Real-time kinetics of surfactant molecule transfer between emulsion particles probed by in situ second harmonic generation spectroscopy

Article abstract of DOI:10.1021/ja2104608

Emulsions are widely used in industrial and environmental remediation applications. The breaking and reformulation of emulsions, which occur during their use, lead to changes in their surface composition as well as their physical and chemical properties. Hence, a fundamental understanding of the transfer of surfactant molecules between emulsion particles is required for optimization of their applications. However, such an understanding remains elusive because of the lack of in situ and real-time surface-specific techniques. To address this, we designed and synthesized the surfactant probe molecules MG-butyl-1 (2) and MG-octyl-1 (3), which contain an n-butyl and an n-octyl chain, respectively, and a charged headgroup similar to that in malachite green (MG, 1). MG is known to be effective in generating second harmonic generation (SHG) signals when adsorbed onto surfaces of colloidal microparticles. Making use of the coherent nature of SHG, we monitored in real-time the transfer of 2 and 3 between oil-in-water emulsion particles with diameters of ～220 nm. We found that 3 is transferred ～600 times slower than 2, suggesting that an increase in the hydrophobic chain length decreases the transfer rate. Our results show that SHG combined with molecular design and synthesis of surfactant probe molecules can be used to measure the rate of surfactant transfer between emulsion particles. This method provides an experimental framework for examining the factors controlling the kinetics of surfactant transfer between emulsion particles, which cannot be readily investigated in situ and in real-time using conventional methods.

Full text of DOI:10.1021/ja2104608

Article
pubs.acs.org/JACS
Real-Time Kinetics of Surfactant Molecule Transfer between
Emulsion Particles Probed by in Situ Second Harmonic Generation
Spectroscopy
YuMeng You,† Aaron Bloomfield,† Jian Liu, Li Fu, Seth B. Herzon,* and Elsa C. Y. Yan*
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States
S
* Supporting Information
ABSTRACT: Emulsions are widely used in industrial and environmental remediation applications. The breaking and
reformulation of emulsions, which occur during their use, lead to changes in their surface composition as well as their physical
and chemical properties. Hence, a fundamental understanding of the transfer of surfactant molecules between emulsion particles
is required for optimization of their applications. However, such an understanding remains elusive because of the lack of in situ
and real-time surface-specific techniques. To address this, we designed and synthesized the surfactant probe molecules MG-butyl-
1 (2) and MG-octyl-1 (3), which contain an n-butyl and an n-octyl chain, respectively, and a charged headgroup similar to that in
malachite green (MG, 1). MG is known to be effective in generating second harmonic generation (SHG) signals when adsorbed
onto surfaces of colloidal microparticles. Making use of the coherent nature of SHG, we monitored in real-time the transfer of 2
and 3 between oil-in-water emulsion particles with diameters of ∼220 nm. We found that 3 is transferred ∼600 times slower than
2, suggesting that an increase in the hydrophobic chain length decreases the transfer rate. Our results show that SHG combined
with molecular design and synthesis of surfactant probe molecules can be used to measure the rate of surfactant transfer between
emulsion particles. This method provides an experimental framework for examining the factors controlling the kinetics of
surfactant transfer between emulsion particles, which cannot be readily investigated in situ and in real-time using conventional
methods.
mulsions are mixtures of two immiscible liquids,
commonly oil droplets in water or water droplets in oil.
Scheme 1. Transfer of Surfactant Molecules from a Donor
Emulsion Droplet to an Acceptor Emulsion Droplet
E
They are often stabilized at their interfaces by surfactants.
Emulsions are widely used in industrial and environmental
remediation applications, including oil recovery and trans-
port,1,2 emulsion polymerization,3,4 drug delivery,5 food
processing, production of cosmetics, water purification,6,7 and
cleaning up oil spills.8 During these processes, the emulsion
droplets are broken and reformed constantly, and the surfactant
molecules undergo continuous reorganization and are rapidly
transferred between emulsion droplets. Because of the large
surface-to-volume ratio of emulsion droplets, such variations
lead to significant changes in the physical and chemical
properties of emulsions.9 Consequently, an understanding of
the fundamental factors governing the kinetics of surfactant
molecule transfer between emulsion particles is critical to the
development of surfactant molecules and emulsion systems.
Probing the kinetics of surfactant molecule transfer between
emulsion particles (Scheme 1) in situ and in real time has been
challenging. This is primarily due to the lack of surface-sensitive
techniques that can isolate the signal of interfacial molecules
from the signal of molecules in the bulk solution. To date, no
real-time in situ kinetic study of surfactant molecule transfer
between colloidal emulsion particles has been reported. The
mass transport of emulsion particles to dispersed phases has
been studied by NMR and fluorescence microscopy,10 and
surfactant molecule transport from bulk phases to the surfaces
of millimeter-sized emulsion droplets has been probed by real-
time measurements of surface tension.11 However, these studies
do not directly probe transfer of surfactant molecules between
colloidal emulsion particles.
In this work, we approached this problem using second
harmonic generation (SHG) spectroscopy combined with
molecular design and synthesis. The surfactant probe (SP)
molecules MG-butyl-1 (2) and MG-octyl-1 (3) (Scheme 2)
were synthesized in two steps by amination12 of di-(p-
bromophenyl)phenylmethane followed by hydride abstraction
[see the Supporting Information (SI)]. These surfactants
contain an n-butyl and an n-octyl chain, respectively, and a
common cationic headgroup similar to that found in malachite
green (MG, 1). MG itself is known to be effective in generating
SHG signals from the surfaces of colloidal microparticles using
fundamental light at 800 nm. Making use of the coherent
Received: November 7, 2011
Published: February 17, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Article
I
SHG also scales linearly with the particle density (ρ), so ISHG ∝
Scheme 2. Structures of Surfactant Probe Molecules
ρN2.
To illustrate how SHG can be used to study surfactant
transfer between emulsion particles, we consider the following
experiment (Scheme 3). At time t < 0, the sample contains a
Scheme 3. SHG Kinetic Measurement of Surfactant
Molecule Transfer between Emulsion Particles: Time
Dependence of ISHG upon Addition of APs to DPs at t = 0
nature of SHG, we measured the rates of transfer of 2 and 3
between oil-in-water emulsion droplets.
SHG, which is similar to sum-frequency generation (SFG), is
a second-order surface-specific process (see the SI).13 Both
SHG and SFG have been applied to the study of oil−water
interfaces.14 The surface sensitivity originates from the selection
rule that second-order optical processes are forbidden in
centrosymmetric media but allowed in non-centrosymmetric
media under the dipole approximation.13 In bulk media,
molecules are randomly oriented and centrosymmetry is
preserved; thus, no SHG signal is generated. In contrast,
because of the asymmetric forces across interfaces, molecules at
interfaces are aligned, thereby breaking the centrosymmetry.
Thus, the second-order polarization, P(2), induced at interfaces
can add up coherently and generate SHG signals:
mixture of emulsion and SPs. If the SP has a UV−vis
absorption in resonance with the SH frequency, it provides a
strong SHG signal. The SHG intensity can be expressed as ρN2,
where ρ is the particle density and N is the surface population
of SPs on the donor emulsion particles (DPs). At t = 0, an
equal volume of plain emulsion particles, termed acceptor
particles (APs), at the same particle density is injected into the
DP sample. Although the overall particle density stays the same,
the density of DPs is halved. Hence, ISHG becomes ρN2/2. For t
> 0, SPs start to be transferred from DPs to APs, and ISHG
decays until a new equilibrium is established. When the SP
concentration is low, one can assume that all SPs adsorb onto
the particle surface during the transfer process, and SPs in the
bulk solution can be neglected. Consequently, the SP molecules
are equally distributed onto APs and DPs at equilibrium.
Because the APs and DPs are now indistinguishable, the
particle density is again equal to ρ, but the surface population is
reduced to N/2. Hence, ISHG becomes ρN2/4. Therefore, when
the SP concentration is low and the particle separation is large,
one can use SHG to observe the transfer of SPs directly.
To analyze the time-dependent ISHG, we can consider the
following simple model:
(2)
(2)
E
∝ P ∝ χ E E
(1)
2ω
ω ω
where Eω is the incident field, E2ω is the second-harmonic (SH)
field, and χ(2) is the second-order susceptibility, which is related
to the microscopic second-order polarizability α(2) by
(2)
(2)
= N⟨α
χ
⟩
(2)
where N is the surface population and the brackets indicate an
average over all molecular orientations. α(2) is related to the
properties of the interfacial molecules. The SHG signal is
enhanced when the fundamental (ω) or SH frequency (2ω)
coincides with an electronic transition of the molecule.
Both SHG and SFG have been extended to the study of
colloidal surfaces.15 Although a colloidal particle is centrosym-
metric, if the size of the particles is in the micrometer or
submicrometer range, the second-order optical field generated
at the particle surface can add up coherently and give a
signal.15e Hence, the coherent SHG signal observed from
particle surfaces is different from incoherent hyper-Rayleigh
scattering, which is due to fluctuations in the molecular
orientation and density in isotropic bulk solutions.13e,15f More
discussion about the coherent addition of the SH field
generated from microparticle surfaces can be found in the SI,
while quantitative descriptions of SH scattering from particle
surfaces have been derived in excellent theoretical studies.16 In
terms of experiments, SHG and SFG have already been used to
study various colloidal systems, including silica particles,15a−c
polymer particles,15d−j carbon black particles,15k droplets,15l
emulsions,15f,m,n clay particles,15o and liposomes,15p−r to obtain
surface populations, adsorption free energies, surface potentials,
and transport properties.
Our method for probing surfactant molecule transfer
between emulsion particles uses the coherent nature of the
SHG signal.17 Since ISHG = |E2ω|2, ISHG is directly proportional
to the square of the surface population (N2) if surfactant
molecules are distributed evenly on the particle surfaces. When
the average separation between the particles is much longer
than the coherence length, the SH electric fields (E2ω)
generated from individual particles add incoherently. Thus,
k
1
DP·SP XooooY AP·SP
k
−1
where DP·SP and AP·SP represent SPs adsorbed on DPs and
APs, respectively. Since DPs and APs are emulsion particles of
the same kind, k1 and k−1 are equal (i.e., k1 = k−1 = k). If the SP
concentration is low, meaning that nearly all of the SPs are
adsorbed onto DPs or APs and the bulk concentration of SPs
can be ignored, the transfer rate can be written as
dN
AP
dt
= k(N − N ) = k(N − 2N
)
DP AP AP
0
(3)
where NDP and NAP are the numbers of SPs per DP and AP,
respectively, and N0 is the total number of SPs per DP for time
t ≤ 0. Solving eq 3 gives
1
2
−2kt
N (1 − e )
N
=
AP
0
which allows ISHG(t) to be written as
ρ
2
ρ
2
ρ
4
2
2
2
−4kt
I
(t) =
N
+
N
=
N
0
(1 + e )
SHG
AP
DP
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The normalized ISHG(t) then becomes
diameter of 226 32 nm. From this we calculated the particle
density used in the SHG experiments as 1.3 × 1010 cm−3, giving
an average particle separation of ∼4 μm.
I
(t)
(t < 0)
1
4
SHG
−4kt
(1 + e
=
)
(4)
I
SHG
To obtain the kinetic data as shown in Scheme 3, 2 or 3 was
first dissolved in water at pH 4, as adjusted by HCl(aq), and
diluted with the emulsion sample in the phosphate buffer to a
SP concentration of 2.3 μM (pH 6.2 0.1) to yield DPs. Next,
a solution of plain emulsion particles was diluted to the same
particle density (1.3 × 1010 cm−3) in the buffer to give APs. The
SHG measurements were carried out using an 800 nm
fundamental beam. The 400 nm SHG signal was detected at
90° with respect to the incident beam (see the SI). The SHG
signal of DPs in a quartz cuvette was monitored for ∼10 min to
ensure adsorption equilibrium. Figure 2 shows that for t < 0,
the DPs generated a stable SHG signal, which was normalized
to 1. At t = 0, an equal volume of APs at the same particle
density eas rapidly injected into the DP solution. The signal
immediately dropped to ∼0.5. Next, for t > 0, ISHG decayed
because of the transfer of SPs from DPs to APs. As t → ∞, ISHG
reached ∼0.25, where a new equilibrium was established and
the SP molecules were equally distributed onto DPs and APs.
Figure 2 a−c shows different signal-to-noise levels due to
different integration times per data point: 5.0 s for MG-octyl-1,
0.5 s for MG-butyl-1, and 0.2 s for MG, which were adjusted
because of the duration of the measurements.
Equation 4 can be used to fit the decay of ISHG(t) as shown in
Scheme 3 to obtain the rate constant, k.
To perform the kinetic measurements, we had to synthesize
the SP molecules. The surfactant probes MG-butyl-1 (2) and
MG-octyl-1 (3), were prepared by Buchwald−Hartwig
coupling12 of 4,4′-dibromotriphenylmethane with equimolar
dimethylamine/methylbutylamine and dimethylamine/methyl-
octylamine mixtures, respectively. Statistical mixtures of
products were formed and then separated by flash-column
chromatography. 4-Dimethylamino-4′-methylbutylaminotriphe-
nylmethane and 4-dimethylamino-4′-methyloctylaminotriphe-
nylmethane were oxidized by treatment with ceric ammonium
nitrate and HCl to afford 2 and 3, respectively. Also obtained
by this sequence were dibutylated MG-butyl-2 (4) and
dioctylated MG-octyl-2 (5). In this study, we only used the
products containing one alkyl chain. They were characterized
by standard spectroscopic techniques (see the SI). Figure 1
shows that the UV−vis spectra of 1−3 were similar, consistent
with the presence of similar cationic head groups.
The ISHG decays as shown in Figure 2 were fitted to eq 4 to
obtain the transfer rate constants k. Each measurement was
repeated at least three times to yield the average k values (7.3
0.2) × 10−5 s−1 for MG-octyl-1, (5.2 1.6) × 10−2 s−1 for MG-
butyl-1, and 0.24 0.03 s−1 for MG. Decreasing the alkyl chain
length from eight to four carbons led to an increase in the rate
by a factor of ∼600. Further shortening the chain from butyl to
methyl increased the rate by a factor of ∼5. The results suggest
that for longer alkyl chains, the transfer rate is lower. Because
we carried out all of the SHG measurements under the same
conditions, the different rates are due to the structural
variations in the alkyl chain. Thus, our results reveal that the
hydrophobic interactions between the surfactant’s alkyl chains
and the oil phase play an important role in the rate-determining
step of the transfer process.
Figure 1. (a) UV−vis absorption spectra of synthesized 1−3 and
commercially available 1, normalized to the absorption at ∼620 nm.
(b) UV−vis absorption spectra of DPs containing 3 (red) and APs
(black). The particle density was 1.3 × 1010 cm−3, and the
concentration of 3 was 2.3 μM.
To ensure that the emulsion system was stable upon addition
of SPs and during the SHG experiments, we monitored the
UV−vis spectra of the DPs and APs. Figure 1b shows UV−vis
spectra of DPs bearing MG-octyl-1 and plain APs. The AP
spectrum is due to Rayleigh scattering. The DP spectrum shows
the UV−vis adsorption of SPs on top of the scattering
background, suggesting that the emulsion was stable upon
addition of MG-octyl-1. The DP and AP spectra were the same
We prepared an oil-in-water emulsion by ultrasonication of a
1:9 (v/v) mixture of 5 mM 1-dodecanol/n-tetradecane solution
and 5 mM sodium dodecyl sulfate/water solution, as described
in the SI. The pH of both the DP and AP solutions was
maintained by a buffer (1 mM phosphate, pH 6.2). Dynamic
light scattering (DLS) (532 nm, ALV-5000, Langen) was used
to measure the size of the emulsion, yielding an average
Figure 2. Time-dependent SHG intensities for (a) MG-octyl-1, (b) MG-butyl-1, and (c) MG. The data points are shown as dots, and the red lines
are fits to eq 4. Each measurement was repeated at least three times, and the average decay times are shown as (3.2 0.4) × 103 s for MG-octyl-1,
5.2 1.6 s for MG-butyl-1, and 1.1 0.1 s for MG.
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before and after the SHG measurements, indicating that the
emulsions were stable during the SHG experiments. Similar
results for MG-butyl-1 and MG are given in the SI.
particles, or desorption−adsorption of detergent molecules.
The SHG method can be readily applied to measure the
activation energy of the transfer process.18 The approach will
also allow investigations of various factors controlling the
kinetics, yielding useful information for optimizing composi-
tions and conditions of emulsion systems for better perform-
ance in their industrial and environmental remediation
applications.
When we derived the equations to analyze the kinetic data,
we assumed the concentration of free SP in the bulk solution to
be negligible relative to the surface population. We verified this
assumption by measuring the adsorption isotherm (see the SI).
The total SP concentration used in the kinetic measurements
was 2.3 μM for t < 0, which became 1.2 μM for t > 0. The
adsorption isotherms (see the SI) confirmed that 2.3 and 1.2
μM were below the adsorption saturation. From the adsorption
isotherm, we estimate that <1% of the total SP was free in
solution, validating our assumption (see the SI). Indeed, Figure
2 shows excellent agreement between the measured and
theoretical ISHG values (1, 0.5, and 0.25 for t < 0, t = 0, and t →
∞, respectively), further verifying the assumption.
The synthetic method used in this study is versatile and can
be used to create a wide range of SPs with varying properties.
The method couples the MG headgroup to one or two alkyl
chains via the Buchwald−Hartwig reaction.12 Hence, it is
possible to use variety of secondary amines that can contain
linear, branched, or cyclic aliphatic or aromatic hydrocarbon
chains and are commercially available or accessible by reported
synthetic procedures. Access to structurally diverse SP
molecules is expected to be useful for studying the effect of
carbon chains on surfactant transfer between colloidal emulsion
particles, yielding practical information for optimizing emulsion
systems and developing new detergent molecules.
Because of a lack of techniques, the transfer kinetics of
surfactant molecules in emulsion systems has remained largely
unexplored. The methodology developed here allows surfactant
molecule transfer between emulsion particles to be probed in
situ and in real time. This is expected to be useful for studying
mechanisms of surfactant transfer between emulsion droplets
and investigating various factors controlling the kinetics.
Besides the effect of the alkyl chain, the method could also
probe the effects of the physical and chemical environments
(e.g, temperature, pH, electrolyte concentration, and surface
potential) on the transfer rate.
Although our synthetic method is limited to detergent
molecules with the MG headgroup, the coherent nature of the
SHG signal could be used to probe detergent molecule transfer
between colloidal emulsion particles in other molecular
systems. In fact, the fundamental beam is not limited to the
output from a Ti:Sapphire laser (∼800 nm), and other light
sources (e.g., in the visible region) could also be used. Hence,
the SHG method can be applied to other detergent molecules,
as long as the headgroups have UV−vis absorptions in
resonance with the fundamental or SH wavelength.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization data, adsorption
isotherms, and UV−vis spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
seth.herzon@yale.edu; elsa.yan@yale.edu
■
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. M. Elimelech and Dr. Z. Meng (Yale
University) for their help with the DLS measurements. This
work was supported by ACS PRF (49557-DNI5 to E.C.Y.Y.)
and the Yale University Setup Fund (to S.B.H.). J.L. was the
recipient of the Anderson Postdoctoral Fellowship.
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