Chemistry Reference
In-Depth Information
TABLE 6.4. Effects of Temperature on MAC of DMAB in Several Surfactant Systems
MAC (g/mol Surfactant)
————————————
Surfactant
At 30
C
At 50
C
MAC
50
/MAC
30
C
9
H
19
COO
Na
þ
0.64
1.19
1.86
C
11
H
23
COO
Na
þ
1.50
2.43
1.62
C
13
H
27
COO
Na
þ
2.71
4.15
1.53
C
12
H
25
NH
3
þ
Cl
4.32
5.63
1.30
ionic nature, etc.). Since changes in temperature are known to affect some of
those characteristics, it should not be surprising to find alterations in the
solubilizing properties of surfactants as a result of modifications in micellar
structure.
2. Changes in temperature can affect the intermolecular interactions between
solvent and solutes (e.g., hydrogen bonding), so that the overall solvent
properties of the liquid for surfactant and additive may be significantly altered.
Data illustrating the effect of temperature changes on the ability of several
alkali soaps to solubilize N,N-dimethyl-aminoazobenzene (DMAB) are listed in
Table 6.4. Interestingly, if one considers the relative change (MAC
50
/MAC
30
)
in the amounts solubilized at 30
C and 50
C, the greatest increase occurs for the
system having the lowest initial solubilizing power at the lower temperature. In a
relative sense, the effect of the temperature increase can be viewed as the poor
getting richer, and the rich not doing too badly.
In a study of the effects of temperature changes on the solubilizing power of
the nonionic surfactant CH
3
(CH
2
)
9
(OCH
2
CH
2
)
12
OCH
3
containing fixed amounts
of n-decane and n-decanol, it was found that in the case of each additive, as the
temperature was increased, the apparent micellar aggregation number increased,
as expected from results in the absence of additives (Table 6.5), and that the number
of additive molecules incorporated per micelle increased. However, examination of
the data shows that the ratio of surfactant to additive molecules in each micelle
remains constant throughout the temperature range, with values of 10 for the
n-decane solubilized in the micellar core and 2.5 for the n-decanol located in
the palisades region.
The effect of temperature changes on the micellization of ionic surfactants is not
as simple a relationship as that found for most nonionic materials, and it is to be
expected that the effects on solubilization will be correspondingly more complex. It
has been reported that micellar solutions of dodecylamine hydrochloride saturated
with xylene passed from a clear, isotropic solution to a turbid dispersion as the tem-
perature was increased. It was noted in Chapter 4 that many ionic surfactants pass
through a minimum in cmc near room temperature; it would be interesting to know
whether a maximum in solubilizing power is attained in the same temperature
region as the minimum in cmc.
