Full Terms & Conditions of access and use can be found athttp://www.tandfonline.com/action/journalInformation?journalCode=lsfm20

Soft Materials

ISSN: 1539-445X (Print) 1539-4468 (Online) Journal homepage: http://www.tandfonline.com/loi/lsfm20

Phase behavior of poly diacetylene mixed with axanthene dye at air–water interface and onto solidsupport

Sudip Suklabaidya, Sekhar Chakraborty, Bapi Dey, D. Bhattacharjee & SyedArshad Hussain

To cite this article: Sudip Suklabaidya, Sekhar Chakraborty, Bapi Dey, D. Bhattacharjee & SyedArshad Hussain (2018): Phase behavior of poly diacetylene mixed with a xanthene dye at air–waterinterface and onto solid support, Soft Materials, DOI: 10.1080/1539445X.2018.1548358

To link to this article: https://doi.org/10.1080/1539445X.2018.1548358

View supplementary material

Published online: 30 Nov 2018.

Submit your article to this journal

Article views: 32

View Crossmark data

Phase behavior of poly diacetylene mixed with a xanthene dye at air–waterinterface and onto solid supportSudip Suklabaidya, Sekhar Chakraborty, Bapi Dey, D. Bhattacharjee, and Syed Arshad Hussain

Thin film and Nanoscience Laboratory, Department of Physics, Tripura University, Agartala, Tripura, India

ABSTRACTPolydiacetylene (PDA) and its derivatives exhibit interesting photophysical properties, specifically,visible colorimetric transformation which makes them promising candidates for applications invast number of fields. PDA is obtained through photopolymerization of diacetylene monomers(DA) using UV-irradiation. UV-irradiation leads to the formation of meta stable nonfluorescent bluephase (blue polymer) which on further irradiation converted to stable auto fluorescent red phase(red polymer) with alternating triple and double bonds in ene-yen motif. Herein diacetylenemonomers 10, 12-tricosadiynoic acid (TCDA) and amphiphilic octadecyl rhodamine B chloride(RhB18) has been used to prepare mixed Langmuir film. The presence of RhB18 at different molarratio produces monolayer supramolecular structure as well as typical trilayer structure. Atomicforce microscopy investigations gave visual evidence of the formation of trilayer. The mixed filmcontaining TCDA mole fraction � 0.8 produces monolayer and TCDA mole fraction � 0.85produces typical trilayer. Photopolymerization is possible only in the trilayer. Thus the phases ofPDA can be tuned depending on the molar ratio of TCDA in the mixed film. This study demon-strates that the incorporation of dye RhB18 with diacetylene monomer TCDA provides a means formodulating the structure and chromatic features of PDA assemblies, giving rise to a newmorphologies and unique optical properties. This may extend the field of application of PDAassemblies for sensing.

ARTICLE HISTORYReceived 23 July 2018Accepted 31 October 2018

KEYWORDSPDA; photopolymerization;Langmuir–Blodgett film;UV-irradiation; BAM

Introduction

In recent years, researchers have shown their intereston substances such as conjugated polymers that spurimmense interest in the field of both fundamental andapplied science. Among a wide variety of conjugatedpolymers, including polythiophenes, polyanilines, poly-pyrroles, and polyphenylene, as well as poly (phenyleneethynylenes), polyacetylenes, polydiacetylenes (PDAs):PDAs have attracted considerable interest in the field ofchemical and biological sensors (1–4). PDAs are one ofthe most impressive quasi-1D π-conjugated polymers,first prepared by Wegner in 1969 (5). PDA has twodistinct phases blue and red phase depending on themicroenvironment and preparation condition (6–8).Accordingly, they have attracted great attention indesigning colorimetric and fluorescence sensors(9–13). In 1993, Charych et al. first explored theirpotential use in sensing applications (14). PDAs canbe prepared from various kind of monomeric assem-blies such as bulk crystals, self-assembled films single

crystal, as nanocomposites components incorporatedinto inorganic host matrices (15) and nano- structuressuch as vesicles, tubes, and ribbons (15–18). Generally,diacetylene (DA) amphiphilic monomers form self-assembled liposome in aqueous solutions; howeverthey can also be spread at the air–water interface toform the Langmuir monolayer and hence Langmuir–Blodgett (LB) films (19–22). Here it is possible to poly-merize the DA layer through both in-situ (at air–waterinterface in LB trough) and ex-situ (after transferringthe floating DA layer to solid support) upon applicationof external stimuli such as UV radiation (7), heat (23–31), mechanical stress (32–34), organic solvent (35–37),electric current (38, 39), and binding of biologicalagents (40–44). Accordingly, LB technique is one ofthe best techniques for yielding well controlled, highlyorganized ultrathin PDA films. Recently it has beenobserved that DA is investigated using LB techniqueby mixing with organic dyes in order to manipulate/modify the phase behavior of PDA in the mixed films(45–50). It has already been reported that this control

CONTACT Syed Arshad Hussain sa_h153@hotmail.com; sahussain@tripurauniv.in Thin film and Nanoscience Laboratory, Department of Physics,Tripura University, Suryamaninagar 799022, Tripura, IndiaColor versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsfm.

Supplementary data for this article can be accessed here.

SOFT MATERIALShttps://doi.org/10.1080/1539445X.2018.1548358

© 2018 Taylor and Francis Group, LLC

on lateral organization can be achieved by connectingthe polar head groups, either by hydrogen bonding viaelectrostatic interaction or through the self-aggregationof dyes (50–52). Gonzalez-Delgado et al. control theorganizations by the self-aggregation of the dyes in themixed films containing dimyristoyl-phosphatidic acidand an amphiphilic hemicyanine dye (SP) in 1:1 molarratio and also mixed monolayers of SP and stearic acidin a 1:1 molar ratio (53, 54).

However, literature survey revealed that there exist onlyfew reports where mixing behavior of different diacetylenederivatives with other dyes have been investigated by LBtechnique (48, 55). Therefore, we felt that investigations onthe mixing behavior of diacetylene derivative mixed withorganic amphiphile may provide valuable informationabout the PDA phase behavior. Accordingly, we investigatethe mixing behavior of diacetylene tricosadiynoic acid(TCDA) with an interesting fluorescent dye Octadecylrhodamine B chloride (RhB18) and hence study thephase behavior of the mixed system by LB technique.TCDA has been specifically selected in our studies due toits strong significant chromatic response compared toother diacetylene derivatives (19, 56, 57). On the otherhand amphiphilic RhB18 is a cationic dye widely used asfluorescent probes (58). Therefore, it is expected thatRhB18 will interact with TCDA electrostatically. Thismay effect the phase behavior of TCDA and hence itschromatic response. Again, being amphiphilic RhB18form stable Langmuir film at air–water interface.

The main objective was to investigate the effect ofmixing on the phase behavior of TCDA when mixedwith RhB18. Normally pure TCDA has two phases: blueand red upon application of external stimuli on Langmuirfilm (8, 59). However, interestingly our investigationsrevealed that in certain cases for the TCDA: RhB18mixed film possess both stable blue and red phases.

Experimental Section

Materials

Diacetylene monomer 10, 12-TCDA and OctadecylRhB18 were purchased from Sigma Chemical Company.Chloroform (99.9%; SRL, India) used as a solvent, was ofspectroscopic grade and its purity were checked by fluor-escence spectroscopy. The molecular structure of TCDAand RhB18 is shown in Figure 1. Active solutions wereprepared by dissolving the diacetylene monomer (DA) inspectroscopic grade chloroform (SRL) and filteredthrough a 0.2 μm (PTFE) nylon filter and purity of thesame was also checked by UV–Vis absorption and fluor-escence spectroscopic techniques and was used withoutfurther purification. Ultrapure water, emanated in our labby aMilliporeMilli-Q unit, having resistivity 18.2MΩ cmwas used as subphase.

Isotherm Measurement and LB Film Formation

Surface pressure versus area per molecule (π-A) iso-therms as well as multilayer film preparation measure-ments were obtained with a commercially available LBfilm deposition instrument (Apex 2000C, ApexInstruments Co., India). The concentrations of thestock solutions for TCDA, RhB18 were 10−3 M. Inorder to have Langmuir films of pure TCDA, pureRhB18 and their mixture with different ratios a smallamount (120 μl, 35 μl) of the dilute solutions of TCDAand RhB18 were spread at the air–water interface of theLB trough (of total area 466.2 cm2) made of Teflonfilled up with ultrapure Millipore water (18.2 MΩ cm)with the help of a micro-syringe and after allowingsufficient time (about 15 min) to evaporate the volatilesolvent, the barrier was compressed slowly at a rate of5 mm/min to obtain the isotherm characteristics. The

Figure 1. Molecular structures of (a) the diacetylene monomer, 10, 12-TCDA and (b) Octadecyl RhB18.

2 S. SUKLABAIDYA ET AL.

surface pressure (π) versus average area available forone molecule (A) was measured by a Wilhelmy platearrangement. Each isotherm was repeated a number oftimes and data for surface pressure−area per moleculeisotherms were obtained by a computer interfaced withthe LB instrument. Before each isotherm measurement,the trough and barrier were cleaned with ethanol andthen rinsed by Milli-Q water. After the completion ofreaction kinetics the film was lifted horizontally atdesired pressure with the fixed position of the barrier.A smooth fluorescence grade quartz plate (for spectro-scopy) was used as solid substrate. The temperature wasmaintained at 24°C throughout the experiment. A UVPen-Ray lamp (λ = 254 nm, 4 W, UVP) was used topolymerize the LB film.

Brewster angle microscopy (BAM) Imaging

In order to visualize the changes in domain structuresand morphology of TCDA and mixed film at the air–water interface BAM was used. In our present commu-nication, BAM images were taken usinga nanofilm_EP4-BAM (Accurion, Serial No.1601EP4030) with a 30 mW laser emitting p-polarizedlight at 532 nm wavelength that directed to the air/water interface at the Brewster angle (53.1°). Thereflected light was passed through a focal lens to ananalyzer at a known angle of incident polarization andthen received by a CCD camera to obtain an image ofmonolayer. The lateral resolution of the image was2 μm, the shutter speed was 1/50 s.

UV-Vis Absorption and Fluorescence Spectroscopy

UV−vis absorption and fluorescence of pure as well asthose of mixed LB films, were recorded by usingabsorption spectrophotometer (PerkinElmer, Lambda25) and fluorescence spectrophotometer (PerkinElmer,LS 55), respectively. The absorption spectra wererecorded at 90° incidence using a clean quartz slide asreference.

Atomic force microscopy

TheAFM images of LB films were taken with a commercialAFM (Innova AFM system, Bruker AXS Pte Ltd.) by usingsilicon cantilevers with a sharp, high-apex-ratio tip (VeecoInstruments). The AFM images given here were obtainedin intermittent-contact (“tapping”) mode.

Results and Discussions

Monolayer Characteristics

In order to study the monolayer forming behavior ofTCDA, and mixed TCDA: RhB18 film, surface pressure-area (π-A) isotherms of pure TCDA and TCDA: RhB18mixtures at air–water interface have been recorded usingLB technique. Figure 2 shows the surface pressure versusarea per molecule (π-A) isotherms of TCDA mixed withRhB18 at different molar ratio along with those of pureTCDA and RhB18 for comparison. Monolayer charac-teristics extracted from the isotherm of Figure 2 areshown in Table 1. The thermodynamic behavior andthe existing nature of interactions among the binarycomponents can be highlighted from the (π-A) iso-therms. The isotherm for RhB18 is a smoothly increasingcurve with a lift of area 1.66 nm2. The lift of area isdetermined according to the method described by Raset al (60) (shown in supporting information Fig. S1). Themolecular areas of 0.80 nm2 at 20 mN/m and 0.60 nm2 at30mN/m of RhB18 is consistent with the reported resultsof Vuorimaa et al (61). The limiting area per molecule ofRhB18 was obtained by extrapolating a line tangent tozero surface pressure, at a steep segment of the isothermand found to be 1.42 nm2. On the basis of molecularmodeling (CPK model) considering the flat orientationof RhB18, we calculated the mean molecular area ofRhB18 as 1.43 nm2, which is very close to the experimen-tally observed value. This indicates the flat orientation ofthe dye molecule (shown in supporting information Fig.S2), keeping the xanthenes moiety of rhodamine mole-cule flat at air–water interface and the alkyl chain verti-cally in upward direction. The isotherm of pure TCDAexhibits a condensed phase collapsing at ca. 12.55mN/m.The limiting area per molecule of TCDA at condensedphase was approximately 0.42 nm2 per molecule. Thecompression of the Langmuir monolayer after collapseleads to a reorganization of the film at a stable phase witha limiting molecular area of ca. 0.14 nm2 per molecule.This value suggests the formation of TCDA trilayer. Thishas been confirmed by comparing the mean molecularareas before collapse and when again reorganizationoccurred after collapses (Table 1). Our later BAM studiesalso support this trilayer formation in pure TCDA andmixed film up to TCDA mole fraction � 0.85. Previousstudies conducted on PDA films at air–water interfacehave also revealed similar trilayer formation (20, 57). Theisotherms corresponding to TCDA: RhB18mixedmono-layer at different molar ratios spaced themselves betweenthose of pure components with lift-off areas rangingbetween those of individual components. This indicatesthe presence of interaction and certain extent of

SOFT MATERIALS 3

miscibility among the binary components in the mixedmonolayer. The area per molecule of the binary systemincreases systematically with increase in mole fractionsof RhB18.

Here the nature of mixed isotherms for mole frac-tions of TCDA ranging from 0.8 to 0.1, curve 5–11 ofFigure 2 were almost similar to that of pure RhB18isotherm (Fig. 2, curve 12). Whereas, for TCDA molefraction > 0.8 the nature of the mixed isotherms (curve2–4 of Fig. 2) were TCDA (curve 1 of Fig. 2) like.Interestingly, the mixed films at these three mole

fractions form trilayers whereas for isotherms recordedwith lower mole fractions of TCDA no traces of tri-layers were obtained (curve 5–11 of Fig. 2). The col-lapse pressure of the mixed binary system increaseswith the increase in molar ratio of RhB18 in themixed monolayer. The TCDA: RhB18 mixedLangmuir monolayer principally sustained by ionicinteractions between anionic TCDA and cationicRhB18.

Additivity and surface phase rule were employed toexplore more information about the interactions betweenthe binary components in the Langmuir monolayer (62).The π-A isotherms are used to have idea about the mis-cibility or the phase separation of these binary componentsin the mixed monolayer (63). The miscibility of mixedmonolayer was determined quantitatively by analyzingthe excess area AE of the mixed monolayer at the air−water interface. It is given by AE = A12 − Aid, with Aid

=A1X1 +A2X2, whereAid is the ideal area permolecule,A1

and A2 are the areas occupied by the monomers of TCDAand RhB18, respectively, and X1 and X2 are the molefractions of the components in the mixtures. A12 is theexperimentally observed area per molecule. In the idealcase, the plot of excess area versus mole fraction isa straight line, whereas, partial miscibility and non-ideality are characterized by deviation from it (AE = A12

− Aid ≠ 0) (63, 64). AE < 0 indicates attractive interactionsbetween the binary components whereas AE > 0 indicates

Figure 2. Surface pressures versus area per molecule (π-A) isotherms of mixed TCDA: RhB18 Langmuir films at different molefractions of TCDA along with pure TCDA (1) and RhB18 (12) isotherm. The mole fraction of TCDA in each film is (2) 0.95 (3); 0.9 (4);0.85 (5); 0.8 (6); 0.7 (7); 0.6 (8); 0.5 (9); 0.4 (10); 0.3 (11); 0.1.

Table 1. Monolayer characteristics of TCDA: RhB18 mixedLangmuir film.

Molefraction ofTCDA

Lift-offarea(nm2)

Meanmolecular

area(nm2/

molecule)

Collapsepressure(mN/m)

Trilayerformedor not

Mean moleculararea after

trilayer formed

0 (pureRhB18)

1.66 1.43 39.3 No –

0.1 1.48 1.32 39.3 No –0.3 1.28 1.11 42 No –0.4 1.21 1.09 40.6 No –0.5 1.01 0.9 40.6 No –0.6 0.88 0.75 38 No –0.7 0.8 0.69 36 No –0.8 0.71 0.6 32.9 No –0.85 0.7 0.58 30.9 Yes 0.20.9 0.66 0.57 23.78 Yes 0.180.95 0.61 0.55 19.11 Yes 0.171 (PureTCDA)

0.45 0.42 12.55 Yes 0.14

4 S. SUKLABAIDYA ET AL.

a repulsive interaction between the binary components inthe mixed monolayer. Figure 3 shows the plot of AE versusmole fractions of TCDA at different surface pressures, viz.,2.5, 5, 7.5, 10 and 12.5 mN/m. For almost all the mixedfilms positive deviation is observed. However, the extent ofpositive deviation varies with mole fraction. This indicatesthat the interaction between TCDA and RhB18 stronglydepends on mixing ratio. Maximum interaction occurredat TCDA mole fraction XTCDA = 0.4.

Brewster angle microscopy

In-situ BAM investigation is one of the most reliabletools to have visual idea about the microstructure of thecomplex monolayer at air–water interface. Domains ofdifferent sizes and shapes in the BAM images ofLangmuir monolayer indicates the phase transition inthe floating layer. BAM analysis provides a deep pock-ets of microscopic information upon the compressionprocess and also give visual evidence about the growthphenomena and domain formation within the film. Inthe present case, BAM images of pure TCDA andTCDA: RhB18 mixed Langmuir monolayer were takenat different stages during π-A isotherm measurement atair–water interface (Fig. 4 and fig. S3- S5 of supportinginformation). BAM images recorded for the floatingTCDA film at different surface pressure before collapsedo not show any organized structures/domain. (Fig. S6of supporting information).

Rather the BAM images revealed continuous smoothuniform structure less surface. Similar features werealso reported in case of TCDA film before any phasechange/collapse occurred in the floating mono-layer (46).

Interestingly, after collapse the BAM images showdistinct organized structures with appearance of cleardendritic nature (Fig. S3 (i)–(iii) of supporting infor-mation). Although in the observed BAM images thebranching in the dendritic structure seems to occurrandomly, however, predominantly they occur alongone direction within the branch. It is assumed thatdiffusion limited aggregation (DLA) occurred in theTCDA film after collapse resulting formation of fractaldendritic structures (46). According to DLA model animmobile cluster progressively growing through aggre-gation of monomer and has been successfully employedto describe the fractal aggregation behavior in varioussystems (65, 66). Interestingly, stopping the barriercompression at a particular surface pressure after theappearance of fractal dendritic structures in the BAMimages showed that slight change in the dendritic struc-tures occurred with the passage of time. This indicatesthe non-equilibrium nature of the growth of theobserved structures in the floating film. As the film iscompressed further, the branches in the observed struc-tures become thicker and larger due to the coalescenceof smaller branches. It is well known that TCDA formtrilayer film after collapse at air–water interface (45).

Figure 3. Excess area per molecule of mixed TCDA: RhB18 Langmuir films as a function of mole fraction of TCDA at differentpressures.

SOFT MATERIALS 5

Our isotherm studies also revealed the same. In theBAM image this small trilayer (45) domains appear aswhite region, which probably due to the greater thick-ness of the reflecting domains. As the film is com-pressed further, the white trilayer structure becomesmore dominant. It is well known that TCDA can bepolymerized in two distinct phases: blue and red undersuitable conditions (8, 59). Based on this phase changes,TCDA molecule have been used to design various sen-sors. In the present work we have mixed a xanthenesdye RhB18 with TCDA and studied the mixed filmbehavior. The main idea was to tune the two phasesof TCDA for its future application.

Interestingly, the BAM studies of TCDA: RhB18revealed that TCDA mole fraction � 0.85 in the

TCDA: RhB18 mixed monolayer only possess definitedendritic structures (Fig. 4 and fig. S4, S5 of supportinginformation). Mixed monolayer having TCDA molefraction < 0.85 do not form such structures (Fig. 5).However, marked difference in the BAM images inpresence of RhB18, clearly confirm the presence ofRhB18 in the mixed films. Based on the mole fractionof TCDA in the TCDA: RhB18 mixed film, differentkinds of structures were observed.

For TCDA mole fraction < 0.85 in the mixed filmthe structure totally disappeared. On the other hand itwas found that the BAM images were covered withsmall domains having dot like structure (Fig. 5). It isinteresting to note that pure TCDA form trilayer/multi-layer after collapse (45). Our previous isotherm studies

Figure 4. BAM images of TCDA: RhB18 = 0.95:0.05 mixed Langmuir film recorded at the indicated points during the (π-A) isothermmeasurement.

6 S. SUKLABAIDYA ET AL.

showed that TCDA: RhB18 mixed film having TCDAmole fraction � 0.85 only formed trilayer/multilayerafter collapse. However, trilayer/multilayer was notformed for the mixed film with TCDA mole fraction< 0.85. In case of pure TCDA it is assumed that DLAis the key factor to form the observed dendritic struc-tures. Therefore, it can be said that mole fraction ofTCDA play a vital role in the DLA process in case ofTCDA: RhB18 mixed films. TCDA mole fraction� 0.85 in the TCDA: RhB18 mixed film is favorablefor DLA, whereas, TCDA mole fraction less than this inthe mixed film is not suitable for the DLA to occur.

Here the BAM images were recorded during pressure-area isothermmeasurement, where area per molecule wasreduced progressively. This implies that the observedmolecular domains grow with compression and at

particular area. Also the shapes as well as the order andsymmetry of the structures depend on the interactionbetween TCDA and RhB18 as well as the extent of crystal-lization process. Studies based onMonte Carlo simulationalso suggested aggregation growth of two dimensionalsquare lattices during surface-pressure measurement(67). TCDA and other diacetylene derivatives polymerizesin presence of UV and form poly diacetylene with twodistinct phases depending on the extent of exposure.Therefore we felt that it would be interesting to studythe BAM images of pure TCDA and TCDA: RhB18mixture under the exposure of UV. Accordingly, wehave measured the BAM of the corresponding floatinglayer after UV-irradiation. Representative images areshown in Figure 6. Here images before and after UV-irradiation are shown for comparison. Interestingly, it has

Figure 5. BAM images of TCDA: RhB18 = 0.8:0.2 mixed Langmuir film recorded at the indicated points during the (π-A) isothermmeasurement.

SOFT MATERIALS 7

been observed that application of UV-irradiation causesan increase in the brightness of all the images compared tothe same before irradiation. Observed increase in bright-ness may be due to the increase in absorption of radiationby the TCDA polymer formed due to UV-irradiation (48,55, 68). In the present case for BAM study, we have usedlaser of wavelength 532 nm. Again our spectroscopicstudies (given in the later section of this manuscript)showed that after polymerization the absorption spectrachanges significantly and maximum absorption lieswithin 542–548 nm region under various condition.This is very close to the laser wavelength. Accordingly

absorption of radiation after polymerization increasesresulting a sharp increase in the reflection and hence thebrightness.

It is important to mention in this regard that duringUV-irradiation process the domain texture as well as thearea of the floating film remain almost unchanged. Also thebrightness of the domains increases after polymerizationand the brightness of the region surrounding the domainsdo not increases. These observations indicated that thepolymerization takes place exclusively with in the dendriticstructures. Interestingly the brightness of the BAM imagesof the TCDA: RhB18mixed film with TCDAmole fraction

Figure 6. BAM images of TCDA and TCDA: RhB18 mixed Langmuir film before and after 1 min UV-irradiation on air–water interface.

8 S. SUKLABAIDYA ET AL.

< 0.85 do not show significant change after UV-irradiation (Fig. S7 of supporting information). Our pre-vious BAM as well as isotherm studies revealed that mixedfilm with TCDAmole fraction < 0.85 do not form trilayerand hence did not show the dendritic structure in the BAMimages (Fig. 5). Therefore, observed almost no increase inthe brightness of the BAM images of the mixed film(TCDAmole fraction < 0.85), clearly indicate that trilayerformation play a crucial role in polymerization. As a wholeBAM study after UV-irradiation give compelling visualevidence of polymerization of TCDA at air–water interface.

Atomic force microscopy

In order to have the visual evidence of trilayer formationwe have studied the LB film using a traditional imagingtechnique AFM. In order to do that pure TCDA andTCDA: RhB18 mixed (TCDA mole fraction = 0.95)Langmuir film were deposited onto smooth silicon wafersubstrate after collapse of the floating film. Before deposi-tion both the films were polymerized to red phase at air-water interface. Corresponding AFM images were shownin Figure 7.

AFM image of pure TCDA (Fig. 7(a)) clearly showedstacks of TCDA polymer strand with clear straight linecracks. These are the characteristics of ordered crystal-line films of PDA (69). Height profile analysis (relative

to the empty substrate) revealed that height of stackslies at around 9 nm. It has been reported that thethickness of the polymerized TCDA monolayer lieswithin the range of 3 ± 0.5 nm (70). Therefore, in thepresent case observed height confirm that TCDA formstacks of trilayer after collapse.

On the other handmorphology of TCDA: RhB18mixedfilm was slightly different than the pure TCDA. Here insome cases width of trilayer strands decreases and in somecase almost continuous film morphology was observed.Due to opposite ionic nature RhB18 and TCDA stronglyinteracted with each other through electrostatic attraction.This may in turn affect the filmmorphology as observed inthe AFM image (Fig. 7(b)). Height profile analysis revealedthat here the film thickness lies near about 10 nm, which isslightly higher than the pure TCDA. Thismay be due to thepresence of RhB18 in the TCDA trilayer stacks. As a wholeAFM studies gave compelling visual evidence of trilayerformation of TCDA after collapse due to polymerization.

Spectroscopic Characterization

UV-Vis Absorption SpectroscopySurface-pressure area per molecule isotherm as well asin-situ BAM studies of pure TCDA and TCDA: RhB18mixed film at air–water interface revealed that trilayeris formed in pure TCDA film at air–water interface

Figure 7. AFM images of (a) pure TCDA film in red phase and (b) mixed TCDA: RhB18 (TCDA molefraction = 0.95) LB film in redphase.

SOFT MATERIALS 9

(45). On the other hand in case of mixed film trilayer isformed up to TCDA mole fraction � 0.85 in TCDA:RhB18 mixed film. However, for the film with TCDAmole fraction < 0.85 no trilayer was formed at air–water interface. Again it is well known that TCDA filmshows two phases upon application of external stimuli(45). In order to check the phase behavior of the mixedfilm we have investigated the UV-Vis absorption spec-tra of pure and mixed LB films transferred on to quartzsubstrate upon application of external stimuli such asUV-irradiation and heat. It is expected that this willgive an insight to the role of trilayer formation as wellas the phase behavior of mixed films and hence willenable us to tune these two phases.

Absorption spectra for pure TCDA and TCDA: RhB18mixed film (TCDA mole fraction = 0.95) after UV-irradiation are shown in Figure 8(a–b) respectively. Allthe films were prepared after collapse of the Langmuirfilm at air–water interface and consequent trilayer forma-tion. Pure TCDA absorption spectrum does not showprominent absorption band in absence of UV-irradiation, whereas, absorption spectrum of pureTCDA film recorded after 1 min UV – irradiationshows prominent band at 645 nm along with a weakhump at 590 nm. These two bands are the characteristicsof blue phase of TCDA film (7). However upon increasingthe UV-irradiation doses the intensities of these twobands decreases gradually and at the same time newband appeared at around 510 and 557 nm. For the filmwith UV-irradiation time one hour or more the 645 nm

band become almost a weak hump and the 510 and557 nm band become maximum intense. It is relevant tomention in this context that upon irradiation of UV lighttopochemical photoploymerization of TCDA occurredand initially form metastable blue phase. Upon increasein extent of UV-irradiation transition from the transientblue phase to stable red phase take place (8, 59). Each ofthese two phases possess two absorption bands, vibronicand excitonic with peaks around 590 and 640 nm for theblue phase and 500 and 550 nm for the red phase respec-tively (7). The transition between blue to red phase maybe monitored by observing the shift of absorption bands.In the present case it is assumed that initially upon appli-cation of UV-irradiation polymerization is occurred andblue phase appeared in the TCDA film. However, due toincrease in the extent of UV-exposure blue phase trans-formed to red phase showing clear indication through theshift of the absorption bands.

On the other hand for the mixed films (TCDA molefraction = 0.95) even in absence of UV-irradiation clearbands at around 565 and 520 nm are observed. Thesetwo bands are mainly due to the presence of RhB18 inthe mixed film. 565 nm band is due to the 0–0 transi-tion of RhB18 and 520 nm band is due to the vibronictransition of RhB18 monomer (71). However, these twobands are slightly shifted toward higher energy regioncompared to the same in case of pure RhB18 LB films(Fig.S8 shown in supporting information). It is relevantto mention in this context that TCDA is anionic andRhB18 is cationic in nature. Therefore, both the

Figure 8. Colorimetric properties of TCDA and TCDA: RhB18 LB film. UV-Vis absorption spectra of (a) pure TCDA film and (b) TCDA:RhB18 = 0.95: 0.05 films following UV-irradiation for the indicated durations. All the films were obtained after collapse pressure andwith horizontal lifting in the Langmuir trough.

10 S. SUKLABAIDYA ET AL.

molecules may interact electrostatically in the mixedsystem. This interaction may affect the organization ofboth the molecules in the mixed films resulting theshifting of RhB18 absorbance as observed. Absorptionspectrum of the same after 1 min UV-irradiationshowed prominent bands at 640 nm in addition to thebands due to RhB18 absorption. This band is due toblue phase of TCDA and indication of the formation ofblue phase due to photochemical polymerization of theTCDA in the mixed films. Interestingly, upon increas-ing the extent of UV-irradiation (> 15 min) the peakdue to blue phase never disappeared. Although natureof absorption bands in the higher energy region getsmodified with respect to both position and shape com-pared to the same in case of 1 min UV-irradiation.Here, a shift of the order of 8 nm is observed with anadditional weak hump at around 490 nm. This indi-cates the formation of red phase of TCDA. That meansboth the red and blue phases co-exist in the mixed film.Although in case of pure TCDA blue phase was meta-stable and only stable red phase was observed. On theother hand, for the mixed film with TCDA mole frac-tion < 0.85 absorption peaks due to the blue phasenever appeared even upon the increase in the UV-irradiation dose (Fig. S9 shown in supporting informa-tion). Our earlier investigation of isotherm and BAMshowed that trilayer was not formed in case of theTCDA: RhB18 mixed film with TCDA mole fraction< 0.85. On the other hand trilayer was formed for themixed film with TCDA mole fraction � 0.85.Therefore, the observed absorption spectra of TCDA:RhB18 mixed film give a clear indication that trilayerformation in the mixed film play a vital role toward thephotochemical polymerization of TCDA in the mixedfilm.

It is relevant to mention that in case of diacetylene film,when the distance between adjacent DAmonomer is of theorder of 4.7–5.2 À and the angle between the diacetylenerod (monomer) and the translational vector (stacking axis)is about 45° (� 5°), photopolymerization occur uponUV-irradiation (21, 72). This results an extended π � conju-gated backbone structure with excellent electronic andoptical properties. Upon application of external stimuliblue to red color shift occurs resulting fluorescence switchon. This blue to red transition is induced by the decrease ineffective conjugation length of PDA (formation of smallerand more condensed unit cell) under lipid chain distortionand disorder (73). Accordingly, distance between adjacentpolymer chain is reduced to 3.9 Å in red phase, which wasof the order of 4.7–5.2 Å in blue phase. This in turn causesa straitening of the alkyl chain in the PDA backbone. In

case of PDA Langmuir film at air–water interface thistransformation has been manifested as the decrease inarea per molecule during transition from monomer toblue and consequent red phase transformation (47). Inthe present case spectroscopic investigations revealed thatfor pure TCDA film upon application of UV-irradiationphotopolymerization occurred in the LB films. Based onthe UV-irradiation time, total transformation from blue tored phase occurred. However, in case of mixed film boththe blue and red phase co-existed after 90 min of UV-irradiation. This may be due to the fact that in the mixedfilm TCDA and RhB18 remained as oppositely chargedionic pairs (anionic TCDA and cationic RhB18).Additional interactions such as hydrophobic interactionbetween the alkyl chain of the molecules may also occur.Therefore presence of RhB18 molecules may oppose thestraightening of the alkyl chain of TCDA after polymeriza-tion to a certain extent. As a result both the blue phase(with inclined chains) and red phase (with straightenchain) co-existed in the mixed films. This has beenshown schematically in Figure 9. On the other hand formixed film with TCDA mole fraction < 0.85 trilayer wasnot formed and hence photoploymerization was notobserved. TCDA and RhB18 molecules are ionicallybonded through electrostatic interaction in the mixedfilms. Therefore, presence of large number of RhB18 mole-cules prohibits the TCDA trilayers formation in the mixedfilms (RhB18 molefraction � 0.2).

In order to check the effect of heat on the phasebehavior of pure TCDA and TCDA: RhB18 mixedfilms, initially the floating film was polymerizedthrough exposure of UV radiation for about 30 s inthe LB trough. This leads to the blue phase of TCDA atair–water interface. After that the photopolymerizedfloating film was transferred onto quartz substrate andtheir absorption spectra were recorded upon exposureto heat at different temperature.

Polymerized pure TCDA film show absorption peakscorresponding to blue phase (Fig. 10(a)). Upon increasein the temperature during heat exposure the intensityof blue phase absorbance decreases and at the sametime absorption peaks corresponding to red phaseappeared. For the film treated at 65°C the absorptioncorresponding to blue phase totally disappeared andonly the absorption due to red phase exist. This indi-cates that at 65°C blue phase of TCDA totally convertedinto red phase.

On the other hand for the mixed films (Fig. 10(b))also showed almost similar trend. Initially, blue phaseexist and at higher temperature 65°C or more the bluephase totally converted to red phase showing presence

SOFT MATERIALS 11

of the peaks due to the red phase only. It is interestingto note that during UV exposure for the mixed filmsalways both the blue and red phase co-existed. Howeverupon heat treatment at higher temperature TCDAshowed only the stable red phase for the mixed films.

Fluorescence SpectroscopyIn order to check the effect of TCDA polymerization aswell as TCDA: RhB18 mixing on fluorescence behaviorof RhB18 as well as TCDA in red phase, we haveperformed the fluorescence analysis of the TCDA:RhB18 films. Our previous investigation revealed thatin case of TCDA: RhB18 mixed film polymerizationwas possible for the film with TCDA molefraction ≥0.85. Therefore for fluorescence study we have chosentwo films with TCDA molefraction 0.95 and 0.7.

Pure RhB18 LB film showed prominent fluorescencewith peak around 595 nm (Fig. 11(a)) which is similarto the earlier reported results (74). Upon application of

UV-irradiation pure RhB18 fluorescence remainedalmost unchanged (figure not shown).

It is well known that pure TCDA show fluorescenceonly at red phase and no fluorescence occurred in bluephase (75). Here also until red phase is achieved almostno fluorescence was observed (Fig. 11(a)). We havechecked by varying the excitation wavelength as well.However, TCDA showed prominent fluorescence onlywhen red phase was achieved with peaks at around 556and 618 nm (Fig. 11(a)). This is in agreement with thereported literature (75). In case of mixed films forfluorescence measurement we have selected two excita-tion wavelengths – 550 nm (absorption maximum ofRhB18) and 485 nm (absorption maximum of TCDA).When λex = 550 nm, in absence of UV-irradiation boththe mixed film showed similar fluorescence behaviorwith fluorescence band at around 595 nm (Fig. 11(b)).This resembles the fluorescence behavior of RhB18onto thin films as reported earlier (74). However

O

COOC18H37

(CH3CH2)2N

R=-CH2-CH3

N+(CH2CH3)2

Cl-

RhB18 structure RhB18 symbol

O

NR2R2NCl-

O

Monomer Blue Polymer Blue and red Polymer

OO

OO

OO

OO

HO

O

H OO

HO

O

HOO

HO

O

H OO

HO

O

HOO

HO

O

HOO

HO

O

HO

O

HO

O

HOO

HO

O

HO

O

HO

O

HOO

HO

O

H

OO

OO

n

n

OO

HO

O

H OO

H OO

HOO

HO

O

H OO

HO

O

HOO

H OO

H

n

n

UV UV

O

NR2R2NCl-

O

H

O

NR2R2NCl-

O

H

O

NR2R2NCl-

O

H

O

NR2R2NCl-

O

H OO

n

OO

OO

O

NR2R2NCl-

O

H

O

NR2R2NCl-

O

H OO

n

Figure 9. Schematic representation of formation of trilayer and polymerization in the TCDA: RhB18 mixed films.

12 S. SUKLABAIDYA ET AL.

upon UV-irradiation the intensity of RhB18 fluores-cence decreases gradually with increase in UV-irradiation time (Fig. 11(b)). This may be due to thefact that upon UV-irradiation TCDA polymerizes andextent of polymerization changes until final stablephase is reached. This polymerization may affect theorganization of RhB18 onto mixed films and hence thefluorescence is affected.

On the other hand marked changes in the fluores-cence spectra of TCDA:RhB18 mixed films with λex= 485 nm is observed (Fig. 11(a)). Here prominentfluorescence with two bands at around 579 and642 nm are observed. These two bands are red shiftedby 23 and 24 nm respectively with respect to pureTCDA fluorescence spectrum. Interestingly these twobands did not occurred when the same film is excitedwith λex = 550 nm i.e. absorption maximum of RhB18(Fig. 11(b)). This clearly suggest that observed fluores-cence for the mixed film (TCDA molefraction = 0.95) isdue to light absorbed by TCDA only. However pre-sence of RhB18 affected the polymerization as well asphase behavior of TCDA to a large extent resultingsuch change in TCDA fluorescence in the mixed films.

On the other hand for the mixed film (TCDA mole-fraction = 0.7) no fluorescence was observed with λex= 485 nm corresponding to the absorption maximumof TCDA (Fig. 11(c)). Only fluorescence due to RhB18was observed when excited at λex = 550 nm corre-sponding to the absorption maximum of RhB18 (figurenot shown). Our previous studies of this manuscript itwas seen that in case of mixed film polymerization andhence red phase was achieved only for TCDA

molefraction ≥ 0.85. On the other hand TCDA hasshown fluorescence only in red phase. Therefore, nofluorescence for the mixed film with TCDA molefrac-tion = 0.7 clearly suggest that no red phase wasachieved for this film.

Conclusion

The TCDA: RhB18 mixed Langmuir monolayer and LBfilms have been characterized by in-situ and ex-situphotopolymerization of DA molecule into PDA.Different dendritic structures have been observed byBAM after the collapse of the monolayer or after thetrilayer or multilayer formation of pure TCDA andmixed TCDA: RhB18 film. The mixed film containingTCDA mole fraction � 0.85 produces typical trilayerand polymerization was possible only in that trilayer,whereas, mixed film containing TCDA mole fraction <0.85 produces monolayer and no polymerization takesplace. Trilayer formation has been confirmed by AFMstudies. We confirm the polymerization of the mixedfilm and pure TCDA by UV absorption and steadystate fluorescence spectroscopy and in-situ BAM analy-sis. The increase in RhB18 mole fraction restricts theformation of trilayer and also the polymerization in themixed film. RhB18 molecule possibly acts as spacerbetween two DA monomers. Thus we demonstratea very simple process depending on the mole fractionof the binary components by which we can tune thephases of PDA. This will enrich the library of PDAmolecules for its future sensing applications.

Figure 10. Absorption spectra of (a) pure TCDA and (b) TCDA: RhB18 = 0.95:0.05 mixed film measured upon heating.

SOFT MATERIALS 13

Acknowledgments

The authors are grateful to DST, for financial support tocarry out this research work through FIST – DST projectref. SR/FST/PSI-191/2014. SAH is grateful to DST, forfinancial support to carry out this research work throughDST, Govt. of India project ref. No. EMR/2014/000234.The authors are also grateful to UGC, Govt. of India forfinancial support to carry out this research work throughfinancial assistance under UGC – SAP program 2016.

The author SC is grateful to CSIR, for financial sup-port to carry out this research work through RA award(No. 09/714(0017)/2016-EMR-I, Dated: 31/03/2017). Theauthors are also grateful to Prof. Amir Berman, BenGurion University of the Negev, Beer-Sheva, Israel, forproving the TCDA sample.

References

1. Lee, S., Kim, J. Y., Chen, X., and Yoon, J. (2016)Chemical Communications, 52:9178–9196.doi:10.1039/C6CC03584A.

2. Reppy, M.A., and Pindzola, B.A. (2007) ChemicalCommunications, 4317–4338. doi:10.1039/b703691d

3. Ahn, D.J., and Kim, J.M. (2008) American ChemicalSociety, 41:805–816. doi:10.1021/ar7002489

4. Yoon, B., Lee, S., and Kim, J.M. (2009) ChemicalSociety Reviews, 38:1958–1968. doi:10.1039/b819539k

5. Wegner, G. (1969) Zeitschrift für Naturforschung,24:824–832. doi:10.1515/znb-1969-0708

6. Fouassier, J.P., Tieke, B., and Wegner, G. (1979) IsraelJournal of Chemistry, 18:227–232. doi:10.1002/ijch.v18:3-4

Figure 11. (a) Steady state fluorescence spectra of TCDA: RhB18 mixed (TCDA mole fraction = 0.95) LB film before and after UV-irradiation along with pure TCDA and RhB18 LB films. λex = 485 nm for pure TCDA and TCDA: RhB18 mixed films. λex = 550 nm forpure RhB18 LB film. (b) Steady state fluorescence spectra of TCDA: RhB18 mixed (TCDA molefraction = 0.95) LB film with differentUV-irradiation time. λex = 550 nm. (c) Steady state fluorescence spectra of TCDA: RhB18 mixed (TCDA molefraction = 0.7) LB film withdifferent UV-irradiation time. λex = 485 nm.

14 S. SUKLABAIDYA ET AL.

7. Lifshitz, Y., Upcher, A., Shusterman, O., Horovitz, B.,Berman, A., and Golan, Y. (2010) Physical ChemistryChemical Physics : PCCP, 12:713–722. doi:10.1039/B915527A

8. Bassler, H. (1984) Photopolymerization of diacetylenes. InPolydiacetylenes; Cantow, H.-J., ed. (Vol. 63, pp. 1–48);Berlin, Heidelberg: Springer.

9. Zhu, C., Liu, L., Yang, Q., Lv, F., and Wang, S. (2012)Chemical Reviews, 112:4687–4735. doi:10.1021/cr200263w.

10. Lee, J., Seo, S., and Kim, J.M. (2012) AdvancedFunctional Materials, 22:1632–1638. doi:10.1002/adfm.v22.8.

11. Yoon, B., Ham, D.Y., Yarimaga, O., An, H., Lee, C.W.,and Kim, J.M. (2011) Advanced Materials,23:5492–5497. doi:10.1002/adma.201103379

12. Lee, J., Yarimaga, O., Lee, C.H., Choi, Y., and Kim, J.M.(2011) Advanced Functional Materials, 21:1032–1039.doi:10.1002/adfm.201002042.

13. Sun, X., Chen, T., Huang, S., Li, L., Peng, H., and Chen, T.(2010) Chemical Society Reviews, 39:4244–4257.doi:10.1039/c001151g

14. Charych, D.H., Nagy, J.O., Spevak, W., and Bednarski, M.D. (1993) Science, 261:585–588. doi:10.1126/science.8342021

15. Lu, Y., Yang, Y., Sellinger, A., Lu, M., Huang, J.,Fan, H., Lopez, G.H., Burns, A.R., Sasaki, D.Y.,Shelnutt, J., and Brinker, C.J. (2001) Nature,410:913–917. doi:10.1038/35073544

16. Lee, S.B., Koepsel, R.R., and Russell, A.J. (2005) NanoLetters, 5:2202–2206. doi:10.1021/nl0513582

17. Batchelder, D.N., Freeman, T.L., Häussling, L.,Ringsdorf, H., Wolf, H., and Evans, S.D. (1994) Journal ofthe American Chemical Society, 116:1050–1053.doi:10.1021/ja00082a028

18. Potisatityuenyong, A., Tumcharern, G., Dubas, S.T.,and Sukwattanasinitt, M.J. (2006) Colloid andInterface Science, 304:45–51. doi:10.1016/j.jcis.2006.08.054

19. Day, D., and Ringsdorf, H. (1978) Journal of PolymerScience: Polymer Letters Edition, 16:205–210.doi:10.1002/pol.1978.130160501

20. Lio, A., Reichert, A., Ahn, D.J., Nagy, J.O.,Salmeron, M., and Charych, D.H. (1997) Langmuir :the ACS Journal of Surfaces and Colloids, 13:6524–6532.doi:10.1021/la970406y

21. Kuriyama, K., Kikuchi, H., and Kajiyama, T. (1998)Langmuir : the ACS Journal of Surfaces and Colloids,14:1130–1138. doi:10.1021/la970831r

22. Zhong, L., Zhu, X., Duan, P., and Liu, M. (2010) TheJournal of Physical Chemistry. B, 114:8871–8878.doi:10.1021/jp1020565

23. Dei, S., Shimogaki, T., and Matsumoto, A. (2008)Macromolecules, 41:6055–6065. doi:10.1021/ma702128a

24. Park, H., Lee, J.S., Choi, H., Ahn, D.J., and Kim, J.M.(2007) Advanced Functional Materials, 17:3447–3455.doi:10.1002/(ISSN)1616-3028

25. Wacharasindhu, S., Montha, S., Boonyiseng, J.,Potisatityuenyong, A., Phollookin, C.,Tumcharern, G., and Sukwattanasinitt, M. (2010)Macromolecules, 43:716–724. doi:10.1021/ma902282c.

26. Dei, S., Matsumoto, A., and Matsumoto, A. (2008)Macromolecules, 41:2467–2473. doi:10.1021/ma702128a

27. Phollookin, C., Wacharasindhu, S., Ajavakom, A.,Tumcharern, G., Ampornpun, S., Eaidkong, T., andSukwattanasinitt, M. (2010) Macromolecules,43:7540–7548. doi:10.1021/ma101264k

28. Wang, X., Sandman, D.J., Chen, S., and Gido, S.P. (2008)Macromolecules, 41:773–778. doi:10.1021/ma702128a

29. Potisatityuenyong, A., Rojanathanes, R.,Tumcharern, G., and Sukwattanasinitt, M. (2008)Langmuir : the ACS Journal of Surfaces and Colloids,24:4461–4463. doi:10.1021/la800354q

30. Ahn, D.J., Chae, E., Lee, G.S., Shim, H., Chang, T., Ahn, K.,and Kim, J.M. (2003) Journal of the American ChemicalSociety, 125:8976–8977. doi:10.1021/ja0263137

31. Kim, J.M., Lee, J., Choi, H., Sohn, D., and Ahn, D.J. (2005)Macromolecules, 38:9366–9376. doi:10.1021/ma051551i

32. Park, D.H., Hong, J., Park, I.S., Lee, C.W., and Kim, J.M.(2014) Advanced Functional Materials, 24:5186–5193.doi:10.1002/adfm.v24.33

33. Nallicheri, R.A., and Rubner, M.F. (1991)Macromolecules,24:517–525. doi:10.1021/ma00002a027

34. Carpick, R.W., Sasaki, D.Y., and Burns, A.R. (2000)Langmuir : the ACS Journal of Surfaces and Colloids,16:1270–1278. doi:10.1021/la990706a

35. Jiang, H., Wang, Y., Ye, Q., Zou, G., Su, W., andZhang, Q. (2010) Sensors and Actuators. B, Chemical,143:789–794. doi:10.1016/j.snb.2009.09.063

36. Yoon, J., Chae, S.K., and Kim, J.M. (2007) Journal ofthe American Chemical Society, 129:3038–3039.

37. Cheng, Q., Stevens, R.C., Cheng, B.Q., and Stevens, R.C. (1997) Advanced Materials, 9:481–483. doi:10.1002/(ISSN)1521-4095

38. Peng, H., Sun, X., Cai, F., Chen, X., Zhu, Y., Liao, G.,Chen, D., Li, Q., Lu, Y., Zhu, Y., and Jia, Q. (2009)Nature Nanotechnology, 4:738–741. doi:10.1038/nnano.2009.264

39. Liang, J., Huang, L., Li, N., Huang, Y., Wu, Y., Fang, S.,Oh, J., Kozlov, M., Ma, Y., Li, F., Baughman, R., andChen, Y. (2012) ACS Nano, 6:4508–4519. doi:10.1021/nn3006812

40. Su, Y.L., Li, J.R., Jiang, L., and Cao, J. (2005) Journal ofColloid and Interface Science, 284:114–119.doi:10.1016/j.jcis.2004.10.003

41. Reichert, A., Nagy, J.O., Spevak, W., and Charych, D.(1995) Journal of the American Chemical Society,117:829–830. doi:10.1021/ja00107a032

42. Charych, D., and Cheng, Q. (1996) Chemical Biology,3:113–120. doi:10.1016/S1074-5521(96)90287-2

43. Kolusheva, S., Shahal, T., and Jelinek, R. (2000)Biochemistry, 39:15851–15859. doi:10.1021/bi000570b.

44. Pan, J.J., and Charych, D. (1997) Langmuir : the ACSJournal of Surfaces and Colloids, 13:1365–1367.doi:10.1021/la9602675.

45. Gaboriaud, F., Golan, R., Volinsky, R., Berman, A., andJelinek, R. (2002) Langmuir : the ACS Journal ofSurfaces and Colloids, 17:3651–3657. doi:10.1021/la0012790

46. Volinsky, R., Gaboriaud, F., Berman, A., and Jelinek, R.(2002) The Journal of Physical Chemistry. B,106:9231–9236. doi:10.1021/jp020393j

SOFT MATERIALS 15

47. Carmona, L.A., Romero, M.T.M., Casares, J.J.G.,Morales, M.P., and Camacho, L. (2013) The Journalof Physical Chemistry. C, 117:21838−21848.

48. Carmona, L.A., Payá, C.R., Espejo, G.G., Romero, M.T.M., Casares, J.J.G., and Camacho, L. (2015) Langmuir :the ACS Journal of Surfaces and Colloids, 31:5333–5344.doi:10.1021/acs.langmuir.5b00175

49. Carmona, L.A., Romero, M.T.M., Casares, J.J.G., andCamacho, L. (2015) Journal of Colloid and InterfaceScience, 459:53–62. doi:10.1016/j.jcis.2015.08.003

50. Jiang, H., and Jelinek, R. (2015) Langmuir : the ACSJournal of Surfaces and Colloids, 31:5843–5850.doi:10.1021/acs.langmuir.5b01697

51. Vollhardt, D., Liu, F., and Rudert, R. (2005) TheJournal of Physical Chemistry. B, 109:17635–17643.doi:10.1021/jp0519870

52. Casares, J.J.G., Miguel, G.D., Morales, M.P.,Romero, M.T.M., Camacho, L., and Muñoz, E. (2009)The Journal of Physical Chemistry. C, 113:5711–5720.doi:10.1021/jp810935x

53. Delgado, A.M.G., Payá, C.R., Carmona, C.R.,Casares, J.J.G., Morales, M.P., Muñoz, E., Romero, M.T.M., Camacho, L., and Brezesinski, G. (2010) TheJournal of Physical Chemistry. C, 114:16685–16695.doi:10.1021/jp1076577

54. Delgado, A.M.G., Casares, J.J.G., Payá, C.R.,Morales, M.P., Romero, M.T.M., Brezesinski, G., andCamacho, L. (2011) The Journal of Physical Chemistry.C, 115:9059–9067. doi:10.1021/jp200769g

55. Carmona, L.A., Romero, M.T.M., Casares, J.J.G.,Morales, M.P., and Camacho, L. (2013) The Journal ofPhysical Chemistry. C, 117:21838–21848. doi:10.1021/jp406397q

56. Wang, S., Ramirez, J., Chen, Y., Wang, P.G., andLeblanc, R.M. (1999) Langmuir : the ACS Journal ofSurfaces and Colloids, 15:5623–5629. doi:10.1021/la990206h

57. Wang, S., Li, Y., Shao, L., Ramirez, J., Wang, P.G., andLeblanc, R.M. (1997) Langmuir : the ACS Journal ofSurfaces and Colloids, 13:1677–1681. doi:10.1021/la9606411

58. Han, Z.X., Zhang, X.B., Li, Z., Gong, Y.J., Wu, X.Y.,Jin, Z., He, C.M., Jian, L.X., Zhang, J., Shen, G.L., andYu, R.Q. (2010) Analytical Chemistry, 82:3108–3113.doi:10.1021/ac100376a.

59. Wegner, G. (1972) Die Makromolekulare Chemie,154:35–48. doi:10.1002/macp.1972.021540103.

60. Ras, R.H.A., Nemeth, J., Johnston, C.T., Dimasi, E.,Dekany, I., and Schoonheydt, R.A. (2004) PhysicalChemistry Chemical Physics : PCCP, 6:4174–4184.doi:10.1039/B405862C

61. Vuorimaa, E., Ikonen, M., and Lemmetyinen, H.(1994) Chemical Physics, 188:289–302. doi:10.1016/0301-0104(94)00231-2

62. Gaines, G.L. (1966) Insoluble Monolayers at Liquid-GasInterfaces. New York: Interscience Publishers.

63. Sadrzadeh, N., Yu, H., and Zografi, G. (1998) Langmuir :the ACS Journal of Surfaces and Colloids, 14:151–156.doi:10.1021/la970956w.

64. Siiw, A. (1990) Acta Polymerica. Neue Bucher/NewBooks. 36:1990.

65. Barabási, A.L., and Stanley, H.E. (1995) Fractal Concepts inSurface Growth. Cambridge: Cambridge University Press.

66. Vicsek, T.S. (1992) Fractal Growth Phenomena.WORLD SCIENTIFIC. doi:10.1142/1407

67. Saito, Y. (1989) Physical Review A, 40(8):4716–4723.68. Roldán-Carmona, C., Giner-Casares, J.J., Pérez-

Morales, M., Martín-Romero, M.T., and Camacho, L.(2012) Advances in Colloid and Interface Science,173:12–22. doi:10.1016/j.cis.2012.02.002

69. Sasaki, D.Y., Carpick, R.W., and Burns, A.R. (2000)Journal of Colloid and Interface Science, 229:490–496.doi:10.1006/jcis.2000.7043

70. Biesalski, M.A., Knaebel, A., Tu, R., and Tirrell, M.(2006) Biomaterials, 27:1259–1269. doi:10.1016/j.biomaterials.2005.08.002

71. Hussain, S.A., Chakraborty, S., Bhattacharjee, D., andSchoonheydt, R.A. (2010) Acta. Part A, Molecular andBiomolecular Spectroscopy, 75:664–670. doi:10.1016/j.saa.2009.11.037.

72. Lifshitz, Y., Golan, Y., Konovalov, O., and Berman, A.(2009) Langmuir : the ACS Journal of Surfaces andColloids, 25:4469–4477. doi:10.1021/la8029038

73. Cho, E., and Jung, S. (2018) Molecules, 23:107.doi:10.3390/molecules23010107

74. Hussain, S.A., and Schoonheydt, R.A. (2010) Langmuir :the ACS Journal of Surfaces and Colloids, 26:11870–11877.doi:10.1021/la101078f.

75. Ahn, D.J., and Kim, J.M. (2008) Accounts of ChemicalResearch, 41:805–816. doi:10.1021/ar7002489

16 S. SUKLABAIDYA ET AL.