- Email: [email protected]
Interaction forces between OH-groups in different solvents as observed by scanning force microscopy
Colloids and Surfaces A: Physicochemical and Engineering Aspects 164 (2000) 175 – 181 www.elsevier.nl/locate/colsurfa
Interaction forces between OH-groups in different solvents as observed by scanning force microscopy G. Papastavrou, S. Akari * Max-Planck-Institut for Colloid- and Interface Science, D-14424 Potsdam, Germany Received 18 December 1998; accepted 19 August 1999
Abstract Interactions between hydroxyl-terminated surfaces were measured on local scale by performing force-displacement measurements using a scanning force microscope (SFM). For this purpose a SFM-tip was modified by self assembly techniques employing alkanethiolates v-substituted with OH-groups. Gold surfaces were functionalized employing the same method. Interaction forces between the OH-terminated tip and samples terminated by OH- or CH3-groups were examined in hexadecane and deionized water. As in water OH/OH-interaction forces have been found to be relatively small, in hexadecane larger forces clearly could be detected. Further investigations by a systematic variation of the solvents have been carried out to clarify the origin of the observed strong adhesion forces between OH-groups in unpolar solvents. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Interaction forces; OH-groups; Scanning force microscopy
Scanning force microscopy (SFM) [1] is becoming an important tool not only for imaging surface topography with high lateral resolution but also to measure forces between the different chemical surfaces [2 – 6]. The recently introduced modification of the SFM-tip in a well defined manner by self-assembly techniques is becoming an efficient method for examining forces between the functionalized tip and chemical/biological surfaces, as well as to detect different chemical species on the sample with high resolution. Using self assembly techniques [7,8] molecules can be covalently attached to the tip to build up a highly * Corresponding author.
ordered monolayer. Because of the relatively small curvature radius of the tip the molecules interacting with the surface can be reduced to a minimum allowing the detection of forces between few molecules. This is an important difference to the surface force apparatus by which forces on a macroscopic scale are measured [9]. The aim of this study is to examine intermolecular forces between OH-groups in different solvents. This is a point of interest since the interaction between these groups should be mainly dominated by hydrogen bonds, which are of large importance in many biological systems and play a main role in supramolecular chemistry. SFM-tips modified by hydroxyl-groups have al-
0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 4 0 3 - 3
176
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181
Fig. 1. Force –displacement curves measured for an OH-tip and OH-sample in water. (b) Histogram of the corresponding adhesion forces. For the insert a smaller interval was used to elucidate the distribution of the adhesion forces.
ready been employed in various other studies, although the interpretations of the observed force profiles are not consistent [10,11]. In the study presented, different solvents were employed in a systematic way to further investigate the interaction mechanisms. In this way it is possible to distinguish between the influence of different factors like hydrogen bonding, solvation forces, film permeation by the solvent and capillarity condensation [9]. For this purpose tips of a SFM were modified by 11-mercapto-undecanol molecules (HS(CH2)11OH) on the bases of self-assembly techniques. In a first step the Si-tips (Nanosensors, Germany) were coated with a gold film of 75 nm thickness by thermal evaporation. A thin layer of Chromium (5 nm), evaporated first serves as adhesion promoter. Simultaneously, gold substrates were prepared by evaporation of gold on glass slides. The tips were then kept in a 1 mM ethanolic solution of mercapto-undecanol [HS-(CH2)11-OH] for at least 12 h. The gold surfaces were modified by the same thiol or dodecanethiol [HS-(CH2)11-CH3] in an analogous way as the tips. The quality of the obtained SAM-films was controlled by contact angle and reflection absorption infrared spectroscopy measurements (RAIRS) for the samples. The force spectroscopy measurements were per-
formed by a standard SFM (D3000, Digital Instruments Santa Barbara, CA). The presented measurements were always acquired pairwise for different solvent or sample combinations. In the case of hexadecane and water the same tip was used for acquiring the force-displacement-curves in the same solvent, only exchanging the samples was necessary. For perfluorodecalin, dodecane and octanol the same samples and tips were used, exchanging the solvent after washing thoroughly with heptane and in the case of octanol with ethanol. To carry out quantitative analysis the spring constants of the cantilevers were determined individually for each cantilever by recording the thermal noise with a spectrum analyzer [12]. Additionally the procedure described above ensures that uncertainties in the spring constant and tip radius do not directly influence a comparison of the obtained adhesion forces. Fig. 1(a) shows a typical force-displacement curve of an OH-modified tip and an OH-surface in water. The histogram (Fig. 1(b)) of the adhesion forces (withdrawing part of the force–displacement curve) shows a maximum at 0.3290.14 nN for the corresponding adhesion forces. Fig. 2(a) shows the interaction of the OH-tip and OH-surface in hexadecane. The corresponding histogram shows a maximum at 1.429
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181
177
Fig. 2. Force – displacement curves measured for the OH-tip and OH-sample in hexadecane. (b) Histogram of the adhesion forces.
0.58 nN for the adhesion force between the OHgroups. Since OH-groups are able to build hydrogen bonds and since hexadecane has a very low dielectric constant, one could conclude that the observed attractive forces are primarily due to electrostatic interactions based on the dipole moment of the OH-groups, resulting in the hydrogen bonds. In contrast, in water the electrostatic interaction is reduced because of its very high dielectric constant and the competition of the water molecules for the free OH-groups. An important aspect influencing the adhesion forces in solution is the possibility that solvent molecules could permeate partially the self-assembled monolayer, as it is reported for acetonitrile [13]. A high order for similar films of alkanethiolates v-substituted with OH-groups was verified by NEXAFS measurements [14]. The employed films have been examined by ex situ RAIRS measurements [15]1 for their molecular order. Nevertheless, a permeation of the solvent at the defects of the monolayer is likely. In principle a configuration of upright standing hexadecane molecules 1
The orientational order of the carbonchains in the thiolsamples has been estimated from the peaks corresponding to the methylene stretching modes at 2918.6 and 2849.8 cm − 1, which are comparable for both samples. The wavenumbers are in good agreement with the results of Porter et al.
in the film is imaginable for this case. Presumably, they will be fixed by van der Waals forces between the carbon chains leading to a compensation of defects in the film structure in analogy to Langmuir Blodgett films [16]. But a permeation of hexadecane in defect-free areas, resulting in a restructuring of the film seems less likely, taking into account the packing density of the SAMs, the size of hexadecane and the weak interaction of the monolayer with an apolar solvent [17]. The simultaneous measurements of CH3-terminated samples also strongly supports the idea that no solvent permeation effects are responsible for the observed adhesion forces (Fig. 3(a, b)). The high adhesion force of an OH-terminated tip on CH3terminated samples (2.6190.53 nN) in water compared to 0.2190.13 nN for the same combination in hexadecane can be explained by hydrophobic interactions in the aqueous environment (Fig. 4(a, b)). The presence of a hydrophobic surface in water leads to a domination of hydrophobic forces over other interaction forces. This effect has been quantitatively examined by measuring the adhesion forces between a hydrophilic tip and two different samples, one hydrophobic the other one hydrophilic in different mixtures of water and ethanol [18]. On the other hand, the low adhesion values in hexadecane are reasonable for the interaction between an uncharged but
178
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181
Fig. 3. Force displacement curve of an OH-tip and CH3-surface in hexdecane. (b) Histogram of the corresponding adhesion forces. For the insert a smaller interval was used to elucidate the distribution of the adhesion forces
polar group and a nonpolar group in the absence of solvent induced restructuring of the film. Furthermore, measurements between hydroxylterminated surfaces were conducted in perfluorodecaline [19]. This solvent also has a low dielectric constant, but possesses a shape which should sterically hinder an intercalation in the film. A comparison of the adhesion forces obtained by the same tip in perfluorodecaline and hexadecane does not show reduced adhesion forces. This further indicates that solvent permeation does not affect the adhesion forces for the densely packed thiol monolayers studied and the solvents employed. Hexadecane is known to build ordered films on different samples, resulting in solvation forces. These forces have been already examined by AFM for highly ordered pyramidal graphite (HOPG) [20,21] and siliconoxide. Studies with aliphatic hydrocarbons on siliconoxide show that the solvation forces are strongly dependent on the temperature and the chain length of the hydrocarbons [22]. Furthermore experiments on siliconoxide show the addition of dodecane to the liquid phase resulted in a complete disappearance of attractive forces. For hexadecane as well as dodecane the measurements were conducted between 22 and 24°C, a temperature region where the
building up a of an ordered layer of hexadecane on siliconoxide was not observed. The adhesion forces in dodecane do not vary significantly from the ones measured before with the same tip in hexadecane (Table 1). From this experiment the contribution of solvation forces can not be excluded, but they should be of minor influence on the adhesion forces between the hydroxyl-terminated tip and sample in hexadecane. Further support that hydrogen bonding may be responsible for the observed adhesion forces comes from experiments in octanol. Fig. 5 represents a typical force–displacement curve between two hydroxyl-terminated surfaces in octanol. For all curves at least one step in the contact regime could be observed, indicating the adsorption of octanol to the interfaces. Similar results have already been described for unmodified tips on mica in a series of short chained alcohols, ranging up to propanol [23]. The adhesion forces obtained when applying low loading forces are strongly reduced compared with the ones measured in hexadecane employing the same tip. When applying a sufficiently high loading force, an additional step in the contact regime and a clear increase in the adhesion forces can be observed. In ethanol a similar adsorption behaviour could be observed (data not shown) [24]. The adsorption of alcohols
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181
179
Fig. 4. Force –displacement curve of an OH-tip and CH3-surface in water. (b) Histogram of the corresponding adhesion forces.
suggests that one should be careful in regarding only the dielectric constant of the medium as promoting factor for hydrogen bonding. Continuum models of adhesion have been shown to be applicable to SFM-measurements at least as an approximation. The JKR-model [25] gives reasonable values for adhesion forces also in the case of typical SFM-tip diameters. By employing the JKR-model, one can roughly estimate if the observed adhesion forces are compatible with hydrogen bonding in the case of OH/OH in hexadecane. Using a tip radius measured by electron microscopy from a tip of the same charge and applying the effective contact radius at pull off a contact area with about 20 functional groups is estimated2. This would also correspond to the contact area of interaction via hydrogen bonds. The work of adhesion can be estimated by the tip radius and the measured adhesion forces. This 2 The effective contact radius at pull off is given by the JKR-theory with:
as =
' 3
3pR 2W12 2K
together with the elasticity module of Gold (K =64 GPa) in the case of OH/OH in HD an effective radius of 4 nm2 for the contact area is calculated. Using an area of 0.214 nm2 per functional group one obtains the number of molecules corresponding to the contact area.
work per area can be converted to an energy per molecule by applying the known surface area per molecule for thiol based SAMs. The obtained binding energy of about 0.14 kcal mol − 1 would correspond to 2–3 hydrogen bonds in the contact area. In this estimation a literature value of 0.8 kcal mol − 1 was considered [26]. Taking the crudeness of the approximation into account, this value is not unreasonable, since it is likely that only the half of the molecules present in the contact area could participate for steric reasons. Also, the probability of building a hydrogen bond is not considered and it is known that the strength of a hydrogen bond strongly depends on the local environment, which likely differs strongly for the here studied SAMs compared with systems studied in liquid and/or gaseous environment. Another important point is the possible absence of degrees of freedom for the hydroxyl groups fixed in the monolayer, resulting in unfavourable binding geometries. However, a different mechanism for the observed adhesion forces is imaginable. In analogy to surface-force-apparatus (SFA) measurements also in our measurements a capillary condensation can lead to an increase of the adhesion forces [27]. To clarify this point in situ preparation of the tips is planned. To conclude we could show that solvents with very low dielectric constant lead to high adhesion
180
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181
Table 1 Adhesion forces between an OH-terminated tip and an OH-terminated sample in different solventsa
Adhesion force (nN)
Adhesion (nN) force in referencec
Perfluorodecaline
Dodecane
Octanol
2.69 9 2.17b n =585 Hexadecane 2.55 90.69 n =992
0.86 90.24 n =956 Hexadecane 0.91 9 0.51 n = 641
0.21 9 0.18 n =407 Hexadecane 2.46 90.79 n =998
a
Hexadecane is given as reference. The hexadecane measurements were obtained with the same tip and sample. For different locations on the sample a large spreading of the adhesion values was observed (contrary to the other entries): (a) 0.59 90.13 n = 281; (b) 4.63 91.21 n=210; (c) 4.64 90.67 n =94. c Measured with the same tip and sample in hexadecane (reference solvent), either before or after measurement in the solvent of column header. b
forces for hydroxyl terminated surfaces, as probed by modified SFM-tips. By performing measurements in different solvents and for different surface combinations the contribution of other factors, like solvent permeation into the monolayers and solvation forces, were estimated. Nevertheless it is shown that the reason for the observed forces has to be interpreted carefully. Using JKRapproximations one could conclude that only few molecules are contributing to the forces measured by scanning force microscopy via hydrogen bonding. On the other hand capillary condensation would be also compatible with the obtained re-
sults. The clear differences in adhesion forces for different solvents can be used to influence the friction forces during operation of the SFM in the lateral force mode. This is a promising approach to enhance the imaging of specific functional groups by SFM [28].
Acknowledgements The authors are indebted to Professor Mo¨hwald for encouraging discussions, revising the manuscript and his continuos support. We thank Dr Hartmann for SEM-measurements and the DFG for financial support.
References
Fig. 5. Force – displacement curve obtained in octanol (OH-tip and OH-surface). The layering of octanol molecules on the interface can be derived from steps in the approach part of the curves (see also insert).
[1] G. Binnig, C. Quate, C. Gerber, Phys. Rev. Lett. 56 (1986) 930. [2] T. Nakagawa, K. Ogawa, T. Kurumizawa, S. Ozaki, Jpn. J. Appl. Phys. 32 (1993) L294. [3] E.-L. Florin, T.V. Moy, H.E. Gaub, Science 264 (1994) 415. [4] C.D. Frisbie, L.F. Rozsnai, A. Noy, M.S. Wrighton, C.M. Lieber, Science 265 (1994) 2071. [5] S. Akari, D. Horn, H. Keller, W. Schrepp, Adv. Mat. 7 (1995) 549. [6] E.W. van der Vegte, G. Hadziioannou, Langmuir 13 (1997) 4357. [7] H. Keller, W. Schrepp, H. Fuchs, Thin Solid Films 210/211 (1992) 799. [8] C.D. Bain, G.M. Whitesides, Angew. Chem. Int. Ed. Engl. 28 (1989) 506.
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181 [9] J. Israelachvili, Intermolecular and Surface Forces, Academic Press, San Diego, 1992. [10] S.K. Sinniah, A.B. Steel, C.J. Miller, J.E. Reutt-Robey, J. Am. Chem. Soc. 118 (1996) 8925. [11] (a) T. Han, J.M. Williams, T.P. Beebe, Anal. Chim. Acta 307 (1995) 365. (b) L.A. Wenzler, G.L. Moyes, G.N. Raikar, R.L. Hansen, J.M. Harris, T.P. Beebe, L.L. Wood, S.S. Saavedra, Langmuir 13 (1997) 3761 [12] J.L. Hutter, J. Bechhoefer, Rev. Sci. Instrum. 64 (1993) 1868. [13] M.R. Anderson, M.N. Evaniak, M. Zhang, Langmuir 12 (1996) 2327. [14] O. Dannenberger, K. Weiss, H.-J. Himmel, B. Jager, M. Buck, C. Woll, Thin Solid Films 307 (1997) 183. [15] M. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 (1987) 3559. [16] G. Brezesinski, M. Thoma, B. Struth, H. Mohwald, J. Phys. Chem. 100 (1996) 3126. [17] D.S. Karpovich, G.J. Blanchard, Langmuir 13 (1997) 4031.
.
181
[18] Unpublished results, paper in preparation. [19] K. Feldman, T. Tervoort, P. Smith, N.D. Spencer, Langmuir 14 (1998) 372. [20] D.L. Klein, P.L. Mc Euen, J.E. Bowen Katari, A.P. Alivisatos, Nanotechnology 7 (1996) 397. [21] S.J. O’Shea, M.E. Welland, T. Rayment, Appl. Phys. Lett. 60 (1992) 2356. [22] T. Pfohl, Ph.D Thesis, University of Potsdam, 1998. [23] Y. Kanda, T. Nakamura, K. Higashitani, Colloids Surfaces A 139 (1998) 55. [24] A. Noy, D.J. Vezenov, C.M. Lieber, Ann. Rev. Mater. Sci. 27 (1997) 381. [25] K.L. Johnson, K. Kendall, A.D. Roberts, Proc. R. Soc. London A 324 (1971) 301. [26] C.L. Perrin, J.P. Nielson, Annu. Rev. Phys. Chem. 48 (1997) 511. [27] H.K. Christenson, J. Colloid Interface Sci. 104 (1985) 234. [28] G. Papastavrou, S. Akari, Nanotechnology, in press
Interaction forces between OH-groups in different solvents as observed by scanning force microscopy G. Papastavrou, S. Akari * Max-Planck-Institut for Colloid- and Interface Science, D-14424 Potsdam, Germany Received 18 December 1998; accepted 19 August 1999
Abstract Interactions between hydroxyl-terminated surfaces were measured on local scale by performing force-displacement measurements using a scanning force microscope (SFM). For this purpose a SFM-tip was modified by self assembly techniques employing alkanethiolates v-substituted with OH-groups. Gold surfaces were functionalized employing the same method. Interaction forces between the OH-terminated tip and samples terminated by OH- or CH3-groups were examined in hexadecane and deionized water. As in water OH/OH-interaction forces have been found to be relatively small, in hexadecane larger forces clearly could be detected. Further investigations by a systematic variation of the solvents have been carried out to clarify the origin of the observed strong adhesion forces between OH-groups in unpolar solvents. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Interaction forces; OH-groups; Scanning force microscopy
Scanning force microscopy (SFM) [1] is becoming an important tool not only for imaging surface topography with high lateral resolution but also to measure forces between the different chemical surfaces [2 – 6]. The recently introduced modification of the SFM-tip in a well defined manner by self-assembly techniques is becoming an efficient method for examining forces between the functionalized tip and chemical/biological surfaces, as well as to detect different chemical species on the sample with high resolution. Using self assembly techniques [7,8] molecules can be covalently attached to the tip to build up a highly * Corresponding author.
ordered monolayer. Because of the relatively small curvature radius of the tip the molecules interacting with the surface can be reduced to a minimum allowing the detection of forces between few molecules. This is an important difference to the surface force apparatus by which forces on a macroscopic scale are measured [9]. The aim of this study is to examine intermolecular forces between OH-groups in different solvents. This is a point of interest since the interaction between these groups should be mainly dominated by hydrogen bonds, which are of large importance in many biological systems and play a main role in supramolecular chemistry. SFM-tips modified by hydroxyl-groups have al-
0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 4 0 3 - 3
176
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181
Fig. 1. Force –displacement curves measured for an OH-tip and OH-sample in water. (b) Histogram of the corresponding adhesion forces. For the insert a smaller interval was used to elucidate the distribution of the adhesion forces.
ready been employed in various other studies, although the interpretations of the observed force profiles are not consistent [10,11]. In the study presented, different solvents were employed in a systematic way to further investigate the interaction mechanisms. In this way it is possible to distinguish between the influence of different factors like hydrogen bonding, solvation forces, film permeation by the solvent and capillarity condensation [9]. For this purpose tips of a SFM were modified by 11-mercapto-undecanol molecules (HS(CH2)11OH) on the bases of self-assembly techniques. In a first step the Si-tips (Nanosensors, Germany) were coated with a gold film of 75 nm thickness by thermal evaporation. A thin layer of Chromium (5 nm), evaporated first serves as adhesion promoter. Simultaneously, gold substrates were prepared by evaporation of gold on glass slides. The tips were then kept in a 1 mM ethanolic solution of mercapto-undecanol [HS-(CH2)11-OH] for at least 12 h. The gold surfaces were modified by the same thiol or dodecanethiol [HS-(CH2)11-CH3] in an analogous way as the tips. The quality of the obtained SAM-films was controlled by contact angle and reflection absorption infrared spectroscopy measurements (RAIRS) for the samples. The force spectroscopy measurements were per-
formed by a standard SFM (D3000, Digital Instruments Santa Barbara, CA). The presented measurements were always acquired pairwise for different solvent or sample combinations. In the case of hexadecane and water the same tip was used for acquiring the force-displacement-curves in the same solvent, only exchanging the samples was necessary. For perfluorodecalin, dodecane and octanol the same samples and tips were used, exchanging the solvent after washing thoroughly with heptane and in the case of octanol with ethanol. To carry out quantitative analysis the spring constants of the cantilevers were determined individually for each cantilever by recording the thermal noise with a spectrum analyzer [12]. Additionally the procedure described above ensures that uncertainties in the spring constant and tip radius do not directly influence a comparison of the obtained adhesion forces. Fig. 1(a) shows a typical force-displacement curve of an OH-modified tip and an OH-surface in water. The histogram (Fig. 1(b)) of the adhesion forces (withdrawing part of the force–displacement curve) shows a maximum at 0.3290.14 nN for the corresponding adhesion forces. Fig. 2(a) shows the interaction of the OH-tip and OH-surface in hexadecane. The corresponding histogram shows a maximum at 1.429
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181
177
Fig. 2. Force – displacement curves measured for the OH-tip and OH-sample in hexadecane. (b) Histogram of the adhesion forces.
0.58 nN for the adhesion force between the OHgroups. Since OH-groups are able to build hydrogen bonds and since hexadecane has a very low dielectric constant, one could conclude that the observed attractive forces are primarily due to electrostatic interactions based on the dipole moment of the OH-groups, resulting in the hydrogen bonds. In contrast, in water the electrostatic interaction is reduced because of its very high dielectric constant and the competition of the water molecules for the free OH-groups. An important aspect influencing the adhesion forces in solution is the possibility that solvent molecules could permeate partially the self-assembled monolayer, as it is reported for acetonitrile [13]. A high order for similar films of alkanethiolates v-substituted with OH-groups was verified by NEXAFS measurements [14]. The employed films have been examined by ex situ RAIRS measurements [15]1 for their molecular order. Nevertheless, a permeation of the solvent at the defects of the monolayer is likely. In principle a configuration of upright standing hexadecane molecules 1
The orientational order of the carbonchains in the thiolsamples has been estimated from the peaks corresponding to the methylene stretching modes at 2918.6 and 2849.8 cm − 1, which are comparable for both samples. The wavenumbers are in good agreement with the results of Porter et al.
in the film is imaginable for this case. Presumably, they will be fixed by van der Waals forces between the carbon chains leading to a compensation of defects in the film structure in analogy to Langmuir Blodgett films [16]. But a permeation of hexadecane in defect-free areas, resulting in a restructuring of the film seems less likely, taking into account the packing density of the SAMs, the size of hexadecane and the weak interaction of the monolayer with an apolar solvent [17]. The simultaneous measurements of CH3-terminated samples also strongly supports the idea that no solvent permeation effects are responsible for the observed adhesion forces (Fig. 3(a, b)). The high adhesion force of an OH-terminated tip on CH3terminated samples (2.6190.53 nN) in water compared to 0.2190.13 nN for the same combination in hexadecane can be explained by hydrophobic interactions in the aqueous environment (Fig. 4(a, b)). The presence of a hydrophobic surface in water leads to a domination of hydrophobic forces over other interaction forces. This effect has been quantitatively examined by measuring the adhesion forces between a hydrophilic tip and two different samples, one hydrophobic the other one hydrophilic in different mixtures of water and ethanol [18]. On the other hand, the low adhesion values in hexadecane are reasonable for the interaction between an uncharged but
178
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181
Fig. 3. Force displacement curve of an OH-tip and CH3-surface in hexdecane. (b) Histogram of the corresponding adhesion forces. For the insert a smaller interval was used to elucidate the distribution of the adhesion forces
polar group and a nonpolar group in the absence of solvent induced restructuring of the film. Furthermore, measurements between hydroxylterminated surfaces were conducted in perfluorodecaline [19]. This solvent also has a low dielectric constant, but possesses a shape which should sterically hinder an intercalation in the film. A comparison of the adhesion forces obtained by the same tip in perfluorodecaline and hexadecane does not show reduced adhesion forces. This further indicates that solvent permeation does not affect the adhesion forces for the densely packed thiol monolayers studied and the solvents employed. Hexadecane is known to build ordered films on different samples, resulting in solvation forces. These forces have been already examined by AFM for highly ordered pyramidal graphite (HOPG) [20,21] and siliconoxide. Studies with aliphatic hydrocarbons on siliconoxide show that the solvation forces are strongly dependent on the temperature and the chain length of the hydrocarbons [22]. Furthermore experiments on siliconoxide show the addition of dodecane to the liquid phase resulted in a complete disappearance of attractive forces. For hexadecane as well as dodecane the measurements were conducted between 22 and 24°C, a temperature region where the
building up a of an ordered layer of hexadecane on siliconoxide was not observed. The adhesion forces in dodecane do not vary significantly from the ones measured before with the same tip in hexadecane (Table 1). From this experiment the contribution of solvation forces can not be excluded, but they should be of minor influence on the adhesion forces between the hydroxyl-terminated tip and sample in hexadecane. Further support that hydrogen bonding may be responsible for the observed adhesion forces comes from experiments in octanol. Fig. 5 represents a typical force–displacement curve between two hydroxyl-terminated surfaces in octanol. For all curves at least one step in the contact regime could be observed, indicating the adsorption of octanol to the interfaces. Similar results have already been described for unmodified tips on mica in a series of short chained alcohols, ranging up to propanol [23]. The adhesion forces obtained when applying low loading forces are strongly reduced compared with the ones measured in hexadecane employing the same tip. When applying a sufficiently high loading force, an additional step in the contact regime and a clear increase in the adhesion forces can be observed. In ethanol a similar adsorption behaviour could be observed (data not shown) [24]. The adsorption of alcohols
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181
179
Fig. 4. Force –displacement curve of an OH-tip and CH3-surface in water. (b) Histogram of the corresponding adhesion forces.
suggests that one should be careful in regarding only the dielectric constant of the medium as promoting factor for hydrogen bonding. Continuum models of adhesion have been shown to be applicable to SFM-measurements at least as an approximation. The JKR-model [25] gives reasonable values for adhesion forces also in the case of typical SFM-tip diameters. By employing the JKR-model, one can roughly estimate if the observed adhesion forces are compatible with hydrogen bonding in the case of OH/OH in hexadecane. Using a tip radius measured by electron microscopy from a tip of the same charge and applying the effective contact radius at pull off a contact area with about 20 functional groups is estimated2. This would also correspond to the contact area of interaction via hydrogen bonds. The work of adhesion can be estimated by the tip radius and the measured adhesion forces. This 2 The effective contact radius at pull off is given by the JKR-theory with:
as =
' 3
3pR 2W12 2K
together with the elasticity module of Gold (K =64 GPa) in the case of OH/OH in HD an effective radius of 4 nm2 for the contact area is calculated. Using an area of 0.214 nm2 per functional group one obtains the number of molecules corresponding to the contact area.
work per area can be converted to an energy per molecule by applying the known surface area per molecule for thiol based SAMs. The obtained binding energy of about 0.14 kcal mol − 1 would correspond to 2–3 hydrogen bonds in the contact area. In this estimation a literature value of 0.8 kcal mol − 1 was considered [26]. Taking the crudeness of the approximation into account, this value is not unreasonable, since it is likely that only the half of the molecules present in the contact area could participate for steric reasons. Also, the probability of building a hydrogen bond is not considered and it is known that the strength of a hydrogen bond strongly depends on the local environment, which likely differs strongly for the here studied SAMs compared with systems studied in liquid and/or gaseous environment. Another important point is the possible absence of degrees of freedom for the hydroxyl groups fixed in the monolayer, resulting in unfavourable binding geometries. However, a different mechanism for the observed adhesion forces is imaginable. In analogy to surface-force-apparatus (SFA) measurements also in our measurements a capillary condensation can lead to an increase of the adhesion forces [27]. To clarify this point in situ preparation of the tips is planned. To conclude we could show that solvents with very low dielectric constant lead to high adhesion
180
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181
Table 1 Adhesion forces between an OH-terminated tip and an OH-terminated sample in different solventsa
Adhesion force (nN)
Adhesion (nN) force in referencec
Perfluorodecaline
Dodecane
Octanol
2.69 9 2.17b n =585 Hexadecane 2.55 90.69 n =992
0.86 90.24 n =956 Hexadecane 0.91 9 0.51 n = 641
0.21 9 0.18 n =407 Hexadecane 2.46 90.79 n =998
a
Hexadecane is given as reference. The hexadecane measurements were obtained with the same tip and sample. For different locations on the sample a large spreading of the adhesion values was observed (contrary to the other entries): (a) 0.59 90.13 n = 281; (b) 4.63 91.21 n=210; (c) 4.64 90.67 n =94. c Measured with the same tip and sample in hexadecane (reference solvent), either before or after measurement in the solvent of column header. b
forces for hydroxyl terminated surfaces, as probed by modified SFM-tips. By performing measurements in different solvents and for different surface combinations the contribution of other factors, like solvent permeation into the monolayers and solvation forces, were estimated. Nevertheless it is shown that the reason for the observed forces has to be interpreted carefully. Using JKRapproximations one could conclude that only few molecules are contributing to the forces measured by scanning force microscopy via hydrogen bonding. On the other hand capillary condensation would be also compatible with the obtained re-
sults. The clear differences in adhesion forces for different solvents can be used to influence the friction forces during operation of the SFM in the lateral force mode. This is a promising approach to enhance the imaging of specific functional groups by SFM [28].
Acknowledgements The authors are indebted to Professor Mo¨hwald for encouraging discussions, revising the manuscript and his continuos support. We thank Dr Hartmann for SEM-measurements and the DFG for financial support.
References
Fig. 5. Force – displacement curve obtained in octanol (OH-tip and OH-surface). The layering of octanol molecules on the interface can be derived from steps in the approach part of the curves (see also insert).
[1] G. Binnig, C. Quate, C. Gerber, Phys. Rev. Lett. 56 (1986) 930. [2] T. Nakagawa, K. Ogawa, T. Kurumizawa, S. Ozaki, Jpn. J. Appl. Phys. 32 (1993) L294. [3] E.-L. Florin, T.V. Moy, H.E. Gaub, Science 264 (1994) 415. [4] C.D. Frisbie, L.F. Rozsnai, A. Noy, M.S. Wrighton, C.M. Lieber, Science 265 (1994) 2071. [5] S. Akari, D. Horn, H. Keller, W. Schrepp, Adv. Mat. 7 (1995) 549. [6] E.W. van der Vegte, G. Hadziioannou, Langmuir 13 (1997) 4357. [7] H. Keller, W. Schrepp, H. Fuchs, Thin Solid Films 210/211 (1992) 799. [8] C.D. Bain, G.M. Whitesides, Angew. Chem. Int. Ed. Engl. 28 (1989) 506.
G. Papasta6rou, S. Akari / Colloids and Surfaces A: Physicochem. Eng. Aspects 164 (2000) 175–181 [9] J. Israelachvili, Intermolecular and Surface Forces, Academic Press, San Diego, 1992. [10] S.K. Sinniah, A.B. Steel, C.J. Miller, J.E. Reutt-Robey, J. Am. Chem. Soc. 118 (1996) 8925. [11] (a) T. Han, J.M. Williams, T.P. Beebe, Anal. Chim. Acta 307 (1995) 365. (b) L.A. Wenzler, G.L. Moyes, G.N. Raikar, R.L. Hansen, J.M. Harris, T.P. Beebe, L.L. Wood, S.S. Saavedra, Langmuir 13 (1997) 3761 [12] J.L. Hutter, J. Bechhoefer, Rev. Sci. Instrum. 64 (1993) 1868. [13] M.R. Anderson, M.N. Evaniak, M. Zhang, Langmuir 12 (1996) 2327. [14] O. Dannenberger, K. Weiss, H.-J. Himmel, B. Jager, M. Buck, C. Woll, Thin Solid Films 307 (1997) 183. [15] M. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 (1987) 3559. [16] G. Brezesinski, M. Thoma, B. Struth, H. Mohwald, J. Phys. Chem. 100 (1996) 3126. [17] D.S. Karpovich, G.J. Blanchard, Langmuir 13 (1997) 4031.
.
181
[18] Unpublished results, paper in preparation. [19] K. Feldman, T. Tervoort, P. Smith, N.D. Spencer, Langmuir 14 (1998) 372. [20] D.L. Klein, P.L. Mc Euen, J.E. Bowen Katari, A.P. Alivisatos, Nanotechnology 7 (1996) 397. [21] S.J. O’Shea, M.E. Welland, T. Rayment, Appl. Phys. Lett. 60 (1992) 2356. [22] T. Pfohl, Ph.D Thesis, University of Potsdam, 1998. [23] Y. Kanda, T. Nakamura, K. Higashitani, Colloids Surfaces A 139 (1998) 55. [24] A. Noy, D.J. Vezenov, C.M. Lieber, Ann. Rev. Mater. Sci. 27 (1997) 381. [25] K.L. Johnson, K. Kendall, A.D. Roberts, Proc. R. Soc. London A 324 (1971) 301. [26] C.L. Perrin, J.P. Nielson, Annu. Rev. Phys. Chem. 48 (1997) 511. [27] H.K. Christenson, J. Colloid Interface Sci. 104 (1985) 234. [28] G. Papastavrou, S. Akari, Nanotechnology, in press