J . Phys. Chem. 1989, 93, 5650-5654
5650
Effect of Urea on Hydrophobic Interaction: Raman Difference Spectroscopy on the C-H Stretching Vibration of Acetone and the C-N Stretching Vibration of Urea Yasuhisa Mizutani,' Keiji Kamogawa,$and Koichiro Nakanishi*st Division of Molecular Engineering, Graduate School of Engineering, Kyoto University, Sakyo- ku, Kyoto 606, Japan, and Institute f o r Molecular Science, Okazaki National Research Institutes, Myodaiji, Okazaki 444, Japan (Received: March 23, 1988; I n Final Form: January 23, 1989)
The mixing state of urea in urea-water binary system and those of urea and acetone in urea-acetone-water ternary system are investigated with Raman difference spectroscopy. The frequency shift of the C-N stretching vibration, A Y C N , in the former linearly depends on the mole fraction of urea, xu,and changes little at lower xu. In the ternary system, the frequency shift of the C-H vibration, AucH, is determined as a function of xA and xu, from which the transfer shift from water to aqueous urea solution, AvcHtranS,is evaluated as a function of xu. The AvCHtransis found to give a major contribution to the AwCH and it is almost proportional to xu. The results reveal that urea molecules attractively solvate acetone single molecule in close vicinity to themselves without changing the acetone-water interaction and that they decrease the acetone-acetone interaction to some extent. This is also confirmed by AuCN behavior. The present study provides microscopic experimental evidence for the solvation effect of urea which has been discussed extensively.
Introduction Physicochemical properties of aqueous urea solution have been extensively studied in relation to its utilization for denaturing pr0teins.l Urea is characterized by its high solubility in water (>20 M at 25 oC)2and a large positive enthalpy change on mixing. Wetlaufer et al. have investigated the effect of urea on the solubilities of hydrocarbons in water and the results show that urea enhances the solubilities of hydrocarbons (except methane).3 Several investigations have also revealed that urea inhibits micellar aggregation of surfactant^.^" Hydrophobic interaction plays a dominant role in the formation of such higher order structures of folded globular proteins7 and molecular assembly. Previous studies on urea have focused on its potential ability to weaken hydrophobic interactions. So far two mechanisms have been proposed by which urea perturbs the mixing state of hydrophobic solutes in aqueous solutions. One is the indirect mechanism where urea changes water structure in favor of the dissolution of hydrophobic solutes,' and the other is the direct one where urea participates in the solvation of hydrophobic solutes in ~ a t e r . ~ , ' ~ The indirect mechanism was proposed by Frank and Franks.' They explained the urea-induced solubility enhancement of hydrocarbons, which was investigated by Wetlaufer et al., in terms of a two-state equilibrium of water structure. This interpretation has been supported by many experimental works."-'5 Nevertheless, there has also been a conflicting experimental interpretation that the effect of the addition of urea to water does neither ,~~ computer promote nor destroy the solvent s t r ~ c t u r e . ' ~Recently, simulations on the urea effect in aqueous media have been carried out by two g r ~ u p s . ' ' ~Both ~ ~ results clearly revealed negligible influence of urea upon water structure. Although the direct mechanism has received much less attention, it could interpret the urea effect, too. Nozaki et al. suggested that mixed clathrate-like structure is formed around hydrocarbon molecules in aqueous solution. This is based on the crystalline clathrate structures between hydrocarbons and urea, which are as widely known as those between hydrocarbons and water.9 Roseman and Rossky have ascribed the enhanced solubilities of hydrocarbons by urea to the smaller free energy of dissolution in mixed solvent caused by the replacement of water by the larger urea molecule in the solvation region.1° For further discussion of these mechanisms, microscopic information about the mixing state of the solute is essential. The intramolecular vibrations reflect the environment around a solute molecule sensitively and Raman difference spectroscopy allows 'Kyoto University. *Institute for Molecular Science.
0022-3654/89/2093-5650$0 1.50/0
us to measure fine frequency shifts of these vibrational bands with high precision. Frequency shifts have quantitatively been investigated for some aqueous solutions of nonelectrolytes involving C-H bonds, and, furthermore, the correlation between frequency shift behavior and the solution structure has been discussed.2w24 In the present study, we measure frequency shifts of the C-N symmetric stretching vibration of urea and of the C-H symmetric stretching vibration of acetone and evaluate magnitudes of acetone-acetone, acetone-urea, and acetone-water interactions in urea-acetone-water ternary system. On the spectroscopic basis, we will discuss the participation of urea in the solvation of acetone.
Experimental Section Apparatus and Measurement Procedure. Details o f the experimental apparatus and procedure were described previously.20,2' Raman spectra were taken with the 514.5-nm line (laser power -300 mW) of an Ar+ ion laser (NEC GLG3200). The light path of the incident radiation was fixed by passing it through several irises prior to focusing it on the sample cell. The scattered ra-
( I ) Tanford, C . Adu. Protein Chem. 1968, 23, 122. (2) Ellerton, H. D.; Dunlop, P. J . Phys. Chem. 1966, 70, 1831. (3) Wetlaufer, D. B.; Malik, S. K.; Stoller, L.; Coffin, R. 1. J . A m . Chem. Soc. 1964, 86, 509. (4) Brunig, W.; Holtzer, A. J . A m . Chem. SOC.1961, 83, 4865. (5) Murkerjee, P.; Ray, A. J . Phys. Chem. 1963, 67, 190. (6) Schick, M. J. J . Phys. Chem. 1964, 68, 3585. (7) Kauzmann, W. Adc. Protein Chem. 1959, 14, 1. (8) Frank, H. S.; Franks, F. J . Chem. Phys. 1968, 48, 4746. (9) Nozaki, Y.; Tanford, C . J . Biol. Chem. 1963, 238, 4074. (10) Roseman, M.; Jencks, W. P. J . A m . Chem. SOC.1975, 97, 631. (11) Finer, E. G.; Franks, F.; Tait, M . J. J . A m . Chem. SOC.1972, 94, 4424. (12) Walrafen, G. E. J . A m . Chem. SOC.1966, 44, 3726. (13) Hammes, G. G.; Schimmel, P. R. J . Chem. Phys. 1967, 89, 442. (14) Barone, G.; Rizzo, E.; Vitagliano, V . J . Phys. Chem. 1970, 74, 2230. (15) Grant, E. H.; Keefe, S. E.; Shack, R. Adu. Mol. Relax. Processes 1972, 4 , 217. (16) Subramanian, S.; Sarma, T. S.; Balasuburamanian, D.; Ahuwalia, J. C. J . Phys. Chem. 1971, 75, 815. (17) Swenson, C. A. Arch. Biochem. Biophys. 1966, 117, 494. (18) Kuharski, R . A,; Rossky, P. J . J . A m . Chem. SOC.1984, 106, 5786. (19) Tanaka, H . ; Touhara, H.; Nakanishi, K.; Watanabe. N. J . Chem. Phys. 1984, 80, 5170. (20) Kamogawa, K.; Tajima, K.; Hayakawa, K.; Kitagawa, T. J . Phys. Chem. 1984, 88, 2494. (21) Kamogawa, K.; Kitagawa, T. J . Phys. Chem. 1985, 89, 1531. (22) Kamogawa, K.; Kitagawa, T. J . Phys. Chem. 1986, 90, 1077. (23) Kamogawa, K.; Kaminaka, S.; Kitagawa, T. J . Phys. Chem. 1987, 91, 222. (24) Kamogawa, K.; Kitagawa, T.Vibrational Spectra and Structure; Vol. 178:Durig, J . R., Ed.; Elsevier: Amsterdam, in press.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 5651
Effect of Urea on Hydrophobic Interaction
[ U r e a ] / mo1.kg-l
T b
+ r-l 0
10
5
0'5
- 1.0 0.0 1200
1000
800
XU
RAMAN SHIFT / cm-' Figure 1. Raman spectra of 12 M (17.8 mol %) (A) and 6.5 M (10.5 mol 8 ) (B) aqueous solutions and the difference spectrum between (A) and (B). The difference spectrum is an expanded scale; the multiplication factor is 21.72. The peak height in the original spectrum (AZO) and the peak-to-valley height in the difference spectrum (AZp) are represented. diation was collected a t 90' to the laser beam, passed through a glass filter (Hoya 0-53) in order to eliminate Rayleigh scattering, and dispersed with a single monochromator (Spex 1704 with a 1200 grooves/mm,f= 1000 mm). The dispersed radiation was detected with an intensified silicon photodiode array detector (PAR Model 1420,OMA3 system) controlled by a microcomputer ( N E C PC9801VM21) and the spectral data were stored in and analyzed with the microcomputer. Sample Preparation. Urea (Nakarai Chemicals, guaranteed reagent) and acetone (Nakarai Chemicals, UV spectroscopic grade) were used without further purification. Water was distilled and deionized. The concentrations of the samples were determined by weight. The prepared solution was left to stand for one night at room temperature and more than an hour in a water bath a t 25.0 O C , and sealed up in a 1 X 1 X 5 cm3 sample cell before measurement. The temperature of the sample was controlled to 25.0 & 0.5 "C. Dam Analysis. Details of the data analysis were described previously.zo The Raman spectra of sample and reference solutions were alternately measured. The background part due to solvent was subtracted by the fitting technique.20 The difference spectra, AI(v), were obtained by the subtraction of reference spectra from sample spectra by
AI(v) =
Isample(v)
- CFcf(v)
(1)
Here P m p l e ( v ) and Pcf(v)are respectively sample and reference spectra after the background correction and C is an adjustable parameter, which is determined so as to make the area of the positive part equal to that of the negative part in AI(v). Difference spectra with higher noise were smoothed with the polynomial curve fitting method proposed by Savitzky and GolayFs By artificially generating the simulated difference spectra with the microcomputer, we determined the calibration curve between Av and the ratio, AIp/AIo, where AIp is the peak-to-valley intensity in the difference spectrum and AIo is the peak height of the sample spectrum (see Figure 1). The observed ratio, AIp/AIo, was converted into a frequency shift, Av, by using the calibration curve.
Expressions for Frequency Shifts and Concentrations Subscripts U, A, and W denote urea, acetone, and water, respectively, and superscripts b and t denote binary and ternary systems, respectively. We express the concentration of each species in the ternary system in such a way that xi and X,are the concentrations of species i relative to water and the whole species, (25) Savitzky, A.; Golay, M. J. E. J . Anal. Chem. 1964, 36, 1627.
02
01
Figure 2. Concentration dependence of frequency shift of the C-N symmetric stretching mode of urea in aqueous media. respectively. While the acetone concentration, xAor X A , is expressed in mole fraction, that of urea, xu, is expressed in mole fraction or molarity. As will be mentioned, reference systems are infinitely dilute solutions of the probe solutes.
Results and Discussion 1. Urea-Water System. We first investigate urea-water binary systems in order to clarify their mixing state. A strong Raman line a t 1003 cm-' of urea is assigned to almost pure symmetric stretching vibration of the C-N bond by calculation of the normal-mode vibrations.26 Contrary to this, the corresponding IR absorption is very weak.26 This highly selective Raman activity shows that the C-N vibration modifies the polarizability rather than the dipole moment. Accordingly, the frequency shift of this band reflects the magnitude of interaction due mainly to the dispersion force of surrounding molecule.27 Moreover, since this band is single and highly symmetric, it is suitable for precise difference analysis. From the above reasons it is possible to discuss the mixing state based on the frequency shift of this C-N bond in the same way as the previous discussions for C-H shift^.^"-^^ Figure 1 shows an example of the Raman spectra of aqueous urea solution and the difference spectrum of the C-N stretching vibration. This symmetrical difference spectrum indicates a small change in the Raman bandwidth.28 We measure the frequency shift of the C-N band by use of 10.5 mol % aqueous urea solution as a reference and extrapolated it to zero mole fraction. Then, AvCNbwas obtained as the frequency shift from this extrapolated value. Figure 2 shows AvCNbobtained by the above procedure. Important features in Figure 2 are a linear change of frequency shift a t higher mole fraction and little change with urea concentration in dilute range ( x u < 0.04). Accordingly, we discuss these features in the next few paragraphs. First, we discuss a linear change of frequency shift a t higher mole fractions. Such linear behavior has been generally obqerved for C-H symmetric stretching vibrations on isotopic dilution.22*B In a binary system composed of species X and Y, the observed frequency shift of species X, Avow, is a function of composition, xx, and is considered to arise from the X-X and X-Y interactions. It is represented as
AVobsd = AvXXXX
AVXYXY
(2)
by neglecting three-body and higher order interactions.z' Avxx and Avxy are called homogeneous and heterogeneous interaction factors, related to the X-X and X-Y interactions, respectively. Their physical meanings are described elsewhere.% Avxx and Avxy (26) Yamaguchi, T.; Shimanouchi, T.; Mizushima, S. Spectrochim. Acta 1957, 10, 120.
(27) Benson, A. M. Jr.; Drickamer, H. G. J . Chem. Phys. 1957.27, 1164. (28) Laane, J. J . Chem. Phys. 1981, 75, 2539. (29) Laane, J.; Kiefer, W. J . Chem. Phys. 1980, 73, 4971.
5652
T h e Journal of Physical Chemistry, Vol, 93, No. 15, 1989
Mizutani et a ] .
are equal to the frequency shifts of X species in the corresponding imaginary neat liquids of X and Y species, respectively, in which X.-X and X.-Y interactions are equivalent to those in a solution of a given concentration, x x . The linear change in the observed frequency shift gives constant Auxx and Auxv values. I n the present system, Auuti and Auuw are -4.5 and +0.2 cm-I, respectively. I n this concentration range, the environments in which the C-N bond of urea interacts with surrounding molecules are expressed in the forms of the ideal mixture of two environments represented by the constant interaction factors described above. The second feature of hCNb is that it shows little change with urea concentration in dilute range (xu < 0.04). Similar behavior has already been observed for the C-H vibration of methanol in aqueous solution, in which clathrate hydrate-like structure is p r o p o ~ e d . ~In~ ideal , ~ ~ solution, eq 2 leads to a linear variation of -luobsd, for X can still contact with X with some probability even in the dilute range. The frequency shift Au is given by Au = A(ye'
+ e")
(3)
where A and y are proper values of the probe molecule and e' and e" are the first- and second-derivative terms of interaction potential, r e ~ p e c t i v e l y . ~In ~ the right-hand side of eq 3, the second-derivative term is much smaller than the first-derivative term.27s31-33So th e AuObsdarising from the dispersion force may be represented as (4) where a x and aIare the polarizability of a probe molecule and that of molecule which interacts with the probe molecule, (i = X or Y). rx, denotes the equilibrium intermolecular distance between X and i molecules in the solution. ax' is the derivative of a x with regard to the symmetric stretching coordinate and is proportional to the square root of the Raman scattering intensity.22*31Technically speaking, r x I means the distance between the bond in question of species X and the interacting site of other species i. It was also revealed experimentally that Avxv near xx = 1 .O correlates with the polarizability of species Y in ethanol solutions as expected from eq 4.23 Since neighbor molecules outside the second hydration shell scarcely contribute to h & s d in eq 4, & b , b s d reflects the local structure around the probe molecule. Accordingly, if relatively stable hydration shells are formed around urea molecules, the contribution to AucNbdue to the interaction between urea molecules should be negligible. Under this situation AuCNb does not vary because a dilution by water simply leads to the decrease of the concentration of hydrated urea molecules and does not perturb the hydration shell around urea. Orita et al. calculated large hydrogen-bonding energy between one urea molecule and five water molecules by the a b initio LCAO SCF Tanaka et al. carried out a molecular dynamics calculation on the hydration of urea in an infinitely dilute solution and the results showed that 4-5 water molecules strongly interact with urea.I9 One of the most probable interpretations is that the small concentration dependence of the frequency shift is due to the rigid hydration shell around urea. Moreover, the mole fraction (-0.05) a t the deflection point means that about 15 water molecules besides these strongly hydrogen-bonded (4-5) water molecules are required for sufficient hydration of eacb urea molecule. 2. Urea-Acetone-Water System. The frequency shifts of urea and acetone molecules in the urea-acetone-water ternary system are observed and discussed in order to clarify the mixing states of urea and acetone molecules in the system. The observed bands are the C-H symmetric stretching vibration band a t 2940 cm-l of acetone22and the C-N symmetric stretching vibration band (30) Okazaki, S.; Touhara, H.; Nakanishi, K . J . Chem. Phys. 1984, 81, 890. (31) Schindler. W.; Jonas. J. J . Chem. Phys. 1980, 73, 3547. (32) Schweizer, K. S . ; Chandler, D. J . Chem. Phys. 1982, 76, 2296. (33) Zakin, M. R.; Herschbach, D. R. J . Chem. Phys. 1986, 85, 2376. (34) Orita. Y.: Pullman, A . Theor. Chim. Acra (Berlin)1977, 45, 257.
F]6 1 i n the system
Figure 3. Schematic diagrams on frequency shifts of the C-H symmetric stretching mode of acetone (a, top) and of the C-N symmetric stretching mode of urea (b, bottom). 1 / m denotes an infinitely dilute state. Details of nomenclature on each frequency shifts are in the text.
0.0
- 1.0
'E \
5 - 2.0
> a
Figure 4. Plots of bCH against acetone concentration, X,. Urea-free bCH, namely that in the acetone-water binary system, are represented by open circle (0).Concentration of urea, xu,is 1 M (1.8 mol %) (a), 2 M (3.5 mol 7%) ( O ) , 3 M (5.1 mol %) (A),4 M (6.7 mol %) (A),6 M (9.8 mol 5%) (O), and 10 M (15.3 mol %) (B), respectively.
of urea. We have obtained the frequency shift of the C-H band, hCH, by regarding an infinitely dilute aqueous acetone solution as a standard, as schematically shown in Figure 3a. The h C N t for urea, which is schematically shown in Figure 3b, is obtained by adopting an infinitely dilute aqueous urea solution as a standard. In this section, we discuss each frequency shift of acetone in steps I-IV in Figure 3a. The important parts are step 111, in which solvation of acetone is discussed, and step IV, in which the acetone-acetone interaction in the presence of urea is discussed. Furthermore, we discuss step IV from the frequency shift of urea. S t e p I. Step I is defined as the process in which the concentration of acetone changes from infinitely dilute value to x,. This is associated with the shift At+-', which are shown by open circles in Figure 4. They were obtained by measurements on aqueous solution (x, < 0.05) and by an extrapolation to xA 0. From a previous it is found that this infinitely dilute state is perturbed from neat acetone liquid by about +7 cm-I. AucH' also expressed in a similar way as eq 2, namely, +
The Journal of Physical Chemistry. Vol. 93, No. 15, 1989 5653
Effect of Urea on Hydrophobic Interaction AucHi =
AuAAbxA
+ AuAWbxW
[Urea] / mol kg-'
(5)
AuCH' decreases lineariy with the mole fraction of acetone by a
factor of -26 cm-' per mole fraction, and this corresponds to AvMb and AuAwbvalues of -26 and 0 cm-I, respectively. The fact that AuAwb = 0 a t xA = 0 is self-evident from the standard state of the frequency shifts. The constant value of AvAWbsuggests that the acetone-water interaction does not change in this concentration range. The value AuAAb in this study corresponds to the value ( A Y A A - A u A B ) = 20 cm-' in the previous where the reference state was neat liquid. Contrary to the urea-water system, saturation of frequency shift on dilution is not observed in this system. Step II. Step I1 is defined as the process in which urea is added to an aqueous acetone solution whose concentration is xA and AucH" is defined as the frequency shift in this process. AuCH"(XA.xU)can be obtained by the measurement of solutions whose concentration are (XA,xU) and (XA,O).Accordingly, the frequency shift of acetone in ternary solution, AucH(XA,XU),can be calculated by the following equation
O'O c
\
- 1.0
15
Fc i
k
I
\
\
- 3.0 0.0
0.1
0.2
X"
A ~ c H ( ~ A , x u=) AVCHYXA) + A~cH"(XA,XU)( 6 ) AuCH(XA,xU)
10
5
0
Figure 5. Plot of transfer frequency shift,
can also be expressed by
centration,
xu.
against urea con-
D a t a were obtained from Figure 4.
is different. Therefore, the indirect model proposed by Frank and Figure 4 shows A u C H ( X A , X U ) as a function of the mole fraction Franks would give no reasonable answer to explain this negligible of acetone, X A . Addition of urea uniformly gives rise to negative A u A W ~ ~ ~ ~ ~ . frequency shift. Next, we evaluate AvAUtrans on the basis of the acetone cluster Step III. Step 111 is defined as the process in which urea is observed in dilute aqueous acetone solution. Since, the aceadded to infinitely dilute aqueous acetone solution and AvcH1ll(~u) tone-acetone interaction factor, AuAAb, is -26 cm-' in the dilute is defined as the frequency shift in this process. Since concentration range (xA < 0.05), substitution of an acetone corresponds to the frequency shift which is associated with the molecule in the cluster with a urea molecule gives rise to an transfer of single acetone molecule from water to aqueous urea imaginary acetone-urea interaction factor, AuAU*, which can be solution, we call it the transfer frequency shift, A u ~ ~ ~ ~The~ ~ ( evaluated x ~ ) . as transfer frequency shift can be obtained by the extrapolation of AUCH(~A,XU to) XA 0: (9)
-.
by eq 4.37 AuAU* is calculated to about -19 cm-' by eq 9 after subtraction of resonance coupling effect involved in AuAA.22*38 This value is almost the same as -16 cm-', which is observed in dilute aqueous acetone solution. As a result, roughly speaking the acetone-urea interaction in the ternary system is almost close to the acetone-acetone interaction in binary aqueous solution. Consequently, the urea molecule seems to be in close proximity to acetone molecule without modifying the acetone-water interaction. Step IV. Step IV is defined as the process in which the acetone concentration is altered from infinitely dilute state to X A in the " as mixed solvent composed of urea and water. A U ~ ~is defined the frequency shift which occurs in this process and is a function of X A and xu. Since AuCH arises from the change of the interaction such as acetone-acetone, acetone-urea, and acetone-water, it is represented as
Figure 4 indicates that the major contribution of AucH(XA,Xu) comes from A u c H ~ ~ ~ " ~This ( x u )suggests . that the interaction between a single acetone molecule and urea molecules is much more important than that between an acetone cluster and urea molecules. The transfer frequency shift is a function of xu and is, in the same way as eq 2, represented by the sum of the contributions both from urea sites and water sites as follows: A u c H ~ ~ ~ " ~ (= x UA) Y
+
A U ~ ~ ~ ~ AvAwtransxW ~ X U
(8)
Here AuAUtrans and AuAWtrans are equal to the frequency shifts of acetone in the corresponding imaginary neat states of urea and water, respectively, in which A-U and A-W interactions are equivalent to those in a solution of a given concentration. Figure 5 shows the transfer frequency shift obtained from eq 7. It decreases almost linearly with urea concentration and gives AuAUtrans N -16 cm-' and AuAWtrans N -0.1 cm-]. This small A u suggests ~ that~ the interaction ~ ~ between ~ acetone and water in the ternary system is almost the same as that in the binary solution and that urea does not cause any appreciable change in acetone-water interaction. The indirect model for urea proposed by Frank and FranksB has been supported by numerous experiments of the urea effect on water structure, such as IH chemical shift," infrared spectra of water, and shear and structural contributions to ultrasonic absorption by several g r o ~ p s . ' ~ , ~ ~ . ~ ~ Our results of AuAWtransshow that the change of acetone-water interaction is small. If urea changes the long-range water structure and perturbs the hydration of acetone extensively, the difference and AuAWtrans will become large. But the result between bAWb (35) Arakawa, K.; Takenaka, N. Bull. Chem. SOC.Jpn. 1967.40, 2139. (36) Arakawa. K.; Takenaka, N.; Sasaki, K. Bull. Chem. SOC.Jpn. 1970, 43, 636.
~
+
AUCH = P u A A t X A AuAU'XU + AuAw'XW (IO) ~ By summarizing the latter two terms in the right-hand side of eq 10 as a term which denotes the contribution of mixed solvent, we obtain the following equation which is similar to that of the binary system
AUCH =
+ AuAStXs
( 10') where the subscript S denotes mixed solvent. On the limit XA 0, AuAs is AuCHtrans, clearly from its physical meaning. AuAAtXA
+
(37) We assume that rxx is nearly equal to rxI and that Aux, linearly depends on ai. This assumption is rough, but practically it causes no serious problem for the discussion of the imaginary interaction factor. (38) Since the contribution from resonance coupling effect in ref 22 is that in neat liquid, it may be smaller than the contribution in the acetone cluster. Then the present result is somewhat overestimated, but this difference in estimation of the contribution from resonance effect does not cause a problem for comparison between AuAu and AvAU*.
5654
Mizutani et al.
The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 0.0 I
receive the interaction from them, as discussed from A V A U " ~ " ~ . AvUA'* is the expected value for urea molecule interacting with acetone after the substitution of urea molecules in a binary system. It is calculated as follows:
I
b(aA
AvUA'* = AvUu a -20 L 0 00
x*
0.05
Figure 6. Plot of AvCNt against urea concentration. Notation of urea concentration is same as in Figure 4.
Figure 4 shows that AvAA' is -17 cm-] over certain XArange in IO M (I5 mol %) aqueous urea solution and its absolute value is smaller than that in aqueous media, A v A A b = -26 cm-'. This indicates that the addition of urea reduces the interaction between acetone molecules to some extent. In Figure 4, A v C H ( X A , x u ) depends linearly on XAwhen XU is 0 or I O M, and the slopes at intermediate concentrations (4-6 M) change from the slope of 10 M solution to that of 0 M solution as XA increases. This suggests that acetone molecules are transferred between two different environments when the urea concentration is intermediate. Next, we discuss this process from the standpoint of urea, Le., from the frequency shift of urea. We define A v c N ' ( X A , x u ) as the frequency shift from that in infinitely dilute aqueous urea solution (see Figure 3b). The results are shown in Figure 6. In the same way as eq I O and lo', we obtain the following equations for AYcN' AvCN'
=
AYUA'XA
+ Avuu'XU + AVUW'XW
(1 1)
Figure 6 shows that AvCN'(XA,XU) decreases linearly with XA. This suggests that urea is proximal enough with acetone molecules to
-
"W)
("u - "W)
AvWb (-4.5 cm-') gives a AvuA'* value of -5.3 cm-'. On the other hand, observed values of AvuA' were -13 cm-' (6 M) and -19 cm-' (10 M), in Figure 6 , which are fairly larger than the imaginary values, AvUA'*. Therefore, AvCNtsuggests that urea is mixing in the vicinity of acetone in the ternary solution. The direct mechanism of the urea effect have been proposed by Nozaki et aL9 and Roseman et a1.I0 Recently, Kuharski et al. carried out a molecular dynamics simulation of a ternary system consisting of 200 water molecules, one urea molecule, and a Lennard-Jones sphere and the results show that urea molecule displaces several water molecules, which is more than that expected from molecular volume of water, from the apolar solvation In conclusion, the present results lead to the picture that urea is in the vicinity of acetone, solvates acetone with water, and weakens acetone-acetone interaction with negligible change of acetone-water interaction. And it should be emphasized that the interaction between urea and a single acetone molecule is the driving force which hinders the acetone association. This conclusion is agreeable with the direct model where urea participates in the solvation of hydrophobic solutes.
Acknowledgment. We gratefully thank Professor Teizo Kitagawa, Institute for Molecular Science, for his encouragement during the present work. They also thank Professor Keitaro Yoshihara of the same institute for kind permission to use the OMA3 system. Registry No. Water, 7732-18-5; urea, 57-13-6; acetone, 67-64-1. (39) Kuharski, R. A,; Rossky,
P.J . Am. Chem. SOC.1984, 106, 5794.
