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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 15925–15935 15925
Cite this: Phys. Chem. Chem. Phys., 2011, 13, 15925–15935
Interfacial water on hydrophobic surfaces recognized by ions and
moleculesw
Masami Shibukawa,* Yoshiki Kondo, Yawara Ogiyama, Keita Osuga and
Shingo Saito
Received 10th March 2011, Accepted 5th July 2011
DOI: 10.1039/c1cp20704k
Recent spectrophotometric and molecular dynamics simulation studies have shown that the
physicochemical properties and structures of water in the vicinity of hydrophobic surfaces differ
from those of the bulk water. However, the interfacial water acting as a separation medium on
hydrophobic surfaces has never been detected and quantified experimentally. In this study, we
show that small inorganic ions and organic molecules differentiate the interfacial water formed on
the surfaces of octadecyl-bonded (C18) silica particles from the bulk water and the chemical
separation of these solutes in aqueous media with hydrophobic materials can be interpreted with
a consistent mechanism, partition between the bulk water phase and the interfacial water formed
on the hydrophobic surface. Thermal transition behaviour of the interfacial water incorporated in
the nanopores of the C18 silica materials and the solubility parameter of the water calculated from
the distribution coefficients of organic compounds have indicated that the interfacial water may
have a structure of disrupted hydrogen bonding. The thickness of the interfacial water or the limit
of distance from the hydrophobic surface at which molecules and ions can sense the surface was
estimated to be 1.25 � 0.13 nm from the volume of the interfacial water obtained by a liquid
chromatographic method and the surface area, suggesting that the hydrophobic effect may extend
beyond the first solvation shell of water molecules directly surrounding the surfaces.
1 Introduction
It has so far been reported that the properties and structures of
water on hydrophobic surfaces differ markedly from those of
the bulk water.1–15 The origin of hydrophobic interaction is
usually attributed to the specific local structure of water
around hydrophobic molecules or assemblies. During the past
decade, nonlinear spectroscopic methods, such as vibrational
sum frequency generation (VSFG) spectroscopy or second
harmonic generation (SHG) spectroscopy, have been applied
to the investigation of the hydrogen bonding and orientation
of interfacial water molecules at vapour/water or organic/
water interfaces6–9 and the existence of specific water structures
has been suggested on hydrophobic surfaces. According to the
pictures of interfacial water molecules obtained by VSFG and
SHG spectroscopy, it is expected that water molecules on
hydrophobic surfaces interact with solute molecules differently
from that in the bulk phase. In other words, the interfacial
water itself is expected to function as a separation medium.
A lot of kinds of hydrophobic materials are used for
separation and/or accumulation of chemical substances in
aqueous solutions. For example, alkyl bonded silica and
polystyrene-divinylbenzene copolymer beads are widely used
as packing materials for reversed-phase liquid chromatography
and solid-phase extraction. Themechanism of chemical separation
in these so-called reversed-phase systems has so far been
interpreted with direct adsorption of solute compounds onto
the hydrophobic surfaces of the packing materials16,17 or
partition into the hydrophobic moieties.18–21 To our knowledge,
any model taking into account the role of interfacial water on
the hydrophobic surfaces as a separation medium in chemical
separations has never been presented. This means that whether
or not the interfacial water itself shows a specific interaction
with solute molecules and how much amount of the interfacial
water is formed on hydrophobic surfaces have not been
clarified yet.
Recently, an interesting chemical separation with hydrophobic
porous materials has been reported by Davankov et al.22–24
They showed that simple inorganic ions are chromatographically
separated on hydrophobic nanoporous materials such as
hypercrosslinked polystyrene and nanoporous carbon particles
using pure water as the eluent and claimed that the separation
selectivity of these nonionic nanoporous materials for inorganic
ions can be interpreted with a size-exclusion mechanism.
Graduate School of Science and Technology, Saitama University,255 Shimo-Okubo, Sakura-ku, Saitama, Japan.E-mail: [email protected]; Fax: +81 48-858-3520;Tel: +81 48-858-3520w Electronic supplementary information (ESI) available: Table S1 andFig. S1 and S2. See DOI: 10.1039/c1cp20704k
PCCP Dynamic Article Links
www.rsc.org/pccp PAPER
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15926 Phys. Chem. Chem. Phys., 2011, 13, 15925–15935 This journal is c the Owner Societies 2011
That is, they explained the separation of Ca2+ and H+ or
SO42� and Cl�, for example, on the basis of the difference in
hydration radii of the ions.
Although hydrophilic polymer gels are entirely different
materials from crosslinked polystyrene and alkyl-bonded silica,
it has been reported that they can also separate inorganic ions
in a manner which appears to be similar to that observed in the
hydrophobic materials. For example, Ogata et al.25,26 showed
that the chromatographic retention volume of magnesium ions
in a Sephadex G-15 column (a crosslinked dextran) depends
on the type of counter-anion in the aqueous eluent, the
retention volume increasing in the order SO42� o Cl� o
NO3�o ClO4
�. They reported that this order corresponds to
the order of the hydration radii of these anions and interpreted
this counter-ion effect with the size-exclusion mechanism.
On the other hand, Shibukawa et al.27,28 indicated that the
effect of the counter-ion as well as the co-ion on the retention
of the analyte ion cannot be interpreted with the size-exclusion
mechanism but with the partition between the bulk water and
the water incorporated in the polymer matrices on the basis of
a stoichiometric ion partition model. They also demonstrated
that the amount of water in a polymer gel that exhibits
different affinity to solute compounds from the bulk water
can be estimated by a liquid chromatographic method29 and
the estimated amount of the water is in good agreement with
the amount of water that shows specific thermal phase transition
behaviour determined by differential scanning calorimetry
(DSC).30–34
Taking into consideration the experimental observations
obtained by liquid chromatography described above, we felt
that the counter-ion and co-ion effects on the retention of the
analyte ion should reflect the separation mechanism on the
hydrophobic surfaces in aqueous media, and the interfacial
water formed on the surfaces can be detected and quantified by
liquid chromatography using small inorganic ions as probes.
In this study, we will show that separation of inorganic ions
on nonionic hydrophobic materials, octadecyl-bonded silica
(C18 silica) particles, in pure aqueous media can be successfully
explained by the partition between the hydrophobic interfacial
water and the bulk water phase and the interfacial water plays
a key role as a separation medium not only for inorganic ions
but also for organic compounds on hydrophobic surfaces.
Thermal analysis of water incorporated in the hydrophobic
nanopores is also performed by DSC in order to elucidate the
states of the interfacial water on the hydrophobic surfaces.
2 Experimental
2.1 Materials
All chemicals used in this study were obtained from commercial
sources and were of reagent-grade unless otherwise stated.
Deuterated water (D2O) for NMR use was purchased from
Wako Pure Chemicals (Tokyo, Japan). Water was purified
subsequently with an Elix-Advantage 3-UV (Nihon Millipore,
Tokyo, Japan) and an Arium 611 DI (Sartorius, Tokyo, Japan).
The columns used were Capcell Pak C18 UG80, UG120
and UG300 (Shiseido, Tokyo, Japan) and L-column2 ODS
(Chemicals Evaluation and Research Institute, Tokyo, Japan).
The particle size of all the packing materials used in this study
was 5 mm. The column size, 150 � 4.6 mm, was also the same
for all the columns used.
2.2 Chromatographic conditions
Chromatographic measurements were performed on an HPLC
system consisting of a Shiseido (Tokyo, Japan) Model Nano-
space SI-1/2001 pump, a GL Science (Tokyo, Japan) Model
DG660B degasser, a Rheodyne (Cotati, CA, USA) Model
7025 loading injector fitted with a 20 mm3 sample loop, a
Shiseido (Tokyo, Japan) Model Nanospace SI-1/2002 UV
detector, and a Shimadzu (Tokyo, Japan) Model RID-6A
refractometric detector. An inductively coupled plasma atomic
emission spectrometry (ICP-AES) instrument, aModel OPTIMA
5300DV (Perkin Elmer, Yokohama, Japan), was used for
detection of alkali and alkaline earth metal ions. The columns
were thermostated at 298 K or 303 K using a Shimadzu Model
CTO-6A column oven. Specific surface areas of the packing
materials were measured with a Horiba (Tokyo, Japan) Model
SA-6200 using the BET nitrogen sorption method.
All the eluents were filtered through a 0.45 mm membrane
filter JHWPO 4700 obtained from Nihon Millipore (Yonezawa,
Japan) before use. Elutions were carried out at a constant flow
rate of ca. 0.5 cm3 min�1. The exact values of the volumetric
flow rate were measured using a volumetric flask. The extra
column volume was determined by measuring the elution volume
of a sample solute through the system from which the column
had been removed. The weight of each column packing material
in the column was determined after the packing material was
quantitatively transferred into a beaker and then dried in an
oven at 363 K until a constant weight was reached. Test
solutions were prepared by dissolving analyte compounds in
the eluent to be used. The detection signal was fed into a CDS
plus data analysis system (LA Soft, Tokyo, Japan).
2.3 DSC measurements
Samples of Capcell Pak C18 UG300 for DSC measurement were
taken from the column packed with the material and kept in
water. The water content of the sample to be submitted for DSC
measurement was adjusted by allowing water to vaporize from
the sample in a desiccator containing silica gel at room tempera-
ture. About 5 mg of each sample was placed in an aluminium
sample vessel to be used as volatile samples and the sample vessel
was sealed hermetically. After the DSC measurement, the vessel
was weighed in order to ensure that there is no water leakage.
A Seiko Instruments (Chiba, Japan) DSC-120 differential
scanning calorimeter equipped with a cooling device was used
to measure the phase transition of water in the sample. DSC
curves were obtained by cooling at the scanning rate of
2 K min�1 from 298 K to 223 K and then heating at
1 K min�1 to 298 K after maintaining 223 K for 10 min.
The temperatures and enthalpies of crystallization and melting
of water in the samples were calibrated using pure water.
After DSC measurements, the sample vessel was punctured
with tweezers and placed in an oven at 363 K to dry samples.
The total water content of each sample, wt (g g�1 dry particle),
was calculated as follows:
wt = Ww/Wp (1)
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where Ww and Wp denote the mass of water in the sample and
that of dry particles, respectively.
2.4 Determination of ion-exchange capacities of Capcell Pak
C18 UG80
2.4.1 Cation exchange capacity. After 30 cm3 of
10 mmol dm�3 RbCl aqueous solution was passed into the
column, the column was swept with pure water until a
rubidium ion was not detected in the eluate. The concentration
of rubidium ions in the eluate was monitored by ICP-AES.
Then, 10 mmol dm�3 KCl aqueous solution was passed into
the column and the eluate was collected in a 25 cm3 volumetric
flask. All rubidium ions were eluted within the first 25 cm3 of
eluate from the column. The concentration of rubidium ions in
this solution was determined by ICP-AES.
2.4.2 Anion exchange capacity. To the column, 30 cm3 of
10 mmol dm�3 KI aqueous solution was passed. The column
was then washed with pure water until an iodide ion was not
detected in the eluate. The elution of iodide ions was monitored
by means of a UV spectrophotometer. Then, 10 mmol dm�3
KClO4 solution was passed into the column and the eluate was
collected into a 25 cm3 volumetric flask. All iodide ions were
eluted within the first 25 cm3 of eluate from the column. The
concentration of iodide ions was determined by absorptiometry
at 226 nm.
3 Results and discussion
3.1 Effects of counter-ion and co-ion on retention of an analyte
ion on a C18 silica column in aqueous media
Shibukawa and Ohta28 developed a stoichiometric model for
two-phase partition of an ionic solute, in which complete
dissociation of ionic salts can be assumed, and demonstrated
that the effects of counter-ion and co-ion in the mobile phase
on the retention of the analyte ion on nonionic hydrophilic
polymer gels such as polyacrylamide can be clearly interpreted
with the partition mechanism. According to the model, the
distribution coefficient of the analyte ion, Ap+, obtained by
elution with an aqueous solution of an electrolyte consisting of
Ym+ and Xn� is given by
log DYXA ¼ 1
nlog KAX �
p
nðmþ nÞ log KYX þ kYXA ð2Þ
where KAX and KYX are equilibrium constants for the equilibria
(3) and (4) shown below, respectively, and kYXA is the logarithm
of the ionic activity coefficient quotient represented by eqn (5)
nApþ þ pXn� ! nApþ þ pXn� ð3Þ
nYmþ þmXn� ! nYmþ þmXn� ð4Þ
kYXA ¼ log
yYXA
yYXA
!yYXY yYX
X
yYXY yYX
X
! pmþn
24
35 ð5Þ
yYXi in eqn (5) is the activity coefficient of ion i on the molarity
scale in a system containing the electrolyte YX and the
quantities or the species in the stationary phase are given as
superscript bar notation.
From eqn (2) we can obtain eqn (6) and (7) representing the
counter-ion and co-ion effects on the distribution coefficient of
the analyte ion, respectively, provided that the activity coefficient
of the single ion in both the phases can be regarded as constant
regardless of the type of the background electrolyte:
log DYXA ¼ p
mþ nlog DWZ
X þ C1A ð6Þ
log DYXA ¼ � p
mþ nlog DWZ
Y þ C2A ð7Þ
where DWZX and DWZ
Y are the distribution coefficients of Xn�
and Ym+ obtained in the WZ electrolyte system arbitrarily
chosen and C1A and C2A are the constants for the counter-ions
of the same valency n� and for the co-ions of the valency m+,
respectively.
If the size-exclusion mechanism governs the distribution of
ionic solutes into the pores, on the contrary, the effects of the
counter-ion and co-ion on the distribution coefficient of the
analyte ion depend on their relative size to that of the analyte
ion.24–27 When the hydration radius of the analyte ion is larger
than those of the counter-ions and the co-ions, the distribution
coefficient of the analyte ion should be determined by the size
of the analyte ion itself and is expected to be independent of
the type of the counter-ion or co-ion. On the other hand, when
the size of the counter-ion, Xn�, is larger than that of the
analyte ion, Ap+, the distribution coefficient of the analyte
ion, DYXA is determined by the size of the counter-ion because
of the requirement of the electrical neutrality. Therefore
DYXA is related to DWZ
X by eqn (8), if the sizes of the ions,
Wq+ and Zr�, are smaller than that of Xn�:
logDYXA = logDWZ
X (8)
When the hydration radius of the co-ion is relatively larger
than that of the analyte ion, the co-ion partially excluded from
the pores should exert the Donnan exclusion effect on the
analyte ion so that the distribution coefficient of the analyte ion
will become greater than that obtained with the background
electrolyte consisting of smaller co-ions. The effect may depend
not only on the relative difference in the hydration radii between
the analyte ion and the co-ion but also on the concentration of
the electrolyte.
These considerations described above indicate that one can
determine which is the predominant separation mechanism,
partition or size-exclusion, by examining the counter-ion and
co-ion effects on the retention of analyte ions of different
charges and sizes. In order to obtain the distribution coefficient
in a liquid chromatographic system, it is necessary to measure
the mobile phase volume and the stationary phase volume.
Shibukawa and Ohta29 have reported that the mobile phase
volume, Vm, in partition chromatography can be determined
by employing ionic solutes as probes based on the ion partition
model presented. The idea of the method is based on the fact
that the ratio of the retention factors of two probe ions with
the same charge is constant regardless of the eluent electrolytes.
This is the case for a system where the activity coefficients of
the probe ions of the identical charges, A and B, can be
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15928 Phys. Chem. Chem. Phys., 2011, 13, 15925–15935 This journal is c the Owner Societies 2011
regarded as the same as each other in the mobile and
stationary phases in the liquid chromatographic systems as
represented by eqn (9) and (10), respectively.
yYXA = yYX
B , yWZA =yWZ
B (9)
yYXA ¼ yYX
B ; yWZA ¼ yWZ
B ð10Þ
According to the modified Debye–Huckel equation given
by Davies,35 eqn (11), the mean rational ionic activity coeffi-
cient of a strong electrolyte dissociating into a cation of
valency z+ and an anion of valency z� in a dilute solution
(r0.1 mol dm�3) depends only on the total ionic strength of
the solution and is independent of the type and concentration
of the coexisting electrolytes:
log f� ¼ �Ajzþz�jffiffiffiIp
1þffiffiffiIp � 0:20I
� �ð11Þ
where A is the proportionality constant (= 0.52 at 303 K) and
I is the ionic strength. Since the ionic strength of the eluent is
kept constant at 0.01 or 0.1 mol dm�3 in this work, the activity
coefficients of the ions with the same charge can be regarded as
being identical. The mobile phase volume is then given by29
Vm ¼VYX
A VWZB � VWZ
A VYXB
VYXA þ VWZ
B � VWZA � VYX
B
ð12Þ
where Vjki is the retention volume of the ion i obtained by
elution with a solution of the electrolyte jk. This equation
reveals that the mobile phase volume can be calculated from
the retention volumes of two equally charged probe ions
determined in two eluent electrolyte systems. The Vm value
thus obtained corresponds to the volume of the bulk water
phase in a chromatographic column. We have demonstrated
that this method gives a reasonable value as Vm in reversed-
phase liquid chromatography as well as in normal-phase liquid
chromatography.29–34,36,37
In this study we selected four commercially available C18
silica particles as sample materials with a hydrophobic surface
and determined the Vm values in the columns packed with the
particles and filled with water. We measured the retention
volumes of IO3�, Br�, NO3
�, I� and SCN� selected as probe
ions by elution with 0.1 mol dm�3 NaCl and NaClO4 aqueous
solutions and obtained the Vm values by substituting the
retention volumes into eqn (12). The Vm values determined
for the C18 silica columns are tabulated in Table 1 together
with the total water volume in the columns, V0, surface area,
As, and pore diameter, dpore.
We determined the V0 values by measuring the retention
volume of deuterated water. It has been reported that isotopic
substitution may affect retention of some organic compounds
such as n-alkanes and aromatic compounds.38,39 This observation
suggests that the retention volume of D2O could be different
from that of H2O or the V0 value. Therefore we also deter-
mined the V0 values by a pycnometric method according to the
following equation:
V0 ¼WtðcÞ �WpðcÞ
rACN
ð13Þ
whereWt(c) andWp(c) denote the total mass of the contents in
the column filled with acetonitrile and that of the dry packing
material, respectively, and rACN is the density of acetonitrile.
Acetonitrile was used instead of water because water would be
forced out of the pores of some C18 silicas used in this study
after the release of the pressure as described in Section 3.3. We
substantiated that the V0 values determined by these two
methods are in good agreement with each other within the
experimental error. In this study we used the V0 values
obtained by measuring the retention volume of D2O for
further calculation because the pycnometric method is tedious
and gives less precise results.
As seen in Table 1, the Vm value is distinctly smaller than the
V0 value for all the columns studied, indicating that there
exists a water phase that interacts with ionic solutes differently
from the bulk water in the C18 silica columns. It has been known
that silanol groups on silica particles cannot be deactivated
completely by usual octadecylsilylation and/or endcapping
methods. Water molecules strongly adsorb on silanol groups
and may form a hydration layer. However, the C18 silica
particles we used were prepared by polymeric end capping or
polymer coating of a silica support so that they have virtually
no residual silanols.40–43 Therefore the water phase detected by
the present method, which functions as the stationary phase
for inorganic ions, can be regarded as being formed on the
bonded layer of the C18 silica particles.
The fact that the Vm values calculated from each combination
of two probe ions are in good agreement with one another
suggests that the water phase recognized as the stationary
phase by all the probe ions is identical and that the differentiation
of inorganic ions takes place by partition of ions between the
bulk water phase and the interfacial water on hydrophobic
surfaces. We have thus calculated the D values for various
inorganic ions as well as several neutral organic compounds by
the following equation:
DYXA ¼ VYX
A � Vm
V0 � Vm¼ VYX
A � Vm
VIWð14Þ
where VIW denotes the volume of the interfacial water.
In Fig. 1, logDKXA , the logD values of Li+, Na+, Rb+,
Mg2+ and Ca2+ on Capcell Pak C18 UG80 when eluted with
Table 1 Mobile phase volume (Vm), total water phase volume (V0), surface area (As), and pore diameter (dpore) for C18 silica columns studied
Columna Vmb/cm3 V0
b/cm3 Asc/m2 column�1 dpore
d/nm
Capcell Pak C18 UG80 1.24 � 0.03 1.47 210 8Capcell Pak C18 UG120 1.37 � 0.03 1.58 149 12Capcell Pak C18 UG300 1.77 � 0.02 1.90 106 30L-column2 ODS 1.41 � 0.02 1.70 218 12
a The particle size of all the packing materials used was 5 mm. The column size, 150 � 4.6 mm, was also the same for all the columns used.b Determined at 303 K. c Determined by the BET method by the use of nitrogen gas. d Given for base silica particles by the manufacturers.
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0.01 mol dm�3 potassium salt (KX, X� = Cl�, Br�, NO3�,
ClO4�, SCN�) solutions are plotted against logDKCl
X ,
the logD values of the anions X� obtained by elution with
0.01 mol dm�3 KCl solution. As described below the amount
of fixed ions on the surface of this packing material is so small
that the ion-exchange and ion-exclusion are completely
suppressed under the ionic strength of 0.01 mol dm�3. All
the plots yielded straight lines and the slopes of the plots are
0.5 and 1.0 for univalent and divalent cations, respectively.
This result agrees well with the prediction from eqn (6),
indicating that the difference in the distribution coefficients
of these metal ions is not caused by the size-exclusion effect.
Particularly, change in the retention order of alkali metal ions
and alkaline earth metal ions with the nature of counter-anion
could not be explained by the size-exclusion mechanism. The
mobile phase volume in size-exclusion chromatography is
the interstitial volume and is expected to be smaller than the
Vm value calculated by eqn (12). However, the dependence of
the retention of alkali and alkaline earth metal ions on the
type of counter-anion in the eluent could not be interpreted
with the size-exclusion mechanism even if the distribution
coefficients were calculated using the interstitial volume instead
of the Vm value.
Fig. 2 illustrates the plots of logDKXA vs. logDKCl
X for anionic
analyte ions, representing the co-ion effect. The plots for
nonionic solutes, thiourea, acetone and uracil are also shown
in Fig. 2. As can be seen from this figure, the distribution
coefficients of the nonionic solutes are independent of the
nature of the anions in the mobile phase, while those of the
analyte anions depend on the type of the co-ion. All the plots
give straight lines and the value of the slope of each plot is in
good agreement with the predicted one from eqn (7). It should
be noted that even the distribution coefficient of the large
analyte ion, (ethylenediaminetetraacetato)copper(II) (Cu(edta)2�),
shows the dependence on the type of the co-ion in the eluent
expected from eqn (7) as well. If the size-exclusion effect is the
predominant factor controlling the separation, the distribution
coefficient of Cu(edta)2� should be independent of the type
of the anion in the eluent. These results indicate that the
size-exclusion effect is not the predominant factor controlling
the separation in the present system. Since the inorganic ions
used in this study are assumed not to interact directly with the
hydrophobic moiety or the alkyl chains, it can be concluded
that the partition between the bulk water and the interfacial
water formed on the hydrophobic surface governs the separation
of ionic solutes on the C18 silica in aqueous media.
When the retention volume of an analyte is less than V0 or
the concentration of the analyte in the solution becomes larger
after contact with the sorbent, it is usually concluded that
these phenomena, sometimes called negative adsorption, are
caused by the size-exclusion or ion-exclusion.22–24,44 Actually
some cations and anions used in this study exhibit the retention
volumes smaller than the V0 value. However this is not caused
by the size-exclusion or ion-exclusion effect but by weaker
affinity of the ions for the interfacial water than for the
bulk water.
The materials which we analyzed in this study are mesoporous
(dpore Z 8 nm), while Davankov et al. used materials in which
the pore diameters are 4 nm or less for separation of inorganic
ions.22–24 The size-exclusion might be effective for such
Fig. 1 Plots of logDKXA of cationic analytes against logDKCl
X .
Values in parentheses give the slopes of the plots. Column: Capcell
Pak C18 UG80. Ionic strength of the mobile phase: 0.01 mol dm�3.
Temperature: 303 K.
Fig. 2 Plots of logDKXA of anionic and nonionic analytes against
logDKClX . For other details see Fig. 1.
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15930 Phys. Chem. Chem. Phys., 2011, 13, 15925–15935 This journal is c the Owner Societies 2011
materials with smaller pores, but the role of interfacial water in
separation processes may not be ignored even for microporous
materials.
The cation exchange capacity of the Capcell Pak C18 UG80
was determined to be 0.13 meq column�1, while the anion
exchange capacity was negligibly small (0.01 meq column�1) in
the present eluent system. The cation exchange sites may be
residual silanol groups, but this value is so small that the
cation exchange adsorption and anion exclusion can be
completely suppressed by adding an electrolyte to the eluent
at the ionic strength of 0.01 mol dm�3 or above. Actually, the
retention volumes of the ions determined at the ionic strength of
0.1 mol dm�3 are the same as those obtained at 0.01 mol dm�3
(see ESIw, Table S1).
We tentatively estimated the amount of water bound to
silanol groups assuming that the cation exchange capacity
represents the amount of silanol groups and one silanol group
binds ten water molecules. The estimated value of the amount
of water in the column is as small as 0.02 mm3, which also
suggests that the interfacial water on the C18 silicas recognized
by inorganic ions is not the hydration layer on silanol groups.
Several investigators examined the effect of the type and
concentration of inorganic anions in the mobile phase on the
retention of cationic organic compounds in reversed-phase
liquid chromatography with C18 bonded silicas.45,46 They
observed the significant difference in the retention when
different counter-anions were applied and concluded that this
effect could be explained by ion-pair formation in the mobile
phase with subsequent retention of the neutral ion-pair.
However, the ion-pair formation between alkali metal ions
or alkaline earth metal ions with the inorganic anions used in
this study is negligible in aqueous solutions the ionic strength
of which is not larger than 0.1 mol dm�3.47 Furthermore the
distribution coefficients for these inorganic ions used as probes
in this study are independent of the ionic strength ranging
from 0.01 to 0.1 mol dm�3 as described above. If the ion-pair
formation followed by adsorption onto the surface governs
the retention of the inorganic ions on the C18 silica column, the
distribution coefficients should increase with an increase in
the concentration of the counter-ions or the ionic strength of
the eluent as reported by Dai and Carr.46 This reveals that the
counter-ion effect shown in Fig. 1 cannot be interpreted with the
ion-pair formation mechanism. In addition, the ‘‘negative
adsorption’’ could not be explained in terms of direct adsorption
of the ion-pair, either.
3.2 Recognition of the interfacial water by organic molecules
In order to clarify whether organic compounds can sense the
interfacial water as inorganic ions do or not, we determined
the distribution coefficients of various small organic com-
pounds on the C18 silica columns using pure water as an
eluent according to the following equation.
D ¼ VR � Vm
VIWð15Þ
where VR is the retention volume of the probe organic
compound. In Fig. 3, logD values of n-alcohols, ketones,
nitriles, nitroalkanes, uracil, thiourea and inorganic anions
obtained on a Capcell Pak C18 UG80 column, logD(UG80), are
plotted against those for a Capcell Pak C18 UG120 column,
logD(UG120). As seen in Fig. 3 the plots give a straight line with
the slope of unity going through the origin, which indicates
that the D value of a probe organic compound obtained on a
Capcell Pak C18 UG80 column is equal to that on a UG120
column although the retention volumes on these two columns
are quite different. The plots for the other combinations of the
columns also give straight lines with the slope of unity going
through the origin (ESIw, Fig. S1).The D value calculated by eqn (15) corresponds to the
equilibrium constant of the partition between the bulk water
phase and the interfacial water as illustrated in Fig. 4(a). On
the other hand, if all the water in the columns could be
Fig. 3 Values of logD for Capcell Pak C18 UG80 plotted against
logD for Capcell Pak C18 UG120. Temperature: 303 K. Symbols:
n = methanol, ethanol, 1-propanol and 1-butanol; &= acetone and
2-butanone; J = acetonitrile and propionitrile;B= nitromethane,
nitroethane and 1-nitropropane; + = uracil; * = thiourea;
K = inorganic anions. The D values for organic compounds were
determined with pure water and those for inorganic anions with
0.1 mol dm�3 NaCl and NaClO4 aqueous solutions.
Fig. 4 Schematic illustration of the distribution mechanisms of a
solute on water/C18 bonded silica surface. (a) Partition between the
bulk water and the interfacial water formed on the surface of the
C18 bonded phase, (b) adsorption/desorption between the water phase
and the surface of C18 bonded phase.
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assumed to be the bulk water, another distribution coefficient,
Dads, can be obtained by the following equation:
Dads ¼VR � V0
Asð16Þ
The Dads value corresponds to the equilibrium constant of the
adsorption/desorption between the water phase and the surface
of the C18 silica as depicted in Fig. 4(b).
Fig. 5 shows the plots of the logDads values for Capcell
Pak C18 UG80 (logDads(UG80)) vs. the values for UG120
(logDads(UG120)) (see ESIw, Fig. S2, for the plots of the other
combinations of the columns). The plots for strongly retained
compounds appear to fall on a straight line, whereas those for
weakly retained solutes such as inorganic anions, thiourea and
methanol obviously deviate from the straight line. The plots
for some ions could not be shown in this figure because they
exhibit the retention volumes smaller than the V0 value so that
the Dads values become negative.
If the retention of the organic compounds could be inter-
preted with the partition between the bulk water phase and the
alkyl bonded phase, the distribution coefficient should be
given by
DC18 ¼VR � V0
VC18ð17Þ
where VC18 is the volume of the C18 bonded layer. Since the
difference between eqn (16) and (17) is only the value of the
denominator, the logDC18(UG80) vs. logDC18(UG120) plots
should also give a curved line similar to that shown in Fig. 5.
Although the columns are different from each other in pore
size and specific surface area, the distribution coefficient of a
solute compound must be independent of these physical para-
meters since the surface chemical structure can be regarded as
identical and it governs the structure of the interfacial water
predominantly. Table 2 lists the ratios of D(UG80)/D(UG120)
andDads(UG80)/Dads(UG120) for the organic compounds studied.
It can also be seen from Table 2 that the D(UG80) and D(UG120)
values are approximately the same as each other independent
of the nature of the organic compound, while the Dads values
obtained on the two columns are quite different especially
for weekly retained solutes. The results shown in Fig. 3 and 5
and Table 2 indicate that on hydrophobic surfaces there exists
a water layer functioning as the stationary phase or the
separation medium not only for inorganic ions but also for
organic molecules.
3.3 Thermal phase transition behaviour of water incorporated
in C18 bonded silica particles
Thermal phase transition behaviour reflects the state or the
structure of water in the environment surrounding it. DSC has
been used extensively to obtain quantitative information on the
different states of water in organic and inorganic hydrogels.48
The studies which have so far been carried out indicate that
water usually exists in three different states within the gel
matrices, which can be defined as follows: free water, which
undergoes similar thermal phase transitions to those of the
bulk water; freezable bound water or intermediate water,
which exhibits a melting/crystallization temperature shifted
with respect to that of the bulk water; and non-freezing water
that does not exhibit a detectable phase transition over the
range of temperatures normally associated with the bulk water.
The latter two states of water are considered to result from
interactions with the gel matrices49–52 or compartmentalization
of water by the small pores of the gel.53–56
However, the thermal transition behaviour of water incor-
porated in hydrophobic materials has scarcely been investi-
gated because the hydrophobic materials will not allow water
to ingress into the pores or cavities. We have thus used C18
silica particles taken from a column packed with them and
filled with water as a sample in a similar manner to that
described in the previous study.36 It has been reported that
water is forced out of the small pores of hydrophobic C18 silica
particles packed in a column when the pressure is released.57,58
We adopted Capcell Pak C18 UG300 as a sample material for
DSC measurements because it has the largest pore size among
the C18 silicas used in this study and keeps water in its pores
even after the release of the pressure.
Fig. 5 Values of logDads for Capcell Pak C18 UG80 plotted against
logDads for Capcell Pak C18 UG120. The solid line shows a line going
through the origin with the slope of unity. See Fig. 3 for experimental
details.
Table 2 Comparison of the ratios of the distribution coefficients oforganic compounds obtained for Capcell Pak C18 UG80 and UG120calculated from eqn (15) and (16)
D(UG80)/D(UG120) Dads(UG80)/Dads(UG120)
Methanol 0.90 0.66Ethanol 0.95 0.731-Propanol 0.99 0.771-Butanol 1.03 0.80Acetonitrile 0.98 0.75Propionitrile 1.04 0.81Acetone 1.05 0.822-Butanone 1.10 0.85Nitromethane 0.98 0.76Nitroethane 1.04 0.811-Nitropropane 1.09 0.85Uracil 0.91 0.70Thiourea 0.93 0.68
Av. � S.D. 1.00 � 0.06 0.77 � 0.06
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Fig. 6 shows the DSC curves of water incorporated in
Capcell Pak C18 UG300 in cooling and heating processes.
Two peaks were observed in the heating curve, while one sharp
peak was observed in the cooling one. The exothermic peak
detected in the cooling process is attributed to the crystalli-
zation of water in the sample and the depression of the
crystallization temperature may be ascribed to supercooling
of water.
The melting temperature of one of the two peaks observed
in the heating process is the same as that of the peak obtained
for pure water. Therefore, this peak can be attributed to
melting of bulk water in the sample. The other peak has its
melting temperature below 273 K so that the water for this
peak is categorized as freezable bound water or intermediate
water. We clarified that the freezable bound water observed
for a porous polystyrene-divinylbenzene copolymer gel,
TSKgel Styrene-250, does not function as the stationary phase
and presumed that it is water isolated or compartmentalized
in the small pores, which has similar properties to those of
the bulk water with respect to the interaction with solute
compounds.36 It has been known that the depression of the
melting temperature is a function of the pore radius.55 The fact
that the melting temperature of the freezable bound water
observed for the TSKgel Styrene-250 is nearly the same as that
of the water incorporated in Capcell Pak C18 UG300 supports
this reasoning because the pore diameter of the former material
is 25 nm and can be assumed to be nearly the same as that of the
latter one.
Fig. 7 shows DSC heating curves obtained in two consecutive
measurements for an identical sample. It can be seen that the peak
area for freezable bound water in the second measurement is
smaller than that in the first one, while the peak area for free water
in the second measurement is larger than that in the first one.
We observed a similar phenomenon for water in TSKgel Styrene-
250.36 The result shown in Fig. 7 indicates that a part of water
incorporated in the pores comes out in the melting or the crystal-
lization process.
The amounts of the free water, wf, and freezable bound
water, wfb, expressed in g g�1 dry particle, were calculated
from the first DSC heating curves as follows:
wf = Q(Z 273 K)/DHWp (18)
wfb = Q(o273 K)/DHWp (19)
where Q(Z 273 K) and Q(o273 K) are the heats absorbed in
the heating process, which are calculated from the areas of the
peaks above and below 273 K in the DSC heating curve,
respectively, and DH is the heat of fusion of water calculated
at various temperatures.50 The content of nonfreezing water,
wn, was calculated by subtracting wf and wfb from the total
content of water, wt, as follows:
wn = wt � wf � wfb (20)
Fig. 8 shows dependence of the values of wf, wfb and wn on wt.
The wf value decreases as wt decreases, while both wfb and wn
are nearly constant at wt=2.0 g g�1 or above. The samples for
which the total water content was less than 2.0 g g�1 showed
smaller wfb and wn values and for the samples of wt o 1.0 g g�1
neither freezable bound water nor nonfreezing water were
detected. This is probably because water in the pores of the
C18 silica particles came out of the pores while the samples
were dried in the desiccator. The wfb and wn values calculated
for the samples of wt = 2.0 g g�1 or more are 0.38 � 0.04 and
0.18 � 0.03 g g�1, respectively.
The amount of the stationary phase water or interfacial
water, wIW, expressed in g g�1 dry particle, can be calculated as:
wIW ¼rWVIW
WpðcÞð21Þ
where rW is the density of water. The wIW value obtained for
Capcell Pak C18 UG300 is 0.13 � 0.02 g g�1 (inserted as a solid
Fig. 6 DSC curves of water incorporated in Capcell Pak C18 UG300.
Fig. 7 DSC heating curves of water incorporated in Capcell Pak C18
UG300. wt = 3.11 g g�1 dry particle.
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line in Fig. 8), which compares with the wn value. This means
that the stationary phase water or interfacial water detected by
liquid chromatography is the water that does not crystallize
even at 223 K. This suggests that the hydrogen network of the
water may be disrupted in the vicinity of the C18 silica surface
and this disordered structure of water can be a cause of the
depression of crystallization.
3.4 Evaluation of the state of the interfacial water on the
surface of C18 silicas by molecular probes
The regular solution is principally defined as a solution in
which there is no specific interaction between solute and
solvent molecules and no change in states of association and
orientation by mixing.59 Thus this simple theory has been
usually considered not to be applicable to the interpretation of
the behaviour of aqueous solutions, which involves a specific
interaction such as hydrogen and coordination bonding. How-
ever, many examples of the solution behaviour involving water
as a component have been reported which can be successfully
interpreted by applying the regular solution theory to the
systems. For example, Black et al. verified that the solubility
of water in various hydrocarbons can be explained semi-
empirically by the regular solution theory.60 Suzuki and his
coworkers61–65 studied the extraction behaviour of various
chelating ligands and their metal complexes in organic
solvent–water biphasic systems. They demonstrated that the
regular solution theory gives a good interpretation of the
dependence of the distribution coefficients of these compounds
on the nature of the organic solvents and the distribution
coefficients can be predicted by solubility parameters of the
compounds and the two phases.
The results shown in the preceding sections reveal that the
structure or the state of water on the hydrophobic surfaces is
different from that of the bulk water and organic molecules as
well as inorganic ions can recognize the difference. We have
thus attempted to evaluate the state of the interfacial water by
estimating the solubility parameter of the interfacial water
from the distribution coefficients of the probe compounds.
If the interfacial water on the hydrophobic surfaces
is assumed to be a homogeneous phase, the distribution
coefficient of a solute compound is given by the following
equation:59
ln DA ¼vA
RT½ðdA � dWÞ2 � ðdA � dIWÞ2�
þ vA1
vIW� 1
vW
� �ð22Þ
where R is the gas constant, T is the absolute temperature,
dA, dW and dIW are the solubility parameters of the solute
compound, the bulk water and the interfacial water, and vA,
vW and vIW are molar volumes of the solute compound,
the bulk water and the interfacial water, respectively. We
calculated the dIW values from the distribution coefficients
and the solubility parameters of the organic compounds used
as probes assuming that the molar volume of the interfacial
water is the same as that of the bulk water. The solubility
parameters of the organic compounds as well as the bulk water
were calculated from the values of the molar enthalpy of
vaporization, DHv, and the molar volume at 298 K.66 The
DHv values for some probe compounds, for which the values
at 298 K are not given, were estimated using the following
empirical equation:67
DHv;T2
DHv;T1
¼ Tc � T2
Tc � T1
� �0:38
ð23Þ
where Tc denotes the critical temperature.
The dIW values obtained are tabulated in Table 3. It should
be noted that all the dIW values calculated from the distribution
coefficients of various organic compounds are in good agree-
ment with one another. This also indicates that the retention
of these small hydrophilic organic molecules on the C18 silicas
can be attributed to the partition between the bulk water phase
and the interfacial water as the first approximation. The mean
value, 45.8 MPa1/2, corresponds to the solubility parameter of
the bulk water at 348 K. This suggests that the interfacial
Fig. 8 Dependence of wf, wfb and wn values on total water content for
Capcell Pak C18 UG300. Symbols: ’ = wf; m = wfb; K = wn.
Table 3 Estimated solubility parameter of the interfacial water on thesurface of Capcell Pak C18 UG300
Solute compound dIW/MPa1/2
Methanol 46.1Ethanol 45.81-Propanol 45.61-Butanol 45.3Acetonitrile 45.9Propionitrile 45.7Acetone 46.12-Butanone 45.9Nitromethane 45.8Nitroethane 45.61-Nitropropane 45.5
Av. � S.D. 45.8 � 0.3
Temperature: 298 K.
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water on the surface of the inner wall of a C18 silica has a
structure of disrupted hydrogen bonding, which appears to be
consistent with the view obtained by the DSC experiments.
The structure-ordering or iceberg model was first proposed
by Frank and Evans1 and the usual view of the hydrophobic
interaction is based on this model. However, the structure of
water on hydrophobic surfaces has recently been discussed in a
rather controversial manner. For example, Richmond et al.
demonstrated by their VSFG spectrometric study that hydro-
gen bonding between adjacent water molecules at organic/
water interfaces such as CCl4/H2O and n-hexane/H2O is weak
in contrast to generally accepted models of water on hydro-
phobic surfaces.6,14 On the other hand, Chandler et al. claimed
that hydrogen bonding of water persists around small apolar
solutes, whereas it is depleted near large apolar solutes or
assemblies.3 It has also been shown by molecular dynamics
simulation that the orientation of the interfacial water molecules
and the hydrogen bonding between these water molecules
strongly depend on the local curvature of the interface; at
the positions of the local concave curvature of the interface,
similar to the surface of small apolar solutes, water molecules
can maintain all the four possible hydrogen bonds, whereas
water molecules located at the positions where the interface is
of convex structure are oriented in such a way that they
sacrifice one of their potential hydrogen bonds.13 These results
suggest that the structure of water on hydrophobic surfaces
may depend not only on the chemical structure of the matrix,
which is in contact with water, but also on the physical
structure or local curvature of the interface. Since the inner
wall of the pores of the C18 bonded silicas should be of uneven
surface, the structure of water incorporated in the pores may
not be uniform.
The structure of the interfacial water may also depend on
the distance from the surface. Therefore the dIW value estimated
from the distribution coefficients of organic compounds is
considered to be the average of the values for the water layers
formed on the hydrophobic surfaces. The dIW values calculated
from the D values of the probes having longer alkyl chains are
slightly smaller than those obtained from the data of the
compounds with shorter chains. This may reveal that the
former molecules distribute more than the latter ones into
the interfacial water layers located closer to the alkyl bonded
layer. More hydrophobic compounds may partition into the
C18 bonded phase although it is very difficult to measure
accurately the retention volume of such a compound under
pure aqueous conditions due to very strong retention. However,
if the molecules deeply partition into the bonded phase and
come into contact with the alkyl chains, the dIW values should
be much less than those shown in Table 2 or be close to the
values for hydrocarbons, i.e.B14 MPa1/2. Therefore relatively
hydrophilic solute molecules used as probes in this study are
considered to be located mainly in the interfacial water layers
on the surface.
3.5 Estimation of the thickness of the interfacial water formed
on the surfaces of C18 bonded silicas
Now that the volumes of the interfacial water have been
obtained, the thickness of the water can be given by VL/As.
We calculated the thickness of the interfacial water for the four
C18 silicas used in this study from the surface areas listed in
Table 1 although these values were determined by the BET
method using nitrogen gas and might be more or less different
from actual surface areas of the particles exposed to water.
The calculated thicknesses of the interfacial water for Capcell
Pak C18 UG 80, 120, 300 and L-column2 ODS are 1.1, 1.4, 1.2
and 1.3 nm, respectively. These values correspond to the
thickness of water layers of 4–5 molecules. It should be noted,
however, that this calculation does not take into account the
hydration of probe molecules and ions. Therefore these values
may be regarded as the limit of distance from the hydrophobic
surface at which molecules and ions can sense the surface.
4 Conclusions
So far, the separation of inorganic ions with nanoporous
hydrophobic materials in aqueous media has been explained
in terms of a size-exclusion mechanism, while the accumulation
and/or separation of organic compounds at a hydrophobic
surface has been interpreted with direct adsorption of solute
molecules onto the surface or partition into the hydrophobic
moieties. In contrast to these explanations, we have presented
in this paper a new consistent view that the interfacial water
formed on the hydrophobic surface functions as a separation
medium.
The liquid chromatographic investigation of the effects of
the counter-ion and co-ion in the eluent on the retention of
various ions on a column packed with octadecyl-bonded silica
particles has clearly shown that the separation of ionic solutes
from pure aqueous solutions is not caused by size-exclusion
but results from partition between the bulk water phase and
the interfacial water on the hydrophobic surface, which has
different affinity for ionic solutes from that of the bulk water
and acts as the stationary phase. Comparison of the distribution
coefficients of organic compounds calculated with the equations
derived from different postulated models indicates that the
interfacial water formed on the hydrophobic surface also
functions as the separation medium for organic compounds
in aqueous media.
Based on the thermal phase transition behaviour of water
incorporated in the pores of octadecyl-bonded silica particles
obtained by DSC measurements, we have concluded that the
interfacial water does not crystallize even at 223 K and this
extreme depression of the freezing point may be caused by
partial disruption of water hydrogen bonding. Furthermore,
we attempted to calculate the solubility parameters of the
interfacial water, dIW, from the distribution coefficients of
n-alcohols, ketones, nitriles and nitroalkanes in order to
elucidate the state of the interfacial water in the vicinity of
the surface of octadecyl-bonded silicas functioning as the
separation medium. The dIW values obtained from all
the organic compounds agree well with one another, being
45.8 � 0.3 MPa1/2. This value corresponds to the solubility
parameter of the bulk water at 348 K, which also suggests that
the interfacial water may have a structure of disrupted hydrogen
bonding network.
The thickness of the interfacial water formed on the hydro-
phobic surface or the distance from the surface at which small
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molecules and ions can sense the surface was calculated from
the volume of the interfacial water determined by liquid
chromatography and the surface area of a C18 silica material.
The value obtained for the four C18 silica materials examined
in this study is 1.25 � 0.13 nm, corresponding to the thickness
of water layers of 4–5 molecules. Our findings suggest that the
hydrophobic effect may extend beyond the first solvation shell
of water molecules directly surrounding the surfaces.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research No. 20350034 from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, and a grant
from Saitama University.
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