Indi an Journal of Chcmi stry Vo l. 45A, Dece illber 2006, pp. 2599-2606

Excess adsorption on hydrophobic and hydrophilic solid-liquid interfaces: Negative adsorption of electrolytes and urea. Part 2

Ebrahilll I-bider", D K Chattorat ", & K P Dash

"Departillent o r Food Tec hnology & Bioc he l11ic7t1 Enginec ring. J:1cbvpur Univers it y. Ko lkata 7000]2. India

"Dcpartlllcnt o r Chcllli stry. [Jose In stitut e. 92. /\ charya Prarui la Chandra Road. Kolka ta 700009. Ind ia

E- I11 ~ lil : dkchattoraj @hotlll ai l.co ill

Receil'ed 22 Sep/ell/ber 2005: rel'isei/ 28 SejJ/('/lIiJer 2006

The ex tent o r hyd rati on o r charcoa l. cati oni c ami ani oni c res in powders in the prese nce and abscnce or sO l11 e inorgan ic salt s and urca has been studi ed using isopi es ti c vapou l' pressure tec hnique. The shapes o r wate r vapour adsorpt ion isotherms 1'0 1' ditlerent so lid s agree wit h type III BET iso therill s. At water acti vity approac hing unit y. maxi mulll values 0 1' water adsorpti on !'lnlll in Illo les pCI' square meter or so lid at 30DC arc in the orde r: cati oni c I-es in > ani on ic res in > charcoal. Stand ard frce cnergies (!'leO). or adsorption o r wat er vapour ha ve :1 lso bee n evalu ated . The negati ve and pos iti ve excess hydrati on or these so lid powe rs ror dirfe rent 1110 le rract ions or Li C I. N:1CI. KCI. Na,SO.\ and urea in the bulk phase have becn es tilll :1 tcd using cbta obtained rro m the isopi esti c ex periments. The rree energies o r excess hy dr~ lti o n 1'0 1' all these systems ha ve been eva luated usin g the integrat ed rorms o r the Gibbs ad sorp tion equati ons. The abso lutc values or watel' and salt bindi ng to these so li ds have been es til11ateci in the bulk phase .

Studies on the adsorpti on of water and electrolytes on solid surfaces are of great importance to understand molecular interacti ons occurri ng in coll oidal sys tems, catalytic processes, adsorpti on chromatography and in many bios urface phenomena . Adsorpti on of water on the surface of solids has been extensively studi ed using van ous phys icochemi cal techniques l

-' .

Competitive adsorpti on of water and elect ro lytes on the surface of powdered so lids such as carbon, alumina and sili ca has been studied ex tensively using vari ous phys icochemical t echniques~ -~. However, concentrati ons of electro lytes are relatively low in most of the systems studied and the ro le of water in the preferential adsorpti on of so lute or so lvent has not been examined quantitatively on thermodynamic grounds. Chattoraj and co- workers have studi ed extensively preferential ad sorption of water vapour in the absence and prese nce of inorganic salts by

d I . I) I . . I 10 .. d pow eree protetns po yamtno aCle s , cattont C an . . t' 11 I d I . 1" '1 ' 11 d anionic sur actants powe ere a umlna -, Sl Ica - an

barium sulphate l] using isopi estic vapour pressure

method.

We report here the ex tent of adsorption of water vapour in the presence and absence of urea and inorganic salts on the surfaces of charcoal , cationic and ani oni c resin using isopi es tic vapour pressure technique. An attempt has been made to calculate the

standard free energies for adsorpti on of electrol ytes in these systems using integrated forms of the Gi bbs adsorpti on equ ati on, and to clea rly identify a suitable standard state of reference fo r the chan ges of free energies of negati ve adsorpti on of electro l ytes of the so lid-liquid interface so th at relati ve affiniti es of these solids for water ane! salts can be co mpared.

Materials and Methods Charcoal was purchased from Sigma Chemi cals

Co., USA . Amberlite res in (H+ exchan ge) IR 120 ( 16-45 mesh ) and aamberlite res in IR 402 CI - exchange (s tandard grade) were obtai ned from FI ub, Germany. Before use, charcoal was washed wi th di sti lied water for remo ving trace of impurities from so lid and then dri ed at 850°C for g h. The dried material s were then stored in a des iccator containing concentrated sulfuri c acid. Res in (CI- particles) was washed with di st ill ed water till the washing had a constant pH 7.0. Resin (H +) was washed several times with 2-4 M aCi so lution to make it a Na+ exchange res in . Both the res ins were air-dri ed and the kept in a vacuum desiccator containing cone. sulfuric acid to remove the last trace of water. After drying, these materi als were kept in a desiccator containing anhydrous CaCl 2.

The electrolytes LiCl , NaCI, KCI and a2SOcj (anal ytical grade) were llsed without furth er

2600 INDI AN J CI-IEM. SEC A . DECEMBER 2006

puri f icati on. Urea was recrys talli sed twice from warm ethanol. Double distill ed water was used throughout the inves ti gati ons.

The surface area of so lid powders of charcoal was determined using adsorpti on o f aceti c acid from aqueous solution. Surface area o f the res ins was determined from palmiti c acid adsorpti on in benzene. These method s arc discu ssed elsewherel-I. The surfaces of charcoal , res in (Na+) and resin (CI -) were

found to be 63.4 ± 2. 1, 55 .0 ± 0.5 and 29.0 ± 1.0 meter square per g o f the powder.

Isopicstic studies

Water vapour adsorpti on by powdered so lid materi als such as sili ca, alu minal2 and barium sulphatel.

1 has been quanti tati ve ly es timated using

isopi es ti c vapour pressure method using a definite weight o f the dried power in an accurately weighed sample bottl e. Lid o f the boltle was then rcmoved and the bottle w ithoul lid was fl oated on cone. sulphuric acid so lution (using suitably a tripod stand) in a spec iall y desi gned des iccator. The desiccator was then evacuated and the H 2S0~ solution inside was stirred by a magneti c stirrer for a week to attain vapour pressure equilibrium between hydrated powder present in the sample bottle and the H"SO~ so lution present in the des iccator (referred as reference so lution). The sample bottl e was th en taken out o f the des icca tor quickly (with lid on) and was weighed. From the diffe rence in weights, moles (II I) o f water vapour adsorbed per kg (or per unit area) of the powder were cal culated. The IH 2S0~ ] in the reference solution was determined titrimetricall y and the corresponding relati ve humidity (p ip .. ) at thi s [acid] was obtained using the standard tabl e. The water acti v ity (a l) at thi s state of isopi es tic equilibrium IS equal to pip .. according to the Raoult 's law.

For the determi nation l". 15 of the ex tent o f excess hydration of the powdered so lid in the presence o f inorganic sa lt or an organic solute (sucrose or urea), a definite amount o f dry salt was taken in a sample bottle. The bottl e was fl oated on the reference soluti on containing same solute present in the sample. The sample in the weighing bottle and the reference solution were brought in isopi es ti c equilibrium as before for a week. Arter thi s, the sample bottl e was taken out from the des iccator (with lid on) and weighed accurately_ From th e dilTerence in weights,

total weight ( Wi) and total moles (n:) of water

present per kg of powdered sample were cal culated.

M oles ( n ~) of sa lt per kg o f powdered so lid was

already known . Total molality m; I' the salt (or

organic solute) in the reference soluti on was es timated by direct weighing o f the definite we ight o f the

so lution to complete dryness at lOS- 110°C. For LiCi and urea, a spec ial method lS-17 was used in place or

dry ing. Total molality I11 ~ of the solute i n th e sample

per kg of th e powder is equal to 1000 n2 ! M in: so that

it s va lue was calculated rrom known values o f n: and

n ~ . However, after adsorpti on o f the salt and water by

one kg of th e so lid powder, frce molality of the so lution in contact with one kg o f powder, will be 1I1~. Thi s free solution was in vapour pressure equilibrium

w ith the reference solution of molality m;. The small

contribution o f the so lid powder to the vapour pressure can be neg lected 15-17 so that we can assume

that /1/2 = m;.

Results and Discussion M oles of water vapour (1/ 1) bound per kg of

charcoal , cati onic and anionic res ins, respecti ve ly at 300 e and 400 e were plotted against water acti v ity (a l) within the total range zero to unity (not shown). A ll these iso therms are sigmoidal in sha e and similar in form which can be expected from the type 3 BET isotherm I2.1]. In all these isotherms, I.h-e abso lute values o f 1/1 at a gi ven value or a l increase with increase in temperature. Thi s indicates th at w i th increase in temperature, more water molecul es arc bound to the solid surface possibl y due to the hydrophobic effect. Thi s al so indi cates th at the adsorpti on in all these cases is endotherm ic in nature. These isotherms may qualitati ve ly be div ided into three parts I.' . Fi [-st part of th e iso therm in the range o f relative hum:c1ity zero to 0.20 is for the formati on o f primary layer o f water of the , olid surface. In thi s region, water molecules are bound strongly to the surface o f solid thu s forming a monol(lyer IO

.12. The

second part o f the curve for relative hLL11 idity 0.30-0.80 is responsible for the formation or new si tes or holes on the solid surfaces. The steep th ird parr of the curve is probabl y due to the condensati on of randomly ori en ed W(lter molecules on the hyd rated surfacc or solids, i .e. the formati on o f multi layers of adsorbate water molecules0. IO.17. For anion ic resin , 1/ 1 is independent o f temperature up to relati ve humidity 0.50. The extrapolated values of 1/1 at a l equal to unit y correspond to the maximum and effecti ve amount of

HALDER ('I (II. : NEGATI VE ADSOR PT ION OF ELECTROLYTES AN D UREA 260 1

water bound by solids and fo r each system it is

referred to as 6.n ~). The v::lIues of 6.n ~) for charcoal,

cationic res in and ani oni c res in at 30 and 40°C are given in Table I . lt may be noted th at values of both

d A () • . . b III an Dnl are pos iti ve or zero; It ca nnot e

obviously nega ti ve. 6.n:) for all sys tems increases

with the increase of temperature (Table I ). The table

shows th at in terms of 6.n:), the adsorbents at 30°C

are in the order: cati on ic resi n > ani oni c res in > charcoal.

However with increase in temperature from 30°C to

40°C, 6.n:) for charcoal increases signifi cantl y

poss ibl y due to hydrophobic interacti on and its magnitude becomes same as th at fo r ani oni c res in .

6.n~ for cat ioni c res in at both temperatures is

significantl y large.

The integral free energy change (6.C::.) due to the

hydrati on of powdered so lid for the change of ([I from zero to unity has been eva luated using Eq. ( I ) deri ved by BuIl 17

-1') on thermodynami c ground :

() f ",=1 11 6.C = - RT --.!... d ([1 \\' 0

([I

... ( I )

From the graphical eva luati on of the area under the

curve obtained by plotting [~) as fun cti on of ([I (/1

between the limit 01 = 0 to 0 1 = I , the valu e of the

integral for each system may be calculated. 6.C~. for

charcoal, cati oni c and ani oni c res ins is given in Table

I. 6.C::' according to Eq. ( I ), is always negati ve

which indicates that the hyd rati on of charcoal , cationi c and ani oni c res in is thermodynamicall y

spontaneous process. Val ues of 6.C::. at 30°C also are

in the order: cati oni c res in > ani oni c res in> charcoal.

But at 40°C, - 6.C::. fo r charcoa l due to the in crease of

the hydrophobic interacti on exceeds that o r ani oni c res in (Ta ble I ).

When powdered so lid as adsorbent in the sa mple bottl e remains in co ntac t wi th water as so lve nt and electrol yte (o r urea) as so lu te at vapour pressure equilibrium with the reference so lution, both solvent and solute will co mpete for interac ti on with aC li ve

spots of solid . Relati ve or excess adsorption r~ of

solute for such syste l11 wi II be ca lcul ated assum ing Eq. (2)9.J:l.

Tab le l - V.llues of (I'- n;') and (-I'-C:: ) for the hydrati on o r

charcoa l. cat ion ic and anion ic resin power in absence in so lute

So li ds Temp. I'-n;'xlO' -(I'-G ~: L xlO· (K) (mole/m' or (U /m' or solid)

solid)

Charcoa l 303 0.338 4.52

313 0.730 10.4

Cati oni c 303 2.00 28.0 res in (C I-)

3 13 4. tO 30.0 Ani oni c 303 0.664 5.52 res in (Na+)

313 0.736 5.65

I Wi I (2) r, = -- (m, - III, ) ... , 1000 ' ,

The superscri pt I for r~ im plicitly indicates that the

adsorpti on of the so lvent co mponent I is taken as

zero. From Eq. (2), we al so note that for 1112 < m ~ ,

r~ for so lute adsorption is pos iti ve. But for 1112 > m ~,

the so l ute adsorpti on r~ becomes negati ve.

Now, at isopi es ti c equilibrium,

Ill? 111 X ~ -- -- .. . (3)

Here measured value 1112 is related to III moles of free solute and " 2 moles of free so lvent respec ti vely per unit area of the so lid powder. XI and X2 are their respecti ve mole fracti ons of the free co mponents in the sampl e bottle at adsorpti on equilibrium .

We can replace m~ , Ill" and W I I by

I 000 n ~ 1000 111 and M I respecti vely so that I' I n l

M I nl MI III

inserting these in Eq. (2), we obtain9

(4) I I X , =n -n - ' 2 I V

, \ I

Here n: and n ~ stand fo r total moles of so lvent (of

molecular weight M I ) and so lute respecti ve ly per unit surface area of the so lid power before adsorpti on. Equati on (4) can be written as9 Eq. 5.

2602 I DIAN J CI-IEM. SEC A. DECEMBER 2006

. .. (5)

so that upon combin ing Eqs (4) and (5), we get:

Thi s equati on has been deri ved earli er by SChay"° and Kipling"1 for the adsorption or liquid mi xtures on vari ous so lid powders. Eq. (6) , can be vvritt en as:

( 2 = _ ~I~ I X-. ,

... (7)

Eqs (6) and (7) show that ri~ and I~ arc not

independent quantiti es and their va lues arc oppos ite in

sign. Also, if I~ is calculated from Eq. (2) using

experimental va lues o f m ~ and 1112, the value o f

excess adsorpti on of so lvent I~ can be evaluated

using Eq . (7) . Relatio s (2), (4) and (5) do not, however, exhibit

expli citl y th e individual contributions o f so lute and

so lvent i ll the measured values o f 112 and I~ . T o

overcome thi s di fricul ty, Chaltoraj el 0 1. "22 have recen tl y shown that the surface phase in contact with liquid is inhomogeneous whereas the bul k phase of binary so lution is always homogeneous in nature. The

inhomogeneous surface phase is composed o f 6.//1

and 6.//2 moles of so lvent and solute respecti ve ly per

unit surface area and it is in contact w ith //1 and //2 moles of so lvent and so lvent forming homogeneous bulk phase of binary so lution. W e can, thu s, writ e:

n: = //1 +6.//1

n ~ = //2 +6.n:>

(8)

(9)

Combining relations (4), (5) , (8) and (9), ori ginal fundamen tal relations ( 10) and ( II ) are obtained:

... ( 10)

. .. ( I I )

These relations indicate how I.: and 112 arc

related to the actu al compos iti ons 6111

and 6.//2 or

so lvent22.21 and so lute, respec ti ve ly . Re lati ons ( 10)

and ( II ) indica te that 6.//1 and 6.//2 rno ics of so lven t

and so lute respecti ve ly actu ~lI l y fo rming inhomogeneous surface phase per unit area are both related to the excess or relati ve adsorption quant i ti es

112 and I~ . If 6.//1 is eq ual to zero, then f rom relati on

( I 0), I~ becomes equal to 6.//2 ' Similarly , i f 6.//2 is

zero, II" = 6.//1 ' furt her, unlike (~ or I I"' 6.//1 and

6.//2 are abso lute va lues whi ch may be posi tive or

zero but never become negati ve. A lso, if under certain

circumstances, I~ varies l inearl y wll h change of

X21X I (or 1112155.5) , th en using Eq . (1 0), va lues of

6.//1 and 6.//2 ' respecti ve ly can be eva luated from the

slope and intercept o f the linear pl ot.

Figures 1-3 show excess adsorption

different so lutes ex pressed in moles p' r kg

(X2/X, )x1 02

<U 0 2 <U r:. u '0 O"l .Y --Q) ~

::J

0 C/>

'0 en Q)

0 E

_N

Co.. I

14

12

10

8

6

4

2

- 2

•

20 40 60 80 10 0

0

•

0

A

20 40 60 80 100

(X/X,)xIOJ

r~ for

of solid

Fi!!. t - PIOl or I", ".I"( x 2) ror ch;m:o;d i 11 p r~ ~;(, J1CC or d i rrcrcill ~ - x,

sol ules al - 30°C: (e) LiCl: (6.) NaCl: (0) KCI: (TI) Na2S04' (+)

Urea.

HA LDER 1:1 ({f .: NEGATIVE A DSORPTION OF ELECTROLYTES AN D UREA 2603

(X 2/XI

)X102

2 0 40 60 80 100

12

c 10 c en (f)

~ 8

~ . ~ .~ c c 0 Q -;;; 6 -;;; u u a 0 O"l 4 16 O"l -"" .:,c; -III ~ :::l :::l

0 "0 en Vl

a a (1) 0 (1)

0 0 E E

- N - '" L -2 L I I

-4

-6~~~~--~--~~ 20 40 60 80 100

(X/X,)XIOJ

Fig. 2--Plol of r; 1'.I"( X ~ J for cal ionic resin in presence of - XI

differenl so lules al 30oe: (0) LiCI: (Ll) NaCl: ([] ) KCI. (IT)

Na~SO." (+) Urea.

powder as fun ctions of X ~ . r~ is negative for hi gher XI

I f• X, (. I ) B I I . va ues 0 - - I.e., 111 , <III, . ut at ower va ues ot XI --

the 1110 1 rati o compositi on, f~ beco mes pos itive (i. e. ,

m~ > III~ ). From Eg. (7), we find that excess adso rpti on

of water fl2 becomes positive when r~ is negative

and vice versa . Further, we note from the fi gures that

r l . I' I . I X , - . , vanes Inear y Wltl--- XI

in wide range of

concentrations . From the slope and intercept of these strai ght lines, absolute values of 1'1111 and 1'1112 for

different systems have bee n eva luated usin g Eq. ( 10). Values of 1'1111 and 1'111~ for different sys tems are given

in Table I.

From Tab le 2, we note that va lues of 1'1111 at 30°C

for different so lid powders are in the order: charcoal: Na2S0~ > KCI > LiCI > urea> NaCl; Cation ic res in:

12 c:

~ 10 u

Q)

o

•

(X2 /X 1h10 2

20 40 60 80 100

•

o 0

•

_E", 0 f-+-+H.r'I------- ---1 c...

I • -2

-4L-~L__L__~_~~

20 40 60 80 100

(X 2 / XI )x103

I (X, J Fig. 3~r l ol of r~ 1'.1" ~ for an iolli c res ill ill presence of

differenl so lules al 30°C (e) LiCI (1'» NaCI, (0) KCI. (TI)

Na~SO.j, (+) Urea.

Na2S0" > KCI > NaCi > LiCI > urea; Anionic res in: Na2S0~ == LiCI > KCI == NaC I > urea.

Further in all cases, co mpari son of values of 1'1111 in

the presence of salt inc reases signifi cantly from thei r

respec ti ve va lues of .6. 11 ~) (Table I). Thus, additi on of

salts increases the abso lute hydration of the solid powder. It has some similarl y with salting- in phenomena observed for in crease in so lubil ity of bio­co lloids in the presence of sal t. The order of elect rolytes observed probab ly depends upon the activity coefficients of electrolytes in the wide range of hi gh electro lyte concentrations bes ides amount of electrolyte binding 1'11l ~ whi ch are differen t for

different surfaces (Ta ble 2).

Dev iation of the pl ots of r~ :Igain st Xc/Xli II

F;gs 1-3 from l;nw;Ly ;n m La;n ,·cg;ons 01 r ~: ) ;s

an indication that 1'1111 or 1'1111 are nOl always

2604 INDIAN J CHEM , SEC A , DECEMBER 2006

Table 2- Hyclrali oll paralllelers or cl ifTerell1 solid surfaces ill presence o f clifTerenl so lules al 30°C

Sol icl surfaces Solule ~1/, x I O)

( 11101 H2O/Ill" solid)

Charcoa l Liel 3.28 NaCI 0.71 KCI 3.63

Na2S0~ 3.68 Urea 1.49

Cal ionic res in LiCI 6.7 NaCI 7.27 KCI 7.34

Na2S0~ 10. 16 Urea 4.46

Ani onic resin LiCI 3.82 NaCi 3.30 KCI 3.39

Na2S0.1 3.80 Urea 2. 10

constants bnt they v"·y now w;tI, change of [ ~: l For a given solute, it is observed th at r~ becomes

positive or negati ve depending on the va lue of Xl /X I. For each solute, there ex ists a definite value of X2/X I

when r~ becomes zero (Figs 1-3). Thi s is the

azeotropic state l 7 for adsorption of so lvent and so lute onto solid surfaces. In thi s state, 11112/11 111 beco mes

eq ual to Xl /XI . Values of m~zco for various so lutes of

the azeotropic states are given in Table 2. Willard Gibbs2~ derived hi s elega nt adsorption

equati on by arbitrary placement of an imaginary dividing plane near the interface between two fluid phases containing multi-componen t mi xtures. For two component so lute and so lvent sys tem in bulk in contact with an air-water interface, this equati on will be:

. . . (12)

Here, y stands for the surface tension per unit length (or surface free energy per unit area) of the interface. Also d~ll and d~2 in Eq. (12) represent chemical potentials of solvent and so lute in the bulk phase which may be taken to be equal to RT d In 01 and RT d In 0 2, respectively. Here (/ 1 and (/2 stand for the activiti es of the solvent and so lute in the bulk phase.

~1/,X I 05 aLeu III 2 ~C" (-~C::, )

(mol solule/ (kJIll -" x 105)

1112 solicl )

8.50 1.44 18_8 2_33 1.82 6n 10.1 1.54 69.4 7.71 1.1 6 59.9 4.4 1.64 17.7

17.4 1.43 2.4 1 15.1 1.1 5 14.5 19.2 1.45 75.9 12.5 0.682 86.2 5.2 0.647 12 1.0

9.11 0.264 18.2 12.5 2.09 15.3 26. 1 4.27 49.0 5.9 0.86 4 1.8

3.3 1 0.875 34.2

Gibbs defined the surface excess qu ant ities r 1 and

r 2 as the difference of total moles of so lvent and

so lute components, respect ively present in the whole system and those present in the bulk separated from imaginary surface phase (a) due to the arbi trary placement of the dividing plane. By changing the pos ition of the dividing plane arbitra rily in side the inhomogeneous a-phase in a suitab le position , 11 may become zero so that Eq . ( 12) becomes:

. . . ( 13)

Superscript of ['~ indicates that the surface excess of

co mponent I has been put to zero in Eq . ( 12). 1~ has

been shown Ins to be in vari ant with respect to the position of the dividing pl ane. Defay and Priggogine26

have shown from thermodyn am ic analy"is that:

... (J4)

Chattoraj ~lI1d co-workers22 have show n that Eq, ( 13) will remain valid if air-water interface is replaced by so lid-water interface. The similarity in form s between relati ons (4) and ( 14) indicates [hat directl y

measured va lue of r~ based on relation (2) is same as

the Gibbs surface excess based on arbitrary pl acement of dividing plane near the interfac ial region.

HALDER el (II.: EGATIVE ADSORPT ION OF ELECTROLYTES AND UREA 2605

Gibbs equation indicates th at by the addition of surface ac ti ve substances in so lution, y is lowered

si nce r~ becomes positi ve whereas on addition of

inorgani c salt y increases since surface excess of salt in the system becomes negative. It can be shown that

all equations related to r~ and r~ di scussed here are

consistent wi th pos i ti ve and negati ve surface excess quantiti es introduced by Gibbs for the adsorpti on of so lute and solvent components at a fluid interfacecc.

The chemical potential ~c of an electrolyte P\,+ Q\'. in the bulk phase may be written as:

. .. ( 15)

Here ~+ and ~L are chemi cal potenti als of cati ons and anions in the bulk phase, respec ti vely. Also, D+ and D. are number of cat ions and anions when one molecule of salt is co mpletely di ssociated in the bul k phase. Putting thi s val ue of ~l ~ in Eq. ( 13), Chattora/7 has shown that:

. . . ( 16)

The mean mole fracti on X± of the electrol yte in bulk can be calculated using:

X ± =(X +.X. ) 1/11. +U. ... ( 17)

Here X+ and X. stand for mole fract ions of cations and anions, respectively, whi ch can be ca l c ulated~ ' from measured value of 1I1 ~ . The mean activity coefficients .ii of the electrolytes for given values of X;,: have been caiculatedusing the procedures foll owed earl ier27 .

One can fix the equati on for the apparent standard

free energy change t.q~:) fo r the excess adsorption of

an electrol yte by integrati on of Eq . ( 16) between limits X± equal to zero and unity so that relation (1 8) wi ll be obtai ned22

.

t.cO =JX.=I dy :lfl 0

[J x,

=-RT(D+ + D. ) o · ~(/~ X+ - r~ In U +X+)] I x ... . . . . 1. 1

.. . ( 18)

The integral part of the ri ght side of Eq . ( 18) stands for free energy of adsorpti on when f ±X± is altered from zero to a particular value of X± and the second part represents the free energy change due to

hypotheti cal diluti on from thi s va lue of X± to unity at

constant val ue of r~.

t.q~:) vari es linearl y with 11 jX; (Fig. 4), so that

extrapolated va lues of t.C Il equal to the standard free energy change for adsorp ti on at X± equal to uni ty have

been calcu lated . t.Co for electrolyte adsorpti on are found from Table 2 to be all posit ive. This indica tes that the excess adsorpti on or electrolytes and urea on the surfaces of charcoal, cat ioni c and ani onic res ill s. res pec ti vely are non·spontaneous processes.

In an alternati ve approach of the Gibbs treatmentcc.c3 for adsorpti on at fluid interface, one can place arbitraril y the dividing plane in the surface

phase a in such a manner so that r; beco mes zero .

In this condition , using Eq. ( 12) we get:

... ( 19)

Thus, we fi nd from Eqs ( 13) and ( 18),

r~ d~l ~ = r~ d~ll ' The excess standard free energy

change flG ::y for the so l vellt adsorpti on (hydrat ion)

per unit surface area can also be obtained by integration of Eqs (13) and ( 19).

20

16

12

8 ....-.. 0'> 4 .:£

"-Y. -.-/ 0 ><

0. .. C> <l - 4

-8

I lb •

. I Fig. 4 - Plot of f\.C",. \ '.1' IX: for ca ti on ic resin in presence of

different so lutes at 30°C: (0) LiCI (rr) NaCI. (f\.) KCI. (e)

N a~SO.j , CA) UI·ea.

2606 IND IAN J CII EM. SEC A. DECEMB ER 2006

() f ~ f X, =I , LlCh), = 1 dy= - II r~ d~l l

= - Jr X, =I r~ d~l, =-f .1', =11 r~ d~l, II - - X, =I - -

. . . (20)

f X. =I

= + - r~ d,Ll , =-LlC" o --

We note that by multipl ying values of LlC" by - I, one

can es timate LlCI~Jy and signs for these quantities

(Table 2) will all be negati ve. We have already menti oned that form of the Gibbs adsorpti on eq uati on will remain same ror Iluid as we ll as solid-liquid interface22. This indicates that surface excess hydrati on presented in Table 2 for different electrolytes and urea, respectively on the surfaces of charcoal , cati onic and ani oni c res ins is spontaneous in

nature. Magnitudes or !::..G 0 (alld LlC,~J)' ) for different

so lutes are comparab le since all these quantiti es refe r to standard state n2 (orJi Xi ) equal to unity .

From Table 2, we can compare the excess

hydrati on (-!::"GI~)Y ) for the solid powder in the

presence of various salts and urea. For di fferent

solids, !::"GI~Y is in the order (for electrolytes and

salts); charcoal KCI > NaCi > Na2S0~ > LiCI > urea, cationic resin: urea > Na2So.1 > KC I > NaCI .> LiCl , anionic res in: KCI > Na2S0~ > Urea > LiCi > NaCI.

E I· - . . . d 'x I I I AGO ar IeI' uSing Isoplest lc ata-, we ca CLl atec U II)'

fo r alumina, sili ca and barium sulphate particles.

Here, (-!::..q~') is in the order; alumina: Na2S0~ >

KC I > NaCI > LiCI , silica : Na2S0~ >KCI > LiCi > NaCI , barium sulphate: a2S0~ > LiCi > aCI > KCI.

This order of electro lytes for all six types of particles is observed to be different from each other. The order of the sa lts does not stri ctly foll ow any regular lytropic series of electrolytes valid for biocolloid precipitation. Poss ibl y, the observed series of electroly tes for each solid particles depend on its hydrophilic and hydrophobic nature of different solid surfaces having different extents of surface heterogeneities and porositi es, surface charge and surface hydration, different orientati ons of surface water, etc.

Acknowledgemcnt INSA Honorary Scientist D. K. Chattoraj is

thankful to Indian National Science Academy, New Delhi for financial assistance. Financial assistance

fro m CSIR and DBT New Delh i. arc al so ack nowledged with thanks.

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