AqA AqW kTln C ln N

where AqW is the heat of wetting of the adsorbent.

0 1

Figure 2. Experimental isotherms of LS (a) and Si-LS (b) adsorption on cellulose (1), titanium dioxide (2), kaolin (3) and calcium carbonate (4).

The physical meaning of the parameter AqA is the energy of bonding of adsorbate molecules with adsorbent surface.

In accordance with characteristics shown in the Table 2, the solids studied can be classified according to their affinity towards lignosulphonates, in decreasing order: Cellulose > TiO2 =Kaolin > CaCO3

TABLE 2. Characteristics of LS and Si-LS adsorption on solids.

Adsorptive

Adsorbent

(mg/g)

AqA (kJ/mol)

Cellulose

25.1

46.1

LS

Kaolin

14.0

32.9

TiO2

10.3

25.4

CaCO3

5.0

n.d.

Cellulose

36.6

51.2

Si-LS

Kaolin

18.0

34.3

TiO2

12.4

33.2

CaCO3

6.3

n.d.

Figure 3. Example of the fit of the Aranovich model to the experimental adsorption data: 1, 2 - experimental data for LS and Si-LS adsorption, respectively, on kaolin; 1a, 2a - model fits.

Figure 3. Example of the fit of the Aranovich model to the experimental adsorption data: 1, 2 - experimental data for LS and Si-LS adsorption, respectively, on kaolin; 1a, 2a - model fits.

The data obtained show that LS adsorbs efficiently on the kaolin surface. At the pH 7 the kaolin surface charge density is about 6 ^Kl/cm2 and specific adsorption of LS reaches up to 14 mg/g (about 1 mg/m2), which is a typical value for the adsorption of polyelectrolyte on the oppositely charged surface. This proves that in the LS-kaolin system non-electrical contributions to the free energy of adsorption are significant. Based on the literature data, hydrophobic interaction could contribute significantly in the adsorption interaction on the kaolin surface. The comparison of adsorption characteristics of LS and Si-LS (Table 2) with their hydrophobicity (values of the Gibbs free energy of polymer macromolecules transfer from aqueous in ethyl acetate phase, -AG°aq ^ oii, were 3.93 and 2.00 kJ/mol for LS and Si-LS, respectively) confirmed this suggestion.

The LS adsorption isotherm on cellulose indicates their high affinity (Figure 2a). It also shows larger values of LS adsorption within the whole concentration region under investigation, in comparison with those obtained for inorganic solids. Cellulosic fibres are negatively charged in aqueous suspensions, mainly due to the dissociation of carboxylic groups. Therefore, the two most important long-range surface forces in fibre-lignin systems should be van der Waals' attraction and electrostatic repulsion. The adsorption of LS by cellulose can be caused by such phenomena as physical adsorption due to van der Waals' forces or hydrophobic interactions and formation of acceptor--donor complexes between lignin and cellulose.

The co-adsorbed spin-probe method used in the present work allowed to show that at low levels of the sorbent surfaces coverage (0 < 0.2) two-component EPR spectra were observed. The outer peaks in these spectra, representing the probe with correlation time of rotational movement, Tc « 10-6 - 10-8 s, was attributed to a flat immobilized position of the probe surrounded by the polymer molecules. Growth of adsorbed LS amount resulted in the increase in the intensity of inner peaks characteristic for the probe having rotational movements with small correlation time, Tc < 10-9 s. For samples prepared by sorption from LS solutions with a concentration value, corresponded to the adsorption plateau, one-component spectra with Tc = 2 x 1011 s were observed, which are characteristic for relatively free movement of the spin probe among tails and loops of adsorbed LS molecules in the first layer (shell). Total combination of EPR spectroscopy data obtained indicates a gradual decrease of the fraction of adsorbed polymer segments laying flat on the surface of adsorbent and an increase in the amount of oriented into the solution loops and tails forming pseudoliquid microphase at the adsorbent surface. These observations are in good compliance with the results obtained recently in computational investigations of the cellulose-lignin assembly.17 Due to a significant contribution of H-bonding interactions, in the first shell, lignin at its low concentration adsorbs flat on the surface in order to maximize the interactions with cellulose. However, such type of orientation of lignin aromatic rings becomes rapidly less apparent for units located at greater distance from the interface.

A significant increase in the adsorption value and energy of adsorption interaction as the result of LS modification by the siliconorganic oligomers (Figure 2a and b, Table 2) indicates the possibility to regulate the adsorption behaviour of LS. The presence of the Al3+ in the siliconorganic block permits the Si-LS molecules approaching closer to the negatively charged adsorbent surface than in the case of non-modified LS.

Besides cellulose and the above mentioned inorganic solids used as filler for production of different kinds of paper, the organic polymer floccul-ants are widely applied in papermaking. Therefore, interaction of LS/Si-LS with polymeric flocculants is a matter of high importance.

The results from the DSC testing of the LS and Si-LS blends with PAA flocculant show (Table 3) that non-modified LS has a partial miscibility with PAA, which reveals in two sets of glass transition points (Tg): one for PAA rich and other for LS rich blends. Modification with siliconorganic oligomers leads to compatibility of the Si-LS and PAA in the whole range of their ratio, that was exhibited in single Tg and almost perfect correlation with the Fox equation.18 The forming one phase structure was characterized by a synergistic flocculation effect.

As the result of enhanced adsorption of Si-LS on the surface of the cellulose and inorganic fillers, its addition into paper composition improves uniformity of distribution of fillers and fine cellulose fibers leading to higher mechanical strength of paper as well as increases fine fibers retention diminishing their coming into environment.

TABLE 3. DSC data from testing of LS-PAA and Si-LS-PAA blends.

Composition

Proportion of the first component (%)

Proportion of the second component (%)

Tg1 (oC)

(oC)

Tg calculated according the Fox equation18

(oC)

PAA and LS

0

100

n.o.

79.1

PAA and LS

10

90

63.4

77.6

73.2

PAA and LS

20

80

62.0

76.0

69.0

PAA and LS

33

66

39.7

62.4

61.2

PAA and LS

50

50

41.7

62.2

55.0

PAA and LS

75

25

41.2

61.1

47.7

PAA and LS

100

0

42.1

n.o.

42.1

PAA and Si-LS

0

100

71.2

n.o.

71.2

PAA and Si-LS

10

90

66.5

n.o.

67.0

PAA and Si-LS

20

80

63.4

n.o.

63.8

PAA and Si-LS

33

66

57.1

n.o.

57.9

PAA and Si-LS

50

50

49.8

n.o.

52.9

PAA and Si-LS

75

25

47.9

n.o.

46.9

PAA and Si-LS

100

0

42.1

n.o.

42.1

*Not observed

0 0

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