Indian J. Phys. 84 (6), 625-629 (2010)
© 2010 IACS
Investigations of RhB18 langmuir monolayer by
fluorescence imaging microscopy
S A Hussain, S Chakraborty and D Bhattacharjee*
Department of Physics, Tripura University, Suryamaninagar-799130, Tripura, India
Imaging Microscopic studies of the formation of domain structure in the mixed Langmuir monolayer of RhB and
Stearic acid (SA) at the air-water interface. Strong repulsive interaction between the unlike components leads to
the phase separation and formation of microcrystalline domains at the air water interface of the Langmuir
monolayer. These domain can be directly visualized using fluorescence imaging microscope.
: This communication reports the surface pressure vs area per molecule isotherm and Fluorescence
Keywords: Isotherms, Langmuir films, Fluorescence Imaging Microscopy,
PACS Nos.: 81.16.Dn, 68.47.Pe
Investigation of Langmuir monolayer on the air-water interface has been proved to be a
relevant and fruitful activity for researchers in diverse fields of science [1-3]. Langmuir
monolayer presents an opportunity to study the dependence of two-dimensional phase
behaviour on molecular interactions and packing [4-7.]. The complexity of the phase and
domain behaviours, both static and dynamic, makes these monolayers exhibit many
examples of two dimensional condensed matter physics phenomena. A wide variety of
molecules can be studied as monolayers, including “simple” single alkyl chain surfactants,
phospholipids, organometallics, polymers and proteins.
The design and fabrication of molecular electronic devices using Langmuir-Blodgett
technology encourages the seeking of a better characterization of the initial monolayer on
the water surface.
Conventional techniques for studying Langmuir monolayer include surface pressure vs
area per molecule isotherm study, surface potential and surface viscosity measurement
S A Hussain, S Chakraborty and D Bhattacharjee
and more recently developed methods such as in-situ FTIR spectroscopic studies as well
as for domain structure visualization, in-situ Fluorescence Imaging Microscope (FIM) and
Brewster Angle Microscope (BAM). [8,9].
When transferred onto solid supports to form mono- and multilayered Langmuir-Blodgett
Films, they have their potential use in the construction of molecular electronic devices.
Most of the envisioned applications require incorporation of organic molecules having
interesting chromophores. Studies on non-linear optical properties ferroelectricity, pyroelectric
properties, photochromic effect are going on using LB films [10-14].
In this communication we have investigated the mixing behaviour of a laser dye Octadecyl
Rhodamine B (RhB) and stearic acid in the mixed Langmuir monolayer on the water
surface. Surface Pressure vs area per molecule isotherm and fluorescence imaging
microscopy (FIM) studies were used to characterize the monolayer films.
Octadecyl Rhodamin B (RhB, 99%,Molecular Probe) and SA (purity>99%) purchased from
Sigma Chemical Company were used as received. Spectroscopy grade chloroform (SRL,
India) was used as solvent and purity of the solvent was checked by fluorescence
spectroscopy before use. Langmuir Blodgett (LB) film deposition instrument (Apex-2000C,
India) was used for the study of isotherm characteristic as well also for the fabrication of
Langmuir monolayer on the water surface of the LB trough by changing various parameters
for fluorescence imaging microscopic study.
Phase contrast Fluorescence Imaging Microscope (FIM) (Model: Motic AE31) is attached
with the LB instrument for in-situ study of the monolayer images.
For the study of isotherm characteristics as well as also for the fabrication of Langmuir
monolayer, triple distilled deionised millipore water was used as subphase and the
temperature was maintained at 240C with pH of the subphase at 6.5 in equilibrium with
atmospheric CO2. Solutions of octadecyl Rhodamine B (RhB), SA and RhB-SA mixture
prepared in chloroform solvent and were spread on the water surface.
After a delay of 15 minutes to evaporate the solvent, the film at the air-water interface
was compressed slowly at the rate of 2x10-3nm2mol-1s-1 to study the surface pressure vs
area per molecule isotherm. FIM images of the monolayers of pure components
(RhB and SA) were taken at 25 mN/m surface pressure. FIM images of mixed monolayer
were taken at various surface pressures starting from 0 mN/m upto 25 mN/m.;
3. Results and discussions
3.1. Surface pressure vs area per molecule Isotherm :
Figure 1 shows the surface pressure vs area per molecule isotherms of pure SA, pure
RhB and RhB-SA mixed film having 0.1 mole fractions of RhB. The isotherm of pure SA
is consistent with the works mentioned in the literature . The area per molecules are
0.23 and 0.21 for pure SA at the surface pressures 15 and 25 mN/m and collapse pressures
Investigations of RhB18 langm uir m onolayer by fluorescence im agine m icroscopy
at about 55mN/m. Isotherm of pure RhB shows an infliction point of about 32 mN/m with
collapse pressure at about 40 mN/m. This infliction point indicates the rearranging and
orientation of RhB molecules at the air-water interface. Isotherm of mixed monolayer of
RhB–SA at 0.1 molefraction of RhB lies in between the isotherm of pure SA and RhB. It
has a collapse pressure at 30 mN/m. This low value of collapse pressure is an indication
of strong repulsive interaction between the unlike components which may result in the
formation of aggregate of like components resulting in the formation of distinct domain of
RhB and SA.
Figure 1. Surface pressure area per molecule isotherms of SA, RhB and RhB-SA (0.1 : 0.9 molar ratio)
mixture. Inset: molecular structure of RhB.
3.2 In-situ Fluorescence Imaging Microscopic (FIM) study of Langmuir monolayer :
Figure 2a and 2b show the FIM images of pure SA monolayer and Pure RhB monolayer
at the air water interface at a surface pressure of 25 mN/m. Due to absence of any
fluorescence, SA monolayer gives a dark image which is uniform throughout the film,
Figure 2. Fluorescence microscopic images of (a) SA and (b) RhB monolayers respectively at the air-
water interface. The scale bar in the lower left represents 10 μm.
S A Hussain, S Chakraborty and D Bhattacharjee
whereas RhB monolayer fluorescence image gives a bright crimson red illumination. This
illumination is uniform throughout the film indicating formation of uniform monolayer of
When mixed monolayer is formed on the air-water interface then due to strong repulsive
interactions between unlike components phase separation occurs leading to the formation
of aggregates and domain structures of micrometer range. Figures 3a to 3e show the
fluorescence microscopic images of mixed Langmuir monolayer taken at various surface
pressures starting from 0 mN/m up to 25 mN/m. It is interesting to note that in the mixed
film, phase separation is started even at 0 mN/m surface pressure (Figure 3a). At 10 mN/
m surface pressure (Figure 3b) distinct circular shaped black patches are observed having
bordered with crimson red colour. The circular black patches are due to SA where as
crimson red coloured bordering areas are due to RhB. At higher surface pressures of 15
mN/m upto 25 mN/m the contrast become opposite. i.e., distinct crimson coloured circular
patches with lightly illuminated background is observed. This brightly illuminated circular
patches are due to the formation of distinct domains of RhB microcrystalline aggregates.
The dimension of these domains ranges typically from about 5 μm to 25 μm. The lightly
illuminated background clearly indicates that RhB domains are pushed up on the SA film
and covers the SA monolayer.
Figure 3. Fluorescence Microscopic Images of Langmuir monolayer of RhB mixed with SA on water
subphase at molar ratio of RhB:SA=0.1:0.9 at different pressures viz (a) 0 mN/m, (b) 5 mN/m, (c) 15 mN/
m, (d) 20 mN/m and (e) 25 mN/m. The scale bar in the lower left represents 10 μm
In conclusion our results show that due to strong repulsive interactions between the unlike
components in the mixed monolayers of RhB-SA, phase separation occurs. This leads to
(a) (b) (c)
Investigations of RhB18 langm uir m onolayer by fluorescence im agine m icroscopy Download full-text
the formation of distinct circular shaped domain structures of RhB micro-crystals. FIM
images at high surface pressure, the dimension of the domain ranges between 5 μm to
The authors are grateful to DST, Govt. of India for providing financial support through the
project no. DST Project No: SR/S2/LOP-19/07.
A Ulman. An Introduction to Ultrathin Organic Films: From Langmuir Blodgett to Self Assemblies
(Academic Press, New York) (1991)
 G LGaines Insoluble Monolayer at Liquid – Gas Interface (John Wiley & Sons, New York) (1966)
M C Petty Langmuir Blodgett Film: An Introduction (Cambridge University Press) (1996)
K J Stine Microscopy Research and Technique 27 439 (1994)
J Y J Unag, D P Parazak, H Y Chiu and K J Stine J. coll. Int. Sci. 171 366 (1995)
M M Lipp, K Y C Lee, A Waring and J A Zasadzinski Biophysical Journal 72 2783 (1997)
K Y C Lee, M M Lipp, D. Y. Takamoto, E Ter-Ovanesyan and J A Zasadzinski Langmuir 14 2567
R K Gupta and K A Suresh Euro. Phys. J. E 14 35 (2004)
 Md N Islam, D Bhattacharjee and S A Hussain Surface Review and Lett. 15 287 (2008)
J W Baur, M F Durstock, B E Taylor, J R Spray, S Reulbach and L Y Chiang Synth. Met. 121 1547
V L Colvim, M C Schlamp and A P Alvistos Nature 307 354 (1994)
M Ferreira, O Onitsuja, A C Fou, B Hsieh and M F Rubner Mater. Res. Symp. Proc. 413 49 (1996)
H Hong, D Davidov, Y Avny, H Chayet, E Z Farggi and R Neumann Adv. Mater. 7 846 (1995)
C Brands, P J Neyman, M Guzy, S Shah, H Wang, H W Gibson, K VanCott R M Davis, C Figura and
J R Heflin Polym. Mater. Sci. Eng. 83 219 (2000)
S Deb, S Biswas, S A Hussain and D Bhattacharjee Chem. Phys. Lett. 405 323 (2005)