THE INFLUENCE OF THE NATURE OF SURFACTANTS
25
k × 103, s–1
the oil phase becomes more hydrophobic; that is, the
transition from butanol to hexanol and then to decane
evens out differences in the hydrophobic properties of
substrates and results in their localization in the same
microregion. A determining reactivity factor then
becomes the electrophilic properties of the reaction
center.
8
3
4
0
3'
A comparison of the catalytic activities of three-
component systems for substrate 3 shows that their
effect increases in the series AOT–decane–water <
SDS–butanol–water < AOT–butanol–water. This ten-
dency is disturbed for substrates 1 and 2 because of
changes in their localization, see above. According to
Table 1, both the nature of the oil phase and the struc-
ture of the surfactant influence the catalytic effect. The
replacement of decane with butanol (the transition from
theAOT–decane–water toAOT–butanol–water system)
increases kobs approximately sixfold for all the three
substrates, and reaction inhibition changes for cataly-
sis. The replacement of SDS with AOT (the transition
from the SDS–butanol–water to AOT–butanol–water
system) increases kobs by a factor of 2.7 for 3. For 2, kobs
increases 13.8 times, and with 1, the effect reaches two
orders of magnitude, which is likely to a greater extent
related to changes in the localization of the reagents
than to the replacement of the surfactant.
10
20
30
2'
120
80
40
1
2
0
10
20
30
σ
Dependences of the observed rate constant for alkaline
hydrolysis of phosphonates (1) 1, (2, 2') 2, and (3, 3') 3 in
surfactant–butanol–decane–water four-component micro-
emulsions on the σ = [butanol]/[surfactant] molar ratio;
0.01 M NaOH, 0.25 M surfactant, W = 13; surfactants:
(1−3) AOT and (2', 3') SDS.
The kinetic data on the SDS–butanol–decane–water
and AOT–butanol–decane–water four-component
microemulsions are shown in the figure. These data
were obtained at various σ values, which characterize
the content of butanol in the systems. The kinetics of
hydrolysis of phosphonate 1 only slightly depends on
the composition of the oil phase. The reactivities of
substrates 2 and 3 change differently depending on the
structure of the surfactant. On the whole, the tendencies
characteristic of three-component solutions are
retained for substrate 3, and the kobs value is somewhat
higher in the system based on AOT over the whole
range of σ variations; this value increases as the frac-
tion of butanol grows larger. The kobs value approaches
the value of the rate constant in theAOT–butanol–water
system as σ increases in microemulsions based onAOT.
Similar reasoning is valid with reference to the
kinetic behavior of substrate 2 in microemulsions based
on AOT. At a large content of decane (90 vol %), the
observed rate constant is close to its value in the AOT–
decane–water system. The kobs value, however,
decreases as the fraction of the alcohol in the oil phase
increases. At the highest σ value corresponding to
70 vol % butanol in the oil phase, kobs = 0.005 s–1,
which is much lower than kobs in the AOT–butanol–
water system (0.121 s–1).
We measured the surface potential of reverse micro-
emulsions based onAOT and SDS. For this purpose, we
studied the acid-base properties of p-nitrophenol in
these systems at various pH values and calculated K‡
by the Henderson–Hesselbach equation,
The reactivity of compound 2 in four-component
microemulsions is higher in systems based on SDS
than in systems based on AOT. In passing from the
SDS–butanol–water three-component system to the
SDS–butanol–decane–water four-component mixture,
the reactivity of phosphonate 2 changes qualitatively.
The addition of decane (30 vol %) sharply increases the
observed rate constant for the alkaline hydrolysis of 2
compared with the three-component system (from
0.008 to ~0.06 s–1), and kobs increases as the fraction of
the alcohol grows larger. It follows that the distribution
of the reagents and the ratio between the factors that
determine the catalytic effect change for phosphonate 2
pKa, obs = pH
(1)
+ log[p-nitrophenol]/[p-nitrophenolate].
The potential was calculated by the equation
pKa, obs = pKa, 0 – FΨ/2.303RT,
(2)
where pKa, 0 is the nonelectrostatic component of the
in passing from the three- to four-component micro- shift of K‡, F = 96486 C/mol is the Faraday constant,
emulsion based on SDS. and R = 8.314 J/(mol K) is the gas constant.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 81 No. 1 2007