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Medium effects in water—ethylene carbonate mixtures
Symposium
abstracts - solute-solute-solvent
x141
interactions
Electrochemistry
Medium Effects in Water-Ethylene tures
Carbonate Mix-
TABLE I. Hydration and Solvation Numbers in HzO-Ethylene Carbonate Solvent Mixtures. EC wt%
Gl,,,n+ Nl,El+
0.00 9.50 20.16 28.98 39.96 49.15 61.35 78.12 100.00
-32.03 -32.49 -35.49 -34.92 -38.49 -40.46 -43.34 -56.85 -
A. A. EL-HARAKANY, H. SADEK and A. M. ABDOU Science Centre for Advancement of Post Graduate Studies, Alexandria University, Alexandria, Egypt
Electromotive the type:
force measurements
of the cell of
Pt, Hz(l atm)/HCl (m), EC(x), Hz0 (100 -x)/
-32.03 -31.46 -30.46 -29.56
-28.19 -26.62 -24.15 -18.42 -
nh
As&+
5.4 5.2 5.1 4.9 4.1 4.4 4.0 3.1 -
-
n,
-1.90
-4.41 -6.93 -10.80 -15.21 -22.14 -38.24 -89.98
0.3 0.7 1.2 1.8 2.5 3.1 6.4 (15.0)
IAgCl,Ag in the temperature range 25 “C to 45 “C at 5 “C intervals have been used to derive the change in the standard free energy (AGa, enthalpy (ma, and entropy (AS,? of transfer of HCl from water to water-EC mixtures. Figure 1 shows that at 25 “C, AG: changes its sign from negative values at concentration of the organic solvent less than 40 wt.%, to positive values at higher percentages of EC. This behaviour is explained on the basis of the change in the internal structure of the mixed solvent. The electrostatic contribution of the Gibbs free energy of transfer is evaluated according to Born’s model. The results show that the electrostatic part of the free energy change plays an important role in the total value of AG: at concentration of EC < 27 wt.%. At higher percentages, the nonelectrostatic free energy change start to have an appreciable effect on AG:. Values of the standard entropy and enthalpy of transfer are found to be always negative and decrease with increasing EC content. This clarifies that the net effect is to increase the entropy loss, and suggests that, in the transfer process, the net amount of order created by HCl as a solute is more marked in the mixed solvent than in pure water. The entropy change of mixed hydration and solvation ASi,Sp+, is evaluated in each mixed solvent. It can be divided into a hydration component AS&r+, and a solvation component A,Y&+, according to the relation:
at
25°C P
AG; / 0
- 200 -
I)
AG; I
12Co at
45OC
P
lml-
m-
600
-
Loo
-
200
-
0
-2i?O
-0
-
L
AGil I 10
30
50
70 EC
%
-
Fig. 1. Variation of AG; and Acd with the wt.% of ethylene carbonate.
where, x2 and x1 are the mole fractions of water and EC respectively.
Table I illustrates the calculated values of the hydration and solvation numbers in each of water-
Posters
Xl42
EC solvent mixture The high value of the solvatlon number m 100 wt % EC may be due to the primary solvatlon layer together with some contrlbutlon from the secndary solvatlon layer
V(NHS)
reflects
the negative
hydration
of -NH;-
group References 1 U L Haldna and H J Kuura, Zh Phys Khzm, 41, 2787
An Estimation of the Volume of Restructured Water Shells around Hydrophobic Solutes
(1967) 2 U Haldna and L Oraste, Orgamc Reactwlty, 14, (1977) 3 M M Karelson, Orgamc Reactwrty, IS, 530 (1976)
345
U HALDNA and L ORASTE Department of Chemrstry, EstomanSSR, USSR
Tartu State
Unwerslty,
Tar&,
Structure of Ahphatlc Alcohols in Electrical Double Layer on Electrode Surface Water IS known to be a largely associated hquld The presence of non-electrolytes or large organic ions, tends to strengthen the hydrogen bonds between the water molecules near the large hydrophobic solute, and a cage or ‘Iceberg’ 1s effectively formed around them This model of the hydrophobic hydration of alkyl groups 1s widely accepted but the thickness of restructured water shells around hydrophobic solutes has not been determined In order to estimate the volume of restructured water per mole hydrophobic solute we used the dlfferenteal conductometry of carefully thermostatted (0 001 “C) solutions [ 1, 21 The experimental procedure consists of measuring the specific electrical conductance (x) of a strong electrolyte solution before and after adding a small amount of hydrophobic solute Provided the equivalent conductance of the strong electrolyte ions m the restructured water shell IS zero [3] the volume of the restructured water shell 1s given by [2,3] Ay103 vs., = --x.c
b3/moll
where Ax IS the change m x, corrected for the Increase m volume and c 1s the molar concentration of the strong electrolyte We have attempted to measure the V,,-values m aqueous solutions of perchlorlc acid for 16 alkylammonium ions The results obtamed can be summarized as follows V,., = n(CH3)*V(CH3)
+ n(CH2)*V(CH1)
+
n(NH;)*V(NH;) where n(l) IS the number of groups I m the ion studled and V(l) 1s the specific volume of hydration shell of l-group In l&20% HC104(w/w), V(CH3) = V(CH2) = 40 _+ 10 [cm3/n(l) mol] and V(NHi) = -48 f 10 [cm3/n(l) mol] The negative value for
U PALM and M VAARTNdU Laboratory of Electrochemistry, Tartu Tartu 202400, Estoman S S R , US S R
State
Uhwerslty,
The structure of solvent m the field of the electrical double layer exerts significant influence on the mner layer propertles on adsorption of Ions and molecules Some theoretical models consldermg dlfferently the mteractlon of solvent dipoles m the mner layer have recently been elaborated [l-3] On the basis of the experlmental data obtamed by us on blsmuth electrode m alcohohc solutions, mamly m ethanol, the analysis of the vahdlty of the cluster of the solvent models [l-3] for the descrlptlon structure m the mner layer has been carried out m this paper Cluster model was chosen smce alcohols belong to the group of associated liquids Accordmg to cluster model solvent exists on the electrode surface m the form of separate chemlsorbed molecules with constant orlentatlon and of associates of molecules (clusters) the orlentatlon of which depends upon electrode charge q and the composltlon of which varies with temperature T By use of the equations of the cluster model [l-3] the electrode charge has been calculated for several temperatures m the mterval from -15 to 50 “C The results of the theoretical calculations were compared with the experlmental data m the form of the plots of the mner layer integral capacity K,-,z against q at several T Experimental values of Koz were found from the dlfferentlal capacity of bismuth m alcohohc solutions of LlC104 at various temperatures On the basis of the calculations it was established, that at T < 2.5 “C the ethanol molecules exist m the mner layer mainly m the form of two-dimensional double associates due to the hydrogen bondmg
abstracts - solute-solute-solvent
x141
interactions
Electrochemistry
Medium Effects in Water-Ethylene tures
Carbonate Mix-
TABLE I. Hydration and Solvation Numbers in HzO-Ethylene Carbonate Solvent Mixtures. EC wt%
Gl,,,n+ Nl,El+
0.00 9.50 20.16 28.98 39.96 49.15 61.35 78.12 100.00
-32.03 -32.49 -35.49 -34.92 -38.49 -40.46 -43.34 -56.85 -
A. A. EL-HARAKANY, H. SADEK and A. M. ABDOU Science Centre for Advancement of Post Graduate Studies, Alexandria University, Alexandria, Egypt
Electromotive the type:
force measurements
of the cell of
Pt, Hz(l atm)/HCl (m), EC(x), Hz0 (100 -x)/
-32.03 -31.46 -30.46 -29.56
-28.19 -26.62 -24.15 -18.42 -
nh
As&+
5.4 5.2 5.1 4.9 4.1 4.4 4.0 3.1 -
-
n,
-1.90
-4.41 -6.93 -10.80 -15.21 -22.14 -38.24 -89.98
0.3 0.7 1.2 1.8 2.5 3.1 6.4 (15.0)
IAgCl,Ag in the temperature range 25 “C to 45 “C at 5 “C intervals have been used to derive the change in the standard free energy (AGa, enthalpy (ma, and entropy (AS,? of transfer of HCl from water to water-EC mixtures. Figure 1 shows that at 25 “C, AG: changes its sign from negative values at concentration of the organic solvent less than 40 wt.%, to positive values at higher percentages of EC. This behaviour is explained on the basis of the change in the internal structure of the mixed solvent. The electrostatic contribution of the Gibbs free energy of transfer is evaluated according to Born’s model. The results show that the electrostatic part of the free energy change plays an important role in the total value of AG: at concentration of EC < 27 wt.%. At higher percentages, the nonelectrostatic free energy change start to have an appreciable effect on AG:. Values of the standard entropy and enthalpy of transfer are found to be always negative and decrease with increasing EC content. This clarifies that the net effect is to increase the entropy loss, and suggests that, in the transfer process, the net amount of order created by HCl as a solute is more marked in the mixed solvent than in pure water. The entropy change of mixed hydration and solvation ASi,Sp+, is evaluated in each mixed solvent. It can be divided into a hydration component AS&r+, and a solvation component A,Y&+, according to the relation:
at
25°C P
AG; / 0
- 200 -
I)
AG; I
12Co at
45OC
P
lml-
m-
600
-
Loo
-
200
-
0
-2i?O
-0
-
L
AGil I 10
30
50
70 EC
%
-
Fig. 1. Variation of AG; and Acd with the wt.% of ethylene carbonate.
where, x2 and x1 are the mole fractions of water and EC respectively.
Table I illustrates the calculated values of the hydration and solvation numbers in each of water-
Posters
Xl42
EC solvent mixture The high value of the solvatlon number m 100 wt % EC may be due to the primary solvatlon layer together with some contrlbutlon from the secndary solvatlon layer
V(NHS)
reflects
the negative
hydration
of -NH;-
group References 1 U L Haldna and H J Kuura, Zh Phys Khzm, 41, 2787
An Estimation of the Volume of Restructured Water Shells around Hydrophobic Solutes
(1967) 2 U Haldna and L Oraste, Orgamc Reactwlty, 14, (1977) 3 M M Karelson, Orgamc Reactwrty, IS, 530 (1976)
345
U HALDNA and L ORASTE Department of Chemrstry, EstomanSSR, USSR
Tartu State
Unwerslty,
Tar&,
Structure of Ahphatlc Alcohols in Electrical Double Layer on Electrode Surface Water IS known to be a largely associated hquld The presence of non-electrolytes or large organic ions, tends to strengthen the hydrogen bonds between the water molecules near the large hydrophobic solute, and a cage or ‘Iceberg’ 1s effectively formed around them This model of the hydrophobic hydration of alkyl groups 1s widely accepted but the thickness of restructured water shells around hydrophobic solutes has not been determined In order to estimate the volume of restructured water per mole hydrophobic solute we used the dlfferenteal conductometry of carefully thermostatted (0 001 “C) solutions [ 1, 21 The experimental procedure consists of measuring the specific electrical conductance (x) of a strong electrolyte solution before and after adding a small amount of hydrophobic solute Provided the equivalent conductance of the strong electrolyte ions m the restructured water shell IS zero [3] the volume of the restructured water shell 1s given by [2,3] Ay103 vs., = --x.c
b3/moll
where Ax IS the change m x, corrected for the Increase m volume and c 1s the molar concentration of the strong electrolyte We have attempted to measure the V,,-values m aqueous solutions of perchlorlc acid for 16 alkylammonium ions The results obtamed can be summarized as follows V,., = n(CH3)*V(CH3)
+ n(CH2)*V(CH1)
+
n(NH;)*V(NH;) where n(l) IS the number of groups I m the ion studled and V(l) 1s the specific volume of hydration shell of l-group In l&20% HC104(w/w), V(CH3) = V(CH2) = 40 _+ 10 [cm3/n(l) mol] and V(NHi) = -48 f 10 [cm3/n(l) mol] The negative value for
U PALM and M VAARTNdU Laboratory of Electrochemistry, Tartu Tartu 202400, Estoman S S R , US S R
State
Uhwerslty,
The structure of solvent m the field of the electrical double layer exerts significant influence on the mner layer propertles on adsorption of Ions and molecules Some theoretical models consldermg dlfferently the mteractlon of solvent dipoles m the mner layer have recently been elaborated [l-3] On the basis of the experlmental data obtamed by us on blsmuth electrode m alcohohc solutions, mamly m ethanol, the analysis of the vahdlty of the cluster of the solvent models [l-3] for the descrlptlon structure m the mner layer has been carried out m this paper Cluster model was chosen smce alcohols belong to the group of associated liquids Accordmg to cluster model solvent exists on the electrode surface m the form of separate chemlsorbed molecules with constant orlentatlon and of associates of molecules (clusters) the orlentatlon of which depends upon electrode charge q and the composltlon of which varies with temperature T By use of the equations of the cluster model [l-3] the electrode charge has been calculated for several temperatures m the mterval from -15 to 50 “C The results of the theoretical calculations were compared with the experlmental data m the form of the plots of the mner layer integral capacity K,-,z against q at several T Experimental values of Koz were found from the dlfferentlal capacity of bismuth m alcohohc solutions of LlC104 at various temperatures On the basis of the calculations it was established, that at T < 2.5 “C the ethanol molecules exist m the mner layer mainly m the form of two-dimensional double associates due to the hydrogen bondmg