S. Cabani, 6 .Conti, and L. Lepori
1030 (8) T. Yasunaga, S. Fujii. and M. Miura, J. Colloid Interface Sci., 30,
D. C. Poland and H. A. Scheraga, J. Coiioid interface Sci., 21, 273
399 (1 969). (9) R. Zana and J. Lang, C. R. Acad. Sci., Ser. C. 266, 893 (1968). (IO) R . Zana and J. Lang, C. R. Acad. Sci., Ser. C, 266, 1347 (1968). ( 1 1 ) E. Graber, J. Lang. and R. Zana, Kolloid-Z. Z. Polym., 238. 470 (1970).
(1966). In principle different relaxation speeds would resuit according to the particular micellar property measured in the process. When u and 141 are not too large, the measured properties will be essentiaily proportional to m3 and thus all give the same relaxation time. 0. Lamm, Ark. Kemi, 18A, no. 9 (1944). Equation 3 would give 6 = 1. A value of fl f 1 reflects a concentration dependence of k,+ and k,- which should be taken into account when higher accuracy is sought. (a) S. N. Wall, W. Karlsson, and E. A. G. Aniansson, unpublished results: (b) P. Mukerjee, J. Phys. Chem., 7%, 565 (1972);J. M. Corkill, J. F. Goodman, T. Walker, and J, Wyer, Proc. Roy. Soc., Ser. A, 312, 243 (1969). D. Attwood, P. H. Elsworthy, and S. B. Kayne, J. Phys. Chem., 74, 3529 (1970);H. Coil, ibid., 74, 520 (1970). By expanding R ( [ 1 ) and the numerator of (23)in powers of t1and comparing the coefficients of the two one finds that gogd compensation would result from a weak s dependence of k,-A, in the intermediate s region. Simple numerical examples will substantiate this conclusion, at least for [ < 0.05. G. Arfken, "Mathematical Methods for Physicists," Academic Press, New York. N. Y., 1966. See also forthcoming doctoral thesis of S. Wall. N. Bjerrum, Kgi. Dan. Vidensk. Selsk. Ski.. 12, no. 4, 7 (1915);2. Anorg. Aiig. Chem., 119, 179 (1921);F. J. C. Rossotti and H. Rossotti, "The Determination of Stability Constants," McGraw-Hill, New York, N . Y . , 1961.
P. J. Sams, E. Wyn-Jones, and J. Rassing, Chem. Phys. Lett., 13.
233 (1972). J. Rassing. p. J. Sams, and E. Wyn-Jones, J. Chem. SOC..Faraday Trans. 2. 69, 180 (1973). J. Oakes, J. Chem. Soc., Faraday Trans. 2, 68, 1464 (1972) K. K. Fox, Trans. Faraday Soc., 67, 2802 (1971). N. M. Atherton and S. J. Strach, J. Chem. SOC., Faraday Trans. 2. 68. 374 119721 -, ?. 'Nskagawa and H. Jizomoto. Kolloid-Z. 2. Polym., 250, 594 \
~
(1972). E. Graber and R. Zana, Kolloid-Z. Z.Polym., 238, 479 (1970). N. Muller, J. Phys. Chem., 76, 3017 (1972). I t has come to the author's attention that a somewhat similar analogy has previously been drawn between a set of consecutive firstorder reactions and a diffusion process.'' J. A. Christiansen, J. Phys. Chem., 15,445 (1932). T, A. Rak, Doctoral Dissertation, University of ,Copenhagen, Munksgaard, Copenhagen, 1959. C. A. J. Hoeve and G. C. Benson, J. Phys. Chem.. 61, 1149
.''
(1957). D. C. Poland and H. A. Scheraga, J. Phys. Chem.. 69, 2431
jl965),
Volumetric Properties of Aqueous Solutions of Organic Compounds. I 11. Aliphatic econdary Alcohols, Cyclic Alcohols, Primary, Secondary, and Tertiary Amines
. Cabani,* G. Conti, lstituto d i Chimica Fisica. Universita di Pisa, Pisa. ltaly
and b. Lepori Lsboratorio di Chimica Quantistica ed Energetica Moiecolare dei CNR, 56100 Pisa, ltaly
(Received August 6, 7973)
Pubiication costs assisted by Consiglio Nazionaie delle Ricerche (Rome)
Apparent molal volumes, a,, in aqueous solution a t 25" were determined with a differential hydrostatic balance for the following series of compounds: secondary aliphatic alcohols, cyclic alcohols, primary, secondary and tertiary n-aliphatic amines. The most interesting results are the following. (i) The contributions to the limiting partial molal volumes, OzO,of methylene groups in straight chain and cyclic compounds were found to be clearly different. Such a clear distinction was not observed in the case of methylene contributions to the molar volumes of the pure liquids. (ii) In the plane h-vz',"(h = slope of the straight line @, = 02' hc, valid in dilute solutions) monofunctional compounds lie on a single correlation line; however, polyfunctional compounds are situated well below this line, the displacement increasing with larger values of 0 2 ' . (iii) The volume change in the proton ionization of the methylamines is found to be nearly constant: A V = 4.7 i 0.7 ml mol-I. For the higher members of the amine series the following A V values are instead observed: 4.6 i 0.4 ml mol-1 (primary amines), 2.5 f 0.3 ml mol-I (secondary amines), 1 i 1ml mol- (tertiary amines).
+
Introduction As part of a systematic investigation of the thermodynamic properties of organic solutes in aqueous solution, we have considered in recent volumetric properties of some cyclic ethers, cyclic amines, and their corresponding hydrochlorides. Preliminary studies were made on the effects associated with the change from open chain to cyclic compounds, on the concentration dependence of The Journal of Physical Chemistry, Vol. 78, No. 10, 1974
apparent molal volumes, a", of mono- and bifunctional solutes, and on the volume changes in the proton ionization process of organic compounds containing nitrogen. In order to gain a clearer picture of the nature of the volumetric behavior, we report here studies of the volumetric properties of the following series of compounds in dilute aqueous solution a t 25": aliphatic secondary alcohols (2-butanol, 3-pentanol, 3-hexanol, 4-heptanol), cyclic
' ~ ~ l u ~ e Properties tric of Aqueous Solutions of Organic Compounds alcohols (cyclopentanol, cyclohexanol, cycloheptanol), primary n-alkyl amines (C,H=+lNHz with 1 5 n 5 7 ) , and also secondary [(GnHa,,.l)~NHwith 1 5 n 5 41 and tertiary amines ~ ~ ~ ~ triethyl~ ~ eandt diethylmethylh ~ ~ ~ , amine) Measurement of apparent molal volumes in aqueous soliltioiis have been previously reported for 2-butano13v4 in water, and for methyl-, dimethyl-, trimethyl- and triethaqueous ~ o l u t i o n . Values ~,~ of the volumes of mi.xing in water were also given for 2-butanol, cyciopentanol, cyelohexaiiol, diethyl- and t r i e t h ~ l a m i n e . ~ ~
~ ~ x ~ ~Sectioar ~ ~ ~ e r ~ t a ~ .Materials. All substances employed were reagent grade cmnmercial products. The alcohols were refluxed over calc.iam hydride and then fractionally distilled. The amines were rectified 0x1 metallic sodium under nitrogen. In all cases the fraction collected showed a glc purity >99.5%. In the case of ammonia, methyl-, ethyl-, dimethyl- and trimethylamine the commercial aqueous solutions were used directly without further purification. The amine cont.en&of these solut,ions was determined by HCl titration. Water used in a11 the experiments was deionized and further distilled from an alkaline potassium permanganate solution. A p p a r a t u s and Measurements. The hydrostatic differential balance used for density measurements (with a precision of I. ppm) and the experimental procedure were pueviously 4described.l 'The apparent, molal volumes of the alcohols were calculated b y
where M is the solute molecular weight, c its molar concentration, do and d the density of the water and the solution, respectively. in t.he case of the amines (B) apparent molal volume values, apyobsd,were first calculated by introducing into eq 1. the molecular weight of the hypothetical hydrate l3.Hz69. The latter were then corrected to Q V values of the free amines using values of the acid dissociation constants of the bases taken from Perrin8 and values of the apparent molal volumes of the amine salts taken from Desnoyers and Arel,s Verrall ,and Conway,lo and Laliberti. and Conway."l- The values of NaC1, NaBr, and NaOH were taken from Desnoyers, et a1.,12 and from Bodanszky and Kmzmann.l3 The correction procedure was described in a previous paper.] As :tn example, Figure 1 shows the trend of the function @'*/(l - a ) us. (1 _-cu)cBo for dimethylamine, where cBo is the stoichiometric concentration of the base and a is the degree of hydrdysis. Values of the function are shown corrected and uncorrected for hydrolysis. At low solute concentra tion marked deviations from linearity occur on chnnging the PPI, values used by 0.1 unit.
+,"
Results and Discussion Table I summarizes the values a t infinite dilution of the partial molal volumes, V2', and the excess molal volumes, IPE, calculated as P E = uZo- Vz, where Vz is the molar volume of the pure substance. Values are also reported of the slope h of the straight line representing the concent,ratian d.ependen.ce, x i dilute solution, of the functions a, and @ ' i ' .-/ (al ) for alcohols and amines, respectively. As is generally rioticed for all the nonelectrolytes so far
i
l
d
I
0
0.2
I
0.4 0.6 (1 - a ) c,o, M.
Figure 1. Apparent molar volumes of dimethylamine in aqueous solution at 25":0 , data uncorrected for hydrolysis ( a = 0); 0, data corrected for hydrolysis: 0 ,from measurements in 0.025 N KOH ( a = 0).
considered, the excess volumes are negative and increase in magnitude with increasing size of the hydrocarbon part of the molecule and concentration. Though this is a well-known behavior, the large amount of data now available suggests some relationship between structure of the compounds and their volumetric properties. Particularly, these data allow one to individuate some general behavior in the dependence of on concentration thus leading to a better understanding of solutesolute interactions in dilute aqueous solut,ion. Methylene Contribution in Cyclic and O p e n Chain Monofunctional Compounds. The linear dependence often found for many thermodynamic properties on the number of carbon atoms, k,in homologous series of unbranched compounds allows one to identify the slope of this function with the average cont,ribution of a methylene group to the specific property. In the case of volumes an average value V(CH2) = 16.6 =k 0.3 ml mol-I is so obtained by applying the relationship V = f ( r k ) to homologous series of monofunctional open chain liquid compounds (n-alkanols, primary ro-aliphatic amines, and secondary n-aliphatic amines). On the contrary the V(CH2) values of monofunctional cyclic compounds (secondary cyclic alcohols, secondary cyclic amines, tertiary cyclic amines, and cyclic ethers) are scattered over the large range 13.5-17.5 ml mol-I. This lack of regularity in the volumetric behavior of the pure liquids precludes the use of excess volumes or the comparison of the values of ~ ( C H Zand ) V(CH2) in different homologous series in order to interpret the volumetric properties of the solutes in aqueous solution. It is more significant to compare the V(CH2) values in aqueous solutions because of the clearer distinction that occurs in the volumetric properties of various molecules present in this state. For example, two separate ranges can be d.istinguished for monofunctional unbranched compounds: O"(CH2) = 15.9 It 0.4 ml mol-1 for open chain compounds and Y"(CH2) = 14.2 f 0.4 ml mol-I for cyclic comp o u n d ~ . "Such ~ a difference can in part be due to intrinsic factors but in part may be also justified assuming that cyclic molecules are more easily introduced into the cluster cavities of the solvent. Unfortunately, the values of partial molal volumes of open chain and cyclic monofunctional compounds are not available in solvents other than water. Such data might be very useful in order to ascertain The Journalof PhysicalChemistry, Vol. 78, No. 10, 1974
S . Cabani, G.Conti, and L.
1032
Lepori
TABLE I: Volumetric Properties of Amines and Alcohols in Water at 25' V?,b
No.of expt
Concn range, M
Ammonia
21
0.03-0.9
Methylamine
18
0.03-0.9
8
0.03-0.6 0.07-0.14 0.03-0.8 0.02-0.7 0.02-0.8 0.01-0.5
Substance
Ethylamine n-Propylamine n-But ylamine n-Pent ylamine n-Hexylamine n-Wept ylamine Dimethylamine
21 32 29 31 30 18 24
0.01-0.1
0.005-0.025 0.02-0.7
Diethylamine
12
0.02-0.5 0.06-0.2 0.02-0.4
Di-n-propylamine Di-n- butylamine Trimethylamine
21 20 19
0.01-0.3 0 .003-0 .02 0.03-0.5
14
0.02-0.5 0.03-0.16 0.03-0.6 0.02-0.7
8
Diethylmethylamine' Triethylamine
9 22
V $ a , ml
mol-'
24.85 f 0.04 24.6 41.68 f 0.04 41.6 41.15 f 0.06c
h,a mil. m o l +
ml mol-! ,'oP
28.25
-3.4
This work
-0.3 i 0.1
47.33
-5.6
66.61 83.15 99.77 116.14 132.8 149.32 69.41
-8.2 -9 .o - 10 .Q -10.4 -11.2
-2.1 f 0.5
104.56
-12.9
This work 14 This work 5,lO This work This work This work This work This work This work This work 14 This work 5,lO This work
f 0.5
137.93 171.03 94.28
-15.3d -14.9 -15.7 - 15.5
This work This work This work
14
-0.1
j ,0.1
58.37 f 0.05 74.12 f 0.04 8 9 . 8 =t0 . 1 105.7 f 0 . 1 121.6 f 0 . 2 137.6 i 0 . 3 59.80 f 0.04 59.1 59.0 f O.lc 58 .6c 91.68 I O . 1 0
-0.9 -1.2 -1.4 -2.0 -3.0 -6.5 -0.5
f 0.1 f 0.1 f 0.4 f 1
123.06 f 0.10 155.4 i 0 . 4 78.8 f 0 . 2 78.4 78.6 f 0.Zc 77.9c 106.77 f 0.10 120.9 & 0 . 1 119.70
-3.2 -25
40 .Oc
2.t 3
f 10 f 0.1
21
-1.5 i 0 . 4
14
This work
-2.3 & 0.3 - 2 . 3 2.t 0 . 4
This work This work
5,lO
139.93
-19.0 -21.8' -5.7
10
10
0.01-0.4 0.01-0.15 0 ,005-0 ,04 0.02-0.2
101.28 i 0.03 117.14 f 0.09 133.2 f 0 . 2 89.06 f 0.02
-2.3 f 0 . 2 -3.2 2.t 1 . 4 -19 f 7 -2.2 f 0.2
108.00 125.47 142.60 91.34
C yclohexanol
7
0.02-0.3
103.54 f 0 . 0 3
- 2 . 5 f 0.2
105.95
C ycloheptanol
18
0.01-0.1
116.88 f 0.04
-3.0 f 0.6
120.26
3-Pentanol 3-Wexanol 4-Heptanol C!yclopentanol
20 16 13
86.66 86.53
$r
7
-1.1 f 0 . 8
92.38
0.02-0.5 0.003-0.1
-11.7 -9.6
0.0 f 0 . 3
-1.7 f 0.1
15
Ref
i 0.1
-0.1
0.03
%Butanol
ml mol-'
-6.8d -6.7 -8.3
-9.4 -2.3 -2.9d -2.4 -2.0d -3.4
7 This work 4 7 This work This work This work This work 7
This work 7 This work
In the case of t,he following amines: n-butylamine (0.4); n-pentylamine ( 0 . 3 ) ;diethylmethylamine (0.45);triethylamine (0.35);the function @*/(I - a) = f [ (1 - a ) c g o 1 was found t o be linear up to the concentration value indicated in parentheses. The molar volumes Vs at 25O of pure substances were evaluated from density data reported io the literature. From measurements in 0,025 N KOH aqueous solutions. At 26.5'. e The VBHCi0 of Et?MeN .HCl, necessary for hydrolysis correction, was obtained from measurements on aqueous solutions of the amine neutralized with HC1. As a result of seven runs in the concentration = (222.81 f 0.01) - (2.15 I 0 . 0 3 ) ~ . range0.03-0.5 M we found: % ~ c i- 1.868
whether the observed difference between the P ( C H 2 ) of open chain and cyclic compounds is prevailingly due to effects of the solutes on the water structure or to geometric factors characteristic of the molecules. The above interpretation assumes a two-state model for water. Such a model has already been adopted in order to interpret the differences between the methylene contribution of open chain and cyclic monofunctional compounds to the hydration entropies16a ( A S h ( C H 2 ) = -3.5 f 1.0 cal d e g - l mol-* for open chain and A S h ( C H 2 ) = -2.6 f 0.3 ea1 deg-l mol-' for cyclic compounds), to the hydration enthalpies*& (AHh(CH2) = -0.95 kcal mol-l for open chain and A H h ( C H z ) = -0.6 kcal mol-l for cyclic compounds) and to the partial molal heat capacities1% ( @ c p ( C H 2 ) = 22 f 2 cal deg-I mol-* for open chain and ( P c p ( C H 2 ) = 18 f 2 cal deg-l mol-1 for cyclic compounds). These differences are in our opinion t o be ascribed to the higher structure promoting effect exhibited by open chain compounds as a result of the larger hydrophobic surface that their alkyl chains expose to the solvent through less restricted rotation. When intrinsic structure factors reduce this internal rotation freedom the contribution of the hydrocarbon part The Journal of Physical Chemistry, Vol. 78, No. 70, 1974
to the thermodynamic property (e.g., partial molal volume or partial molal heat capacity) for the open chain compound approaches the value which is observed for the corresponding cyclic compound. In this respect an interesting comparison can be made between the pairs diethylamine-pyrrolidine and diethylmethylamine-N-methylpyrrolidine for which the values = 13.8 ml mol-l and Sacp = 36.6 cal deg-I mol-l and the values S v z o = 9.5 ml mol-' and 6@cp= 19.9 cal deg-l mol-I are observed, respectively. The lower value of 6 % of~ cyclization for the tertiary amine was interpreted1@ assuming that the presence of the methyl group reduces the rotational freedom of the remaining ethyl groups which are thus less effective in promoting water structure. This same effect can explain the differences now observed for SVz' of cyclization of secondary with respect to tertiary amines. In other words a reduction of the degrees of internal freedom of the molecule is reflected by the partial molal thermodynamic properties primarily because of a reduced ability of the hydrophobic part of the molecule to promote structural order in the solvent. Volume Change in Proton Ionization. Values of i4V corresponding to the reaction BH+ -* B +- W+ for a large
So2'
Volumetric Properties of Aqueous Solutians of Organic Compounds
4 033
TABLE XI: T h e r m o d y n a m i c Functions for Proton Ionization f r o m P r o t o n a t e d Aminles inr Aqueous Solution a t 25 Base
PIP,
YBEXO,
ml mol - 1
ml mol-1
X
AV, ml mol-'
AH, kcalmol-1
AS,cal deg - 1 mol-'
AC,, cal deg --I
mol -1
l l _ _ l _
Ammonia
Ethylarnine n-Propylamine
24 .6a 24.8" 40 . W 41.6" 41.7c 58 .4c 74..1 c
n-Butylamine
89.8C
110.27
Br
4.3
105. 7c 121 .6c 137.6~ 59 . S C 5 9 ,l a 58.6j 91.7c 123. lC 155 , 4c 78 .8c 78 ,41z 77.9f 106.8c 120 . g C
126.li 142 .Ot 157 .gi 72.5f 7 3 . la 72.51 106 . 7 d 138. 7d 170.7d 90.6, 91 .7(h 90.6f 122.8C 138.6f 186.81
Br Br Br
4.3 4 "3 4.4 5.1 3.8 3.9 2.8 2.2 2.5 6.0 4.5 5 .O 1.8 0.1
~ e t ~ y ~ ~ ~ ~ i n e
n-Pent ylamine n-Hexylamine n-Heptylamine Dimethylamine Diethylamine Di-n-propylamine Di-a-butylamine Trimethylamine Diethylmethylamine Triethylamine Tripropylamine
*
Reference 14. Reference 17. Reference 22. Reference 16.
e
This work.
36 .Oa 42.6< 53.8, 55.5" 60 .8% 77.7i 94 1%
C1
Br Cl Cl Br Br Br
6.4 7 .O 4.1 3.9 6.6 5.4 4.7
c1 c1 c1 c1 c1 c1 c1 c1
c1 c1 c1
c1
12.480 12 .42e 13 " 090 13 .2gh
-0.45b
-3
-0.60e
-0.w
13 .71h 13 67e 13 . 84h 13 ,66j 13 . 8 0 k 13 .88e 13 .98jG 13 .8ge 13.98h
-2.71% -2.56 -2.0'1 -2.s -2.w
11.868 12 .04Ii 12 .73" 1 3 .17h 13 .66h 8.81Q
-9.50
-7.3" -6.2'' -5.7h -35.2g
1 0 .5ge
-13.5e
I
-4.70
v~~~~
-
8 .OQ
-4.1'1
-2.1e -_1 . P
7 .$e
2 .3e
-2.1e -1 . 8 h
18.5l 23 .l*
-8.7"
Reference 11. e Reference 18. f Reference 10. 0 Reference 19. Reference 20, Reference 9.
number of amines are reported in Table 1117-22as well as the values of enthalpy, entropy, and heat capacity changes associated with this process. In the calculation of AV the values V H C ~=' 17.83 ml mol-1 and = 24.71 nil mol-I were assumed.23 Examination of these data shows the following. (a) For the first terms of the various series ( z . e , CH3NH2, (CM3)2NH, (CI33j3'N) the ionization volume has an almost constant value: A V = 4.7 f 0.7 ml mol-l whereas the AH, A S , and A C , values vary markedly as the numbier of methyl groups bonded to nitrogen is varied. (b) As the alkyl clhain length is increased the A V rapidly reaches a constant value 4.6 rt: 0.4 ml mol-l for primary amines, 2.5 & 0.3 ml mol- for secondary amines and 1 i 1 ml mol-I for tertiary amines. Analogously the changes of other thermodynamic functions associated with the process rapidly reach constant values: AH = 13.8 f 0.1,13.2 3s 0.4, 10.6 kcal A S = -2.2 z!= 0.4, -6.4 f 0.8, -13.5 cal deg-l mol I; AC, = 4.9 & 2.5, 20 f 1, 38 f 5 cal deg-l an0l-l for proton ionization of primary, secondary and tertiary protonated amines respectively. (c) When going from the first to the higher members within each series the A V and A C , values decrease while the A S and AN values increase. Finally, the data in (b) indicate that in going from primary ---* secondary tertiary amines, the values of A V , AH, and '1sdecrease whereas the values of A C , increase. Observation (b) allows one to deduce for intramolecular distances greater than two bonds removed from the nitrogen center that the water-hydrocarbon part and waterhydrophilic center interactions may be considered, as a first approximation, independent. Therefore the thermodynamics of the proton ionization process o f charged nitrogen centers is better characterized by AX values ( X =
.Oh
19.11 20.91
43.80 33.6l 44,7e Reference 21.
V, H , S , and C,) of amines containing alkyl groups other than methyl. It would be very useful to know the separate value of partial molal properties of the free and protonated bases, but difficulties arise, for ionic species, owing to the ambiguity of assigning individual ionic contributions. However an analysis of @cPvalues of a number of primary, secondary, and tertiary amines and their hydrochlorides established that the trend in AC, of the ionization process is mainly due to the fact that the contribution to the partial molal heat capacity by the charged nitrogen center decreases significantly as the number of alkyl radicals bonded to nitrogen is increased.l@ This is in agreement with the interpretation given by Evans and HamannZ4 to the trend of the ionization entropies. According to these authors the ammonium ion is surrounded by a zone of ordered water; on substituting the hydrogen atoms bonded to nitrogen with alkyl groups the entropy loss of the solvent is lowered, thus causing the partial molar entropy of the acid cation to iiicrease relative to that of the neutral base. There are good reasons to believeZ5 also that the progressive lowering of the values of A V of ionization as more and more alkyl groups are bonded to nitrogen is virtually determined by interactions of water with the charged nitrogen centers; i.e , the larger the number of bonded alkyl groups, the weaker is the electrostrictioii effect and the smaller the volrame increment associated with proton ionization. It is not readily understood why the values of A V of ionization in the series o f methylamines are almost independent of the number o f methyl groups bonded to nitrogen, whereas the AH, A s , and .IC, values vary markedly with this number, more so than in the case of bulkier substituents. Dependence of o n Concentration. As already obThe Journal of Physical Chemisfr}/, Vol. 78, NO. 10, 1974
S. Cabani, G. Conti, and L. Lepori
S 034
-6
hydrocarbon region o f two or more solute molecules. This would cause the lifetime o f solutes in the c l u s t e r cavities t o be enlarged with t h e c o n s e q u e n t macroscopic effect of a v o l u m e decrease. T h e e x p o n e n t i a l trend of the plot finally suggests that t h i s effect o f s t a b i l i z a t i o n o f w a t e r c l u s t e r s by solute-sol u t e i n t e r a c t i o n s i s probably cooperative.
/
I-
@ /
N '
p'
Acknowledgment.
This work
t h r o u g h financial assistance
h a s been s u p p o r t e d from the Consiglio Nazionale
delle R i c e r c h e (C.N.R.).
References and Notes
+J--e-7b '
'
80 100 ' V,', ml mol-'.
I20
'
I40
'
Figure 2. Plot of h vs. 0,' for organic solutes in aqueous solution at 25". Monofunctional compounds (0):1, methanol at 20" (ref 3, 2 6 ) ; 2, ethanol at 20" (ref 3, 26); 3, 1-propanol at 20" (ref 3, 2 6 ) : 4, l-.butanol (ref 4 ) , 2-methyl-2-propanol (ref 4 ) , c y ciopentanol; 5, 1-pentanol at 20" (ref 3, 2 6 ) ; 6, 2-propanol at 20" (ref 3, 2 6 ) ; 7, 2-methyl-I-propanol (ref 4 ) , 2-butano1, n-buthylamine; 8, ethylene oxide at 10' (ref 27), ethyleneimine (ref 1 ) : 9, propylene oxide and trimethylene oxide (ref 28); 10, tetrahydrofuran and pyrrolidine (ref 1 ) , n-propylamine; ll,tetrahydropyran and 2-methyltetrahydrofuran (ref 1 ) ; 12, 2,5-dimethyitetrahydrofuran (ref 1 ) ; 13, azetidine (ref 1 ) ; 14, piperidine (ref I ) , diethylnmine; 15. hexamethylenimine (ref 1) , n-pentylamine; 1 6 , heptarnethylenimine (ref 1 ) ; 17, 1-methylpyrrolidine (ref 1 ) ; 18, 1-methylpiperidine (ref 1); 19, 3-pentano1, cyclohexanol; 20, cycloheptanol, 3-hexanol; 21, methylamine; 22, ethylamine, dimethylamine; 23, triethylamine; 24, n-heptylamine; 25, trimethylamine; 26, n-hexyiamine, di-n-propylamine; 27, diethylmethylamine. Polifunctional compounds ( 0 ) : 28, glycine (ref 29); 29, urea (ref 2 9 ) ; 30, thiourea (ref 29); 31, 1,3-dioxolane (ref 1 ) ; 32, glycerol (ref 30); 33, 1,4-dioxane (ref l ) , 1,3-dioxane (ref 2 8 ) , urethane (ref 29); 34, pyrocatechol (ref 29), resorcinol (ref 2 9 ) , hydroquinone (ref 29) ; 35, hexamethylenetetraamine (ref 31).
servedl in dilute s o l u t i o n the aVvalues decrease with c o n c e n t r a t i o n a c c o r d i n g t o the r e l a t i o n s h i p & = 0 2 ' hc. The value of the slope h varies c o n s i s t e n t l y when going from monofunctional t o bifunctional compounds o f the s a m e molecular d i m e n s i o n s . A m o n g m o n o f u n c t i o n a l comp o u n d s it depends o n the e x t e n s i o n of the alkyl chain. Examination of a plot of h U S 0 2 ' (Figure 2)26-31 indicates that a l l s a t u r a t e d monofunctional compounds l i e on a sing l e curve which shows an exponential increase of t h e a b s o l u t e value of h with i n c r e a s i n g Uz0 values. P o l y f u r i c tional compounds having similar limiting partial molal volumes as t h e m o n o f u n c t i o n a l c o m p o u n d s h a v e less negative values o f h. Moreover, in t h e latter case t h e h v a l u e o n l y changes s l i g h t l y with 02'. The f a c t that ox1 t h e plane h - 0,'a s i n g l e c o r r e l a t i o n l i n e r e p r e s e n t s the volumetric behavior of different types of m o n o f u n c t i o n a l . compounds, suggests the s o l u t e - s o l u t e i n t e r a c t i o n s occurring via the functional groups t o be p r o b a b l y n e g l i g i b l e in dilute s o l u t i o n . The p r o g r e s s i v e volu m e l o w e r i n g as t h e s o l u t e c o n c e n t r a t i o n i s i n c r e a s e d is t h u s m a i n l y t o be a s c r i b e d t o a prolongation o f t h e a v e r age l i f e of w a t e r c l u s t e r s w h i c h a r e i n c l u d e d between the
+
The Journal of Physical Chemistry, Voi. 78, No. IO, 1974
(1) S. Cabani, G. Conti, and L. Lepori, J. Phys. Chem., 76, 1338 (1972). (2) S . Cabani, G. Conti, L. Lepori, and G. Leva, J . Phys. Chem.. 76, 1343 (1972) (3) M. E. Friedman and H . A. Scheraga, J . Phys. Chem., 69, 3795 (1965), (4) F. Franks and H. T Smith, J . Chem. Eng. Data. 13, 538 (1968): Trans. Faraday Soc., 64, 2962 (1968), (5) B. E. Conway and R. E. Verrall, J . Phys. Chem.. 70, 3952 (1966) (6) B. E. Conway, R. E. Verrall, and J. E. Desnoyers. Trans Faraday Soc.. 62, 2738 (1966). (7) K . R. Brower, J. Peslak, and J. Eirod, J . Phys. Chem., 73, 207 (1969). (8) D. D. Perrin, "Dissociation Constants of Organic Bases in Aqueous Solution," Butterworths, London, 1965. (9) J. E. Desnoyers and M. Arel, Can. J . Chem. 45, 359 (1967). 10) R. E. Verrall and 8 . E. Conway, J. Phys. Chem., 70, 3961 (1966). 11) L. H. Laliberte and B. E. Conway,J. Phys. Chem.. 74, 4116 (1970). 12) J. E. Desnoyers, M. Arel, G. Perron, and C. Jolicoeur, J. Phys. Chem., 73, 3346 (1969). 13) A . Bodanszkyand W. Kauzmann,J. Phys. Chem., 66,177 (1962). 14) S . D. Hamann and S. C. Lim, Aust. J . Chem.. 7, 329 (1954). 15) These data were obtained considering: (i) for open chain compounds 28 molecules belonging to the series of n-alkanols, secondary alkanols, primary and secondary n-aliphatic amines, primary and secondary n-aliphatic protonated amines: (ii) for cyclic compounds 21 molecules belonging to the series of secondary and tertiary cyclic amines, cyclic alcohols, cyclic ethers, secondary and tertiary protonated cyclic amines. The first term of each series was not taken into consideration. 16) (a) S. Cabani, G. Conti, and L. Lepori, Trans. Faraday Soc., 67, 1943 (1971); (b) S. Cabani, G. Conti, A . Martinelli, and E. Matteoli,
