Bergaya F. Handbook of Clay Science

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comparison with the radiation wave length (l ¼ 546 nm), P(q) is a constant E1, B the

second osmotic virial coefﬁcient, and hM

i the weight-average molecular weight of

the disper sed particles. K



is the optical constant, equivalent to the scattering length

for neutrons or X-rays and given by



¼ 2p



1

ð18Þ

where

is the refractive index of the solvent, d

n=dc the refractive index increment,

and N the Avogadro number. For interacting particles, the Rayleigh ratio can be

expressed by

Rðq; cÞ¼K



cMPðqÞGðq; cÞð19Þ

where G(q) is the interference function which depends on particle concentration. For

q-0 G(0, c) depends on the isothermal osmotic compressibility (dp/d c)

by the relation:

Gð0; cÞ¼

ðdp=dc

ð20Þ

where T is the absolute temperature, k the Boltzmann’s constant and c

the particle

number density. R

is plotted as a function of q for concentrations varying from 10 to

2.5 g/L. This method permits the molecular weight hM

i of Laponite particles to be

determined, and their diameter (ca. 25 nm) and thickness (ca. 1 nm) to be evaluated.

There is evidence to indicate that the particles are completely dispersed at these clay

concentrations.

DLS. This technique provides information on the Brownian motion in a sample by

analysing the ﬂuctuation of the scattered light intensity (Berne and Pecora, 1990).

The DLS data yield the autocorrelation function of intensity g(t) in a normalized

form, given by

gðtÞ¼

Ið0ÞIðtÞ



ð21Þ

where I(0) is the intensity measured at some arbitrary time, I(t) that after a delay of t

and hIi the time-averaged intensity. I(t) is measured at a ﬁxed angle.

gðtÞ1 ¼ e

2Dq

ð22Þ

In case of monodisperse or weakly polydisperse suspensions, g(t) is related to the

translational diffusion D (¼ kT=f ) of the particles, where k is the Boltzmann con-

stant and f (¼ 6pZR

) the collective friction factor. Z is the solvent viscosity and R

the hydrodynamic effective particle radius.

Chapter 12.5: Small-Angle Scattering Techniques896

Experiments were performed by Avery and Ramsay (1986) for three clay contents

(10, 15 and 30 g/L) and also at different times after preparation (2.5, 8 and 29 h). In

order to extract the translational diffusion D from the correlation function , ln g(t)is

plotted vs. q

t. If the particles move independently without any interaction, and

without changing the dispersion viscosity, the decay is almost linear over the whole

q-range. This was observed for the lowest clay content and after 2.5 h. For higher

contents and at longer t values, a certain deviation from linearity indicated a pro-

gressive decrease of the effective translational coefﬁcient D. These authors sugge sted

that the particles became spatially constrained in an equilibrium structure due to the

mutual interaction of their double layers.

SANS. Aqueous dispersions of Laponite containing increasing amounts of D

were studied by Avery and Ramsay (1986). The SANS spectra obtained at the lowest

clay concentration (25 g/L) followed Guinier’s law only for the smallest q,aspre-

dicted by Eq. (8a) for non-interactin g particles. The gyration radius computed from

the slope of Guinier’s plot: ln q

I(q) vs. q

, corresponded to disks with a thickness of

1 nm. Thus, the Laponite particles were well dispersed. At all other concentrations,

interaction effects were observed, conﬁrming the beginning of gelation.

Similarly, Rosta and Von Gunten (1990) investigated aqueous dispersions of Na

Laponite RD in KCl solutions at different ionic strength in the concentration range of

0.025–5.0 g/L, using static and DLS. By combining these techniques with membrane

ﬁltration and electron microscopy, they observed that the majority of the sol particles

occurred in the form of particles, composed of 2–4 parallel layers with a lateral

extension of about 30 nm and an average molecular weight of 30007500 kg/mol. The

authors also found a large deviation of the interparticle interference function G(0,c)

from 1, clearly indicating strong double-layer interactions between the particles. The

hard body and electrostatic contributions to the collective friction factor were eval-

uated by ﬁrst-order virial expressions. The electrostatic friction e

E,0

at inﬁnite di-

lution varied inversely with the concentration of an added electrolyte and was always

larger than the usual Stokes friction factor, e

, at the ionic strengths considered

(o0.0025 M). Electrostatic coupling between the colloidal particles contributes sig-

niﬁcantly to the collective friction.

Thompson and Butterworth (1992) investigated Laponi te XLG by electron mi-

croscopy, electron diffraction, ultracentrifuge analysis and SAXS. Concerning the

size and the dispersion state of the particles, they came to the same conclusions as

Rosta and Von Gunten (1990). They emphasized the necessity of careful attention to

the preparation of Laponite dispersions and the choice and control of pH.

This brief survey clearly indicates that for natural smectites (montmorillonites)

the conditions for investigating sol–gel transition by SAS techniques are relatively

restricted. Because of the high anisotropy of the clay mineral layers, the large size

distribution of particles, the determination of the smallest q values (to derive the

overall size of particles and their interaction distance range) involves making a series

12.5.2. SAS by Clay Minerals 897

of simpliﬁcations. The majority of studies on sol–gel transition mechanisms were

therefore performed with Laponite dispersions.

D. Modelling Sol– Gel Transition in Laponite Dispersions

Mourchid et al. (1995) combined USAXS, SAXS (CEA, Saclay, France) and rheo-

logy experiments on Laponite dispersions with cryofracture and ﬂow birefringence

observations. The experimental SAS spectra were compared with spectra computed

for hypothetical dispersion structures deduced from the equilibrium properties of

dispersions modelled by Monte Carlo simulations. The general purpose of these

numerical simulations was to evaluate the organization of clay particles induced by

excluded volume effects and electrostatic repulsion between the diffuse double layers.

The most interesting parts of this work are the phase diagram, (ionic strength vs. clay

concentration), describing the sol–gel transition, and the state equations (Fig. 12.5.8).

The curves of osmotic pressure vs. Laponite concentration showed a break cor-

responding to the sol–gel transition followed by a pseudo-plateau separating the

liquid and the gel phase. This was explained by an isotropic–nematic (I–N) transition

Fig. 12.5.8. Experimental equations of state for Laponite dispersions. The plots show var-

iation of osmotic pressure P with clay concentration C at different ionic strengths I of NaCl:

(K) I ¼ 10

–4

M; (

): I ¼ 5 10

–3

M; (m): I ¼ 10

2

M. From Mourchid et al. (1995).

Chapter 12.5: Small-Angle Scattering Techniques898

(Onsager transition). However, the phase separation as predicted by this theory was

not observed along the sho rt pseudo-plateaus of Fig. 12.5.8 . In explanation, the

authors admitted the possibility of a thermodynamic transition coinciding with a

mechanical phase transition resulting in a frustrated I–N transition.

Another important result, deduced from the structure simulation, was that the

most energetically favourable conﬁguration between the two ﬁrst neighbouring

Laponite particles was the perpendicular orientation without any edge-to-face con-

tact. This could be compatible with a certain number of like card-house structures

(van Olphen, 1977) observed in ﬂocculated dispersions by electron microscopy. The

USAXS spectra indicate repulsive effects between particles and the absence of in-

terparticle contacts. Dijkstra et al. (1997) also discussed a Monte Carlo model for the

structure and gelation of smectite clay dispersions. They conﬁrmed the edge-to-face

particle conﬁguration without any contacts.

Kroon et al. (1996) made a DLS study of sol–gel transition in Laponite disper -

sions at various clay mineral concentrations (1–3.5%), on both sides of the gelling

time, T

. Measurements of the static part of scattering combined with the DLS

results showed the slowing down of the dynamics of a homogeneous sol until T

Thus, the sol–gel transition threshold was marked by a drastic change in the static

part of the scattered intensity. In the gel phase, the function showed a power law

decay, with a concentration-dependent scaling exponent. The authors showed the

strong similarity with the scenarios predicted by the mode-coupling theory of the

structural glass transition. However, transition should lead to a fragile glass and

should not be far off an isotropic–nematic transition.

Using SANS (ILL, Grenoble), SALS and USAXS (CEA, Saclay), Pignon et al.

(1997) showed that Laponite XLG dispersions have fractal structures over very large

length scales when the gels are observed over time. Parallel rheometric measurements

reveal two characteristic scale lengths in the gel behaviour. The authors suggested

that the gel is composed of subunits measuring a few tens of nm. The subunits

combine to form dense aggregates measuring ca. 1 mm. At larger scale length, these

micrometer-sized aggregates form isotropic fractal arrangements of dimension D,

which increase with particle volume fraction. The isotropic fractal arrangement may

control the macroscopic mechanical behaviour. Fig. 12.5.9 illustrates the structural

evolution of a Laponite dispersion as a function of gelation time, derived from static

light scattering patterns.

Mourchid and Levitz (1998) used SALS and USAXS to study long-term gelation

of Laponite dispersions. A plot of log I(q) vs. log q showed a linear decrease with a

slope 2oao3 for the smallest q values. The authors concluded that this phenom-

enon involved a long-term mechanism leading to the formation of glassy systems

with a fractal structure. However, such structure is not observed if the starting

chemical parameters are stable during the experiment. For example, a decrease in pH

causes a release of divalent and trivalent cations, which in turn, promotes aggre-

gation. Therefore, chemical modiﬁcation of the colloid particles can play a central

role in the evolution of the system.

12.5.2. SAS by Clay Minerals 899

Pignon et al. (2000) investigated the structures of Laponite dispersions and the

deposits that form during ﬁltration at a transmembrane pressure of 0.5 bar using

SANS (ILL, Grenoble) and SALS. They compared the gel structures to the respec-

tive fractal structures of the deposits.

Levitz et al. (2000) showed that at low ionic strength (I ¼ 3.10

5

M) and low-clay

contents (0.82, 1.02 and 1.85%), the Laponite dispersions gave USAXS spectra with

correlation peaks that are compatible with long-range electrostatic stabilization.

Close inspection of these correlation peaks indicate that the individual particles are

not homogeneously dispersed in the space. Such results contrast strongly with the

behaviour at high ionic strength (I>10

4

M).

To evaluate the nematic order in dilute dispersions, Lemair e et al. (2002) per-

formed SAXS experiments on oriented samples of Laponite gel, at the European

Synchrotron Radiation Facilities (ESRF) in Grenob le. They used a 3%(w/w)

dispersion, obtained by slow evaporation of a 1% dispersion. The spectra were

recorded on a two-dimensional detector. The nematic order parameter, S, was de-

rived from the scattered intensity of an oriented sample. At a clay content of 3%, S is

similar to that found for classical liquid crystals.

Cousin et al. (2002) followed a new way to determine the gel structure of Laponite

dispersions along the transition pseudo-plateau of Fig. 12.5.8. Magnetic spherical

colloidal particles of maghemite were added to the Laponite dispersions for SANS

q (Å

-1

)

I(q)

-1.8

-3

-1

Fig. 12.5.9. Static light scattering for Laponite dispersions at different equilibrium gelation

times. Volume fraction of clay, 1.2%; ionic strength, 10

–3

M NaCl; and pH ¼ 9.5. From

Pignon et al. (1997).

Chapter 12.5: Small-Angle Scattering Techniques900

studies (ILL, Grenoble, France). The scattered intensity is given by

ðqÞ¼I

ðqÞþI

ðqÞð23Þ

where I

(q) is the total scattering by the dispersion, I

(q) the scattering by the

Laponite particles, I

(q) the scattering of the dispersed magnetic maghemite particles

and I

(q) an intensity contribution due to the coupling between Laponite and

maghemite particles.

By varying the ratio of D

O/water, the contrast between Laponite particles and

the solvent was extinguished and I

(q) was reduced to I

(q). As the maghemite

particles are spherical, the interference function G(q) was obtained from Eqs. (11)

and (12). Thus, the distributions of the maghemite particles in the gel could be

determined for different Laponite contents. Concentrations and ionic strengths

of dispersions were selected to follow the pseudo-plateau of sol–gel transition

(Fig. 12.5.8). SANS and viscosity measurements showed that the magnetic probes

were homogeneously dispersed in Laponite dispersions except in the region of the

plateau where they were enriched in domains of low Laponi te contents, indicating

that the Laponite dispersions are microscopically biphasic. The authors concluded

that this structure could be consistent with a frustrated I–N transition or a spinodal

decomposition.

DLS and shear rejuvenation experiments on a dilute Laponite dispersion (3%, w/w)

in pure water were performed by Bonn et al. (2002). Shear rejuvenation is given

as the decay of the correlation function g(t)

–1

vs. time (as measured by DLS),

when different share rates are applied to a dispersion after ageing (e.g., 1 h) for

different shearing times. The reversion of the dispersion state after shearing is com-

pared to the dispersion states previously observed after different ageing times. The

results suggest that the dynamic behaviour of such a colloid is very close to that of

aglassysystem.

The overall conclusion that can be drawn is that the sol–gel transition in Laponite

dispersions may be explained in term s of a glassy transition with some speciﬁc

characteristic probably due to particle anisotropy.

Dispersions in Organic Solvents

Ho et al. (2001) used WAXS and SANS to study the structure of organophilic clay

minerals and their interaction with various organic solvents. Both types of experiments

were performed at the NIST, NCNR (USA). Na

-montmorillonite was exchanged

with ditalloyl dimethylammonium ions, consisting of a mixture of long-chain

alkylammonium ions (ca. 65% C

,30%C

and 5% C

). The organic solvents

chosen covered a range of solubility parameters. The organo-clay samples were used as

received and after puriﬁcation. In both cases, the layers were fully exfoliated in chlo-

roform, while the particles retained their layer structure and expanded to a similar

extent in benzene, toluene and p-xylene. However, the extracted (puriﬁed) material had

a stronger tendency to gel. At the same clay concentration, the swollen particles of this

12.5.2. SAS by Clay Minerals 901

sample were also thinner, and therefore more numerous, than those of the unpuriﬁed

counterpart. The SANS curves in the lowest q-range (from 0.04 nm

–1

) followed a type

of scale law (q

a

), with a depending on the degree of dispersion, very similar to that in

aqueous dispersions. The lateral size of the organically modiﬁed montmorillonite

particles, estimated by atomic force microscopy, is in the range of 0.4–1.0 mm.

12.5.3. APPLICATION OF SAS TO PILLARED CLAYS AND

CLAY–POLYMER NANOCOMPOSITES

Some examples of pillared clay minerals and clay–polymer na nocomposites are

given, showing the different structural details that can be obtained by matching

SANS spectra, modelling SAXS curve proﬁles and by measuring SANS under shear.

A. Structure of Pillared Clay Minerals and Organo-Clays

Pinnavaia et al. (1984) studied two samples of pillared montmorillonite by SANS

(PLUTO, Harwell, UK). The samples were prepared by exchanging the interlayer

ions with Al

(Keggin) cations at 70 1C, and heating the product at 350 1C (see

Chapter 7.5). The problem was to measure the sizes of pores between the clay

mineral particles, on the one hand, and pores between the Al

cations or Al

pillars in the interlayer space, on the other. The scattering from spaces between

particles was suppressed by contrast-matching with a mixture of C

and C

The scattering from pillars could be completely suppressed by ﬁlling the interlayer

space with C

. Although the SANS patterns were not thoroughly analysed, the

authors clearly showed how the different pores types can be distinguished and how

the pillars are distributed within the interlayer space.

Steriotis et al. (2002) used SANS contrast-matching to study montmorillonite and

beidellite pillared with Mg

/Al

,Al

or Keggin cation s. They were able to

determine the distances between pillars in the interlayer space.

Hanley et al. (1997) investigated the dispersion of Laponi te in cetyl trimethyl-

ammonium bromide (CTAB) solution by SANS and DLS. The exp eriments were

performed at NIST, NCNR (USA ). The SANS patterns of the surfactant-free dis-

persion conﬁrmed that the Laponite particles were scattered independently. After an

addition of CTAB close to four times the cation exchange cap acity (CEC) of the clay

mineral, contrast matching by H–D exchange in the solvent indicated that the

CTAB–Laponite complex retained a disk morphology, despite the large excess of

CTAB. An analysis of the DLS correlation function revealed two conﬁgurations:

(i) free clay mineral particles; and (ii) CTAB-exchanged particles containing surfact-

ants in excess of the CEC, probably enough to form a stable bilayer. The effect of

CTAB concentration on aggregation of clay mineral particles, and subsequent re-

dispersion was also reported and the inﬂuence of clay minerals on possible CTAB

micelle formation was brieﬂy discussed.

Chapter 12.5: Small-Angle Scattering Techniques902

Khatib et al. (1995) characterized dimethyl dioctadecylammonium (DDA)-

exchanged montmorillonite by SAXS, and by adsorption of nitrogen and methanol.

The SAXS patterns were analysed by comparison with simulated patterns for inter-

stratiﬁcation of reacted and unreacted layers. The interlayer DDA cations were

assumed to be tilted at different angles to the silicate surface. Under methanol vapour

pressures the mobility of the alkyl chains was enhanced. After dispersion in liquid

methanol, the sample became homogeneous as indicated by a strong increase in

structural order.

B. Clay-polymer Nanocomposites

Schmidt et al. (2002) studied the effe ct of shear on the viscoelastic behaviour of

clay–polymer hybrids using Rheo-SANS. The sample was a 3% (w/w) dispersion of

Laponite LRD in a 1.5% aqueous solution of polyethylene oxide (PEO). With an

increasing shear rate an anisotropic scattering pattern was developed due to orien-

tation of clay mineral particles in the shear ﬁeld. Under shear, an unusual alignment

of clay mineral particles was observed along the ﬂow direction. The shear-induced

orientation of the macromolecules and the particles were measured in D

O. SANS

measurements of contrast-matched samples revealed the orientation of the polymer

molecules alone. As the shear rate increased, the clay mineral pa rticles orient ﬁrst,

and then the PEO chains become more and more stretched. The recovery from

anisotropy is much faster than expected from simple Brownian motion of only the

clay mineral particles in a medium of the same viscosity. This behaviour would

indicate dyn amic coupling of the polymer chains to the clay mineral.

12.5.4. CONCLUSIONS

All scattering techniques are capable of making in situ observations of ﬁnely

divided materials, in particular colloidal dispersions, without causing any damage to

the samples.

SAXS. Many SAS studies were performed by X-rays because X-r ay techniques

preceded neutron scattering and the scatte ring contrast of clay mineral layers is high

for X-rays. Unlike neutrons, X-rays do not give any background of inelastic scat-

tering at small angles. The matching method based on the anomal ous scattering near

the absorption edges is a useful feature. A number of synchrotron-radiation centres

with specially designed spectrometers were developed, making the technique avail-

able to non-specialists.

SANS. The primary advantage of neutrons is their very weak absorption by clay

mineral dispersions and powders. The samples can be as thick as 1 cm whereas they

cannot be thicker than 0.2–0.5 mm for SAXS. Thus, dry clay mineral samples can be

12.5.4. Conclusions 903

investigated by SANS, and particular SAS ex periments such as Rheo-SANS, can be

performed. The added advantage of using neu trons is the possibility to change the

scattering contrast by varying the D:H ratio in water and organic molecules.

SALS. This technique can be used to investigate dispersions at a large length scale,

and thus provide a relatively large overview of the samples than do other techniques.

But the range of samples that can be studied by SALS is limited because dispersions

have to be transparent to light. Nevertheless, the use of SALS was probed for clay

colloid studies in combination with DLS and rheological measurements. The

Fraunhofer scattering range (cf. Table 12.5.1) is interesting for granulometric meas-

urements, which can be made in conjunction with sedimentation experiments, as

illustrated by the work of Pabst et al. (2000) on kaolins.

SAXS, SANS and SALS, when used together, allow a multiscale sample explo-

ration to be done (from nanometer to several tens of micrometers) under static and

dynamic conditions. In many cases, the macroscopic behaviour of materials can be

explained on the basis of the structure of the system in the nanometer dimension.

In this non-exhaustive review of SAS techni ques we indicated the possibility and

potential of using these methods to characterize clay mineral disper sions. The sol–gel

transition in such systems, for example, is now reasonably well understood. Sample

preparation, however, must be carefully controlled if consistent results are to be

obtained, and valid data interpretation is to be achieved. SAS techniques were long

used to study aqueous colloidal dispersions of ‘‘pure’’ clay minerals. It is only rel-

atively recently that other clay mineral systems, notably organo-clays and smec-

tite–polymer composites, were investigated by scattering methods.

REFERENCES

Andrews, D.E., Schmidt, P., van Olphen, H., 1967. X-ray studies of interactions between

montmorillonite platelets. Clays and Clay Minerals 13, 321–330.

Auvray, L., Auroy, P., 1991. Scattering by interfaces: variations on Porod’s law. In: Lindner, P.,

Zemb, T. (Eds.), Neutron, X-ray and Light Scattering. Elsevier, Amsterdam, pp. 199–221.

Avery, R.G., Ramsay, J.D.F., 1986. Colloidal properties of synthetic hectorite clay disper-

sions. II. Light and small-angle neutron scattering. Journal of Colloid and Interface Science

109, 448–454.

Bacon, G.E., 1975. Neutron Diffraction. Clarendon Press, Oxford.

Bale, H.D., Schmidt, P.W., 1984. Small-angle X-ray scattering investigation of submicroscopic

porosity with fractal properties. Physical Review Letters 53, 596–599.

Ben Rhaiem, H., Pons, C.H., Tessier, D., 1987. Factors affecting the microstructure of

smectites: role of cation and history of applied stresses. In: Schultz, L.G., van Olphen, H.,

Mumpton, F.A. (Eds.), Proceedings of the International Clay Conference, Denver, 1985.

The Clay Minerals Society, Bloomington, IN, pp. 292–297.

Chapter 12.5: Small-Angle Scattering Techniques904

Berne, B.J., Pecora, R., 1990. Dynamic Light Scattering: With Applications to Chemistry,

Biology and Physics, Reprint Edition. Krieger, Malabar, FL.

Bohren, C.F., Huffman, D.R., 1998. Absorption and Scattering of Light by Small Particles.

Wiley, New York.

Bonn, D., Tanase, S., Abou, B., Tanaka, H., Meunier, J., 2002. Laponite: aging and shear

rejuvenation of a colloidal glass. Physical Review Letters 89, 015701.

Calas, G., Basset, W.A., Petiau, J., Steinberg, M., Tchoubar, D., Zarka, A., 1984. Some

mineralogical applications of synchrotron radiation. Physics and Chemistry of Minerals 11,

17–36.

Callaghan, I.C., Ottewill, R.H., 1974. Interparticle forces in montmorillonite gels. Faraday

Discussions of the Chemical Society 57, 110–118.

Carrado, K.A., Thiyagarajan, P., Song, K., Winans, R.E., 1998. Anomalous small-angle

X-ray scattering studies of interlayer heavy metal ions in clay minerals. Chemistry of

Materials 10, 1130–1134.

Cebula, D.J., Thomas, R.K., White, J.W., 1980. Small angle neutron scattering from dilute

aqueous dispersions of clay. Journal of the Chemical Society, Faraday Transactions 1 (76),

314–321.

Cotton, J.P., 1991. Introduction to scattering experiments. part I. In: Lindner, P., Zemb, T.

(Eds.), Neutron, X-ray and Light Scattering. Elsevier, Amsterdam, pp. 1–37.

Cousin, F., Cabuil, V., Levitz, P., 2002. Magnetic colloidal particles as probes for the de-

termination of the structure of Laponite dispersions. Langmuir 18, 1466–1473.

Dijkstra, M., Hansen, J.P., Madden, P.A., 1997. Statistic model for the structure and gelation

of smectite clay suspensions. Physical Review E 55, 3044–3053.

Faisandier-Cauchois, K., Pons, D.H., Tchoubar, D., 1995. Analyse de la structure multi-

chelle des dispersions de montmorillonite. PIRSEM Report, France, p. 10.

Faisandier, K., Pons, D.H., Tchoubar, D., Thomas, F., 1998. Structural organization of Na-

and K-montmorillonite suspensions in response to osmotic and thermal stresses. Clays and

Clay Minerals 46, 636–648.

Glatter, O., 1991. Small angle scattering and light scattering, part I. In: Lindner, P., Zemb, T.

(Eds.), Neutron, X-ray and Light Scattering. Elsevier, Amsterdam, pp. 33–82.

Glatter, O., Kratky, O. (Eds.), 1982. Small Angle X-ray Scattering. Academic Press,

London.

Guinier, A., Fournet, G. (Eds.), 1955. Small-Angle Scattering of X-rays. Wiley, New York.

Hall, P.L., Churchman, G.J., Theng, B.K.G., 1985. Size distribution of allophane unit par-

ticles in aqueous suspensions. Clays and Clay Minerals 33, 345–349.

Hanley, H.J.M., Muzny, C.D., Butler, B.D., 1997. Surfactant adsorption on a clay mineral:

application of radiation scattering. Langmuir 13, 5276–5282.

Hanley, H.J.M., Straty, G.C., Tsvetkov, F., 1994. A small-angle neutron scattering study of a

clay suspension under shear. Langmuir 10, 3362–3364.

Ho, D.L., Briber, R.M., Glinka, C.J., 2001. Characterisation of organically modiﬁed clays

using scattering and microscopy techniques. Chemistry of Materials 13, 1923–1931.

Hofer, M., 1991. Basic concepts in static and dynamic light scattering. Application to colloids

and polymers, part VI. In: Lindner, P., Zemb, T. (Eds.), Neutrons, X-ray and Light

Scattering. Elsevier, Amsterdam, pp. 301–324.

Keren, R., Klein, E., 1995. Sodium/calcium-montmorillonite suspension and light scattering.

Soil Science Society of America Journal 59, 1032–1035.

References 905