Adsorption of Thiacyanine Dyes on Silver Halide Nanoparticles: Study

Silver halide particles were synthesized in a water-in-oil microemulsion consisting of AOT (sodium bis-2ethylhexylsulfosuccinate)/n-heptane/water. Dif...
0 downloads 0 Views 260KB Size
Download PDF
Langmuir 2000, 16, 597-606

597

Adsorption of Thiacyanine Dyes on Silver Halide Nanoparticles: Study of the Adsorption Site L. Jeunieau,* V. Alin, and J. B. Nagy Laboratoire de Re´ sonance Magne´ tique Nucle´ aire, Faculte´ s Universitaires Notre-Dame de la Paix, 61, rue de Bruxelles, 5000 Namur, Belgique Received February 4, 1999. In Final Form: July 13, 1999 Silver halide particles were synthesized in a water-in-oil microemulsion consisting of AOT (sodium bis-2ethylhexylsulfosuccinate)/n-heptane/water. Different molecules of thiacyanines were adsorbed on these particles. These molecules can be adsorbed on the particles by two main sites of adsorption: either by an interaction between the sulfur atom and the silver ion or by a Coulombic interaction between the nitrogen atom and the halide ion. To distinguish the two adsorption sites, particles with different excesses of charge have been used. It has been concluded that the adsorption site depends on the structure of the molecules: the sulfur is more reactive in the presence of an electron donor moiety and more for the thiacarbocyanine than for the thiacyanine. The adsorption between the sulfur and the silver ion is also more favorable on the silver bromide particles than on the silver chloride particles.

1. Introduction Cyanine dyes are currently used in the photographic industry as spectral sensitizers. Indeed, while photographic silver halide grains have an intrinsic absorption in the blue region, the sensitivity can be expanded to the entire visible region by adding a spectral sensitizer. In fact, an electron is transferred from the dye molecules to the silver halide particles.1 These dyes can form J-aggregates characterized by a bathochromic shift with respect to the absorption band of the adsorbed monomer. The dye molecules are adsorbed in a flat geometry in the monomer form,2 with a maximum interaction between the dye and the silver halide surface. By increasing the dye concentration, the dye molecules interact with each other and adopt a conformation perpendicular to the surface in order to have a maximum interaction between the dye molecules and especially their π electrons. Despite the great utility of such a system, the site of adsorption of the thiacarbocyanine is not well established. If we look at the general structure of a thiacarbocyanine dye, two types of adsorption are possible: the first occurs by a Coulombic interaction between the nitrogen atom and the halide ion; and the second involves the interaction between the sulfur atom and the silver ion. As in many cases the group W in Figure 1 is an alkylsulfonate, a third interaction is possible between the alkylsulfonate and the silver ion. The adsorption is generally attributed to a Coulombic interaction between the nitrogen atom and the halide ion. This originates from the fact that the adsorption is reversible.3 Indeed, this interaction is of a physisorption type. Furthermore, measurements of ionic conductivity have shown that the conductivity increases with the adsorption of thiacarbocyanine on AgBr grains.4,5 This has been interpreted by the repulsion of silver ions at the (1) Tani, T. Photographic Sensitivity; Oxford Series on Optical and Imaging Science; Oxford University Press: New York, 1995; Chapter 5. (2) James, T. H. The Theory of the Photographic Process, 4th ed.; Macmillan Publishing Co.: New York, 1977; p 235. (3) Tani, T. J. Imaging Sci. 1985, 29, 165. (4) Takada, S.; Tani, T. J. Appl. Phys. 1974, 45, 4767. (5) Tani, T.; Takada, S. Photogr. Sci. Eng. 1974, 18, 620.

Figure 1. General structure of (n ) 0) thiacyanine and (n ) 1) thiacarbocyanine.

surface kink sites on the grain surface into an interstitial position. Concomitantly, the adsorption by the sulfonate moieties could be excluded as the variation of the number of sulfonate moieties does not lead to a variation of the ionic conductivity.6 Another argument in favor of the Coulombic interaction is the positive charge of the sulfur in a thiacyanine. The positive charge on the sulfur has been observed by theoretical calculation and by the study of another class of dye, the benzothiolinium:7 these dyes have a sulfur atom in the place of the nitrogen in the thiacarbocyanine. By adsorption of benzothiolinium on silver bromide the ionic conductivity is increased. It is thus considered that the sulfur bears a positive charge and repels the interstitial silver ions. A last argument in favor of the Coulombic interaction is the increase of the dye adsorption with the pAg, i.e., for a lower concentration in silver. But some dyes, such as 1,1′-diethyl-2,2′-carbocyanine iodide, are adsorbed at a high concentration in silver.8 This adsorption has been attributed to nonspecific van der Waals forces, which supplement the electrostatic forces. On the other hand, in favor of the interaction between the silver and the sulfur are the articles from Bird et al. who proposed a chemisorption between the silver and the sulfur.9,10 The geometric factors essentially determine the type of chemisorption. More recently, Kawasaki11 deter(6) Tani, T. J. Imaging Sci. 1990, 34, 143. (7) Tani, T.; Yamashita, H.; Tanaka, C.; Tanaka, J. J. Imaging Sci. Technol. 1992, 36, 124. (8) West, W.; Carroll, B. H.; Whitcomb, D. H. Chem. Phys. 1952, 56, 1054. (9) Mastropaolo, D.; Potenza, J.; Bird, G. R. Photogr. Sci. Eng. 1974, 18, 450. (10) Gray, W. E.; Brewer, W. R.; Bird, G. R. Photographic Sci. Eng. 1970, 14, 316.

10.1021/la990114s CCC: $19.00 © 2000 American Chemical Society Published on Web 10/16/1999

598

Langmuir, Vol. 16, No. 2, 2000

mined by XPS measurements, that the interaction between the 5,5′-dichloro-9-ethyl-thiacarbocyanine occurs by an interaction between the sulfur and the silver ion on silver bromide. The silver halide particles used are synthesized in a microemulsion medium12-14 (the AOT(sodium bis-2-ethylhexylsulfosuccinate)/n-heptane/water microemulsion) and have a size of 46 Å for the silver bromide particles and 32 Å for the silver chloride particles. The medium of adsorption is thus drastically different from the classical one: the particles have AOT adsorbed on their surface in place of gelatine and are surrounded by heptane and water in place of water. The different charge of the particles is controlled by using different excesses of salts: excess of silver ions in the case of positive particles and excess of halide ions in the case of negative particles. The chosen excess is 50% and corresponds approximately to the percentage of atom at the surface of the particles. The control of the charge of the particles is easier with these nanoparticles: the percentage of atom at the surface of the particles is greater (for example, for particles of 0.4 µm, the percentage of atom at the surface of the particles is approximately only 0.9%), and the particles are in a confined medium, so we can more surely express the hypothesis that the excess of salt is adsorbed on the surface of the particles. The purpose of the work is to study the site of adsorption of different thiacarbocyanine dyes. For this purpose different dye molecules with different structures have been used (Figure 1). The different dyes have a similar basic structure and differ only by one substituent. For example, dye 3 differs from dye 1 by the replacement of chlorine atoms by phenyl groups.

Jeunieau et al. The structures of the various dyes used are shown in Figure 2. For the dye adsorption, 4.3 × 10-2 mL of a 10-3 M solutions of the dye in methanol was added to 5 mL of the AOT solution in heptane. This mixture was finally added to 10 mL of the colloidal suspension of silver halide and stirred for 30 min. Time zero was taken at the end of the 30 min. The weight percent of the final microemulsion was AOT 7.16%, water 0.61%, heptane 91.93%, and methanol 0.30%. The number of dye molecule per particle on AgBr was equal to 21 and to 8 on AgCl. This number was calculated by using the number of dye molecules and the number of silver halide particles obtained by dividing the total weight of the silver halide by the weight of one particle (obtained by using the volume and the density of bulk silver halide). Most of the experiments for both silver halide particle preparation and dye adsorption were carried out at 20 °C; for the other experiments the temperature is indicated. A Uvikon 930 UV-spectrophotometer was used for measuring the UV-visible absorption spectra. The wavelength precision was 0.2 nm. The optical path was 1 cm. The reference in the absorbance measurements was a 0.12 M solution of AOT in heptane. The UV-visible spectra were decomposed using software developed locally. The spectral assignment was made as follows. First, a UV-visible spectrum of the dye in the same AOT/nheptane/water microemulsion was taken; the observed maximum of absorption corresponds to the monomer in solution. A small bathochromic shift is observed with respect to the dye in a diluted solution in methanol: e.g., for dye 1, the λmax is 603.2 nm in the microemulsion and λmax ) 578.8 nm in methanol. Second, a low concentration of the dye (10-4 M) was used in order to obtain the spectrum of the dye adsorbed in the monomeric form on the AgBr (or AgCl) particles. The wavelengths at the maximum of absorbance of the monomer in solution and of the adsorbed monomer are listed in Table 1. The size of the particles was determined from transmission electron micrographs taken on a Philips 301 TEM which were handled to obtain the particle size distribution using the appropriate software.

2. Experimental Procedures Silver halide particles were synthesized in microemulsion systems composed of AOT (sodium bis-2-ethylhexylsulfosuccinate)/n-heptane/water.15,16 The microemulsions were prepared from n-heptane (Aldrich 99+% HPLC grade), AOT (Sigma 99%) and aqueous solutions of silver nitrate (Janssens Chimica) using bidistilled water and potassium bromide (or chloride) (Merck Uvasol spectroscopy). The concentrations of the solutions were 0.063 M for the silver nitrate and 0.064 M for the potassium bromide (or chloride). The microemulsions were prepared with a 0.12 M solution of AOT in n-heptane and the aqueous solutions were added in order to have a ratio w ) [H2O]/[AOT] of 3.1. A total of 5 mL of the microemulsion containing silver nitrate is poured into 5 mL of the microemulsion containing potassium bromide (or chloride). The diameter of the particles was equal to 46 Å for the silver bromide particles and 32 Å for the silver chloride particles. Particles with an excess of 50% in bromide, or in silver have also been synthesized, the corresponding sizes are 58 and 63 Å, respectively. For the particles of silver chloride, the size of the particles with an excess of 50% in chloride is 42 Å and that for the particles with an excess of 50% in silver is 65 Å. (11) Kawasaki, M. IS&T’s 50th Annual Conf.; Proceeding of the Imaging Science and Technology, Cambridge, 1997; p 102. (12) Monnoyer, Ph.; Fonseca, A.; B.Nagy, J. Colloid Surf. 1995, 100, 233. (13) B.Nagy, J.; Barette, D.; Fonseca, A.; Jeunieau, L.; Monnoyer, Ph.; Piedigrosso, P.; Ravet-Bodart, I.; Verfaillie, J.-P.; Wathelet, A. In Nanoparticles in Solids and Solutions; An Integrated Approach to their Preparation and Characterisation, Fendler, J., De´kany, I., Eds.; Kluwer: Dordrecht, The Netherlands, 1996; p 71. (14) Monnoyer, Ph.; B.Nagy, J.; Buschman, V.; Fonseca, A.; Jeunieau, L.; Piedigrosso, P.; Van Tendeloo, G. In Nanoparticles in Solids and Solutions; An Integrated Approach to their Preparation and Characterisation, Fendler, J., De´kany, I., Eds.; Kluwer: Dordrecht, The Nethrlands, 1996; p 505. (15) Cabos, C.; Delord, P. J. Appl. Cryst. 1979, 12, 502. (16) Rouvie`re, J.; Couret, J. M.; Lindheimer, M.; Dejardin, J. L.; Marrony, R. J. Chem. Phys. 1979, 76, 3.

3. Results and Discussion To obtain information on the adsorption sites of the dye molecules, particles with different surface charges were used. Three types of particles are specially investigated: the “neutral” particles, with the same quantity of bromide and silver, the “positive” particles with an excess of 50% in silver, and the “negative” particles with an excess of 50% in bromide. This excess value corresponds approximately to the percentage of atom at the surface. The comparison between the various results will allow us to distinguish between the different adsorption sites. Indeed, if the adsorption occurs by an interaction between the silver ion and the dye, the adsorption will be favored by a primary adsorption of an excess of silver; in the opposite case, if the adsorption occurs by an interaction between the dye and the halide ion, the adsorption will be favored by a primary adsorption of an excess of halide. A third possibility cannot be excluded, for which one should be aware of the fact that the dye in solution may be either in the trans or the cis isomeric form.17,18 If the dye is adsorbed in a cis form, the two adsorption sites would be in contact with the silver halide: a Coulombic interaction between the nitrogen and the halide ion and on the other side of the molecule an interaction between the sulfur and the silver ion. In this case the adsorption will be favored for the neutral particles. The distinction between an adsorption which occurs by an interaction between the alkylsulfonate and the silver ion and an interaction which occurs between the sulfur and the silver ion will be made by comparison between dyes 1 and 2. These two dye (17) West, W.; Pearce, S.; Grum, F. J. Phys. Chem. 1967, 71, 1316. (18) Noukakis, D.; Van der Auweraer, M.; Toppet, S.; De Schryver, F. C. J. Phys. Chem. 1995, 99, 11860.

Silver Halide Adsorption Site for Thiacyanine Dyes

Langmuir, Vol. 16, No. 2, 2000 599

Figure 2. Structures of the dyes used. Table 1. Maximum Absorbance of the Monomer in Solution and of the Adsorbed Monomer

dye 1 dye 2 dye 3 dye 4 dye 5

λmax of the monomer in solution (nm)

λmax of the adsorbed monomer (nm)

603.2 602.6 571.2 432.8 433.4

620.0 618.6 585.9 445.0 446.7

molecules have the same structure except for the presence of a propylsulfonate in the case of dye 1. 3.1. Interaction Between the Dye and the Salt in the Microemulsion Medium. To correlate the modification of the UV-visible spectra due to the surface charge with the site of adsorption, care has been taken to study the influence of the salt on the absorption spectrum in the microemulsion medium. In fact, it can be thought that the salt in excess adsorbs on the surface of the particles, but it cannot be totally excluded that a part of the salt remains in solution. Furthermore, this will give us an insight on the possible interaction between the sulfur and the salt. It is well known that the silver can form complexes with molecules containing sulfur.19-21 A UV-visible absorption spectrum has been taken of the dye in the (19) Laing, D. K.; Pettit, L. D. J. Chem. Soc., Dalton Trans. 1975, 2297. (20) Barnes, D.; Laye, P. G.; Pettit, L. D. J. Chem. Soc. (A) 1969, 2073. (21) Ahrland, S.; Chatt, J.; Davies, N. R.; Williams, A. A. J. Chem. Soc. 1958, 264.

microemulsion medium with a silver nitrate concentration corresponding to the excess of salt and this spectrum has been compared with the spectrum of the dye in the microemulsion and in the presence, in this case, of potassium bromide salt, to take into account an eventual salt effect. The dye absorbance decreases in the presence of the silver nitrate, indicating a complexation. In fact, if silver nitrate is added directly to the solution of dye 1 in methanol, the color changes from purple to yellow after 1 day. The color change stems from the appearance of the dye-Ag+ complex. Oppositely, no influence of the potassium bromide has been observed. Some dyes showed also a band corresponding to the J-aggregate; these dyes seem thus to have a greater interaction with the silver ion. The J-aggregates are observed in the case of dye 1 and dye 4. A small shoulder is also observed in the case of dye 2. The greater reactivity of sulfur of dye 1 with respect to dye 3 can stem from the mesomeric donor effect of the phenyl group compared to the electron withdrawing effect of chlorine. The electronic delocalization is thus greater in dye 1 and therefore the polarizalibity of the dye is greater and by consequence the sulfur is softer. As the interaction between sulfur and silver can be seen as a soft-soft interaction, this one is favored for dye 1. If we compare dye 1 and dye 2, we have to remember that the sulfur is positively charged in such molecules and that the positive charge increases in the presence of a positive nitrogen. If the sulfonate group of the propylsulfonate comes close to the nitrogen atom, the positive charge of the nitrogen is decreased and thus the sulfur becomes less positive. This can explain the greater

600

Langmuir, Vol. 16, No. 2, 2000

interaction between the sulfur and the silver ion in the case of dye 1. This effect can also be detected in the case of dyes 4 and 5. The propylsulfonate and the butylsulfonate do not adopt the same stable conformations. The propylsulfonate can adopt a conformation which brings the nitrogen and sulfur atoms nearer. However, due to steric considerations the butylsulfonate cannot adopt such a conformation and thus the positive charge on the nitrogen is less neutralized by the butylsulfonate than by the propylsulfonate groups. This allows us already to emphasize the great importance of the electron donating effect of phenyl groups by comparison with the neutralization effect of the propylsulfonate: dye 2 forms only a small amount of J-aggregate, while dye 3 does not show any J-aggregate formation. Another important conclusion is that the interaction between the silver ion and the dye does not occur by an interaction between the alkylsulfonate moieties and the silver ion. This has already been observed by comparing the coordinating capacity of sodium benzenesulfonate with that of sulfonated sulfides.20 In fact, the coordinating capacity of benzenesulfonate was negligible compared with that of the sulfonated sulfides, and it was thus assumed that complex formation takes place through the bivalent sulfur atom. Different factors allow us to assert this: first, a J-aggregate is observed in the case of dye 2, second, the influence of the phenyl (or chlorine) substituent is more important than the propylsulfonate moieties. This leads us to already exclude an adsorption by the alkylsulfonate on the silver halide particles; this interaction is not sufficiently important. 3.2. Adsorption on the Silver Halide Particles. The different dye molecules have been adsorbed on particles of silver bromide and silver chloride and on particles with different excesses of salt. All the dyes could be adsorbed on the silver halide nanoparticles and the different UVvisible absorption spectra are decomposed. Two sets of data are of particular interest: the variations of the absorbance of the monomer in solution and the variation of the maximum of absorbance of the J-aggregate, which is related to the size of the J-aggregate. The maximum of absorbance increases with increasing size of the Jaggregate. In fact, by comparing the absorption maximum of J-aggregates obtained under various conditions, it has been assumed that the packing of the dye molecules (slipping angle, intermolecular distance in the aggregates, the variation of which can lead to different maxima of absorbance22,23,24) does not depend on the time or the surface charge. This hypothesis could be incorrect when comparing the J-aggregates adsorbed on particles of different charges. Therefore the determination of the adsorption site is not carried out by using this factor alone. The variation of the absorbance of the J-aggregate is more complicated to interpret due to the sedimentation of the particles. In fact, by its adsorption the dye partially removes the surfactant from the surface of the particles, provoking the instability of the particles.25,26 The aggregation of the dye molecules in J-aggregates increases this phenomenon, the surfactant being removed from a large continuous surface of the particles. 3.2.1. Adsorption of Dye 1. 3.2.1.1. Adsorption on Particles of Silver Bromide. Figure 3 shows the variation of the absorbance of the monomer in solution and of the (22) Norland, K.; Ames, A.; Taylor, T. Phot. Sci. Eng. 1970, 14, 295. (23) Nolte, H. J. Chem. Phys. Lett. 1975, 31, 134. (24) Czikkely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (25) Jeunieau, L.; B.Nagy, J. Appl. Organomet. Chem. 1998, 12, 341. (26) Jeunieau, L.; B.Nagy, J. Colloids Surf. A 1999, 151, 419.

Jeunieau et al.

Figure 3. Variation of the absorbance of the monomer in solution and of the J-aggregate as a function of time and of the absorbance maximum of the J-aggregate as a function of time in the case of dye 1 adsorbed on AgBr particles.

J-aggregate and the variation of the maximum of absorbance of the J-aggregate as a function of time. The absorbance of the monomer in solution being the lowest in the case of the neutral particles, the amount of adsorbed dye is thus the highest on these particles. However, the absorbance of the J-aggregate decreases the fastest on these particles, due to the sedimentation of the particles. The high instability on the particles prevents the conclusion that the adsorption of the dye is more favored in the case of the neutral particles. In fact, the sedimentation of the particles can lead to a displacement of the equilibrium of adsorption. The reaction of adsorption can be written in this way

AgBr + Dye H Dyeadsorbed If the particles with dyes on their surface sediment out, the equilibrium of adsorption will be displaced to the right side, and more dye will be adsorbed on the particles. This lower stability of the particles can be explained by the stabilization of the particles by a charge effect in the case of the positive and negative particles. In fact, the nanoparticles can be stabilized in two ways: by adding a surfactant or a polymer which is adsorbed at the surface of the particles, or by a charge effect in which the particles

Silver Halide Adsorption Site for Thiacyanine Dyes

Langmuir, Vol. 16, No. 2, 2000 601

Figure 5. UV-visible absorption spectra of dye 2 adsorbed on silver bromide and on particles with an excess of 50% in silver or in bromide and spectrum of the dye in the microemulsion medium.

Figure 4. Variation of the absorbance maximum of the monomer in solution and of the J-aggregate as a function of time and variation of the maximum of absorbance of the J-aggregate as a function of time in the case of dye 1 adsorbed on particles of silver chloride.

of like charge will repel each other.27 In our case the surfactant stabilizes the particles, and in addition, in the presence of either an excess of silver or bromide the second stabilization factor takes place: the charge effect. The absorbance of the monomer in solution is of the same order in the case of the particles with an excess of bromide or silver. But the maximum absorbance of the J-aggregate, which is related to the size of the aggregate is the highest in the case of the positive particles. The interaction between the sulfur and the silver leads thus to the highest size of the J-aggregate. The smallest size is observed on the negative particles. Thus it is concluded that the adsorption is more favored in the case of the positive particles and the adsorption occurs predominantly by an interaction between the sulfur and the silver ion. 3.2.1.2. Adsorption on Particles of Silver Chloride. The absorbance of the monomer in solution is the lowest in the case of the particles with an excess of chloride (Figure 4). As the decrease of the absorbance of the J-aggregate is the greatest in this case, a sedimentation effect cannot be excluded. The size of the J-aggregate is also the highest on these particles and the smallest on the neutral particles. (27) Everett, D. H. Basic Principles of Colloid Science; Royal Society of Chemistry: Cambridge, 1988; p 45.

As the order of the size of the J-aggregates (related to the maximum of absorbance) is the same as the order given by the amount of adsorbed dye, it can be concluded that adsorption is favored in the case of the negative particles and the adsorption occurs essentially by a Coulombic interaction between the nitrogen and the chloride ion. However, the adsorption is not very disfavored in the case of the positive particles. The size of the J-aggregates (as the maximum of absorbance of the J-aggregate is a little smaller) is only a little smaller in this case (see below). The adsorption can thus occur as well by an interaction between the nitrogen and the chloride ion. It can be thought that adsorption on the neutral particles occurs at both sites. However, the adsorption of the dye in the form of a trans isomer can be excluded: the adsorption is not favored in the case of the neutral particles. 3.2.2. Adsorption of Dye 2. This dye molecule is strongly adsorbed on the silver halide particles, and a rapid sedimentation of the particles is observed. This higher adsorption can come from the lower solubility of this dye in water. It has been previously determined28 that the particles are surrounded by a layer of water. As a consequence the dye is more in contact with water than with heptane and it is its solubility in water which governs its adsorption and not its solubility in heptane. 3.2.2.1. Adsorption on Particles of Silver Bromide. From Figure 5, which shows the different UV-visible absorption spectra of dye 2, it can be clearly seen that the adsorption is favored in the case of the particles with an excess of bromide. So the main interaction must be a Coulombic interaction between the nitrogen and the bromide ion. Figure 6 shows the different absorption spectra with different percentages of excess in bromide. It can be seen that a 25% excess in bromide is enough to favor the adsorption by comparison with the particles of silver bromide. It must be emphasized that the adsorption is not totally impeded by an excess of silver (Figure 5). This shows again that the adsorption does not occur by a sole interaction between the propylsulfonate moieties and the silver ions. If it were the case, the adsorption would be totally impeded. 3.2.2.2. Adsorption on Particles of Silver Chloride. Figure 7 shows the UV-visible absorption spectra of dye 2 adsorbed on particles of silver chloride and on positive and negative particles. The adsorption is favored by an (28) Jeunieau, L.; B.Nagy, J. Langmuir, submitted.

602

Langmuir, Vol. 16, No. 2, 2000

Jeunieau et al.

Figure 6. UV-visible absorption spectra of dye 2 adsorbed on particles of silver bromide with different excesses in bromide.

Figure 8. Variation of the absorbance of the monomer in solution and of the maximum absorbance of the J-aggregate as a function of time in the case of dye 3 on particles of silver bromide.

Figure 7. UV-visible absorption spectra of dye 2 adsorbed on silver chloride and on particles with an excess of 50% in silver or in chloride.

excess of chloride: the J-aggregates are of greater size (the maximum of absorbance of the J-aggregate is greater). The adsorption occurs thus by an interaction between the nitrogen and the chloride ion. 3.2.3. Adsorption of Dye 3. 3.2.3.1. Adsorption on Particles of Silver Bromide. The absorbance of the monomer in solution is the lowest in the case of the positive particles and the highest in the case the neutral particles (Figure 8). The variation of the size of the J-aggregate follows the same sequence as the amount of dye adsorbed. The adsorption is thus favored in the case of the positive particles and occurs preferentially by an interaction between the silver ion and the sulfur atom. 3.2.3.2. Adsorption on Particles of Silver Chloride. Adsorption has also been carried out on particles of silver chloride. In this case, the adsorption is clearly disfavored by an excess of silver; no J-aggregates are observed on these particles except at the very beginning of the reaction (Figure 9). In fact, in this case the dye is desorbed from the particles, the absorbance of the monomer in solution increases, and by consequence the absorbance of the adsorbed monomer decreases. In the case of the neutral particles, the absorbance of the monomer in solution is slightly lower than in the case of the negative particles, but the size of the J-aggregate

is smaller (Figure 10). The small difference between the neutral particles and the negative particles can come from the streric effect of the propylsulfonate. In fact, the adsorption on the negative particles seems to be slower, at the beginning it is only favored after a lapse of time. For the neutral particles, it is possible that a small amount of molecules is adsorbed by an interaction between the sulfur atom and the silver ion and this permits a faster adsorption reaction. The adsorption in a cis isomer form, which is performed by both sites, can be excluded; in this case, the size of the J-aggregate would stay the highest. It can thus be concluded that the adsorption occurs by a Coulombic interaction between the nitrogen atom and the chloride ion. 3.2.4. Adsorption of Dye 4. 3.2.4.1. Adsorption on Particles of Silver Bromide. Figure 11 shows the variation of the absorbance of the monomer in solution and the variation of the maximum of absorbance of the J-aggregate as a function of time. The three curves of the variation of the absorbance of the monomer in solution show a desorption effect after a certain lapse of time. This can come from the sedimentation of the particles; in this case the particles sediment and the dye can be desorbed during the sedimentation. The desorption is the highest in the case of the negative particles. However, the decrease of the absorbance of the J-aggregate is not the fastest in this case, so the desorption cannot be totally explained by the sedimentation of the particles. The size of the J-aggregate is the highest in the case of the negative particles and the lowest for the neutral particles (the size follows the λmax values, as explained below). The adsorption is thus favored by an excess of bromide more than by an excess of silver. The adsorption seems thus to occur by an interaction between the nitrogen atom and the bromide ion. However, the interaction between the silver ion and the sulfur atom cannot be excluded, the adsorption on the positive particles being more favorable than for the neutral particles.

Silver Halide Adsorption Site for Thiacyanine Dyes

Figure 9. Variation of the absorbance of the monomer in solution, of the adsorbed monomer and of the J-aggregate as a function of time in the case of dye 3 adsorbed on particles of silver chloride.

Figure 10. Variation of the maximum of absorbance of the J-aggregate as a function of time in the case of dye 3 adsorbed on particles of AgCl.

3.2.4.2. Adsorption on Particles of Silver Chloride. The adsorption is clearly disfavored by an excess of silver in this case; the J-aggregates can only be observed on these particles after 400 h (Figure 12). The adsorption seems to be slightly favored by an excess of chloride; the size of

Langmuir, Vol. 16, No. 2, 2000 603

Figure 11. Variation of the absorbance of the monomer in solution and of the J-aggregate as a function of time and variation of the maximum of absorbance of the J-aggregate as a function of time in the case of dye 4 adsorbed on silver bromide particles.

the J-aggregates is slightly higher in this case. The adsorption thus occurs essentially by an interaction between the nitrogen atom and the silver ion. It is amazing that the size of the J-aggregate is the highest on the particles with an excess of silver. This can come from a LaMer growth of the J-aggregate,29 in which the nucleation of the aggregate is the limiting step and its growth is easier. At the beginning the adsorption on the positive particles is disfavored but after nucleation the interaction between the dye molecules becomes important and the growth of the J-aggregate is exceptionally favored. 3.2.5. Adsorption of Dye 5. 3.2.5.1. Adsorption on Particles of Silver Bromide. The adsorption is disfavored in the case of the neutral particles. The adsorption of the monomer in solution is the highest in this case and the size of the J-aggregate is the lowest (Figure 13). The highest absorbance of the J-aggregate in this case could stem from a lower sedimentation of these particles probably due to the lowest size of the J-aggregates. If we compare the other two cases, the absorbance of the monomer in solution is lower in the case of the positive (29) Steiger, R.; Aebischer, J.-N.; Haselbach, E. J. Imaging Sci. 1991, 35, 1.

604

Langmuir, Vol. 16, No. 2, 2000

Figure 12. Variation of the absorbance of the monomer in solution and of the J-aggregate as a function of time and variation of the maximum of absorbance of the J-aggregate as a function of time in the case of dye 4 adsorbed on particles of AgCl.

particles. However, the size of the J-aggregate is greater for the negative particles, the interaction thus occurs between the bromide ion and the nitrogen atom. The greater size of the J-aggregate arises only after a lapse of time. This can be due to a steric effect, the sulfur atom being more accessible than the nitrogen atom. This can explain the fact that the absorbance of the monomer in solution is lower for the positive particles; the dye is more rapidly adsorbed and the sedimentation effect occurs earlier and therefore more dye is adsorbed. 3.2.5.2. Adsorption on Particles of Silver Chloride. The dye adsorption is very low on the particles of silver chloride; no J-aggregates are observed on these particles. The absorbance of the monomer in solution is the lowest in the case of the negative particles and the absorbance of the adsorbed monomer is the highest in this case; it can thus be concluded that the adsorption is favored in the case of the negative particles and is carried out by an interaction between the chloride ion and the nitrogen atom (Figure 14). 3.3. Comparison Between the Different Dye Molecules. To compare the size of the J-aggregate, their maximum absorbance has been related to their size in the case of a circular J-aggregate. The size of a given

Jeunieau et al.

Figure 13. Variation of the absorbance of the monomer in solution and of the J-aggregate of dye 5 adsorbed on AgBr as a function of time and of the maximum of absorbance of the J-aggregate as a function of time.

J-aggregatecanbededucedfromspectroscopicproperties.30-32 The spectral shift for a linear and cyclic aggregate is a function of the aggregate size N:

(N - 1)/N ) ∆vN/∆v∞

(1)

for a cyclic aggregate, where ∆vN and ∆v∞ are the spectral shift of N-mer and ∞-mer with respect to the adsorbed monomer, respectively. The size of the J-aggregate is greater in the case of dye 4 and 5 (Figure 15). This can be due to the smaller size of these dye molecules, more dye molecules can be adsorbed on the small surface of the particles. On the other hand, the dye-dye molecular interaction is favored for dyes 4 and 5, the aromatic parts being separated only by one (30) Moll, J. Forschungbericht 214: Exciton-Dynamics in J-Aggregates of an Organic Dye; Bundesanstalt fu¨r Materialforschung und -pru¨fung: Berlin, 1995. (31) Tani, T.; Suzumoto, T.; Kemnitz, K.; Yoshihara, K. J. Phys. Chem. 1992, 96, 2778. (32) Muenter, A.; Brumbaugh, D. V.; Apolito, J.; Horn, L. A.; Spano, F. C.; Mukamel, S. J. Phys. Chem. 1992, 96, 2783.

Silver Halide Adsorption Site for Thiacyanine Dyes

Figure 14. Variation of the absorbance of the monomer in solution and of the adsorbed monomer as a function of time for dye 5 on particles of AgCl.

Figure 15. Variation of the average number of dye molecules per J-aggregate adsorbed on AgBr particles as a function of time.

methine group. In fact, cyanines with a short polymethine chain more easily form J-aggregates.33 The size of the J-aggregate is greater in the case of dye 1 in comparison with dye 3. This can be due to the presence of the phenyl group in dye 1 which increases the van der Waals interaction between the dye molecules. The variation of the size of the J-aggregate is not the same in the case of dye 4 and dye 5. In the case of dye 4, the size of the J-aggregate reaches a constant value after a certain time of adsorption, while for dye 5 the size of the J-aggregate increases continuously. Dye 5 is less adsorbed on the particles than dye 4. This difference of variation can come from the higher difficulty of nucleation of the J-aggregate on the particles for dye 5 than for dye 4. If the growth is easier, the J-aggregate will be formed on a low number of particles and will grow on a limited number of particles. (33) Tytyulkov, N.; Fabian, J.; Mehlhorn, A.; Dietz, F.; Tadjer, A. Polymethine Dyes Structure and Properties St Kliment Ohridski University Press: Sofia, 1991; p 121.

Langmuir, Vol. 16, No. 2, 2000 605

Table 2 summarizes the different interactions described above. In addition, the reaction of adsorption has also been carried out on these particles in the presence of an excess of 100% in silver ion. In such a condition there is a competition between the dye adsorption and the complexation with the silver ion in solution. In all of the cases the adsorption is much lower if not totally impeded. This is an argument in favor of the adsorption of the excess of silver ion. In fact, if the silver nitrate in excess stays in solution it will complex the cyanine dye which will not be adsorbed on the particles and this complexation will cause a decrease of the adsorption. Furthermore, the equilibrium constant of the reaction of adsorption has been calculated at the end of the reaction. This is not a pure indication of the adsorption alone for the sedimentation effect also influences this value. These different results show that an interaction of S-Ag is more favorable for the silver bromide particles than for the silver chloride particles. This stems from the difference of the softness of the silver ion in silver bromide compared to silver chloride. In fact, the polarizability of AgBr is 6.63 × 10-24 cm3 and that of AgCl is 5.34 × 10-24 cm3 as calculated from the equation of Clausius-Mosotti34 by using the corresponding refraction indices,35 the softness of silver ion is thus more important for the silver bromide particles. This can explain the observation that the S-Ag interaction which is a soft-soft interaction is more favorable in the case of silver bromide. The size of the J-aggregate is greater on the particles of AgBr than on AgCl, except in the case of dye 3, and the equilibrium constant is also higher for AgBr in all cases. The greater dye adsorption on AgBr than on AgCl can stem from two factors. The first is the smaller polarizability of the silver chloride. This hypothesis has been used to explain better adsorption of dyes on AgI than on AgBr particles.27 The second factor is the better adsorption of the AOT on silver chloride than on silver bromide. This has been suggested in a previous study of the AgCl synthesis in AOT/n-heptane/water microemulsion systems.36 The greater size of the J-aggregate on AgCl for dye 3 can come from the presence of chlorine in the dye molecules; these atoms interact with the silver chloride and favor a greater size of the J-aggregate. The same argument could also be used in the case of dyes 4 and 5, but in this case the difference between AgBr and AgCl has perhaps a greater importance. The structure of the dye is also very important for the type of interaction. Dye 1 shows the most important silversulfur interaction. This can be due to the mesomeric donor effect of the phenyl moieties with respect to the attractive effect of chlorine. The electronic density of sulfur is thus greater in the case of dye 1 and this explains the greater importance of the S-Ag interaction. In the case of dye 2, the lower S-Ag interaction can come from the positive charge on the nitrogen which is not canceled by a negative charge (such as propylsulfonate in dye 1). Dye 4 shows a lower S-Ag interaction than dye 3 and the attribution of the adsorption site is a little ambiguous in the case of dye 4. The difference between the two dye molecules is the length of the methine bridge. In dye 4, the conjugated system is smaller, thus the electronic (34) Israelachvilli, J. Intermolecular and Surface Forces; Academic Press: Suffolk, 1995; p 79. (35) CRC Handbook of Physics and Chemistry, 51th ed.; CRC Press: Boca Raton, FL, 19XX. (36) Jeunieau, L. Bachelor Thesis, FUNDP, 1995.

606

Langmuir, Vol. 16, No. 2, 2000

Jeunieau et al.

Table 2. Summary of the Different Interactions Between the Dyes and the AgBr and AgCl Nanoparticles and the Silver Ions in Solutions and the Average Size of the J-Aggregates adsorption on AgBr

Na

adsorption on AgCl

Na

K∞ AgBrb

K∞ AgClb

100% excess in silver

dye 1

interaction S-Ag

4.3

3.7

36.6

4.3

no adsorption

J-aggregate

dye 2 dye 3 dye 4

interaction N-Br interaction S-Ag interaction N-Br interaction S-Ag not excluded interaction N-Br

7.3 2.5 12.2

interaction N-Cl and interaction Ag-S interaction N-Cl interaction N-Cl interaction N-Cl

4.4 8.5 9.3

s 9.6 6.3

s 2.2 3.1

no adsorption low adsorption no adsorption

few J-aggregate s J-aggregate

13.0

interaction N-Cl

s

2.5

s

low adsorption

s

dye 5

complexation with silver ions in solution

a

Average number of molecules per aggregates in the case of a cyclic aggregate at the end of the reaction of adsorption, 1 h after the reaction in the case of dye 2. b Equilibrium constant at the end of the reaction of adsorption. This value has been obtained following K ) (A0 monomer-A∞ monomer)/A∞ monomer.

densities are more localized on the atoms,37 and the sulfur atom is softer (better interaction with silver ion in solution) and the nitrogen is more positive leading to a better Coulombic interaction. As the reactivity of the two adsorption sites is increased in the same sense, it is difficult to explain the difference between the two dye molecules. Dye 4 is more adsorbed than dye 5, and this can be explained by two factors. The reactivity of sulfur is lower in dye 5 for the lower cancellation of the positive charge of the nitrogen by the alkylsulfonate and the positive charge of nitrogen is less accessible in the case of dye 5. So the two sites of adsorption are of lower reactivity in this molecule. It must be noticed that the site of adsorption does not follow the same order as the interaction of dye with silver ion in solution. Nevertheless, it is correlated with the absence of adsorption in the presence of an excess of 100% in silver. If the interaction with silver ion in the microemulsion medium is high (presence of J-aggregate), no adsorption is observed in the presence of an excess of 100% in silver. The choice of the adsorption site is governed by the difference of reactivity of the sulfur in comparison with the reactivity of nitrogen. In solution, the interaction is only governed by the reactivity of the sulfur site toward the silver ions. Finally, from the apparent adsorption constant values (K∞) in Table 2, the following sequence can be established for the adsorption on AgBr nanoparticles: dye 1 > dye 3 > dye 4 > dye 5. (37) Andre´, J.-M.; Mosley, D. H.; Andre´, M-C.; Champagne, B.; Clementi, E.; Fripiat, J. G.; Leherte, L.; Pisani, L.; Vercauteren, D. P.; Vracko, M. Exploring Aspects of Computational Chemistry Concepts; Presses Universitaires de Namur: Namur, 1997; p 183.

4. Conclusions A general conclusion can be drawn on the adsorption of the thiacyanine molecules: the site of adsorption depends on the dye structure. In this paper, it has been shown that an interaction between the sulfur and the silver ion cannot be excluded. This is not necessarily in contradiction with the results of the ionic conductivity. Indeed, the dye used in those studies2,3 is the 1,1′-ethyl9-methylthiacarbocyanine and it is possible that this dye is adsorbed by an interaction between the nitrogen atom and the bromide ion. Furthermore, in the case of an interaction between the sulfur atom and the silver ion, a positive charge is carried on the sulfur and this can repel the silver ions in the vicinity of the silver which is involved in the interaction with the sulfur. The greater amount of dye adsorbed when the pAg increases can come from the interaction between the dye in solution and the silver ion. It has been shown that an excess of 100% AgNO3 forbids the adsorption. In our case, this corresponds to an amount of silver ions of two monolayers while in the case of the classical nanoparticles, this corresponds to a lower concentration in silver. For example, with particles of 0.4 µm an excess of silver of only 1.8% is sufficient to prevent dye adsorption. Acknowledgment. L.J. thanks F.R.I.A. for financial help. The authors thank Agfa-Gevaert for providing the dyes used and Paul Callant for fruitful discussion. LA990114S