Dynamic light scattering study of aggregation in aqueous solutions of five amphiphiles

Accepted Manuscript Dynamic light scattering study of aggregation in aqueous solutions of five amphiphiles

Jacobo Troncoso, Katerina Zemánková, Aida Jover PII: DOI: Reference:

S0167-7322(17)31416-2 doi: 10.1016/j.molliq.2017.06.022 MOLLIQ 7468

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

5 April 2017 2 June 2017 5 June 2017

Please cite this article as: Jacobo Troncoso, Katerina Zemánková, Aida Jover , Dynamic light scattering study of aggregation in aqueous solutions of five amphiphiles, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.06.022

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ACCEPTED MANUSCRIPT Dynamic light scattering study of aggregation in aqueous solutions of five amphiphiles. Jacobo Troncoso, Katerina Zemánková, Aida Jover. Departamento de Física Aplicada, Universidad de Vigo, Campus Ourense, 32004

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Ourense, Spain.

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Departamento de Química Física, Universidad de Santiago de Compostela, Campus de

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Lugo, Lugo, Spain.

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Abstract

Aqueous dilute solutions of 2-butoxyethanol, 1,2-hexanediol, tert-butanol, 11-butylamine are analized through the dynamic light scattering

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pentylamine and

technique. The mutual diffusion coefficient is determined against amphiphile

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composition, showing, in all cases, clear minima in the dilute region. Using the Stokes-

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Einstein relation, the mean size of the aggregates is also obtained as a function of mole fraction. For enough concentrated solutions, structures in the range 2-20 Å are observed.

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It is found that both their size and the minimum amphiphile mole fraction for which aggregation is observed are strongly correlated with the hydrophobic character of the

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amphiphile.

Keywords: water solutions, amphiphiles, dynamic light scattering, aggregation, hydrophobicity

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Introduction

The study of dilute aqueous solutions of low molecular weight amphiphiles has attracted a large amount of work since long ago [1-2]. The behavior of these systems has revealed to be largely anomalous as compared to the expected behavior for a typical

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binary mixture. [3-33] Three clearly differentiated regions have been observed. [16-33] For very high dilutions the amphiphile is hydrated, that is, surrounded by water

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molecules. This significantly changes the water structure, inducing a counterintuitive

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decrease of the system entropy, a phenomena called hydrophobic hydration. As amphiphile dilution becomes lower, it was hypothesized to start forming molecular

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aggregates, i.e. nanoscopic regions in which the amphiphile composition is larger than

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in the bulk solution. These structures could be though as imperfect micelles, since water is not supposed to be largely excluded from them. If amphiphile composition becomes even larger, these structures disappear and the solution ceases to present such

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anomalous behavior.

Most of the works devoted to these questions have been focused only in a limited

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number of amphiphiles. Among them, tert-butanol and 2-butoxyethanol seemed to present a markedly strong anomalous behavior, fact that motivated an extensive study of

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such aqueous solutions to be carried out through a wide variety of experimental techniques [3-12, 16-21, 34-42]. One might think that these two amphiphiles could present some peculiarities that could make their aqueous solutions, in some way, special. However, in two recent papers [41-42], strong anomalies in heat capacity were found for a large set of amphiphiles in aqueous solution. The authors concluded that tert-butanol and butoxyethanol are not singular cases, but they can be included in a general picture for aqueous amphiphile solutions at least as regards to heat capacity behavior: it was found that other molecules with large enough hydrophobic tails also 2

ACCEPTED MANUSCRIPT show strong anomalies. As a rule, when the number of methylene groups is over two, clear non-regular behavior in heat capacity was observed. Moreover, the region where heat capacity anomalies happen moves towards more dilute regions as hydrophobic character of the amphiphile becomes stronger. Encouraged by these results, we have determined the mutual diffusion coefficients using the dynamic light scattering

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technique (DLS) for five amphiphiles for which large heat capacity anomalies have

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been observed: 2-butoxyethanol 1, 2-hexanediol, tert-butanol, 1-pentylamine and 1-

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butylamine. The study was carried out as a function of amphiphile composition in the dilute region and at 298.15 K. Clear signs of aggregation phenomena are observed and

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DLS data allow the mean size of the aggregates to be estimated. The analysis of these results reveals new insights about the microscopic processes involved in dilute aqueous

Experimental

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2.

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solutions of amphiphiles.

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MilliQ water was used to make the solutions. All other chemicals were obtained from Sigma-Aldrich with the following purities: 2-butoxiethanol (>99%), tert-butanol

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(>99.7%), 1,2-hexanediol (>98%), 1-pentylamine (>99%) and 1-butylamine (99.5%). Mixtures were made by weighing in a Mettler AE250 balance, with an uncertainty of

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0.1 mg, and prior to DLS measurements they were passed several times through Teflon 0.2 m filters in order to remove solid impurities and dust particles. A Malvern 4700 apparatus, equipped with an 75 mW Argon laser at 488 nm was used. All DLS measurements were done with the fotodetector located at 90º and temperature set to 298.15 K. Each experiment correlogram has been used to obtain the first-order autocorrelation function. Most studied samples can be fitted to a single exponential decay, given by the next equation:

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ACCEPTED MANUSCRIPT 𝑔1 (𝑡) = 𝐴 + 𝐵𝑒 𝑡/𝑡0

(1)

where, A, B and t0 are fitting coefficients. From the relaxation time t0, the mutual diffusion coefficient D was obtained: 1

0𝑞

(2)

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4𝜋𝑛 𝜆

𝜃

sin⁡( 2)

(3)

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𝑞=

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with q given by:

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𝐷=𝑡

being  the light wavelength (488 nm),  the scattering angle (90º) and n the refractive

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index of the solution. The latter quantity was estimated using the relation: (4)

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𝑛 = 𝜙𝑛1 + (1 − 𝜙)𝑛2

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where  is the amphiphile volume fraction and n1, n2 are the refractive indexes for pure liquids that form the mixture. This is a quite satisfactory definition for the refractive

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index of binary systems, and it has been proved to be highly accurate; deviations under 1 % have been found, even for strongly non-ideal mixtures [43-44]. Except for water,

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refractive indexes were measured at 488 nm using an Abbe refractometer. The obtained values are given in Table 1. For solutions in the very diluted regime, we found that g1 can be no longer represented by Eq. (1), but a second exponential with much larger relaxation time t1 must be added: 𝑔1 (𝑡) = 𝐴 + 𝐵𝑒 𝑡/𝑡0 + 𝐶𝑒 𝑡/𝑡1

(5)

This phenomenon was extensively studied [45-49], concluding that large structures, with linear dimensions above 100 nm, are formed. In Fig. 1, g1 was represented for two 4

ACCEPTED MANUSCRIPT solutions, in order to show the way in which these mesoscopic structures are manifested over the autocorrelation function. Since we are only interested in the small aggregates, although we had to use Eq. (5) for some mixtures, we present only the diffusion data obtained from the quick relaxation time t0.

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Dynamic viscosity  was determined using an Anton-Paar AMVn falling-ball viscometer. It was obtained by measuring the time tf that a steel ball lasts in falling

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through a capillary filled with the sample. Dynamic viscosity was obtained using the

(6)

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𝜂 = 𝐾(𝜌𝑏 − 𝜌𝑙 )𝑡𝑓

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equation:

where b is the ball density (7.751 g·cm-3), l the density of the liquid, calculated by

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interpolation from [42] and K, the apparatus constant, determined by calibration with water. More information about this experimental methodology can be found elsewhere

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[50].

3. Results and Discussion

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Figure 2 and Table 2 show the results obtained for the mutual diffusion coefficients against amphiphile mole fraction. Although there are some works which report D for 2-

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butoxyethanol and tert-butanol, only three among them present this quantity at 298.15 K and within the composition interval of interest. Figure 2 shows the comparison with these data: quite good agreement was obtained for both systems, fact that confirms the reliability of the followed experimental methodology. Mutual diffusion coefficient shows minima for all amphiphiles. The mole fraction of this minimum is around 0.05 for 2-butoxyethanol, 1-butylamine and 1,2-hexanediol, but for tert-butanol it goes toward the much large value of 0.19, and for 1-pentylamine, it moves to quite smaller composition, around 0.02. As for its magnitude, 2-butoxyethanol 5

ACCEPTED MANUSCRIPT shows the smaller one, around 5·10-11 m2·s-1 followed by 1-pentylamine and 1,2hexanediol with minima around 7·10-11 m2·s-1. Tert-butanol presents larger values, with minimum of 9·10-11 m2·s-1, and 1-butylamine shows the highest one, 20·10-11 m2·s-1, more than twice larger than the other amphiphiles. From viscosity measurements, an estimation of aggregate size can be calculated from

𝑘 𝑇

𝐵 𝜉 = 6𝜋𝜂𝐷

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the correlation length of concentration fluctuations:

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(7)

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being kB the Boltzmann constant, and  the dynamic viscosity. This expression is nothing but the Stokes-Einstein relation applied to particles with radius equal to 

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Viscosity was obtained from literature [51-56] except for 1-pentylamine, experimentally

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determined in this work, and given in Table 3.

Figure 3 gives the obtained results. It is worth to note that there are some Small Angle

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X-ray Scattering [16, 34] and Small Angle Neutron Scattering [35] data for tert-butanol

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+ water system which allows the comparison between the obtained correlation lengths to be performed. Both curves are quite similar; for instance literature data yields a maximum  value of 0.55 nm at x=0.14, highly coincident with our values (0.58 m at

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x=0.15). All systems show quite similar behavior against composition: no aggregates

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are observed until amphiphile mole fraction exceeds a threshold, then the aggregate size quickly increases until it reaches a maximum, and for higher mole fractions, it slowly decreases with composition. The composition at which aggregates start to be observed, show a clear tendency: for 1-pentylamine a mole fraction as small as 0.006 is obtained, followed by 1,2-hexanediol and 2-butoxyethanol, 1-butylamine and finally, tert-butanol, with a large value of 0.06. This composition seems to be analogous to the critical concentration in micellar systems: for values lower than this threshold aggregates do not appear. Therefore, a quite sharp change in the system is expected to be observed around 6

ACCEPTED MANUSCRIPT this mole fraction, which would affect the physical properties of these aqueous solutions. Several thermodynamic analyses for these amphiphiles have been carried out [17-33, 41-42], observing anomalies in third order derivatives of the thermodynamic potential. The peaks of these anomalies are temperature dependent, developing in the (x,T) plane,

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the so called Koga line. This line is strongly affected by the amphiphile chemical nature,

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concretely with its “degree of hydrophobicity” [41-42], which is determined by the

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number of pure hydrophobic groups that the amphiphile has. For instance, 1pentylamine would present four pure hydrophobic groups, since the first methyl group

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is strongly affected by the nitrogen atom, and therefore it shows an important asymmetry in its charge distribution. It was found [41-42] that these Koga lines have

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higher slope and are more skewed to low mole fractions as the hydrophobic character of the amphiphile becomes stronger. This follows the same trend as that observed in this

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work: we show the larger aggregates at lower compositions for the amphiphiles with

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higher degree of hydrophobicity; as it becomes lower, aggregates are smaller and appear at higher amphiphile concentrations. In fact, it was argued [41-42] that the composition

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obtained from Koga line is the one that marks the beginning of aggregation, i. e. the above-cited composition threshold. It is possible to obtain the mole fraction calculated

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from these lines at 298.15 K and compare them with our data. Figure 4 shows that there is a clear correspondence between the composition obtained from Koga lines, xK, and the value at which aggregates start to appear, i.e the lower composition for which the  could be obtained, xmin. Finally, it must be noted the strong correlation between aggregate size and hydrophobic nature of the amphiphile: 1-pentylamine, shows the highest  (2 nm) , followed by 2-butoxyethanol (1.6) nm, 1,2-hexanediol (1.2 nm), and, finally the lowest value were observed for tert-butanol and 1-butylamine (0.6 nm). 2-

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ACCEPTED MANUSCRIPT butoxyethanol forms larger aggregates than 1,2-hexanediol, even though it should show a slightly lower hydrophobicity. This deviation from the general tendency could come from the fact that 2-butoxyethanol + water system present a lower consolute critical point with coordinates xc=0.0598 and Tc=321.65 K [50], which could affect the size of the observed aggregates.

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In this context, it must be noted that several experimental procedures have been

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developed in order to give a quantitative estimation the of the hydrophobic/hydrophilic

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character of a solvent such as partition coefficient evaluation [57], water content determination in phase-separated solutions [58] or glycerol and 1-propanol probing,[25-

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29]. This last methodology allowed Koga establishing a classification of solutes in water [31] as hydrophobes, hydrophiles or amphiphiles. Using this scheme, all liquids

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studied in this work would be hydrophobes. It must be noted that these names must be taken in the context of Koga’s work, since almost all classified molecules are

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commonly named amphiphiles, since they have hydrophilic and hydrophobic moieties.

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None of the compounds studied by Koga is purely hydrophobic, as, for instance, hexane, probably due to its very small solubility in water makes the proposed

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methodology very difficult to be applied. Therefore, our results strongly support the statement that the anomalies in the

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thermodynamic behavior of aqueous solutions of amphiphiles have their origin in the appearance of aggregates, which would be though as a pre-micellar molecular arrangement. However, it must be noted an important difference between typical micellar systems and those studied in this work: no complex structures at high values of the amphiphile appear, as it is often obtained in solutions that form micelles. The addition of amphiphile destroys the aggregation, probably due to an increase of these molecules outside the aggregates, which would make the medium to present a greater

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ACCEPTED MANUSCRIPT affinity for them, and thus, amphiphiles would migrate from these pre-micellar structures to the bulk solution, making them to become smaller and as amphiphile composition increases.

4. Conclusion

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The dynamic light scattering results for the studied amphiphiles revealed the appearance

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of aggregates with correlation lengths in the range 2-20 Å. Even though the studied

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systems show important differences as regards the dependence of the size against amphiphile mole fraction, the global behavior is quite similar. It was found a clear

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correlation between both the aggregate size and the amphiphile mole fraction at which

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these structures start to appear with the hydrophobic character of the amphiphile.

Acknowledgements

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Authors want to thank Spanish Ministry of Economy and Competitiveness for financial

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support under Grants No. FIS2011-29614 (J. Troncoso and K. Zemánková) and MAT2013-45657-P (A. Jover). Authors acknowledge Claudio A. Cerdeiriña for helpful

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References

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discussions.

[1] F. Franks, D.J.G. Ives, Q. Rev. 20 (1966) 1. [2] F. Franks, J.E. Desnoyers, Water Sci. Rev. 1 (1985) 171. [3] G. W. Euliss, C. M. Sorensen, J. Chem. Phys. 80 (1984) 4767-4773. [4] T.M. Bender, R. Pecora, J. Phys. Chem. 92 (1988) 1675. [5] R. C. Castillo, H. C. Domínguez, M. Costas, J. Phys. Chem., 94 (1990) 8731. [6] D.S. Wilcox, B.M. Rankin, D. Ben-Amotz, Faraday Discuss. 167 (2013) 177.

9

ACCEPTED MANUSCRIPT [7] D. Ben-Amotz, B.M. Rankin, B. Widom, J. Phys. Chem. B 118 (2014) 7878. [8] D. Ben-Amotz, J. Phys. Chem. Lett. 6 (2015) 1696. [9] B.M. Rankin, D. Ben-Amotz, S.T. van der Post, H.J. Bakker, J. Phys. Chem. Lett. 6 (2015) 688. [10] T. Kato, J. Phys. Chem. 89 (1985) 5750.

PT

[11] K Mochizuki, SR Pattenaude, D Ben-Amotz, J. Am. Chem. Soc. 138 (2016) 9045.

RI

[12] S. R. Pattenaude, BM Rankin, K Mochizuki, D Ben-Amotz, Phys. Chem. Chem.

SC

Phys. 18 (2016) 24937.

[13] J.G. Davis, K. P. Gierszal, P. Wang, D. Ben-Amotz, Nature 491 (2012) 582.

NU

[14] B.M. Rankin, D. Ben-Amotz, B. Widom, Phys. Chem. Chem. Phys. 17 (2015) 21960.

MA

[15] D Ben-Amotz, Annu. Rev. Phys. Chem. 67, (2016) 617. [16] K. Koga, Chem. Phys. Lett. 111 (1984) 176.

D

[17] Y. Koga, J. Phys. Chem. 100 (1996) 5172.

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[18] Y. Koga, Solution Thermodynamics and its Application to Aqueous Solutions: A Differential Approach, Elsevier, Amsterdam, 2007.

(2009) 5885.

CE

[19] Y. Koga, P. Westh, Y. Moriya, K. Kawasaki, T. Atake, J. Phys. Chem. B 113

AC

[20] K. Yoshida, S. Baluja, A. Inaba, Y. Koga, J. Chem. Phys. 134 (2011) 214502. [21] K. Yoshida, A. Inaba, Y. Koga, J. Sol. Chem. 43 (2014) 663. [22] Y. Koga, K. Nishikawa, P. Westh, J. Phys. Chem. B 111 (2007) 13943. [23] Y. Koga, Y. Miyakaki, Y. Nagano, A. Inaba, J. Phys. Chem. B 112 (2008) 11341. [24] Y. Koga, P. Westh, K. Nishikawa, S. Subramaniam, J. Phys. Chem B 115 (2011) 2995. [25] T. Kondo, Y. Miyazaki, A. Inaba, Y. Koga, J. Phys. Chem. B 116 (2012) 3571.

10

ACCEPTED MANUSCRIPT [26] T. Morita, P. Westh, K. Nishikawa, Y. Koga, J. Phys. Chem. B 116 (2012) 7328. [27] Y. Koga, Phys. Chem. Chem. Phys. 15 (2013) 14548. [28] Y. Koga, , Phys. Chem Chem. Phys. 15, (2013) 14548. [29] Y. Koga, P. Westh, Phys. Chem Chem. Phys. 16, (2014) 335. [30] K. Yoshida, P. Westh, A. Inaba, M. Nakano, Y. Koga, J. Mol. Liq. 202 (2015) 40.

PT

[31] Y. Koga, J. Mol. Liq. 205 (2015) 31.

RI

[32] H. Ohgi, H. Imamura, K. Yonenaga, T. Morita, K. Nishikawa, P. Westh, Y. Koga,

[33] Y. Koga, J. Mol. Liq. 219 (2016) 1006.

SC

J. Mol. Liq. 224 (2016) 401.

NU

[34] K. Nishikawa, H. Hayashi, T. Iijima, , J. Phys. Chem. B 93 (1989) 6559. [35] D. Subramaniam, D.T. Boughter, J.B. Klauda, M.A. Anisimov, Faraday Discuss.

MA

167 (2013) 217.

[36] G. D’Arrigo, J. Teixeira, J. Chem. Soc. Faraday Trans. 86 (1990) 1503.

D

[37] D.T. Bowron, J.L. Finney, A.K. Soper, J. Phys. Chem. B 102 (1998) 3551.

PT E

[38] R. Sinibaldi, C. Casieri, S. Melchionna, G. Onori, A. Segre, S. Viel, L. Mannina, F.D. Luca, J. Phys. Chem. B 110 (2006) 8885.

(2012)114509.

CE

[39] L. Comez, L. Lupi, M. Paolantoni, F. Picchio, D. Fioretto, J. Chem. Phys. 137

AC

[40] D. Subramanian, J.B. Klauda, P.J. Collins, M.A. Anisimov, J. Phys. Chem. B 118(2014) 5994.

[41] K. Zemánková, J. Troncoso, C. A. Cerdeiriña, L. Romaní, Chem. Phys. Lett. 640 (2015) 184–187. [42] K. Zemánková, J. Troncoso, C. A. Cerdeiriña, L. Romaní, M. A. Anisimov, Chem. Phys. 472 (2016) 36–43. [43] P. Brocos, A. Piñeiro, R. Bravo, A. Amigo, Phys. Chem. Chem. Phys. 5 (2003) 550-557. 11

ACCEPTED MANUSCRIPT [44] M. A. Iglesias-Otero, J. Troncoso, E. Carballo, L. Romaní, J. Chem. Thermodyn. 40 (2008) 949-956. [45] M. Sedlák, J. Phys. Chem. B 110 (2006) 4329–4338. [46] M. Sedlák, J. Phys. Chem. B 110 (2006), 4339–4345. [47] M. Sedlák, J. Phys. Chem. B 110 (2006), 13976–13984.

PT

[48] M. Sedlák, D. Rak, J. Phys. Chem. B 2013, 117 (2013) 2495–2504.

RI

[49] M. Sedlák, D. Rak, J. Phys. Chem. B 118 (2014) 2726–2737.

SC

[50] M. Souto-Caride, J. Troncoso, J. Peleteiro, E. Carballo, and L. Romaní, Phys. Rev. E 71 (2005) 041503.

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[51] D.-R. Chiou, S.-Y. Chen, L.-J. Chen, J. Chem. Eng. Data 55 (2010) 1012–1016. [52] P. K. Kipkemboi, A. J. Easteal, Can. J. Chem. 72 (1994) 1937-1945.

MA

[53] M. S. Paez, O. Julio, M. Méndez, D. Pérez, C. M. Romero, Rev. Colomb. Quim., 37 (2008) 105-119.

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[54] P. Jarosiewicz, G. Czechowski, J. Jadzyn, Z. Naturforsch. 59a (2004) 559-562.

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[55] M. A. Saleh, S. Akhtar, A. R. Khan, Phys. Chem. Liq., 39 (2001) 85-97. [56] M.-J. Lee, S.-M. Hwang, Y.-C. Kuo, J. Chem. Eng. Data 38 (1993) 577-579

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[57] L. Ropel, L. S. Belveze, S. N. K. V. Aki, M. A. Stadtherr, J. F. Brennecke, Green Chem., 7, (2005) 83.

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[58] S. Saita, Y. Kohno, H. Ohno, Chem. Commun., 49,(2013)93

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ACCEPTED MANUSCRIPT Table 1. Refractive index data of pure compounds at 488 nm. n 1.419 1.386 1.412 1.441 1.401

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Solute 2-Butoxyethanol tert-Butanol 1-Pentylamine 1,2-Hexanediol 1-Butylamine

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Table 2. Mutual diffusion coefficient for the studied systems.

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1,2-Hexanediol D/1010 2 -1 x m ·s 0.0100 4.32 0.0148 2.38 0.0175 2.12 0.0204 1.33 0.0252 1.08 0.0347 0.94 0.0399 0.85 0.0489 0.71 0.0631 0.70 0.0729 0.71 0.0801 0.73 0.1002 0.83 0.1093 0.84 0.1438 0.94 0.2037 1.14

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1-Pentylamine D/1010 2 -1 x m ·s 0.0063 6.96 0.0081 2.66 0.0096 3.08 0.0098 1.20 0.0109 3.15 0.0129 0.84 0.0167 0.76 0.0189 0.77 0.0198 0.82 0.0218 0.72 0.0268 0.74 0.0290 0.70 0.0329 0.80 0.0369 1.01 0.0415 0.96 0.0487 1.09 0.0578 1.46 0.0972 2.12 0.1389 2.88

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tert-Butanol D/1010 2 -1 x m ·s 0.0602 3.19 0.0755 1.75 0.0979 1.22 0.1209 1.10 0.1507 0.97 0.1932 0.91 0.2081 0.94 0.2484 0.91 0.2930 1.35 0.3003 1.30 0.4188 2.34 0.5049 3.96

1-Butylamine D/1010 2 -1 x m ·s 0.0420 2.80 0.0456 2.36 0.0490 2.24 0.0542 2.24 0.0617 2.30 0.0680 2.32 0.0750 2.39 0.0828 2.49 0.0994 2.76 0.1187 2.89 0.1316 3.13

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2-Butoxyethanol D/1010 2 -1 x m ·s 0.0186 1.74 0.0242 1.68 0.0309 0.92 0.0428 0.67 0.0604 0.43 0.0658 0.47 0.0961 0.52 0.0990 0.55 0.1035 0.69 0.1163 0.59 0.1416 0.84 0.1774 0.96 0.2126 1.44 0.2407 1.24

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Table 3. Dynamic viscosity for 1-pentylamine + water system. /mPa·s

x1 0.0133 0.0207 0.0254 0.0516 0.0812 0.0952 0.1268 0.1588

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1.193 1.401 1.528 2.150 2.590 2.715 2.918 3.040

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Figure 1. First-order correlation function for two 1-pentylamine aqueous solutions: black circles x= 0.0329, white circles x=0.0167.

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Figure 2. Mutual diffusion coefficients for aqueous mixtures against mole fraction of the amphiphile for (a) 2-butoxyethanol, (b) 1-pentylamine; (c) 1,2-hexanediol; (d) 1butylamine; (e) tert-butanol. Black and white symbols represent this work and literature data [3-5], respectively. Lines are only guides for the eye.

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Figure 3. Correlation length of concentration fluctuations  for aqueous mixtures against mole fraction of the amphiphile for: (a) 2-butoxyethanol, (b) 1-pentylamine; (c) 1,2hexanediol; (d) 1-butylamine; (e) tert-butanol. Lines are only guides for the eye. 18

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Figure 4. Minimum mole fraction of the amphiphile at which aggregates were observed,

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xmin, against amphiphile mole fraction at 298.15 K of the Koga line, xK.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights - Dynamic light scattering experiments were carried out for five amphiphiles in water. - Aggregates in the range 2-20 Å were observed. - These nanoscopic structures start to appear at a threshold composition.

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- Both aggregate size and threshold composition are strongly correlated with the hydrophobic character of the amphiphile.

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