Study of the Influence of Alkyl Chain Cation-Solvent Interactions on Water Structure in 1 , 3-Butanediol-Water Mixture by Apparent Molar Volume Data

The densities of 1,3-butanediol-water mixtures and some tetraalkylammonium iodide salt solutions in these solvent mixtures at different concentrations (0.02 M-0.14 M) have been determined at 298.15 K using magnetic float densitometer technique. Then apparent molar volumes ФV of the electrolytes in above solvent mixtures were calculated. The apparent molar volumes of transfer ∆Φv 0 (tr) were also calculated and the ion-ion / ionsolvent interactions are then discussed on the basis of changes in the Masson’s slope and apparent molar volumes of transfer data.


Introduction
The knowledge of apparent molar and partial molar volumes of electrolyte has been utilized to study ion-ion and ion-solvent interactions in aqueous and non-aqueous solvents of different dielectric constant by a number of scientists [1][2][3][4][5][6] .Masson 7 found that the apparent molar volumes of electrolytes, Ф V , vary with the square root of molar concentration, √c, (over a wide concentration range) by the linear relation.
Where, Sv is the experimental slope.The discovery of negative slope of R 4 NI-type salts in aqueous solutions by Frank 8 and then by Wen and Saito 9 could lead to findings that there were R 4 N + ion-water hydrophobic interactions present in such solutions.The negative slope was explained by Frank on the basis of his hypothesis according to which the water structure is enforced around the alkyl chains of the R 4 N + ions.The decrease of the apparent molar volume Ф V with increase in concentration is due to accommodation of the R 4 N + ions inside the cages of water molecules of the enforced water structure system.The scientists, in the past, working on apparent molar volumes [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] attempted to find out the nature of Masson's slope (Sv).The change in slope shows the change in volume of solution.Positive slope corresponds to the increase in volume while negative slope corresponds to the decrease in volume on increasing the salt concentration.The increase or decrease in volume occurs due to structural changes in solvent molecules on addition of salts.It stuck in our curious mind to see what type of changes may occur in solution and what would be the possible reasons which are responsible for these changes.
We have examined the behaviour of different tetra alkyl ammonium iodide salts in 1,5pentanediol-water mixtures in our earlier communication 15 .In order to establish a definite picture of the influence of tetraalkyl chain-water interactions, it became necessary to carry out the similar studies in, another some what similar system, 1,3-butanediol-water mixture.In this communication, therefore, 1,3-butanediol-water binary solvent mixtures, having different water contents, have been selected for the study of effect of ion solvent interactions in solution of R 4 NI salts.
The density data have been analysed by means of Massons's equation.The apparent molar volume, Ф V, apparent molar volume at infinite dilution, Ф V 0 and slopes Sv have been interpreted in terms of ion-ion and ion-solvent interactions.
The Ф V 0 determined by Ф V values have been utilized to estimate apparent molar volumes of transfer at infinite dilution, ∆Ф V 0 (tr) for various tetraalkylammonium iodide salts from water to aqueous solutions of 1,3-butanediol.

Experimental
The binary solvent mixture of 1,3-butanediol-water, having 20, 40, 60 and 80% water (v/v) were prepared using conductivity water.The dielectric constants of these solvent mixtures of specified compositions have not been reported in the literature.Therefore, these were determined graphically, by plotting dielectric constants against composition (0% and 100% composition selecting as the two extreme points and then joining these two points by a straight line).The estimated values of dielectric constant of intermediate compositions were noted and are given in Table 1.Table 1.Estimated values of dielectric constant of 1,3-butane diol-water mixtures used as a solvent at 298. 15  The densities d 0 of these solvent mixtures were measured at 298.15 K by magnetic float densitometer using equation d 0 = (W + w + f .I) / (V + w / d Pt ) (2) Here W = weight of float, w = weight put on the float, f = weight equivalent current (g/Ampere), V = vol. of float, d Pt = density of platinum.The d 0 data have been summarized in Table 2.
Table 2 12, and 0.14 M were prepared in 20, 40, 60 and 80% water in 1,3-butanediol mixtures one by one taking one electrolyte and one solvent composition at a time.Me 4 NI salt was excluded due to solubility restrictions.The density, d, of these solutions were also measured at 298.15 K using Eq.2.The volumes of transfer parameters ∆Ф V 0 (tr) have also been calculated for this system by using the formula: is the apparent molar volume at infinite dilution in the mixed solvent and Ф V 0 (W) is the apparent molar volume, at infinite dilution, in pure solvent (in this case water).Ф V 0 (W) data have been taken from the literature 10 .The ∆Ф V 0 (tr) values are summarized in Table 4.The interactions have also been discussed on the basis of this property at the end.

Results and Discussion
The apparent molar volumes, Ф V , were then calculated using d and d 0 values for each electrolyte and for each concentration by using the equation: Here M is the molecular weight and c is the molar concentration of the electrolyte Then Ф V vs. √c curves were drawn for all the four electrolytes in each four solvent composition.These curves are shown in Figure 1 to Figure 4.The curves were found to be all straight lines so the Masson's equation is applicable for all the salts for entire range of concentration selected.Фv,

Study of the Influence of Alkyl Chain Cation-Solvent Interactions 1326
The experimental slope Sv as given in the Masson's Eq.1 was also calculated for each curve.If Ф V increases with increases in concentration, the slope comes out to be positive while the slope assumes the negative value if Ф V decreases on increasing the solute concentration.The Sv-data with their sign (+ve or -ve) have been summarized along with the solvent composition in Table 3. From the Figure 1, the trend of Ф V indicates that the apparent molar volume increases with increase in electrolyte concentration for all the four electrolytes, eg, Et 4 NI, Pr 4 NI, Bu 4 NI and Pen 4 NI in the 20% solvent mixture.Hence Sv is positive for each of them.2).
If the water content is further increased to 60 or even 80%, the Ф V values of two higher tetraalkylammonium salts Bu 4 NI and Pen 4 NI show a decreasing trend.Hence they give negative slope while the two lower tetra alkyl ammonium salts Et 4 NI and Pr 4 NI give positive slope (Figure 3 & 4).The overall picture is very much clear if one observes the data of Table 3 carefully.If this Table is examined critically, it is evident that the slopes go on decreasing, row-wise from Et 4 NI to Pen 4 NI and also down the column from 20 to 80% water in 1,3butanediol mixture.By increasing the higher percentage of water content in the mixtures the slopes even change sign from positive to negative in case of Bu 4 NI and Pen 4 NI.This type of behaviour shown by the slope, Sv, may be explained as below The higher tetraakylammonium ions enforce the structure of water more than the lower tetraakylammonium ions i.e. the relative power of enforcement of water structure in cationwater hydrophobic ion-solvent interactions is Et 4 N + < Pr 4 NI + < Bu 4 N + < Pen 4 N + Thus Pen 4 NI salt has greater influence on water molecules and elongates the water structure more than the Bu 4 NI salt does.Similarly Bu 4 NI salt has greater influence on water molecules and elongates the water structure more than the Pr 4 NI salt does.But the effect of Bu 4 NI salt is less than that of Pen 4 NI on the water solvent molecules and so on.The effect of enforcement of water molecule structure is least in case of Et 4 NI salt.The more stretching of the water molecules means the more cavities or void spaces formed in the solvent molecules.
In 20% water, since the water content is very small, only a fewer cavities are available which are filled up instantly as the salt concentration is increased and the addition of salt increases the volume hence Ф V increases with increase in concentration of salt resulting the positive Sv values in all the four electrolytes.As the water content further increased from 20 to 40% in the solvent mixture, hence more water molecules are available.In such a mixture, three salts Et 4 NI, Pr 4 NI and Bu 4 NI are not able to enforce the water structure much due to lesser amount of water present and so the increase of electrolyte concentration increases the Ф V values and so a positive slope is obtained in these cases also.The fourth salt Pen 4 NI has greater influence, owing to its largest size, on the structure of water molecule in cation-water hydrophobic ion-solvent interactions.So comparatively water molecules form a network of structure consisting of more void spaces.Thus the addition of Pen 4 NI salt in 40% water solvent mixture does not contribute towards the volume.The molecules of Pen 4 NI, because of large number of cavities inside the water structure network, hide themselves in the cavities and thus there is no contribution of it toward the volume.So the Ф V decreases on the addition of salt.Hence the negative Sv value is obtained in case of Pen 4 NI in 40% water mixture.
In 60 and 80% water rich mixtures, the plenty of water molecules are available.The higher tetraalkylammonium ions, namely Bu 4 NI and Pen 4 NI exert the greater influence on the water molecules and hence the structure of water molecules is enforced to the extent that the addition of salt does not fill up all the void spaces created inside the water structure network in dilute solutions.Thus the increase in concentration in case of these two salts does not increase the volume and hence Ф V decreases with increase in the electrolyte concentration in case of Bu 4 NI and Pen 4 NI salts.Therefore Sv values are found to be negative in these cases.The effect is much more stronger in case of Pen 4 NI than Bu 4 NI.So the value of Sv is more negative in the former case than in the later case.The values of Sv down the columns go on decreasing, it is because of little enhancement in the values of dielectric constant which is responsible in weaker ion-ion interactions (Low dielectric constant of the medium favours strong ion-ion interaction while the high dielectric constant favours weak ion-ion interaction).

Conclusion
The Ф V 0 values (Table 4) are +ve and large for the tetraalkylammonium salts in all the composition of aqueous solution of 1,3-butanediol, showing, thereby, the presence of weak ion-solvent interactions.Increase in Ф V 0 and ∆Ф V 0 (tr) values with decreasing concentration of 1,3-butanediol (or increasing concentration of water) may be attributed to increase in electrostriction in presence of water in the solvent mixture.Thus the electrostriction effect, which brings about the shrinkage in the volume of the solvent is increased in pure water as compared with that in the mixed solvent.Since electrostriction primarily reflects electrolyte-solvent interaction, it can be inferred that electrolyte -solvent interaction increases (while ion-ion interaction decreases) with decrease in 1,3-butanediol content (or increase in water content) in the solvent mixture.From the above discussion, it is evident that the Frank's hypothesis of enforcement of water structure by large tetraalkylammonium salts not only is applicable in pure water solutions but seems to be applicable in non-aqueous -water mixture system also.

Table 3 .
Sv-values of tetraalkylammonium iodide salt solutions in different compositions of 1,3-butanediol-water mixtures As the water content is increased to 40% in 1,3-butanediol the apparent molar volume decreases with increasing electrolyte concentration in case of Pen 4 NI, therefore the slope becomes negative in this case of Pen 4 NI while the other salts Et 4 NI, Pr 4 NI and Bu 4 NI, still give positive value of slope (Figure