0012-5008/05/0010- © 2005 Pleiades Publishing, Inc.0199

Doklady Chemistry, Vol. 404, Part 2, 2005, pp. 199–202. Translated from Doklady Akademii Nauk, Vol. 404, No. 5, 2005, pp. 638–641.Original Russian Text Copyright © 2005 by Fedotov, Tret’yakov, Ivanov, Kuklin, Islamov, Putlyaev, Garshev, Pakhomov.

Contemporary soil science almost does not addressthe structural organization of soil colloids. Soils aretreated in terms of a three-phase physical model,according to which solid soil particles form a porousnetwork partially filled with soil solutions [1].

It has been demonstrated [2–6] that soil solutionsare structured colloids and that they affect many prop-erties of soils. However, these studies did not concerncolloidal structures per se and structural rearrange-ments occurring in gel systems were deduced fromvariations in the properties of the soils.

Our goal in this work was to find methods withwhich we could obtain information on soil colloidalstructures. Carrying out an electron microscopic inves-tigation of soil solutions, we discovered fractal colloi-dal structures. We used small-angle neutron scatteringin our pioneering investigation of rearrangementsoccurring in the colloidal structures of air-dry soils andchanges in the fractal dimension of the colloids inresponse to water addition.

Samples were collected from high-humus layers ofa soddy-podzolic soil from the Yakhroma River flood-plain, gray forest soils from the Vladimir region,leached Kuban chernozem, and a greenhouse substrate.

The electron microscopic experiments were carriedout on a Carl Zeiss Supra 50 VP scanning electronmicroscope (SEM) equipped with an autoemissionsource operated at accelerating voltages of 3–10 kVand with an InLens secondary electron detector.

The test samples were soil solutions extracted fromthe soils at 10 atm and diluted 100- to 1000-fold. Thesolutions were applied to atomic smooth fresh fracturesurfaces of mica substrates. Carbon was sputtered on thetest samples in a Leybold Univex 300 thermal evaporator.

The fractal dimension of the test samples was deter-mined using small-angle neutron scattering. Fractal

objects are known to have specific small-angle neutronscattering patterns: log–log plots of scattering intensityversus pulse energy over a fairly wide range of pulseenergies are straight lines [7]:

(1)

For mass fractals, the

x

value, i.e., the fractal familyindex, coincides with the fractal dimension.

The measurements were carried out on a YuMOspectrometer installed at the fourth channel of an IBR-2 pulsed reactor. Since a two-detector system was used(Fig. 1), the range (as well as the dynamic range) of thescattering vector magnitude

Q

ran from 0.007 to

0.6

Å

–1

for neutron wavelengths from 0.7 to

5

Å and detector–sample distances of 3.6 and 12.97 m for the nearer andthe farther detectors, respectively [7, 8].

Test samples used in the small-angle neutron scat-tering experiments were placed in Hellma cells with auseful thickness of 2 mm; the beam size was 14 mm.The cells were mounted inside a thermobox maintainedat

25°ë

[9].Primary data processing was performed using SAS

software [10]. The data were normalized to a vanadiumreference in order to obtain the spectra on the absolutescale [11].

The test samples used in the small-angle neutronscattering experiments were air-dry soils and soils con-taining ordinary or heavy water in an amount of eighth-to nine-tenths of the least water capacity.

First, we studied the colloidal structures extractedfrom soils together with soil solutions using electronmicroscopy. The most interesting results were obtainedfor chernozem.

The electron micrographs in Fig. 2 show representa-tive colloidal structures observable in the soil solutionsextracted from chernozem. Similar structures werefound in the silt fraction of the soddy-podzolic soil.Individual colloid particles (with sizes from 50 to200 nm) are aggregated into clusters; the exterior struc-ture of these clusters is identical to that of fractal clus-ters, in particular, of clusters formed according to diffu-sion-controlled aggregation [12, 13]. We can assumethat colloidal soil solutions have a fractal structure.However, we cannot yet conclude that fractal structures

I k( ) x k.log–∼log

Fractal Structures of Soil Colloids

G. N. Fedotov*,

Academician

Yu. D. Tret’yakov**, V. K. Ivanov**, A. I. Kuklin***, A. Kh. Islamov***, V. I. Putlyaev*,

A. V. Garshev*, and E. I. Pakhomov*

Received May 30, 2005

* Moscow State Forestry University,ul. Pervaya Institutskaya 1, Mytishchi-5, Moscow oblast, 141005 Russia

** Moscow State University, Vorob’evy gory, Moscow, 119992 Russia

*** Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia

CHEMISTRY

200

DOKLADY CHEMISTRY

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2005

FEDOTOV

et al

.

exist in soil solutions or in the silt fractions of soils;such structures could have appeared during samplepreparation for the electron microscopic experiments.

We checked our assumption using small-angle neu-tron scattering. Neutron scattering patterns intrinsic tofractal objects were observed for all of the soils studied[13]; the self-similarity range averaged 1.5 orders ofmagnitude. Figure 3 displays a sample neutron scatter-ing curve for chernozem.

The results of the data processing (table) were inter-preted in terms of a relationship between the fractal

dimension of a colloidal system and the evolution of itsstructure [12, 13].

In some cases, the fractal dimensions of the soilsapproach the maximal possible value (

D

= 3). This factindicates that the colloidal fractal clusters are extremelybranched. We should note that the fractal dimension issignificantly affected by the water content of the soil(the water content is responsible for variations in thecharacteristics of colloidal soil solutions). The fractaldimension in greenhouse substrate samples decreasedfrom

D

= 2.9 to

D

= 2.55 in response to increasing

1

2

3

4

5

67

8

9

1011

1213

1415

16

17

9b

9‡ l2

l1

Fig. 1.

Block scheme of the YuMO spectrometer: (

1

) reflector, (

2

) reaction zone with a decelerator, (

3

) shutter, (

4

) exchangeablecollimator, (

5

) disc of the exchangeable collimator, (

6

) inlet to the vacuum collimator tube, (

7

) adjustable collimator, (

8

) disc ofthe adjustable collimator, (

9

) sample table, (

9a

) goniometric unit, (

9b

) thermobox, (

10

) thermostat, (

11

) sample magazine,(

12

,

14

) vanadium detector standards, (

13

,

16

) scattering detectors, (

15

) detector mask, and (

17

) detector in the direct beam.

Fractal dimensions of the colloidal components of soils

State of the soil Soddy-podzolic soil Gray forest soil Chernozem Greenhouse substrate

Air-dry – 2.90

±

0.10 2.95

±

0.10 2.90

±

0.10

H

2

O-containing 3.10

±

0.10 2.65

±

0.10 2.80

±

0.10 2.55

±

0.10

D

2

O-containing 2.75

±

0.10 – 2.75

±

0.10 2.40

±

0.10

DOKLADY CHEMISTRY

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FRACTAL STRUCTURES OF SOIL COLLOIDS 201

water content of the samples. The other soil samplesstudied in this work showed similar decreases in theirfractal dimensions as their water content increased.

In accordance with previous data [4–8], waterabsorption by soil samples increases the average dis-tance between monomeric colloid particles that consti-

tute fractal clusters. Our results show that the specificcontent of colloid particles in moist soils is lower thanin air-dry soils. The greenhouse substrate shows astronger decrease in the colloid particle content uponmoistening, apparently, due to the fact that its watercontent after moistening is almost three times that ofchernozem or the soddy-podzolic soil.

(a) (b) 200 nm2

µ

m

Fig. 2.

Micrographs of soil solution extracted from chernozem, diluted 100-fold, and applied to mica substrates. Magnification:(a)

×

30000 and (b)

×

100000.

0.1

0.10.01

1

0.01

10

100

I

, cm

–1

Intensity

Q

,

Å

–1

Scattering vector magnitude

Air-dry chernozemChernozem with

H

2

O

Chernozem with

D

2

O

Fig. 3.

Small-angle neutron scattering intensity vs. scattering vector magnitude for chernozem samples.

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Measurements of soil samples with deuterated watermake organic colloid particles and silicic acid colloidparticles invisible to this technique. The measured frac-tal dimensions for all soils decreased as a result. Thesoddy-podzolic soil experienced the greatest decrease,as expected because of the increased silicic acid contentof this soil.

In summary, we have found evidence to support theexistence of fractal colloidal structures in soils. This isdirect experimental evidence for the existence of self-organization in soils, which has been hypothesizedrepeatedly for open systems.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research (project nos. 03–04–48216, 04–04–48586, and 05–04–48655) and the Moscow Govern-ment (grant no. 1.2.24).

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