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Colloids and Surfaces A: Physicochem. Eng. Aspects 354 (2010) 51–55
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Organization and properties of a novel amphiphilic crown-ether dye in monolayers at the air/water interface S.Yu. Zaitsev a,b , A.A. Turshatov a,b , D. Möbius a,∗ , S.P. Gromov c , M.V. Alfimov c a b c
Department of NanoBiophotonics, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37070 Göttingen, Germany Moscow State Academy of Veterinary Medicine and Biotechnology, Acad. Skryabina Str. 23, Moscow 109472, Russia Photochemistry Center, Russian Academy of Sciences, Novatorov Str. 7a, Moscow 119421, Russia
a r t i c l e
i n f o
Article history: Received 31 July 2009 Received in revised form 22 September 2009 Accepted 26 September 2009 Available online 2 October 2009 Keywords: Amphiphilic dye Monolayers Benzodiaza-15-crown-5 Isotherms Reflection spectra
a b s t r a c t The novel amphiphilic benzodiaza-15-crown-5-styryl dye (BI68) forms stable insoluble monolayers at the air/water interface (collapse pressure 47 mN/m) and on aqueous subphases containing alkali metal or heavy metal salts (collapse pressures in the range of 38–45 mN/m, respectively). The organization of the dye monolayer depends on chromophore association and interactions with salts in the subphase as observed by measuring surface pressure–area and surface potential–area isotherms, reflection spectra and by Brewster angle microscopy observations. Immiscibility was found in two-component monolayers of BI68 and random copolymers of styrene with methacrylic acid and with N-vinylpyrrolidone, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ultrathin films (monolayers, bi- and multilayers) of various crown ethers have been investigated intensely in recent years because they can serve as simple and efficient models for molecular recognition and interaction phenomena at interfaces (for example, ion recognition and interaction in biological membranes [1]). In some papers they were considered as interesting nanomaterials and as sensoring components of various devices. Moreover, modification of the crown ethers with photosensitive groups leads to multifunctional derivatives, which are particularly useful for the construction of various supramolecular systems of fundamental and applied importance. After our first publication devoted to the investigation of monolayers of amphiphilic derivatives of dibenzo-18-crown-6 [2], the literature in this field has been summarized in the well-known review of Lednev and Petty [3]. Our more recent studies [4–8] on monolayers and Langmuir–Blodgett (LB) films of a series of amphiphilic crown-ether styryl dyes with variable size of the polyether ring and substituents in the chromophoric part demon-
∗ Corresponding author. E-mail addresses: [email protected] (S.Yu. Zaitsev), [email protected] (D. Möbius). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.09.050
strated the possibility of preparing photosensitive films with the capability to selectively bind particular alkali and heavy metal cations. The aim of this work is the investigation of surface-active, ionselective and photosensitive properties of the novel amphiphilic benzodiaza-15-crown-5-styryl dye (BI68) in monolayers at the air/water interface. 2. Experimental The novel amphiphilic benzodiaza-15-crown-5-styryl dye BI68 (see scheme below) was obtained by the condensation of the heterocyclic salt containing the long alkyl substituent with formyl derivative of benzoazacrown ether in boiling acetonitrile for 5 h. The isolation procedure of BI68 will be published together with other details of the synthesis and complete list of all of the synthesised analogs (derivatives) in a specialized Russian journal within a few years. Starting reagents were prepared as described earlier [9,10]. The dye was obtained in 15% yield, m.p. 112–115 ◦ C. 1 H NMR (Bruker, 500 Hz, DMSO-d6 , 25 ◦ C) ı: 0.89 (t, 3H, CH3 , J = 6.7), 1.28 (m, 26H, 13CH2 ), 1.42 (br. s, 4H, 2CH2 ), 1.48 (t, 6H, 2CH3 , J = 6.7), 2.51 (m, 12H, 6CH2 ), 3.65 (m, 8H, 4CH2 ), 4.32 (s, 4H, 2CH2 ), 4.36 (s, 4H, 2CH2 ), 4.96 (dd, 2H, CH2 N, J = 7.2), 7.25 (m, 1H, H-5 ), 7.74 (m, 2H, H-2 , H-6 ), 7.79 (t, 2H, H-6, J = 7.7), 7.88 (t, 1H, H-5, J = 7.9), 8.18
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(d, H-a, J = 15.7), 8.30 (d, 1H, H-7, J = 8.5), 8.41 (d, 1H, H-b, J = 15.7), 8.46 (d, 1H, H-4, J = 8.0), 10.36 (s, 1H, NH). Anal. Calcd. for C44 H69 ClN4 O8 S·H2 O: C, 60.91; H, 8.25; N, 6.46. Found: C, 60.72; H, 8.18; N, 6.39. Styrene, methacrylic acid and N-vinylpyrrolidone were purified by vacuum distillation. The random copolymers styrene-methacrylic acid (St/MA), containing 34 mol% MA and styrene-N-vinylpyrrolidone (St/VPD) containing 41 mol% VPD were synthesised by radical copolymerisation in the following conditions: [AIBN] = 0.03 mol/l, T = 60 ◦ C, and 7% conversion. The percent compositions of St/MA copolymers were determined by conductometric titration of acidic groups and of St/VPD by elementary analysis. Molecular weights Mn (St/MA) = 8.17 × 104 and Mn (St/VPD) = 6.99 × 104 were
Fig. 1. Surface pressure–molecular area (a) and surface potential–molecular area isotherms (b) of monolayers of the crown-ether dye BI68 on water (1) and 1 mM solutions of KClO4 (2), Cu(ClO4 )2 (3), and AgClO4 (4).
Fig. 2. Normalized reflection spectra of monolayers of the crown-ether dye BI68 at different surface pressures and subphases. Subphase: (a) water at: (1) 5 mN m−1 , (2) 15 mN m−1 , and (3) 30 mN m−1 ; (b) 1 mM KClO4 at: (1) 5 mN m−1 , (2) 15 mN m−1 , and (3) 30 mN m−1 ; (c) 1 mM AgClO4 at: (1) 5 mN m−1 , (2) 15 mN m−1 , and (3) 30 mN m−1 .
S.Yu. Zaitsev et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 354 (2010) 51–55
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Fig. 3. BAM images of monolayers of BI68, of two-component BI68/St-VPD (20 mol% BI68) and of two-component BI68/St-MA (20 mol% BI68) on water: (a, c, and e) and on 1 mM solution of Cu(ClO4 )2 : (b, d, and f); surface pressure 10 mN/m, image size 5 mm × 6.5 mm.
determined by osmometric measurements in dioxane, at T = 30 ◦ C. The salts KClO4 (99+%), AgClO4 (99.9%), Hg(ClO4 )2 (98%) and Pb(ClO4 )2 (99.995%) were purchased from Aldrich. The water was distilled and cleaned with a Milli-Q filtration unit of Millipore Corp. Surface pressure ()–molecular area (A) and surface potential (V)–molecular area (A) isotherms of the dye monolayers were recorded on a rectangular trough (dimensions 11.0 cm × 38.0 cm × 0.8 cm) made from polytetrafluoroethylene provided with a 2.0 cm wide filter paper Wilhelmy balance and vibrating plate condenser (with Pt plate, 1.5 cm diameter, operating at 400 Hz). The 1.0 mM dye solution (35 l) was spread onto water or various 1.0 mM aqueous salt solutions (20 ◦ C), and the monolayers were compressed by moving the barrier with a constant speed of about 16 cm2 /min. Monolayers of the dye BI68 were characterized by Brewster angle microscopy (BAM) using a MiniBAM of Nanofilm Technologie NFT, Göttingen, Germany. The wavelength of the laser diode emission of this instrument was 660 nm. Reflection spectra of monolayers at constant surface pressure were measured under nor-
mal incidence of light with a modified spectrometer of the type described earlier [11]. The difference R of the reflectivities of the monolayer-covered surface and the bare surface is normalized with respect to the surface density according to Rnorm = R·A, and this normalized reflectivity difference is plotted in the figures. 3. Results and discussion The novel crown-ether dye BI68 forms stable monolayers on water (Fig. 1a, curve 1) with a collapse pressure of 47.0 mN/m and area at collapse of 0.22 nm2 /molecule as well as on various salt solutions, for example on 1.0 mM solutions of KClO4 (Fig. 1a, curve 2), collapse pressures 45.0 mN/m and area 0.18 nm2 /molecule, or of Cu(ClO4 )2 (Fig. 1a, curve 3), and AgClO4 (Fig. 1a, curve 4), with the same collapse pressure 38.0 mN/m and area at collapse of 0.22 nm2 /molecule. Similar isotherms were found in the case of Hg2+ (as an example of other heavy metal cations) or Ca2+ (as an example of the alkali-earth metal cations), data not shown here. The area per molecule is larger in the whole surface pressure range on water than in the presence of salts in the aqueous subphase at a
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concentration of 1 mM. This is evidence of different molecular organizations of the dye in the presence of salts in the aqueous subphase as compared to water. The surface potential on water (Fig. 1b, curve 1) has a value of V = 240 mV at an area of A = 1.2 nm2 mainly due to the positive charge of the benzothiazolium ring. The surface potential increases upon compression and remains approximately constant in the range 0.7 nm2 > A > 0.3 nm2 . When V remains constant during compression, the apparent molecular dipole moment (including the contribution of the diffuse double layer) app = V·A decreases. A possible reason is molecular reorientation. Around A = 0.25 nm2 , V rises steeply probably due to collapse. The surface potential isotherms on 1 mM solutions of KClO4 , Cu(ClO4 )2 , and AgClO4 (Fig. 1b, curves 2–4, respectively) are similar to each other and differ considerably from that on water. In these three cases, V ≈ 0 V at A = 0.8 nm2 and the surface potential is smaller in the whole range of area until approaching collapse around A = 0.25 nm2 . There is a steep rise between A = 0.6 nm2 and 0.5 nm2 followed by a weekly rising part of the isotherms. The condensing effect of the three salts on monolayers of BI68 may be caused by an interaction of the anion ClO4 − , present in the subphase at a concentration of 1 mM, with the positively charged benzothiazolium ring. Since this anion is not hydrated it may form an ion pair with the positive charge thereby reducing the surface potential as well as the repulsion between the amphiphilic molecules. On water, BI68 may dissociate with the perchlorate ion as part of the diffuse double layer. An interaction of the different cations with the amphiphilic dye is not very probable since the macrocycle has two–NCH3 groups that may even be protonated at pH ≈ 5.4 of the aqueous subphase. The situation is different from that of a similar dye with a 15-crown-5 macrocycle [7]. Important information on chromophore association and orientation as well as interactions of the dye with ions in the aqueous subphase is gained by measuring reflection spectra of the dye monolayers on distilled water and the various salt solutions. In all cases a strong band is observed in the range of 400–550 nm (Fig. 2). The reflection spectra in the case of distilled water subphase (Fig. 2a) are dominated by a relatively intense band with maximum at 462 nm that may be attributed to the monomer transform of the dye. By deconvolution (assuming Gaussian shape of the bands) a new sharp band of low intensity with maximum at 509 nm is found. This may be attributed to an aggregate of the dye. The low intensity reflection band with maximum of about 330–350 nm is attributed to the monomer cis-form of dye. Upon compression of the monolayer from 5 mN/m to 15 mN/m (curve 2) and 30 mN/m (curve 3), the normalized reflection intensity decreases indicating an increase of the chromophore tilt. In the case of 1 mM aqueous solutions of KClO4 (Fig. 2b), there is a relatively high intensity reflection band with maximum of 442–444 nm and a weak long wavelength shoulder. The intensities at the maximum are somewhat smaller than on pure water, indicating a stronger tilt of the chromophores at the corresponding surface pressures. Almost the same spectral properties are found for the dye in the case of 1 mM aqueous solutions of AgClO4 (Fig. 2c). In summary, the organization of BI68 monolayers in the presence of salts differs from that on pure water. A similar topography of BI68 in monolayers at = 10 mN/m on water and on 1 mM Cu(ClO4 )2 , is found by BAM (compare Fig. 3a and b). On water small bright domains surrounded by a darker homogeneous phase are just visible (Fig. 3a). The nature of the corresponding phase transition is not clear since a nearly horizontal part that is typical for a LE-LC transition is missing in the –A isotherm. The BAM image of the monolayer on Cu(ClO4 )2 (Fig. 3b) differs from that on water slightly, mainly by an increase of the area fraction of bright domains. This may be due to higher surface density of the dye at 10 mN/m on Cu(ClO4 )2 as compared to water, see Fig. 1a.
In order to obtain homogeneous monolayers of BI68 in an appropriate matrix, two-component monolayers of the dye with the two different random copolymers styrene/acrylic acid and styrene/Nvinylpyrrolidone, respectively, were prepared and investigated. The styrene/acrylic acid copolymer, due to the presence of carboxylate groups, may separate the BI68 molecules and prevent the formation of bright domains in contrast to the neutral copolymer styrene/N-vinylpyrrolidone. Surface pressure–area isotherms of the two-component monolayers with different molar fractions of BI68 are shown in Fig. 4a for styrene/acrylic acid and 4b for styrene/N-vinylpyrrolidone. The molar fractions of BI68 based on the monomer subunit of the polymer are 0 (curves 1), 0.2 (curves 2), 0.333 (curves 3), 0.5 (curves 4), 0.667 (curves 5), and 1 (curves 6). The small area per polymer subunit in the absence of dye 1 may be due to coiling of the polymer. Plots of average area vs. molar fraction of the dye at different surface pressures (not shown here) reveal for both polymers a linear dependence that indicates either ideal mixing or phase separation of the two components (immiscibility). BAM images at the air/water interface at 10 mN/m of the two-component monolayers (Fig. 3c for St-VPD and Fig. 3f for St-MA) show a reduced density of bright domains, however, the domains may be attributed to the dye phase. It is surprising, that the positively charged BI68 molecules are not molecularly redistributed in the St-MA matrix via interaction with the carboxyl groups. On the
Fig. 4. Surface pressure–molecular area isotherms of two-component monolayers for various molar fractions of the dye BI68 (based on the monomer subunit of the copolymers) on water: BI68/St-VPD (a) and BI68/MA (b), molar fractions of the dye: 0 (curve 1), 0.2 (curve 2), 0.333 (curve 3), 0.5 (curve 4), 0.667 (curve 5) and 1 (curve 6).
S.Yu. Zaitsev et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 354 (2010) 51–55
surface of a 1 mM Cu(ClO4 )2 solution, the topographies of the pure dye monolayer and of the two-component monolayers (see Fig. 3b, d, and f) are very similar. This observation supports the interpretation of immiscibility in the two-component monolayers of dye 1 and the two polymers, respectively. 4. Conclusion The amphiphilic benzodiaza-15-crown-5-styryl dye BI68 forms stable insoluble monolayers at the air/water interface as well as on 1 mM aqueous solutions of various salts. The molecular organization, in particular tilting of the chromophores, depends on surface pressure and on interactions with the salts dissolved in the subphase. In two-component monolayers of the dye and two different polymers, phase separation of dye and matrix occurs. Acknowledgments Some parts of this work were supported by grants from RFBR, Russian Ministry of Science and Education and the Deutsche Forschungsgemeinschaft (436 RUS 113/686). The authors are grateful to Mrs. I.E. Baronova for sample preparation and technical assistance.
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References [1] J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995. [2] S.Yu. Zaitsev, V.V. Lutsenko, V.P. Zubov, Bioorganic Chemistry 9 (4) (1983) 567–569 (Russian). [3] I.K. Lednev, M.C. Petty, Adv. Mater. 8 (1996) 615. [4] S.Yu. Zaitsev, V.P. Vereschetin, S.P. Gromov, O.A. Fedorova, M.V. Alfimov, H. Huesmann, D. Möbius, Supramol. Sci. 4 (1997) 519. [5] S.Yu. Zaitsev, V.P. Vereschetin, E.A. Baryshnikova, S.P. Gromov, O.A. Fedorova, M.V. Alfimov, H. Huesmann, D. Möbius, Thin Solid Films 329 (1998) 821. [6] S.Yu. Zaitsev, E.A. Baryshnikova, T.I. Sergeeva, S.P. Gromov, O.A. Fedorova, O.V. Yescheulova, M.V. Alfimov, S. Hacke, W. Zeiss, D. Möbius, Colloids Surfaces 171 (2000) 283. [7] S.Yu. Zaitsev, E.A. Baryshnikova, T.I. Sergeeva, S.P. Gromov, O.A. Fedorova, O.V. Yescheulova, M.V. Alfimov, S. Hacke, W. Zeiss, D. Möbius, Thin Solid Films 372 (2000) 230. [8] S.Yu. Zaitsev, T.I. Sergeeva, E.A. Baryshnikova, S.P. Gromov, O.A. Fedorova, M.V. Alfimov, S. Hacke, D. Möbius, Colloids Surfaces A 198–200 (2002) 473. [9] O.A. Fedorova, Y.V. Fedorov, A.I. Vedernikov, O.V. Yescheulova, S.P. Gromov, M.V. Alfimov, L.G. Kuz’mina, A.V. Churakov, J.A.K. Howard, S.Yu. Zaitsev, T.I. Sergeeva, D. Möbius, New J. Chem. 26 (2002) 543. [10] O.A. Fedorova, A.I. Vedernikov, I.E. Baronova, O.V. Yescheulova, S.P. Gromov, M. Orrit, D. Möbius, U. Lehmann, H. Meyer, J. Chem. Phys. 85 (1986) 4966. [11] M. Orrit, D. Möbius, U. Lehmann, H. Meyer, J. Chem. Phys. 85 (1986) 4966.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Organization and properties of a novel amphiphilic crown-ether dye in monolayers at the air/water interface S.Yu. Zaitsev a,b , A.A. Turshatov a,b , D. Möbius a,∗ , S.P. Gromov c , M.V. Alfimov c a b c
Department of NanoBiophotonics, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37070 Göttingen, Germany Moscow State Academy of Veterinary Medicine and Biotechnology, Acad. Skryabina Str. 23, Moscow 109472, Russia Photochemistry Center, Russian Academy of Sciences, Novatorov Str. 7a, Moscow 119421, Russia
a r t i c l e
i n f o
Article history: Received 31 July 2009 Received in revised form 22 September 2009 Accepted 26 September 2009 Available online 2 October 2009 Keywords: Amphiphilic dye Monolayers Benzodiaza-15-crown-5 Isotherms Reflection spectra
a b s t r a c t The novel amphiphilic benzodiaza-15-crown-5-styryl dye (BI68) forms stable insoluble monolayers at the air/water interface (collapse pressure 47 mN/m) and on aqueous subphases containing alkali metal or heavy metal salts (collapse pressures in the range of 38–45 mN/m, respectively). The organization of the dye monolayer depends on chromophore association and interactions with salts in the subphase as observed by measuring surface pressure–area and surface potential–area isotherms, reflection spectra and by Brewster angle microscopy observations. Immiscibility was found in two-component monolayers of BI68 and random copolymers of styrene with methacrylic acid and with N-vinylpyrrolidone, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ultrathin films (monolayers, bi- and multilayers) of various crown ethers have been investigated intensely in recent years because they can serve as simple and efficient models for molecular recognition and interaction phenomena at interfaces (for example, ion recognition and interaction in biological membranes [1]). In some papers they were considered as interesting nanomaterials and as sensoring components of various devices. Moreover, modification of the crown ethers with photosensitive groups leads to multifunctional derivatives, which are particularly useful for the construction of various supramolecular systems of fundamental and applied importance. After our first publication devoted to the investigation of monolayers of amphiphilic derivatives of dibenzo-18-crown-6 [2], the literature in this field has been summarized in the well-known review of Lednev and Petty [3]. Our more recent studies [4–8] on monolayers and Langmuir–Blodgett (LB) films of a series of amphiphilic crown-ether styryl dyes with variable size of the polyether ring and substituents in the chromophoric part demon-
∗ Corresponding author. E-mail addresses: [email protected] (S.Yu. Zaitsev), [email protected] (D. Möbius). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.09.050
strated the possibility of preparing photosensitive films with the capability to selectively bind particular alkali and heavy metal cations. The aim of this work is the investigation of surface-active, ionselective and photosensitive properties of the novel amphiphilic benzodiaza-15-crown-5-styryl dye (BI68) in monolayers at the air/water interface. 2. Experimental The novel amphiphilic benzodiaza-15-crown-5-styryl dye BI68 (see scheme below) was obtained by the condensation of the heterocyclic salt containing the long alkyl substituent with formyl derivative of benzoazacrown ether in boiling acetonitrile for 5 h. The isolation procedure of BI68 will be published together with other details of the synthesis and complete list of all of the synthesised analogs (derivatives) in a specialized Russian journal within a few years. Starting reagents were prepared as described earlier [9,10]. The dye was obtained in 15% yield, m.p. 112–115 ◦ C. 1 H NMR (Bruker, 500 Hz, DMSO-d6 , 25 ◦ C) ı: 0.89 (t, 3H, CH3 , J = 6.7), 1.28 (m, 26H, 13CH2 ), 1.42 (br. s, 4H, 2CH2 ), 1.48 (t, 6H, 2CH3 , J = 6.7), 2.51 (m, 12H, 6CH2 ), 3.65 (m, 8H, 4CH2 ), 4.32 (s, 4H, 2CH2 ), 4.36 (s, 4H, 2CH2 ), 4.96 (dd, 2H, CH2 N, J = 7.2), 7.25 (m, 1H, H-5 ), 7.74 (m, 2H, H-2 , H-6 ), 7.79 (t, 2H, H-6, J = 7.7), 7.88 (t, 1H, H-5, J = 7.9), 8.18
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(d, H-a, J = 15.7), 8.30 (d, 1H, H-7, J = 8.5), 8.41 (d, 1H, H-b, J = 15.7), 8.46 (d, 1H, H-4, J = 8.0), 10.36 (s, 1H, NH). Anal. Calcd. for C44 H69 ClN4 O8 S·H2 O: C, 60.91; H, 8.25; N, 6.46. Found: C, 60.72; H, 8.18; N, 6.39. Styrene, methacrylic acid and N-vinylpyrrolidone were purified by vacuum distillation. The random copolymers styrene-methacrylic acid (St/MA), containing 34 mol% MA and styrene-N-vinylpyrrolidone (St/VPD) containing 41 mol% VPD were synthesised by radical copolymerisation in the following conditions: [AIBN] = 0.03 mol/l, T = 60 ◦ C, and 7% conversion. The percent compositions of St/MA copolymers were determined by conductometric titration of acidic groups and of St/VPD by elementary analysis. Molecular weights Mn (St/MA) = 8.17 × 104 and Mn (St/VPD) = 6.99 × 104 were
Fig. 1. Surface pressure–molecular area (a) and surface potential–molecular area isotherms (b) of monolayers of the crown-ether dye BI68 on water (1) and 1 mM solutions of KClO4 (2), Cu(ClO4 )2 (3), and AgClO4 (4).
Fig. 2. Normalized reflection spectra of monolayers of the crown-ether dye BI68 at different surface pressures and subphases. Subphase: (a) water at: (1) 5 mN m−1 , (2) 15 mN m−1 , and (3) 30 mN m−1 ; (b) 1 mM KClO4 at: (1) 5 mN m−1 , (2) 15 mN m−1 , and (3) 30 mN m−1 ; (c) 1 mM AgClO4 at: (1) 5 mN m−1 , (2) 15 mN m−1 , and (3) 30 mN m−1 .
S.Yu. Zaitsev et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 354 (2010) 51–55
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Fig. 3. BAM images of monolayers of BI68, of two-component BI68/St-VPD (20 mol% BI68) and of two-component BI68/St-MA (20 mol% BI68) on water: (a, c, and e) and on 1 mM solution of Cu(ClO4 )2 : (b, d, and f); surface pressure 10 mN/m, image size 5 mm × 6.5 mm.
determined by osmometric measurements in dioxane, at T = 30 ◦ C. The salts KClO4 (99+%), AgClO4 (99.9%), Hg(ClO4 )2 (98%) and Pb(ClO4 )2 (99.995%) were purchased from Aldrich. The water was distilled and cleaned with a Milli-Q filtration unit of Millipore Corp. Surface pressure ()–molecular area (A) and surface potential (V)–molecular area (A) isotherms of the dye monolayers were recorded on a rectangular trough (dimensions 11.0 cm × 38.0 cm × 0.8 cm) made from polytetrafluoroethylene provided with a 2.0 cm wide filter paper Wilhelmy balance and vibrating plate condenser (with Pt plate, 1.5 cm diameter, operating at 400 Hz). The 1.0 mM dye solution (35 l) was spread onto water or various 1.0 mM aqueous salt solutions (20 ◦ C), and the monolayers were compressed by moving the barrier with a constant speed of about 16 cm2 /min. Monolayers of the dye BI68 were characterized by Brewster angle microscopy (BAM) using a MiniBAM of Nanofilm Technologie NFT, Göttingen, Germany. The wavelength of the laser diode emission of this instrument was 660 nm. Reflection spectra of monolayers at constant surface pressure were measured under nor-
mal incidence of light with a modified spectrometer of the type described earlier [11]. The difference R of the reflectivities of the monolayer-covered surface and the bare surface is normalized with respect to the surface density according to Rnorm = R·A, and this normalized reflectivity difference is plotted in the figures. 3. Results and discussion The novel crown-ether dye BI68 forms stable monolayers on water (Fig. 1a, curve 1) with a collapse pressure of 47.0 mN/m and area at collapse of 0.22 nm2 /molecule as well as on various salt solutions, for example on 1.0 mM solutions of KClO4 (Fig. 1a, curve 2), collapse pressures 45.0 mN/m and area 0.18 nm2 /molecule, or of Cu(ClO4 )2 (Fig. 1a, curve 3), and AgClO4 (Fig. 1a, curve 4), with the same collapse pressure 38.0 mN/m and area at collapse of 0.22 nm2 /molecule. Similar isotherms were found in the case of Hg2+ (as an example of other heavy metal cations) or Ca2+ (as an example of the alkali-earth metal cations), data not shown here. The area per molecule is larger in the whole surface pressure range on water than in the presence of salts in the aqueous subphase at a
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concentration of 1 mM. This is evidence of different molecular organizations of the dye in the presence of salts in the aqueous subphase as compared to water. The surface potential on water (Fig. 1b, curve 1) has a value of V = 240 mV at an area of A = 1.2 nm2 mainly due to the positive charge of the benzothiazolium ring. The surface potential increases upon compression and remains approximately constant in the range 0.7 nm2 > A > 0.3 nm2 . When V remains constant during compression, the apparent molecular dipole moment (including the contribution of the diffuse double layer) app = V·A decreases. A possible reason is molecular reorientation. Around A = 0.25 nm2 , V rises steeply probably due to collapse. The surface potential isotherms on 1 mM solutions of KClO4 , Cu(ClO4 )2 , and AgClO4 (Fig. 1b, curves 2–4, respectively) are similar to each other and differ considerably from that on water. In these three cases, V ≈ 0 V at A = 0.8 nm2 and the surface potential is smaller in the whole range of area until approaching collapse around A = 0.25 nm2 . There is a steep rise between A = 0.6 nm2 and 0.5 nm2 followed by a weekly rising part of the isotherms. The condensing effect of the three salts on monolayers of BI68 may be caused by an interaction of the anion ClO4 − , present in the subphase at a concentration of 1 mM, with the positively charged benzothiazolium ring. Since this anion is not hydrated it may form an ion pair with the positive charge thereby reducing the surface potential as well as the repulsion between the amphiphilic molecules. On water, BI68 may dissociate with the perchlorate ion as part of the diffuse double layer. An interaction of the different cations with the amphiphilic dye is not very probable since the macrocycle has two–NCH3 groups that may even be protonated at pH ≈ 5.4 of the aqueous subphase. The situation is different from that of a similar dye with a 15-crown-5 macrocycle [7]. Important information on chromophore association and orientation as well as interactions of the dye with ions in the aqueous subphase is gained by measuring reflection spectra of the dye monolayers on distilled water and the various salt solutions. In all cases a strong band is observed in the range of 400–550 nm (Fig. 2). The reflection spectra in the case of distilled water subphase (Fig. 2a) are dominated by a relatively intense band with maximum at 462 nm that may be attributed to the monomer transform of the dye. By deconvolution (assuming Gaussian shape of the bands) a new sharp band of low intensity with maximum at 509 nm is found. This may be attributed to an aggregate of the dye. The low intensity reflection band with maximum of about 330–350 nm is attributed to the monomer cis-form of dye. Upon compression of the monolayer from 5 mN/m to 15 mN/m (curve 2) and 30 mN/m (curve 3), the normalized reflection intensity decreases indicating an increase of the chromophore tilt. In the case of 1 mM aqueous solutions of KClO4 (Fig. 2b), there is a relatively high intensity reflection band with maximum of 442–444 nm and a weak long wavelength shoulder. The intensities at the maximum are somewhat smaller than on pure water, indicating a stronger tilt of the chromophores at the corresponding surface pressures. Almost the same spectral properties are found for the dye in the case of 1 mM aqueous solutions of AgClO4 (Fig. 2c). In summary, the organization of BI68 monolayers in the presence of salts differs from that on pure water. A similar topography of BI68 in monolayers at = 10 mN/m on water and on 1 mM Cu(ClO4 )2 , is found by BAM (compare Fig. 3a and b). On water small bright domains surrounded by a darker homogeneous phase are just visible (Fig. 3a). The nature of the corresponding phase transition is not clear since a nearly horizontal part that is typical for a LE-LC transition is missing in the –A isotherm. The BAM image of the monolayer on Cu(ClO4 )2 (Fig. 3b) differs from that on water slightly, mainly by an increase of the area fraction of bright domains. This may be due to higher surface density of the dye at 10 mN/m on Cu(ClO4 )2 as compared to water, see Fig. 1a.
In order to obtain homogeneous monolayers of BI68 in an appropriate matrix, two-component monolayers of the dye with the two different random copolymers styrene/acrylic acid and styrene/Nvinylpyrrolidone, respectively, were prepared and investigated. The styrene/acrylic acid copolymer, due to the presence of carboxylate groups, may separate the BI68 molecules and prevent the formation of bright domains in contrast to the neutral copolymer styrene/N-vinylpyrrolidone. Surface pressure–area isotherms of the two-component monolayers with different molar fractions of BI68 are shown in Fig. 4a for styrene/acrylic acid and 4b for styrene/N-vinylpyrrolidone. The molar fractions of BI68 based on the monomer subunit of the polymer are 0 (curves 1), 0.2 (curves 2), 0.333 (curves 3), 0.5 (curves 4), 0.667 (curves 5), and 1 (curves 6). The small area per polymer subunit in the absence of dye 1 may be due to coiling of the polymer. Plots of average area vs. molar fraction of the dye at different surface pressures (not shown here) reveal for both polymers a linear dependence that indicates either ideal mixing or phase separation of the two components (immiscibility). BAM images at the air/water interface at 10 mN/m of the two-component monolayers (Fig. 3c for St-VPD and Fig. 3f for St-MA) show a reduced density of bright domains, however, the domains may be attributed to the dye phase. It is surprising, that the positively charged BI68 molecules are not molecularly redistributed in the St-MA matrix via interaction with the carboxyl groups. On the
Fig. 4. Surface pressure–molecular area isotherms of two-component monolayers for various molar fractions of the dye BI68 (based on the monomer subunit of the copolymers) on water: BI68/St-VPD (a) and BI68/MA (b), molar fractions of the dye: 0 (curve 1), 0.2 (curve 2), 0.333 (curve 3), 0.5 (curve 4), 0.667 (curve 5) and 1 (curve 6).
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surface of a 1 mM Cu(ClO4 )2 solution, the topographies of the pure dye monolayer and of the two-component monolayers (see Fig. 3b, d, and f) are very similar. This observation supports the interpretation of immiscibility in the two-component monolayers of dye 1 and the two polymers, respectively. 4. Conclusion The amphiphilic benzodiaza-15-crown-5-styryl dye BI68 forms stable insoluble monolayers at the air/water interface as well as on 1 mM aqueous solutions of various salts. The molecular organization, in particular tilting of the chromophores, depends on surface pressure and on interactions with the salts dissolved in the subphase. In two-component monolayers of the dye and two different polymers, phase separation of dye and matrix occurs. Acknowledgments Some parts of this work were supported by grants from RFBR, Russian Ministry of Science and Education and the Deutsche Forschungsgemeinschaft (436 RUS 113/686). The authors are grateful to Mrs. I.E. Baronova for sample preparation and technical assistance.
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