Thermodynamic functions of solutions of homologous dicarboxylic acids in water + acetone mixtures from surface tension measurements

Thermodynamic functions of solutions of homologous dicarboxylic acids in water + acetone mixtures from surface tension measurements

RIHDPHASE EOglUgll ELSEVIER nuia Phase Equilibria 134 (1997) 267-276 Thermodynamic functions of solutions of homologous dicarboxylic acids in water ...

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RIHDPHASE EOglUgll ELSEVIER

nuia Phase Equilibria 134 (1997) 267-276

Thermodynamic functions of solutions of homologous dicarboxylic acids in water + acetone mixtures from surface tension measurements Upendra Nath Dash, Braja Kishore M o h a n t y Department of Chemist~', Utkal Uni~'ersity, Bhubaneswar, Vani Vihar 751004, India Received 13 July 1996; accepted 25 January 1997

Abstract Surface tensions of solutions of oxalic, malonic, succinic, glutaric and adipic acids in water and water + acetone (Xace~o,e = 0.016, 0.033, 0.072, 0.117 and 0.171) mixtures at different temperatures for varying ranges of concentrations have been measured by means of the ring detachment method. The data were analyzed using Gibbs equations and various thermodynamic functions have been evaluated. The results reveal the nature of solute adsorption at the interface in these solvents. © 1997 Elsevier Science B.V. Keywords: Interfacial tension; Enthalpy; Gibbs energy; Homologous oxalic acid; Water + acetone

1. Introduction In this paper, we report work which is part of a wider study of the effect of increasing the size of the hydrophobic side chain in homologous fatty dicarboxylic acids on the free energy of surface-active molecules, both in aqueous solutions and in aqueous acetone solutions. A significant amount of work--both theoretical and experimental--has been reported on the surface tensions of aqueous inorganic electrolytes [1,2]. Some previous work on the effect of increasing the size of the hydrophobic side chain in a series of amino acids in aqueous solutions at 298.15 K has already been reported [3]. The present account is concerned with the effects which the homologous dicarboxylic acids, such as (CHz)n(COOH) 2, where n = 0, 1, 2, 3 or 4, have on the surface tension of water and on water + acetone mixtures in the absence of adsorbed surfactant at different temperatures, such as at 288.15, 298.15, 308.15 and 318.15 K.

2. Experimental details Oxalic acid (BDH, AnalaR, 99.5%) was dehydrated by heating it up to 100°C and keeping it in a vacuum desiccator for 18-20 h. Malonic acid (Merck, GR, 99.5%) was recrystallized three times 0378-3812/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 1 2 ( 9 7 ) 0 0 0 4 6 - 0

268

U. Nath Dash, B. Kishoru Mohantv / Fluid Phase Equilibria 134 (1997) 267-276

from benzene-ether that contained 5% of light petroleum (boiling point, 60-80°C); succinic acid (BDH, AnalaR, 99.5%) was used without further purification; and glutaric (Fluka AG, 99%) and adipic acid (SRL, 99% pure) were recrystallized three times from chloroform and acetone respectively. The purities of the acids were checked by titration against standard alkali. The purities of these acids were found to be approximately 99.5%. The acids were kept in a vacuum desiccator over anhydrous calcium chloride until required. The acetone (BDH, AnalaR containing 0.1% organic impurities and a water content of 1%) was purified [4] under reflux with successive quan!:ities of potassium permanganate until the violet colour persisted. It was then dried with anhydrous potassium carbonate filtered from the desiccant fractionated. The water content reduced by storage over a type 4A molecular sieve. Conductivity water (sp. cond. = 10 6 S cm 1 ) was used for prepariug the water + acetone mixtures of required compositions. The acetone content in the mixed solvents was accurate to within + 0.01%. The solutions were prepared on a molal basis, by dissolving known weights of the acids in appropriate weights of the relevant solvent, i.e. water or water + acetone mixtures. The solutions were then kept for 2 h in a water thermostat maintained at the required temperature (accurate to within _+0.05 K) before use for surface tension measurements. The surface tensions were measured by the ring detachment method using a du Nouy tensiometer (Win S-53, Winson Co, Calcutta) which was enclosed in air thermostat. The solutions were placed in a double-walled glass vessel through which water circulated from a water thermostat at the required temperature. Before each measurement, the platinum ring was cleaned by flaming. The planarity of the ring was checked by viewing its mirror image in the surface of the liquid. The calibration of the tensiometer was carried out using the surface tension values of pure water at the experimental temperature, as available in the literature [5]. At least five observations were made for each solution and the average was taken as the final surface tension value for the given solution.

3. Results and discussion The values of the surface tension y were measured for solutions of the five homologous dicarboxylic acids in water and water + acetone mixtures at 288.15, 298.15, 308.15 and 318.15 K. Typical plots that show the variation of y with the square root of the molality are shown in Fig. 1. It can be seen that the acids markedly lower the surface tension of water, indicating that the surface layers of the solutions are enriched in the corresponding acid solute. The lowering of surface tension is again noticed by increasing the concentration and temperature. This qualitative picture leads to the conclusion that the acids under study can be said to be positively adsorbed at the interface. The positive adsorption is further strengthened by increasing the acetone content in water. It can be observed that the surface tension decreases with increasing the size of the hydrophobic chain in the dicarboxylic acids in all the solvents at all temperatures investigated. The surface tension y of solutions of the dicarboxylic acids in water and water + acetone mixtures at different concentrations were fitted to an equation of the form [6] Y = To + Am2 + Bm3/2 + Cm

(1)

over the concentration range studied for the acid solutions, where To is the surface tension in pure solvent, and A, B and C are constants characteristic of the acids. The values of A, B and C are given in Table 1. Typical plots of ( Y - To)~ m vs. m I/2 are shown in Fig. 2.

269

U. Nath Dash, B. Kishore Mohanty / Fluid Phase Equilibria 134 (1997) 267-276

"° /

,.of S3'o r

o~

s2

46"5~ 38.5 ~0 37.5 j:

3S.5} 34.5[

o'.i

0.3

,(~

(m011/='kg-1/z)

Fig. 1. Variation of surface tension 3' with m ~/z at 298.15 K: (1) oxalic acid in water; (2) malonic acid in Xaceto.e = 0.016; (3) succinic acid in Xa~et.... = 0.033; (4) glutaric acid in X~ceto.e = 0.072: (5) adipic acid in Xac~to.~ = 0.117.

The stoichiomolalities m have been converted to mean ionic activities a+, using mean ionic activity coefficients 3' + as follows [5]:

a ~ = a( m) = ( Qo~my + ) ~

(2)

-O'4 l

'7"

-

03< "

";'E -0.2 Z

~. - 0-1 o

i o.1

0-2 ml/z (mo 11/2k9-l/z)

0-3

Fig. 2. Plot o f ( y - y o ) / m vs. m I/2 at 298.15 K: ( l ) oxalic acid in Xacetone 0.171; (2) malonic acid in Xacelone succinic acid in xacetone = 0.072; (4) glutaric acid in x~ceto, o = 0.033; (5) adipic acid in X.cetone = 0.016. =

=

0.117; (3)

U. Nath Dash, B. Kishore Mohanty / Fluid Phase Equilibria 134 (1997) 267-276

270 Table 1

The values of constants A, B and C of Eq. (1) for h o m o l o g o u s oxalic acids in water and w a t e r + 0.033, 0.072, 0.117 and 0.171) mixtures at different temperatures A

B

acetone

C

A

B

x ~ t o , ~ = 0.00, T = 298.15 K - 13.788(75) 6.365(51) - 15.720(84) 7.249(46) - 17.644(85) 8.130(59) -21.037(86) 9.680(61) -22.970(87) 10.560(62)

(Xaceton e =

C

Oxalic acid Maloinic acid Succinic acid Glutaric acid Adipic acid

Xacetone : 0 . 0 0 , - 14.367(78) - 16.301(86) - 18.234(88) -20.650(91) -23.066(93)

T = 288.15 K 6.630(57) 7.515(59) 8.398(60) 9.503(59) 10.608(58)

-0.832(6) -0.941(6) - 1.048(7) - 1.184(7) - 1.319(7)

Oxalic acid Malonic acid Succinic acid Glutaric acid Adipic acid

Xacetone = 0 . 0 0 , T = 3 0 8 . 1 5 K - 13.304(63) 6.144(51) - 15.720(71) 7.249(52) - 17.210(76) 7.925(54) - 19.633(75) 9.031(54) -21.570(83) 9.917(55)

-0.773(6) - 0.908(6) -0.990(7) - 1.126(7) - 1.234(8)

-

4.737(49) 5.401(53) 6.287(51) 6.951(52) 7.615(54)

-0.601(5) -0.681(6) - 0.789(6) -0.871(6) - 0.952(6)

Oxalic acid Malonic acid Succinic acid Glutaric acid Adipic acid

x ~ t o , ~ = 0.016, T = 288.15 K - 11.057(59) 5.113(49) -12.511(68) 5.778(51) - 14.449(76) 6.663(52) - 16.386(86) 7.548(54) - 18.809(87) 8.655(54)

-0.646(5) -0.727(6) 0.835(6) - 0.944(6) - 1.080(6)

x~o~to~~ = 0.016, T = 298.15 K - 10.573(69) 4.892(50) -12.026(72) 5.556(52) - 13.479(73) 6.220(53) - 14.933(73) 6.884(53) - 17.386(74) 7.549(54)

-0.619(6) -0.701(6) -0.781(7) - 0.863(7) - 0.944(7)

Oxalic acid Malonic acid Succinic acid Glutaric acid Adipic acid

x~c~to,,~ = 0.016, T = 308.15 K - 10.331(58) 4.782(42) - 11.783(59) 5.446(44) - 13.237(60) 6.110(46) - 14.691(72) 6.774(47) - 16.144(75) 7.438(47)

-0.605(6) -0.687(6) -0.768(7) -0.849(7) -0.931(7)

x~c~to,~ = 0.016, T = 318.15 K - 10.088(57) 4.671(41) - 11.542(58) 5.335(41) - 12.996(61) 5.999(42) - 15.449(70) 6.663(43) - 15.902(73) 7.327(48)

- 0.592(6) - 0.674(7) -0.755(7) -0.836(7) -0.917(8)

Oxalic acid Malonic acid Succinic acid Glutaric acid Adipic acid

Xacetone = 0.033, T = 288.15 K - 9.362(45) 4.339(39) - 10.816(53) 5.003(40) - 12.269(58) 5.668(41) - 13.722(64) 6.331(44) - 15.176(76) 6.995(45)

- 0.552(5) -0.663(6) -0.714(6) - 0.795(6) -0.876(7)

x~ceto,e = 0.033, T = 298.15 K - 9.119(44) 4.229(38) - 10.573(52) 4.892(39) - 12.026(57) 5.557(42) - 13.480(62) 6.221(36) - 14.933(73) 6.885(36)

- 0.538(5) -0.619(6) -0.701(6) - 0.782(6) -0.863(7)

Oxalic acid Malonic acid Succinic acid Glutaric acid Adipic acid

Xaceto,e = 0.003, T = 308.15 K -8.732(42) 4.051(38) - 10.185(53) 4.715(39) - 11.639(60) 5.379(42) - 13.092(62) 6.044(44) - 14.545(66) 6.704(45)

-0.516(5) -0.598(6) -0.679(7) - 0.760(7) -0.841(7)

X~etone = 0.033, T = 318.15 K -8.586(42) 3.985(38) - 10.040(44) 4.649(40) - 11.493(42) 5.313(41) - 12.947(50) 5.977(43) - 14.401(55) 6.641(44)

-0.508(5) -0.590(6) -0.671(6) - 0.752(7) -0.833(7)

Oxalic acid Malonic acid Succinic acid Glutaric acid Adipic acid

Xac~to,e = 0.072, T = 288.15 K -8.781(42) 4.073(38) - 10.234(54) 4.738(39) - 11.671(58) 5.397)41) - 13.141(62) 6.066(42) - 14.421(66) 6.677(43)

-0.519(6) - 0.600(7) - 0.681(7) - 0.763(7) -0.840(8)

xac~to,e = 0.072, T = 298.15 K -8.635(41) 4.007(39) - 10.089(43) 4.671 (41 ) - 11.542(55) 5.335(42) - 12.996(60) 5.999(44) - 14.449(65) 6.663(49)

-0.511(7) - 0.592(7) -0.673(7) -0.755(8) -0.836(8)

Xaceton e = 0 . 0 0 ,

10.234(61) 11.688(68) 13.625(71) 15.078(73) 16.532(75)

T = 318.15

0.016,

-0.800(6) -0.908(6) - 1.016(7) - 1.206(7) - 1.314(7)

K

U. Nath Dash, B. Kishore Mohanty / Fluid Phase Equilibria 134 (1997)267-276

271

Table 1 (continued) A

B

C

A

B

C

x,,c~ro,~ - 0 . 0 7 2 , T = 3 1 8 . 1 5 K

x~to, ~ = 0.072, T = 308.15 K Oxalic acid

- 8.490(44)

3.941(31 )

- 0.503(6)

- 8.296(44)

3.852(33)

Malonic acid

- 9.943(53)

4.605(33)

- 0.584(6)

- 9.749(55)

4.516(35)

- 0.492(5) - 0.573(6)

Succinic acid

- 11.397(57)

5.269(34)

-0.665(6)

- 11.202(56)

5.180(39)

-0.655(6)

Glutaric acid

- 12.850(67)

5.933(38)

-0.747(7)

- 12.656(67)

5.844(41)

-0.736(6)

Adipic acid

- 14.304(78)

6.597(38)

-0.828(7)

- 14.109(69)

6.508(44)

-0.817(6)

Oxalic acid

Xaccto,e = 0 . 1 1 7 , T = 2 8 8 . 1 5 K - 7.763(42) 3.609(30)

- 0.462(5)

X~eto, ~ = 0 . 1 1 7 , T = 2 9 8 . 1 5 K - 7.527(44) 3.410(30)

- 0.438(5)

Malonic acid

- 8.764(54)

4.063(32)

- 0.517(6)

- 8.295(55)

3.852(33)

Succinic acid

- 9.701 (65)

4.494(33)

- 0.571 (6)

- 9.264(65)

4.295(34)

- 0.546(6)

Glutaric acid

- 10.670(75)

4.937(35)

- 0.625(7)

- 10.234(70)

4.738(36)

- 0.600(7)

Adipic acid

- 11.369(77)

5.379(36)

- 0.679(7)

- 11.203(75)

5.180(37)

- 0.654(7)

Oxalic acid

X~,ceto,e = 0 . 1 1 7 , T = 3 0 8 . 1 5 K - 6.794(45) 3.166(29)

- 0.408(5)

Xaceton e = 0 . 1 1 7 , T -- 3 1 8 . 1 5 K - 6.261(46) 2.923(27)

Malonic acid

- 7.763(52)

3.609(31 )

- 0.462(6)

- 7.231(45)

Succinic acid

-8.732(57)

4.051(32)

-0.516(6)

Glutaric acid

-9.701(68)

4.494(32)

-0.571(7)

Adipic acid

- 10.669(72)

4.937(33)

Oxalic acid

Xaceto, e = 0 . 1 7 1 , T = 2 8 8 . 1 5 K - 7.036(42) 3.277(29)

- 0.492(6)

- 0.378(5)

3.366(28)

- 0.432(6)

- 8.199(45)

3.808(30)

-0.486(6)

- 9.168(67)

4.250(30)

-0.540(7)

-0.625(7)

- 10.137(70)

4.693(31)

-0.595(7)

- 0.422(5)

Xaceton ~ = 0 . 1 7 1 , T = 2 9 8 . 1 5 K - 6.503(41 ) 3.033(28)

- 0.392(5)

Malonic acid

- 8.005(53)

3.719(30)

- 0.475(5)

- 7.473(52)

3.476(29)

- 0.446(5)

Succinic acid

- 8.974(64)

4.162(33)

- 0.530(6)

- 8.441(54)

3.919(30)

- 0.500(6)

Glutaric acid

- 9.943(65)

4.605(33)

- 0.584(6)

- 9.410(65)

4.361 (30)

- 0.554(6)

Adipic acid

- 10.912(70)

5.048(34)

-0.638(6)

- 10.379(70)

4.804(31)

-0.609(6)

Oxalic acid

Xaceto, e = 0. l 7 l, T = 3 0 8 . 1 5 K - 6.115(41 ) 2.856(27)

- 0.370(5)

xaceto, e = 0 . 1 7 l, T = 3 1 8 . 1 5 K - 5.389(40) 2.524(26)

Malonic acid

- 7.084(53)

3.299(29)

- 0.424(5)

- 6.358(52)

- 0.330(5)

2.967(28)

- 0.384(5)

Succinic acid

- 8.053(63)

3.741 (30)

- 0.478(5)

- 7.327(62)

3.410(29)

- 0.438(6)

Glutaric acid

- 9.022(64)

4.184(31 )

- 0.533(6)

- 8.296(63)

3.852(30)

- 0.492(6)

Adipic acid

- 9.992(69)

4.627(32)

- 0.587(6)

- 9.265(65)

4.295(30)

- 0.546(6)

The f i g u r e s in p a r e n t h e s e s a r e t h e s t a n d a r d e r r o r in A. B a n d C to the order of l 0

3

where a(m) is the activity (on the molal scale) of the solute, and we have

Q = (v,,+v __ )l/v v

- -

3,+- y++3,5 We also have that v ( = v + + v ) is the total number of moles of ions (v+ is the number of moles of positive ions and v is the number of moles of negative ions) given by 1 tool of the solute, such as homologous dicarboxylic acids. The mean ionic molality m_+ can be defined by [5] (3)

m+=(v"+v " + _ )(o~m) ~

where m + = v + o ~ m , measurements.

m

= v

cem

and a is the degree of dissociation obtained from the conductance

272

U. Nath Dash, B. Kishore Mohanty / Fluid Phase Equilibria 134 (1997) 267-276

The mean ionic activity coefficient y ± was determined by means of the Debye-Huckel limiting law log y ± = -SIz+lz_lI '/2

(4)

where and z+ and z_ are the valencies of the ion, S is the Debye-Huckel constant (1.8246 × 106/(DT) 3/2) and I is the ionic strength of the solution given by 1

l = ~Emiz

2

i

(5)

Here, m~ is m+ or m_ and is obtained using Eq. (3). Using Eqs. (3)-(5) and taking Q = 41/3 for homologous dicarboxylic acids, the mean ionic activities a+ were determined from Eq. (2). Assuming that the Gibbs equation applies to the above homologous decarboxylic acids, the surface excess F 2 can be obtained from /"2 =



dy

2RT

da+

(6)

or

/"2 = 2.3026RT

(7)

dlog a±

where a+ is the bulk mean ionic activity of the acid as evaluated by means of Eq. (2), and dy/da+ and d y / ( d log a ±) are the slopes of the plots of y vs. a + and y vs. log a_+, as shown in Fig. 3 and Fig. 4 respectively. The values of F 2 (shown in Table 2) are positive for all the homologous dicarboxylic acids in all the solvents. As expected, the values of /'2 decrease with increasing

67"0I 66-0~ 5~.5 I 51-5

~ 46"5 ;E

"~ 4 5 - 5 "0 3 9 . 0 :

36"0

35.0 0

i

0.01

O-OZ

a!(mOI Kg1) Fig. 3. Plot o f y vs. a + at 298.15 K: (1) oxalic acid in xacet..... = 0.117; (2) malonic acid in x~cetone = 0.072; (3) succinic acid in X~cetone = 0.033; (4) glutaric acid in x~c~ton~= 0.016; (5) adipic acid in water.

U. Nath Dash, B. Kishore MohanO, / Fluid Phase Equilibria 134 (1997) 267-276

6-/-0 I 66"0~

273

5

5~'5 t 4

,~ %

46.5

"~0.0-45.5

3

~9.0 2

~B.O

36.o 35"o

i

-~'0

i

-2-5

i

-1"5

-?..o

Log
=

0.072; (3) s u c c i n i c

=

temperature and also decrease with increasing chain length of the dicarboxylic acids (see Figs. 5 and 6). It is evident from Figs. 1, 3 and 4 that the variation of y with molality or mean ionic activity is of

Table 2 V a l u e s of the t h e r m o d y n a m i c p a r a m e t e r s A H ° (kJ tool

i), A G O (kJ m o l

z) A S 0 (kJ mol

i K - 1) and F 2 for h o m o l o g o u s

o x a l i c acid s in w a t e r and w a t e r + a c e t o n e (Xaceton~ = 0.016, 0.033, 0.072, 0.117 and 0.171) m i x t u r e s at 298.15 K AH °

AG O

O x a l i c acid

- 28.6

M a l o n i c acid S u c c i n i c acid G l u t a r i c acid

A S ° X 10 z

F 2 X 10 8

AH °

AG O

A S ° X 10 2

F 2 X l0 s

Xaceto,e = 0.00 - 111.6 29

18.8

- 30.8

Xacetone = 0.016 - 120.7 30

18.6

-36.2 -50.2 -60.2

- 146.4 -206.8 -240.9

37 53 60

18.0 17.6 17.5

-39.1 -53.9 -65.7

- 158.4 -225.6 -270.3

40 57 69

17.9 17.2 17.1

A d i p i c acid

-70.1

-296.8

76

17.1

-75.9

-322.6

83

17.0

O x a l i c acid M a l o n i c acid

-39.1 -42.5

Xacetone = 0.033 - 154.8 39 - 172.4 43

18.4 17.8

-44.4 -49.1

Xacetone = 0.072 - 172.2 43 - 195.1 49

18.8 18.2

S u c c i n i c acid

-60.7

-256.3

66

17.4

-70.4

-291.6

74

17.4

G l u t a r i c acid A d i p i c acid

- 72.9 -88.0

- 307.2 -371.3

79 95

17.0 16.9

- 80.9 -98.4

- 341.5 -416.5

87 106

17.1 17.0

O x a l i c acid M a l o n i c acid S u c c i n i c acid G l u t a r i c acid

-52.1 - 56.8 -78.7 -89.6

Xacetone = 0.117 - 193.2 47 - 216.4 54 -297.8 73 -366.9 93

19.9 19.2 18.7 17.6

-59.7 - 68.5 90.6 - 101.9

Xacetonc = 0.171 -228.6 57 - 248.5 60 -350.5 89 -407.6 102

20.9 20.6 19.4 18.8

A d i p i c acid

- 107.4

-444.9

17.3

- 126.0

-520.6

18.5

113

132

U. Nath Dash, B. Kishore Mohamy / Fluid Phase Equilibria 134 (1997) 267 276

274

9-3 '22 21 ?.0

C =o 0

19 18

1"7 16 15

T(K)

Fig. 5. Variation of /"2 with T: (1) oxalic acid in x~.~t.... = 0.171; (2) malonic acid in x,c~to~~ = 0.117; (3) succinic acid in x~to~ ~ = 0.072; (4) glutaric acid in x~,,,~ = 0.033; (5) adipic acid in x~to° ~ = 0.016. i n v a r i a b l y s i m i l a r p a t t e r n at all t e m p e r a t u r e s . T h e r e f o r e , d y / d a + is i n d e p e n d e n t o f t e m p e r a t u r e f o r a g i v e n d i c a r b o x y l i c acid. K e e p i n g this s i m p l i f i c a t i o n in m i n d , w e c a n w r i t e dF 2 - -

ai

dy

-

dT

(8)

2RT 2 da+

C o n s i d e r i n g the m o d e l as a d o p t e d [2] to a p p l y to the p r e s e n t data, an a l t e r n a t i v e e x p r e s s i o n f o r the t e m p e r a t u r e c o e f f i c i e n t o f /'2 c a n be w r i t t e n in the f o r m d/' 2

dT

a+ -

RT 2 A H ° z

(9)

w h e r e z is the t h i c k n e s s o f the s u r f a c e p h a s e e x i s t i n g in e q u i l i b r i u m w i t h the b u l k p h a s e , a n d A H ° is

?.1 ~.0

19 18 17 ~6 15 NUMBER OF..CH~-GROUPS

Fig. 6. Variation of F 2 with number of - C H ~ - groups in water at (1) 288.15 K, (2) 298.15 K, (3) 308.15 K and (4) 318.15 K.

U. Nath Dash, B. Kishore Mohan~ / Fluid Phase Equilibria 134 (1997) 267-276

275

the change in standard enthalpy in transferring 1 mol of the dicarboxylic acid from the bulk to the surface region, neglecting temperature variation. Comparing Eq. (9) with Eq. (8), we obtain dy da+

- 2AH°z

(10)

which enables us to determine the values of A H °, provided that the value of z is known. However, in the absence of any definite knowledge about the z value in the present investigation, the calculation of A H ° presents some difficulties. Consequently, as a measure of good approximation, the value of A H ° was calculated for various values of z (z = 0.5-5 nm at intervals of 0.5 nm) from Eq. (10) at different temperatures; for z = 0.5 nm, a greater linearity of the plots of d y / d a + ) vs. A H ° was observed. The values of A H ° calculated for z = 0.5 nm are shown in Table 2, along with the values of AG o, i.e. the change in standard Gibbs free energy derived by means of

r2-

a+_ AG°z

(11)

The values of AS °, i.e. the change in standard entropy, were computed from the Gibbs-Helmholtz equation as A H o - AG O AS° T

(12)

and are also shown in Table 2. The negative values of AG o and A H ° (Table 2) indicate that the adsorption of the homologous dicarboxylic acids is stabilized and that the adsorption process is exothermic in nature. The increasingly negative values of AG o and A H ° from oxalic to adipic at all temperatures and in all solvents suggest that the stabilization increases with increasing hydrophobic chain length of the dicarboxylic acids. The positive AS ° values indicate that the adsorbed molecules tend to be disordered on the surface region; this 'disorderedness' increases as the chain length of the acids increases in all solvents and at all temperatures. This may be attributed to the fact that the adsorbed molecules become more disordered with increasing hydrophobic chain length.

4. Notation a !

A,B,C I m R S T Z Z+ Z_

mean ionic activity constants of Eq. (1) ionic strength of the solution stoichiomolality gas constant per mole Debye-Huckel constant temperature in kelvin thickness of the surface phase existing in equilibrium with the bulk phase valency of the positive ion valency of the negative ion

276

AH ° AG O AS o

U. Nath Dash, B. Kishore Mohanty / Fluid Phase Equilibria 134 (1997) 267-276

standard molal enthalpy change standard molal Gibbs free energy change standard molal entropy change

Greek letters a

Y T0 T+ P+ b'

degree of dissociation surface tension of the solution surface tension of the pure solvent mean ionic activity coefficient surface excess number of moles of positive ion given by 1 mol of the solute number of moles in negative ion given by 1 mol of the solute

Acknowledgements The authors are thankful to the UGC, New Delhi, for financial support under the DRS programme.

References [1] [2] [3] [4]

R. Aveyard and S.M. Saleem, J. Chem. Soc. Faraday Trans. I 71 (1975) 1609. M.J. Hey, D.W. Shield, J.M. Speight and M.C. Will, J. Chem. Soc. Faraday Trans I 77 (1981) 123. J.R. Pappenheimer, M.P. Lapie and J. Wyman, J. Am. Chem. Soc. 58 (1936) 1851. B.S. Furniss, A.J. Hannaford, V. Rogers, P.W.G. Smith and A.R. Tatchell, Textbook of Practical Organic Chemistry, Longman, London, 1984, p. 275. [5] R.A. Robinson and R.H. Stokes, Electrolyte Solutions, Butterworth, London, 1955, p 30. [6] U.N. Dash and N.N. Pasupalak, Indian J. Chem., in press.