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Introduction to SAXS at SSRL

John A Pople

Stanford Synchrotron Radiation Laboratory,
Stanford Linear Accelerator Center, Stanford CA 94309

Everything You Ever Wanted to Know About

But Were Afraid to Ask

SAXS

Scattering or Microscopy?

• Local detail

• Surface detail

• Faithfully represents
local complexities

Microscopy good for:

E.g. if objective is to
monitor the degree to
which Mickey’s nose(s)
and ears hold to a circular
micromorphology… use
microscopy not scattering

Scattering or Microscopy?

• Global parameters,
distributions; 1

order

• Different sample states

• Non destructive sample
preparation

• In-situ transitional studies

Scattering good for:

Solid

Melted & Sheared

Recrystallized

Ag-Au dealloyed in 70% HNO

0.01

0.10

1.00

10.00

100.00

1000.00

0.01

0.1

Log Q

Log I

1 min
5 min
15 min
60 min
720 min

Forming a bi-continuous
porous network with
ligament width on the
nanoscale by removing
the less noble element
from a binary alloy, in this
case Ag-Au (multiple
films for trans scattering)

200 nm

5 mins
in conc
HNO

60 mins

Complementary Scattering and Microscopy

Scattering: Neutrons or Photons?

X-rays

Neutrons

Neutron scattering: Deuteration allows species selection

X-ray scattering:
Relatively small sample quantities required
Relatively fast data acquisition times - allows time resolved
effects to be characterized

Sensitive to nuclear
scattering length contrast

Sensitive to electron density
contrast

Scattering: Neutrons or Photons?

Neutrons: Deuteration
allows species selection

Atom

Scattering length Incoherent scattering

(x 10

)

(x 10

)

-0.374

0.667

This essentially permits a dramatic
alteration to the ‘visibility’ of the tagged
elements in terms of their contribution to
the reciprocal space scattering pattern

X-ray Scattering: Transmission or Reflection?

Transmission geometry appropriate
for:

•

Extracting bulk parameters,

especially in deformation

•

Weakly scattering samples:

can vary path length

Need to be conscious of:

Constituent elements, i.e. absorption cutoffs
Multiple scattering
Area of interest: surface effect or bulk effect

linear eicosanol (C

O, MW = 298)

Study phase transitions of Langmuir
monolayers of mixed fatty alcohols in
terms of molecular branching and
surface tension

branched eicosanol (C

O, MW = 298)

increasing surface pressure: 0 to 40 mN/m

q (/nm)

Linear Eicosanol

Langmuir
trough

Gold mirror

x-ray path

Surface
tension
sensor

Rheology of Straight and Branched Fatty Alcohols

X-ray Scattering: SAXS or WAXS?

No fundamental difference in physics: a consequence of chemistry

WAXS patterns contain data
concerning correlations on an intra-
molecular, inter-atomic level

SAXS patterns contain data concerning
correlations on an inter-molecular level:
necessarily samples where there is
macromolecular or aggregate order

As synthesis design/control improves,
SAXS becomes more relevant than
ever before

X-ray Scattering: SAXS or WAXS?

Experimental consequences

WAXS: Detector close to sample, consider:

• Distortion of reciprocal space mapping
• Thermal effects when heating sample
• No ion chamber for absorption

SAXS: Detector far from sample, consider:

• Absorption from intermediate space
• Interception of appropriate q range

Face centered cubic

Recognizing Reciprocal Space Patterns: Indexing

Real

space

packing

Reciprocal

space

image

(unoriented

domains)

Body centered cubic

Hexagonal

Normalized
peak positions

≡

1; =

√

2; =

√

≡

1; =

√

4/3; =

√

8/3

≡

1; =

√

3; =

√

Recognizing Reciprocal Space Patterns:

Preferential Orientation

Real

space

packing

Reciprocal

space

image

Randomly

aligned rods

Preferentially

aligned rods

Hydrated DNA

Extracting Physical Parameters from X-ray data

I(q)

)

Extracting Physical Parameters from X-ray data

Molecular size: Radius of gyration (R

)

Guinier plot

ln I(q) / q

I(q) = I(0) exp [-q

/ 3]

Guinier region: q < 1 / R

lnI

)

Extracting Physical Parameters from X-ray data

Molecular conformation: Scaling exponent

Guinier plateau

Intermediate

region

)

ln q

Rod

Sphere

Coil in
good solvent

-1

-5/3

-4

Gradient of profile in
intermediate region
implies fractal dimension
of scattering unit

Molecular Conformation in Dentin

SAXS pattern

pulp

DEJ

John H Kinney

Department of Preventive and Restorative Dental Sciences,
University of California, San Francisco, CA 94143

Molecular Conformation in Dentin

1.2

1.4

1.6

1.8

2.2

0.5

1.5

Distance from pulp (mm)

aling

one

16G213
16G224
17G246

Shape change of mineral crystallites from needle-like to plate-like from pulp to
dentin-enamel junction (DEJ).

pulp

DEJ

1.4

1.8

2.2

0.5

1.5

Distance from pulp (mm)

Scaling exponent

needle-like

plate-like

0.2

0.4

0.6

0.8

q / nm

-1

I(q)

Control tooth

DI tooth

4 5

Dentinogenesis imperfecta (DI) teeth
shown to exhibit impaired development
of intrafibrillar mineral: characteristic
scattering peaks are absent from the
diseased tooth.

Molecular Conformation in Dentin

Extracting Physical Parameters from X-ray data

Kratky plot

Molecular conformation: Persistence length of coiled chain

I(q) q

persistence length

= 6 / (

q*)

Extracting Physical Parameters from X-ray data

)

Azimuthal profile

Molecular orientation: Orientation parameter P

(cos

)> =

∫

I(s,

) P

(cos

) sin

∫

I(s,

) sin

Normalized:

-0.5 < P

< 1

Orientation parameters: 0 < P

< 0.3

Axis of orientation

Measuring the degree and inclination of preferential molecular
orientation in a piece of injection molded plastic (e.g. hip replacement
joints). ~ 1500 WAXS patterns

Molecular Orientation in Injection Moldings

Marks the injection point

SSRL Beamline 1-4: SAXS Materials Science

shutter

beam
defining
slits

ion
chambers

sample
stage

guard
slits

CCD detector

optical rail & table

supply

X-rays