Reactive uptake of N2O5 on aqueous H 2so4 solutions coated with 1-component and 2-component monolayers

Article abstract of DOI:10.1021/jp8005469

Reactive uptake of N₂O₅ on aqueous sulfuric acid solutions was studied in the presence of 1-component (octadecanol) and 2-component (octadecanol + phytanic acid) monolayers. In the 1-component monolayer experiments, the reactive uptake coefficient depended strongly on the molecular surface area of the surfactant. Also, the 1-component monolayer showed significant resistance to mass transfer even when the fractional surface coverage of the surfactant was less than 1. For example, a monolayer of 1-octadecanol with a fractional surface coverage of 0.75 decreased the reactive uptake coefficient by a factor of 10. This is consistent with previous studies. In the 2-component monolayer experiments, the reactive uptake coefficient depended strongly on the composition of the monolayer. When the monolayer contained only straight-chain molecules (1-octadecanol), the reactive uptake coefficient decreased by a factor of 42 due to the presence of the monolayer. However, when the monolayer contained 0.20 mole fraction of a branched surfactant (phytanic acid) the reactive uptake coefficient only decreased by a factor of 2. Hence, a small amount of branched surfactant drastically changes the overall resistance to reactive uptake. Also, our results show that the overall resistance to reactive uptake of 2-component monolayers can be predicted reasonably accurately by a model that assumes the resistances to mass transfer can be combined in parallel.

Full text of DOI:10.1021/jp8005469

J. Phys. Chem. A 2008, 112, 4625–4635
4625
Reactive Uptake of N2O5 on Aqueous H2SO4 Solutions Coated with 1-Component and
2-Component Monolayers
L. M. Cosman and A. K. Bertram*
Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall, VancouVer,
British Columbia V6T 1Z1, Canada
ReceiVed: January 19, 2008; ReVised Manuscript ReceiVed: February 27, 2008
Reactive uptake of N2O5 on aqueous sulfuric acid solutions was studied in the presence of 1-component
(octadecanol) and 2-component (octadecanol + phytanic acid) monolayers. In the 1-component monolayer
experiments, the reactive uptake coefficient depended strongly on the molecular surface area of the surfactant.
Also, the 1-component monolayer showed significant resistance to mass transfer even when the fractional
surface coverage of the surfactant was less than 1. For example, a monolayer of 1-octadecanol with a fractional
surface coverage of 0.75 decreased the reactive uptake coefficient by a factor of 10. This is consistent with
previous studies. In the 2-component monolayer experiments, the reactive uptake coefficient depended strongly
on the composition of the monolayer. When the monolayer contained only straight-chain molecules
(1-octadecanol), the reactive uptake coefficient decreased by a factor of 42 due to the presence of the monolayer.
However, when the monolayer contained 0.20 mole fraction of a branched surfactant (phytanic acid) the
reactive uptake coefficient only decreased by a factor of 2. Hence, a small amount of branched surfactant
drastically changes the overall resistance to reactive uptake. Also, our results show that the overall resistance
to reactive uptake of 2-component monolayers can be predicted reasonably accurately by a model that assumes
the resistances to mass transfer can be combined in parallel.
Introduction
g10) when particles contain a large amount of organic material
in addition to inorganic material. One possible explanation for
Heterogeneous reactions between aerosol particles and gas
phase species can play a key role in the atmosphere.1–11 One
heterogeneous reaction that has been studied extensively is the
reaction between N2O5 and aqueous particles:
these results is that the organic material formed a coating on
the aqueous droplets, and this coating limited the transfer of
N2O5 molecules across the air-aqueous interface.52
Recently, several research groups have investigated the effect
of organic monolayers on reactive uptake of N2O5 on aqueous
solutions or particles.37,38,44–46,51 Park et al.45 studied N2O5 uptake
on H2SO4 solutions and showed that γ decreased by a factor of
2.5 in the presence of hexanol monolayers and decreased by a
factor of 1.5 in the presence of butanol monolayers. Thornton
and Abbatt44 investigated N2O5 uptake on sea salt aerosols and
determined that the uptake coefficient decreased by a factor of
3–4 in the presence of hexanoic acid monolayers. McNeill et
al.,38 studied uptake of N2O5 on aqueous salt aerosols, and they
observed a decrease in γ by a factor of 7.5 in the presence of
monolayers formed from sodium dodecyl sulfate37 and a factor
of 20 in the presence of monolayers formed from sodium
oleate.38 More recently, we measured the reactive uptake of
N2O5 on H2SO4 solutions coated with 1-octadecanol, 1-hexa-
decanol, and stearic acid and observed a decrease in the reactive
uptake coefficient by a factor of 61, 55, and 17, respectively.51
In contrast, we found that an insoluble, branched monolayer
containing 16 carbon atoms (phytanic acid) did not have an
effect on γ within our experimental error.51 The structures of
1-octadecanol and phytanic acid are shown in Table 1.
N2O5(g) + H2O(l) aerosol8 2 HNO3(aq)
(R1)
Modeling studies have demonstrated that the rate for this
reaction needs to be known accurately to predict tropospheric
concentrations of NOx, O3, and OH.2,6,12
Often the efficiency of N2O5 heterogeneous reactions is
described by the reactive uptake coefficient (γ), which is defined
as the fraction of collisions with a surface that leads to the
irreversible loss of the gas-phase species due to a reaction. Since
the initial modeling studies that demonstrated the atmospheric
importance of the N2O5 heterogeneous reaction, there has been
extensive laboratory investigations of the reactive uptake
coefficient of N2O5 on aqueous inorganic solutions and
particles.13–24 These studies have shown that N2O5 reactive
uptake is large (γ is between 0.02 and 0.15) on aqueous
inorganic particles.
In addition to inorganic material, tropospheric particles can
also contain a significant fraction of organic surfactants.25 These
surfactants can form organic monolayers at the air-aqueous
interface,26–36 and depending on the composition and degree of
compression of these organic monolayers, they may limit the
transfer of molecules across the air-aqueous interface.26–28,37–51
Possibly related, recent field measurements over the northeast
United States by Brown et al.52 showed that the reactive uptake
coefficient of N2O5 can decrease significantly (by a factor of
Related to the above, Folkers et al.41 investigated the uptake
of N2O5 on aqueous NH4HSO4 particles coated with organics
produced by the ozonolysis of R-pinene. In these studies, it was
observed that an organic film approximately 15 nm thick reduced
the reaction probability of N2O5 by approximately a factor of
5. Also, Badger et al.53 investigated N2O5 uptake on aqueous
ammonium sulfate aerosols containing humic acid (a surfactant).
For aerosols containing only 6% humic acid by dry mass, a
* To whom correspondence should be addressed.
10.1021/jp8005469 CCC: $40.75
2008 American Chemical Society
Published on Web 04/29/2008

4626 J. Phys. Chem. A, Vol. 112, No. 20, 2008
Cosman and Bertram
TABLE 1: Structure of Organic Molecules Used in This Studya
a Melting point for 1-octadecanol was obtained from Sigma-Aldrich. The melting point for phytanic acid has not been reported, but it is a
liquid at room temperature.
decrease in reactivity of more than a factor of 2 was observed
compared with the case for single-component ammonium
sulfate. Analysis of the uptake coefficients using the water
concentration data showed that the change in reactivity could
not be explained by the change in water content alone. The
authors attributed the larger than expected change in reactivity
to the formation of organic films by the humic acid surfactants.53
Despite the recent progress on the effect of organic mono-
layers on N2O5 reactive uptake, much remains to be done. For
example, in most of the previous studies only single-component
monolayers were investigated. More work on multicomponent
monolayers (monolayers containing more than one type of
organic surfactant) is needed. Also, much remains to be learned
to understand the mechanism by which monolayers and films
affect N2O5 uptake. Uptake studies as a function of molecular
surface area of the surfactant, the two-dimensional (2D) phase
of the organic monolayer, and temperature would be useful to
better understand the physical chemistry. This type of informa-
tion is also necessary to accurately extrapolate laboratory data
to atmospheric conditions.
chain surfactant) and phytanic acid (a branched surfactant). This
mixed monolayer is of atmospheric relevance because mono-
layers in the atmosphere may consist of more than one type of
surfactant. It is also reasonable to expect that some atmospheric
monolayers will consist of mixtures of straight-chain and
branched surfactants. Prior to our uptake measurements, we first
determine if 1-octadecanol-phytanic acid monolayers are
miscible or immiscible, which is necessary for understanding
our uptake data.57 To determine the miscibility, we measure
the surface pressure-area isotherms of the 2-component mono-
layer. The analysis of this data allows us to show that
1-octadecanol-phytanic acid monolayers are immiscible. After
proving this 2-component monolayer is immiscible, we study
the uptake coefficient of N2O5 on aqueous H2SO4 solutions in the
presence of these monolayers as a function of the composition
of the monolayer (i.e., mole fraction of phytanic acid in the
2-component monolayer). The results from these studies are
presented below and compared to predictions based on two
different models used to describe mass transfer across the air-
aqueous interface in the presence of a 2-component monolayer.
The atmospheric implications of these results are also discussed.
As mentioned above, we recently studied N2O5 reactive
uptake on aqueous sulfuric acid solutions coated with 1-octa-
decanol, 1-hexadecanol, stearic acid, and phytanic acid.51 In
these studies, we only investigated 1-component monolayers,
and for each organic monolayer we only measured the reactive
uptake at one surface pressure, which corresponds to one
molecular surface area of the surfactant (i.e., packing density
of the surfactant). Experiments as a function of molecular
surface area with the same surfactant were not carried out. The
current paper is an extension of our previous work and consists
of two series of measurements. In the first series of measure-
ments, we investigate the uptake of N2O5 on aqueous sulfuric
acid solutions coated with a 1-octadecanol monolayer (a
1-component monolayer) as a function of the molecular surface
area of the surfactant. This allows us to isolate the effect of
molecular surface area on the reactive uptake coefficient. In
addition, the results from this study allow us to investigate
whether or not the 2D phase of the surfactant monolayer has a
large impact on uptake. Monolayers can go through a series of
2D phase transitions (see below), and in the 1-octadecanol
experiments as a function of molecular surface area mentioned
above the monolayer undergoes a phase transition. This allows
us to observe how the 2D phase alters the reactive uptake
coefficient. Below, we present these results and also discuss
the results in terms of the Accessible Area Theory,54 which has
been used in the past to predict the effect of organic monolayers
on water evaporation.48,55,56
Experimental Section
Rectangular Channel Flow Reactor and Determination
of Reactive Uptake Coefficients. A rectangular channel flow
reactor46,51 coupled to a chemical ionization mass spectrometer51
is used to measure heterogeneous reaction rates for the uptake
of N2O5 on aqueous solutions coated with organic monolayers.
The strength of the rectangular channel flow reactor configu-
ration is that we can determine the surface pressure and
molecular surface area (i.e., packing density) of the organic
monolayer before and after each kinetic measurement.
Figure 1 is a schematic of the rectangular channel flow reactor
(RCFR). A detailed description of the flow reactor and flow
dynamics in the reactor can be found in our previous work.46,51
Our flow reactor builds on earlier experiments, where a
Langmuir trough was used for heterogeneous studies.58,59 Our
flow reactor is made entirely from aluminum and is temperature
controlled by circulating coolant through open channels in the
body of the reactor. All interior aluminum walls are coated with
halocarbon wax to minimize loss of N2O5 to the walls. A glass
trough is located on the bottom surface of the reactor, which is
filled with the aqueous solution. The reactive surface is 7.5 cm
in width and 22 cm in length. This aqueous solution can be
coated with an organic monolayer. The height of the headspace
above the liquid surface (Figure 1b) is in most cases less than
1 cm. As shown in Figure 1a, N2O5 enters the flow cell through
a stainless steel, T-shaped injector that slides just above the
In the second series of measurements, we focus on a
2-component monolayer consisting of 1-octadecanol (a straight-

H2SO4 Solutions Coated with 1- and 2-Component Monolayers
J. Phys. Chem. A, Vol. 112, No. 20, 2008 4627
Figure 1. (a) A top view sketch of the rectangular channel flow reactor without the aluminum cover. The liquid solution is placed in a quartz
trough, which fits smoothly inside the flow reactor. (b) A side view of the rectangular channel flow reactor coupled to a CIMS.
surface of the liquid. The T-shaped injector is equipped with
six exit holes (0.2 mm in diameter) facing the top of the reactor,
which distribute N2O5/He evenly along the width of the flow
cell. The carrier gas (He) enters the flow reactor through inlets
at the back of the flow cell. The gas stream within the reactor
first flows against a barrier to ensure mixing before reaching
the liquid surface. The temperature of the liquid and the gas
above the liquid are measured using thermocouples, and in all
cases the liquid and the gas are within ( 0.5 K of each other.
The pressure inside the flow cell is measured using an MKS
baratron at the exit of the flow reactor (see Figure 1b).
Prior to entering the flow reactor, the carrier gas was passed
through a carbon filter (Supelco, Supelcarb HC) and a Drierite
(W.A. Hammond Drierite Co. Ltd.) trap cooled with liquid
nitrogen to remove any possible organic contamination. The
carrier gas then passed over a water reservoir held at a fixed
temperature to adjust the relative humidity (RH) of the carrier
gas. The RH was adjusted so that it matched the relative
humidity over the aqueous 60 wt % sulfuric acid solution,
calculated using the AIM model.60–62 The RH of the carrier gas
was verified with a dew point hygrometer.
min-1 at STP of dry He prior to entering the flow reactor through
the T-shaped injector. Total mass flow rates inside the flow
reactor ranged from 260 to 360 cm3 min-1 at STP and total
pressures were between 2.6 to 3 Torr. Laminar flows were
achieved under these conditions as Reynold’s numbers ranged
from 0.4 to 0.8.
A chemical ionization mass spectrometer (CIMS) was con-
nected to the exit of the flow cell, which was used to measure
the change in the gas phase reactant concentration as a result
of reactive uptake at the liquid surface.46,51,63,64 N2O5 was
detected as NO3- after its chemical ionization by I-.15,21,65 Trace
amounts of CH3I diluted in 1250-1500 cm3 min-1 at STP of
N2 were passed through a polonium-210 source (NRD, model
Po-2031) to form I-. For N2O5 detection in the presence of H2O,
the chemical ionization region was biased to -122 V to
decluster weakly bound ion-H2O clusters.15,21 N2O5 concentra-
tions ranged from 2.1 × 1010 to 1.1 × 1011 molec cm-3 in these
experiments. N2O5 concentrations were based on the I- + N2O5
chemical ionization reaction rate that has been reported in the
literature.65
The open channel above the liquid surface has a rectangular
geometry. The flow dynamics of our system have been
characterized in our previous publication using computation fluid
dynamics simulations.46 These calculations show that the carrier
gas reaches a fully developed laminar flow in less than 1.5 cm,
which is much shorter than the length of the reactive surface.
See our previous publication for further discussion on the gas
flow dynamics in the system.46
The reactive uptake coefficients were determined from the
irreversible removal of N2O5 as a function of injector position
in the RCFR. Shown in Figure 2 are plots of the natural
logarithm of the N2O5 signal as a function of reaction time for
the loss of N2O5 on coated and uncoated aqueous 60 wt %
sulfuric acid solutions at 273 ( 1 K. The data for each uptake
experiment is represented with a linear fit, which yields the
observed first-order rate constant (kobs) from the slope of each
data set. The first order wall loss rate (kw) was calculated from
kobs using a procedure developed by Knopf et al.46 This
procedure corrects for any concentration gradients that can
develop in the flow reactor as a result of a fast heterogeneous
reaction at the surface.
N2O5 was produced by reacting a small flow of NO2 with an
excess of O3. O3 was generated by passing a flow of O2 over
an ultraviolet source (Jelight, model 600). To remove water
vapor from the O2 carrier gas, a Drierite trap was placed
immediately before the UV lamp. NO2 was passed through a
P2O5 trap to remove trace amounts of water prior to reaction
with O3. N2O5 produced by this reaction flowed through an
additional P2O5 trap to reduce the concentration of nitric acid
and was then collected and stored in a glass trap immersed in
an ethanol bath cooled to 193 K. N2O5 condensed as white
crystals inside the glass trap.
The determination of kw from kobs requires diffusion coef-
ficients for N2O5 in He (D
) and N2O5 in H2O (D
N2O5-
N2O5-He
H2O). The values used for DN O -He and DN O -H O at 273 K were
2
5
2
5
2
289 and 72 Torr cm2 s-1, respectively. These values were taken
from Knopf et al.46 and were calculated based on molecular
parameters.66–68 The diffusion coefficient (D) for N2O5 in a
binary mixture of helium and water is then given by69
A saturated flow of N2O5 between 7-8 cm3 min-1 at standard
temperature and pressure (STP) was mixed with 70–80 cm3

4628 J. Phys. Chem. A, Vol. 112, No. 20, 2008
Cosman and Bertram
P
PHe
H2O
D
N2O5-H2O
1
D
)
+
(1)
D
N2O5-He
where PHe and PH O are the partial pressures of helium and
2
water vapor in the flow cell, respectively.
The reactive uptake coefficient (γ) was determined from kw
using70,71
1
γ
c
4kw
S
V
1
2
)
·
+
(2)
where c is the mean molecular velocity of N2O5, S is the reactive
surface area inside the flow cell, and V is the volume above
this area.
Preparation of 1-Component Monolayers for the Reactive
Uptake Measurements. The first set of experiments involved
N2O5 uptake experiments on 1-component monolayers (specif-
ically 1-octadecanol) as a function of molecular surface area of
the surfactant (i.e., packing density of the surfactant). We
prepared monolayers with various molecular surface areas by
varying the amount of organic surfactant added to the surface.
Because the total area of the surface was known accurately and
the total amount of organic surfactant added to the surface was
known accurately, the molecular surface area of the surfactant
molecules could be accurately calculated. The molecular surface
area of the surfactant was also verified by measuring the surface
tension of the films before and after the reactive uptake
measurements (see below). More details on preparation of
the monolayers are as follows: first, the surface of the aqueous
acid was thoroughly cleaned using a PTFE nozzle aspirator to
remove any organic contamination on the surface prior to
preparing an organic monolayer. Second, solutions of 1-octa-
decanol were prepared by dissolving the surfactant in chloroform
(∼1 mg cm-3) to make organic solutions. Finally, monolayers
were prepared by depositing a known volume of the organic
solution on a clean aqueous sulfuric acid surface. The chloro-
form was allowed time to evaporate, leaving behind an organic
monolayer with a known molecular surface area of the
surfactant.
Figure 2. Natural logarithm of the observed N2O5 signal as a function
of reaction time. Symbols show representative data for a series of
1-octadecanol monolayers with varying molecular surface area. Solid
squares, a molecular surface area of 20.7 Å2 molec-1; open squares,
21.2 Å2 molec-1; open triangles, 22.6 Å2 molec-1; and solid triangles,
24.2 Å2 molec-1. The dashed lines represent the corresponding linear
fits to the data.
To verify the molecular surface area of the surfactants, we
measured the surface tension of the prepared films using the
Wilhelmy plate method prior to and after the uptake measure-
ments. From the surface tension, we calculated the surface
pressure, and then from the surface pressure we determined
the molecular surface area of the surfactant molecules from the
pressure-area isotherm of octadecanol on aqueous sulfuric acid
solutions. The pressure-area isotherm for 1-octadecanol, which
was determined in previous experiments51 using a commercial
Langmuir film balance, is illustrated in Figure 3a (solid curve).
The isotherm shows the relationship between the surface
pressure of the film and the molecular surface area of the
surfactant, so once the surface pressure is known, the molecular
surface area can be determined. The molecular surface areas
determined from the surface tension measurements were always
within a few percent of the molecular surface areas calculated
from knowledge of the mass of organic surfactant added to the
surface. Also shown in Figure 3a are the molecular surface areas
at which reactive uptake coefficients were determined in this
study (open symbols). The solid symbol represents the condi-
tions used in our previous study for 1-octadecanol monolayers.51
More details on the surface tension measurements are as
follows:51 surface tension measurements were carried out with
a platinum Wilhelmy plate (23.32 mm perimeter) connected to
a tensiometer (NIMA Technology, model PS4). The platinum
plate was cleaned using a 1:1 solution of H2SO4 and HNO3
Figure 3. Surface pressure-area isotherm for 1-octadecanol51 (panel
a) and reactive uptake coefficients (γ) for N2O5 on organic coated
aqueous 60 wt % sulfuric acid at 273 ( 1 K plotted as a function of
molecular surface area of 1-octadecanol (panel b). The solid line in
panel a represents the variation of surface pressure (π) with a change
in molecular surface area. S′, S, S*, and L2* represent the different 2D
phases of the monolayer (see text for further details).76 Also shown is
the collapse pressure (πc) of the 1-octadecanol monolayer. The open
symbols in panel a represent the experimental conditions (surface
pressures and molecular surface areas) at which reactive uptake
experiments were performed for N2O5 on aqueous 60 wt % H2SO4 at
273 ( 1 K coated with 1-octadecanol. The solid square in panel a
represents experimental conditions used previously by Cosman et al.51
for 1-octadecanol monolayers.
prior to each measurement, followed by thorough rinsing with
purified water. The Wilhelmy plate was hung from the tensi-
ometer and was immersed in the aqueous acid solution contained
in the temperature controlled flow cell. The force on the
Wilhelmy plate was measured while the plate was immersed
and detached from the liquid solution contained in the quartz

H2SO4 Solutions Coated with 1- and 2-Component Monolayers
J. Phys. Chem. A, Vol. 112, No. 20, 2008 4629
trough. The surface tension was calculated from the maximum
difference in the force on the plate between immersion and
separation from the solution.51,72,73 The surface tension in the
presence of the film was measured before and after each kinetic
experiment and used to calculate the surface pressure.
Measurements of Surface Pressure-Area (π-A) Isotherms
of the 2-Component Monolayers. The second set of experi-
ments involved N2O5 uptake experiments on a 2-component
monolayer (consisting of 1-octadecanol and phytanic acid) as
a function of mole fraction of phytanic acid in the monolayer
(xphytanic). First, in order to characterize these monolayers, which
was necessary for interpreting the uptake results, we measured
surface pressure-area isotherms57 as a function of mole fraction
of phytanic acid in the monolayer, using a commercial Langmuir
film balance (NIMA Technology, model 611). The main point
of these experiments was to determine if the 2-component
monolayer was immiscible or miscible.
the isotherm for 1-octadecanol on 60 wt % H2SO4 at 273 K,51
and the symbols represent the surface pressures and molecular
surface areas at which reactive uptake experiments were
performed. Figure 3a illustrates that as the molecular surface
area decreases, the monolayer undergoes a series of phase
transitions shown by a change in the slope of the isotherm.57,75
These phase transitions correspond to different degrees of
ordering of the organic surfactants on the surface.75,76 At large
molecular surface areas (greater than approximately 40 Å2
molec-1) the films exist as a 2D gas on the aqueous acid surface,
with molecules on the surface exerting relatively little force on
each other due to sufficient separation.57 For decreasing mo-
lecular surface areas the monolayers undergo several phase
transitions until they reach their collapse pressure, πc (which
occurs at approximately 19 Å2 molec-1 for 1-octadecanol).57
In Figure 3 the different phase regions (defined by discontinuities
in the slope of the isotherm) are labeled as S′, S, S*, and L2*,
based on previous studies of 1-octadecanol monolayers on
water.76 The different phases correspond to different orientations
of the organic chains and also different unit cells. Traditionally,
S′ and S would be classified as a condensed solid phase and S*
and L2* would be classified as a liquid condensed phase.57 In
our experiments, we studied N2O5 uptake in the presence of S*
and L2* phases. The main difference between these two phases
is a change in the tilt angle of the hydrocarbon chains in the
monolayer.76
Figure 3b shows the reactive uptake coefficients (γ) in the
presence of 1-octadecanol monolayers determined in our studies
using different molecular surface areas. The error bars for γ
take into account reproducibility in the kinetic experiments and
a 20% uncertainty in the diffusion coefficients.
Figure 3 shows that the reactive uptake coefficient depends
strongly on the molecular surface area of the organic monolayer
with the reactive uptake coefficient decreasing as the molecular
surface area was decreased. This observation is consistent with
previous studies of the rate of water evaporation through organic
monolayers.56,57,77 In these previous studies, it was generally
observed that the resistance to evaporation increased as the
molecular surface area of the surfactant decreased.
Figure 3 also suggests that the reactive uptake coefficient does
not change drastically when the phase of the monolayer converts
from S* to L2*. The γ values shown in Figure 3b increase
smoothly with increasing molecular surface area of the surfactant
with no apparent step function change observed upon transition
between monolayer phases S* and L2*. This is also generally
consistent with studies of the rate of water evaporation through
organic monolayers. For example, La Mer et al.77 showed that
the resistance to water evaporation increased smoothly (i.e., no
apparent step function change in the evaporation resistance was
observed) as monolayers underwent phase transitions from
condensed liquid to solid phases. Our results are also not
surprising as the surface pressure changes relatively smoothly
between S* to L2*, and the structural differences between S*
to L2* are thought to be relatively minor. Because of experi-
mental constraints, we were not able to compress the monolayer
to molecular surface areas beyond that at equilibrium. Hence,
we were not able to monitor the change in reactive uptake
coefficient when going from the phase S* to S, where the surface
pressure changes most steeply.
The Langmuir film balance consisted of a PTFE trough (with
dimension of 20 by 30 cm), two movable barriers, and a surface
pressure sensor (NIMA Technology, model PS4) with a
platinum plate. The experimental procedure that was followed
is described in detail by Myrick and Franses.74 Briefly, the
trough was filled with an aqueous sulfuric acid solution, and
the surface of the acid solution was cleaned thoroughly using
an aspirator. A known volume of an organic solution (containing
1-octadecanol and phytanic acid dissolved in chloroform) was
added to the clean H2SO4-H2O surface. The chloroform was
allowed to evaporate leaving behind a known number of
surfactant molecules on the surface. The surface pressure was
then recorded as the moveable barriers reduced the available
surface area, resulting in a surface pressure-area isotherm.57
Surface pressure-area isotherms for 2-component monolayers
were measured at a constant compression rate of 20 cm2 min-1
.
Preparation of the 2-Component Monolayers for the
Reactive Uptake Measurements. After characterizing the
1-octadecanol-phytanic acid monolayers, we then investigated
the reactive uptake of N2O5 on aqueous 60 wt % H2SO4
solutions in the presence of these monolayers. Measurements
were carried out as a function of the mole fraction of phytanic
acid in the monolayer. For these experiments, the surface
pressure of the films was kept 21 ( 2 mN m-1, which is less
than the first collapse pressure of the 2-component monolayer
(see below for more details). Monolayers with a surface pressure
of 21 ( 2 mN m-1 were prepared by dissolving a known amount
of 1-octadecanol and phytanic acid (with a fixed ratio) in
chloroform. Drops of this solution were then added to a clean
aqueous sulfuric acid surface until the desired surface pressure
was reached, which was verified by measuring the surface
tension of the solution. The surface tension was measured with
the procedure discussed above.
Chemicals. Helium (Praxair, 99.999% Purity), nitrogen
(Praxair, 99.999%), nitrogen dioxide (Matheson, 99.5%), oxygen
(Praxair, 99.5%), diphosphorus pentoxide (Aldrich, 97%),
1-octadecanol (Sigma-Aldrich, 99%), phytanic acid (Sigma-
Aldrich, 96%), chloroform (Fischer, 99+%), and sulfuric acid
(Fischer, 95+%) were all used as supplied.
Results and Discussion
N2O5 Reactive Uptake in the Presence of a 1-Component
Monolayer (1-Octadecanol). The reactive uptake of N2O5 on
aqueous H2SO4 was measured in the presence of a monolayer
of 1-octadecanol at varying molecular surface areas of the
surfactant. The surface pressure-area isotherm for 1-octadecanol
is shown in Figure 3a.51 The solid line in Figure 3a illustrates
In Figure 3, we have plotted γ versus the molecular surface
area of the organic monolayer. Another way to present the
reactivity data is versus the fractional surface coverage. Here,
we define the fractional surface coverage as the molecular
surface area of the surfactant normalized to the molecular surface

4630 J. Phys. Chem. A, Vol. 112, No. 20, 2008
Cosman and Bertram
Figure 5. Reactive uptake coefficients for N2O5 on aqueous sulfuric
acid in the presence of organic monolayers as a function of molecular
surface area. Open squares, 1-octadecanol (this study); solid right facing
triangles, phytanic acid (this study); solid squares, 1-octadecanol
(Cosman et al.);51 solid triangles, stearic acid (Cosman et al.);51 solid
circles, phytanic acid (Cosman et al.);51 solid left facing triangles,
1-hexadecanol (Cosman et al.);51 solid diamonds, butanol (Park et al.);45
solid stars, hexanol (Park et al.);45 and solid inverted triangles,
1-octadecanol (Knopf et al.).46 In panel a, the ordinate is the reactive
uptake coefficient in the presence of the organic film (γfilm), and in
panel b the ordinate is γfilm normalized to reactive uptake coefficient
for the uncoated solution (γuncoated).
Figure 4. Reactive uptake coefficients for N2O5 on organic coated
aqueous 60 wt % sulfuric acid at 273 ( 1 K plotted as a function of
the fractional monolayer surface coverage. In this figure, the reactive
uptake coefficient in the presence of the organic monolayer (γfilm) is
normalized to reactive uptake coefficient for the uncoated solution
(γuncoated) to illustrate the change in reactive uptake coefficient due to
the presence of the monolayer. Open squares, 1-octadecanol (this study);
solid square, 1-octandecanol (Cosman et al.).51 Panel a gives the ordinate
in a linear scale whereas panel b shows the ordinate in a log scale.
Our current data for 1-octadecanol is represented with open
symbols, and all the other data (obtained with soluble and
insoluble monolayers) is represented by solid symbols. First,
our current data generally fits well the trend observed with all
the previous data.45,46,51 Second, considering all the data, there
appears to be a strong correlation between the reactive uptake
coefficient and the molecular surface area. This trend was
observed in our previous publication,51 and our current data adds
more support to this conclusion.
The fact that the data in Figure 5 correlates well with the
surface area is broadly consistent with the Accessible Area
Theory.54 The Accessible Area Theory has previously been used
to predict the effect of organic monolayers on the evaporation
rate of water. This theory captures well the general trends in
evaporation of water in the presence of several long-chain
alcohols. However, the agreement between theory and experi-
ments is not quantitative.48,56 The Accessible Area Theory
suggests that transport occurs only through open sections of
the surface. Such sections may be formed through random
fluctuations or by incomplete packing of the film. One limitation
of this theory is that it does not consider the effect of chain
length on the mass transport of gases.54
area of the surfactant at the collapse pressure, πc. Shown in
Figure 4 is the reactive uptake coefficient in the presence of
the organic monolayer (γfilm) normalized by the uptake coef-
ficient for the uncoated case (γuncoated) as a function of this
fractional surface coverage. Figure 4 shows that the fractional
surface coverage does not have to be 1 to significantly decrease
the reactive uptake coefficient. Even at 0.75 of the maximum
surface coverage (i.e., fractional surface coverage ) 0.75), the
monolayer still decreases the reactive uptake coefficient by a
factor of 10. Park et al.45 and McNeill et al.,38 also performed
a similar analysis to the above (although they defined the
fractional surface coverage slightly differently). Our conclusion
mentioned above is similar to the conclusions reached by these
authors previously. For example, McNeill et al.37 observed a
significant decrease in γ (approximately a factor of 3 decrease)
even when the fractional surface coverage was 0.08 for a NaCl
aerosol coated with sodium dodecyl sulfate monolayers.
In Figure 5, we compare our 1-octadecanol results with
previous measurements of N2O5 reactive uptake measured for
aqueous sulfuric acid solutions coated with 1-component
monolayers. For this comparison we use the molecular surface
area on the x-axis, since in our previous work we have shown
that the uptake results for coated aqueous sulfuric acid solutions
correlate better with the molecular surface area of the surfactant
compared to other parameters such as surface pressure.51 Panel
a displays γfilm and panel b displays the ratio of γfilm to the
reactive uptake coefficient of the uncoated solution (γuncoated).
Note that Figure 5 should not be considered as a rigorous
test of the Accessible Area Theory because the figure largely
separates the short chain molecules (right hand side in Figure
5) from the long chain molecules (left hand side in Figure 5
with the exception of phytanic acid). More work is needed to
fully test the Accessible Area Theory. Studies of the reactive

H2SO4 Solutions Coated with 1- and 2-Component Monolayers
J. Phys. Chem. A, Vol. 112, No. 20, 2008 4631
Figure 6. Surface pressure-area isotherms for organic monolayers on aqueous 60 wt % sulfuric acid at 273 ( 1 K. Panel a: the isotherm for
1-octadecanol51 (xphytanic ) 0) represented by the solid line and phytanic acid51 (xphytanic ) 1) shown as the dashed line. πc,
and πc,
octadecanol
phytanic
represent the surface pressure at which monolayers of pure 1-octadecanol and pure phytanic acid collapse, respectively. Panel b: the isotherm for
2-component monolayers of 1-octadecanol and phytanic acid. The solid line, dashed line, and bold solid line represent the isotherms for compositions
of xphytanic ) 0.05, 0.2, and 0.7, respectively. Isotherms for xphytanic ) 0.1 and 0.4 have been omitted for clarity.
uptake of N2O5 in the presence of long-chain molecules with
large molecular surface areas would be useful. From Figure 5,
we conclude that the trend in reactive uptake coefficient of N2O5
with molecular surface area is broadly consistent with the
Accessible Area Theory (i.e., a correlation between the reactive
uptake coefficient and the molecular surface area is observed).
More work is needed to determine if the theory is quantitative.
Also, note that in Figure 5 we have only plotted data measured
with aqueous sulfuric acid solutions. As we discussed in our
previous manuscript,51 data measured with a sea salt or NaCl
subphase are not consistent with the trend shown in Figure 5.
More work is needed to understand this apparent conflict.
Surface Pressure-Area Isotherms of 2-Component Mono-
layers and the Miscibility of These Monolayers. A 2-com-
ponent monolayer may be miscible or immiscible. An immis-
cible monolayer can be thought of as made up of two separate
monolayers, whereby one component forms patches of a
monolayer distributed within a monolayer of the second
component.57 The main point of this section is to determine if
the 2-component system we studied (1-octadecanol-phytanic
acid monolayer) is immiscible.
Figure 7. The collapse pressures of 1-octadecanol-phytanic acid
monolayers on aqueous 60 wt % sulfuric acid at 273 ( 1 K as a function
of composition (xphytanic). Solid triangles and solid squares represent
the first collapse pressure (π1c,mixture) and second collapse pressure
(π2c,mixture) for the 2-component monolayers, respectively. The open
triangle and open square represent the collapse pressure of pure phytanic
acid and pure 1-octadecanol monolayers, respectively.
Shown in Figure 6 are the pressure-area isotherms for the
1-octadecanol-phytanic acid monolayers (with various xphytanic
values) as well as the isotherms for the pure components (i.e.,
pure 1-octadecanol and phytanic acid). The π-A isotherm for
1-octadecanol (see Figure 6a) shows several “kinks” in the
isotherm due to 2D phase transitions, as discussed above.57,75,76
As the molecular surface area decreases, the monolayer
undergoes several phase transitions until it reaches its collapse
pressure (labeled in Figure 6 as πc,octadecanol),51 and a further
compression of the film results in the formation of a new bulk
phase on the surface.57
Panel b of Figure 6 illustrates the π-A isotherms for the
2-component monolayers. The isotherms for the mixtures appear
to be intermediate between the pure component isotherms and
also exhibit two discontinuities that we assign as collapse
pressures of the monolayers, labeled π1
and π2
.
c,mixture
c,mixture
The π-A isotherms for xphytanic ) 0.1 and 0.4 have been omitted
for clarity.
One useful tool for establishing the miscibility of a 2-com-
ponent monolayer is the interfacial phase rule.57,78,79 According
to this phase rule, when a monolayer is immiscible it will exhibit
two separate collapse events that correspond to the collapse
pressures of the individual pure components, independent of
monolayer composition.57 If, however, the two components of
the monolayer at the interface are miscible, the surface pressure
at which the monolayer collapses is dependent on the monolayer
composition.57 Figure 7 shows the collapse pressure for the
2-component monolayers of 1-octadecanol and phytanic acid
as a function of monolayer composition (i.e., xphytanic), as well
as the collapse pressures for the single component monolayers.
The solid symbols represent the collapse pressures for the
The behavior for the branched surfactant, phytanic acid, is
significantly different than for the straight-chain surfactant,
1-octadecanol (see Figure 6a). Between 45 Å2 molec-1 and
approximately 80 Å2 molec-1, the branched-chain monolayer
is in a liquid expanded state.51,57 At a surface pressure of 26
mN m-1 the monolayer collapses (labeled πc,phytanic in Figure
6a), and further compression of the film results in the formation
of liquid lenses in equilibrium with a monolayer at a molecular
surface area of 44.5 Å2 molec-1 51
.

4632 J. Phys. Chem. A, Vol. 112, No. 20, 2008
Cosman and Bertram
2-component monolayer and the open symbols represent the
collapse pressures for the 1-component systems. Figure 7 shows
that the 1-octadecanol-phytanic acid monolayers exhibit two
collapse pressures that are consistent with the collapse pressures
of the pure component systems and are roughly independent of
composition, as one would expect for an immiscible monolayer.
The scatter in π2c,mixture as a function of xphytanic is not surprising.
π2c,mixture corresponds to the collapse pressure of 1-octadecanol
in the 2-component monolayer. In this case, the 1-octadecanol
is in a 2D solid phase just before collapse. It is well known
that the collapse pressure of a 2D solid phase can vary
significantly from experiment to experiment and is sensitive to
trace levels of impurities in the monolayer because the collapse
of a 2D solid phase is a nucleation event with a large kinetic
barrier.57 Also the scatter observed in Figure 7 is typical for
immiscible monolayers.80
Another useful tool for investigating the miscibility of a
monolayer is the average molecular surface area in the
2-component monolayer and the excess area in the 2-component
monolayer. If the 2-component monolayer is completely im-
miscible, then the average molecular surface area of the
surfactant molecules in this monolayer should be related to
the molecular surface area in the 1-component monolayers at
the same surface pressure, according to the following equa-
tion:57
A12,predicted ) A1x1 + A2x2
(3)
Figure 8. Panel a shows the measured molecular surface area (solid
symbols) and the predicted molecular surface area (dashed line) for
2-component monolayers with π ) 21 ( 2 mN m-1 as a function of
composition (xphytanic). Panel b displays the excess area (Aex) as a function
of composition for 1-octadecanol-phytanic acid monolayers. See text
for further details.
where A12,predicted is the average molecular surface area predicted
for an immiscible 2-component monolayer. A1 and A2 are the
molecular surface areas in the 1-component monolayers 1 and
2, respectively, and x1 and x2 are their respective mole fractions.
For eq 3 to apply, A12,predicted, A1, and A2 need to be evaluated
at the same surface pressure. In Figure 8a, we plot A12,predicted
as well as the measured average molecular surface area in the
2-component monolayer, A12,measured, evaluated at a surface
pressure of 21 mN m-1. A12,measured was determined from
the surface pressure-area isotherms shown in Figure 6b. In
Figure 8a, symbols represent A12,measured and the line represents
A12,predicted. The good agreement between the two gives more
support for an immiscible monolayer. To further explore this
point, we also calculated the excess area, Aex, as a function of
monolayer composition (evaluated at 21 mN m-1) for the
2-component monolayer. The excess area is a measure of
nonideality of the monolayer, and is given by80,81
Aex ) A12,measured - A12,predicted
(4)
where A12,measured and A12,predicted are as defined above. 2-Com-
ponent monolayers that are completely immiscible have Aex)
0.80 Positive or negative values for the excess area indicate
nonideal mixing in the 2-component systems.81 Figure 8b shows
the excess areas as calculated using eq 4. The values of Aex do
not appear to vary in any systematic way from zero. In fact,
the error bars for all data points overlap zero except for one.
A final way we explored the miscibility of the monolayer
was by comparing our measured pressure-area isotherms with
predictions. If the monolayers are completely immiscible, then
the isotherms of the 2-component monolayer should be a linear
combination of the 1-component monolayers, scaled to the mole
fractions of the single components. In Figure 9, we have shown
two of our measured isotherms, along with predictions (shown
as dashed lines) assuming the monolayers were immiscible. The
good agreement gives further support that our monolayers are
immiscible.
Figure 9. Surface pressure-area isotherms for binary mixtures of
1-octadecanol and phytanic acid performed on aqueous 60 wt % sulfuric
acid at 273 ( 1 K. Examples of experimentally measured isotherms
for 2-component monolayers containing xphytanic ) 0.1 (bold solid line)
and xphytanic ) 0.7 (solid line) are shown. The dashed lines are the
predicted isotherms based on a linear combination of the isotherms for
pure 1-octadecanol51 and pure phytanic acid.51
The above evidence gives strong support that the 1-octa-
decanol-phytanic acid monolayers are completely immiscible
and hence one can think of the monolayers as forming patches
of phytanic acid distributed within a monolayer of 1-octadecanol
(or vice versa). At the first collapse pressure (π1c,mixture), phytanic
acid is squeezed out resulting in a monolayer that contains only
1-octadecanol until the second collapse pressure (π2c,mixture) at
which the 1-octadecanol collapses as well. Because at the first
collapse pressure all the phytanic acid is squeezed out of the
monolayer, we performed all our uptake measurements using a

H2SO4 Solutions Coated with 1- and 2-Component Monolayers
J. Phys. Chem. A, Vol. 112, No. 20, 2008 4633
TABLE 2: Measured Reactive Uptake Coefficients for N2O5
on 60 Wt % Aqueous Sulfuric Acid Solutions at 273 ( 1 K
Coated with 1-Octadecanol-Phytanic Acid Monolayers at 21
( 2 mN m-1a
molecular surface
area (Å2/molec )
lower
limit
upper
limit
xphytanic π (mN m-1
)
γ
0
21.1
19.7
21.7
20.1
21.4
21.9
21.5
21.3
22.1
23.5
26.3
32.2
38.1
47.2
0.00116 0.00091 0.00141
0.00579 0.00405 0.00752
0.0109 0.00761 0.0141
0.0217 0.0174 0.0261
0.0352 0.0246 0.0457
0.0625 0.0410 0.0841
0.0663 0.0439 0.0894
0.05
0.1
0.2
0.4
0.7
1
a The average molecular surface areas for each 2-component
monolayer (determined from the pressure-area isotherms) are also
included.
surface pressure of 21 ( 2 mN m-1. This ensures that we still
had two components in the monolayer.
N2O5 Reactive Uptake in the Presence of a 2-Component
Monolayer. The reactive uptake of N2O5 on aqueous 60 wt %
H2SO4 solutions at 273 K was measured in the presence of
1-octadecanol-phytanic acid monolayers. 2-Component mono-
layers with a mole fraction of phytanic acid (xphytanic) equal to
0, 0.05, 0.1, 0.2, 0.4, 0.7, and 1 were studied. Reactive uptake
coefficients and average molecular surface areas for each binary
mixture studied are reported in Table 2. The upper and lower
limits for γ in Table 2 take into account 20% error in the
diffusion coefficients.
The reactive uptake coefficients determined in these studies
are also shown in Figure 10. This figure shows that γ increases
as xphytanic increases. This was expected as the uptake coefficient
on a H2SO4-H2O solution coated with a phytanic acid mono-
layer (at a surface pressure of 21 ( 2 mN m-1) is much larger
than for the same solution coated with a 1-octadecanol mono-
layer (at a surface pressure of 21 ( 2 mN m-1). As discussed
in our previous publication,51 the reason phytanic acid has a
smaller effect on the reactive uptake coefficient is because of
the branched structure of the molecule, which prevents the
surfactant from packing densely on the surface (or achieving a
small molecular surface area).
Figure 10. The reactive uptake coefficient for N2O5 on aqueous sulfuric
acid in the presence of 1-octadecanol-phytanic acid monolayers (γmixed
film) as a function of mole fraction of phytanic acid in the monolayer,
xphytanic. Solid squares represent the average γ value for each corre-
sponding monolayer. The error bars represent 2σ. The bold dashed line
represents the prediction based on eq 5 whereas the bold dash dot line
represents the prediction based on eq 6.57 The shaded regions reflect
the uncertainty in the predictions based on the uncertainty in γ1 (xphytanic
) 0) and γ2 (xphytanic ) 1). In panel a, the ordinate is the reactive uptake
coefficient in the presence of the organic film (γmixed film), and in panel
b the ordinate is γmixed film normalized to reactive uptake coefficient for
the uncoated solution (γuncoated).
Two different models have been suggested to explain the mass
transfer of species across a 2-component organic monolayer.
Gaines57 suggests that for 2-component immiscible monolayers
the resistance to mass transfer of patches of unmixed monolayers
might be expected to combine as resistances in parallel.
This model can be expressed with the following equation:57
eq 6 for our experimental conditions. The shaded regions in
Figure 10 reflect the uncertainty in the model predictions based
on the uncertainty in γ1 and γ2. Our data is consistent with the
model for reactive uptake in the presence of an immiscible
monolayer presented by Gaines57 (eq 5). However, our data does
not agree well with the model prediction for a miscible
monolayer (eq 6). Our data is the first case where the results
for an immiscible monolayer are consistent with eq 5. More on
this is included below.
Related to this discussion, several researchers have investi-
gated the evaporation of water coated with 2-component organic
monolayers.55,77,84–90 Researchers have generally found that the
resistance to water evaporation due to an ideal, miscible
monolayer can be explained with eq 6.77,90 For immiscible
monolayers, none of the water evaporation studies could
be explained by eq 5 (or eq 6).48,56,85,87,89 For example, several
studies have investigated the evaporation resistance of 1-octa-
decanol-cholesterol monolayers (which are expected to be
immiscible) and have shown resistances that are appreciably
lower than those predicted by eq 5.85,89 For example, deviations
from model predictions by up to a factor of 2.2 were observed
by Barnes et al.85 Possible explanations for why evaporation
resistance measurements through immiscible monolayers are not
γ12 ) x1γ1 + x2γ2
(5)
where x1 and x2 are the mole fractions of component one and
component two in the 2-component monolayer, and γ12, γ1, and
γ2 represent the reactive uptake coefficients for the 2-component
and 1-component monolayers, respectively (all evaluated at the
same surface pressure).
In contrast, Barnes and LaMer82 used the Energy Barrier
Theory82–84 to predict the resistance to mass transfer through
an ideal, miscible monolayer. In terms of reactive uptake
coefficients, this model can be expressed with the following
equation:82
1
γ12
1
1
ln
) x1 ln
+ x2 ln
(6)
(
)
( )
( )
γ1
γ2
The dashed lines in Figure 10 are the result of plotting eq 5,
whereas the dash-dot lines in Figure 10 are the result of plotting

4634 J. Phys. Chem. A, Vol. 112, No. 20, 2008
Cosman and Bertram
consistent with eq 5 include partial miscibility or mass transfer
occurring mainly at grain boundaries.48
reactive uptake. These measurements showed that when the
monolayer contained 100% straight-chain molecules, the de-
crease in the reactive uptake coefficient was approximately a
factor of 42 due to the presence of the monolayer. However,
our results showed that when the mole fraction of branched
surfactant was only 0.20 the decrease in the reactive uptake
coefficient was only a factor of 2 (down from 42). Hence, a
small amount of branched surfactant drastically changes the
overall resistance to reactive uptake on aqueous sulfuric acid
solutions. This highlights the importance of understanding the
composition of mixed organic monolayers in acidic atmospheric
particles. Also, our results showed that the overall resistance to
reactive uptake of immiscible monolayers can be predicted
reasonably accurately using eq 5, which assumes the resistances
to mass transfer can be combined in parallel. This equation may
be useful for making predictions of reactive uptake of aqueous
particles coated with multicomponent monolayers in the
atmosphere.
The difference in structure between cholesterol and octade-
canol seems to be greater than the difference between octade-
canol and phytanic acid. This may also be related to why eq 5
fits our octadecanol-phytanic acid data and not the previous
octadecanol-cholesterol data. Note that the reactive uptake
coefficient of N2O5 on aqueous sulfuric acid solutions coated
with 1-octadecanol-phytanic acid monolayers may also deviate
slightly from the predictions based on eq 5, but our work shows
that this deviation is less than the uncertainty in our measure-
ments as illustrated by Figure 10.
In addition to water evaporation studies, Gilman and Vaida42
studied the uptake of acetic acid in the presence of mixed
monolayers consisting of 1-triacontanol and cis-9-octadecen-
1-ol, which are expected to form immiscible monolayers. These
authors studied equimolar mixtures and observed that the mixed
films had permabilities that were between that of the single-
component films that comprise the mixture, which is consistent
with our findings. However, the applicability of eqs 5 and 6 to
their results were not studied.
Because the transfer of molecules across 2-component
monolayers is an important step toward a complete understand-
ing of the mechanism of mass transfer across the air-aqueous
interface in the presence of monolayers, more work in this
direction is required. Further experiments that would be
beneficial could include studies of N2O5 uptake on the same
2-component immiscible monolayers studied in the water
evaporation experiments where the results deviated significantly
from eq 5.
Acknowledgment. This work was funded by the Natural
Science and Engineering Research Council of Canada, NSERC,
the Canada Foundation for Innovation, CFI, and the Canada
Research Chair Program. The authors thank Daniel Knopf for
his help during the initial stages of this project.
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The first part of this manuscript focused on the reactive uptake
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