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Riveros, D.C., Vargas, E.F. and Hefter, G. (2015) Enthalpies of dissolution of long chain-length alkyltrimethylammonium

bromide salts in water at temperatures from 278.15 to 308.15K. The Journal of Chemical Thermodynamics,

81. pp. 109-115.

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Accepted Manuscript

Enthalpies of dissolution of long chain-length alkyltrimethylammonium bro-mide salts in water at temperatures from 278.15 K to 308.15 K

Diana C. Riveros, Edgar F. Vargas, Glenn Hefter

PII: S0021-9614(14)00303-6DOI: http://dx.doi.org/10.1016/j.jct.2014.09.018Reference: YJCHT 4049

To appear in: J. Chem. Thermodynamics

Received Date: 11 July 2014Revised Date: 11 September 2014Accepted Date: 13 September 2014

Please cite this article as: D.C. Riveros, E.F. Vargas, G. Hefter, Enthalpies of dissolution of long chain-lengthalkyltrimethylammonium bromide salts in water at temperatures from 278.15 K to 308.15 K, J. Chem.Thermodynamics (2014), doi: http://dx.doi.org/10.1016/j.jct.2014.09.018

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1

Enthalpies of dissolution of long chain-length alkyltrimethylammonium

bromide salts in water at temperatures from 278.15 K to 308.15 K

Diana C. Riveros,a Edgar F. Vargas

a* and Glenn Hefter

b

a Laboratorio de Termodinámica de Soluciones, Departamento de Química, Universidad de

los Andes, Bogotá, D.C., Colombia

b Chemistry Department, Murdoch University, Murdoch WA 6150, Australia

* Corresponding author, e-mail: [email protected]

Keywords: enthalpy, solution, heat capacity, alkyltrimethylammonium ions, hydrophobic,

calorimetry, micelle

Abstract

Molar enthalpies of dissolution, ∆slnHm, of decyl-, dodecyl- tetradecyl- and hexadecyl-

trimethylammonium bromide salts (RMe3NBr) in water have been measured

calorimetrically as a function of concentration and temperature. Standard state values,

ο∆ msln H , were obtained by extrapolation to infinite dilution and showed a smooth variation

with the number of carbon atoms, nC, in the alkyl chain R, unlike their shorter congeners

(nC ≤ 6). The corresponding molar isobaric heat capacity changes, ο∆ mp,slnC , were calculated

from the temperature dependence of ο∆ msln H and also varied smoothly with nC but were

more than an order of magnitude greater than those of their shorter congeners, probably

reflecting a marked increase in hydrophobicity with increasing nC. The behaviour of the

2

RMe3NBr salts was found to parallel broadly that of the symmetrical tetraalkylammonium

salts, R4NBr.

3

1. Introduction

Alkyltrimethylammonium salts (RMe3NX) have been widely used in chemical,

biochemical, pharmaceutical and industrial studies as model compounds for investigating

the effects of cationic hydrophobicity on solution structure and other properties. A major

advantage of these salts is that the alkyl chain length can be systematically varied over a

very wide range without incurring the solubility or synthetic limitations of, for example, the

popular symmetrical tetraalkylammonium salts (R4NX). Not surprisingly therefore, the

thermodynamic and transport properties of the RMe3NX salts in water have been studied

extensively. Measurements have included molar volumes [1-7], electrical conductivities

[4,5,7-9], heat capacities [3,4,7,10], osmotic coefficients [7,11] and dilution enthalpies

[12,13]. Studies relating to the micellar behaviour of RMe3NX salts in water and other

solvents have also been reported [14,15].

The molar enthalpy of dissolution of a solid in a solvent, ∆slnHm, is a sensitive probe of

solution behaviour that, unlike many thermodynamic properties, can be measured with

good accuracy down to very low solute concentrations. Such measurements for the shorter

chain-length alkyltrimethylammonium bromide salts (alkyl = ethyl to hexyl) have been

reported as a function of solute concentration and temperature by Tamaki and Furuya [16].

Their results have shown that ∆slnHm does not vary monotonically with the carbon chain

length, exhibiting instead a more complex pattern of behaviour. On the other hand, the

corresponding isobaric heat capacity changes, ∆slnCp,m, were found increase reasonably

smoothly from R = Me to Hx [16]. Interestingly, broadly similar (but differing in detail)

trends in both ∆slnHm and ∆slnCp,m are observed for the related tetraalkylammonium

bromides in water [17-19].

4

It is of interest to extend the measurements of ∆slnHm for the alkyltrimethylammonium

bromides to cations of longer chain lengths than those studied by Tamaki and Furuya [16]

to establish whether or not the patterns observed for the less hydrophobic cations are

continued. Accordingly, the present paper reports molar enthalpies of dissolution in water

for four alkyltrimethylammonium bromides CnH2n+1(CH3)3NBr, which will be abbreviated

throughout as CnMe3NBr, with long, even-numbered alkyl chains (alkyl = decyl to

hexadecyl). The solution enthalpies for C10Me3NBr and C12Me3NBr were measured at

concentrations below the CMC. For C14Me3NBr the measurements straddled the CMC

while for C16Me3NBr all measurements, except for the lowest concentration, were above

the CMC. The measurements were made at four temperatures from 278.15 K to 308.15 K to

yield values of the corresponding isobaric heat capacity changes.

2. Experimental

2.1 Reagents

The alkyltrimethylammonium bromide salts were obtained from commercial sources.

Decyltrimethylammonium and dodecyltrimethylamonium bromides (C10Me3NBr and

C12Me3NBr) were purchased from Alfa Aesar (USA), while tetradecyltrimethylammonium

and hexadecyltrimethylammonium bromides (C14Me3NBr and C16Me3NBr) were bought

from Sigma (USA) and Merck (Germany), respectively. All four salts were dried for ~1 h

at a pressure of ~100 torr at T = 350 K then stored in a desiccator over freshly-activated

silica gel. According to the suppliers each salt had a mass fraction purity of ~0.99; this was

confirmed by potentiometric titration against a standard solution of Ag+(aq). Water was

doubly distilled, first from alkaline KMnO4 solution and then from water alone. Its

5

conductivity was ≤ 2 µS⋅cm–1. The specifications of the chemical substances are given in

table 1.

2.2 Apparatus and procedures

Enthalpies of solution of the target salts in water were measured using a semi-adiabatic

solution-reaction calorimeter described in detail elsewhere [20]. The calorimeter vessel was

immersed in a thermostat developed in our laboratory whose temperature was controlled

with a Julabo LC6 unit to better than ±0.005 K at all temperatures studied. The calorimeter

was tested by measuring the dissolution enthalpies in water of potassium chloride (KCl(s))

at T = 298.15 K and sodium chloride (NaCl(s)) at 308.15 K, according to literature

recommendations [21, 22]. The values obtained, (17.46 ± 0.21) kJ⋅mol-1 and (3.09 ± 0.04)

kJ·mol-1

, respectively, were in excellent agreement with the corresponding literature values

of (17.584 ± 0.017) kJ⋅mol-1 [21] and (3.068 ± 0.008) kJ⋅mol-1[22]. Solute masses were

determined without buoyancy correction using an analytical balance with a precision of

0.01 mg. The molar solution enthalpies, ∆slnHm, of the alkyltrimethylammonium bromide

salts in water were measured in the approximate concentration range of (1 to 9) mmol⋅kg-1.

The experimental uncertainty of ∆slnHm was less than 1 %, with the exception of

C12Me3NBr at 278.15 K where the experimental uncertainty was less than 3 %.

3. Results

The values obtained for ∆slnHm for the present alkyltrimethylammonium bromide salts in

water at temperatures (278.15, 288.15, 298.15 and 308.15) K are listed along with their

standard uncertainties in tables 2 to 5. Standard state (infinite dilution) molar enthalpies of

6

solution, ο∆ msln H , were obtained at each temperature, by fitting the relevant experimental

�slnHm(m) values with an equation proposed by Pitzer et al. [23, 24]:

( ) ( ) ( ) ( ) IβHIfbIbAzzmH // +∆==+ν−∆ οmsln2121HXMmsln 1ln2 (1)

where f (I1/2

) is introduced to represent the left-hand side of eq (1) and where b = 1.2 is a

constant [23], AH is the Debye-Hückel constant for enthalpies, I (= 0.5 Σ mizi2) is the

stoicheometric ionic strength of the solution, β is a temperature-dependent empirical

parameter and other symbols have their usual meanings [23, 24]. The values of AH/RT =

(0.62663, 0.70535, 0.80185 and 0.9072) kg-1/2⋅mol-1/2 at (278.15, 288.15. 298.15 and

303.15) K, respectively were taken from Archer and Wang [25]. The values of ο∆ msln H and

β (table 6) were obtained by means of weighted (× σ∆H–2

) linear regressions of f (I1/2

)

against I at each temperature. Representative plots (at 298.15 K) are given in figure 1, while

the results obtained at all temperatures are summarized in table 6, along with their standard

uncertainties.

The temperature dependence of ο∆ msln H over the range 278.15 ≤ T/K ≤ 308.15 was found to

be approximately linear for all of the present salts. Accordingly, the isobaric heat capacity

change accompanying the dissolution process, ο∆ mp,slnC was assumed to be independent of

temperature and the ο∆ msln H (T) values were fitted using the equation:

TCaH οmp,slnο

msln ∆+=∆ (2)

where a is an empirical parameter. The numerical values of ο∆ mp,slnC are given in table 6.

7

4. Discussion

4.1 Enthalpies of solution

Although the variation of ∆slnHm with solute concentration (tables 2–5) was rather small:

typically 0.99 in most cases

(figure 1, but see also section 4.3 below). At 278.15 K, the lowest temperature investigated,

the ∆slnHm values of all of the present salts in water (table 2) were negative and became

increasingly exothermic with increasing concentration, corresponding to a negative β

parameter. In contrast, at (288.15, 298.15 and 308.15) K, the values of ∆slnHm (tables 3–5)

were positive and became more endothermic with increasing concentration, corresponding

to a positive β. The standard state enthalpies, ο∆ msln H , and the corresponding empirical β

values obtained via eq (1) are listed in table 6.

While solution enthalpies varied little with solute concentration, the effects of temperature

are dramatic (figure 2). For example, increasing the temperature from 278.15 K to 308.15 K

caused ο∆ msln H (C16Me3NBr) to become more positive by over 100 kJ⋅mol-1

changing from

strongly exothermic to strongly endothermic. Similar, if somewhat smaller, changes in

ο∆ msln H were observed for the other salts (figure 2). Such large changes indicate large

positive values of ο∆ mp,slnC , which will be further discussed below (section 4.2).

To the best of our knowledge, no studies of the enthalpies of solution in water for the

C10Me3NBr to C16Me3NBr salts have been reported in the literature and so no direct

8

comparisons with the present results are possible. On the other hand, as noted earlier,

ο∆ msln H values for C1Me3NBr to C6Me3NBr have been determined [16, 26] over a similar

range of temperatures to the present study. For convenience, those results are plotted in

figure 3 and show that ο∆ msln H for these salts do not vary monotonically with respect to the

number of carbon atoms in the alkyl chain. Instead it was found that:

C1Me3NBr > C5Me3NBr > C2Me3NBr > C6Me3NBr > C4Me3NBr > C3Me3NBr

at all temperatures investigated (although, as is apparent from figure 3, the ordering might

well change over a wider T range). In contrast to the behaviour of the shorter-chain salts,

figure 4 shows that the ο∆ msln H values for the present RMe3NBr salts vary systematically

with nC, the number of carbon atoms in the alkyl chain R, at all of the investigated

temperatures, even when those values change from endothermic to exothermic.

The differences and similarities between the shorter (nC ≤ 6) and the longer (10 ≥ nC ≥ 16)

alkyl chain salts is better illustrated in figure 5. With the exception of C5Me3NBr, all of the

ο∆ msln H values for CnMe3NBr lie on a more-or-less smooth but complex curve as nC

increases. Also shown in figure 5 are the ο∆ msln H values for the R4NBr salts (R = Me to

Pe). Interestingly, these quantities show very similar behaviour to the present RMe3NBr

salts, including the sharp increase in ο∆ msln H going from nC = 4 to nC = 5.

4.2 Heat capacity changes

The change in the molar isobaric heat capacity accompanying dissolution, ο∆ mp,slnC ,

obtained via eq (2) are listed for the present salts in table 6. These values are also plotted in

9

figure 6 along with the corresponding literature values for C1Me3NBr to C6Me3NBr [16,

26]. Unlike ο∆ msln H (figure 5), the ο∆ mp,slnC values vary reasonably smoothly with

increasing chain length for both shorter (nC ≤ 5) and longer (10 ≥ nC) alkyl groups. More

remarkably, the heat capacity changes for the longer chain length cations are more than an

order of magnitude greater than those of the shorter ones. While some of this change is a

reflection of the increasing number of bonds in the cations, there is clearly a significant

change in the effect of the addition of a single –CH2– group to RMe3N+ with increasing

alkyl chain length (figure 6). This might reflect a curling up of the longer alkyl chains on

themselves, an increase in water structure with increasing cation hydrophobicity, or some

degree of sub-micellar aggregation (nano-heterogeneity). In this context, it would be useful

to have the ο∆ mp,slnC values for the C7 to C9 salts.

Figure 6 also shows the available literature

data for ο∆ mp,slnC for the symmetrical

tetraalkylammonium salts (R4NBr, nC ≤ 5). These values also vary smoothly with

increasing nC although they seem to have more in common with the longer chain RMe3NBr

salts than the shorter ones. It is noteworthy that the average change per –CH2– group in

ο∆ mp,slnC (R4NBr) going from R = Me to Pe is more than five times the corresponding value

of ο∆ mp,slnC (RMe3NBr) rather than the expected four. This again suggests some degree of

interaction amongst the alkyl chains, or a significant increase in the structure of water

around the increasingly hydrophobic cations, or incipient nano-heterogeneity, or some

combination of all of these effects. As ο∆ mp,slnC values reflect the differences between the

heat capacities of the solid and the relevant solution little more can be said about them at

this stage, in the absence of the appropriate data for the solids.

10

4.3 Micellar behaviour

The alkyltrimethylammonium bromide salts studied here are considered to be surfactants

and their critical micelle concentrations (CMCs) in water have been reported by several

authors [5, 7-9, 14]; the available data are summarized in table 7. As would be anticipated

for genuinely independent measurements, the results are somewhat scattered but they are

sufficiently consistent to draw some useful conclusions. As was mentioned in the

introduction section, all solution enthalpies for C10Me3NBr and C12Me3NBr were measured

at concentrations below the CMC (compare the data in tables 2, 3 and 7). For C14Me3NBr,

the measurements straddled the CMC (compare tables 4 and 7), while for C16Me3NBr all

measurements, except at the lowest concentration, were above the CMC (compare tables 5

and 7). Given that the slopes of ∆slnHm vs. I curves either side of the CMC would be

expected to differ significantly [5], the results obtained suggest that the magnitude of the

molar enthalpy of aggregation (micellization), ∆aggHm, is very small compared with that of

∆slnHm and thus has almost no effect on it. The CMC values in table 7 do indeed show

almost no dependence on temperature (although the range is small) consistent with ∆aggHm

≈ 0. Much more interestingly, close inspection of the ∆slnHm data for C14Me3NBr at T =

298.15 K (table 5) particularly when compared with the corresponding results for the other

salts (figure 1), does show some evidence of a small break in slope. This break is more

easily seen in the plot of ∆slnHm against √m shown in figure 7. The CMC estimated from the

intersection of the two straight lines corresponds to m = 2.9 mmol⋅kg–1, which is in

reasonable agreement with the values given in table 7.

11

5. Conclusions

The present measurements indicate that there are both significant differences (especially

with respect to ο∆ msln H ) and similarities (with respect toο∆ mp,slnC ) between the

thermodynamic behaviour upon dissolution in water of the shorter (nC ≤ 6) and longer (nC ≥

10) alkyl chain RMe3NBr salts. The remarkable increase (by more than an order of

magnitude) in ο∆ mp,slnC as nC increases from 6 to 10 reflects a major change in the

hydrophobicity of the RMe3N+ ions. Analogous measurements on the intermediate chain-

length salts would be interesting.

Acknowledgement

The authors wish to thank the Faculty of Science of the Universidad de los Andes for

financial support.

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12

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13

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14

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Table 1. Specification of chemical samples

Compound Source Purification Method Mass fraction

purity Analysis method

C10Me3NBr Alfa Aesar Vacuum dried 0.99 Potentiometric titration

C12Me3NBr Alfa Aesar Vacuum dried 0.99 Potentiometric titration

C14Me3NBr Sigma Vacuum dried 0.99 Potentiometric titration

C16Me3NBr Merck Vacuum dried 0.99 Potentiometric titration

Water Doubly distilled

16

Table 2. Molar enthalpies of solution of the present alkyltrimethylammonium bromides in

water at T = 278.15 Ka.

m

/mmol⋅kg-1 ∆slnHm

/ kJ⋅mol-1 m


/ kJ⋅mol-1 C10Me3NBr C12Me3NBr

b

0.940 –16.3 1.265 –17.6

1.868 –16.6 2.346 –18.2

3.346 –17.0 3.038 –18.6

4.044 –17.2 4.070 –18.7

4.873 –17.7 6.241 –19.1

5.756 –17.9 6.991 –19.8

6.452 –18.2 7.941 –19.8

7.839 –18.3 5.110 –18.6

C14Me3NBr C16Me3NBr

1.122 –26.8 1.054 –39.2

2.010 –27.0 1.679 –39.6

2.792 –27.5 2.590 –39.5

3.903 –27.5 3.199 –39.7

4.431 –28.1 4.198 –39.7

5.073 –28.7 4.920 –39.7

6.201 –29.1 6.542 –39.8

7.033 –29.5 7.055 –40.4

7.619 –29.9

Second run

C10Me3NBr C12Me3NBrb

1.105 –16.4 1.344 –17.5

1.990 –16.5 2.109 –18.1

3.784 –17.0 3.215 –18.3

5.531 –17.9 4.355 –18.5

7.436 –18.3 6.999 –18.9

8.357 –18.7 8.216 –19.0

C14Me3NBr C16Me3NBr

0.868 –26.8 1.167 –39.1

1.899 –27.0 1.558 –39.5

3.458 –27.5 3.197 –39.6

5.080 –28.7 4.901 –39.7

6.819 –29.5 6.208 –40.4

8.001 –29.9 8.325 –40.6

17

a The phase state of the salts is crystalline. Pressure atmospheric p = 0.07466 MPa. Standard uncertainties u

are u(T) = 0.01 K, u(p) = 1 kPa, u(m) = 0.005 mmol⋅kg-1, and the combined expanded uncertainty Ur(∆H) =

0.01.b The combined expanded uncertainty was Ur(∆H) = 0.03, (0.95 level of confidence)

18



m


/ kJ⋅mol-1 m



1.105 7.9 1.619 12.1

2.099 8.0 1.889 12.2

3.209 8.0 3.035 12.3

4.428 8.1 3.784 12.2

5.182 8.1 4.572 12.3

6.115 8.1 5.798 12.8

7.460 8.3 6.614 12.8

8.123 8.4 7.333 13.0

9.415 8.4 8.681 13.4

C14Me3NBr C16Me3NBr

0.875 17.4 1.166 20.7

1.967 17.4 1.615 20.8

2.637 17.5 2.484 20.8

3.535 17.6 3.221 21.0

4.565 17.6 4.104 21.1

5.242 17.7 4.886 21.2

5.914 18.0 5.474 21.2

6.899 18.1 6.263 21.6

8.002 18.3



0.01, (0.95 level of confidence)

19



m

/ mmol⋅kg-1 ∆slnHm

/ kJ⋅mol-1 m



1.105 33.9 1.376 37.3

1.990 34.0 1.979 37.4

2.990 34.0 2.793 37.4

3.784 34.1 3.721 37.8

4.754 34.5 4.808 38.3

5.531 34.5 6.041 38.4

6.846 34.6 6.809 38.5

8.357 35.1 7.937 38.7

8.180 38.8

C14Me3NBr C16Me3NBr

1.262 41.4 0.787 48.2

1.995 41.5 1.563 48.5

2.704 41.8 2.464 48.3

3.818 42.2 3.073 48.2

4.245 42.7 3.853 48.8

5.084 43.0 4.805 49.2

6.068 43.6 5.149 49.3

6.700 43.8 5.714 49.8

7.474 44.3



0.01 (0.95 level of confidence)

20



m


/ kJ⋅mol-1 m



1.349 47.2 1.350 56.1

2.539 47.2 2.317 56.4

5.178 47.4 2.988 56.6

6.424 47.5 3.580 56.6

7.278 47.6 4.781 56.7

7.407 47.6 5.846 56.7

10.041 47.6 6.791 57.9

11.117 48.0 7.550 58.0

8.543 58.3

C14Me3NBr C16Me3NBr

0.885 58.0 0.820 64.9

1.602 58.2 1.816 65.3

2.884 58.3 3.362 65.6

3.589 58.3 4.317 66.1

4.783 58.4 5.036 66.1

5.064 58.7 5.767 66.5

5.739 58.7 6.342 66.6

6.570 58.6 7.653 66.8

7.590 59.0



0.01 (0.95 level of confidence)

21

Table 6. Standard molar enthalpies of solution in water of the present

alkyltrimethylammonium bromides along with the empirical parameter β(T) from eq (1) at

various temperatures and the corresponding standard isobaric heat capacity change at T =

298.15 K.

Temperature

/K

ο∆ msln H /

kJ⋅mol-1

∆Ηu a

/ kJ⋅mol-1

β

/ kJ⋅kg⋅mol-2

βua

/ kJ⋅kg⋅mol-2

ο∆ mp,slnC

/ J⋅K-1⋅mol-1

pCu∆

a

/ J⋅K-1⋅mol-1

C10Me3NBr

278.15 –15.8 0.1 –353 10

288.15 7.8 0.1 51 7

298.15 33.4 0.1 158 18 2110 140

308.15 46.8 0.1 80 9

C12Me3NBr

278.15 –17.7 0.1 –219 28

288.15 11.6 0.1 166 19

298.15 36.8 0.1 223 8 2610 150

308.15 54.8 0.3 420 48

C14Me3NBr

278.15 –26.1 0.1 –499 13

288.15 16.9 0.1 142 18

298.15 40.6 0.1 450 17 2690 260

308.15 57.8 0.1 134 5

C16Me3NBr

278.15 –38.8 0.1 –246 23

288.15 20.5 0.1 119 10

298.15 47.4 0.1 330 12 2770 660

308.15 64.8 0.1 242 12

a Uncertainties are the standard errors in the parameters calculated by the Table Curve 2D software [29].

Pressure atmospheric p = 0.07466 MPa. Standard uncertainties u are u(T) = 0.01 K, u(p) = 1 kPa.

22

Table 7. Literature data for the critical micelle concentrations (CMC) of the present

alkyltrimethylammonium bromide salts in water at various temperatures.

T = 293.15 K 298.15 K 303.15 K

Salt CMC/ CMC/ CMC/

mmol·L-1 mmol·kg-1 mmol·L-1

C10 Me3 NBr 67.6[8]

C12 Me3 NBr 10.01[5]

15.4[8]

; 15.2[7]

11.9[5]

C14 Me3 NBr 3.30[5]

3.79[8]

; 3.6[14]

; 3.84[9]

4.05[5]

; 3.91[9]

C16 Me3 NBr 0.98[5]

0.955[8]

; 0.9[14]

1.01[5]

23

Figure 1. Representative plots of f(I1/2

), the left hand side of eq (1), against ionic strength I

for the present alkyltrimethylammonium bromide salts at T = 298.15 K: (■) C10Me3NBr;

(×) C12Me3NBr; (▲) C14Me3NBr; (�) C16Me3NBr.

30

35

40

45

50

55

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

f(I

1/2

)/

kJ·

mol-

1

I / mmol·kg-1

24

Figure 2. Standard molar enthalpies of solution of the present alkyltrimethylammonium

bromide salts in water as a function of temperature: (■) C10Me3NBr; (×) C12Me3NBr; (▲)

C14Me3NBr; (�) C16Me3NBr. Lines are visual guides only; some have been omitted for

clarity.

-50

-30

-10

10

30

50

70

275 280 285 290 295 300 305 310

∆sl

nH

° m/

kJ·

mol-

1

T / K

25

Figure 3. Literature values [16,26] of the standard molar enthalpies of solution in water of

alkyltrimethylammonium bromide salts at various temperatures: (×) Me4NBr; (○)

EtMe3NBr; (●) PrMe3NBr; (�) BuMe3NBr; (�) PeMe3NBr; (�) HxMe3NBr. Lines are

visual guides only.

5

7

9

11

13

15

17

19

21

23

25

283 288 293 298 303 308 313

∆sl

nH

οm

/ k

J·m

ol-

1

T / K

26

Figure 4. Standard molar enthalpies of solution in water of the present

alkyltrimethylammonium bromide salts as a function of the number of carbon atoms in

their alkyl chain at various temperatures: (×) 278.15 K; (■) 288.15 K; (▲) 298.15 K; (�)

308.15 K. Lines are visual guides only.

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

9 10 11 12 13 14 15 16∆sl

nH

°m

/ k

J·m

ol-

1

nC

27

Figure 5. Standard molar enthalpies of solution in water at T = 298.15 K of alkyltrimethyl-

ammonium bromide salts: (●) this work; (○) literature data [16,26]; and of symmetrical

tetraalkylammonium bromides, (�) literature data [18,26, 27]. Lines are visual guides only.

-20

-10

0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18

∆sl

nH

om

/ k

J·m

ol-

1

nC

28

Figure 6. Standard heat capacity changes for dissolution in water,ο∆ mp,slnC , at T = 298.15 K

of alkyltrimethylammonium bromide salts: (●) this work; (○) literature data [16,26]; and

tetraalkylammonium bromide salts: (�) literature data [28]. Lines are visual guides only.

-500

0

500

1000

1500

2000

2500

3000

0 2 4 6 8 10 12 14 16 18

∆sl

nC

op,m

/ J·

K-1

·mol-

1

nC

29

Figure 7. Molar enthalpy of solution of tetradecyltrimethylammonium bromide at T =

298.15 K as a function of the square root of its molality. Lines are weighted linear

regressions.

41.0

41.5

42.0

42.5

43.0

43.5

44.0

44.5

0.03 0.04 0.05 0.06 0.07 0.08 0.09

∆sl

nH

m/

kJ·

mol-

1

m1/2 / (mol·kg-1)1/2

30

Highlights:

- The enthalpies of solution of surfactants in water were measured

- The character exothermic or endothermic depends on the temperature

- The hydrophobicity of the cations increases significantly between 6 to 10 carbons

Cover page author's versionlong chain-length alkyltrimethylammonium bromide salts

murdoch research repository · 2015. 2. 20. · 1 enthalpies of dissolution of long chain-length...

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