- Email: [email protected]
The orientation of formamide at a mercury surface
Eleetroanalytwal Chemtstry and Interracial Electrochemistry, 53 (1974) 479-480 © Elsevier Sequoia S.A., Lausanne - Pnnted m The Netherlands
479
PRELIMINARY NOTE
T h e o r i e n t a t i o n o f f o r m a m i d e at a m e r c u r y s u r f a c e
ROGER PARSONS Department of Physical Chemistry, The Untversity, Bristol BS8 1 TS (England)
(Recewed 10th May 1974)
The orientation of solvent molecules at an electrode surface Is of considerable interest in the study of the electrode-solution interface. One of the most direct methods for studying this is that of calculating the temperature coefficient of the dipole potential from measurement of the temperature coefficient of the potential of zero charge (p.z.c.). This was first carried out by Randles and Whiteley I for the mercury-water interface with the conclusion that the oxygen atom of the water molecule was preferentially oriented towards the mercury at the p.z.c. A similar determination for methanol was made by Garnish and Parsons 2 and later by Koczorowskl and Figaszewski 3 . The sign of the temperature coefficient was found to be the same as that for water (although the values &ffer) indicating that m methanol also the negative end of the molecule is preferentially oriented toward the mercury electrode. Formamide provides a particularly interesting case because the large solvent hump occurring at a substantial negative charge (a = - 8 pC cm-2) suggests an even stronger orientation of the formamlde molecule in this direction. It is the purpose of this note to calculate the temperature coefficient of the dipole potential of formamide at the p.z.c. Measurements of the temperature dependence of the potential difference across the cell Cu I Agl AgCl(sat) I 0.1 mol 1-1 KC1 [ Hg(o=0) I Cu m formamlde
(I)
were made by Nancollas et al. 4 with the results E = - 0 . 4 7 7 V at 5°C, E = -0.467V at 25°C and E = -0.442V at 45°C. These yield a temperature coefficient d E / d T " + 0.85 mV K- ~. By an analysis following that of Randles and Whiteley it can be shown that the temperature coefficient of the interfacxal potential at the p.z.c, is F d A~b(a = 0)/dT
=
--SAgCI + SAg
+ SC1- -
R lnaCl- - R T d In aCl-/dT + F dE/dT
(1) where the symbols have their usual significance although it must be noted that Randles and Whiteley wrote cell (I) in the reverse direction so that the last term m eqn. (1) has the
480
PRELIMINARY NOTE
o p p o s i t e sign F r o m L a t i m e r s S -,,tgt_ ~,l = "~ ~ - 97 cal* ~K- ~ tool- 1 ' SAg = 41 32 cal K - L m o l S"Cl- in f o r m a m l d e was e s m n a t e d b y Crlss e t al 6 , , to be 10 1 cal K- 1 m o V ~ T h e activity c o e f f i c m n t o f the chloride ion was a s s u m e d to be equal to the m e a n a c n v l t y c o e f f i c i e n t o f NaC1 at 0.1 mol 1-1 m f o r m a m l d e 8 T h e t e m p e r a t u r e c o e f f l m e n t o f the a c n w t y c o e f f i c i e n t was assumed to be small so t h a t the p e n u l m n a t e t e r m in eqn. ( l j was neglected. Hence d A ~ ( o = 0 ) / d T was f o u n d to be + 2 3 m V K- 1 This p o s m v e value for the t e m p e r a t u r e c o e f f i c i e n t indicates t h a t f o r m a m l d e is also o r i e n t e d w i t h the negative e n d o f the & p o l e t o w a r d s t h e m e r c u r y surface at the p z c Its large m a g m t u d e suggests t h a t the & p o l e p o t e n t i a l is larger t h a n t h a t in w a t e r or m e t h a n o l . B o t h o f these facts me c o n s i s t e n t w i t h the presence o f a solvent h u m p at a negative charge o f - 8 / I C c m - ' - T h u s the m t e r p r e t a u o n o f this h u m p as a result o f solvent r e - o r i e n t a t i o n 4"9 is c o n f i r m e d REFERENCES 1 2 3 4 5 6 7
I E B Randles and K Whlteley, Tmns Farada)' Soc , 52 11956) 1509 J D Garnish and R Parsons, Tmn* Faraday Soc. 63 (1967) 1754 Z Koczorowskl and Z. F~gaszewskl, Rocz Chcm, 44 (1970) 191 G tt Nancollas, D S Rind and C A. Vincent, J Play's Chem, 70 (1966) 3300 W L Latimer, Oxtdatton Potentials PrenUce-ttall, New York, 1952 C M Cnss, R.P Held and E. Luksha, J Plo's Chem. 72 (1968) 72 C M Crass and M Salomon m A K Covington and T Dackenson (Lds), Pll~ ~tcal Chemtstt 3, o f Organtc Soh,ent S.vstenls, Plenum Press, London and Nex~ York, 1973 8 Yu M Povarov, Yu M Kessler and A I Gorbanev, ElektrOkhmuya, i (1965) 1174 9 E. DutMcWlCZ and R Parsons, J Electtoanal Chem 11 (1966) 196
*1 cal = 4 184 J
479
PRELIMINARY NOTE
T h e o r i e n t a t i o n o f f o r m a m i d e at a m e r c u r y s u r f a c e
ROGER PARSONS Department of Physical Chemistry, The Untversity, Bristol BS8 1 TS (England)
(Recewed 10th May 1974)
The orientation of solvent molecules at an electrode surface Is of considerable interest in the study of the electrode-solution interface. One of the most direct methods for studying this is that of calculating the temperature coefficient of the dipole potential from measurement of the temperature coefficient of the potential of zero charge (p.z.c.). This was first carried out by Randles and Whiteley I for the mercury-water interface with the conclusion that the oxygen atom of the water molecule was preferentially oriented towards the mercury at the p.z.c. A similar determination for methanol was made by Garnish and Parsons 2 and later by Koczorowskl and Figaszewski 3 . The sign of the temperature coefficient was found to be the same as that for water (although the values &ffer) indicating that m methanol also the negative end of the molecule is preferentially oriented toward the mercury electrode. Formamide provides a particularly interesting case because the large solvent hump occurring at a substantial negative charge (a = - 8 pC cm-2) suggests an even stronger orientation of the formamlde molecule in this direction. It is the purpose of this note to calculate the temperature coefficient of the dipole potential of formamide at the p.z.c. Measurements of the temperature dependence of the potential difference across the cell Cu I Agl AgCl(sat) I 0.1 mol 1-1 KC1 [ Hg(o=0) I Cu m formamlde
(I)
were made by Nancollas et al. 4 with the results E = - 0 . 4 7 7 V at 5°C, E = -0.467V at 25°C and E = -0.442V at 45°C. These yield a temperature coefficient d E / d T " + 0.85 mV K- ~. By an analysis following that of Randles and Whiteley it can be shown that the temperature coefficient of the interfacxal potential at the p.z.c, is F d A~b(a = 0)/dT
=
--SAgCI + SAg
+ SC1- -
R lnaCl- - R T d In aCl-/dT + F dE/dT
(1) where the symbols have their usual significance although it must be noted that Randles and Whiteley wrote cell (I) in the reverse direction so that the last term m eqn. (1) has the
480
PRELIMINARY NOTE
o p p o s i t e sign F r o m L a t i m e r s S -,,tgt_ ~,l = "~ ~ - 97 cal* ~K- ~ tool- 1 ' SAg = 41 32 cal K - L m o l S"Cl- in f o r m a m l d e was e s m n a t e d b y Crlss e t al 6 , , to be 10 1 cal K- 1 m o V ~ T h e activity c o e f f i c m n t o f the chloride ion was a s s u m e d to be equal to the m e a n a c n v l t y c o e f f i c i e n t o f NaC1 at 0.1 mol 1-1 m f o r m a m l d e 8 T h e t e m p e r a t u r e c o e f f l m e n t o f the a c n w t y c o e f f i c i e n t was assumed to be small so t h a t the p e n u l m n a t e t e r m in eqn. ( l j was neglected. Hence d A ~ ( o = 0 ) / d T was f o u n d to be + 2 3 m V K- 1 This p o s m v e value for the t e m p e r a t u r e c o e f f i c i e n t indicates t h a t f o r m a m l d e is also o r i e n t e d w i t h the negative e n d o f the & p o l e t o w a r d s t h e m e r c u r y surface at the p z c Its large m a g m t u d e suggests t h a t the & p o l e p o t e n t i a l is larger t h a n t h a t in w a t e r or m e t h a n o l . B o t h o f these facts me c o n s i s t e n t w i t h the presence o f a solvent h u m p at a negative charge o f - 8 / I C c m - ' - T h u s the m t e r p r e t a u o n o f this h u m p as a result o f solvent r e - o r i e n t a t i o n 4"9 is c o n f i r m e d REFERENCES 1 2 3 4 5 6 7
I E B Randles and K Whlteley, Tmns Farada)' Soc , 52 11956) 1509 J D Garnish and R Parsons, Tmn* Faraday Soc. 63 (1967) 1754 Z Koczorowskl and Z. F~gaszewskl, Rocz Chcm, 44 (1970) 191 G tt Nancollas, D S Rind and C A. Vincent, J Play's Chem, 70 (1966) 3300 W L Latimer, Oxtdatton Potentials PrenUce-ttall, New York, 1952 C M Cnss, R.P Held and E. Luksha, J Plo's Chem. 72 (1968) 72 C M Crass and M Salomon m A K Covington and T Dackenson (Lds), Pll~ ~tcal Chemtstt 3, o f Organtc Soh,ent S.vstenls, Plenum Press, London and Nex~ York, 1973 8 Yu M Povarov, Yu M Kessler and A I Gorbanev, ElektrOkhmuya, i (1965) 1174 9 E. DutMcWlCZ and R Parsons, J Electtoanal Chem 11 (1966) 196
*1 cal = 4 184 J