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Interactions in aqueous solutions of alkylureas. Excess enthalpies at 25• for butylurea and its isomers.
59
JournelofMolecularLiquids, 21(1983)59-65 Elsevier Science Publishers B.V.,Amsterdam-F’rintedin The Netherlands
INTERACTIONS IN AQUEOUS SOLUTIONS OF ALRYLUREAS. EXCESS ENTHALPIES AT 25' FOR BUTYLURBA AND ITS ISOMERS. VINCENZO ABATE, CUIDO BARONE, PASQUALE CACACE, GIUSEPPINA CASTRONUOVO AND VITTORIO ELIA.
ISTITUTO CHIMICO - UNIVERSITY OF NAPLES, VIA MEZZOCANNONE 4, 80134 NAPLES ITALY
ABSTRACT The heats of dilution in water of three isomerit alkylureas (normal-bu= tylurea, tert-butylureaand tetramethylurea)were determined at 25'C. A cornparis& of the concentration coefficients of the excess enthalpies, with those of other alkylureas in water makes possible to identify, dif= ferent behaviours, both determined primarily by hydrophobic interactions One, shown by the and urea-hydrocarbons,water-mediated,interactions. n-butylurea,is similar to that of diluted or moderately concentrated solu= tions of alkanols in water. Another is shown by the tert-butylureaand te= tramethylurea.The kind of aliphatic substituents (linear or branched) and their position determine probably large differences in the cooperativity and effectiveness of the interactions.
INTRODUCTION Studies on diluted or moderately concentrated aqueous solutions of non-electrolytesprovide information about molecular interactions in the mixtures of polar compounds. In recent years the role of water, as an es= sential partner of solute-solute interactions,has been often outlined and clarifiedp-43. In these systems, in fact, the solute-solute,solutesolvent and solvent-solventinteractionshave comparable intensities and time scales. Among the simple nonelectrolytes,alkylureas are peculiar in that they have two kinds of functional group..which perturb water in different ways, generating distincts interactions. In the present paper, we report on the excess enthalpies in water of three isomeric alkylureas, namely mono-n-bu= tylurea (MBU), mono-tert-butylurea(tBU) and 1,1',3,3'-tetramethylurea (TMU). These alkylureas have a variety of arrangements of the hydrophobic regions with respect to the hydrophilic part of the molecule. The excess enthalpies obtained are compared to other thermodynamicand physico-the= mica1 properties of the other two isomers, 1,3-diethylurea (1,3 DEU) and l,l'-diethylurea (1,l' DEU) and of other urea derivatives and urea-like compounds previously studied 2,4-lg. The excess enthalpy of a ! inary non-symmetric solution can be defined using the molality scalep5-171 as:
HE (m) =
H(m)
-
Hy
-
$4
0167s7322/83/$03.00 0 1983 E1sevierSciencePubhshersB.V.
(1)
60
where H and HE are referred to an amount of solution containing 1 kg of solvent and m moles of solute, Hy is the standard enthalpy of 1 kg of wa= ter and 2; is the limiting partral molal enthalpy of the solute. The ex= cess properties can be represented as power series expansions. A form often used, for the excess free energy, enthalpy, entropy, etc., is: GE
(m)
=
giim2
+
giiim 3
+
.........
(2)
HE
(m)
=
hiim2
+
hiiim3
+
.........
(2')
TSE
(m)
=
Tsiim2
+
Tsiiim3
+
.........
(2”)
In the MC Millan and Mayer approachB,15-181, the coefficients of these expansions take the meaning of virial coefficients, and they account for the pair, triplet, and higherorder interactions among solute particles, respectively. These coefficients account implicitly for any change in solute-solvent interactions and in the state of solvent water. However the pertubations arising from the introduction of the solute into i$e solvent at infinite dilution are included in the reference values 'E" *, H2, etc.. EXPERIMENTAL MBU and tBU (K&K products) were crystallized from ethanol and dried in vacua. TMU (Fluka product) was distilled under vacuum. All the solutions were freshly prepared by weight, using bidistilled and degassed water. Calo= rimetric measurements were carried out using an LKB 2107-112 standard batch and an LKB 10700-l standard flow microcalorimeters, at 25.00 + 0.01 "C. In the batch case. the experimental heats Q, evaluated by ihe integra= converted into the-heats tion of the ~~~ti2;~kqfaopfs~Xe~~;,g~ramI-are of dilution ,$HdiL (m'-_,
m)
=
-Q/w,
(3)
where w is the weight of water in the final solutions and m' and m are the in1.&. ral and final molalities. With the flow microcalorimeter, the va= lues of the AHdil are obtained as follows: AHdil
(m'cpm)
=
-(dQ/dt)Pw
(3')
where dQ/dt is evaluated from the instrumental deviations, normalized by electrical calibrations for each set of runs. P is the total mass flow-ra= te of water. Checks were performed to assure th: reproducibility of the data obtained through the two techniques. For a dilution process, in which a solute species passes from a initial mola= lity m' to a final one m, the heat of dilution is given by p,12-141 :
__$_
a
Hdil
(ms_
m)
=
_&
HECd
= hii(m-m')
A suitable fitting of Eq.(4) of the excess enthalpy.
- _$
HE(m’)=
+ hiii(m2-mV2)
to the data, then gives the virial
+... (4
coefficients
RESULTS The experimental calorimetric data concerning the three substances exami= ned, are given in Table A, B and C of the Appendix. In "'able I the enthalpic contributions & to the virial coefficients are reported, together with their 95% confidence limits.
61 TABLE I - Coefficients of the Excess Enthalpies of Aqueous Solutions of Alkylureas and Urea-like Solutes at 298.15 K.
Solute and molality range
tert-butylurea (o-0.15) tetramethylurea (O-4) l,l-diethylurea (O-1.5) n-butylurea (o-0.7) 1,3-diethylurea (O-2) n-propylurea (O-2) ethylurea (O-2) 1,3-dimethylurea (O-2) 1,1-dimethylurea (O-2.7) methylurea (O-2) urea (O-2) thiourea (O-1.2) biuret (o-0.16)
aUnits:
Jmol-l(mo1
kg-l)-1
h
3632 2032 791 1039 1011 292 160 35 38 -85 -351 -970 .2120
a
h
i373d ~80 +44 *31 +78 i52 a14 *14 210 *2 +3e fl4 +61
-6220 -139 -48 420 139 92 37 78 21 21 21 70
iii
b
' dthe reported
"H.C.c
tl9 sod +20 i26 A39 *34 ~26 &6 ~16 +4 f2 i6e a11
’ bUnits: Jm01-~(mol kg-1)-2
bon atoms in the alkyl chains confidence indices for istance, Refs.
ii
4 4 4 4 4 3 2 2 2 1 0 0 0
Ref.
This work 11 11 14 This work
’ 'number of car=
uncertainties
are the 95%
; eother similar data are reported in other papers:see, [3,4,10,16,17,20]
and the literature
quoted
therein.
Both are obtained from Eq.(4) by a least square procedure. The polynomial expansions of highest degree, whose coefficients exceed their own 95% con= fidence limits, are chosen. In the same Table, data relative to other al= kylureas and urea-like substances are shown for comparison. The numerical values of the h coefficients depend clearly on the number and stereochemii stry of the s&stituents. As expected, the hii values, for the three corn= pounds studied, are positive and higher than those of the coefficients of other alkylureas. However the hiii values are different. In fact, for TMU and tBU the h.iii are large and negative, whereas for MBU hiii is large but positive. Thus, the urea family of solutes in water can be split into three groups: - urea-like solutes (biuret, thiourea, urea) characterized by values negative for hii and positive for hiii [4,10,12,131 ;
- solutes of second group, which are similar the monohydric
alcohols [15,16,19,20] , even though they can be regarded as "mixed" solutes rather than "soluble hydrocarbons"[l,l6] . Their solutions are chara= cterized by positive values of both hii and hiii [2,14] . Monomethylu= rea is actually intermediate between these two group as shown by a careful analysis of the trend of the excess properties with the con= centration[lO j . Linear monosubstitued alkylureas (monoethyl, monopro=
62 pyl, monobutylurea), dimethylureas belong to this group;
(s;mmetric and asymmetric)
- solutes of the third group have values for hiii.
positive
and 1,3 DEU
for hii but negative
DISCUSSION In preceding papers on this subject 12,4,5,8-141 we discussed the behaviour of aqueous solutions of the first two groups just described. The properties of aqueous solutions of alkylureas (except monomethylurea are assumed to be de= termined primarily by hydrophobic interactionsp,15,16 j . However the urea re= sidue-alkyl residue and the urea-urea, water mediated interactions play impor= tant roles. The latter are predominant in the case of monomethylureapO] and the only ones in the case of urea-like solutesp,l2,13]. The most widely stu= . According to the Friedman and died are the hydro hobic interactionsu,Zl-231 Franks model[l5,16 4 a spontaneous overlap of the hydration cospheres surrounding the hydrophobic groups occurs, in the concentration process, with a release of solvent to the bulk. This release of water to a state richer in enthalpy and entropy, can justify the sign of the contribution to the pairwise interaction . . coefficients (Tsii, hii70) found for all the aqueous solutions of prevailingly hydrophobic solutes (alkylureas, alcohols, alkylamides, dipeptides, etc..) [2,11,14-16,19,20,25,26]. In all these cases the ositive values of Tsii over= w&m the positive ones of the hii, giving giiC0 P2,8,14-16,19,20,24,251. For the urea-like solutes we recently suggestedC4,10,12,13] that a spontaneous pro= cess of coalescence of the "distorted" or " destructured" hydration cospheres of the urea takes place when the solute concentration increases. This process can explain the signs of the pairwise interaction coefficients, all negative, since water would pass in this case from a state richer in enthalpy and entropy to the bulk. In other words, the favourable solute-solute interaction (giiL0) should be seen as a solvent-mediated interaction, not as the formation of H-bon= ded dimers and polymers, as hypothesized by other models. The close similarity of the thermodynamic properties of thiourea and urea solutions C12Jas well as the spectroscopic properties of urea and amide aqueous solutions [5,6,26,27-J sup= port these considerations. The urea-hydrocarbon interactions in water have become clear only recently. The properties of ternary solutions of water and urea with alcoholsD7,28], respectively seem to be determi= ethers[3], ketonesC31 and hydrocarbons[29,30) ned partially by a contribution acting in the same direction as the hydropho= bit interactions. The same conclusion can be reached indirectly by considering the (-CH2-)-urea contributions, which were evaluated by Wood, Lilley and their colleagues by means of the group contribution approach from the pairwise in= teraction coefficients gi.9 h.. and Tsij of a series of binary and ternary aqueous solutions[20,24,2sJ. I3 According to the models previously discussed, an additional release of water from the hydrophobic interaction cospheres was suggested as one of the sources of the observed properties of these ternary solutions[3,17,28]. This effect could be promoted by the disruptive action of urea on the more "ordered" cospheres of the hydrophobic moieties. A comparison of the excess enthalpies relative to the ternary systems water-urea-alkanols [17,28] with those relative to the binary and ternary aqueous solutions of alkanols[16,20], seems to indicate that the interactions of tert-butanol with other alkanols and with itself are weak, whereas the in= teractions with urea are stronger than those involving only the linear alcohols. This could imply that the hydrophobic hydration cospheres of branched alkyl chains contribute relatively little to hydrophobic interactions since they are relatively unchanged whith respect to increasing concentration. On the other hands the addition of urea promotes a substantial breaking up of these cospheres
63
large release of "structured"water to the bulk state,which is richer in enthalpy and entropy. It is possible that binary aqueous solutions of tBU,TkBfand 1,l DEU, whose alkyl groups are arranged more compactly, behave as the water - urea - tert-bu= tan01 solutions, since they show the same effects. These effects are strengthe= ned by the coexistence of the urea residue and alk.1 chain on the same molecu= le. The weak effect of the hydrophobic interaction could be partially (for 1,l DEU) or more than compensated (for TMU and tBU) by the disruptive action of urea residues on the cospheres of the alkyl chain of the other molecules. The main difference between this third group of alkylureas and the second one is the negative value of the hiii coefficients. It is possible that these solutions have a different behaviour which can be explained by means of an association model, which does not apply to the second group. If the association model is assumed to be a step reversible polymerization of a unlimited number of ideal species, with each step characterizedby the same value for the association constant K, and by the same heat of association AH; , it is possible to demon= and a very
strate that Eq. (2) can be transformed
HECd
=
Ka AHim
into Eq. (5) -
2,: AH"am3
[31,32] +
:
5Kz AHim'
+
. . .. (5)
where the h coefficients assume alternate signs, as in the present case. However, the very low values of the K, evaluated for 1,l DEU and TMU (0.030 and 0.034 respectively) and the fact that any explicit change in the solvent has been neglected, suggest that such semiideality models be treated with cautions and be used only as a framework for discussion. The positive values of theAHi may just be an indication that hydrophobic forces still prevail and that solvent water must be involved in the process. In the case of tBU 1 h*** iii(t(hiiI gives more reasonable value for K, andAH: (0.856 and 4'250 Jmolrespecti= vely). Thus, the association model cannot be excluded "a priori" without other evidence.
Appendix Table A- Heats of Dilution of Aqueous Solutions of Monobutylurea (MBU) at 298.15 K.
m'
m 0.5477 0.5391 0.4371 0.2149 0.3442 0.1566 0.1561 0.2529 0.1261 0.2090 0.1022 0.1321 0.0667
0.6670 0.6670 0.6670 0.6670 0.5494 0.5494 0.5494 0.3809 0.3809 0.3199 0.3199 0.1974 0.1974
AHdil
/m
(Jmol
-1
)
179 195 341 647 283 516 535 169 325 135 255 78 149
rr=7 All
measurements
were performed
with the batch microcalorimeter.
64
Table B - Heat of Dilution of Aqueous Solutions of tert-butylurea (tBU) at 298.15 K. m’
m
_
dHdil /m (Jmol-1 )
0.1402 0.1402 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399
0.0636 0.0456 0.1138 0.1019 0.0967 0.0886 0.0873 0.0746 0.0684 0.0554 0.0383 0.0206 0.0110
204 244 50.8 78.2 86.7 109 113 144 185 209 241 316 344
0.1528 0.1475 0.1428 0.1379 0.1325 0.1222 0.1221 0.1158 0.1120 0.1025 0.0938
0.0706 0.0682 0.0660 0.0639 0.0614 0.0748 0.0566 0.0721 0.0688 0.0630 0.0581
186 171 171 177 174 107 170 104 115 101 89.4
u = 8.8 The first series of measurements were performed with the batch microcalori meter, the second with the flow apparatus.
Table C - Heat of Dilution of Aqueous Solutions of Tetramethylurea (TMU) at 298.15 K. m' 3.9795 2.9958
2.9958
1.3003 1.3003 1.0321 0.7266 0.7266 0.7266 0.7266 0.4035 3.4982 a 2.6801 a 1.6241 a 0.1567 a
m ‘2.3387 1.8441 0.3046 0.8264 0.4035 0.1219 0.4358 0.2231 0.1649 0.1598 0.0803 1.4231 1.1374 0.7170 0.0755
_C\Hdil /m (Jmol-1) 2013 1584 4277 818 1607 1781 553 982 1115 1111 649 2724 2169 1486 162
aThese experiments were performed with the batch microcalorimeter.
65 REFERENCES F. Franks in M.N. Jones (Ed.),Biochemical Thermodynamics, Vol. 1, Chap.2, 1 Elsevier, Amsterdam, 1979. 2 G. Barone, G. Castronuovo,V. Crescenzi, V. Elia and E. Rizzo, J.Solution Chem. 7 (1978) 179. 3 F. Franks and M.D. Pedley, J. Chem. Sot. Faraday Trans. I 77 (1981) 1341. and V. Elia, J. Chem. Sot. Faraday 4 G. Barone, P. Cacace, G. Castronuovo Trans.1 77 (1981) 1569. G. Barone, E. Rizzo and V. Vitagliano, J. Phys. Chem. 74 (1970) 2230. E.C. Finer, F. Franks and M.J. Tait, J. Am. Chem. Sot. 94 (1972) 4424. P.R. Philip, G. Perron and J.E. Desnoyers, Can. J.Chem. 52 (1974) 1709. G. Barone, E. Rizzo and V. Volpe, J. Chem. Eng. Data 21 (1976) 59. G. Barone, G. Castronuovo, C. Della Volpe and V. Elia, J. Solution Chem. 6 (1977) 117. 10 G. Barone, G. Castronuovo, V. Elia and A. Menna, J. Solution Chem. a (1979) 157. 11 G. Barone, G. Castronuovo, A. Ceshro and V. Elia, J. Solution Chem. 9 (1980) 867. 12 G. Barone, P. Cacace, G. Castronuovo and V. Elia, Gazz.Chim.Ital. 110 (1980) 215. 13 G. Barone, P. Cacace, G. Castronuovo and V. Elia, Can. J. Chem. 59 (1981) 1257. 14 V. Abate. G. Barone. G. Castronuovo. V. Elia and P. Masturzo, Gazz. Chim. Ital. 111 (1981) as: 15 H.L. Friedman and C.V. Krishnan, J. Solution Chem. 2 0973) 119. 16 F. Franks.M.D.Pedlev and D.S. Reid, J. Chem. Sot. Faraday Trans. I 5 (1976) ia4. _ 17 J.E. Desnoyers, G. Perron, L. Avedikian and J:P. Morel, J. Solution Chem. 5 (1976) 631. jr. and J.E. Mayer, J. Chem. Phys. 13 (1945) 276. la W.G. McMillan 19 B.Y. Okamoto, R.H. Wood and P.J. Thompson, J. Chem. Sot. Faraday Trans. I 74 (1978) 1990. 20 J.J. Savage and R.H. Wood, J. Solution Chem. 5 (1976) 733. New York, 1973. 21 C. Tanford, The Hydrophobic Effect, Wiley-Interscience, 22 A. Ben-Naim, Hydrophobic Interactions, Plenum Press, New York, London, 1980. 23 A.H. Clark, F. Franks, M.D. Pedley and D.S. Reid, J. Chem. Sot. Faraday Trans. I 73 (1977) 290. 24 R.H. Wood and L.H. Hiltzik, J. Solution Chem. 9 (1980) 45. 25 G.M. Blakburn, T.H. Lilley and E. Walmsley, J. Chem. Sot. Faraday Trans. I 76 (1980) 915. 26 I.M. Klotz and J.S. Franzen, J. Am. Chem. Sot. 84 (1962) 3461. 27 28 29 30 31 32
G.E. Walrafen, J. Chem. Phys. 44 (1966) 3726. G. Barone, G. Castronuovo and V. Elia, Adv. Molec. Relax. Interac.Proces= ses, 23 (1982) 279. D.B. Wethlaufer, S.K. Malik, L. Staller and P.L. Coffin, J. Am. Chem. Sot. 86 (1964) 508. A. Ben-Naim and M. Yaacobi, J. Phys. Chem. 78 (1974) 170. S-J. GilI and E.L. Farquhar, J. Am. Chem. SOC. 90 (1968) 3039. A. Cesaro, E. Russo and D. Tessarotto, J. Solution Chem. 9 (1980) 221.
JournelofMolecularLiquids, 21(1983)59-65 Elsevier Science Publishers B.V.,Amsterdam-F’rintedin The Netherlands
INTERACTIONS IN AQUEOUS SOLUTIONS OF ALRYLUREAS. EXCESS ENTHALPIES AT 25' FOR BUTYLURBA AND ITS ISOMERS. VINCENZO ABATE, CUIDO BARONE, PASQUALE CACACE, GIUSEPPINA CASTRONUOVO AND VITTORIO ELIA.
ISTITUTO CHIMICO - UNIVERSITY OF NAPLES, VIA MEZZOCANNONE 4, 80134 NAPLES ITALY
ABSTRACT The heats of dilution in water of three isomerit alkylureas (normal-bu= tylurea, tert-butylureaand tetramethylurea)were determined at 25'C. A cornparis& of the concentration coefficients of the excess enthalpies, with those of other alkylureas in water makes possible to identify, dif= ferent behaviours, both determined primarily by hydrophobic interactions One, shown by the and urea-hydrocarbons,water-mediated,interactions. n-butylurea,is similar to that of diluted or moderately concentrated solu= tions of alkanols in water. Another is shown by the tert-butylureaand te= tramethylurea.The kind of aliphatic substituents (linear or branched) and their position determine probably large differences in the cooperativity and effectiveness of the interactions.
INTRODUCTION Studies on diluted or moderately concentrated aqueous solutions of non-electrolytesprovide information about molecular interactions in the mixtures of polar compounds. In recent years the role of water, as an es= sential partner of solute-solute interactions,has been often outlined and clarifiedp-43. In these systems, in fact, the solute-solute,solutesolvent and solvent-solventinteractionshave comparable intensities and time scales. Among the simple nonelectrolytes,alkylureas are peculiar in that they have two kinds of functional group..which perturb water in different ways, generating distincts interactions. In the present paper, we report on the excess enthalpies in water of three isomeric alkylureas, namely mono-n-bu= tylurea (MBU), mono-tert-butylurea(tBU) and 1,1',3,3'-tetramethylurea (TMU). These alkylureas have a variety of arrangements of the hydrophobic regions with respect to the hydrophilic part of the molecule. The excess enthalpies obtained are compared to other thermodynamicand physico-the= mica1 properties of the other two isomers, 1,3-diethylurea (1,3 DEU) and l,l'-diethylurea (1,l' DEU) and of other urea derivatives and urea-like compounds previously studied 2,4-lg. The excess enthalpy of a ! inary non-symmetric solution can be defined using the molality scalep5-171 as:
HE (m) =
H(m)
-
Hy
-
$4
0167s7322/83/$03.00 0 1983 E1sevierSciencePubhshersB.V.
(1)
60
where H and HE are referred to an amount of solution containing 1 kg of solvent and m moles of solute, Hy is the standard enthalpy of 1 kg of wa= ter and 2; is the limiting partral molal enthalpy of the solute. The ex= cess properties can be represented as power series expansions. A form often used, for the excess free energy, enthalpy, entropy, etc., is: GE
(m)
=
giim2
+
giiim 3
+
.........
(2)
HE
(m)
=
hiim2
+
hiiim3
+
.........
(2')
TSE
(m)
=
Tsiim2
+
Tsiiim3
+
.........
(2”)
In the MC Millan and Mayer approachB,15-181, the coefficients of these expansions take the meaning of virial coefficients, and they account for the pair, triplet, and higherorder interactions among solute particles, respectively. These coefficients account implicitly for any change in solute-solvent interactions and in the state of solvent water. However the pertubations arising from the introduction of the solute into i$e solvent at infinite dilution are included in the reference values 'E" *, H2, etc.. EXPERIMENTAL MBU and tBU (K&K products) were crystallized from ethanol and dried in vacua. TMU (Fluka product) was distilled under vacuum. All the solutions were freshly prepared by weight, using bidistilled and degassed water. Calo= rimetric measurements were carried out using an LKB 2107-112 standard batch and an LKB 10700-l standard flow microcalorimeters, at 25.00 + 0.01 "C. In the batch case. the experimental heats Q, evaluated by ihe integra= converted into the-heats tion of the ~~~ti2;~kqfaopfs~Xe~~;,g~ramI-are of dilution ,$HdiL (m'-_,
m)
=
-Q/w,
(3)
where w is the weight of water in the final solutions and m' and m are the in1.&. ral and final molalities. With the flow microcalorimeter, the va= lues of the AHdil are obtained as follows: AHdil
(m'cpm)
=
-(dQ/dt)Pw
(3')
where dQ/dt is evaluated from the instrumental deviations, normalized by electrical calibrations for each set of runs. P is the total mass flow-ra= te of water. Checks were performed to assure th: reproducibility of the data obtained through the two techniques. For a dilution process, in which a solute species passes from a initial mola= lity m' to a final one m, the heat of dilution is given by p,12-141 :
__$_
a
Hdil
(ms_
m)
=
_&
HECd
= hii(m-m')
A suitable fitting of Eq.(4) of the excess enthalpy.
- _$
HE(m’)=
+ hiii(m2-mV2)
to the data, then gives the virial
+... (4
coefficients
RESULTS The experimental calorimetric data concerning the three substances exami= ned, are given in Table A, B and C of the Appendix. In "'able I the enthalpic contributions & to the virial coefficients are reported, together with their 95% confidence limits.
61 TABLE I - Coefficients of the Excess Enthalpies of Aqueous Solutions of Alkylureas and Urea-like Solutes at 298.15 K.
Solute and molality range
tert-butylurea (o-0.15) tetramethylurea (O-4) l,l-diethylurea (O-1.5) n-butylurea (o-0.7) 1,3-diethylurea (O-2) n-propylurea (O-2) ethylurea (O-2) 1,3-dimethylurea (O-2) 1,1-dimethylurea (O-2.7) methylurea (O-2) urea (O-2) thiourea (O-1.2) biuret (o-0.16)
aUnits:
Jmol-l(mo1
kg-l)-1
h
3632 2032 791 1039 1011 292 160 35 38 -85 -351 -970 .2120
a
h
i373d ~80 +44 *31 +78 i52 a14 *14 210 *2 +3e fl4 +61
-6220 -139 -48 420 139 92 37 78 21 21 21 70
iii
b
' dthe reported
"H.C.c
tl9 sod +20 i26 A39 *34 ~26 &6 ~16 +4 f2 i6e a11
’ bUnits: Jm01-~(mol kg-1)-2
bon atoms in the alkyl chains confidence indices for istance, Refs.
ii
4 4 4 4 4 3 2 2 2 1 0 0 0
Ref.
This work 11 11 14 This work
’ 'number of car=
uncertainties
are the 95%
; eother similar data are reported in other papers:see, [3,4,10,16,17,20]
and the literature
quoted
therein.
Both are obtained from Eq.(4) by a least square procedure. The polynomial expansions of highest degree, whose coefficients exceed their own 95% con= fidence limits, are chosen. In the same Table, data relative to other al= kylureas and urea-like substances are shown for comparison. The numerical values of the h coefficients depend clearly on the number and stereochemii stry of the s&stituents. As expected, the hii values, for the three corn= pounds studied, are positive and higher than those of the coefficients of other alkylureas. However the hiii values are different. In fact, for TMU and tBU the h.iii are large and negative, whereas for MBU hiii is large but positive. Thus, the urea family of solutes in water can be split into three groups: - urea-like solutes (biuret, thiourea, urea) characterized by values negative for hii and positive for hiii [4,10,12,131 ;
- solutes of second group, which are similar the monohydric
alcohols [15,16,19,20] , even though they can be regarded as "mixed" solutes rather than "soluble hydrocarbons"[l,l6] . Their solutions are chara= cterized by positive values of both hii and hiii [2,14] . Monomethylu= rea is actually intermediate between these two group as shown by a careful analysis of the trend of the excess properties with the con= centration[lO j . Linear monosubstitued alkylureas (monoethyl, monopro=
62 pyl, monobutylurea), dimethylureas belong to this group;
(s;mmetric and asymmetric)
- solutes of the third group have values for hiii.
positive
and 1,3 DEU
for hii but negative
DISCUSSION In preceding papers on this subject 12,4,5,8-141 we discussed the behaviour of aqueous solutions of the first two groups just described. The properties of aqueous solutions of alkylureas (except monomethylurea are assumed to be de= termined primarily by hydrophobic interactionsp,15,16 j . However the urea re= sidue-alkyl residue and the urea-urea, water mediated interactions play impor= tant roles. The latter are predominant in the case of monomethylureapO] and the only ones in the case of urea-like solutesp,l2,13]. The most widely stu= . According to the Friedman and died are the hydro hobic interactionsu,Zl-231 Franks model[l5,16 4 a spontaneous overlap of the hydration cospheres surrounding the hydrophobic groups occurs, in the concentration process, with a release of solvent to the bulk. This release of water to a state richer in enthalpy and entropy, can justify the sign of the contribution to the pairwise interaction . . coefficients (Tsii, hii70) found for all the aqueous solutions of prevailingly hydrophobic solutes (alkylureas, alcohols, alkylamides, dipeptides, etc..) [2,11,14-16,19,20,25,26]. In all these cases the ositive values of Tsii over= w&m the positive ones of the hii, giving giiC0 P2,8,14-16,19,20,24,251. For the urea-like solutes we recently suggestedC4,10,12,13] that a spontaneous pro= cess of coalescence of the "distorted" or " destructured" hydration cospheres of the urea takes place when the solute concentration increases. This process can explain the signs of the pairwise interaction coefficients, all negative, since water would pass in this case from a state richer in enthalpy and entropy to the bulk. In other words, the favourable solute-solute interaction (giiL0) should be seen as a solvent-mediated interaction, not as the formation of H-bon= ded dimers and polymers, as hypothesized by other models. The close similarity of the thermodynamic properties of thiourea and urea solutions C12Jas well as the spectroscopic properties of urea and amide aqueous solutions [5,6,26,27-J sup= port these considerations. The urea-hydrocarbon interactions in water have become clear only recently. The properties of ternary solutions of water and urea with alcoholsD7,28], respectively seem to be determi= ethers[3], ketonesC31 and hydrocarbons[29,30) ned partially by a contribution acting in the same direction as the hydropho= bit interactions. The same conclusion can be reached indirectly by considering the (-CH2-)-urea contributions, which were evaluated by Wood, Lilley and their colleagues by means of the group contribution approach from the pairwise in= teraction coefficients gi.9 h.. and Tsij of a series of binary and ternary aqueous solutions[20,24,2sJ. I3 According to the models previously discussed, an additional release of water from the hydrophobic interaction cospheres was suggested as one of the sources of the observed properties of these ternary solutions[3,17,28]. This effect could be promoted by the disruptive action of urea on the more "ordered" cospheres of the hydrophobic moieties. A comparison of the excess enthalpies relative to the ternary systems water-urea-alkanols [17,28] with those relative to the binary and ternary aqueous solutions of alkanols[16,20], seems to indicate that the interactions of tert-butanol with other alkanols and with itself are weak, whereas the in= teractions with urea are stronger than those involving only the linear alcohols. This could imply that the hydrophobic hydration cospheres of branched alkyl chains contribute relatively little to hydrophobic interactions since they are relatively unchanged whith respect to increasing concentration. On the other hands the addition of urea promotes a substantial breaking up of these cospheres
63
large release of "structured"water to the bulk state,which is richer in enthalpy and entropy. It is possible that binary aqueous solutions of tBU,TkBfand 1,l DEU, whose alkyl groups are arranged more compactly, behave as the water - urea - tert-bu= tan01 solutions, since they show the same effects. These effects are strengthe= ned by the coexistence of the urea residue and alk.1 chain on the same molecu= le. The weak effect of the hydrophobic interaction could be partially (for 1,l DEU) or more than compensated (for TMU and tBU) by the disruptive action of urea residues on the cospheres of the alkyl chain of the other molecules. The main difference between this third group of alkylureas and the second one is the negative value of the hiii coefficients. It is possible that these solutions have a different behaviour which can be explained by means of an association model, which does not apply to the second group. If the association model is assumed to be a step reversible polymerization of a unlimited number of ideal species, with each step characterizedby the same value for the association constant K, and by the same heat of association AH; , it is possible to demon= and a very
strate that Eq. (2) can be transformed
HECd
=
Ka AHim
into Eq. (5) -
2,: AH"am3
[31,32] +
:
5Kz AHim'
+
. . .. (5)
where the h coefficients assume alternate signs, as in the present case. However, the very low values of the K, evaluated for 1,l DEU and TMU (0.030 and 0.034 respectively) and the fact that any explicit change in the solvent has been neglected, suggest that such semiideality models be treated with cautions and be used only as a framework for discussion. The positive values of theAHi may just be an indication that hydrophobic forces still prevail and that solvent water must be involved in the process. In the case of tBU 1 h*** iii(t(hiiI gives more reasonable value for K, andAH: (0.856 and 4'250 Jmolrespecti= vely). Thus, the association model cannot be excluded "a priori" without other evidence.
Appendix Table A- Heats of Dilution of Aqueous Solutions of Monobutylurea (MBU) at 298.15 K.
m'
m 0.5477 0.5391 0.4371 0.2149 0.3442 0.1566 0.1561 0.2529 0.1261 0.2090 0.1022 0.1321 0.0667
0.6670 0.6670 0.6670 0.6670 0.5494 0.5494 0.5494 0.3809 0.3809 0.3199 0.3199 0.1974 0.1974
AHdil
/m
(Jmol
-1
)
179 195 341 647 283 516 535 169 325 135 255 78 149
rr=7 All
measurements
were performed
with the batch microcalorimeter.
64
Table B - Heat of Dilution of Aqueous Solutions of tert-butylurea (tBU) at 298.15 K. m’
m
_
dHdil /m (Jmol-1 )
0.1402 0.1402 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399 0.1399
0.0636 0.0456 0.1138 0.1019 0.0967 0.0886 0.0873 0.0746 0.0684 0.0554 0.0383 0.0206 0.0110
204 244 50.8 78.2 86.7 109 113 144 185 209 241 316 344
0.1528 0.1475 0.1428 0.1379 0.1325 0.1222 0.1221 0.1158 0.1120 0.1025 0.0938
0.0706 0.0682 0.0660 0.0639 0.0614 0.0748 0.0566 0.0721 0.0688 0.0630 0.0581
186 171 171 177 174 107 170 104 115 101 89.4
u = 8.8 The first series of measurements were performed with the batch microcalori meter, the second with the flow apparatus.
Table C - Heat of Dilution of Aqueous Solutions of Tetramethylurea (TMU) at 298.15 K. m' 3.9795 2.9958
2.9958
1.3003 1.3003 1.0321 0.7266 0.7266 0.7266 0.7266 0.4035 3.4982 a 2.6801 a 1.6241 a 0.1567 a
m ‘2.3387 1.8441 0.3046 0.8264 0.4035 0.1219 0.4358 0.2231 0.1649 0.1598 0.0803 1.4231 1.1374 0.7170 0.0755
_C\Hdil /m (Jmol-1) 2013 1584 4277 818 1607 1781 553 982 1115 1111 649 2724 2169 1486 162
aThese experiments were performed with the batch microcalorimeter.
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