Solvation excesses of humic acid in water solutions of urea
S. Karabaev1, I. Gainullina1, А. Harchenko1, M. Satarova1, S. Lugovskoy2, А. Pendin3
1
Kyrgyz national university, Kyrgyzstan, karabaev_s@mail.ru
Ariel university center of Samaria, Israel, svetlanalu@ariel.il
3
St.peterburg state university, Russia, pendina@mail.ru
2
ABSTRACT
Humic acids was extracted from brown coals of Kara-Keche deposit
(Kyrgyzstan) and characterized. Solubility of humic acid in water solutions of urea
was studied. Solvation excess of urea over water on humic acid in solution was
calculated. The possibility of simultaneous binding of urea molecules on three
independent centers of humic acids was shown. The estimation of humic acid
dimension in water solutions of urea was given.
Key words
Solvation excess, humic acid, solubility, urea, Gibbs surface excess,
autoexcess.
Introduction
In A. Pendin,s works [1-2], devoted to the solvation excess concept was
shown that for the nonelectrolytes solutions where admissible reactions are ones of
homo and hetero association, the fundamental equation (1) can be grounded:
 d ln y k 


  Г ik( j ) (1),
 d ln ai  T , P ,a
l
where Г
K
i( j)
- solvation excess (SE) of i particle under j one in solvation surrounding
of k particles; y k - k particle rational coefficient of activity; a i - i particle activity; T temperature; P - pressure; l  i, l  j .
The physical mean of solvation excess (SE) in (1) equation of i particle over j is
determined by equation (2).
r
Ni C
k
Г i( j) 
(  ki   kj )4r 2 dr (2),

V 0
where rC - radius of molecular correlation, N i -number of i particles in the volume V,
 ki (  kj ) - radial function of distribution.
So, solvation excess (SE) Г ik( j ) - is average statistical number of i particles on which
solvation shell of k particles is richer in comparison with j particles, proceed from
composition of binary solvent i-j. If in solvation surrounding of k particle relation of i
and j particle numbers are equal of it relation in the volume of solution non indignant
by k particle, then Гik( j )  0 . In this case preferential solvation is absent.
Equations (3) are consequence of equation (2), where xi , x j , x k - mole parts of
i, j, k components [1, 2].
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l
Xi K
Xi K
K
K
Г j (l ) ;  X K Г iK( j )  0 (3)
Г 
Г j (i ) ; Г i ( j )  Г i (l ) 
Xj
Xj
K 1
Existence of binding equations (3) result in that in binary system from for solvation
excesses only one independent, in three component system from eighteen excesses
only for excesses are independent. Range of permissible values of Г ik( j ) is determined
by conditions of stability of concentration changes [2].
Concept of solvation excess was successfully used for description of
preferential solvation in usual multicomponent liquid solutions [3-5]. At the same
time solvation excess Г ik( j ) is analog of Gibbs adsorption in the phase volume. It can
be used for description of preferential solvation effects in solutions of natural
biopolymers, when notion of molecule transforms in molecular ensemble. In
according with this, the aim of our work is studying of preferential solvation of humic
acid (gk) in water solutions of urea (s). Equations (4) and (5) can be written in
accordance with equations (1) and (3) for research system:
  lg YW 
w
s

  (Г w(s)  Г s(w) ) (4)
  lg aW  T , P
K
i( j)
  lg Ygk0 
x w gk
 Г gk
 Г s(w) (5)


w(s) 
xs
  lg aW  T , P ,agk
where
Г ww ( s(Г) ss ( w) -) autoexcesses in binary solvent water-urea,
Ygk0 - limiting
coefficient of activity of humic acid in water solution of urea , Ygk0  1 at a w  1
(Mole dole of urea x s  0 ).
Experiment
Humic acid was extracted from analytical test of brown coal (Kara-Keche
deposit, Kyrgyzstan) using method of Orlov [6]. Dimensions of coal particles were
about 60 mesh. According to methodic coal was decalcinated to release of calcium
ions, which form with humic acid difficult solving humats. Humic substances were
extracted from decalcinated analytical test of coal using 0.1 M solution of NaOH. For
this purpose mixture was mixed during 15-20 minutes on rotator, after that suspension
was settled during 24 hours. Dark colored alkaline solution was separated of coal by
decantation and centrifugation (4000 rotations in a minute). The obtained alkaline
extract was acidified to pH =1-2 using concentrated sulfuric acid to coagulation of
humic and himatomelanic acids. Washed out with distilled water precipitate were
washed by ethanol for separation of himatomelanic acids. The precipitated humic acid
was recrystallized and dried. Set of physico-chemical characteristics of humic acid
was defined for establishment of its classification signs.
Infrared spectra of analytical test of humic acid from Kara-Keche coal was
measured on spectrometer «Nicolett Avvator» in spectral range of 4000-400 сm-1 in
standard KBr based tablets.
As it can be seen from Fig.1 IR spectra of humic acid has a band having a
sharp maxima at 3418 cm-1caused by valent vibrations of OH groups of water,
binding with intermolecular hydrogen bonds. Band at 1610cm-1 is related to
deformation vibrations of OH groups of water. Duplet of weak intensity at 2923 and
2852 cm-1 is caused by valent vibrations of СН, –СН2–and –СН3 moieties. Bands
having some unclear maxima at 1384 and 694 cm-1 are related to deformational
vibrations of these groups. Absorption of –СОО- ion is observed at 1400-1380 сm-1.
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Bend in the range about 1500-1400 cm-1 can be explained by presence of –СОО- ion
in the simple. Possibly valent vibrations of –С=С- aromatic and aliphatic structures,
including conjugated with С=О groups, can influence on absorption in the range
1700-1500 сm-1. Deformation vibrations of ОН groups in water and NH in amines
are observed at the same range. Bands having maxima at 2348 and 2283 cm-1 are

related to the valent vibrations of  NH 2 and -С≡N groups. Band having maxima at
1245 cm-1 is related to the valent vibrations of С-О in carboxyl groups. Maxima in
region of 1200 – 1050, 950-900 сm-1 are related to the deformational vibrations of
alcohols groups –ОН. However, triplet at the same range of spectra having maxima at
1098, 1031 and 1007 сm-1 is caused by valent vibrations of Si-O groups,
characterizing minerals of caolinit group. It is confirmed by maxima at 754 сm-1,
related to the deformational vibrations of these groups. Band with maxima at 911сm1
is also related to the deformational vibrations of О-Н groups of alcohols and to the
deformational vibrations of Ме…ОН…О, band at 532 сm-1 is assigned to vibrations
of Si-O-Me. In IR-spectra of humic acid, bands lying in the range 469 – 411 сm-1 are
characterized by different sulfur- and halogen containing groups, which are minor in
composition of humic acid.
Fig.1 IR-spectra of humic acid from Kara-Keche
Moist ( W a ), ash content ( A a ), element composition ( С а , H а , N a , О а , S а ) of
analytical test of humic acid from Kara-Keche coal were defined using methodic [7].
Analysis of experimental data is based on statement that mass of humic acid simple
adds from mass of organic (CNSO) and inorganic (ash content, hygroscopic water)
parts.
Average arithmetical data of several parallel results are presented in tabl.1.
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Table1
Ash content, moist, element composition of humic acid from Kara-Keche coal
for analytical test of humic acid
С а ,%
60,60
С Г ,%
70,90
Nа, %
О а ,%
W a ,%
S а ,%
H а ,%
3,44
1,04
0,94
19,65
3,63
In calculation on arid, ash discharge mass of humic acid
H
S Г ,%
N Г ,%
О Г ,%
H Г ,%
C
3,56
1,22
1,10
23,22
0,60
A a ,%
10,70
O
C
0,25
Comparison of data of Fig. 1 and tabl. 1 with results of works [8, 9] and
analysis of Van-Krevelen diagrams for atomic relations allow us to assume that
studying simple of humic acid from Kara-Keche coal corresponds to the complex of
typical signs of molecular ensemble with maximal contents of aromatic carbon (70%)
and practically full absence of sugar fragments. Obviously, character of solubility of
humic acid from Kara-Keche coal in water solutions of urea is conditioned by
presence of hydrophobic polycondensing structures.
Isothermal saturation of water solutions of urea by studying simple of humic
acid from Kara-Keche was carried out at Т=2980К and continuous mixing during 24
hours. Phases were separated by centrifugation at 4000 rotations in minute. Solubility
sat.
of humic acid ( CGK
) in water solutions of urea was determined by spectrophotometer
at the wave length   400 nm . E-value of humic acid in water solutions of urea is
Е  22,9 л  г 1см 1 , l  1. Results of experimental data of solubility are presented in
tabl. 2.
Table 2
Solubility and limiting coefficient of activity of humic acid
in water solutions of urea (s) at T  298 0 K
xs
0
0,0026
0,0052
0,0079
0,0106
0,0134
C gk (г / л)
0,00241
0,00247
0,00285
0,00296
0,00297
0,00302
Г s ( w)  1010
0
-0,04
-0,09
-0,14
-0,18
-0,24
lg YW
lg aW
0,00000
0,00016
0,00031
0,00047
0,00063
0,00079
0,00000
-0,00097
-0,00195
-0,00297
-0,00400
-0,00507
lg Ygk0
0,0000
-0,0107
-0,0728
-0,0893
-0,0908
-0,0980
Quantities of lg Ygk0 in water solutions of urea (tabl.2) were calculated by equation (6).
lg Ygk0  lg
C gk ( w)
(6)
C gk ( s )
Values of logarithms of activities (a w ) and coefficients of activity (Yw ) of water in
water solutions of urea and values of Gibbs relative surface excesses of urea over
water ( Г s (w) ) were taken in the work [5].
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Results and discussion
Data of tabl.2 are the basis of calculations of autoexcesses in binary solvent
and solvation excesses of humic acid in water solutions of urea by equations (4) and
(5). For this purpose dependences lg YW = f( lg aW ), lg Ygk0 = f( lg aW ) were
described by polynoms with following analytical differentiation. As it can be seen
from Fig.2 urea, having bipolar nature in water solutions, is surface-nonactive
component, because values of Г s ( w)  0 (curve 3). At the same time, weak
homoassociation of binary solvent components takes place in water solutions of urea,
as calculated values of autoexcesses are positive Г ss( w)  Г ww( s )  0 (curve 2). As it
can be seen from Fig.2 solvation excess of urea over water on humic acid is positive
Г sgk( w)  0 (curve 1) and more than Г ss( w)  Г ww( s ) . This result testifies the preferential
concentration of micro non homogeneities of binary solvent, enriched by urea, on
humic acid. So, surface- nonactive urea is solvatoactive to humic acid in water
solution. It is obvious, that it is explained by inter molecular interactions of urea with
reaction centers of molecular ensemble of humic acid.
Fig.2 Solvation and surface excesses in the system water-urea-humic acid at 2980К
1- solvation excess of urea over water on humic acid( Г sgk( w) ); 2-autoexcess in water
solution of urea ( Г ss( w)  Г ww( s ) ); 3-relative surface excess of Gibbs on the border
water solution of urea – steam ( Г s ( w)  1010 ).
For definition of urea molecules number, simultaneously binding with humic
acid from Kara-Keche coal, analysis of values of solvation excesses Г sgk( w) was carried
out in coordinates of Scetcherd plot and within the frames of Hill,s semi empirical
method [10]. Results of this examination are presented in the Fig.3.
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Fig.3. Hill,s plot of solvation excess of humic acid in the water solution of
urea at 2980К
As it can be seen from Fig.3 Hill,s coefficient (  H ) is equal to unit. It is explained by
binding of urea molecules on three independent centers of studied simple of humic
acid from Kara-Keche coals. So far as constant of humic acid association with three
molecules of urea is not big (К=6,3), we can consider that energy of biopolymer
interaction with components of binary solvents is smaller than among particles of
solvent. In this case phenomenological relation between solvent ( Г sgk( w) ) and surface
( Г s (w) ) excesses can be written as equation (7) [4].
P
gk
Г sgk( w)  4rgk2 N A  Г s(w)  1Г s(w)
(rgk )   2 Г s(w)
(7)
The first number ( 4rgk2 N A  Г s(w) ) of equation (7) is the solvation excess of surfaceactive component of binary solvent on the surface of cavity ( rP ). Radius of this cavity
is equal to radius of humic acid in the water solution of urea ( rgk ). The first correction
P
gk
1Г s(w)
(rgk ) accounts the crookedness of this cavity. The second correction  2 Г s(w)
accounts the contribution of intermolecular interactions of humic acid with urea in the
value of Г sgk( w) at the bringing of biopolymer in cavity. According to Tolmen [11] at
P
rP  rgk , 1Г s(w)
(rgk )  0 . At the same time, the value of second correction can be
gk
equal to three molecules of urea, simultaneously related with humic acid  2 Г s(w)
 3.
So, equation (7) can be written as equation (8)
rgk 
gk
(Г s(w)
 3)
4  N A  Г s(w)
(8)
Results, carried out by equation (8) are presented in the Fig.4.
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Fig. 4. Radius of humic acid ( rgk ) as function of composition
of water solution of urea( X S ) at 2980К
As it can be seen from Fig. 4 radius of humic acid monotonously decreases from 310
to 120 nanometers with increasing of urea mole part in the
interval 0,0014  X S  0,0134 . It is observed the antibat dependence of Г sgk( w) from
binary solvent composition (Fig. 4). Possibly it is related with micelle nature of macro
molecular (humic acid) in the water solution of low molecular bipolar ligand.
Increasing of concentration of this ligand lead to increasing of dielectric penetration
of binary solvent.
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