92168.pdf

Colloid stability 239
negative, the dispersion should flocculate on cooling below the
^-temperature (entropic stabilisation).
These effects have been observed for both aqueous and non-
aqueous media and good correlation between the point of incipient
flocculation and the ^-temperature is well established^112. The
transition from stability to instability usually occurs over a very
narrow temperature range (1 or 2 K). Enthalpic stabilisation tends to
be the more common in aqueous media and entropic stabilisation the
more common in non-aqueous media. Owing to the elastic effect,
aggregation into a deep primary minimum does not take place (as is
possible with lyophobic sols) and redispersion takes place readily on
reverting to better than 0-solvent conditions,

Table 8.4 Classification of sterically stabilised dispersions and comparison of critical

flocculation temperatures (c.f.t) with theta-temperatures"^2 (By courtesy of Academic

Press Inc.)

Stabiliser

Poly

(ethylene

oxide)

Poly

(acrylic

acid)

Polyiso-

butylene

A/,710^3

10

96

1000

9.8

51.9

89.7

23

150

Dispersion

medium

0.39moldrrT^3

MgS0 4 (aq.)

0.2 mol dm~^3

HCl(aq.)

2-methyl

butane

Classification

enthalpic

entropic

enthalpic

c.f.t. /K

318 ±2

316 ± 2

317 ± 2

287 ±2

283 ±2

281 ± 1

325 ± 1

325 ± 1

0/K

315 ±3

287 ±5

325 ± 2

Several quantitative theories of steric stabilisation have been
developed over the last few decades^112 '^114 '^210 "^212.
The forces between sterically stabilised particles have been
measured with a compression cell (see page 223)^213 , and have been
shown (as expected in the light of the foregoing discussion) to be
short-range, with a range comparable with twice the contour length
of the lyophilic chains. For sterically stabilised systems the total
interaction energy can be written as

+ + (8.23)