Full text of DOI:10.1007/BF02900464

NOTES
of NaCl exposed to water vapor. Using ellisometry, Bea-
glehole et al.[11] have determined the thickness of the ad-
sorbed water film on mica as a function of the relative
vapor pressure of water. When neither the tip nor the sam-
ple is modified, the adhesion force between the tip and the
sample is a sum of all the attractive forces acting on the
cantilever, including van der Waals attraction, electrostatic
interactions and capillary force. In most cases, the van der
Waals force and electrostatic interactions are very small
and can be neglected without introducing serious errors.
Therefore, the capillary force resulting from the conden-
sation of water which forms a liquid bridge between the
tip and sample is the dominating factor of the adhesion
force, and this can be proved by the fact that the adhesion
force in liquid can be drastically decreased. In 1989,
Weisenhorn et al.[12] found that the adhesion force be-
Effect of humidity on the sur-
face adhesion force of inor-
ganic crystals by the force
spectrum method
TANG Jing, WANG Chen, LIU Maozi, SU Ming
& BAI Chunli
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080,
China
Correspondence should be addressed to Bai Chunli (e-mail: CLBAI@
infoc3.icas.ac.cn)
Abstract
Effect of relative humidity on the surface adhe-
sion force of several inorganic crystals of mica, CaF2 and
KCl was studied by atomic force microscopy (AFM). The
results showed that the magnitude of surface adhesion force
is mainly dependent on the surface free energy of the ad-
sorbed liquid film, but almost independent of the thickness
of the film. Furthermore, the deliquescence on the crystal
surface was investigated, which demonstrated the capability
of the force spectrum method to monitor changes in ionic
concentrations of adsorbed liquid film in real-time.
tween the tip and mica could decrease from about 100 h
10ꢀ9 N in air to 1×10ꢀ9 N in water.
It is long supposed that under ambient conditions,
the adhesion force between the tip and a sample is af-
fected by temperature, the thickness, the surface free en-
ergy of the adsorbed water film, etc. To make this problem
clear is of essential importance to the quantification of the
force spectrum method. Yet up to now, disparity has been
shown in different experiments. According to Thundat et
al.[13], when the relative humidity increased from 15% to
80%, the adhesion force between Si3N4 tip and mica in-
creased from 4.5×10ꢀ9 N to about 10×10ꢀ9 N. In a more
recent article of Eastman et al.[14], it is reported that the
adhesion forces between unmodified, hydrophilically and
hydrophobically modified Si3N4 tips and mica were
measured at different humidities. However, their results
showed that for one tip, the magnitude of the adhesion
force did not change with humidity, but the force in-
creased as the hydrophilicity of the tip increased.
As stated above, to make certain what factors influ-
ence the adhesion force between the tip and sample is of
significant help to investigations of other systems by the
force spectrum method. In order to confirm if the thick-
ness of adsorbed film and ions in the film affect the adhe-
sion force, we selected three inorganic crystals mica, CaF2
and KCl as our targets whose adsorbed water films have
different qualities, and measured their adhesion forces as
humidity changed.
Keywords: mica, CaF2, KCl, adhesion force, relative humidity, AFM.
Since its invention in 1986[1], AFM has become a
powerful tool for investigation of surface on atomic scale.
The morphology of a sample surface is obtained based on
the interaction between the tip and sample. When the tip
approaches or separates from the sample, the interaction
between them can be detected by the deflection of the
cantilever and thus comes out the force-distance curve or
force spectrum. The force needed to separate the tip com-
pletely from the sample surface is called adhesion force.
Using the force spectrum method to measure the adhesion
force between the chemically or biologically modified tip
or/and sample, we can get the relevant chemical or bio-
logical information. So far, much work has been done in
ü
this area[2 7]. For example, Florin et al.[2] have measured
the adhesion force between individual ligand-receptor
pairs such as avidin-biotin and iminobiotin-biotin, and
Lee et al.[3] have made a direct measurement of the forces
between complementary strands of DNA. Using chemi-
cally modified tips, Werts et al.[4] determined the local
surface reaction conversion during a chemical surface
modification.
1
Instruments and methods
The atomic force microscopy we used is commer-
cially produced by Molecular Imaging Corporation, USA.
It has a chamber which can be sealed and thereby makes it
possible for us to monitor the environmental conditions
(i.e. relative humidity and temperature). Relative humidity
between 20% and 80% was controlled by mixing streams
of dry nitrogen and wet nitrogen (prepared by bubbling
nitrogen gas through distilled water) at an appropriate
flow ratio and the relative humidity above 80% could be
One of the advantages of AFM is that it can be used
in ambient conditions, and in ambient air, a thin film of
water exists on most surfaces[8]. For instance, using a
scanning polarization force microscopy, Hu et al.[9] di-
rectly observed the process of condensation and evapora-
tion of water on mica at room temperature. They[10] also
studied the structure formed on the (100) cleavage surface
912
Chinese Science Bulletin Vol. 46 No. 11 June 2001

obtained by putting distilled water in the chamber. The
humidity was monitored by a digital humidity-sensitive
sensor (produt of the Beijing Kunlun Hai’an Center). The
adhesion force at every humidity (f1%) is a statistical
average of numerous measurements. Experiments were
conducted at 25ć.
slightly, and above the humidity of 60%, the adhesion
2
Results and discussion
From many repetitive experiments on mica, we ob-
tained similar results as follows: as the relative humidity
increased, the surface adhesion force on mica did not
change; when we lowered the humidity, we could get al-
most completely reversible results. A representative
group of data is shown in fig. 1 for specific illustration.
Fig. 2. Adhesion forces between AFM tip and CaF2 surface under
various relative humidities.
Fig. 3. Adhesion forces between AFM tip and KCl surface under vari-
ous relative humidities.
Fig. 1. Adhesion forces between AFM tip and mica surface under
various relative humidities.
force increased dramatically with small increase in hu-
midity. The different performances of CaF2 and KCl are
again due to the difference in the surface free energy of
the adsorbed water film. CaF2 hardly dissolves in water
Beaglehole et al.[11] have ellipsometrically deter-
mined the thickness of adsorbed water film on mica as a
function of the relative vapor pressure of water at 18ć.
Their results showed that the thickness of the water film
on mica increased steadily from the relative humidity of
20%; especially after the humidity reached 80%, the
thickness of water film increased more sharply and at 95%,
the film was thicker than 1 u 10ꢀ9 m. The saturated vapor
pressure of water at our experiment temperature 25ć is
1.5u103 Pa higher than that at 18ć[15], so at every relative
humidity at 25ć, the thickness of the water film should
( at 18ć, the solubility in 100 g water is only 1.50 mg,
and at 40ć, 1.67 mg[16]), and when the adsorbed water
film grew thicker with the increasing humidity, almost no
ions dissolved in the film. As the result, the surface free
energy did not change with humidity and so did the adhe-
sion force. Contrarily, KCl dissolves in water very easily
(at 25ć, the solubility in 100 g water is 35.98 g[16]) and in
air it deliquescences readily. Using the infrared spectro-
scopic photometry, Peters et al.[17] provided the adsorption
isotherm of water on the surface of NaCl(100). They
found that at the humidity of 50%, there were ions dis-
solved in the adsorbed water film. Since KCl and NaCl
are very similar in their solubility (at 25ć, the solubility
of NaCl is 36.08 g[16]), we can infer the similar result for
KCl. When the water film became thicker with humidity,
more and more K+and Clꢀ would dissolve in the film.
Once there are ions in water, the ions will attract water
molecules to the inner part of the solution and the extra
energy to overcome the electrostatic interaction will in-
crease the surface free energy, which in turn will lead to
the increase in the adhesion force. Fig. 3 shows that when
humidity was below 60%, since the amount of ions in the
film is very small, the surface free energy increased very
be thicker than its counterpart at 18ć[11]. However, as
shown in fig. 1, the adhesion force did not change even
when the humidity increased from 80% to 95%. Therefore,
we conclude that the adhesion force is almost unaffected
by the thickness of the water film and thus must be mainly
dependent on the surface free energy of the film. This can
be further proved by the following results from CaF2 and
KCl.
Figs. 2 and 3 respectively show the surface adhesion
force on CaF2 and KCl as a function of relative humidity.
From fig. 2 we can see that just like mica, the adhesion
force on CaF2 remained almost constant as the relative
humidity increased; while fig. 3 shows that the adhesion
force on KCl changes with humidity. When the humidity
increased from 20% to 60%, the adhesion force increased
Chinese Science Bulletin Vol. 46 No. 11 June 2001
913

NOTES
slightly and so did the adhesion force. After the humidity
was raised to above 60%, ions dissolved in the film made
the surface free energy increase much and hence the adhe-
sion force increased dramatically.
4. Werts, M. P. L., van der Vegte, E. W., Hadziioannou, G., Surface
chemical reactions probed with scanning force microscopy,
Langmuir, 1997, 13: 4939.
5. Noy, A., Frisbie, C. D., Rozsnyai, L. F. et al., Chemical force mi-
croscopy: exploiting chemically-modified tips to quantify adhe-
sion, friction, and functional group distribution in molecular as-
sembly, J. Am. Chem. Soc., 1995, 117: 7943.
6. Stuart, K., Hlady, V., Effects of discrete protein-surface interac-
tions in scanning force microscopy: adhesion force measurement,
Langmuir, 1995, 11: 1368.
The different effects of humidity on the adhesion
force on CaF and KCl showed again that in AFM, when
2
the liquid film is continuous, the adhesion force between
the tip and sample is mainly dependent on the surface free
energy of the film, but almost independent of the thick-
ness of the film.
7. Chen, X., Davies, M. C., Roberts, C. J. et al., Recognition of pro-
tein adsorption onto polymer surfaces by scanning force micros-
copy and probe surface adhesion measurements with protein-
coated probes, Langmuir, 1997, 13: 4106.
8. Derjaguin, B. V., Churaev, N. V., Structure of water in thin layers,
Langmuir, 1987, 3: 607.
9. Hu, J., Xiao, X. D., Ogletree, D. F. et al., Imaging the condensa-
tion and evaporation of molecularly thin films of water with
nanometer resolution, Science, 1995, 268: 267.
10. Dai. Q., Hu, J., Salmeron, M., Adsorption of water on NaCl(100)
surfaces: Role of atomic steps, J. Phys. Chem. B, 1997, 101: 1994.
11. Beaglehole, D., Christenson, H. K., Vapor adsorption on mica and
silicon: Entropy effects, layering, and surfaces forces, J. Phys.
Chem., 1992, 96: 3395.
12. Weisenhorn, A. L., Hansma, P. K., Forces in atomic force micros-
copy in air and water, Appl. Phys. Lett., 1989, 54: 2651.
13. Thundat, T., Zheng, X. Y., Chen, G. Y. et al., Role of relative hu-
midity in atomic force microscopy imaging, Surface Science,
1993, 294: L939.
14. Eastman, T., Zhu, D. M., Adhesion forces between sur-
face-modified AFM tips and a mica surface, Langmuir, 1996, 12:
2859.
The magnitude of the measured adhesion force may
differ from one tip to another. This may be caused by two
factors. (ν) Different tips have different radii; and from
the equation Fcap = 4SꢁRJꢁcosTꢁ>ꢂꢃ@ (Fcap, capillary force; R,
radius of contact area; J, surface tension or surface free
energy of the liquid; Tꢁ, contact angle of the liquid-solid
interface), we can know that this will make the capillary
force between the tip and sample different and thus pro-
duces different adhesion forces. (ξ) Different properties
of tips such as contaminants adsorbed on the tip and the
real value of the elastic constant can also result in differ-
ent adhesion forces. Therefore, definite calibration of the
properties of the tip is very important to the quantification
of the force spectrum method.
3
Conclusion
In summary, by the force spectrum method, the sur-
face adhesion force of mica, CaF2 and KCl were measured
as the relative humidity changed. It is concluded that
when the adsorbed liquid film is continuous, the adhesion
force between the tip and sample is mainly dependent on
the surface free energy of the adsorbed liquid but almost
unaffected by its thickness.
15. Soviet Union Chemistry Handbook (ĉ) (Chinese edition by Tao
Kun), Beijing: Science Press, 1963.
16. Soviet Union Chemistry Handbook (ċ)(Chinese edition by Tao
Kun), Beijing: Science Press, 1963.
17. Peters, S. J., Ewing, G. E., Thin film water on NaCl(100) under
ambient conditions: an infrared study, Langmuir, 1996, 12: 6345.
18. Fu, X.-C. Shen, W.-X., Yao, T.-Y., Physical Chemistry(Ċ)(in
Chinese), Beijing: Higher Education Press, 1993.
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(Received September 1, 2000)
914
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