing both stabilizing and destabilizing properties of the suspension which occur above and below the particular concentration of surfactant. The purpose of the present study is to elucidate the nature of the interaction of cationic surfactant with the adsorption sites on mica surfaces. It is also the purpose of the present study to determine the standard conditions for obtaining well-defined reproducible hydrophobic surfaces suited to our AFM surface force studies. Similar studies have also been carried out by Chen et al. (8). Their fine analyses by XPS, contact angle, and surface force measurements also revealed the kinetic behavior of cationic surfactant at mica/solution interface as well as that of equilibrium. They too have found that the adsorption of cationic surfactant on mica surface is an extremely slow process. The principal difference of the present studies from that of Chen is that the preparative method of the “adsorbedspecimen” which leads to different conclusions. In Chen’s studies, the mica specimen was withdrawn from the adsorption bath at 25°C and was subjected to XPS and contact angle analyses after drying without rinsing, while the mica surface in the present studies was adsorbed at 5°C rinsed with purified water after withdrawn from adsorption bath to remove mobile molecules prior to the XPS and contact angle analyses. The combination of the information obtained by both methods should help to resolve the complex behavior of surfactant molecules at solid/solution interfaces.
The nature of adsorbed cationic amphiphiles at the mica/solution interface was studied by XPS and contact angle measurements. The elemental analyses of freshly cleaved mica surfaces by XPS showed that the potassium atoms on the surface lattice of mica are not necessarily distributed equally to each surface on cleavage. The adsorbed cationic amphiphile molecules remaining on mica surfaces after rinsing with distilled water were found to be anchored to the surface by ion-exchange, replacing surface potassium and/or other cations. The ratio of adsorbed cationic amphiphile molecules with single alkyl chains to the maximum potassium ions on mica surface was estimated to be twice as large as that of amphiphiles having two alkyl chains. The contact angle of water drops placed on the adsorbed surface showed a gradual decrease with the elapse of time due to the dissolution of adsorbed surfactant into the water drop; however, the decrease was not observed for those mica surfaces when aged for more than 3 days in the adsorption bath. The anchoring of adsorbed molecules by ion-exchange was found to occur extremely slowly, however; the anchored molecules may not easily be desorbed when rinsed with deionized water. The time dependent anchoring of adsorbed molecules was studied in terms of adsorption time, alkyl chain length, and concentration of cationic surfactant. © 1999 Academic Press Key Words: adsorption; cationic surfactant; potassium ion; cesium ion; ion-exchange capacity of mica; hydrophobicity; contact angle; wettability; XPS; ESCA.
INTRODUCTION
The adsorption of anionic (1, 2) or cationic surfactants (3– 8) on solid surfaces positively or negatively charged in water were known to form two- or three-dimensional molecular aggregates (1– 8) at the interface above and below the cmc of corresponding surfactants. The adsorbed molecules which are in direct contact with a mica surface are expected to be bound by ion-exchange and modify the solid surface to be hydrophobic while the molecules in the outermost part of the surface aggregates are held by hydrophobic interaction and render the
EXPERIMENTAL
Materials, Methods, and Procedures The octadecyltrimethylammonium bromide and other homologues and analogues of alkylammonium cations used in the present studies are abbreviated as 1Cn TAB for alkyltrimethylammonium bromide or 2Cn DAB for dialkyldimethylammonium bromide hereafter. These compounds, having purities of 98%, were purchased from Tokyo Kasei Kogyo Co., Japan, and were used without purifying further. The adsorption was carried out at 5°C throughout these experiments as an optimum
1 The paper fulfills in part the requirement of Ph.D. degree of Bangyin Li at Tokyo Metropolitan University in February 1997. 2 To whom correspondence should be addressed.
25
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
26
LI ET AL.
FIG. 1. Relative intensity of the XPS signal of potassium atoms for “Mica”, “H1-Mica”, and “K1-Mica”.
condition to ensure the reproducible array of adsorbed molecules as judged through AFM observation. The wettability of mica surface covered with adsorbed surfactant is estimated by measuring advancing contact angle of water droplets of 2–5 ml placed on the mica surface in a humidified glass box. The crystal of Muscovite mica (from India) was cleaved in aqueous surfactant solution to avoid any contamination from ambient atmosphere and to allow surfactant molecules to adsorb on the newly exposed surfaces. The specimens of mica were withdrawn from the aqueous surfactant solution at appropriate times, rinsed for 2 min under a stream of deionized water of about 1 liter to remove surfactant solution together with weakly bound surfactant molecules and
leaving the molecules chemically anchored. The water used was purified by a Milli-Q system of Millipore Corporation, U.S.A. Hydrochloric acid (ca. 35%), potassium chloride (99.9%), and cesium chloride (99.0%) used were all purchased from Wako Pure Chemical Industries, Japan. Both XPS and contact angle measurements were made for dried specimen of mica. The contact angle of water drops was determined goniometrically in the field of optical microscope as an average of eight drops randomly placed on mica surface of 10 3 20 mm. The contact angle measurements were carried out in atmospheres of both open laboratory air and a clean humidified environment in a glass chamber. No significant difference was noted for these environmental changes. The desorption problems inevitably encountered during the rinsing process will be discussed later in detail. The X-ray photoelectron spectrometer used was ESCA-750 of Shimadzu Co., Japan. The X-ray of MgK a was used and the intensity of photoelectron was measured at a take-off angle of 90°. The XPS spectrum elucidated the change of atomic composition of the surface: the atomic densities of Si, Al, K, Cs, N, Cl, and Br were determined before and after the surface modification of mica. RESULTS AND DISCUSSION
Anchoring of Adsorbed Molecules by Ion-Exchange Figure 1 shows the XPS spectrum indicating the change of potassium densities of mica surfaces with and without the modification by other cations. The surface density of K atoms is expressed in Fig. 1 as I1K, indicating the intensity of photoelectron emitted by K atoms located at the surface zone of mica specimen. Those filled circles plotted above the mark “Mica” of the horizontal axis represent the K atom densities observed for each surface of mica freshly cleaved
FIG. 2. XPS spectra of nitrogen (a) and cesium (b) atoms for the surfaces “Mica”, “Cs1-Mica”, and “1C18TAB-Mica”.
TIME DEPENDENT ANCHORING OF CATIONIC SURFACTANT ON MICA
27
FIG. 3. XPS spectra of bromine (a) and chlorine (b) atoms for the mica surfaces modified with various hydrophobic cations.
in air. Large fluctuation shows that the K atoms are not necessarily shared equally between the two surfaces on cleavage. The filled triangles above the mark “H1-Mica” indicate the decrease of surface K atoms when the mica surface was ion-exchanged with aqueous hydrochloric acid (pH 3) for 6 h to wash out all K ions located at the outer lattice sites of mica accessible to H1 ions in the solution (8 –11). The fluctuation of the signal I9K observed for “Mica” is minimized by removing those ion-exchangeable potassium atoms unevenly shared on cleavage and leaving those nonexchangeable atoms located at the inner lattice sites of the mica although it accompanied the decrease of I1K. By treating “H1-Mica” with aqueous KCl solution of 5.0 3 1022 M for 6 h at room temperature, the fluctuation of I1K is largely minimized while the intensity I1K itself is maximized as shown by the filled diamonds for “K1-Mica”. Figure 2a and b represent the XPS spectra of IN1s and ICs3d observed for modified mica surfaces with various cations to show that the first layer of adsorbed cationic surfactant is bound to mica surface by ion-exchange. The abbreviations of “Cs1Mica”, “2C18DAB-Mica”, “1C18TAB-Mica”, and “4C1ACMica” indicated on each curve represent the type of modi-
fication made to the original “Mica” surface, namely, the symbol 2C18DAB represents that the mica surface was modified with adsorbed dioctadecyldimethylammonium bromide, and 4C1AC with tetramethylammonium chloride, respectively. Figure 2b shows that the spectrum of cesium atoms ICs3d observed for “Cs1-Mica” disappeared for “2C18DAB-Mica”, “1C18TAB-Mica”, and “4C1AC-Mica” adsorbed surfaces, indicating that the cesium cation was replaced with adsorbed surfactant. The increase of I1K for the process from “H1-Mica” to “K1-Mica” should represent the maximum number of adsorption sites accessible to the cationic surfactant molecules by ion-exchange and is indicated in Table 1 as DI1K. Expressed together with DI1K in Table 1, is the value I1Cs indicating the surface density of cesium atoms determined by XPS for “Cs1-Mica” which was obtained by treating “H1-Mica” with the aqueous cesium chloride solution of 5.0 3 1022 M. It is worth noting here that the value of DI1K is good compared with I1Cs emphasizing that the surfactant cations are adsorbed to the same ionexchange sites as Cs1 cations. The amount of adsorption was also calculated from the IN1s data of XPS spectra for those hydrophobic cations which were listed in Table 2.
FIG. 4. Aging of the contact angle of water drop placed on modified surfaces of 1C18TAB-Mica (a) and 2C18DAB-Mica (b). Each curve was obtained by immersing the mica specimen in 1C18TAB or 1C18TAB 0.1 mM aqueous solution for different lengths of time at 5°C.
28
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FIG. 5. Equilibrium contact angle of water drop placed on mica surfaces modified with two kinds of surfactant 1Cn TAB-Mica (a) and 2Cn DAB-Mica (b) plotted against adsorption time.
These values are consistent with molecular area data analyzed from AFM images taken for the same samples. The observed AFM images, not shown here suggest that the adsorbed surfactant molecules are aligned epitaxially to the surface lattice of cleaved mica. Details of the AFM studies will be published in a separate paper. Figure 3a and b shows the XPS spectra of Br3p and Cl2p atoms for various adsorbed surfaces. No bromine nor chlorine atoms were detected for those mica surfaces to which alkylammonium cations were adsorbed. This means that these organic cations did not accompany their bromide or chloride counter anions when adsorbed. Quantitative calculations of the amount of adsorption, Table 2, indicate that the amount of single-chain cationic surfactant adsorbed is twice as large as that expected from DI1K as estimated in Table 1. The fact that the counter anion was not detected in the present analysis may be due to the possibility that one half of the 1C18TAB molecules were exchanged with cat-
ions other than potassium. It is not certain at this moment whether the hydronium ion is taking the place of the potassium ion. The physical adsorption of surfactant molecules forming a bimolecular layer which accompanies the counter bromide anions may be possible, although these may be lost during the rinsing process during sample preparation. Figure 4a and b shows that the contact angle of water drops on the adsorbed surface is not stable, and changes with time, due probably to the desorption of adsorbed molecules into the water drop. The decrease of contact angle should begin at the moment of placing water drops on the adsorbed surface. The contact angle measured decays with time over the range of 2 to 3 h. It is interesting to note that this decay behavior is very much dependent on the length of immersion time of mica specimen in adsorption bath. For those adsorbed surfaces obtained in adsorption times less than 2 min, the observed contact angle is unstable, decaying rapidly from 76 to 56° for 1C18TAB-Mica (a) and from 83
FIG. 6. Schematic illustration of the time dependent anchoring process of adsorbed molecules as viewed from contact angle change.
FIG. 7. Equilibrium contact angle of adsorbed mica surface obtained for various single chained homologues of cationic surfactant at varied concentrations.
29
TIME DEPENDENT ANCHORING OF CATIONIC SURFACTANT ON MICA
FIG. 8. Effect of chain length of adsorbed cationic surfactant 1Cn TAB (a) and 2Cn DAB (b) on the maximum contact angle of water drop placed on adsorbed mica surface at 5°C.
to 72° for 2C18DAB-Mica (b) within 60 min, while for those surfaces of adsorption time longer than 96 h, the contact angle became stable over prolonged time of 2–3 h. The change of contact angle was from 82 to 79° for 1C18TABMica (a) and from 88 to 87° for 2C18DAB-Mica (b). The equilibrium contact angle values for 1C18TAB-Mica and 2C18DAB-Mica were plotted in Fig. 5 (a and b) as a function of adsorption time. The time dependent anchoring behavior of adsorbed surfactant in the present studies suggests that the ion exchange adsorption of cationic surfactant on cleaved mica surface proceeds very slowly and requires more than 90 h to be anchored in 0.1 mM solution at 5°C and to show a stable contact angle after rinsing. Such slow anchoring may involve many complex rate-determining processes, e.g., slow diffusion of molecules at the interface, hindered penetration and/or overturn of surfactant molecules in the adsorbed film, as depicted schematically in Fig. 6. Figure 7 shows the change of equilibrium contact angle with the concentration of adsorbed surfactant for various single chained homologues of cationic surfactants. The adsorption time was 72 h and the temperature was chosen at 5°C as the optimum conditions to give reproducible results. The maximum contact angle increases with alkyl chain length and shows the tendency to saturate for all surfactant homologues tested. The maximum contact angles attained in Fig. 7 are plotted in Fig. 8 as a function of alkyl chain length for both single chained 1CnTAB (a) and double chained 2CnDAB (b) cationic surfactants. Both of these figures show
that these surfactants do not endow the mica surface with the hydrophobicity of solid paraffin wax, which is as high as 105–106° (12). In Fig. 8b, filled circles represent the mica surface cleaved in air before immersion in surfactant solution, while the open circles are for the mica surfaces cleaved in surfactant solution. The agreement of both results suggests the absence of any serious airborne contamination of the surface when the sample was cleaved in air. CONCLUSION
The surface density of potassium ions on cleaved mica surfaces was unevenly shared between two surfaces newly formed by cleavage; however, the density became equal when the surface was treated with aq. KCl solution. Anchoring of cationic surfactant on cleaved mica surfaces by ion-exchange was found to be an extremely slow process. It takes more than 70 h to saturate the exchange capacity of a mica surface at 5°C at a surfactant concentration of 0.1 mM. The ion-exchange of cationic surfactant with potassium, cesium, and the other interfacial cations was confirmed by XPS. When the adsorbed mica surface is withdrawn from aqueous surfactant solution soon after immersion and rinsed with water, the adsorbed molecules are easily washed away and the surface remains hydrophilic, while surfaces equilibrated for a period of more than 70 to 90 h in surfactant solution at 5°C showed stable hydrophobicity. The adsorbed molecules do not desorb into the water drops placed on the surface for contact angle measure-
TABLE 1 Relative Intensities of the XPS Signal for Potassium and Cesium Atoms on “Mica”, “H1-Mica”, “K1-Mica”, and “Cs1-Mica” Surfaces Surfaces analyzed I1K XPS intensities DI1K I1Cs
“Mica”
“H1-Mica”
0.181 6 0.008 0.164 6 0.003 DI1K 5 [I1K] K12Mica 2 [I1K] H12Mica 5 0.023
“K1-Mica”
“Cs1-Mica”
0.187 6 0.002 0.0224 6 0.003
30
LI ET AL.
TABLE 2 Relative XPS Intensities of Nitrogen and Potassium Atoms for Adsorbed Mica Surfaces
Cations adsorbed
I1N
I1N/DI1K
Density of adsorbed cation (molecules/nm2)
2C18DAB 1C18TAB 4C1AC
0.0252 0.0438 0.0393
1.1 1.9 1.7
2.3 4.1 3.7
DI1K 5 0.023
ment. The maximum hydrophobicity given to the mica surface by the adsorbed cationic surfactant is 80 –90°, compared with that of paraffin wax which is 110°. The present paper offers evidence of slow interfacial processes in agreement with Chen et al. (8), and should help to elucidate the complex mechanism of slow anchoring processes of organic cations at solid/solution interfaces.
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