Page 250 - Introduction to Colloid and Surface Chemistry

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
2
flocculation temperatures (c.f.t) with theta-temperatures" (By courtesy of Academic
Press Inc.)
Stabiliser A/,710 3 Dispersion Classification c.f.t. /K 0/K
medium

Poly 10 0.39moldrrT 3 318 ±2
(ethylene 96 enthalpic 316 ± 2 315 3
±
oxide) 1000 MgS0 4(aq.) 317 ± 2
Poly 9.8 3 287 ±2
(acrylic 51.9 0.2 mol dm~ entropic 283 ±2 287 ±5
acid) 89.7 HCl(aq.) 281 ± 1
Polyiso- 23 2-methyl 325 ± 1
butylene 150 butane enthalpic 325 ± 1 325 ± 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
213
measured with a compression cell (see page 223) , 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)