Quantitative treatment of surface potentials in Langmuir films from aromatic amphiphiles

Article abstract of DOI:10.1016/S0009-2614(01)00175-0

It is shown that the surface potentials of Langmuir monolayers from aromatic compounds can be interpreted using the three-layer capacitor model of Demchak and Fort, with the same local dielectric constants employed for aliphatic compounds. Based on new da

Full text of DOI:10.1016/S0009-2614(01)00175-0

30 March 2001
Chemical Physics Letters 337 >2001) 11±17
www.elsevier.nl/locate/cplett
Quantitative treatment of surface potentials in Langmuir ®lms
from aromatic amphiphiles
b
c
Patrycja Dynarowicz-èaßtka a, , Ailton Cavalli , Demetrio A. Silva Filho ,
*
ꢀ
Piotr Milart a, M. Cristina dos Santos c, Osvaldo N. Oliveira d
a
Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Krakoꢀw, Poland
Universidade Estadual do Norte Fluminense, Campos dos Goytacazes, 21949 Rio de Janeiro, RJ, Brazil
b
c
ꢀ
Universidade Estadual de Campinas ± Instituto de Fõsica Gleb Wataghin, Campinas, SP, Brazil
ꢀ
d
Instituto de Fisica de Sa~o Carlos, Universidade de Sa~o Paulo, CP 369, CEP 13560-970, Sa~o Carlos, SP, Brazil
Received 9 November 2000; in ®nal form 10 January 2001
Abstract
It is shown that the surface potentials of Langmuir monolayers from aromatic compounds can be interpreted using the
three-layer capacitor model of Demchak and Fort, with the same local dielectric constants employed for aliphatic
compounds. Based on new data for monolayers from an acid, amine and ester compounds with an aromatic hydrophobic
part, we show that the dielectric constant for the monolayer/water interface region is 6Æ1. Good agreement between
experiment and theory was obtained by taking the dielectric constant of the monolayer/interface as 3.0Æ0.6, which are
essentially the same as those obtained for long tail aliphatic compounds. Ó 2001 Elsevier Science B.V. All rights reserved.
DV ˆ ꢀ1=Ae0†‰l1=e1 ‡ l2=e2 ‡ l3=e3Š;
ꢀ1†
1. Introduction
where A is the area per molecule, e0 is the vacuum
permittivity, l1 is the vertical component of the
e€ective dipole moment due to reorientation of
water dipoles caused by the introduction of ®lm-
forming molecules at the water interface, and l2
and l3 are the vertical components of group dipole
moments in the hydrophilic and hydrophobic
parts of the molecule, respectively. Each of such
dipoles is embedded in a medium with a local di-
electric permittivity, ei.
Using experimental data for non-ionised
Langmuir monolayers of p-terphenyls, Demchak
and Fort obtained quantitative agreement between
theory and experiment by assuming that the di-
electric constants were e3 ˆ 5:3 for the air/mono-
layer interface, e2 ˆ 7:6 for the monolayer/water
The surface potential technique has been used
in the characterization of Langmuir monolayers
for several decades, but progress in the quantita-
tive interpretation of measured values has been
rather slow. Aturning point was the publication in
1974 of the Demchak and Fort >DF) three-layer
capacitor model [1], in which the surface potential,
DV , was assumed to arise from the dipole mo-
ments in distinct parts of the monolayer and with
such parts possessing distinct local dielectric con-
stants, according to Eq. >1):
* Corresponding author. Fax: +48-12-340515.
E-mail address: ucdynaro@cyf-kr.edu.pl >P. Dynarowicz-
èaßtka).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 > 0 1 ) 0 0 1 7 5 - 0

12
P. Dynarowicz-èaßtka et al. / Chemical Physics Letters 337 92001) 11±17
interface and that l1=e1 ˆ ‡0:04 D [1]. This ap-
proach was later applied to adsorbed >Gibbs)
monolayers formed by small, water-soluble am-
phiphilic molecules [2±6] as well as for Langmuir
®lms from insoluble amphiphiles [7,8], while a
similar 2-layer model was used by Vogel and
compounds with head groups other than the
carboxylic acid, thus allowing us to also estimate
the dielectric constant at the monolayer/water in-
terface, e2. This is done with monolayer results
from 50-phenyl-1,10:30; 100-terphenyl tail attached to
the amino >ANH2) or methyl ester >ACOOCH3)
head group, which were synthesised with this
purpose. Both compounds are capable of stable
®lm formation at the free water surface. The
analysis of such experimental data provided a
complete picture for the interpretation of surface
potentials for aromatic as well as aliphatic com-
pounds. The implications of this achievement are
discussed along with the limitations in the appli-
cation of the DF approach.

Mobius [9]. Oliveira et al. [7], in particular, dem-
onstrated that for aliphatic compounds the value
of e3 ˆ 5:3 suggested by Demchak and Fort was
far too high to explain experimental data from
halogenated compounds. It was then shown [7]
that for aliphatic compounds, agreement with the
experiments required the following parameters:
l1=e1 ˆ À0:065 D, e2 ˆ 6:4; e3 ˆ 2:8. It is worth
mentioning that the latter value for e3 is corrobo-
rated by a theoretical treatment by Taylor and
Bayes [10]. Progress has also been made in the
interpretation of the double-layer contribution to
the surface potential in ionized monolayers, for it
has been shown [11] that at least for weakly ion-
ized monolayers spread on aqueous subphases
with monovalent ions, the simple Gouy±Chapman
theory may provide good agreement with experi-
mental results.
In summary, the monolayer surface potentials
of simple aliphatic compounds could be explained
quantitatively, but the question of whether di€er-
ent parameters should be employed for aromatic
compounds remained. Testing the DF approach
for the latter compounds was not possible for a
number of years because surface potential data
were not available for simple, purely aromatic
compounds for which the contributions from the
group dipole moments could be estimated. Re-
cently, however, we have succeeded in synthesising
a series of aromatic homologues of polyphenyl
carboxylic acids from which stable Langmuir
monolayers could be obtained [12]. Comparing the
e€ective dipole moments of such acids di€ering in
their hydrophobic parts, we were able to show [13]
that the local dielectric constant at the monolayer/
air interface, e3, is 3:0 Æ 0:6, i.e. essentially the
same as for aliphatic molecules >2.8 [7]). Important
for such an achievement was the possibility of es-
timating the dipole moment components for rela-
tively large molecules, which is now possible using
semi-empirical quantum chemistry methods [14].
In this Letter, we present results for aromatic
2. Experimental
2.1. Synthesis
50-phenyl-1,10:30; 100-terphenyl-4-carboxylic acid,
abbreviated as PTCA>compound 1, Scheme 1)
was synthesised according to the procedure de-
scribed in [15]. Methyl 50-phenyl-1,10:30; 100-terphe-
nyl-4-carboxylate ꢀ3) was readily obtained by
alkylation of the sodium salt of the parent carb-
oxylic acid [15] under phase transfer catalysis
>Scheme 2). 50-phenyl-1,10:30; 100-terphenyl-4-amine
ꢀ5) was prepared by reduction of the appropriate
nitro compound, which was synthesised applying
Scheme 1. Chemical structure of 50-phenyl-1,10:30; 100-terphenyl-
4-carboxylic acid, in short PTCA>compound 1).

P. Dynarowicz-èaßtka et al. / Chemical Physics Letters 337 92001) 11±17
13
ortho of rings at 30 and 50); 7.76 and 8.14 >2d, 4H,
AA0BB0; Jortho ˆ 8:3 Hz, protons of the ring with
COOCH3 group); 7.80 >s, 2H, protons 20 and 60);
7.82 >s, 1H, proton 40).
2.1.2. 50-Phenyl-1,10:30; 100-terphenyl-4-amine ꢀ5)
0
Avigorously stirred suspension of 4 -nitro-50-
phenyl-1,10:30; 100-terphenyl ꢀ4) >0.35 g, 1 mmol)
and tin powder >0.5 g) in ethanol >10 ml) was
heated to 80°C. Concentrated hydrochloric acid
>5 ml) was slowly added during 2.5 h. The mixture
became almost clear. Then it was cooled to room
temperature and ®ltered. The ®ltrate was poured
into stirred 10% sodium hydroxide solution >ca. 50
ml). The separated amine was ®ltered o€, washed
with water and a small amount of cold ethanol.
For further puri®cation, the crude product was
suspended in ethanol >20 ml) and re¯uxed for 30
min. The colorless solid was separated and dried in
vacuum at 80°C. An analytical sample was ob-
tained by vacuum sublimation at ca. 1 mm Hg.
Yield: 0.28 g >87%). M. p. 152.3°C. Anal. Calc. for
C24H19N: %C 89.67; H 5.97; N 4.36. Found: %C
Scheme 2. Synthetic path to methyl 50-phenyl-1,10:30; 100-ter-
phenyl-4-carboxylate >compound 3).
the Zimmermann and Fischer method [16]
>Scheme 3).
2.1.1. Methyl 50-phenyl-1,10:30; 100-terphenyl-4-carb-
oxylate ꢀ3)
0
Amixture of sodium 5 -phenyl-1,10:30; 100-ter-
phenyl-4-carboxylate ꢀ2) >0.37 g, 1 mmol), methyl
iodide >0.70 g, 5 mmol), TEBA>0.1 g) and dry
acetone >10 ml) was stirred and re¯uxed for 4 h.
After cooling to room temperature, a small
amount of colorless material was separated by
®ltration and washed with acetone. The solvent
was removed at reduced pressure. The crude
product was treated with water, ®ltered o€, wa-
shed with water and cold methanol >5 ml). It was
puri®ed by recrystallization from methanol and
dried in air. Yield: 0.26 g >72%). M. p. 132.3°C.
Anal.Calc. for C26H20O2: %C 85.67; H 5.54.
89.94; H 6.05; N 4.46. IR >KBr) õ [cmÀ1] 3444, 3369
ꢀ
>NH); 3029 >CH); 1618 >NH); 1590, 1517 >aro-
1
matic rings); 1291 >NH). H NMR >DMSO-d6) d
[ppm] 5.30 >s, 2H, NH2); 6.69 and 7.56 >2d, 4H,
AA0BB0; JAB ˆ 8:5 Hz, protons of the ring with
ANH2 group); 7.40 >t, 2H, Jortho ˆ 7:3 Hz, protons
para of rings at 30 and 50); 7.50 >t, 4H, Jortho
ˆ
Found: %C 85.87; H 5.63. IR >KBr) õ [cmÀ1] 3061,
7:3 Hz, protons meta of rings at 30 and 50); 7.73
>t, 1H, Jmeta ˆ 1:4 Hz, proton 40); 7.76 >d, 2H,
Jmeta ˆ 1:4 Hz, protons 20 and 60); 7.84 >d, 4H,
Jortho ˆ 7:3 Hz, protons ortho of rings at 30 and 50).
ꢀ
2998, 2954, 2841 >CH); 1718 ꢀC@O†; 1608, 1594
1
>aromatic rings); 1276 ꢀCAO†. H NMR ꢀCDCl3†
d [ppm] 3.95 >s, 3H, OCH3); 7.40 >t, 2H,
Jortho ˆ 7:3 Hz, protons para of rings at 30 and 50);
7.49 >t, 4H, Jortho ˆ 7:3 Hz, protons meta of rings
at 30 and 50); 7.69 >d, 4H, Jortho ˆ 7:3 Hz, protons
2.2. Langmuir monolayers
Spreading solutions were prepared by dissolv-
ing the investigated compound >ꢁ0.5 mg/ml) in
freshly distilled, spectroscopic grade chloroform.
Ultrapure water from a Nanopure >in®nity) cou-
pled to a Milli-Q water puri®cation system
ꢀresistivity ˆ 18:2 MX cm† was used as a sub-
phase. To obtain non-ionised ®lms, PTCAmethyl
ester was spread on pure water, while the carb-
oxylic acid >PTCA) and amine were spread on
aqueous acidic >10À3 M. HCl) and basic >10À3 M.
NaOH) solutions, respectively. Monolayers were
Scheme 3. phenyl-1,10:30; 100-terphenyl-4-amine >compound 5).

14
P. Dynarowicz-èaßtka et al. / Chemical Physics Letters 337 92001) 11±17
spread on a KSV-5000 LB trough ꢀtotal area ˆ
730:5 cm2† placed on an anti-vibration table in a
class 10 000 clean room. The surface pressure of
the monolayer was measured to an accuracy of 0.1
m N mÀ1 with a Wilhelmy plate >made of chro-
matography paper, ash-less Whatman Chr 1)
connected to an electrobalance. Simultaneously,
the surface potential was monitored using a
vibrating plate located ca. 2 mm above the water
surface. The reference electrode, made from
platinum foil, was placed in the water subphase.
The surface potential measurements were repro-
ducible to Æ10 mV. Monolayers were usually
compressed with a barrier speed of 25 mm/min
>equivalent to a compression rate of 7:5 Â
Poland. Melting points were determined with Mel-
Temp II melting point apparatus in open capil-
laries and are uncorrected.
3. Results and discussion
Fig. 1 presents surface pressure ꢀp†, electric
surface potential ꢀDV † and e€ective dipole mo-
ment ꢀl?†-area isotherms of the non-ionised ®lms
of PTCA ꢀ1), its methyl ester ꢀ3) and amine ꢀ5).
The data for PTCA>Fig. 1a) have already been
reported and are reproduced from a previous
Letter [13]. The potential >solid line) and e€ective
dipole moment >dotted line) already start to in-
ꢁ2
ꢁ2
1017 A = min) at 20°C.
crease at ca. 60 A =molecule, when the surface
pressure >dashed-dotted line) is still zero. Upon
further compression, the surface potential and ef-
fective dipole moment increase steeply, reach a
maximum and then decrease when the surface
pressure is already non-zero. For a detailed de-
scription and interpretation of the isotherms, the
readers are referred to [12]. The maximum in sur-
face potential/e€ective dipole moment appears at
2.3. Characterization methods
IR spectra were recorded on a Bruker IFS48
spectrometer as KBr pellets. 1H NMR spectra
were taken at 500.13 MHz with a Bruker AMX500
spectrometer using DMSO-d6 or CDCl3 as sol-
vents and TMS as an internal standard. Elemental
analyses were carried out in the Regional Labo-
ꢁ2
the same molecular area ꢀ36 Æ 3 A =molecule†,
ꢀ
ratory of Physicochemical Analyses, Krakow,
regardless of the kind of hydrophilic head group of
Fig. 1. Surface pressure ꢀp† >dashed-dotted), surface potential ꢀDV † >solid) and e€ective dipole moment ꢀle?xp† area >A) isotherms of
PTCA>compound 1), its methyl ester >compound 3) and amine >compound 5) spread on 10À3 M HCl aq. >compound 1), water
>compound 3) and 10À3 M NaOH aq. >compound 5) at 20°C; compression speed 25 mm/min.

P. Dynarowicz-èaßtka et al. / Chemical Physics Letters 337 92001) 11±17
15
Table 1
Comparison of experimental and calculated dipole moments of investigated aromatic amphiphiles
E€ective dipole moment, l? [D]
Dipole moments in vacuum, l [D]a
Compound
Polar group >P)
lc?alc
le?xp
lz
lxÀy
ltotal
b
c
d
ACOOH
ACOOCH3
ANH2
0.54
0.44
0.29
0.53
0.45
0.27
2.675
2.10
1.20
1.127
1.40
1.25
2.903
2.50
1.73
a Calculated with HyperChem.
b The values are given within an experimental error of Æ0.03.
c Component of the dipole moment normal to the interface.
d Component of the dipole moment parallel to the interface.
Model 1) parameterization [18±20]. The normal
components depend on whether a cis or trans
con®guration is assumed for the carboxylic and
ester head group. Table 2 contains the values of l2
obtained using the above-mentioned procedure.
Solving equations of type >2) for all three
combinations of pairs, the mean local dielectric
constant e2 was found to be 6.0Æ1.0, if the cis
con®guration is assumed for both acid and ester
head groups. For trans conformation, a reasonable
value >6.6) is obtained only when ester and acid are
solved in pairs; in the other cases e2 is very high
>14.3 and 17.7 when the acid and ester are solved
in pair with the amine, respectively). It is worth
pointing out that e2 ˆ 6:0 Æ 1:0 for the aromatic
compounds matches very well with the value of 6.4
found by Oliveira et al. [7] for aliphatic amphi-
philes. Using the average value of e3 ˆ 3:0, as
obtained for a series of PTCAderivatives [13], and
obtaining l3 theoretically using HyperChem, the
remaining parameter, l1=e1, was found to be
)0.065 Æ 0.015 D for good agreement to be ob-
tained with the experimental results. Similarly to
[13], l3 was calculated by subtracting the normal
component of the dipole moment of the respective
hydrophilic group ꢀl2† from the normal counter-
part of the dipole moment of the free molecule of
PTCA, its methyl ester and amine ꢀlz†. Table 1
shows the comparison with experimental data,
when the three average parameters were employed,
viz. e2 ˆ 6:0, e3 ˆ 3:0 and l1=e1 ˆ 0:065 D.
the molecule. At this area, the molecules are as-
sumed to take nearly vertical >to the surface) ori-
entation as the e€ective >normal) dipole moment
has its maximum value. Using the Demchak and
Fort model and assuming that the contributions
from the hydrophobic group ꢀl3=e3† and of the
oriented water dipoles ꢀl1=e1† are independent of
the head group, the di€erence in the total, experi-
mentally measured dipole moment ꢀle?xp† can be
ascribed to di€erences in the contribution from the
head group of the molecule ꢀl2=e2†. In order to
determine e2, pairs of Eq. >1) have to be solved for
monolayers of molecules with identical hydro-
phobic group and di€erent hydrophilic heads.
Assuming the same contribution from hydrated
water, the resulting equation has the following
form:
ꢀla À lb†=e2 ˆ l?exp a À le?xp b
:
ꢀ2†
2
2
In the above formula, the superscripts a and b
refer to two distinct compounds in the pair used
for comparison, and le?xp is the product ꢀDVAe0†.
The maximum values of the experimental e€ective
dipole moments for the investigated compounds
ꢀle?xp† are compiled in Table 1, which also includes
the calculated values, as discussed later. One also
need to estimate the dipole moment from the hy-
drophilic groups, l2. Because the interactions with
the neighbouring dipoles and with the medium as a
whole are incorporated in the dielectric constant,
l2 is the vertical component of the dipole moment
in vacuum. These values may be obtained by
subtracting the CÀH dipole moment >0.4 D [17])
‡
from the normal component of the dipole moment
of the free molecule ꢀlz† for benzoic acid, methyl
benzoate and aniline, respectively, calculated with
1
HyperChem Professional Release 5.1, AMolecular Visual-
ization and Simulation Software Package, Hypercube, Gaines-
ville, Florida, 1998.
1
HyperChem in the semi-empirical AM1 >Austin

16
P. Dynarowicz-èaßtka et al. / Chemical Physics Letters 337 92001) 11±17
Table 2
Calculated values of dipole moments of free molecules of benzoic acid, methyl benzoate and aniline
Compound
Dipole moments in vacuum, l [D]a
b
c
lz
lxÀy
ltotal
Benzoic acid
Methyl benzoate
C6H5ACOOH
C6H5ACOOCH3
C6H5ANH2
cis 2.13
trans 4.42
1.10
1.30
1.40
2.42
cis 1.60
trans 3.89
2.11
1.54
Aniline
a Calculated with HyperChem.
b Component of the dipole moment normal to the interface.
c Component of the dipole moment parallel to the interface. Values presented in the table are for the cis con®guration of the hy-
drophilic group.
0.70
Thus it is evident that the electrical parameters
for monolayers of aliphatic and aromatic amphi-
philes are practically the same. This generality may
imply that any type of material can be analyzed,
provided that one can estimate the group dipole
moments with some degree of accuracy. In this
context, the use of semi-empirical methods, as in
this work, may allow extension of the analysis to
larger molecules, such as the phospholipids that
are widely investigated, and whose surface poten-
tial data have not been explained quantitatively in
an unequivocal way. There is one important limi-
tation, however, in the cases where the assump-
tions inherent in the DF approach are not valid.
For instance, as emphasised in [13], when highly
polar groups are incorporated into the hydro-
phobic part, one can no longer assume that the
dielectric constant of a given medium is indepen-
dent of the molecular group. Neither can one as-
sume that a contribution from one layer does not
a€ect the other one, which is an important tenet of
the DF model. The revised Taylor and Bayes
model [21] clearly shows that the value of e2 de-
pends on the dipole species incorporated in the
molecule. They had shown earlier [10] that the
layers can be considered as non-interacting and
thus treated independently >which is the basic as-
sumption of the DF model) only for long-chain
molecules where headgroup and tailgroup dipoles
are more than about 10 methylene units apart.
Since it is known that the hydrophobicity of a
phenylene group is comparable to that of an ali-
phatic fragment with 4Æ0.5 methylene groups [22],
the polyphenyl compounds investigated here,
whose hydrophobic part contains four benzene
rings, can be treated as analogues of alkanoic
amphiphiles with 16 methylene groups. Therefore,
PTCA, its methyl ester and amine are suitable
materials for the DF model, which justi®es the
good agreement between theory and experiment
reported here.
Iwamoto and co-workers [23±25] have also de-
veloped theoretical models for monolayer ®lms,
where it is found that the dielectric constant de-
pends on the alignment and dipole density of the
array of dipoles assumed to represent the mono-
layer [23]. They did not distinguish between the
contributions from the hydrophilic and hydro-
phobic groups, but rather treated the molecular
moments as point dipoles associated with tilted
rod-like molecules arranged on a hexagonal lattice
[23,24]. Considering the randomizing e€ect of
thermal energy on molecular orientation and con-
formation, the relative permittivity of a simple
long-chain alkanoic acid monolayer was found to
be 1 in the expanded region and ca. 2 in the con-
densed state, which con®rms earlier ®ndings by
Taylor and Bayes [10]. Moreover, they also found
that for the estimation of the local dielectric con-
stant account should be taken of the dielectric an-
isotropy inherent in the monolayers owing to the
symmetry breaking at the air/water interface [25].
It should be stressed that the parameters ob-
tained here to input in the DF model are only
average values. It is clear that small changes in
such values still lead to good agreement between
theory and experiment, provided that the other
parameters are also altered. Nevertheless, one may

P. Dynarowicz-èaßtka et al. / Chemical Physics Letters 337 92001) 11±17
17
[7] O.N. Oliveira Jr., D.M. Taylor, T.J. Lewis, S. Salvagno,
C.J.M. Stirling, J. Chem. Soc. Faraday Trans. 1 85 >1989)
1009.

[8] J.G. Petrov, E.E. Polymeropoulos, H. Mohwald, J. Phys.
Chem. 100 >1996) 9860.
be sure that the dielectric constants are within
certain ranges, which are between 5 and 8 for e2
and between 2 and 4 for e3, and these ®gures could
be used as ®rst input in a more re®ned analysis,
where the assumptions of the DF models are re-
laxed. This can be done using models such as those
of [21,23±25], by allowing the dielectric constant to
vary with the molecular group, in addition to in-
vestigating the in¯uence of a polar group in the
hydrophobic tail on the dipole contributions of the
other layers. From the experimental point of view,
surface potential data will be required of com-
pounds where the hydrophilic and hydrophobic
parts are altered in a systematic way.

[9] V. Vogel, D. Mobius, J. Colloid Interface Sci. 126 >1988)
408.
[10] D.M. Taylor, G.F. Bayes, Phys. Rev. E 49 >1994) 1439.
[11] D.M. Taylor, O.N. Oliveira Jr., H. Morgan, Chem. Phys.
Lett. 161 >1989) 147.
[12] P. Dynarowicz-èaßtka, A. Dhanabalan, A. Cavalli, O.N.
Oliveira Jr., J. Phys. Chem B 104 >2000) 1701.
[13] P. Dynarowicz-èaßtka, A. Cavalli, D.A. Silva Filho, M.C.
dos Santos, O.N. Oliveira Jr., Chem. Phys. Lett. 326 >2000)
39.
[14] M.J.S. Dewar, The Molecular Orbital Theory of Organic
Chemistry, McGraw-Hill, New York, 1969.
[15] J. Czapkiewicz, P. Dynarowicz, P. Milart, Langmuir 12
>1996) 4966.
[16] T. Zimmermann, G.W. Fischer, J. Prakt. Chem 329 >1987)
975.
Acknowledgements
[17] J.W. Smith, Electric Dipole Moments, Butterworth, Lon-
don, 1955, p. 98.
The authors acknowledge the ®nancial support
from FAPESP and CNPq >Brazil).
[18] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart,
J. Am. Chem. Soc. 107 >1985) 3902.
[19] M.J.S. Dewar, K.M. Dieter, J. Am. Chem. Soc. 108 >1986)
8075.
References
[20] J.P. Stewart, J Comp. Aided Mol. Design 4 >1990) 1.
[21] D.M. Taylor, G.F. Bayes, Material Sci and Engineering C-
Biomimetic and Supramolecular Systems 8±9 >1999) 67.
[22] N.M. van Oss, P. Kok, T.A.B.M. Bolsman, Tenside Surf.
Det. 3 >1992) 175.
[1] R.J. Demchak, T.J. Fort Jr., J. Colloid Interface Sci. 46
>1974) 191.
[2] P. Dynarowicz, M. Paluch, J. Colloid Interface Sci. 107
>1985) 75.
[23] M. Iwamoto, Y. Mizutani, A. Sugimura, Phys. Rev. B. 54
>1996) 8186.
[3] P. Dynarowicz, M. Paluch, J. Colloid Interface Sci. 129
>1989) 379.
[24] C.-X. Wu, M. Iwamoto, Phys. Rev. B. 55 >1997) 10922.
[25] C. Wu, W. Zhao, M. Iwamoto, J. Chem. Phys. 112 >2000)
10548.
[4] P. Dynarowicz, Coll. Surf. 42 >1989) 39.
[5] P. Dynarowicz, Coll. Polym. Sci 267 >1989) 941.
[6] M. Paluch, P. Dynarowicz, J. Colloid Interface Sci. 115
>1987) 307.