- Email: [email protected]
Self association of sodium salicylate system
Chemical Physics 293 (2003) 211–216 www.elsevier.com/locate/chemphys
Self association of sodium salicylate system Subir Kumar Das a, Shrabani Das b, B. Nandi Ganguly a,*, Amarnath Maitra b a
Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Calcutta 700064, India b Chemistry Department, University of Delhi, Delhi 110007, India Received 14 April 2003; in final form 19 June 2003
Abstract The aggregation behavior of sodium salicylate (SS) in aqueous solutions has been studied by positronium annihilation technique. From the change of the pick-off life time as well as the intensity of the ortho-positronium component, it has been concluded that the minimal hydrotropic concentration (MHC) of SS lies in the range 0.9–1.0 M concentration. To support the above contention, the molecular association of SS has also been investigated by the changes in light scattering intensity, conductivity as well as NMR data. These experiments conjointly infer that the stacking of SS molecules appears to start beyond 0.9 M concentration. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Hydrotropism; Positronium annihilation; Light scattering intensity; HNMR spectra; Stacking of sodium salicylate
1. Introduction Neuberg in 1916 [1] discovered that aqueous solutions of alkali metal salts of benzoic acid, salicylic acid and various other hydroxy aromatic acids displayed a peculiar phenomenon, viz., the enhanced power of solubilizing sparingly soluble substances in water or dissolving solutes that were otherwise insoluble in water. This remarkable power of solubilization was considered, presumably, to be due to salting in effect [2] in the medium. The extraordinary solubilizing power of a class of compounds was referred to as hydrotropism and the chemicals were called hydrotropes [2].
*
Corresponding author. Fax: +91-33-3374637. E-mail address: [email protected] (B.N. Ganguly).
The initial investigations [3,4] on the hydrotropic effect emphasized the molecular structure of the hydrotrope molecule and their primary association phenomenon vis-a-vis their comparison to the surfactant association. Rath [4] focused his attention on their intermolecular stacking or similar aggregation pattern. The idea of molecular aggregation of hydrotropes is the cardinal issue because it allows hydrotrophy to be viewed as a multimolecular process rather than as a specific complexation or medium effect, like co-solvency or the salting in effect. Looking to this interesting possibility of aggregational pattern and the stacking of the molecules, various esteemed instrumental and analytical methods were sought of which the important application of a microprobe like Positronium (Ps), a bound state of eþ and e is used to
0301-0104/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-0104(03)00315-X
212
S.K. Das et al. / Chemical Physics 293 (2003) 211–216
elucidate the microphase changes in the aqueous sodium salicylate medium, analogous to the micellar solutions studied earlier [5,6]. The positronium annihilation technique serves as an important diagnostic tool to investigate the changes taking place in a medium due to molecular association. The method is based on the formation of Ps as well as the interaction of Ps with the medium molecules. The main features of Ps reactions are laid down by their average lifetime and decay modes of the pick-off component (s3 ¼ 1=k3 ), which bear the signature of the electronic (chemical) microenvironment. In this preliminary investigation on hydrotropes, an attempt has been made to follow the changes in lifetime of the Ps component, the inhibition effect of its intensity and the Ps–solute interaction in aqueous solution which remarkably draws attention towards a phase change in the system. Further, the light scattering studies generally adopted to study aggregated systems like the micelles and macromolecules serving as scattering centres showed interesting results. The SS solution was subsequently probed by conductivity measurements pertaining to the extent of the molecular association phenomenon in the process. Finally, the structural and dynamic changes involved in the aggregation and stacking of SS are probed by HNMR spectroscopy. The emerged effect of the investigations led by the assembly of the instrumental methods jointly direct towards a minimal hydrotropic concentration (MHC) conjecture.
tional fast–slow coincidence circuit, with time-toamplitude converter and a multi channel analyser. The positron lifetime spectrometer used in these experiments had a time resolution (FWHM) of 260 ps for the prompt Co60 c rays at the positron experimental window settings. The measurements have been mainly confined to the determination of the long-lived pick-off component s3 and its intensity I3 [7]. All the measurements were performed at room temperature at 298 K, at normal pressure conditions, accumulating more than 106 counts in the spectrum. The data analysis was done with the help of the PATFIT program [8]. Solutions of concentrations 0.1–1.3 M of SS were used for conductivity and light scattering measurements. The same solutions were prepared in D2 O for NMR measurements. The light scattering rate (rate in kHz) for SS solutions were taken in a Brookhaven BI8000 spectrometer fitted with B1200 SM goniometer. An argon air cooled laser was operated at 488 nm as light source and the measurements were done at a scattering angle of 90°. The HNMR spectra were recorded on Bruker Advance-300 spectrometer (300 MHz) and the chemical shifts were expressed in ppm and J (Hz). Conductivity measurements were done on Elico digital conductometer with a conductivity cell of cell constant 1.0.
3. Results and discussion 2. Experimental
3.1. The positronium annihilation method
A pure form of sodium salicylate was used after recrystallization and the aqueous solutions were made from Millipore water. The source of positrons was Na22 (5 micro curie) deposited on a Mylar film and sealing was done by using another identical Mylar flim on top of it. This source was put in a cylindrical specimen container containing the SS solution (degassed), placed between the two detectors viz. BaF2 coupled with PM T, XP2020Q. Positron lifetime measurements have been made using a conven-
A set of experiments were performed with SS in pure water and the changes of the longest pick-off lifetime s3 was observed as a function of its concentration. The variation in the lifetime component and its intensity are given in Fig. 1. These results demonstrate the abrupt changes in the intensity, I3 of the long lived pick-off positronium component after a certain level of concentration, i.e., around 0.9 M SS in aqueous solution, while the changes in the life time s3 are suggestive of subtle change (yet very systematic) at the same
S.K. Das et al. / Chemical Physics 293 (2003) 211–216
213
Fig. 1. The pick-off life time of Ps vs sodium salicylate concentration (error in the s3 is 10 ps) and the percentage intensity I3 , of the pick-off component of Ps vs sodium salicylate concentration (error 0.2). The solid line with a constant slope marks the region of hydrotropism.
concentration. It may be noteworthy to mention here that the earlier osmotic pressure data on SS were interpreted to indicate a weak tendency for association of this compound in water [2]. However, the changes in the pick-off rate (s3 ¼ 1=k3 ) could be expressed in a better way through a electronic overlap interaction as shown in the following paragraph. In our result, the positron annihilation data show clear breaks in the trends of the curves which signify changes in the physical status of SS solution that can be interpreted in terms of collective molecular arrangement analogous to micellar or reverse micellar systems [5,6,9]. However, the difference noticed here is that the level of concentration of the compound is comparatively high to denote the changes in the physical properties through positronium parameters. This information is in conformity with the fact that though the molecular association is reminiscent of the formation of self assemblies, the hydrotropic aggregates differ from micelles in exhibiting a higher molecular concentration characteristic of that substance [10]. From the results (Fig. 1), it is found that after 0.9 M concentration, the life time s3 shows an enhancement with a constant slope against the concentration of SS.
The pick-off lifetime of Ps [11] depends on the average electron density of the medium, at the specific site of Ps, i.e., Z 1 ¼ k3 ¼ 4pr02 c jwPs j2 qðrÞ d3 r; ð1Þ s3 where q, being the electron density which is integrated over space at the site of Ps, r0 being the classical radius of the electron, c is the velocity of light and wPs is the Ps wave function. This pick-off decay rate k3 has been substantially affected (as compared to the pick-off lifetime of pure water being 1.85 ns) beyond a critical concentration of SS and has shown a decreasing trend. This infers a rarefaction of the electronic environment, at the Ps annihilation site, due to some structural reorganization in the presence of SS molecules, beyond 0.9 M concentration level. Or, in the other way, the enhancement in the lifetime s3 means a less interactive domain for Ps (a low electron density region), the electronic interaction being engaged in the self organizing or stacking of the molecules. Essentially, the overlap integral in Eq. (1) is affected due to the inter/intra molecular polarization interaction of hydroxylic and carboxylic groups of SS molecule. A similar indication has been noted in the Ps intensity, I3 vs SS concentration curve (Fig. 1). The
214
S.K. Das et al. / Chemical Physics 293 (2003) 211–216
Inhibition in the Ps formation process means removal of the electrons from the scene. A prior engagement or polarization of the electrons in the medium due to structural reorganization, such as in the present case, would cause a decline in the intensity of the formation of Ps (Fig. 1), which is operative just around 0.9 M SS concentration in this case.
as scattering centers, a regular increase in the photon counting rate (kHz) was accordingly observed initially denoting the association of the SS molecules, as the intensity of the scattered light Ih is proportional to the polarizability (aÞ and molecular weight of the scattering particles. However, with a further increase in concentration of SS in the medium, a dramatic impact arises, presumably due to a continuous stacking effect or a cosolvency of SS molecules in the medium resulting in a single continuum that nullifies the scattering effect as light is not scattered by a perfectly homogeneous medium. This phenomenon is of significance for the expression of hydrotropism [2,13] which is in striking contrast to merely the aggregation phenomenon of the molecules as is the case for micellization or the lamellar structure of the liquid crystalline phase.
3.2. Light scattering
3.3. Conductivity measurements
Fig. 2 shows the photon counting rate of the scattered light which increases initially from 0.1 M SS solution but eventually decreases very abruptly at 0.7 M in the same solution and continues as a plateau region beyond 0.9 M. Since the macromolecules (in this case the associated molecules) could cause the inhomogenity with differing refractive indices to the bulk medium and thus act
Conductivity of aqueous SS increases steadily as its concentration increases in the medium, but the slope of the curve changes beyond 0.9 M concentration of SS (Fig. 3). As such, hydrotropic solution shows high electrical conductance (2) and in this particular case, the slope of the conductivity curve turns very gradual near 0.9 M, signifying an
Fig. 2. The intensity of the scattered light vs concentration of sodium salicylate.
Fig. 3. The conductance vs concentration of sodium salicylate (solid line).
Ps formation probability in a liquid medium is the product of the concentrations Ce ðr; tÞ and Cp ðr; tÞ, of the electron and positron at a given point r, at a given time t, multiplied by the rate constant of positronium formation Kep and integrated over the specific region and over all time, which is expressed as [12] Z 1 Z Pps ¼ dt d3 rKep Ce ðr; tÞCp ðr; tÞ: ð2Þ 0
S.K. Das et al. / Chemical Physics 293 (2003) 211–216
onset of associative mechanism of the molecules in the aqueous medium which is unlike any concentrated aqueous electrolyte solution. 3.4. NMR studies The 300 MHz HNMR spectra of SS in D2 O showing the chemical shifts in J (Hz) have been presented in Fig. 4. The four protons of the benzene ring exhibit NMR peaks between 6 and 8 ppm and the peak positions of these protons are shifted to upfield with the increased concentration of SS in the original experimental spectrum. Such a concentration dependent chemical shift of resonance peaks of molecules were ascribed to a stacking type of interaction [14] in the medium. Also, at lower concentrations of SS, precisely in the concentration range 0.1–0.7 M, the narrow and splitted signals were exhibited owing to slow relaxation process of the system, while at higher concentration (>0.7 M) single broad signals are increasingly prominent, due to the stacking of molecules, ensuing restricted dynamics is expected and thus line broadening effect was observed [15]. Rapid reorientation of the monomer or dimer
215
molecules, on the contrary is responsible for narrow and sharp peaks. NMR data depicts the structural and the chemical environmental effects, i.e., below 0.7 M concentration, SS exists in the form of monomer or loose dimers, where as at P 0.7 M concentration face to face stacking structure predominates. Interestingly, it is pertinent to point out that the single line broadening effect in the NMR spectra has been observed around 0.7 M which is slightly lower than the critical effect observed in other experiments. This could be attributed to the reduction in hydrogen bonding in the close proximity (intra molecular) due to the isotopic effect of D2 O system. In these aggregates, the benzene rings are stacked one above the other with the substituent carboxylic and hydroxyl groups oriented in such a way, so as to minimise the steric hindrance and columbic repulsion. It is quite imperative that the full rotation of benzene substituents accounts for the sharp multiplets, at the lower concentration.
4. Conclusion The studies indicate that positron annihilation spectroscopy is a viable probe to study the molecular association phenomenon not only in the case of surfactant systems, but also in the studies of hydrotropism in aqueous media. The results supported by other experimental techniques such as conductivity, light scattering and HNMR spectroscopy reveal that positron annihilation data measures very precisely, the loose interaction of hydrotrope molecules during the aggregation process. In addition, these experiments also reveal that a hydrotropic medium is completely homogenous, isotropic and highly conductive system.
References
Fig. 4. The HNMR spectra of sodium salicylate solution in D2 O at different concentrations showing the chemical shift in J (Hz).
[1] C. Neuberg, Biochem. Z. 76 (1916) 107. [2] See a Review article D. Balasubramanian, E. Friberg Stig, in: E. Matijevic (Ed.), Surface and Colloid Science, vol. 15, Plenum Press, New York, 1993. [3] H. Freundlich, G.V. Slottman, Biochem. Z. 188 (1927) 101. [4] H. Rath, Tenside 2 (1965) 1.
216
S.K. Das et al. / Chemical Physics 293 (2003) 211–216
[5] Yan Ching Jean, Hans J. Ache, J. Am. Chem. Soc. 47 (1977) 257; J. Phys. Chem. 80 (1976) 1693. [6] S.K. Das, G.B. Nandi, Radiat. Phys. Chem. 47 (1996) 257. [7] G.B. Nandi, P. Sen, Appl. Radiat. Isot. 38 (1987) 681; V.S. Subrahmanyam, G.B. Nandi, P. Sen, Radiat. Phys. Chem. 38 (1991) 213. [8] P. Kirkegaard, N.J. Pedersen, E. Eldrup, PATFIT-88, RISO M-2740, DENMARK, 1989. [9] A.A. Badwan, L.K. El-Khordagui, A.M. Saleh, S.A. Khalil, Int. J. Pharm. 13 (1983) 67.
[10] D. Balasubramanian, V. Srinivas, V.G. Gaikar, M.M. Sharma, J. Phys. Chem. 93 (1989) 3865. [11] D.M. Schrader, Y.C. Jean (Eds.), Positron and Positronium Chemistry, Elsevier, Amsterdam, 1988, p. 159. [12] T. Mukherjee, S.K. Das, G.B. Nandi, P. Sen, R.B. Dutta, Radiat. Phys. Chem. 50 (1997) 363. [13] S.E. Friberg, L. Rydhag, Tenside 7 (1970) 80. [14] N. Foster, J. Magn. Reson. 56 (1984) 140. [15] Koji Kano, Nakajima Takeshi, Maso Taki, Hashimoto Shizunobu, Bull. Chem. Soc. Jpn. 60 (1987) 1281.
Self association of sodium salicylate system Subir Kumar Das a, Shrabani Das b, B. Nandi Ganguly a,*, Amarnath Maitra b a
Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Calcutta 700064, India b Chemistry Department, University of Delhi, Delhi 110007, India Received 14 April 2003; in final form 19 June 2003
Abstract The aggregation behavior of sodium salicylate (SS) in aqueous solutions has been studied by positronium annihilation technique. From the change of the pick-off life time as well as the intensity of the ortho-positronium component, it has been concluded that the minimal hydrotropic concentration (MHC) of SS lies in the range 0.9–1.0 M concentration. To support the above contention, the molecular association of SS has also been investigated by the changes in light scattering intensity, conductivity as well as NMR data. These experiments conjointly infer that the stacking of SS molecules appears to start beyond 0.9 M concentration. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Hydrotropism; Positronium annihilation; Light scattering intensity; HNMR spectra; Stacking of sodium salicylate
1. Introduction Neuberg in 1916 [1] discovered that aqueous solutions of alkali metal salts of benzoic acid, salicylic acid and various other hydroxy aromatic acids displayed a peculiar phenomenon, viz., the enhanced power of solubilizing sparingly soluble substances in water or dissolving solutes that were otherwise insoluble in water. This remarkable power of solubilization was considered, presumably, to be due to salting in effect [2] in the medium. The extraordinary solubilizing power of a class of compounds was referred to as hydrotropism and the chemicals were called hydrotropes [2].
*
Corresponding author. Fax: +91-33-3374637. E-mail address: [email protected] (B.N. Ganguly).
The initial investigations [3,4] on the hydrotropic effect emphasized the molecular structure of the hydrotrope molecule and their primary association phenomenon vis-a-vis their comparison to the surfactant association. Rath [4] focused his attention on their intermolecular stacking or similar aggregation pattern. The idea of molecular aggregation of hydrotropes is the cardinal issue because it allows hydrotrophy to be viewed as a multimolecular process rather than as a specific complexation or medium effect, like co-solvency or the salting in effect. Looking to this interesting possibility of aggregational pattern and the stacking of the molecules, various esteemed instrumental and analytical methods were sought of which the important application of a microprobe like Positronium (Ps), a bound state of eþ and e is used to
0301-0104/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-0104(03)00315-X
212
S.K. Das et al. / Chemical Physics 293 (2003) 211–216
elucidate the microphase changes in the aqueous sodium salicylate medium, analogous to the micellar solutions studied earlier [5,6]. The positronium annihilation technique serves as an important diagnostic tool to investigate the changes taking place in a medium due to molecular association. The method is based on the formation of Ps as well as the interaction of Ps with the medium molecules. The main features of Ps reactions are laid down by their average lifetime and decay modes of the pick-off component (s3 ¼ 1=k3 ), which bear the signature of the electronic (chemical) microenvironment. In this preliminary investigation on hydrotropes, an attempt has been made to follow the changes in lifetime of the Ps component, the inhibition effect of its intensity and the Ps–solute interaction in aqueous solution which remarkably draws attention towards a phase change in the system. Further, the light scattering studies generally adopted to study aggregated systems like the micelles and macromolecules serving as scattering centres showed interesting results. The SS solution was subsequently probed by conductivity measurements pertaining to the extent of the molecular association phenomenon in the process. Finally, the structural and dynamic changes involved in the aggregation and stacking of SS are probed by HNMR spectroscopy. The emerged effect of the investigations led by the assembly of the instrumental methods jointly direct towards a minimal hydrotropic concentration (MHC) conjecture.
tional fast–slow coincidence circuit, with time-toamplitude converter and a multi channel analyser. The positron lifetime spectrometer used in these experiments had a time resolution (FWHM) of 260 ps for the prompt Co60 c rays at the positron experimental window settings. The measurements have been mainly confined to the determination of the long-lived pick-off component s3 and its intensity I3 [7]. All the measurements were performed at room temperature at 298 K, at normal pressure conditions, accumulating more than 106 counts in the spectrum. The data analysis was done with the help of the PATFIT program [8]. Solutions of concentrations 0.1–1.3 M of SS were used for conductivity and light scattering measurements. The same solutions were prepared in D2 O for NMR measurements. The light scattering rate (rate in kHz) for SS solutions were taken in a Brookhaven BI8000 spectrometer fitted with B1200 SM goniometer. An argon air cooled laser was operated at 488 nm as light source and the measurements were done at a scattering angle of 90°. The HNMR spectra were recorded on Bruker Advance-300 spectrometer (300 MHz) and the chemical shifts were expressed in ppm and J (Hz). Conductivity measurements were done on Elico digital conductometer with a conductivity cell of cell constant 1.0.
3. Results and discussion 2. Experimental
3.1. The positronium annihilation method
A pure form of sodium salicylate was used after recrystallization and the aqueous solutions were made from Millipore water. The source of positrons was Na22 (5 micro curie) deposited on a Mylar film and sealing was done by using another identical Mylar flim on top of it. This source was put in a cylindrical specimen container containing the SS solution (degassed), placed between the two detectors viz. BaF2 coupled with PM T, XP2020Q. Positron lifetime measurements have been made using a conven-
A set of experiments were performed with SS in pure water and the changes of the longest pick-off lifetime s3 was observed as a function of its concentration. The variation in the lifetime component and its intensity are given in Fig. 1. These results demonstrate the abrupt changes in the intensity, I3 of the long lived pick-off positronium component after a certain level of concentration, i.e., around 0.9 M SS in aqueous solution, while the changes in the life time s3 are suggestive of subtle change (yet very systematic) at the same
S.K. Das et al. / Chemical Physics 293 (2003) 211–216
213
Fig. 1. The pick-off life time of Ps vs sodium salicylate concentration (error in the s3 is 10 ps) and the percentage intensity I3 , of the pick-off component of Ps vs sodium salicylate concentration (error 0.2). The solid line with a constant slope marks the region of hydrotropism.
concentration. It may be noteworthy to mention here that the earlier osmotic pressure data on SS were interpreted to indicate a weak tendency for association of this compound in water [2]. However, the changes in the pick-off rate (s3 ¼ 1=k3 ) could be expressed in a better way through a electronic overlap interaction as shown in the following paragraph. In our result, the positron annihilation data show clear breaks in the trends of the curves which signify changes in the physical status of SS solution that can be interpreted in terms of collective molecular arrangement analogous to micellar or reverse micellar systems [5,6,9]. However, the difference noticed here is that the level of concentration of the compound is comparatively high to denote the changes in the physical properties through positronium parameters. This information is in conformity with the fact that though the molecular association is reminiscent of the formation of self assemblies, the hydrotropic aggregates differ from micelles in exhibiting a higher molecular concentration characteristic of that substance [10]. From the results (Fig. 1), it is found that after 0.9 M concentration, the life time s3 shows an enhancement with a constant slope against the concentration of SS.
The pick-off lifetime of Ps [11] depends on the average electron density of the medium, at the specific site of Ps, i.e., Z 1 ¼ k3 ¼ 4pr02 c jwPs j2 qðrÞ d3 r; ð1Þ s3 where q, being the electron density which is integrated over space at the site of Ps, r0 being the classical radius of the electron, c is the velocity of light and wPs is the Ps wave function. This pick-off decay rate k3 has been substantially affected (as compared to the pick-off lifetime of pure water being 1.85 ns) beyond a critical concentration of SS and has shown a decreasing trend. This infers a rarefaction of the electronic environment, at the Ps annihilation site, due to some structural reorganization in the presence of SS molecules, beyond 0.9 M concentration level. Or, in the other way, the enhancement in the lifetime s3 means a less interactive domain for Ps (a low electron density region), the electronic interaction being engaged in the self organizing or stacking of the molecules. Essentially, the overlap integral in Eq. (1) is affected due to the inter/intra molecular polarization interaction of hydroxylic and carboxylic groups of SS molecule. A similar indication has been noted in the Ps intensity, I3 vs SS concentration curve (Fig. 1). The
214
S.K. Das et al. / Chemical Physics 293 (2003) 211–216
Inhibition in the Ps formation process means removal of the electrons from the scene. A prior engagement or polarization of the electrons in the medium due to structural reorganization, such as in the present case, would cause a decline in the intensity of the formation of Ps (Fig. 1), which is operative just around 0.9 M SS concentration in this case.
as scattering centers, a regular increase in the photon counting rate (kHz) was accordingly observed initially denoting the association of the SS molecules, as the intensity of the scattered light Ih is proportional to the polarizability (aÞ and molecular weight of the scattering particles. However, with a further increase in concentration of SS in the medium, a dramatic impact arises, presumably due to a continuous stacking effect or a cosolvency of SS molecules in the medium resulting in a single continuum that nullifies the scattering effect as light is not scattered by a perfectly homogeneous medium. This phenomenon is of significance for the expression of hydrotropism [2,13] which is in striking contrast to merely the aggregation phenomenon of the molecules as is the case for micellization or the lamellar structure of the liquid crystalline phase.
3.2. Light scattering
3.3. Conductivity measurements
Fig. 2 shows the photon counting rate of the scattered light which increases initially from 0.1 M SS solution but eventually decreases very abruptly at 0.7 M in the same solution and continues as a plateau region beyond 0.9 M. Since the macromolecules (in this case the associated molecules) could cause the inhomogenity with differing refractive indices to the bulk medium and thus act
Conductivity of aqueous SS increases steadily as its concentration increases in the medium, but the slope of the curve changes beyond 0.9 M concentration of SS (Fig. 3). As such, hydrotropic solution shows high electrical conductance (2) and in this particular case, the slope of the conductivity curve turns very gradual near 0.9 M, signifying an
Fig. 2. The intensity of the scattered light vs concentration of sodium salicylate.
Fig. 3. The conductance vs concentration of sodium salicylate (solid line).
Ps formation probability in a liquid medium is the product of the concentrations Ce ðr; tÞ and Cp ðr; tÞ, of the electron and positron at a given point r, at a given time t, multiplied by the rate constant of positronium formation Kep and integrated over the specific region and over all time, which is expressed as [12] Z 1 Z Pps ¼ dt d3 rKep Ce ðr; tÞCp ðr; tÞ: ð2Þ 0
S.K. Das et al. / Chemical Physics 293 (2003) 211–216
onset of associative mechanism of the molecules in the aqueous medium which is unlike any concentrated aqueous electrolyte solution. 3.4. NMR studies The 300 MHz HNMR spectra of SS in D2 O showing the chemical shifts in J (Hz) have been presented in Fig. 4. The four protons of the benzene ring exhibit NMR peaks between 6 and 8 ppm and the peak positions of these protons are shifted to upfield with the increased concentration of SS in the original experimental spectrum. Such a concentration dependent chemical shift of resonance peaks of molecules were ascribed to a stacking type of interaction [14] in the medium. Also, at lower concentrations of SS, precisely in the concentration range 0.1–0.7 M, the narrow and splitted signals were exhibited owing to slow relaxation process of the system, while at higher concentration (>0.7 M) single broad signals are increasingly prominent, due to the stacking of molecules, ensuing restricted dynamics is expected and thus line broadening effect was observed [15]. Rapid reorientation of the monomer or dimer
215
molecules, on the contrary is responsible for narrow and sharp peaks. NMR data depicts the structural and the chemical environmental effects, i.e., below 0.7 M concentration, SS exists in the form of monomer or loose dimers, where as at P 0.7 M concentration face to face stacking structure predominates. Interestingly, it is pertinent to point out that the single line broadening effect in the NMR spectra has been observed around 0.7 M which is slightly lower than the critical effect observed in other experiments. This could be attributed to the reduction in hydrogen bonding in the close proximity (intra molecular) due to the isotopic effect of D2 O system. In these aggregates, the benzene rings are stacked one above the other with the substituent carboxylic and hydroxyl groups oriented in such a way, so as to minimise the steric hindrance and columbic repulsion. It is quite imperative that the full rotation of benzene substituents accounts for the sharp multiplets, at the lower concentration.
4. Conclusion The studies indicate that positron annihilation spectroscopy is a viable probe to study the molecular association phenomenon not only in the case of surfactant systems, but also in the studies of hydrotropism in aqueous media. The results supported by other experimental techniques such as conductivity, light scattering and HNMR spectroscopy reveal that positron annihilation data measures very precisely, the loose interaction of hydrotrope molecules during the aggregation process. In addition, these experiments also reveal that a hydrotropic medium is completely homogenous, isotropic and highly conductive system.
References
Fig. 4. The HNMR spectra of sodium salicylate solution in D2 O at different concentrations showing the chemical shift in J (Hz).
[1] C. Neuberg, Biochem. Z. 76 (1916) 107. [2] See a Review article D. Balasubramanian, E. Friberg Stig, in: E. Matijevic (Ed.), Surface and Colloid Science, vol. 15, Plenum Press, New York, 1993. [3] H. Freundlich, G.V. Slottman, Biochem. Z. 188 (1927) 101. [4] H. Rath, Tenside 2 (1965) 1.
216
S.K. Das et al. / Chemical Physics 293 (2003) 211–216
[5] Yan Ching Jean, Hans J. Ache, J. Am. Chem. Soc. 47 (1977) 257; J. Phys. Chem. 80 (1976) 1693. [6] S.K. Das, G.B. Nandi, Radiat. Phys. Chem. 47 (1996) 257. [7] G.B. Nandi, P. Sen, Appl. Radiat. Isot. 38 (1987) 681; V.S. Subrahmanyam, G.B. Nandi, P. Sen, Radiat. Phys. Chem. 38 (1991) 213. [8] P. Kirkegaard, N.J. Pedersen, E. Eldrup, PATFIT-88, RISO M-2740, DENMARK, 1989. [9] A.A. Badwan, L.K. El-Khordagui, A.M. Saleh, S.A. Khalil, Int. J. Pharm. 13 (1983) 67.
[10] D. Balasubramanian, V. Srinivas, V.G. Gaikar, M.M. Sharma, J. Phys. Chem. 93 (1989) 3865. [11] D.M. Schrader, Y.C. Jean (Eds.), Positron and Positronium Chemistry, Elsevier, Amsterdam, 1988, p. 159. [12] T. Mukherjee, S.K. Das, G.B. Nandi, P. Sen, R.B. Dutta, Radiat. Phys. Chem. 50 (1997) 363. [13] S.E. Friberg, L. Rydhag, Tenside 7 (1970) 80. [14] N. Foster, J. Magn. Reson. 56 (1984) 140. [15] Koji Kano, Nakajima Takeshi, Maso Taki, Hashimoto Shizunobu, Bull. Chem. Soc. Jpn. 60 (1987) 1281.