Received: August 10, 2015 | Accepted: August 30, 2015 | Web Released: September 10, 2015
CL-150753
Collisional Reaction of Liquid Droplets: Amidation of Dansyl Chloride
Observed by Fluorescence Enhancement
Tomoko Suzuki and Jun-ya Kohno*
Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588
(E-mail: jun-ya.kohno@gakushuin.ac.jp)
The collisional reaction of droplets is observed by time-
resolved fluorescence spectroscopy. Colliding droplets of the
reactant solutions are irradiated with a pulsed laser, and the
resulting fluorescence spectra and images of the colliding
droplets are observed as a function of the elapsed time from
the collision. The amidation reaction of dansyl chloride with
isopropylamine is observed through fluorescence enhancement
on a microsecond time scale. The present method enables us to
measure the early stages of reactions in solutions.
N(CH3)2
N(CH3)2
(CH3)2CHNH2
SO2Cl
SO2NHCH(CH3)2
Figure 1. Amidation reaction of dansyl chloride.
The kinetics of chemical reactions between two species in
solution can only be investigated by mixing two solutions
because the two reactants must be separated before the reaction
to be able to set a reaction start time. Kinetic measurements can
then performed by observing the time evolution of the concen-
trations of the species of interest in the solution. Rapid mixing of
the two solutions is particularly important for kinetic measure-
ments of fast reactions to ensure that the solution is homoge-
neously mixed on a time scale shorter than that of the reaction.
The stopped-flow method is commonly used to observe the
reaction between two species in solution, whereby the two
reactant solutions are delivered to an observational cell through
a mixer, and the composition of the reactant and/or product
species are analyzed. Kinetic data are obtained by this method
because the reaction time corresponds to the distance from the
mixer to the analysis point. This method has been applied to
various reactions, such as the association of two species and
protein folding.1 However, the stopped-flow method typically
suffers from an initial dead time of ca. 1 ms, which is required
to obtain homogeneous mixing of the two reactant solutions.
Recent technical improvements have reduced the dead time to
tens of microseconds.2-5
Liquid droplets can also be employed for the measurement
of chemical reactions between two reactants in solution, such as
the reaction between two trapped reactant droplets in a liquid6
and a droplet falling on the surface of a reactant solution.7
Aerosol droplets have also been used to investigate chemical
reactions. Simpson et al. have observed chemical reactions
during the merging of two droplet streams of H2SO4 and NaOH
aqueous solutions by Raman spectroscopy.8-10 However, it is
likely that microscopic analysis of the droplets will yield more
detailed information on the reaction.
Most recently, we measured the coloring reaction of phenol-
phthalein induced by the collision of droplets of phenolphthalein
in ethanol and aqueous NaOH.13 Through these studies, the
developed laser spectroscopic technique has proved particularly
useful for the investigation of the collisional reaction of droplets.
In the present study, we applied this technique to observe
the amidation reaction of dansyl chloride (DNS-Cl) (Figure 1).
DNS-Cl reacts with primary amines, amino acids, and proteins
to produce dansyl amide (DNS-amide), which fluoresces more
intensely than the reactants. This fluorescence can be used for
the quantitative analysis of the reactant and product species.14-18
In the present research, we investigate the initial kinetics of the
amidation reaction induced by the collision of droplets of DNS-
Cl in ethanol solution (DNS-Cl/EtOH) and isopropylamine
(IPA) droplets. Kinetic measurement within 20 ¯s is achieved by
this method.
A detailed description of the droplet-collision apparatus has
been reported previously.11-13 The apparatus and the exper-
imental procedures employed in the present study are described
briefly as follows. The apparatus was constructed around a
microscope, which was used to observe droplets tens of
micrometers in size. Droplets were produced from reservoirs
of the sample solutions using a set of piezo-driven nozzles
(Microdrop, MD-K-130), which were triggered independently
by electric pulses supplied from a pulse generator. A white light-
emitting diode (LED) was used as a strobe light to aid the
imaging of droplet collision. The LED was mounted under the
collision region and thus illuminated the colliding droplets
from beneath. The objective lens of the microscope above the
collision region focused the light for imaging. The duration of
the LED pulse was set to 1 ¯s, which was the time resolution of
the image measurement. The pulse generator used to trigger
droplet generation was also synchronized with the LED pulses
with a variable delay. A series of droplet-collision images were
recorded by changing the LED timing with respect to droplet
generation. The images in the series show different droplets;
however, because of the sufficiently small variation between
the droplets generated, they show the collision dynamics of the
droplets. In the present study, the impact parameter was set close
to zero; thus, the colliding droplets had cylindrical symmetry
We have also investigated the collision dynamics of liquid
droplets. In our previous study, we observed the temporal
sequence of the collision of water and ethanol droplets strobo-
scopically, and observed the appearance of a characteristic
protrusion upon collision.11 We then observed the Raman spectra
of colliding droplets at various times after collision and
elucidated the mechanism of protrusion formation by deducing
the composition of the protrusion from the Raman spectrum.12
© 2015 The Chemical Society of Japan | 1575
Suzuki, Tomoko
