Lower critical solution temperature (LCST) and theta temperature of aqueous solutions of nonionic surface active agents of various polyoxyethylene chain lengths
Abstract:Cloud points of aqueous solutions of homogeneous poly(oxyethylene)dodecyl ether derivatives (C12(OE)n : 1"/= 2-8) and the apparent theta temperature TO ap were determined from the abrupt changes in optical transmittance and the temperature dependence of the second viriai coefficient obtained by light scattering measurements. It was found that the lower critical solution temperature (LCST) shifts to a lower temperature and lower concentration as the number of oxyethylene units in a molecule decreases. Because o… Show more
“…In particular, for j = 6, 7, and 8, the narrow temperature range over which the relative variance changes rapidly from 0 to 0.5 corresponds to a sphere-to-cylinder micellar shape transition. The experimentally determined shape transition temperatures (indicated by the various arrows in Figure ) are 16, 34, and 50 °C for C 12 E 6 , C 12 E 7 , and C 12 E 8 , respectively. As can be seen, the theory is capable of predicting the micellar shape transition behavior quite accurately. 5 Predicted relative variance of the micellar size distribution of C 12 E j ( j = 5, 6, 7, and 8) micelles in aqueous solution as a function of temperature (solid lines).…”
Section: Resultsmentioning
confidence: 98%
“…As can be seen, the theory is capable of predicting the micellar shape transition behavior quite accurately. 5 Predicted relative variance of the micellar size distribution of C 12 E j ( j = 5, 6, 7, and 8) micelles in aqueous solution as a function of temperature (solid lines). The arrows denote the experimentally determined shape transition temperatures for j = 6, 7, and 8. 54 .…”
The McMillan−Mayer theory of multicomponent solutions is
utilized to formulate a statistical-thermodynamic description of surfactant solution behavior from which
quantitative predictions of micelle
formation, micellar size distribution, and micellar solution phase
separation can be made. Specifically,
a model is constructed for the Gibbs free energy of the micellar
solution, which is divided into ideal and
excess contributions. The advantage of this approach is that it
enables a systematic analysis of various
models of intermicellar interactions. In this paper, we focus on
micelles of nonionic surfactants which
exhibit both repulsive and attractive interactions. The repulsive
interactions are described using excluded-volume considerations, while the attractive ones are modeled using a
mean-field description. Utilizing
this statistical-thermodynamic framework, expressions for the chemical
potentials of each of the solution
components are obtained and used, along with the principle of multiple
chemical equilibrium, to calculate
the micellar size distribution and its moments. An analysis of the
effect of excluded-volume interactions
on the monomer and micelle concentrations and on the weight-average
aggregation number of micelles
which exhibit one-dimensional (cylindrical) growth indicates that these
steric interactions promote micelle
formation and growth. Interestingly, in the limit of extensive
cylindrical micellar growth, we recover the
well-known expressions for the micellar size distribution and its
moments corresponding to the popular
phenomenological “ladder model”, with modified “ladder model”
parameters which are explicit functions
of the excluded-volume parameters. In addition, quantitative
predictions of the critical micellar
concentration, the polydispersity of the micellar size distribution,
and phase separation characteristics are
presented and found to compare favorably with available experimental
data for aqueous micellar solutions
of alkyl poly(ethylene oxide) nonionic surfactants.
“…In particular, for j = 6, 7, and 8, the narrow temperature range over which the relative variance changes rapidly from 0 to 0.5 corresponds to a sphere-to-cylinder micellar shape transition. The experimentally determined shape transition temperatures (indicated by the various arrows in Figure ) are 16, 34, and 50 °C for C 12 E 6 , C 12 E 7 , and C 12 E 8 , respectively. As can be seen, the theory is capable of predicting the micellar shape transition behavior quite accurately. 5 Predicted relative variance of the micellar size distribution of C 12 E j ( j = 5, 6, 7, and 8) micelles in aqueous solution as a function of temperature (solid lines).…”
Section: Resultsmentioning
confidence: 98%
“…As can be seen, the theory is capable of predicting the micellar shape transition behavior quite accurately. 5 Predicted relative variance of the micellar size distribution of C 12 E j ( j = 5, 6, 7, and 8) micelles in aqueous solution as a function of temperature (solid lines). The arrows denote the experimentally determined shape transition temperatures for j = 6, 7, and 8. 54 .…”
The McMillan−Mayer theory of multicomponent solutions is
utilized to formulate a statistical-thermodynamic description of surfactant solution behavior from which
quantitative predictions of micelle
formation, micellar size distribution, and micellar solution phase
separation can be made. Specifically,
a model is constructed for the Gibbs free energy of the micellar
solution, which is divided into ideal and
excess contributions. The advantage of this approach is that it
enables a systematic analysis of various
models of intermicellar interactions. In this paper, we focus on
micelles of nonionic surfactants which
exhibit both repulsive and attractive interactions. The repulsive
interactions are described using excluded-volume considerations, while the attractive ones are modeled using a
mean-field description. Utilizing
this statistical-thermodynamic framework, expressions for the chemical
potentials of each of the solution
components are obtained and used, along with the principle of multiple
chemical equilibrium, to calculate
the micellar size distribution and its moments. An analysis of the
effect of excluded-volume interactions
on the monomer and micelle concentrations and on the weight-average
aggregation number of micelles
which exhibit one-dimensional (cylindrical) growth indicates that these
steric interactions promote micelle
formation and growth. Interestingly, in the limit of extensive
cylindrical micellar growth, we recover the
well-known expressions for the micellar size distribution and its
moments corresponding to the popular
phenomenological “ladder model”, with modified “ladder model”
parameters which are explicit functions
of the excluded-volume parameters. In addition, quantitative
predictions of the critical micellar
concentration, the polydispersity of the micellar size distribution,
and phase separation characteristics are
presented and found to compare favorably with available experimental
data for aqueous micellar solutions
of alkyl poly(ethylene oxide) nonionic surfactants.
“…Many amphiphilic oligo(ethylene glycol) derivatives show phase separation upon heating. These include nonionic surfactants, [5] oligo(ethylene glycol) methacrylate polymers, [6,7] as well as more complex architectures such as amphiphilic dendrons, [8] dendrimers, [9] and branched amphiphiles bearing oligo(ethylene glycol) chains. [10] Many of these structures are more hydrophobic than PEG, resulting in lower cloud points than that of PEGs.…”
Section: Introductionmentioning
confidence: 99%
“…[18,20,21] The effect of hydrophobe content and of added salt are well-known also for non-ionic surfactants. [5,22] The effect of added salt on LCST depends on the molar mass of PEG, and hence the amphiphilic systems stabilized sterically by high-molar-mass PEG are more resistant against salting-out effect than their lowmolar-mass counterparts. [20] The results above indicate that also the number of arms has a similar effect, as a higher number of PEG chains provides a more effective shielding for the hydrophobic core from unfavorable interactions with the aqueous phase.…”
Amphiphilic star-shaped oligo(ethylene glycol)s with a hydrophobic bile acid core and varying number of hydrophilic arms have been made. Their thermal behavior in aqueous solutions depends on the number rather than the length of the arms. The two-armed lithocholate derivative showed the strongest tendency for association and exhibited the lowest cloud point (79 °C) of the oligomers made, as well as another phase separation at a lower temperature (31 °C). The "double thermosensitivity" arising both from the salt-dependent LCST of the oligo(ethylene glycol) segments and the temperature-responsive self-assembly of amphiphilic bile acid derivative provides an interesting path in the design of bile acid-based smart materials.
“…Extensive research on the effect of the alkyl length and the size of headgroup on phase behavior and self-assembly were carried out in the past few decades. [5,6] Particular attention was paid to the combination of C n EO m with other surfactants such as cationic and anionic surfactants.…”
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