Lignin selective dyes: quantum-mechanical study of theircharacteristics

Franc Perdih • Anton Perdih

Received: 23 November 2010 / Accepted: 30 May 2011 / Published online: 11 June 2011

� Springer Science+Business Media B.V. 2011

Abstract The mechanism of dye-bonding to the

lignin component of the fibre was checked using the

quantum-mechanical and crystallographic approach.

The calculated data support the conclusion that for

the selectivity of cationic dyes for lignin, a high

partial negative charge on accessible substituents on

the periphery of the molecule is the most important,

but not an exclusive factor. Besides the delocalized

positive charge, what is also important for the

interaction of a dye with hemicellulose, is the dye’s

ability to be involved in hydrogen bonds, whereas for

the interaction with lignin also its ability of stacking

to it.

Keywords Lignin � Cationic dyes � Ab initio �Mulliken charges

Introduction

The interactions between dyes and lignocellulose

fibres have received considerable interest. Srebotnik

and Messner (1994) examined the possibilities of

observing the delignification process with differential

staining of wood cross-sections using the principle of

selective staining of lignin with Safranine dye and of

the lignin-free part of fibres with Astra Blue dye. Yu

et al. (1995) used selective staining of cellulose

components to determine the pore structure of fibres.

Liu et al. (1999) followed the inhomogeneity of

delignification by measuring the change in fluores-

cence after staining fibres with selected dyes. Moss

et al. (1999) observed the degree of delignification by

measuring the intensity of natural fluorescence of

lignin in the fibres using CLSM (Confocal Laser

Scanning Microscopy).

In a recent paper, Drnovsek and Perdih (2005a)

presented the results of a study of a number of dyes

for their selectivity at pH 5.5 to lignin, hemicellulose,

and cellulose contained in fibres derived from the

kraft softwood pulp. They separated the studied dyes

into three groups: dyes having suitable affinity either

for lignin, hemicellulose or the cellulose part of

fibres. The results obtained on fibres derived from a

kraft pulp were evaluated also by staining model

substances mimicking either lignin, hemicellulose or

cellulose. Then they explained the mechanism of dye

bonding to functional groups (active sites) on the

fibre surface.

They observed that cationic dyes beyond all doubt

have a higher affinity for the lignin-containing fibres

than the anionic dyes, and the presence of positive

charge is undoubtedly of the highest importance for

the dye affinity for lignin. But they observed also

F. Perdih (&) � A. Perdih

Faculty of Chemistry and Chemical Technology,

University of Ljubljana, Askerceva 5, 1000 Ljubljana,

Slovenia

e-mail: franc.perdih@fkkt.uni-lj.si

123

Cellulose (2011) 18:1139–1150

DOI 10.1007/s10570-011-9558-3

other phenomena that were explained by two addi-

tional features. One of them was high electron density

in some peripheral functional groups, especially

when enhanced by methyl or other alkyl groups.

Another one is the steric shielding contributed by the

aforementioned methyl or other alkyl groups. Oil

soluble dyes indicated that the hydrophobicity of

methyl groups is of minor importance than the effect

of the number of methoxy groups.

These conclusions were re-checked against some

standard fibre analyses, determinations of charge, and

ESCA measurements of kraft fibres pre-bleached

with oxygen, followed by hydrogen peroxide or

ozone using the method of Principal Component

Analysis. The majority of data variance was

explained by the lignin content in fibres and by

polarity (hydrophilicity vs. hydrophobicity) of func-

tional groups. The results of staining by using

cationic dyes did not correlate with the quantity of

anionic (mainly carboxylic) groups in fibres at pH 5.5

(Drnovsek et al. 2005).

In addition, the results of dying were compared to

other methods used for similar purposes (Drnovsek

and Perdih 2005b, c). The cationic dyes’ absorbancies

were in correlation with the amount of the active sites

belonging to lignin (mostly phenolic groups) and

elucidated the differences between the course of

delignification caused by either ozone or peroxide.

Cationic dyes adsorbed on hemicellulose (and

lignin) containing cellulose fibres indicate their self

aggregation both in the applied solution as well as on

fibres (Peterlin et al. 2009).

We checked the above conclusions regarding the

importance of the high electron density in some

peripheral functional groups in the ionic dyes using

the quantum-mechanical methods. We analyzed also

available crystallographic data about some of these

dyes to see whether these methods independently

either confirm or refute some of the above-mentioned

explanations.

Experimental data

Experimental data were taken from a previous paper

(Drnovsek and Perdih 2005a: Table 3). In their study

a set of dyes used for detailed evaluation of the

affinity of dyes using optical reflectance was selected

by preliminary test of dye affinity using capillary rise.

There, unbleached kraft pulp from spruce at kappa

number 21 was used as lignin containing L fibers.

Part of L fibers were delignified with acidic perman-

ganate (Liu et al. 1999). This atypical delignification

laboratory procedure was applied to selectively

remove lignin and preserve the carbohydrates as

much as possible undamaged (Li and Gellerstedt

1998a, b). The resulting fibers were denoted as H

fibers. To obtain a-cellulose of the same origin, the

hemicelluloses were removed from a part of the H

fibers using the standard procedure (Tappi 203 om-

93). The resulting fibers were denoted as C fibers.

Dyes

Following ionic dyes were considered in the present

quantum-mechanical study (for structures and nota-

tions see Fig. 1):

AdO Acridine Orange, C.I. 46005, C.I. Basic

Orange 14

AuO Auramine O, C.I. 41000, C.I. Basic Yellow 2

AzB Azure B, C.I. 52010

CV Crystal Violet, C.I. 42555, C.I. Basic Violet 3

E Eosin, C.I. 45380, C.I. Acid Red 87

EE Ethyl Eosin, C.I. 45386, C.I. Solvent Red 45

ER Ethyl Rhodamine B, C.I. 45175, C.I. Basic

Violet 11

MB Methylene Blue, C.I. 52015, C.I. Basic Blue 9

MR Madder Red, C.I. 12245, C.I. Basic Red 76

PF Proflavine

pR Para-rosaniline, C.I. 42500, C.I. Basic Red 9

RB Rhodamine B, C.I. 45170, C.I. Basic Violet

10

SO Safranine O, C.I. 50240, C.I. Basic Red 2

SY Straw Yellow, C.I. 12719, C.I. Basic Yellow

57

Th Thionine, C.I. 52000

Dyes are identified by their common name, Colour

Index Constitution Number and Colour Index Gen-

eric Name (Colour index 1971).

To see the influence of attachment of methyl

groups onto amino nitrogens on the partial charge on

functional groups, all methyl derivatives of Thionine

and Para-rosaniline were also considered in quantum-

mechanic study. The notation of derivative presents

only the number of methyl groups on each amino

nitrogen (for structures and notations see Figs. 2

and 3).

1140 Cellulose (2011) 18:1139–1150

123

Computational procedure

Full geometry optimisations with no symmetry

constraints and Mulliken charge calculations were

performed at the Hartree–Fock (HF) and three-

parameter hybrid density functional method with

the Lee–Yang–Parr correlation functional approxi-

mation (B3LYP) level of theory. The Dunning

correlation-consistent polarized valence double-n(cc-pVDZ) basis set was employed for all elements.

This basis set was found to give good results when

used for dye molecules treatment (Chen and Chieh

2003; Chen et al. 2005; Fonseca et al. 2008; Matsuura

et al. 2008). Calculations were carried out using

Gaussian 03 set of programs (Frisch et al. 2003).

Mulliken population analysis, as also all other

population analyses are arbitrary schemes for assign-

ing charges. Atomic charges are not a quantum

mechanical observable and different population anal-

yses use different schemes for partitioning the

electron density obtained by quantum mechanical

calculations among the atoms in a molecular system.

Mulliken charge values obtained by population

analysis vary with basis set (Leach 1996).

Fig. 1 Molecular

structures and notations of

dyes under investigation.

Nitrogens part of aromatic

rings are marked as N1 resp.

N2, quaternary nitrogens are

marked as N3, azo nitrogens

are marked as N4 and N5.

Oxygens part of aromatic

rings are marked as O1.

Nitrogens in non-equivalent

–NEt2 groups in ER and RB

are marked as N and N0.Dyes are identified by their

common name, Colour

Index Constitution Number

and Colour Index Generic

Name (Colour index 1971)

Cellulose (2011) 18:1139–1150 1141

123

Both methods (HF and B3LYP) gave the same

trends of highly correlated results. For data in Table 2

R(HF, B3LYP) = 0.9734, S = 0.055, F = 1190.9.

See for example also Fig. 6 where correlation

between HF and B3LYP data is R = 0.9976,

S = 0.033, F = 1253.5. For this reason, only results

obtained by the HF method are presented in other

parts of the paper.

X-ray structures of dyes were obtained from the

Cambridge Structural Database (CSD 2008). Mercury

program (Macrae et al. 2008) was used for inspection

of stacking modes and calculations of distances

between parallel average inter-planes. Average

inter-planes were calculated for phenyl-azo-naphthyl,

naphthyl and phenyl moieties in each (substituted)

1-phenyl-azo-2-naphthol.

Results and discussion

The impetus for this study was the selectivity of a

selection of ionic dyes for lignin obtained previously

(Drnovsek and Perdih 2005a). The dyes are listed in

Table 1. The selectivity of dyes is expressed either as

the difference of fibre reflectance at kmax of the dye in

question (L–H) or as its ratio (L/H). For easier

comparison, the L–H values are multiplied by 103.

Where (L–H) 9 103 & L/H, the dye doesn’t stain the

lignin-free fibres appreciably. Where (L–H) 9

103 [ L/H, the dye stains also the lignin-free fibres.

The larger the difference between (L–H) 9 103 and

L/H, the more that dye stains the lignin-free fibres

compared to lignin containing fibres. Regarding their

selectivity for the lignin containing fibres, these dyes

can be divided into three groups:

Crystal Violet, Ethyl Rhodamine B, Methylene

Blue, Azure B (L/H [ 1000);

Safranine O, Auramine O (1000 [ L/H [ 100);

Madder Red, Ethyl Eosin, Straw Yellow, Para-

rosaniline, Acridine Orange (L/H \ 100).

Staining C-fibres: Acridine Orange � Para-rosan-

iline [ Madder Red [ Ethyl Rhodamine B [ Straw

Yellow [ Ethyl Eosin [ Auramine O � Crystal

Violet, Methylene Blue, Azure B, Safranine O.

Staining H-fibres: Acridine Orange [[[ Para-ros-

aniline [ Madder Red � Straw Yellow [ Auramine

O [ Ethyl Eosin, Safranine O, Azure B, Methylene

Blue, Ethyl Rhodamine B, Crystal Violet.

Thus, the dye Acridine Orange stains appreciably

also the C- and H-fibres. To some extent also Para-

rosaniline and Madder Red do this. On the other

hand, Ethyl Rhodamine B, Straw Yellow, Ethyl

Eosin, and Auramine O stain C-fibres to a small

extent, whereas Straw Yellow and Auramine O stain

to some extent the H-fibres.

The question is where these dyes’ properties stem

from.

All these dyes, except (possibly) Ethyl Eosin, carry

a mono-positive charge. But also at pH 5 Ethyl Eosin

can be to a small degree in its cation form. Therefore,

Fig. 2 Molecular structures and notations of methyl deriva-

tives of Thionine under investigation. Nitrogen that is part of

aromatic ring is marked as N1

Fig. 3 Molecular structures and notations of methyl deriva-

tives of Para-rosaniline under investigation. The triphenyl-

methane carbon is marked as M

1142 Cellulose (2011) 18:1139–1150

123

being in the cation form cannot be the reason for the

dyes’ different affinity towards different types of fibers

(obtained from the same delignified source).

Correlation with the number of functional groups

in the dyes in question obtained in the present study

indicates some relationship of staining the L-fibres as

well as the L–H and L/H values with the number of

dialkylamino groups (r * 0.7).

Isolated molecules quantum mechanical data

To answer which atoms or groups in the dyes,

effective in lignin staining, are responsible for this,

one needs to consider the charge distributions in the

dyes from Table 1, which are presented in Table 2.

Table 2 summerizes the results obtained by the

HF/cc-pVDZ method. Mean charge on carbons in CH

groups that are part of aromatic system (C Ar*) (and

to the surroundings) is low and in any case \0.1

charge units. The charge on nitrogen atoms depends

on their position and substitution. Nitrogen atoms

which are part of the aromatic system (N1 in AdO,

AzB, MB, SO, and SY; N2 in SY) bear a partial

charge of -0.23 to -0.40 units. Amino nitrogens

(ArNH2 in pR and SO) have the charge of about

-0.16, alkylamino nitrogen in AzB about -0.34,

whereas the dialkylamino nitrogens (ArNR2 in AdO,

AzB, MB, CV, AuO, ER) about -0.6. Oxygen atoms

which are part of the aromatic system (O1 in ER and

EE) bear a partial charge of about -0.4. Phenolic

oxygen atoms involved in intramolecular hydrogen

bonds with the azo group (in MR and SY) have the

charge of about -0.25, the free ones (ArOH in EE0

and EE?) about -0.2, whereas those in aryl methoxy

groups (ArOR in MR) about -0.47. In aryl-carbox-

ylic groups, the carbonyl atoms (C=O(O)) have the

charge about -0.34, the OH oxygens about -0.24,

whereas in the ester groups derived from them the

carbonyl atoms have the charge of about -0.35 to

-0.4 and the OR oxygens about -0.45.

In the cation of Crystal Violet, for example, which

is highly selective for lignin (Drnovsek and Perdih

2005a), the nitrogen atoms have a partial negative

charge of -0.609 by HF/cc-pVDZ. In the cation of

Para-rosaniline, which is less selective for lignin and

stains appreciably also hemicellulose and cellulose,

the partial charge on the amino nitrogen is -0.164.

The difference of local charges in pairs of dyes

having different number of alkyl groups on nitrogens

resp. oxygens attached to the otherwise identical

aromatic structure is presented in Table 3.

The dyes which stain lignin more effectively

(Drnovsek and Perdih 2005a) have in fact more

negatively charged amino or carboxylic substituents

Table 1 Experimental selectivities for lignin

Dyes C.I. Selectivity

(L–H) 9 103 L/H (L–H) 9 103/(L/H)

Crystal Violet 42555 2,818 2,820 1.00

Para-rosaniline* 42500 1,081 20 53.24

Methylene Blue 52015 1,530 1,530 1.00

Azure B 52010 1,271 1,270 1.00

Ethyl Rhodamine B 45175 1,685 1,690 1.00

Ethyl Eosin 45386 24 25 0.96

Auramine O 41000 290 146 1.99

Safranine O 50240 468 499 0.94

Acridine Orange* 46005 1,309 2.3 573.92

Madder Red* 12245 570 26 22.11

Straw Yellow* 12719 87 23 3.82

From Table 3 in Drnovsek and Perdih (2005a)

C.I. Colour Index Constitution Number (Colour index 1971)

L kraft softwood pulp, H pulp L treated with KMnO4 (Drnovsek and Perdih 2005a)

* These dyes stain to some extent also hemicellulose and cellulose

Cellulose (2011) 18:1139–1150 1143

123

Ta

ble

2C

alcu

late

dM

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SY

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r*0

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60

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t 2-

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sin

cati

on

1144 Cellulose (2011) 18:1139–1150

123

than their less effective counterparts, whereas the

differences in charges of their aromatic systems are

small. The higher negative charge is in all cases the

consequence of the alkyl substitution of those func-

tional groups. The more effective dyes have by [0.2

charge units per atom more negative functional

groups (or [0.1 unit per free electron pair).

In N-methylated Thionine (Fig. 4) with increasing

methyl substitution of amino nitrogens, the partial

positive charge on the ring sulphur and on ring carbons,

as well as the partial negative charge on the ring

nitrogen negligibly (by around 0.01 resp. 0.001 units)

decreases, whereas the charge on amino nitrogen

decreases by approx. 0.2 units per the first methyl

group added to it and by approx. 0.25 units per the

second methyl group added to it. The charge on the

other amino nitrogen is only slightly (by a few 0.001

units) influenced by substitution of the former one.

The charge on the ring nitrogen is more negative than

on the nitrogen of the amino groups, whereas it is less

negative than on nitrogens of the methylamino and

especially dimethylamino groups. Thus, if besides the

overall positive charge of the dye molecule the charge

on amino groups is responsible for the affinity for

lignin, then the ring nitrogen would contribute to it

more than an amino nitrogen. This would explain the

affinity of Thionine in Drnovsek and Perdih (2005a).

In N-methylated derivatives of Para-rosaniline

(Fig. 5) the charge on amino nitrogen decreases by

approx. 0.2 units per the first methyl group added to it

and by approx. 0.25 units per the second methyl group

added to it, as well. The charge on another amino

nitrogen is only slightly (by about 0.001 unit) influenced

by substitution of the former one. In the aromatic

system, the carbons affected by methylation of the

amino groups are first of all the carbons bearing them

(by approx. 0.0045 units), followed by those carbons

connected to them. Thus, also in these cases methylation

of amino nitrogens enhances the local negative charge

on them much more than elsewhere in the molecule.

In quaternary ammonium substituents observed in

the dyes Madder Red and Straw Yellow, which are

formally monopositively charged, the nitrogens carry

by the HF/cc-pVDZ method a partial negative charge

of about -0.53 (and by B3LYP/cc-pVDZ about

-0.28). They are, however, sterically inaccessible

and this may be one of the reasons for the low

staining ability of these dyes.

Table 3 Calculated differences in Mulliken charges of atoms

between dyes of similar structure containing different numbers

of alkyl groups on N resp. O

Dyes* Atom

C Ar* N N0 N1 S

AdO-PF -0.032 20.461 0.001

MB-Th -0.053 20.463 -0.004 -0.018

MB-AzB -0.015 -0.001 20.254 -0.002 -0.001

CV-pR 0.020 20.445

N N0 O1 (CO)O-R

RB-ER 0.004 -0.033 0.019 -0.001 20.220

O O0

EE-–E- -0.002 -0.018 -0.012 -0.001 20.219

EE0–E0 -0.001 -0.007 -0.012 -0.002 20.211

EE?–E? 0.003 -0.010 -0.004 -0.004 20.192

Method: HF/cc-pVDZ. For structures and atom notation see

Fig. 1

Dyes*: Dye pairs of the same molecular backbone structure

differing only in number of N-alkyl or O-alkyl substituents

Bold Differences in Mulliken charges of functional groups

between two dyes of similar structure that seem important for

the effect of staining lignin containing fibres

C Ar*: mean charge on carbons in CH groups that are part of

aromatic system (and exposed to the surroundings)

S, N1, O1: charge on S or N or O atom which is part of the

aromatic system

N, N0, O, O0: different (non-equivalent) amino nitrogens resp.

hydroxy oxygens in the same molecule

EE-, E-—monoanion, EE0, E0—neutral, EE?, E?—cation

-0.6

-0.4

-0.2

0

0.2

0.4

0,0 1,0 1,1 2,0 2,1 2,2

par

tial

ch

arg

e

S

C Ar*

NH2

N1

NHMe

NMe2

HF

Fig. 4 Calculated partial charges on functional groups in

isolated cations of methylated Thionine. Method: HF/cc-

pVDZ. The numbers on x-axis represent the number of methyl

groups on amino nitrogen: 0,0—Thionine; 1,0—Azure C;

2,0—Azure A; 2,1—Azure B; 2,2—Methylene Blue. For

structures and atom notation see Fig. 2. C Ar*—mean charge

of carbons in CH groups that are part of aromatic system (and

exposed to the surroundings)

Cellulose (2011) 18:1139–1150 1145

123

Somewhat similar is the situation in Safranine O,

where the quaternary nitrogen is part of the aromatic

system. Although Safranine O has a charge of only

-0.159 on its –NH2 groups, which is similar to the

charge of -0.164 in the less selective Para-rosaniline,

the presence of the aromatic nitrogen being charged

by -0.581 (N2) contributes to its higher staining

ability.

The nitrogens of the azo groups do not seem to

contribute much to the staining affinity, especially if

involved in the intramolecular hydrogen bonds. This

is in line with the ineffectivity of oil-soluble non-

ionic azo dyes (Drnovsek and Perdih 2005a), where

additional methoxy groups have a much more

pronounced effect than the azo groups.

Whereas Fluorescein has no affinity for lignin and

Eosin a negligible one, Ethyl Eosin has a noticeable

affinity, although it is low compared to those of

dialkylamino groups containing cationic dyes

(Drnovsek and Perdih 2005a). Eosin and Ethyl Eosin

differ on the one hand in that Eosin has a free

carboxylic group whereas in Ethyl Eosin it is

esterified and carries a higher negative partial charge,

which is especially pronounced in its cationic form.

On the other hand, it can be concluded from their

chromatographic behavior (Perdih 1990) that at the

same pH the Ethyl Eosin has a higher tendency to be

in the cationic form than Eosin. Both of these effects

are in favour of the affinity of Ethyl Eosin for lignin.

Among the tested Rhodamines this effect is less

pronounced since both Rhodamine B and its deriv-

ative Ethyl Rhodamine B have two diethylamino

groups, which have a major contribution to the

affinity for lignin.

An arylmethoxy oxygen has an almost twice as

great partial charge density as the corresponding

phenolic oxygen. This seems to be the reason for the

affinity for lignin of some oil soluble dyes (Drnovsek

and Perdih 2005a) as well as of Madder Red, Table 3.

The correlation between L–H representing the

experimental data, Table 1, and the sum of calcu-

lated charges per free electron pair of heteroatoms

on the periphery of the dye molecule, is presented in

Fig. 6. The correlation is quite good (B3LYP:

R = 0.9696; HF: R = 0.9768). This indicates that

the partial negative charge on some substituents on

the periphery of cationic dyes is important for the

selectivity of tested dyes for lignin. It also indicates

that this partial negative charge is not the only

factor affecting it.

Dye aggregates

Since some cationic dyes adsorbed on hemicellulose

(and lignin) containing cellulose fibres indicate their

self aggregation both in the applied solution as well

as on fibres (Peterlin et al. 2009), we surveyed the

available crystallographic data about these dyes since

the aggregated molecules are the basis of the crystals.

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0,0,

0

1,0,

0

1,1,

0

1,1,

1

2,0,

0

2,1,

0

2,1,

1

2,2,

0

2,2,

1

2,2,

2

par

tial

ch

arg

e

M

av. 4

av. 2,6

av. C Ar

av. 3,5

NH2

av. 1

NHMe

NMe2

HF

Fig. 5 Calculated partial charges on functional groups in

isolated cations of methylated Para-rosaniline. Method: HF/cc-

pVDZ. The numbers on x-axis represent the number of methyl

groups on amino nitrogen: 0,0,0—Para-rosaniline; 2,2,2—

Crystal Violet. For structures and atom notation see Fig. 3.

M the triphenylmethane carbon, av. average values of charges

of carbon atoms, 1 bound to M, 2,6 bound to 1, 3,5 bound to

2,6, and 4 bound to 3,5 and N. av. C Ar average charge on

carbon atoms in the aromatic system

-2

-1.5

-1

-0.5

0

0 0.5 1 1.5 2 2.5 3

L-H

sum

of

par

tial

ch

arg

es B3LYP

HFSYAuO

SO

AzB

AdO

MB

ER

CV

Fig. 6 Relation between L–H (difference of reflectance of a

lignin containing and lignin free fibre) and the sum of

calculated charges per free electron pair of heteroatoms on

the periphery of the dye molecule. Abbreviations see Fig. 1.

B3LYP: R = 0.9696, S = 0.063, F = 94.2. HF: R = 0.9768,

S = 0.104, F = 125.1. Correlation between HF and B3LYP

data is: R = 0.9976, S = 0.033, F = 1,253.5

1146 Cellulose (2011) 18:1139–1150

123

From the crystal data about the Acridine Orange

cation (Obendorf et al. 1976; Reddy et al. 1979;

Mattia et al. 1984, 1995; Kuban et al. 1985; Gordon

et al. 2003), the Proflavine cation (Jones and Neidle

1975; Shieh et al. 1980, 1982; Swaminathan et al.

1982; Aggarwal et al. 1984; Ara and El Bahij 2004;

Varga et al. 2007), Methylene Blue (Marr et al. 1973;

Kahn-Harari et al. 1973; Endres et al. 1977; Snaa-

thorst et al. 1981; Enescu et al. 2000; Sours et al.

2002; Kavitha et al. 2004; Li et al. 2005; Soneta and

Miyamura 2006; Raj et al. 2007), Eosin (Harrison

et al. 2007), Rhodamine B (Fujii et al. 1995;

Nishikiori and Fujii 1997; Soneta et al. 2006), Ethyl

Rhodamine G (Wang et al. 1997; Fun et al. 1997; Liu

et al. 1998; Adhikesavalu et al. 2001; Zhang et al.

2001), Para-rosaniline (Koh and Eriks 1971; Liu et al.

2002; Xie 2008), and Crystal Violet (Spangler et al.

1989; Lovell et al. 1999; Lacour et al. 2002; Soneta

et al. 2006), as well as the substituted 1-phenyl-azo-2-

naphthols (Grainger and McConnell 1969; Olivieri

et al. 1989; Gilli et al. 2005; Schmidt et al. 2008;

Guggenberger and Teufer 1975; Whitaker 1977,

1978; Salmen et al. 1988; Diamantis et al. 1992;

Liu et al. 1997, 2005; Yatsenko et al. 2001; Gilli et al.

2002; Schmidt et al. 2007) we can conclude that the

dye’s stacking in aggregates is very variable and in

general quite adaptable to its micro environment. A

variety of point-to-face conformations can be

observed. They span from the parallel to the T-shaped

alignment. In the cases of parallel stackings the syn

orientation or anti orientation is observed, in which

like atoms either eclipse or remain staggered.

However, several other characteristic features can

be noticed. For example, a dimethylamino substituent

is in most cases almost coplanar to the aromatic ring

to which it is attached. It is sometimes involved in

hydrogen bonds of the C–H���X type but not of the

ArR2N���H–X type. It is usually involved in stacking

patterns as an important part of them. An arylamino

group is involved in hydrogen bonds of the N–H���Xtype and very rarely of the ArH2N���H–X one.

Concerning the effect of the ester group, there is

no useful indication in the checked crystal data. For

the effect of the methoxy group, what might be

indicative is the average inter-plain distance (A) in

the parallel parts of the stack of the 1-phenyl-azo-

naphthyl moiety in substituted 1-phenyl-azo-2-naph-

thols: 2-OMe \ 3-OMe \ H * 4-F \ 2,4-NO2 *2-F * 4-Cl \ 4-NMe2 \ 4-NO2 \ 2-NO2, Table 4.

The other stacking characteristics as well as the

ketohydrazone-azoenol tautomerism, however, vary

widely in this set of dyes, so a definitive conclusion

on this question is not yet possible.

From these crystallographic data it can be con-

cluded that the dyes which at pH 5.5 stain also

hemicellulose, i.e. Acridine Orange and Para-rosan-

iline, are N–H type donors of hydrogen bonds. The

dyes which stain lignin have adaptable patterns of the

stacking ability.

Evaluating the results of dye sorption, however,

we have to be cautious when comparing results

obtained in acidic (Drnovsek and Perdih 2005a, b, c;

Drnovsek et al. 2005) versus alkaline media (Fardim

et al. 2002; Fardim and Holmbom 2003; Peterlin

et al. 2009).

Table 4 Smallest average inter-plane distances (A) in the

parallel parts of the stack in substituted 1-phenyl-azo-2-naph-

thols sorted by the distance between two nearest 1-phenyl-azo-

naphthyl moieties

Smallest average plane distance between the

moieties

Substituenta 1-Phenyl-azo-2-naphthyl Phenyl Naphthyl

2-OMe 3.22 3.24 2.98

3-OMe 3.29 3.24 3.23

H 3.35 3.29 3.11

4-F 3.35 3.40 3.41

2,4-NO2 3.40 3.33 3.46

3.40 3.48 3.34

Avg. 3.40 3.40 3.40

2-F 3.41* 3.38* 3.42*

4-Cl 3.41 3.43 3.40

4-NMe2 3.34 3.24 3.54

3.53 3.53 3.23

Avg. 3.43 3.39 3.39

4-NO2 3.45 3.49 3.41

2-NO2 3.51* 3.50* 3.49*

X-ray structures of dyes were obtained from the Cambridge

Structural Database (CSD 2008). Calculations of distances

between parallel average inter-planes were performed using the

Mercury program (Macrae et al. 2008)

Avg. Average for that compound

* Half of the distance between two parallel planes separated by

a non-parallel planea Substituent on the 1-Phenyl-moiety

Cellulose (2011) 18:1139–1150 1147

123

Conclusion

From the experimental evidence it was known that

cationic dyes have in general higher selectivity for

lignin and hemicelluloses than neutral dyes. Since the

charge on dyes is of great importance, we chose to

study charge distribution on isolated ionic dyes using

the Mulliken population analysis. With this method,

based on electrostatic interactions, we obtained good

correlation with experimental data regarding the dyes

selectivities. Mulliken charges on isolated ionic dyes,

crystallographic data and experimentally determined

selectivity of dyes for hemicellulose resp. lignin,

indicate that for the interaction of a dye with

hemicellulose, in addition to the positive charge, the

dye’s ability to be involved in hydrogen bonding

plays an important role. For the interaction of a dye

with lignin, in addition to the positive charge, a high

partial negative charge on accessible substituents and

also its ability of stacking is important. The higher

negative charge is in all tested cases the consequence

of the alkyl substitution of the arylamino or phenolic

functional groups. The more effective dyes have

functional groups by[0.2 charge units per atom more

negative than their non-alkylated derivatives. These

findings support experimental evidence that alkyl-

ation of –NH2 or –OH groups substantially increases

dye’s selectivity for lignin and decreases dye’s ability

to stain polysaccharides.

Mulliken population analysis on isolated ionic

dyes gives us the insight into electrostatic interac-

tions, which are one among many important contri-

butions to the interactions between chemical species.

In order to include into the account also hydrogen

bonding and p–p interactions between dyes and

hemicelluloses/lignin, additional quantum-mechani-

cal investigation that includes also electron correla-

tion is necessary. Although Hunter and Saunders

(1990) in their seminal work used a very simple

model placing point charges in order to explain the

attractive interactions between molecules containing

p-systems, for accurate determination of interaction

energies high level computational approach that

includes electron correlation is needed. When going

from isolated ionic dyes properties to the interactions

between dyes and hemicelluloses/lignin, beside that

additional questions have to be solved such as:

(a) how to include into a computational consideration

polymeric molecules (hemicelluloses and lignin);

(b) how to properly describe the structure of lignin,

because of its heterogeneity and lack of a defined

primary structure; and (c) what kind of hemicellulose

and lignin models would be proper to use in order to

get reliable results. Our endeavors in this direction

are in progress.

Acknowledgments Financial support from the Slovenian

Research Agency (ARRS) through project P1-0175 is

gratefully acknowledged. We are grateful to the National

Institut of Chemistry, Ljubljana (Slovenia) for computational

facilities.

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