The literature data bear witness to strong synergistic effects in mixed aqueous solutions of alkylamine
oxides and alkyl sulfates. Physicochemical properties of these systems depend essentially on pH, which
is due to protonation of amine oxide. Interplay of several types of intermolecular interactions including
chemical ones results in complex phase diagrams. New experimental data presented in the paper refer
to the solid−liquid equilibrium (the Krafft boundary), the composition of crystallizing complexes, and the
critical micelle concentration (cmc) in mixed aqueous solutions containing dimethyldodecylamine oxide
and sodium or magnesium dodecyl sulfate. The temperature maximum suggesting the formation of a 1:1
amine oxide−dodecyl sulfate complex is observed at the curves of dissolution temperature versus surfactant-based composition; the shape of the curves indicates that complexes of other stoichiometries can be formed
in solutions enriched with amine oxide. The dissolution temperature for the 1:1 mixture (and correspondingly,
the concentration of the complex in the solution) grows with growing acidity. In both systems under study,
the dissolution temperature becomes 48 °C at a pH of about 5 and does not change on further pH lowering.
Chemical analysis has shown that the solid phase precipitated from basic solutions contains metal ions
whereas at pH ≤ 5 the complex between the protonated amine oxide and dodecyl sulfate anion is crystallized.
The cmc value for 1:1 acidic mixtures does not depend on the cation nature and is 2 orders of magnitude
lower than that for solutions at the natural pH (which are slightly basic). The pseudophase model taking
into account the reactions of amine oxide protonation and 1:1 complex formation reproduces satisfactorily
the pH effect on the solubility diagrams in the dimethyldodecylamine oxide−sodium dodecyl sulfate−water system.
The effect of additives on the temperature of precipitation of a
nonionic surfactant from micellar solutions
is studied experimentally and on the basis of thermodynamic
relationships for the pseudophase separation
model. The derived formulas connect the decrease in the
precipitation temperature (ΔT effect)
produced
by amphiphilic additives with the composition of mixed micelles
“basic surfactant−additive” and also with
the temperature and the heat of the dissolution of the individual
surfactant. Experimental data on the
dissolution temperatures of crystalline
N-dodecanoyl-N-methylglucamine (MEGA-12) in
aqueous micellar
solutions containing additives of various chemical nature are
presented; the data for systems with alkanol
C2−C4 additives relate to a wide range of the
surfactant and alkanol concentrations. The enthalpies
of
dissolution of the crystalline MEGA-12 in water and water−alkanol
mixtures determined by the Calvet
calorimetry are given. The dependencies of the ΔT
effect on the concentrations of MEGA-12 and alkanol
(ethanol, propanol, or butanol) calculated from the model are in good
agreement with the experimental
data. The composition of mixed micelles formed by MEGA-12 and an
alkanol C2−C4 is found from the
ΔT data with the help of the model; the distribution of
the alkanols between the micellar and aqueous
pseudophases is evaluated. Nonideality of the aqueous alkanol
solutions is taken into account in the model
calculations. The results of measurements for aqueous and
aqueous−alkanol mixtures of MEGA-12
containing salt additives evidence that the addition of salts
(potassium, sodium, and ammonium chlorides,
nitrates, carbonates, and sulfates) to aqueous solutions of MEGA-12
does not change significantly the
temperature of the solid surfactant−micellar solution equilibrium,
but a pronounced decrease of the
temperature is observed when the salts are added to aqueous MEGA-12
mixtures containing alkanols,
which is in correspondence with the model predictions.
The induced smectic A phase which appears in binary liquid crystalline systems has been investigated. Phase diagrams, clearing enthalpies, heat capacities and mixing enthalpies of two binary mixtures of a terminal polar component and of a weakly polar one are presented. The influence of the alkyl chain length, steric and EDA-type interactions on the formation of induced smectic A phase are discussed.Die induzierte smektische A Phase, die in binlren flussig-kristallinen Systemen auftritt, wurde untersucht. Phasendiagramme, Klarenthalpien, Warmekapazitaten und Mischungsenthalpien von zwei biniiren Mischungen einer polaren mit einer schwachen polaren Komponente werden vorgestellt. Der EinfluB der Alkylkettenlinge, der sterischen und EDA-Wechselwirkung auf die Bildung induzierter smektischer A Phasen wird diskutiert.
The mixing enthalpies of binary mixtures of nematic liquid crystals are studied using a Calvet calorimeter. The systems studied are: 4-ethoxybenzylidene-4'-n-butylaniline-4-n-pentylphenyl-4'-methoxybenzoate and 4-n-butyl-4'-methoxyazoxybenzene-4-n-pentyl-4'-cyanobipheny~. In the second system the induced smectic A phase occurs in the medium concentration range.The results are discussed in terms of molecular properties and interactions.
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