Non-ideality in Born-free energy of solvation in alcohol water and dimethylsulfoxide acetonitrile mixtures: Solvent size ratio and ion size dependence

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1 J. Chem. ci., Vol. 9, No. 5, eptember 007, pp Indian Academy of ciences. Non-ideality in Born-free energy of solvation in alcohol water and dimethylsulfoxide acetonitrile mixtures: olvent size ratio and ion size dependence HMANT K KAHYAP and ANJIT BIWA* Department of Chemical, Biological and Macromolecular ciences, and Unit for Nanoscience and Technology,.N. Bose National Centre for Basic ciences, JD Bloc, ector III, alt Lae City, Kolata ranit@bose.res.in M received 7 June 007; accepted July 007 Abstract. ecent extension of mean spherical approximation (MA) for electrolyte solution has been employed to investigate the non-ideality in Born-free energy of solvation of a rigid, mono-positive ion in binary dipolar mixtures of associating (ethanol water) and non-associating (dimethylsulfoxide acetonitrile) solvents. In addition to the dipole moments, the solvent size ratio and ion size have been treated in a consistent manner in this extended MA theory for the first time. The solvent solvent size ratio is found to play an important role in determining the non-ideality in these binary mixtures. maller ions such as Li and Na show stronger non-ideality in such mixtures compared to bigger ions (for example, Cs and Bu 4 N ). The partial solvent polarization densities around smaller ions in tertiary butanol (TBA) water mixture is found to be very different from that in other alcohol water mixtures as well as to that for larger ions in aqueous solutions of TBA. Non-ideality is weaer in mixtures consisting of solvent species possessing nearly equal diameters and dipole moments and is reflected in the mole fraction dependent partial solvent polarization densities. Keywords. Non-ideality; Born-energy; dipolar-mixtures; unequal size; MA.. Introduction olvent mixtures are important reaction media as one can tune the polarity, solubility, etc. of a solution by altering the composition. In the traditional barrier crossing model where an activation barrier separates the reactant and product, the reaction time scale is given by the rate at which the reactant crosses the barrier frequency. olvation environment around a reactant affects the barrier-crossing rate both via solvent arrangement and time-dependent re-arrangement of solvent particles. As inter-diffusion and preferential solvation significantly slows down the dynamics in binary mixtures than in pure solvents, 40 solvent effects on chemical reactions are expected to be different in binary liquid mixtures. In addition to simple chemical reactions, transport property such as ion mobility is also significantly modified in binary mixtures. For example, limiting ionic conductivity of uni-positive alali metal ions tertiary butanol (TBA) *For correspondence water exhibits a non-monotonic alcohol mole fraction dependence. 4 Moreover, bigger ion such as tetrabutyl ammonium ion (Bu 4 N ) shows a linear mole fraction dependence in the same mixture. 4 This different composition dependence originates partly from the nature of solvation of these individual ions and partly from the modification of the solution dynamics. In order to understand solvent effects on these simple events in binary mixtures, one therefore needs to investigate the solution structure in terms of intraand inter-molecular liquid correlations and modifications of solution dynamics. Anomalous physical properties of alcohol water mixtures at room temperature have been studied extensively over the years Different structures and mechanism for formation of those structures in alcohol water solutions at low to high alcohol concentrations have been proposed everal experimental studies indicate that at low alcohol content water structure is stronger than that in pure water This is then used to explain the anomalous decrease in entropy, 4,4 anomalous increase in sound 9

2 9 Hemant K Kashyap and anit Biswas attenuation coefficient 44 and emergence of an endothermic pea in the partial heat of solution of several crystalline salts in aqueous alcohol solutions. 70,7 Further experimental support 7 to this idea came from the fact that partial molal heat of solution showed an even larger endothermic pea when the relevant experiments were carried out at ~ 77 K. At higher alcohol concentration, however, the water structure breas down and the characteristic chainlie structure of pure alcohol predominates At high water content, an infinitely dilute third component (an ion or a neutral solute) competes with the alcohol molecules to be properly surrounded by water structure This is dictated by the relative interaction strengths between the solute and the solvent molecules of different species. As a result, the structure forming or breaing ability of alcohol in water is modified in presence of an ion or a solute Moreover, specific interactions among these components may favour enriching of one component over the other in the first few solvation shells giving rise to what is nown as preferential solvation. In such a situation one would lie to as the following questions. First, will there be any preferential solvation and thus non-ideality in the absence of any solute solvent and solvent solvent specific interactions? This can indeed be a situation as evidenced by the recent simulation studies of non-ideality in viscosity for model binary mixtures. 7,74 econd, how solvent size disparity would affect the preferential solvation and thus the non-ideality? Third, what are the effects of solute size on the non-ideality? Fourth, can one develop a theoretical formalism which will be simple and analytically tractable yet capable of describing the liquid structure in these complex mixtures, at least, qualitatively? ecently, we have extended the mean spherical approximation 75 (MA) framewor in order to investigate answers to these questions. The extended MA incorporates the difference in solvent diameters and dipole moments as well as ion size systematically for the first time. 76 In this article, we apply this extended MA formalism to study the solvent size ratio dependence of non-ideality in Born-free energy of solvation of an infinitely dilute ion in two different types of binary mixtures at different mole fraction of the solvent components: mixtures containing associating (that is, H-bonding) solvents and those made up of non-associating solvents. We consider mono-hydroxy alcohol such as ethanol and water in the first category, and dimethyl sulfoxide (DMO) mixture and acetonitrile in the second. The DMO acetonitrile is interesting because the larger component (DMO) possesses higher dipole moment. 5,77 This leads to a competition in pacing around a dissolved ion in DMO acetonitrile mixture. Moreover, the molecular diameters of these solvents are similar 5,77 and hence the non-ideality in polarization structure (and hence in Born-free energy of solvation) is expected to be weaer. In contrast, the dipole moment of water is slightly larger than the alcohol studied here but the size is considerably smaller. This assists water molecules to be preferably chosen in the first solvation shell and hence the non-ideality is liely to be stronger. Therefore, larger ions would be able to accommodate the larger solvent components leading to a more homogeneous solvation structure. This, in turn, will render the non-ideality weaer. It is to be noted here that the preferential solvation in model binary mixtures have been studied earlier. 78,79 In these studies either the size disparity of all the species was not treated systematically or the solvent size ratio was not incorporated. However, these studies indicated that specific interactions among the components are not crucial for giving rise to non-ideality. 78 In the present, wor we are studying solvent structure around an ion in a binary mixture of fluids made up of dipolar hard spheres with unequal radii and dipole moments We have, therefore, used the MA framewors provided by Isbister and Bearman, 8 Chan et al 80 and Blum et al Here we model the solvent molecules of different species as dipolar hard spheres with single point dipoles located at the centers. 80 Ions are considered as hard spheres carrying point charges of one unit at the respective centers. 80 The ions are present at infinite dilution. The electroneutrality is maintained, as usually done in the MA. The molecular diameters of solvent molecules have been calculated from the van der Waals space filling model. 77 xperimental values of dipole moments are used to specify different solvent components. 5 xperimental crystallographic ionic radii are used to denote the sizes of the ions studied. 5 Mole fraction dependent densities are taen from relevant experiments 44,88 89 and used to calculate the mole fraction dependent properties of these binary mixtures. As we use the MA framewor in our formalism, the analytical tractability of the former is retained. This maes the present theory simple to use in cases where intra and inter-molecular correlations are required as inputs. However, the correlations obtained

3 Born-free energy of solvation in alcohol water and dimethylsulfoxide acetonitrile 9 by the present theory would be at best semi-quantitative as MA is nown to wor rather poorly for strongly dipolar systems. 80 The organization of the rest of the paper is as follows. We describe the theoretical formulation in the next section. This will be brief as the details have been given elsewhere. Numerical results and discussions are given in III. Concluding remars are provided in IV.. Theoretical formulation We shall present here briefly the relevant theory as details have been discussed in ref. 76. Let us consider a solution of an electrolyte of equal-sized ions in a binary dipolar mixture. Let i and be the hard sphere diameters of the two types of solvent molecules with dipole moments μ i and μ, respectively. The solvent number densities are ρ i and ρ. Here, i, and μ i μ. The electro-neutrality is maintained by setting ρ αzα = 0, () α where ρ α denotes the number density of ionic species α with charge z α and diameter α =. The interaction potentials are given by αβ α β u () r = z z e / r, r > =, (a) αβ u (, r ω ) = z eμ / r, r > = ( )/ (b) u ( ω, r ) = z eμ / r, iβ β i r > = ( )/ (c) iβ u ( ω, r, ω ) = μ μ D / r, i i i r > = ( )/, (d) i i where α, β indices are for ions and i, indices are for dipoles. D = ˆ μ( ω ˆˆ ) ( rr I ) ˆ μ( ω), which arises due to the angle dependent parts of the interaction potentials. ˆ( ) ˆ i = μωi r. In above equations, e represents the elementary charge, I denotes a unit tensor, and ˆr is the unit vector from the molecule denoted by the first index towards the one denoted by second index. The orientation of ith type dipole is described by ω i. Now the Ornstein Zernie (OZ) relation can be used to obtain the MA solution for the correlation functions in an electrolyte solution of binary dipolar mixture. 80 The relevant OZ expressions along with the MA closure relations are discussed in ref. 80. ubsequently, the algebra described in Chan et al 80 leads to the following expressions for the wave number () and orientation dependent static correlation functions ( ) = c ( ) ρ c ( ) ( ), αβ αβ αβ β ρ c (, ω ) ( ω, ) (a) α β ω (, ω ) = c (, ω ) ρ c ( ) (, ω ) α ρ c α(, ω) ( ω,, ω) ω (b) ( ω, ) = c ( ω, ) ρ c ( ω, ) ( ) iβ iβ i β ρ i ω ω ω c (,, ) h β (, ) ω, (c) ( ω,, ω ) = c ( ω,, ω ) i i ρc i( ω, ) (, ω ) ρ c i( ω,, ω) ( ω,, ω) ω. (d) Carrying out the angular convolution in (a d) and using the charge neutrality condition, and setting ρc = ρ z, (4) ρ = ρ, (5) I we obtain the following coupled set of equations for the correlation functions due to the electrostatic interactions between ions, ion-dipole and dipoles 76 C C C C ( ) = c ( ) ρ c ( ) ( ) αβ αβ c α β ρc ( ) α β( ) (6a)

4 94 Hemant K Kashyap and anit Biswas C ( ) = c ( ) ρ c ( ) ( ) c α ρc ( ) ( ) (6b) C ( ) = c ( ) ρ c ( ) ( ) iβ iβ c i β ρ c i( ) β ( ) (6c) ( ) = c ( ) ρ c ( ) ( ) i i c i ρ c i( ) ( ). (6d) Now using the algebra described in ref. 76 we obtain the following expression for the Born-free energy of solvation for an ion r r r Born = ρ d Ω (, ω) g (, ω) α α α ω 4π ze = ρω drhα () r, (7) where the second equality follows because hα () r = 0for r < α. We now need to find α is a constant that needs to be determined. quation (7) and the above discussion now provide the following expression of Born-energy ( Born ) for an ion dissolved in binary mixture of dipolar solvents with unequal size and different dipole moments Born α z e = ρ μ H α α ze = ρμ Hα. (9) Now we need to calculate H α( = H α). Baxter factorization 80,90 gives for x > = ( α )/ H ( x) = Q ( x) ρ dyq ( y) Hα ( x y) From (8) when < x xh = Q ( x). (0) ρh α dyq ( y)( x y), () where Q (x) depends solely on the dipolar mixture properties and is given by, 76,79,80 α drh α (r) Qi ( x) = ai ( x i )( x i ) bi ( x i ), () for evaluating Born. This is calculated from the iondipole correlation function as follows 79,80 π H ( x) = drp ( x / r) rh ( r), where x = π x drhα ( r) = xhα for x α, (8) π H = drh ( r), α where the values of a i and b i are described in ref. 76. quation () consists of two coupled simultaneous equations in H α when =, and α is fixed (a given ionic species). Using Baxter s method 90 we obtain the following expression for C ( x) C ( x) = Q ( x), x ρ dyq( y) Qα ( x y) with closure relation, x () C ( x) = πeβμ, x. (4) Therefore, from () and (4), we obtain

5 Born-free energy of solvation in alcohol water and dimethylsulfoxide acetonitrile 95 πeβμ = Q ( x) ρ dyq( y) Qα ( x y), x (5) quation (5) contains two coupled simultaneous equations in Q (x) when =, for a fixed α. Using the expressions of Q (x) given in ref. 76 and carrying out the relevant algebra, from () we obtain the following expressions for as, H H α α Q ( )[ I I ] α α α α Qα( α)[ αi I ] =, (6) [ α αi I ][ α αi I ] [ I I ][ I I ] α α Q ( )[ I I ] α α α α Qα( α)[ αi I ] =, (7) [ α αi I ][ α αi I ] [ I I ][ I I ] α α where I i, I i and I i are given in ref. 76. ubstitution of H α (from (6) and (7)) into (9) provides the required expression for Born. Born free energy (F Born ) of solvation is then calculated from the following relation 80 F Born e = de( /). (8) 0 Born The expressions for partial polarization densities of constituent solvents around an ion with charge z α e at infinite dilution has been shown to be given by 79,80 Pα() r = Pα () r = Ω Ω 4π ρ ( )( ˆ dωgα r,ω ( ω) r ) (9) where P α (r) is partial polarization density of th species. ince hα = hα one can write the above equation as Pα() r = ρμ hα() r = ρμ hα() r d = ρμ Hα() r Hα() r, π r r dr (0) where Hα ( r) is given by 79,80 H () r = rh, r, (a) H () r = Q() r ρ dsq() s Hα( r s), r >. r >. (b) α. Numerical results and discussion As discussed in the Introduction, we would now present numerical results of two different mixtures, namely; ethanol water and DMO acetonitrile mixtures. The relevant parameters required for calculations are summarized in table. All the calculations done here are at 98 5 K. The non-ideality in mole fraction (x ) dependent Born free energy of solvation of an ion is expressed as follows 76,78,79 Δ F ( x ) = F ( x ) F ( x = 0) Born Born Born x [ F ( x = ) F ( x = 0)] Born Born = FBorn ( x ) [( x ) FBorn ( x = 0) xfborn ( x = )] () The second equality of () indicates that the excess Born-free energy of solvation is the deviation in Born-free energy of solvation for an ion at a mole fraction (x ) in a binary mixture from the sum of the mole fraction weighted Born-free energies in the pure Table. olvent parameters used in the calculation. Dipole Diameter* olvent moment (D) (0 8 cm) Water thanol Acetonitrile (ACN) Dimethylsulfoxide (DMO) 4 5 *The diameters of Li and C 4 ions used in the calculations are 4 and 0 Å, respectively

6 96 Hemant K Kashyap and anit Biswas components. We would use () to investigate the mole fraction dependent non-ideality in ΔF Born in ethanol water and DMO acetonitrile mixtures. Figure shows the mole fraction dependence of Born-free energy of solvation, F Born and excess Bornfree energy of solvation, ΔF Born in these mixtures for two ions of very different diameters, Li and tetrabutyl ammonium ion (C 4 ). The Born-free energy of solvation, F Born (upper panel) indicate different mole fraction dependence in ethanol water and DMO acetonitrile mixtures. The insensitivity (or very wea dependence) of F Born on DMO mole fraction in DMO acetonitrile mixture (filled symbols) is a manifestation of competition between pacing and electrostatic interactions. The dipole moment of DMO is little larger than that of acetonitrile and hence ion dipole interaction would favor DMO more in number in the first solvation shell of an ion. However, pacing constraint (repulsion) would not favour disfavor DMO as its size is slightly larger than acetonitrile. Therefore, a delicate balance between these two interactions renders the mole fraction depend- ence of F Born a very wea one. Note also that even in ethanol water mixture (open symbols), the mole fraction dependence in F Born for C 4 (triangles) is very wea. This indicates that C 4 being large in size can accommodate larger solvent molecules more favourably than smaller ions, resulting much less concentration fluctuations in the nearest neighbour solvent arrangements. As expected, the values of F Born for these ions in pure solvents match exactly with predictions from the wors of Chan et al 80. The lower panel of figure shows the mole fraction dependence of the excess Born-free energy of solvation, ΔF Born for Li and C 4 in ethanol water and DMO acetonitrile mixtures. Note that the slope of ΔF Born for DMO acetonitrile mixture is opposite to that found in ethanol water mixtures. imilar results have also been found for smaller ions in TBA water mixtures when we studied aqueous solutions of methanol and TBA. 76 The positive values of ΔF Born clearly indicates that DMO molecules around a small ion such as Li is increasingly disfavored due to Figure. Born-free energy of solvation F Born (upper panel) and excess Born-free of solvation ΔF Born (lower panel) for Li (circles) and C 4 (triangles) ions as a function of mole fraction of second (larger) component (x ) in ethanol water (open symbols) and dimethylsulfoxide acetonitrile (solid symbols) mixtures. Figure. Partial polarization densities of water P water (r) and ethanol P toh (r) (both scaled by BT / w ) around Li and C 4 ions as a function of distance r (scaled by water diameter w ) from the centre of the ion at 0 0 (solid line), 0 50 (short dashed line) and 0 70 (dotted dashed line) mole fractions of ethanol in ethanol water mixtures. For discussion, see text.

7 Born-free energy of solvation in alcohol water and dimethylsulfoxide acetonitrile 97 larger size. This is indeed a manifestation of dominance of repulsive interactions in determining the solvation structure around a solvated ion. Another important aspect of this figure (lower panel) is that the absolute magnitude of ΔF Born is 4 times less for Li in DMO acetonitrile mixture than that in ethanol water solution. This indicates that the solvent size ratio indeed plays a crucial role in determining the extent of non-ideality in binary mixtures. This is one of the new results of the present scheme described here. Also, the position of the extremum of DMO acetonitrile curve occurs at almost 50:50 composition whereas that for ethanol water occurs at ~ 0 mol fraction of ethanol. This further proves that the solvent size ratio not only determines the extent of non-ideality in Born-free energy of solvation in binary mixtures, but determines also the location of the extremum on the mole fraction axis. We next present the numerical results on the partial solvent polarization densities around Li and C 4 in these mixtures at three different mole fractions of either ethanol or DMO. Figure shows the partial solvent polarization densities in ethanol water mix- tures. While the upper panels describe the partial polarization densities of water around Li and C 4, those of ethanol are shown in the lower panels. Note that at the lowest ethanol mole fraction (0 0), both the first and second solvation shells are clearly visible whereas the second solvation shell disappears with the increase in alcohol mole fraction in the mixture. As expected, the pea value of the water polarization density decreases with the increase in ethanol concentration and vice-versa. The results for DMO acetonitrile mixtures are shown in figure. As observed in the ethanol water mixture (figure ), the polarization pea also decreases here as one increases the DMO (larger component) concentration. However, the second solvation shell is well formed for most of the cases in this mixture. This is in contrast to what has been observed in ethanol water mixtures (figure ). This is again due to the larger size of the solvent components constituting the binary mixtures. We show in figure 4 the mole fraction dependence of partial polarization peas for these two mixtures. The decrease or increase of partial solvent polarization pea is non-linear for ethanol water mixtures, Figure. Partial polarization densities of acetonitrile P ACN (r) and dimethylsulfoxide P DMO (r) (both scaled by BT / ACN ) around Li and C 4 ions as a function of distance r (scaled by water diameter ACN ) from the centre of the ion at 0 0 (solid line), 0 50 (short dashed line) and 0 70 (dotted dashed line) mole fractions of DMO in DMO ACN mixtures. For discussion, see text. Figure 4. Upper panels: Comparison of pea values of partial polarization of water P water (pea) and ethanol P toh (pea) for Li (open circles) and C 4 (open triangles) ions as a function of mole fraction x of ethanol in ethanol water mixture. Lower panel: Comparison of pea values partial polarization of acetonitrile P ACN (pea) and dimethylsulfoxide P DMO (pea) for Li (solid circles) and C 4 (solid triangles) ions as a function of mole fraction x of DMO in CAN DMO mixture.

8 98 Hemant K Kashyap and anit Biswas the extent of non-linearity being very wea for C 4. As discussed earlier here and elsewhere, 76 the nonlinearity is a manifestation of preferential solvation and hence this explains the microscopic origin of the strong non-ideality observed for smaller ions in ethanol water and other alcohol water mixtures. In DMO acetonitrile mixture, however, the peas of the partial solvent polarization densities decrease or increase almost linearly for both the ions showing a very wea preferential solvation. This is again due to the comparable sizes and dipole moments of DMO and acetonitrile molecules. 4. Conclusion Let us first summarize the main results of the paper. We have used the recently extended MA formalism 76 to study the Born-free energy of solvation and non-ideality associated with it in associating (alcohol water) and non-associating (DMO acetonitrile) solvent mixtures. The solvent dipoles are treated as dipolar hard spheres and ions as hard spheres with unit charge at centers. The electrolyte concentration has been infinitely dilute. The sizes of every species in the solution and the dipole moments of the different components have been systematically incorporated. The ethanol water mixtures show greater non-ideality than DMO acetonitrile mixtures. The slope of the excess Born-free energy of solvation is found to be opposite in DMO acetonitrile mixture than that in ethanol water solutions. The extremum positions of the excess Born-free energy of solvation versus mole fraction (of bigger component) curves have been found to be affected significantly by the solvent size ratio. Also, the larger ion exhibits smaller nonideality. ince we treat these binary mixtures within the MA framewor, the present approach can neither distinguish between ions of different signs (cation or anion) nor can deal with any specific interactions and shape (say, spherical or ellipsoid) asymmetry that are present in real mixtures. Also, MA is a linear theory which is nown to fail for strongly polar systems. Therefore, the intra and inter-molecular correlations derived in this present wor can be at best semiquantitative in nature. For an accurate description of these quantities one should use a better and nonlinear theories that are liely to be more numerically involved. The present approach, in contrast, is simple and analytically tractable yet captures the physics qualitatively correctly. This is definitely a positive aspect of the present wor given the complexity of real solution mixtures. Acnowledgements We are deeply indebted to Profs Biman Bagchi and Kanan Bhattacharyya for their interest and constant encouragement. We sincerely than Prof. Arup K aychaudhuri for his constant support. We than Ms P Ghoshal for experimentally measuring the mole fraction dependent densities of DMO acetonitrile mixtures. Financial assistance from the Council of cientific and Industrial esearch (CI), India and Department of cience and Technology (DT), India are gratefully acnowledged. HKK thans NBNCB for a research fellowship. eferences. Fleming G and Wolynes P G 990 Phys. Today 4 6. Hynes J T 985 The theory of chemical reactions (ed.) M Baer (CC: Boca aton) 4. Hynes J T 985 Ann. ev. Phys. Chem Grote F and Hynes J T 980 J. Chem. Phys Bagchi B and Biswas, 999 Adv. Chem. Phys Bagchi B and Gayathri N 999 Adv. Chem. Phys Biswas and Bagchi B 996 J. Chem. Phys Nitzan A 988 Adv. Chem. Phys Bagchi B 987 Int. ev. Phys. Chem Bagchi B and Oxtoby D W 98 J. Chem. Phys Kim K and Fleming G 988 J. Phys. Chem Velso P, Waldec D H and Fleming G 98 J. Chem. Phys othenberger G, Negus D K and Hochstrasser M 98 J. Chem. Phys Asano T, Furuta H and umi H 994 J. Am. Chem. oc umi H and Asano T 995 J. Chem. Phys umi H 995 J. Mol. Liq Berne B, Berovec M and traub J 988 J. Phys. Chem traub J and Berne B 986 J. Chem. Phys traub J, Berovec M and Berne B 986 J. Chem. Phys Polla J. Chem. Phys Carmeli B and Nitzan A 98 Phys. ev. Lett Carmeli B and Nitzan A 98 Phys. ev. Lett. 5. Chandra A and Bagchi B 99 Adv. Chem. Phys Bagchi B and Chandra A 989 J. Chem. Phys Bagchi B and Chandra A 989 Chem. Phys. Lett Chandra A and Bagchi B 989 J. Phys. Chem Chandra A and Bagchi B 989 J. Chem. Phys

9 Born-free energy of solvation in alcohol water and dimethylsulfoxide acetonitrile Chandra A and Bagchi B 990 J. Phys. Chem Biswas and Bagchi B 996 J. Phys. Chem Biswas and Bagchi B 996 J. Phys. Chem Biswas and Bagchi B 997 J. Chem. Phys Biswas and Bagchi B 997 J. Am. Chem. oc Biswas, Nandi N and Bagchi B 997 J. Phys. Chem Bagchi B and Biswas 998 Acc. Chem. es Chandra A 995 Chem. Phys. Lett. 5 ; Chandra A 998 Chem. Phys Chowdhuri and Chandra A 00 J. Chem. Phys ; Chowdhuri and Chandra A Gupta and Chandra A 007 J. Chem. Phys Day T J F and Patey G N 999 J. Chem. Phys Li H, Arzhantsev and Maroncelli M 007 J. Phys. Chem. B Gardeci J A and Maroncelli M 999 Chem. Phys. Lett Broadwater T L and Kay L 970 J. Phys. Chem Frans F and Ives D J G 966 Q. ev. Chem. oc Zeidler M D 97 In Water, a comprehensive treatise (ed.) F Frans (New Yor: Plenum) vol., p Brai M and Kaatze U 99 J. Phys. Chem Kaatze U, Brai U, cholle F D and Pottel 990 J. Mol. Liq Kaatze U, Pottel and chmidt P 988 J. Phys. Chem ; Kaatze U, Pottel and chmidt P ; Kaatze U, Menzel K and Pottel 99 J. Phys. Chem Woow D and Czarneci M A 005 J. Phys. Chem. A Murthy N 999 J. Phys. Chem. A Iwasai K and Fuiyama T 979 J. Phys. Chem. 8 46; Iwasai K and Fuiyama T 977 J. Phys. Chem Dixit, Crain J, Poon W C K, Finney J L and oper A K 00 Nature (London) oper A K and Finney J L 99 Phys. ev. Lett Bowron D T, oper A K and Finney J L 00 J. Chem. Phys Turner J and oper A K 994 J. Chem. Phys D Angelo M, Onori G and antucci A 994 J. Chem. Phys gashira K and Nishi N 998 J. Phys. Chem. B Yoshida K and Yamaguchi T 00 Z. Naturforsch. A Nishiawa K, Kodera Y and Iiima T 987 J. Phys. Chem Koga Y 984 Chem. Phys. Lett Finney J L, Bowron D T and oper A K 000 J. Phys. Condens. Matter A 60. Bowron D T, Finney J L and oper A K 998 J. Phys. Chem. B Bowron D T and Moreno D 005 J. Phys. Chem. B Bowron D T and Moreno D 00 J. Chem. Phys uliss G W and orensen C M 984 J. Chem. Phys ato T and Buchner 00 J. Chem. Phys Tanaa H and Gubbins K 99 J. Chem. Phys Ferrario M, Haughney M, McDonald I and Klein M L 990 J. Chem. Phys Palinas G, Hawlica and Heinzinger K 99 Chem. Phys Waisaa A, Komatsu and Usui Y 00 J. Mol. Liq Naanishi K, Iari K, Oazai and Touhara H 984 J. Chem. Phys Arnett M and McKlevey D 966 J. Am. Chem. oc Arnett M and McKlevey D 965 J. Am. Chem. oc Arnett M, Bentrude W G, Bure J J and Duggleby P M 965 J. Am. Chem. oc rinivas G, Muheree A and Bagchi B 00 J. Chem. Phys Muheree A, rinivas G and Bagchi B 00 Phys. ev. Lett Gray C G and Gubbins K 984 Theory of molecular liquids (Oxford: Clarendon) 76. Kashyap H K and Biswas 007 J. Chem. Phys. 7 (submitted for publication) 77. dward J T 970 J. Chem. du Chandra A and Bagchi B 99 J. Chem. Phys Morillo M, Den C, Burgos F and anchez A 000 J. Chem. Phys Chan D Y C, Mitchel D J and Ninham B 979 J. Chem. Phys Wertheim M 97 J. Chem. Phys Isbister D and Bearman 974 J. Mol. Phys Adelman A and Deutch J M 97 J. Chem. Phys Blum L 974 J. Chem. Phys Cummings P T and Blum L 986 J. Chem. Phys Blum L and Wei D Q 987 J. Chem. Phys Wei D and Blum L 987 J. Chem. Phys Fort J and Moore W 965 Trans. Faraday oc Densities of DMO acetonitrile binary mixtures at few mole fractions that were not available in ref. 88 have been determined experimentally by us using a compact density-cum-sound velocity analyzer (Model DA5000) from Anton Paar, GmbH, Austria. Density data available in the ref. 88 have also been retaen in order to chec the reproducibility. 90. Baxter J 970 J. Chem. Phys