Apparent molal volumes of alcohols in aqueous solutions at different temperatures

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1 Indian Journal of Cheistry Vol. 3A, February pp Apparent olal volues of alcohols in aqueous solutions at different teperatures M V Kaulgud". S S Dhondge' & A G Mohari!" Departent of Chcsity, Nagpur University Capus, Nagpur 00\0 Received 9 Septeber 199; accepted 6 Septeber 199 Apparent olal volues. ($ v ) of ethyl. ethyl. n-propyl, isopropyl. n-butyl. isobutyl, see-butyl and lcrt-hutyl alcohols in aqueous solutions are reported at,, 6, 8, and looe for the first tie. The trends of clv against olality () curves have been interpreted in ters of effect of dissolved solute (alcohol) on structure of water. It has been concluded that lower alcohols even at lower teperatures get dissolved in the natural cavities available in the water structure whereas higher alcohols have difficulty in finding suitable cavities in water structure and hence tend to indicate phase separation. The results have also been interpreted in ters of predoinance of solute-solute interactions at lower teperatures. Voluetric properties of aqueous solutions of alcohols have been extensively studied t-. It is generally observed that there is a loss in volue of alcohols on dissolution in water i.e. the liiting partial olal volue ($~) is saller than the volue of the pure liquid solute at the sae teperature. ley5 explained such results by expressing the view that water contains ice like cavities ready for sall nonpolar solute olecules to enter. This approach is supported by copressibility studies by Kaulgud and Rao at different teperatures" (5, 5 and 5 C). However very little inforation is available regarding the therodynaic properties of alcohols between 0 and woe. It has always been interesting to study the behaviour of dissolved solute in aqueous solution around the teperature of axiu density of water (TMD, C) as changes in the extent of hydrogen bonding between water olecules caused by the dissolved solute olecules are bound to be ore predoinant at TMD. With this view we have undertaken the easurent of densities of aqueous solutions of alcohols, naely, MeOH, toh, n-proh, i-proh, n-buoh, i-buoh, ~BuOH and t-buoh below TMD i.e., at, and above TMD, 6, 8 and 10, to obtain apparent olal volues (+vl as a function of concentration and other derived paraeters. Present address 'Departent of Cheistry, S.K.P. College, Karnptee, Nagpur. "Departent of Cheistry. Dharapeth Science College,. Nagpur. Materials and Methods The alcohols i.e. ethyl, ethyl, n-propyl, isopropyl, n-butyl, s-butyl iso-butyl, and z-butyl (all A.R. grade) were further purified by standard ethod? The densities agreed well with literature to better than ± g/rnl for all the alcohols". The density easureents for aqueous solution of alcohols were carried out by using the single float ethod? and Sartorius.odel 7 single pan/sei-icrobalance having facility to weigh below the pan. Doubly distilled water was used throughout the experiental work. In this ethod a weighed quantity (- 170 g) of doubly distilled water was taken in the stainless steel cylinder (capacity 500 rnl). The glass float (plunger) weighted with ercury (volue - 3 l) was suspended fro the pan of the balance by a thin nylon thread into the distilled water taken in the cylinder. The cylinder was kept in a constant teperature bath. The teperature of bath was kept constant to ± C by circulating the coolant liquid fro MK-70 cryostat (accuracy ± 0.0 C). To begin with, the required aount of alcohol was added in the water in the stainless steel cylinder. The solution was stirred with the glass stirrer, and allowed to attain constant teperature. The constant weight of the plunger was noted in this solution. In this way by adding the solute successively to water, 8-10 readings were taken. The density easureents of each alcohol in aqueous solution were ade in -3 different runs. Consid-' ering the accuracy of teperature (± C), weight ( ± 0.01 g) and the volue of the float ( -

2 KAULGUD et al.: APPARNT MOLAL VOLUMS OF AQ. ALCOHOL SOLUTIONS AT VARIOUS TMPRATURS ~ 37 $ 36., ō 69 <; 68 o Fig. i-plots of apparent olal volue (~J against olality () of aqueous ethyl alcohol. 1, OC;, C; 3, 6 C;, 8 C and 5, looc 67 0~----::OJ.., '--:0:-l: 8:---:-';' :;----;-" ';;-6 - -'.;---;-;-; -ts Fig. 3-Plots of apparent olal volue (~J against olality () of aqueous n-propyl alcohol. I, C;, C; 3. 6 C;. WC ''0 '$. :;::: o J Fig. -Plots of apparent olal volue (+.) against olality () of aqueous ethyl alcohol. l, C;, C; 3, 6 C;, goc and 5, urc 3 rnl) the accuracy in the density easureents was of the order of ± 10 pp. The volue of the plunger was deterined at different teperatures first by weighing it in air and then in doubly distilled water. It was calibrated by easuring the densities of aqueous solutions of sodiu chloride at 5 C. It was observed that our density values are in good agreeent with the literature. 0' 0'8 ', Fig. -Plots of apparent olal volue (+v) against olality () of aqueous isopropyl alcohol. 1, OC;, C; 3, 6 C;. s-c Calculations of derived paraeters The apparent olal volue (~J of the solutes have been calculated using the following equation (do - d) M +v= dd, +-;;... (1) where M is the olecular weight of the solute and is its olality. do and d are the densities of solvent and solution respectively.

3 108 INDIAN J CHM. Se. A, FBRUARY '-0., (5 -Q 83 "& 8 0 0' 0' Fig. 5-Plots of apparent olal volue ($v! against olality () of aqueous n-butyl alcohol. 1, C;, C; 3, 6 C;, 8 C \" <5 1' 0 0' Fig. 7-Plots of apparent olal volue ($v! against olality () of aqueous see-butyl alcohol. 1, C;, C; 3, 6 C;, 8 C 87 0' 0' 0'6 0' ' Fig. 6-Plots of apparent olal volue ($v! against olality () of aqueous isobutyl alcohol. 1, C;, C; 3, 6 C;, 8 C 1 ō " For nonelectrolytes if ~ v against is plotted, one gets the liiting apparent olal volues (~e). Although we plotted ~ v against, we do not intend to deterine ~ ~ as the ~ vs has been extrapolated fro higher concentration. ~~ obtained thus are not considered to be reliable and hence are at interpreted in this work. Results and Discussion Figure (1-8) show the variation of ~ v as a function of concentration () of all the alcohols at all teperatures studied in this work. It is observed that: (i) The apparent olal volue (~J goes through a iniu in the case of MeOH, toh, n-proh and (-BuOH. (ii) ~v goes initially through a axiu in the case of i-proh, n-buoh, i-buoh and s-buoh 81~---L--~~--~3~---~---=5 Fig. 8-Plots of apparent olal volue ($v! against olality () of aqueous tert-butyl alcohol. 1, C;, C;' 3, 6 C;, 8 C and show distinct trends of iniu at higher concentrations. The case of n-proh is interesting in that trends of positive d~jd at low teperature ( C) are discernible but not pronounced. It is known that all the above alcohols exhibit negative liiting slopes (d~/ d)_o at StC. In view of this the findings of the present work for slopes between 0-1O e o ust be tered as reark-

4 KAULGUD et al.: APPARNT MOLAL VOLUMS OF AQ. ALCOHOL SOLUTIONS AT VARIOUS TMPRATURS 109 Table l=-the values of (d kl dt)'c and b for all the alcohols Copound (dkldt) (" bx 10-3 b»: 10-3 (q.3) (Lit.) MeOH toh n-proh i-proh n-buoh i-buoh s-buoh I-BuOH a-wada & Ueda, ref. 6 b-kaulgud ref " " ", -.95 b ", -6.91" No data ' ', -7.0 b ", b ", able. The above behaviour of alcohols at lower teperature around teperature of axiu density (TMD) can be rationalized as follows. The lower alcohols like, MeOH, toh, and n- PrOH are accoodated in the cavities of water olecules ultiately showing loss of volue on dissolution even at low teperatures; I-BuOH being a syetrical olecule also shows loss in volue. But branch chained alcohols like i-proh, n- BuOH and s-buoh show positive d~/ d. This indicates the difficulty of these olecules in getting accoodated in the cavities presuably on account of non-availability of cavities of suitable size at low teperatures. Thus at low teperature these olecules of higher alcohols prefer to stay together indicating stronger solute-solute interactions. However, in concentrated region voids are ade in the water structure due to gradual destruction of water structure wherein alcohol olecules are accoodated showing negative d~/ d. The above interpretation is supported by true liiting copressibility (~~) values reporeted" at 5 C. It has been observed that lower alcohols show either positive or sall negative values, whereas higher alcohols show large negative values. However, (-BuOH though large being syetrical shows a sall negative value. Franks and Sith II have shown that all butanols have negative for ~ v with olality < 0.1 at OSC and 5 C. The present positive slopes of ~ v curves for higher alcohols suggest a possible iniu below our lowest concentration (0.1 ) in ~v curves. Overall ~v against curves then thus reseble iscelle foring systes. The peculiar behaviour of these alcohols at low teperatures ight be due to the effect of dissolved solutes on the structure of water around its TMD also. It has been shown by Wada and Ueda that the structural effects of dissolved solue on the TMD of water can be quantitatively studied I~. The observed change in TMD (68) can be written as su of two ters ~8id and ~8str ~8id represents the decrease in TMD when the solution behaves ideally i.e. ~ Vx M= 0 and ~8str. is the contribution to change in TMD when 6 Vx M ~ O. By knowing the olar volue (01 ) and coefficient of expansion (a ) of pure liquids it is possible to evaluate ~8str' It has been shown by Wada and Ueda that ~8str can be expressed in the for of a quadratic equation with respect to solute ole fraction (x ) as:... () where a and b are constants. Kaulgud 1.1 further proved that the values of a and b can also be calculated fro voluetric easureents at different \ teperatures. Thus dldt( P" ) 0 a= ". and (1- X) a I V: - dkldt b;: ( 1 - X ) a I V:'... (3) where Po is the liiting excess volue at C and dkldt is the teperature coefficient of slopes of ~v against ole fraction (d~/ dx )! in dilute solutions. 0l* and a 1 are the olar volue and coefficient of expansion of pure water at 3.98 C. Positive and negative a values show respectively the structure strengthening and structure breaking ability of the dissolved solute. Kaulgud showed that positive values of b reflect solute-solute interactions predoinant at higher teperature and negative b values reflect solute-solute interactions predoinant at lower teperature. In Table 1 are collected the (dkldt)o and b values coputed fro our voluetric data at different teperature around C. It is observed that our values are in good agreeent with literature, except for i-buoh. In the case of i-buoh the sign is the sae but the agnitude of our value is quite high. The concentration range studied by us and reported in the literature overlaps in all the cases. Thus fro the above b values it is concluded that in the case of MeOH, toh, n-proh, n-buoh, i- BuOH, s-buoh and (-BuOH, solute-solute interactions are ore predoinant at lower teperature. However, the agnitude sees to be higher in the case of higher alcohols. i-proh ust be having solute-solute interactions ore predoinant at higher teperature. Higher negative b values for higher alcohols also indicate phase separation at lower teperature which is supported by positive liiting dcp vi d.

5 110 INDIAN J CHM. Se. A, FBRUARY 1995 Kozac et al" coputed B coefficients fro q. (). InYl=Bxi+Cx~... () where YI is the solvent activity coefficient, B and C are the pairwise and triplet coefficients and x is the ole fraction of the solute. It has been observed that B-cofficient goes on increasing as chain length increases. However, for MeOH and [- BuOH it is negative suggesting again loss in volue on dissolution in water. The increase in the B-cofficient values as chain length increases also suggest the increase in extent of hydrophobic hydration. It has been suggested that in the higher concentration range higher alcoholds for alcohol iscelles. Acknowledgeent Thanks are due to Dr. KJ. Patil for helpful discussion. References 1 Friedan & Scheraga H A, J phys Chern, 69 (1965) Alexander D M, J che eng Daia, (1959) 5. 3 Franks F & Sith H T, Trans Faraday Soc, 6 (1968) 96. Nakanishi N, Bull che Soc Japan, 33 (1960) ley D D, Trans FaradaySoc, 35 (1939) Kaulgud M V & Rao K S M, J Chern Soc Faraday Trans I, 75 (1979) Weissberger A & Proskaur S, Techniques of organic chernistry(interscience, New York, 1950) Vol VII. 8 Riddick J A & Bunger W B, (Wiley Interscience, New York 1970). 9 Kaulgud M V, Bhagade V S & Shrivastava A, J che Soc Faraday Trans 1,78 (198) Jolicour C & Lacrox G, Can J Chern, 5 (1976) Franks F & Sith H T,J che eng Data, 13 (1968) Wada G & Ueda S, Bull che Soc Japan, 35 (196) 66, Kaulgud M V, J che Soc FartldJJy Ths, 86 (1990) Kozak J J, Knight W S & Kauzann W J, Chern Phy, 8 (1968)675.