n-Hexanol Molecular Clusters Formed

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Aggregation Number in Water / n-Hexanol Molecular Clusters Formed in Cyclohexane at Different Water / n-Hexanol / Cyclohexane Compositions Calculated by Titration 1H-NMR Mario E. Flores, Toshimichi Shibue, Natsuhiko Sugimura, Hiroyuki Nishide, and Ignacio Moreno-Villoslada J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b08848 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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The Journal of Physical Chemistry

Aggregation Number in Water / n-Hexanol Molecular Clusters Formed in Cyclohexane at Different Water / n-Hexanol / Cyclohexane Compositions Calculated by Titration 1H-NMR Mario E. Flores,1 Toshimichi Shibue,2 Natsuhiko Sugimura,2 Hiroyuki Nishide,3 and Ignacio Moreno-Villoslada1* 1

Instituto de Ciencias Químicas, Facultad de Ciencias, Universidad Austral de Chile,

Valdivia, Chile. Email: [email protected] 2

Materials Characterization Central Laboratory, School of Science and Engineering,

Waseda University, Tokyo 169-8555, Japan 3

Department of Applied Chemistry, School of Science and Engineering, Waseda

University, Tokyo 169-8555, Japan Abstract Upon titration of n-hexanol / cyclohexane mixtures of different molar compositions with water, water / n-hexanol clusters are formed in cyclohexane. Here, we develop a new method to estimate the water and n-hexanol aggregation numbers in the clusters that combines integration analysis in 1D 1H-NMR spectra, diffusion coefficients calculated by diffusion ordered NM R spectroscopy, and further application of the Stokes-Einstein equation to calculate the hydrodynamic volume of the clusters. Aggregation numbers of 5 – 15 molecules of n-hexanol / cluster in the absence of water were observed in the whole range of n-hexanol / cyclohexane molar fractions studied. After saturation with water, aggregation numbers of 6 – 13 n-hexanol and 0.5 – 5 water molecules / cluster were found. 1 ACS Paragon Plus Environment


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O-H and O-O atom distances related to hydrogen bonds between donor / acceptor molecules have been theoretically calculated using the density functional theory. The results show that at low n-hexanol molar fractions, where a robust hydrogen bond network is held between n-hexanol molecules, addition of water makes the intermolecular O-O atom distance shorter, reinforcing molecular association in the clusters, whereas at high nhexanol molar fractions, where dipole-dipole interactions dominate, addition of water makes the intermolecular O-O atom distance longer, weakening the cluster structure. This has a correlation with experimental NMR results, which show an increase on the size and aggregation number in the clusters upon addition of water at low n-hexanol molar fractions, and a decrease on these magnitudes at high n-hexanol molar fractions. In addition, water produces an increase on the proton exchange rate between donor / acceptor molecules at all n-hexanol molar fractions.

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1. Introduction Due to the role of long-chain alcohol molecules as non-ionic co-surfactants in water-in-oil microemulsions,1-5 the investigation of the dynamics and behavior of such molecules in nonpolar solvents is of interest.6-8 They tend to associate in polymolecular clusters as a consequence of intermolecular hydrogen bonding and dipole-dipole interactions.9-13 In addition, alcohols in nonpolar solvents increase the solubility of water in these solvents,14-15 since water molecules can form complexes with the alcohol molecules integrating the hydrogen bond network. Indeed, the number of water molecules in the mixture influences the dynamics of alcohol association.11, 16-19 NMR spectroscopy allows, apart from exploring the chemical environment of molecules, obtaining information about their dynamics and self-association in solution.7, 2025

In the past few years, we have investigated by advanced NMR techniques alcohol

association in n-hexanol / cyclohexane mixtures.4,

11-12

Increasing the n-hexanol molar

fraction (Xhexanol) in such mixtures produces a change on the molecular dynamics, since the main forces that stabilize the clusters shift from hydrogen bonding to dipole-dipole interactions.11 We have investigated, in addition, the ability of an n-hexanol / cylohexane system presenting a well-structured hydrogen bond network to incorporate structured water molecules.4, 11 Although the maximum number of water molecules per alcohol molecules able to integrate the molecular cluster could be calculated, the aggregation numbers of nhexanol (Nh) and water (Nw) in the aggregates remain unknown. In this context, we have recently shown that Nh in n-hexanol / cyclohexane mixtures may be easily obtained correlating chemical shifts of hydroxyl protons in 1D 1H-



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NMR spectroscopy with DFT calculations at low Xhexanol.12 However, the method was not sensitive to calculate Nh at Xhexanol higher than 0.15, since the correlation found presented a profile very flat in these cases, making the chemical shifts of hydroxyl protons subjected to hydrogen bonding in the clusters indistinguishable if Nh takes values higher than 6 molecules / cluster. In order to overcome this problem, a new method to calculate Nh and Nw in clusters of water and long-chain alcohols in nonpolar solvent is developed and applied to the water / n-hexanol / cyclohexane system studied at different Xhexanol. After distinguishing the fraction of bound water from that of free water in the mixtures by 1D 1H-NMR, the method is based on the calculation of the diffusion constants of the mixture components by DOSY 1

H-NMR technique, and further application of the Stokes-Einstein equation. Titration with

water of n-hexanol / cyclohexane mixtures of different composition up to the maximum amount of water able to be incorporated in the clusters is performed. Then, the corresponding aggregation numbers in these mixtures are evaluated and compared with those of the original n-hexanol / cyclohexane initial mixtures, in the absence of added water. The influence of incorporated water to change the structure of the aggregates is also analyzed correlating the experimental 1H-NMR chemical shifts with DFT calculations of O-O and O-H atom distances of the molecules subjected to hydrogen bonding. 2. Experimental 2.1. Reagents. Commercially available cyclohexane (Sigma Aldrich, MW 84.16 g / mol, density 0.7796 g / cm3), anhydrous n-hexanol (TCI, MW 102.17 g / mol, density 0.8152 g / cm3), and acetone-d6 (Sigma Aldrich) were used without further purification.


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Deionized water was used for titration experiments of n-hexanol / cyclohexane mixtures of different initial Xhexanol. 2.2. Equipment. 1D and 2D 1H-NMR measurements were made at 298 K on an AVANCE600 spectrometer (Bruker). 2.3. Procedures. NMR. Samples containing n-hexanol and cyclohexane (1.5 mL) at different compositions were prepared in 5 mm diameter NMR tubes provided with a 2 mm diameter inner tube with acetone-d6, which was used for magnetic field locking system. Xhexanol will be referred to identify sample composition. The samples were sonicated for 1 minute in order to obtain a homogeneous mixture. Then, the different samples were titrated with water successively adding aliquots of water, followed by sonication, and analysis of 1D 1H-NMR spectra. Titration experiments were conducted in order to elucidate the maximum amount of water able to integrate water / n-hexanol clusters. Once this magnitude is calculated, diffusion ordered NMR spectroscopy (DOSY) analyses were made to the samples containing the maximum amount of water and no water. DOSY experiments were made under a stimulated echo sequence using bipolar gradients and a longitudinal eddy current delay. Diffusion delays of 40 ms and a gradient pulse length of 3 ms were applied in order to obtain appropriate curves (25 points) for inverse Laplace transformation. Theoretical Calculations. Calculations were carried out with the Gaussian09 program.26 Because of computational resource limits, methanol was used as n-hexanol model. Reported water / methanol binary cluster structures optimized by B3LYP density functional with the D3 dispersion correction and QZVP basis set were used.27 Theoretical NMR chemical shifts were calculated as the difference of isotropic shielding of reference molecules tetramethylsilane (0.00 ppm) and benzene (7.26 ppm), with the density 5 ACS Paragon Plus Environment


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functional M06-2x and gauge-invariant atomic orbital (GIAO) method combined with 6311+G(d,p) basis set.28-29 The integral equation formalism variant polarizable continuum model (IEFPCM) was used for cyclohexane solvent model. 3. Results and Discussion 3.1. Solubility of water in n-hexanol / cyclohexane mixtures. Titration with water of different n-hexanol / cyclohexane mixtures at different initial Xhexanol produced 1HNMR spectra that may show a variable number of water signals (see Figure S1). As a matter of example, Figure 1 shows the 1D 1H-NMR spectra of water / n-hexanol / cyclohexane at an original Xhexanol of 0.177 containing different amounts of water. At low water content, only one water signal appeared, as can be seen in Figure 1a, assigned to bound water, since its chemical shift is highly upfield shifted with respect to free water. From a certain threshold, which changes for each initial Xhexanol studied, another peak appears at around 4.5 ppm, which is assigned to free water protons (Figure 1b), corresponding to the core of formed micelles, a bicontinuous phase, or a microscopic separate phase.

Figure 1. 1H-NMR spectra of water / n-hexanol / cyclohexane at Xhexanol 0.177 and 0.1 % (v/v) (a) and 0.7 % (v/v) (b) showing respectively one and two signals for water protons. 6 ACS Paragon Plus Environment

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The signals corresponding to free water, bound water, and n-hexanol hydroxyl group appear separated from each other at any composition studied. This allows determining the n-hexanol / bound water molar ratio from the respective well-formed peak intensities, without needing integrating the broad signal corresponding to free water.

Figure 2. Ternary phase diagram of water / n-hexanol / cyclohexane mixtures at initial Xhexanol of 0.058 ( ), 0.087 ( ), 0.177 ( ), 0.301 ( ), 0.463 ( ), 0.564 ( ), 0.775 ( ) and 0.923 ( ). The highest area corresponds to multiphase systems. Titration was stopped when added water only increases the intensity of the free water peak. The composition of the samples studied is plotted in Figure 2 in a ternary system diagram. The solid line indicates the composition at which the clusters are considered saturated with water for each initial Xhexanol. In this report, we studied, then, the domains of the ternary phase diagram situated below this solid line, moving from the bottom edge of the phase diagram to the upper corner as the amount of added water increases. The maximum number of water molecules per 100 molecules of n-hexanol, calculated from the integrals of the bound water signal, increases linearly as a function of



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Xhexanol as can be seen in Figure 3a, indicating a higher ability to incorporate water in the clusters as the polarity of the media increases.

Figure 3. Maximum number of bound water molecules per 100 n-hexanol molecules (a) and molar ratio between n-hexanol and maximum bound water (b) as a function of the initial Xhexanol. Considering the information given in Figure 3a, the average amount of n-hexanol molecules which surrounds one bound water molecule at the saturation point can be calculated. The results are shown in Figure 3b. At initial Xhexanol of 0.058, we estimate a maximum of 11 – 12 n-hexanol molecules surrounding one water molecule, and this magnitude decreases as the initial Xhexanol increases, achieving a value around 2 at initial Xhexanol of 0.923. 3.2. Aggregation number in the clusters. Structuration of n-hexanol molecules forming hydrogen bond networks produces a slow-down on the molecular motion, a fact that can be corroborated by the determination of diffusion coefficients by DOSY NMR. The molecular volumes of cyclohexane (108.0 cm3 / mol), n-hexanol (125.3 cm3 / mol), and the parent hydrocarbon n-hexane used as control (131.6 cm3 / mol) are similar, so that similar diffusion coefficients are expected if the molecules move freely. The molecular 8 ACS Paragon Plus Environment

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volume was estimated making use of the molecular weight and the densities of the molecules.30-31 The diffusion coefficients of the solutes relative to that of cyclohexane are plotted in Figure 4 as a function of the molar fraction of n-hexane (Xhexane) or Xhexanol, in the absence and in the presence of water at the saturation point. An example of the signal intensity evolution versus gradient strength is shown in Figure S2, where it can be seen that the sensitivity of the DOSY technique to calculate small differences on the diffusion coefficients associated to changes in the microstructure of the clusters is remarkable.

Figure 4. Diffusion coefficients obtained by DOSY for n-hexane in cyclohexane ( ), nhexanol in cyclohexane ( ), n-hexanol in water / cyclohexane ( ), and water ( ) in nhexanol / cyclohexane relative to the diffusion coefficients of cyclohexane as a function of Xhexanol or Xhexane. It can be seen that the relative diffusion coefficient of n-hexane in cyclohexane ranges between 1.0 and 1.2, which indicates that the alkane is not aggregated, but homogenously mixed with cyclohexane. On the contrary, the relative diffusion coefficient of n-hexanol, bearing the hydroxyl moiety, is 0.4 – 0.5-fold smaller than that of the solvent



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cyclohexane, indicating that n-hexanol is forming clusters upon self-interaction. It can be seen that the diffusion constant of bound water is very similar to that of n-hexanol, ranging between 0.3 and 0.8 x 10-9 m2s, one order of magnitude lower than free water molecules in liquid water, whose diffusion coefficient is 2.3 x 10-9 m2s at 298 K,32 consistent with the interpretation that water is forming part of the hydrogen bond network in the clusters. On the contrary, free water associated to the second band of water appearing in the 1D 1HNMR spectra (see Figure1b) around the saturation point showed diffusion constants in the order of 1.6 x 10-9 m2s, similar to free water molecules in liquid water. The diffusion coefficients can be used to calculate Nh in the clusters, according to the Stokes-Einstein equation (equation (1)): D = kT/6πηR

(1)

where D is the diffusion coefficient, R is the hydrodynamic radius of the aggregates, k is the Boltzmann constant, T is temperature, and η is the solvent viscosity. For spherical particles,

RD = kT/6πη

(2)

is constant in each formulation, so that the ratio of diffusion coefficients may be easily converted to a ratio of hydrodynamic radii, following Dh / Dc = Rc / Rh

(3)

where the subscripts h and c refer to n-hexanol and cyclohexane, respectively. From the relative diffusion coefficients shown in Figure 4 and using equation (3) we estimate that the hydrodynamic radius of the aggregates fall in the range of 1.8 – 2.6-fold higher than that of


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cyclohexane, and the relative volume is 5.8 – 17.6-fold higher. Assuming that cyclohexane self-diffuses without undergoing strong molecular association, and that the size of the clusters can be approximated to the sum of the size of the n-hexanol molecules comprising them, Nh can be calculated from the formula Nh / Nc = (Dh / Dc)3 x (MWh / MWc) x (dh / dc)

(4)

where MW is the molecular weight, and d is the density, and Nc = 1 according to our previous assumption. Note that diffusion constants are given through DOSY measurements obtained in the range of the ms, so that the Nh obtained are averaged numbers at this time scale. In addition, the accuracy of the aggregation numbers calculated is related to the high sensitivity of the DOSY NMR technique to calculate the diffusion coefficients, overcoming the problem of lack of sensitivity obtained by using only DFT and 1D 1H-NMR chemical shift data.12 Figure 5 shows the calculated Nh with and without bound water. The contribution of water molecules to the total volume of the clusters has been considered in order to calculate Nh, showing very low differences if assumed negligible due to the small amount of water molecules incorporated and to the significant smaller size (18.0 cm3 / mol). The results show that, in the absence of added water, Nh increases with Xhexanol up to a Xhexanol of 0.8, shifting from 5 molecules / cluster at the lowest Xhexanol to almost 15 molecules / cluster at Xhexanol of around 0.8, after which a decrease is observed for nhexanol-reacher compositions. For higher clarity, the actual values are given in Table 1.



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Figure 5. Nh for n-hexanol / cyclohexane mixtures ( ), and for water / n-hexanol / cyclohexane mixtures ( ), estimated through equation (4), and Nw for water / n-hexanol / cyclohexane mixtures ( ) estimated by the experimental n-hexanol / water ratio, as a function of Xhexanol. Nh for pure n-hexanol estimated from literature data ( ).33 The distribution of cyclic and linear n-hexanol aggregate lengths in pure n-hexanol at 298 K has been reported from data obtained through a configurational-bias Monte Carlo simulation based on the transferable potential for phase equilibria-united atom force field.33 The authors report the values of the fraction of molecules forming linear aggregates (0.73), and closed cyclic aggregates (0.27). They also report the mean number of hydrogen bonds per molecule of n-hexanol (1.87). Since molecules forming cyclic aggregates present two hydrogen bonds, as well as molecules in linear aggregates, except those of the head and tail of the aggregates, the simple calculation 0.73 x (2 + 2 x (Nh - 2)) + 0.27 x 2 x Nh = 1.87 x Nh

(5)

allows concluding that the average Nh in pure n-hexanol ranges between 11 and 12. This result finds good agreement with the tendency found with our results if the Xhexanol is extrapolated to 1.0 in the absence of water, as can be observed in Figure 5.


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Table 1. Aggregation numbers in systems of different composition.

Xhexanol

Nh in n-hexanol /

Nh in water / n-hexanol /

Nw in water / n-hexanol /

cyclohexane mixtures



(molecules / cluster)



0.058

5.3

6.7

0.6

0.087

7.0

7.1

0.7

0.177

10.2

9.7

1.4

0.301

11.0

12.3

2.4

0.463

13.1

12.7

3.1

0.564

13.8

12.8

4.0

0.633

14.2

13.0

4.2

0.775

14.5

11.5

4.7

0.886

13.2

9.9

4.3

0.923

12.5

8.3

3.8



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3.3. Influence of water in the cluster size and structure. Nh after incorporation of water to the clusters follows a similar tendency that in the absence of water up to a Xhexanol of 0.5. It has been reported for n-hexanol / cyclohexane systems that at an Xhexanol of around 0.5 the entropy of activation for viscous flow takes positive values, indicating that both molecules are in a less ordered environment compared to solutions at Xhexanol lower than 0.5.34-35 Our own results distinguish that beyond Xhexanol of 0.2, dipolar interactions become more important than hydrogen bonding in the dynamic behavior of n-hexanol in cyclohexane.11 Thus, up to Xhexanol of 0.5, water binds tightly to the n-hexanol hydrogenbond network. For Xhexanol values higher than 0.5, water should contribute to weaken the clusters, which produce a reduction in Nh at Xhexanol values higher than 0.7, as can be seen in Figure 5 and Table 1. From the results of n-hexanol / bound water ratio shown in Figure 3b and the Nh shown in Figure 5 for water / n-hexanol clusters, Nw can be calculated. The results, also shown in Figure 5 and Table 1, indicate an increase on Nw in the clusters as Xhexanol increases up to near 0.8, and a decrease on Nw at higher Xhexanol values. The chemical shifts of water and n-hexanol hydroxyl protons in the range of compositions studied are respectively plotted in Figures 6a and 6b as a function of the water content (in v/v %). The chemical shifts of water molecules are shifted downfield in all cases with respect to that of bulk water, which appears at 4.7 ppm, corroborating their structured nature, integrating the hydrogen bond network formed by the alcohol. The chemical shift of bound water increases as the initial Xhexanol increases, indicating a higher probability of hydrogen bond formation. The increase of the water content for each initial Xhexanol condition also produces a downfield shift of the water proton signal, which can be due to an increase of the water-to-water hydrogen bonding. Similarly to water protons, the


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signal corresponding to the hydroxyl proton of the alcohol is shifted as the initial Xhexanol increases. The addition of water to the n-hexanol / cyclohexane systems produces abrupt downfield shifts at lower initial Xhexanol condition, almost no shift when the initial Xhexanol is 0.564, and slight upfield shifts for higher initial Xhexanol values. This is consistent with molecular associations dominated by dipolar interactions for n-hexanol-rich systems, of Xhexanol over 0.5, so that the hydrogen bonds lengths between associated n-hexanol molecules became slightly higher.

Figure 6. 1H-NMR chemical shift of bound water (a) and hydroxyl proton of n-hexanol (b) as a function of the water content for initial Xhexanol of 0.058 ( ), 0.087 ( ), 0.177 ( ), 0.301 ( ), 0.463 ( ), 0.564 ( ), 0.775 ( ) and 0.923 ( ). Theoretical calculations helped understanding the dynamics of water in the water / n-hexanol / cyclohexane system. Due to limited computational resources, methanol was used as alcohol model. A geometry optimized water / methanol binary system with compositions ranging from water / methanol 10 / 1 to 1 / 10 were used for theoretical NMR chemical shift calculation.27 Figure 7a shows the correlation between intramolecular O-H atom distance and theoretical NMR chemical shift. As can be observed, the NMR chemical 15 ACS Paragon Plus Environment


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shift is a sensitive probe of O-H bond distance change, and linear regression analysis indicated that a downfield shift of 0.1 ppm correlates with an increase of the atom distance of 0.046 Å. Figure 7b shows the correlation between intermolecular O-O atom distance and theoretical NMR chemical shift. The linear regression analysis indicated that a downfield shift of 0.1 ppm correlates with a decrease of 0.56 Å on the O-O atom distance. Thus, the downfield shifting at lower initial Xhexanol condition by the addition of water indicates shorter intermolecular bond distances in water / methanol binary clusters. These results indicated that additional water made n-hexanol association tighter. The slightly upfield shifts at higher initial Xhexanol condition by the addition of water indicate longer intermolecular bond distance in the clusters, indicating that titration of water makes aggregated molecules to loose, separate, or break. This explains that at high Xhexanol the diffusion coefficients of water and the alcohol increasingly differ, allowing faster movements for the smallest water molecules in the clusters, as can be seen in Figure 4.

Figure 7. Correlation between theoretical NMR chemical shifts and atom distance in water / methanol clusters. (a): intramolecular O-H atom distance in hydroxyl group; (b): intermolecular O-O distance; ( ) in methanol molecules; ( ) in water molecules. Linear adjustements: (a) (y = 0.0046x + 0.9569, R² = 0.9909); (b) (y = -0.056x + 3.085, R² = 0.9737). 16 ACS Paragon Plus Environment

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Figure 8. Differences in 1H-NMR chemical shifts between hydroxyl proton of n-hexanol and bound water as a function of water content for initial Xhexanol of 0.058 ( ), 0.087 ( ), 0.177 ( ), 0.301 ( ), 0.463 ( ), 0.564 ( ), 0.775 ( ) and 0.923 ( ). The observed chemical shift difference between water and n-hexanol hydroxyl protons is an indication of the proton exchange rate.6, 15 Proton exchange was observed by preliminary experiments where titration with deuterium water reduced the signal intensity of the alcohol hydroxyl proton (data not shown). A decrease on the difference of the chemical shift between both types of protons by adding water to the n-hexanol / cyclohexane system indicates that the exchange rate becomes faster. This is the case at every initial Xhexanol, as can be seen in Figure 8. At low water content, differences on around 1 ppm (600 Hz) are found for the signals of both exchangeable protons. A decrease on this difference to a plateau at 0.7 – 0.8 ppm (420 – 480 Hz) was observed at every initial Xhexanol values. At lower initial Xhexanol values, this decrease is abrupt, whereas tends to a smoother decay at increasing initial Xhexanol values. This phenomenon correlates with the change on the stabilization mechanism of the system previously observed, shifting from tightly bound molecules by hydrogen bonds to less tightly bound molecules subjected to dipole-dipole interactions, showing higher molecular mobility, but slower increase of the hydrogen exchange rate.11 17 ACS Paragon Plus Environment


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Conclusions. We have successfully estimated the aggregation numbers in water / n-hexanol clusters (Nh and Nw, respectively) formed in cyclohexane at different initial Xhexanol conditions by 1H-NMR, combining 1D 1H-NMR peak integration of n-hexanol and water protons, the information of relative diffusion coefficients of n-hexanol and cyclohexane obtained by DOSY NMR, and the further use of the Stokes-Einstein equation to calculate the hydrodynamic radius of the clusters. In the water / n-hexanol / cyclohexane ternary systems, saturation with water impacts both the aggregation numbers and the structure of the clusters. Nh ranged between 5 and 15 for n-hexanol / cyclohexane binary systems, and between 6 and 13 for water / n-hexanol / cyclohexane ternary systems, whereas Nw ranged between 0.5 and 5 for the latter systems. This simple methodology highlights the potential of NMR spectroscopy, in particular sensitive DOSY experiments, to study molecular dynamics in complex systems and calculate important magnitudes such as aggregation numbers in clusters. Supporting Information. 1D 1H-NMR spectra upon titration experiments and signal intensity versus gradient strength used in DOSY experiments to calculate diffusion coefficients are shown in the Supporting Information. Acknowledgements.


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This work was supported by FONDECYT No. 1050899, Doctoral Fellowship CONICYT (Chile) No. 21100111 (M.F.), and the Research Institute for Science and Engineering, Waseda University.



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TOC Graphic


n-Hexanol Molecular Clusters Formed

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