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Small-angle neutron scattering study of aqueous solutions of 1,7-heptanediol
Journal of Molecular Liquids 118 (2005) 141 – 143 www.elsevier.com/locate/molliq
Small-angle neutron scattering study of aqueous solutions of 1,7-heptanediol L. Alma´sya,*, N.K. Sze´kelya, A. Lena, Cs. Muzsnayb, K.N. Kira´lyb a Research Institute for Solid State Physics and Optics, P.O. Box 49, Budapest 1525, Hungary BabesS -Bolyai University, Faculty of Chemistry and Chemical Engineering, str. Arany Ja´nos 11, Cluj-Napoca, Romania
b
Available online 20 August 2004
Abstract We report results of small-angle neutron scattering (SANS) measurements on aqueous solutions of 1,7-heptanediol in the molarity range 0.11–1.5 mol dm3 at room temperature. The experimental data show aggregation of the diol molecules within the whole studied concentration range. The scattering curves could be accurately described by the structural model of random concentration fluctuations. The results suggest that 1,7-heptanediol molecules in aqueous solution aggregate in a rather loose way. D 2004 Elsevier B.V. All rights reserved. Keywords: Aqueous solution; Nonionic surfactant; Concentration fluctuation; Micelle; SANS
1. Introduction Small-angle X-ray and neutron scattering studies on the aqueous solutions of primary alcohols indicate a tendency of increasing alcohol aggregation with increasing length of the alkyl chain [1–4]. This aggregation is usually attributed to the hydrophobic interaction—the solute molecules form direct contacts by their hydrocarbon chains—while the hydrophilic OH groups participate in hydrogen bonds with the surrounding water shell. The arrangement of hydrophobic and hydrophilic groups on the alcohol molecules is probably one of the main factors that govern their mixing behavior with water. Both longer alkanols and 1,2-alkanediols have distinct hydrophobic and hydrophilic parts, and indeed, many structural and thermodynamic investigations indicate that they can form micelle-like aggregates in water [2,5–8]. Other diols, in particular, those with hydroxyl groups in medial positions or with both OH groups in the two terminal positions, do not have distinct hydrophobic parts; their mixing behavior with water therefore can be expected to differ from that of the 1,2-diols. * Corresponding author. E-mail address: [email protected] (L. Alma´sy). 0167-7322/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2004.07.030
There are only a few experimental studies in the literature dealing with structural investigations of aqueous diol solutions. A series of positron annihilation investigations of aqueous solutions of various diols have been performed by Jerie et al. [9–11]. By comparing 1,2-hexanediol and 1,6hexanediol solutions, they suggested that at low concentrations, both solutes exhibit hydrophobic hydration, but at the same time, they participate in the hydrogen bond network of water [9]. This picture may be valid for the 1,6-hexanediol solutions, but it contradicts the assumption of micellar aggregation of 1,2-hexanediol. In aqueous solutions of 1,2-butanediol, formation of clathrate-like hydrates was suggested [10,11], while no evidence of such behavior was found in 1,3- and 1,4-butanediol solutions. Unfortunately, in these studies, the diol–diol interactions were not considered explicitly, although they should play an important role in forming the structure of the solutions. The small-angle neutron scattering (SANS) technique is a useful tool for measuring the extent of aggregation of molecules in a multicomponent solution, if the sizes of the aggregates are of the order of 1–100 nm [2,4,6,7,12]. Recently, D’Arrigo et al. [6,7] carried out SANS investigations on aqueous solutions of several diols and triols. They found aggregation of many diols having distinct
142
L. Alma´sy et al. / Journal of Molecular Liquids 118 (2005) 141–143
hydrophilic and hydrophobic parts. However, observations on some other types of diols indicated that such a molecular shape of the solute is not always necessary for aggregation. In particular, 1,7-heptanediol was found to aggregate above its critical micelle concentration (molar; 0.12 mol dm3), but the form of the aggregates was difficult to obtain from the rather featureless scattering curves. In our previous SANS investigations of aqueous solutions of small organic molecules, we used a simple solution model in which the distribution of the species resembles statistical concentration fluctuations [4,12]. In the present work, we repeated the SANS measurements on aqueous solutions of 1,7-heptanediol reported in Ref. [6], extending the studied concentration range in order to characterize the structure of the solutions on the nanometerlength scale.
2. Experimental Small-angle neutron scattering on solutions of 1,7heptanediol in heavy water was measured on the SANS instrument Yellow Submarine installed on the cold neutron beam line at the Budapest Research Reactor [13]. The scattered intensity at small angles is proportional to the square of the contrast between the scattering length densities of the species in a two-component solution. By using heavy water as solvent, the contrast could be increased substantially, thus improving the precision of the experiment. The measurements were performed at 25 8C, on four samples with heptanediol molarities 0.11, 0.33, 0.66 and 1.5 mol dm3. The solutions were prepared by weighing from commercial 1,7-heptanediol (Fluka, purityN97%) and heavy water of deuterium content higher than 99 at.%. The samples were measured in 2-mm-thick quartz cells. The counting time was 20 min for each solution. The measured scattering curves were corrected for the sample transmission, the scattering from the cell and the room background. The normalization to the detector efficiency and conversion of the measured scattering to absolute scale was performed by measuring the scattering from a light water sample. The range of scattering vectors q was 0.06–0.41 21 [ q=(4p/k)sinH; k is the neutron wavelength and 2H is the scattering angle]. The average statistical error of the measured data points for the 1.5 molar solution was 0.5%, and the uncertainty of converting to absolute scale was about 10%.
3. Results and discussion The experimental scattering curves obtained at 25 8C are shown in Fig. 1. Enhanced scattering at small angles is observed for all solutions, indicating the nonhomogeneous distribution of the species in the solution. The shape of the scattering curve of the 0.66 mol dm3 solution resembles the SANS curve of 0.7 M 1,7-heptanediol solution shown in
Fig. 1. SANS scattering curves of 1,7-heptanediol solutions measured at 25 8C. M denotes the concentration in mol dm3. The solid lines are fits to the Ornstein–Zernike model.
[6]. The small-angle scattering of the 0.33 mol dm3 solution is appreciably higher than that observed by D’Arrigo et. al. [6] at 20 8C; this may be due to the different experimental temperatures. The usual way of analyzing SANS experimental data (see, e.g., Ref. [2]) consists of modeling the sample structure by assuming various theoretical models, and of calculating the corresponding scattering curves. Then the calculated curves are compared with the experimental data, and the validity of the model can be checked by analyzing the consistency of the fitted curves with the experimental data. The SANS technique is sensitive to the distribution of the species in a liquid mixture of small molecules. Molecules in a two-component mixture can arrange themselves in various structures forming inhomogeneous distribution of the scattering length density, which leads to enhanced smallangle scattering. One possibility is that the molecules selforganize in compact micelle-like aggregates, the hydrophobic alkyl chains being turned inside the micelle and the OH groups turned outside, contacting with water. The
L. Alma´sy et al. / Journal of Molecular Liquids 118 (2005) 141–143 Table 1 Results of the least squares fitting of Eq. (1) to the SANS data of 1,7heptanediol – heavy water mixtures Concentration, mol dm3
I 0, cm1
n, 2
Bg, cm1
m2
0.33 0.66 1.5
0.043F0.011 0.149F0.002 1.027F0.008
2.1F0.5 3.4F0.1 8.0F0.1
0.078F0.011 0.114F0.002 0.212F0.001
3.9 1.1 1.7
143
presently proposed model does not consider micelles or similar compact aggregates, but assumes a continuous distribution of the components, similar to an interconnected diol network. Hydrogen bonds between the diols and water and also between the neighboring diol molecules can probably form in addition to the conventional closecontact hydrophobic interactions.
m2 is the variance of the residuals.
scattering of such aggregates can be modeled as scattering from spherical objects, as was done in several SANS studies of aqueous alcohol solutions [2,6,7]. Another possibility for the arrangement of the two species in the mixture resembles the random concentration fluctuations of the components. In this case, the distribution of the scattering length density (roughly the distribution of the solute molecules) is assumed to be similar to the density distribution in a pure liquid in a state not far from the critical point [14]. This structure is well described by the Ornstein– Zernike model of concentration fluctuations, and the corresponding scattering function is given by Eq. (1): I ð qÞ ¼
I0 þ Bg 1 þ q2 n2
ð1Þ
where I 0 is the coherent forward scattering intensity, n is the correlation length that is the measure of the decay of the correlation in the scattering length density distribution and Bg is a flat background term. The latter accounts for the contribution of the incoherent scattering from the hydrogen and deuterium atoms in the mixture and a possible residual background not subtracted in the course of the correction procedures. The experimental scattering curves were fitted to Eq. (1) by the method of weighted least squares using three adjustable parameters. The results obtained are collected in Table 1 (solutions 0.33, 0.66 and 1.5 mol dm3), and the fitted curves are shown in Fig. 1. In the case of the 0.11 mol dm3 solution, the poor counting statistics and the low coherent scattering intensity prevented the use of unconstrained fitting; only the enhanced small-angle scattering can be seen in the measured data (Fig. 1). The Ornstein–Zernike model function fits the measured scattering curves within their experimental uncertainty rather well, the values of the computed m2 parameter are close to 1. Therefore, we conclude that 1,7-heptanediol– water mixtures are adequately described by random concentration fluctuations, and there is no need to use more complicated models. The model used in Ref. [6] assumed micellar particles with attractive interactions and the five adjustable parameters could be estimated only with rather large uncertainty. Despite the difference between the two approaches, our analysis supports the conclusions of D’Arrigo et al. [6] according to which bdryQ micelles cannot be present in the solution, only mixed aggregates containing water and diol molecules are formed. The
4. Conclusions Small-angle neutron scattering measurements have been performed on heavy water solutions of the nonionic surface active compound 1,7-heptanediol at 25 8C in the molarity range 0.11–1.5 mol dm3. All the studied 1,7-heptanediol solutions are spatially nonhomogeneous. Aggregation of the alcohol molecules takes place even at the lowest concentration studied (0.11 mol dm3). The structure of the 1,7heptanediol solution was modeled by assuming loosepacked alcohol aggregates which can be described by the model of random concentration fluctuations. The good agreement of the scattering curves, calculated from this model, with the experimental data indicates that 1,7heptanediol molecules aggregate in a loose way, possibly forming a network structure interconnected by hydrogen bonding.
Acknowledgements This work has been supported by the EC, under contract No. HPRI-CT-1999-00099 and ICA1-CT-2000-70029. Cs. Muzsnay is grateful to the foundation Domus Hungarica for a research fellowship.
References [1] K. Nishikawa, T. Iijima, J. Phys. Chem. 97 (1993) 10824. [2] G. D’Arrigo, J. Teixeira, J. Chem. Soc., Faraday Trans. 86 (1990) 1503. [3] I. Shulgin, E. Ruckenstein, J. Phys. Chem., B 103 (1999) 872. [4] L. Alma´sy, G. Jancso´, L. Cser, Appl. Phys., A 74 (2002) S1376 (Suppl.). [5] Y. Kato, Chem. Pharm. Bull. 10 (1962) 771. [6] G. D’Arrigo, R. Giordano, J. Teixeira, Langmuir 16 (2000) 1553. [7] G. D’Arrigo, R. Giordano, J. Teixeira, Eur. Phys. J., E 10 (2003) 135. [8] S.M. Hajji, M.B. Errahmani, R. Coudert, R.R. Durand, A. Cao, E. Taillandier, J. Phys. Chem. 93 (1989) 4819. [9] K. Jerie, A. Baranowski, J. Glin´ski, J. Przybylski, J. Radioanal. Nucl. Chem. 246 (2000) 343. [10] K. Jerie, A. Baranowski, J. Glin´ski, K. Orzechowski, J. Radioanal. Nucl. Chem. 241 (1999) 265. [11] K. Jerie, A. Baranowski, J. Glin´ski, K. Orzechowski, Acta Phys. Pol., A 99 (2001) 393. [12] L. Alma´sy, L. Cser, G. Jancso´, J. Mol. Liq. 101 (2002) 89. [13] L. Rosta, Appl. Phys., A 74 (2002) S52 (Suppl.). [14] H.E. Stanley, Introduction to Phase Transitions and Critical Phenomena, Clarendon Press, Oxford, 1971.
Small-angle neutron scattering study of aqueous solutions of 1,7-heptanediol L. Alma´sya,*, N.K. Sze´kelya, A. Lena, Cs. Muzsnayb, K.N. Kira´lyb a Research Institute for Solid State Physics and Optics, P.O. Box 49, Budapest 1525, Hungary BabesS -Bolyai University, Faculty of Chemistry and Chemical Engineering, str. Arany Ja´nos 11, Cluj-Napoca, Romania
b
Available online 20 August 2004
Abstract We report results of small-angle neutron scattering (SANS) measurements on aqueous solutions of 1,7-heptanediol in the molarity range 0.11–1.5 mol dm3 at room temperature. The experimental data show aggregation of the diol molecules within the whole studied concentration range. The scattering curves could be accurately described by the structural model of random concentration fluctuations. The results suggest that 1,7-heptanediol molecules in aqueous solution aggregate in a rather loose way. D 2004 Elsevier B.V. All rights reserved. Keywords: Aqueous solution; Nonionic surfactant; Concentration fluctuation; Micelle; SANS
1. Introduction Small-angle X-ray and neutron scattering studies on the aqueous solutions of primary alcohols indicate a tendency of increasing alcohol aggregation with increasing length of the alkyl chain [1–4]. This aggregation is usually attributed to the hydrophobic interaction—the solute molecules form direct contacts by their hydrocarbon chains—while the hydrophilic OH groups participate in hydrogen bonds with the surrounding water shell. The arrangement of hydrophobic and hydrophilic groups on the alcohol molecules is probably one of the main factors that govern their mixing behavior with water. Both longer alkanols and 1,2-alkanediols have distinct hydrophobic and hydrophilic parts, and indeed, many structural and thermodynamic investigations indicate that they can form micelle-like aggregates in water [2,5–8]. Other diols, in particular, those with hydroxyl groups in medial positions or with both OH groups in the two terminal positions, do not have distinct hydrophobic parts; their mixing behavior with water therefore can be expected to differ from that of the 1,2-diols. * Corresponding author. E-mail address: [email protected] (L. Alma´sy). 0167-7322/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2004.07.030
There are only a few experimental studies in the literature dealing with structural investigations of aqueous diol solutions. A series of positron annihilation investigations of aqueous solutions of various diols have been performed by Jerie et al. [9–11]. By comparing 1,2-hexanediol and 1,6hexanediol solutions, they suggested that at low concentrations, both solutes exhibit hydrophobic hydration, but at the same time, they participate in the hydrogen bond network of water [9]. This picture may be valid for the 1,6-hexanediol solutions, but it contradicts the assumption of micellar aggregation of 1,2-hexanediol. In aqueous solutions of 1,2-butanediol, formation of clathrate-like hydrates was suggested [10,11], while no evidence of such behavior was found in 1,3- and 1,4-butanediol solutions. Unfortunately, in these studies, the diol–diol interactions were not considered explicitly, although they should play an important role in forming the structure of the solutions. The small-angle neutron scattering (SANS) technique is a useful tool for measuring the extent of aggregation of molecules in a multicomponent solution, if the sizes of the aggregates are of the order of 1–100 nm [2,4,6,7,12]. Recently, D’Arrigo et al. [6,7] carried out SANS investigations on aqueous solutions of several diols and triols. They found aggregation of many diols having distinct
142
L. Alma´sy et al. / Journal of Molecular Liquids 118 (2005) 141–143
hydrophilic and hydrophobic parts. However, observations on some other types of diols indicated that such a molecular shape of the solute is not always necessary for aggregation. In particular, 1,7-heptanediol was found to aggregate above its critical micelle concentration (molar; 0.12 mol dm3), but the form of the aggregates was difficult to obtain from the rather featureless scattering curves. In our previous SANS investigations of aqueous solutions of small organic molecules, we used a simple solution model in which the distribution of the species resembles statistical concentration fluctuations [4,12]. In the present work, we repeated the SANS measurements on aqueous solutions of 1,7-heptanediol reported in Ref. [6], extending the studied concentration range in order to characterize the structure of the solutions on the nanometerlength scale.
2. Experimental Small-angle neutron scattering on solutions of 1,7heptanediol in heavy water was measured on the SANS instrument Yellow Submarine installed on the cold neutron beam line at the Budapest Research Reactor [13]. The scattered intensity at small angles is proportional to the square of the contrast between the scattering length densities of the species in a two-component solution. By using heavy water as solvent, the contrast could be increased substantially, thus improving the precision of the experiment. The measurements were performed at 25 8C, on four samples with heptanediol molarities 0.11, 0.33, 0.66 and 1.5 mol dm3. The solutions were prepared by weighing from commercial 1,7-heptanediol (Fluka, purityN97%) and heavy water of deuterium content higher than 99 at.%. The samples were measured in 2-mm-thick quartz cells. The counting time was 20 min for each solution. The measured scattering curves were corrected for the sample transmission, the scattering from the cell and the room background. The normalization to the detector efficiency and conversion of the measured scattering to absolute scale was performed by measuring the scattering from a light water sample. The range of scattering vectors q was 0.06–0.41 21 [ q=(4p/k)sinH; k is the neutron wavelength and 2H is the scattering angle]. The average statistical error of the measured data points for the 1.5 molar solution was 0.5%, and the uncertainty of converting to absolute scale was about 10%.
3. Results and discussion The experimental scattering curves obtained at 25 8C are shown in Fig. 1. Enhanced scattering at small angles is observed for all solutions, indicating the nonhomogeneous distribution of the species in the solution. The shape of the scattering curve of the 0.66 mol dm3 solution resembles the SANS curve of 0.7 M 1,7-heptanediol solution shown in
Fig. 1. SANS scattering curves of 1,7-heptanediol solutions measured at 25 8C. M denotes the concentration in mol dm3. The solid lines are fits to the Ornstein–Zernike model.
[6]. The small-angle scattering of the 0.33 mol dm3 solution is appreciably higher than that observed by D’Arrigo et. al. [6] at 20 8C; this may be due to the different experimental temperatures. The usual way of analyzing SANS experimental data (see, e.g., Ref. [2]) consists of modeling the sample structure by assuming various theoretical models, and of calculating the corresponding scattering curves. Then the calculated curves are compared with the experimental data, and the validity of the model can be checked by analyzing the consistency of the fitted curves with the experimental data. The SANS technique is sensitive to the distribution of the species in a liquid mixture of small molecules. Molecules in a two-component mixture can arrange themselves in various structures forming inhomogeneous distribution of the scattering length density, which leads to enhanced smallangle scattering. One possibility is that the molecules selforganize in compact micelle-like aggregates, the hydrophobic alkyl chains being turned inside the micelle and the OH groups turned outside, contacting with water. The
L. Alma´sy et al. / Journal of Molecular Liquids 118 (2005) 141–143 Table 1 Results of the least squares fitting of Eq. (1) to the SANS data of 1,7heptanediol – heavy water mixtures Concentration, mol dm3
I 0, cm1
n, 2
Bg, cm1
m2
0.33 0.66 1.5
0.043F0.011 0.149F0.002 1.027F0.008
2.1F0.5 3.4F0.1 8.0F0.1
0.078F0.011 0.114F0.002 0.212F0.001
3.9 1.1 1.7
143
presently proposed model does not consider micelles or similar compact aggregates, but assumes a continuous distribution of the components, similar to an interconnected diol network. Hydrogen bonds between the diols and water and also between the neighboring diol molecules can probably form in addition to the conventional closecontact hydrophobic interactions.
m2 is the variance of the residuals.
scattering of such aggregates can be modeled as scattering from spherical objects, as was done in several SANS studies of aqueous alcohol solutions [2,6,7]. Another possibility for the arrangement of the two species in the mixture resembles the random concentration fluctuations of the components. In this case, the distribution of the scattering length density (roughly the distribution of the solute molecules) is assumed to be similar to the density distribution in a pure liquid in a state not far from the critical point [14]. This structure is well described by the Ornstein– Zernike model of concentration fluctuations, and the corresponding scattering function is given by Eq. (1): I ð qÞ ¼
I0 þ Bg 1 þ q2 n2
ð1Þ
where I 0 is the coherent forward scattering intensity, n is the correlation length that is the measure of the decay of the correlation in the scattering length density distribution and Bg is a flat background term. The latter accounts for the contribution of the incoherent scattering from the hydrogen and deuterium atoms in the mixture and a possible residual background not subtracted in the course of the correction procedures. The experimental scattering curves were fitted to Eq. (1) by the method of weighted least squares using three adjustable parameters. The results obtained are collected in Table 1 (solutions 0.33, 0.66 and 1.5 mol dm3), and the fitted curves are shown in Fig. 1. In the case of the 0.11 mol dm3 solution, the poor counting statistics and the low coherent scattering intensity prevented the use of unconstrained fitting; only the enhanced small-angle scattering can be seen in the measured data (Fig. 1). The Ornstein–Zernike model function fits the measured scattering curves within their experimental uncertainty rather well, the values of the computed m2 parameter are close to 1. Therefore, we conclude that 1,7-heptanediol– water mixtures are adequately described by random concentration fluctuations, and there is no need to use more complicated models. The model used in Ref. [6] assumed micellar particles with attractive interactions and the five adjustable parameters could be estimated only with rather large uncertainty. Despite the difference between the two approaches, our analysis supports the conclusions of D’Arrigo et al. [6] according to which bdryQ micelles cannot be present in the solution, only mixed aggregates containing water and diol molecules are formed. The
4. Conclusions Small-angle neutron scattering measurements have been performed on heavy water solutions of the nonionic surface active compound 1,7-heptanediol at 25 8C in the molarity range 0.11–1.5 mol dm3. All the studied 1,7-heptanediol solutions are spatially nonhomogeneous. Aggregation of the alcohol molecules takes place even at the lowest concentration studied (0.11 mol dm3). The structure of the 1,7heptanediol solution was modeled by assuming loosepacked alcohol aggregates which can be described by the model of random concentration fluctuations. The good agreement of the scattering curves, calculated from this model, with the experimental data indicates that 1,7heptanediol molecules aggregate in a loose way, possibly forming a network structure interconnected by hydrogen bonding.
Acknowledgements This work has been supported by the EC, under contract No. HPRI-CT-1999-00099 and ICA1-CT-2000-70029. Cs. Muzsnay is grateful to the foundation Domus Hungarica for a research fellowship.
References [1] K. Nishikawa, T. Iijima, J. Phys. Chem. 97 (1993) 10824. [2] G. D’Arrigo, J. Teixeira, J. Chem. Soc., Faraday Trans. 86 (1990) 1503. [3] I. Shulgin, E. Ruckenstein, J. Phys. Chem., B 103 (1999) 872. [4] L. Alma´sy, G. Jancso´, L. Cser, Appl. Phys., A 74 (2002) S1376 (Suppl.). [5] Y. Kato, Chem. Pharm. Bull. 10 (1962) 771. [6] G. D’Arrigo, R. Giordano, J. Teixeira, Langmuir 16 (2000) 1553. [7] G. D’Arrigo, R. Giordano, J. Teixeira, Eur. Phys. J., E 10 (2003) 135. [8] S.M. Hajji, M.B. Errahmani, R. Coudert, R.R. Durand, A. Cao, E. Taillandier, J. Phys. Chem. 93 (1989) 4819. [9] K. Jerie, A. Baranowski, J. Glin´ski, J. Przybylski, J. Radioanal. Nucl. Chem. 246 (2000) 343. [10] K. Jerie, A. Baranowski, J. Glin´ski, K. Orzechowski, J. Radioanal. Nucl. Chem. 241 (1999) 265. [11] K. Jerie, A. Baranowski, J. Glin´ski, K. Orzechowski, Acta Phys. Pol., A 99 (2001) 393. [12] L. Alma´sy, L. Cser, G. Jancso´, J. Mol. Liq. 101 (2002) 89. [13] L. Rosta, Appl. Phys., A 74 (2002) S52 (Suppl.). [14] H.E. Stanley, Introduction to Phase Transitions and Critical Phenomena, Clarendon Press, Oxford, 1971.