Barbara Briggs

DIFFERENTIAL SCANNING MICROCALORIMETRIC STUDY OF VESICLES IN AQUEOUS-SOLUTIONS FORMED BY DIMETHYLDIOCTADECYLAMMONIUM BROMIDE

Differential scanning microcalorimetric data for dimethyldioctadecylammonium bromide (DOAB) in dilute aqueous solutions prepared using different protocols including an ethanol injection method are compared. Reproducible data are obtained for solutions prepared using a hot-water method. Scans for very dilute solutions show evidence for processes controlled by both inter- and intra-vesicular interactions. An extremum in the scans near 47 °C is assigned to a gel to liquid crystal transition involving local domains within each vesicle comprising ca. 130 DOAB monomers.

Micelle reorganisation characterised by differential scanning calorimetry for aqueous solutions containing hexadecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide

Over a small concentration range in the 10–2 mol dm–3 region, the differential isobaric heat capacities δCp(sln; T) of aqueous solutions containing hexadecyltrimethylammonium bromide (CTAB) show extrema near 310 K when measured as a function of temperature over the range 209 ⩽T/K⩽ 360. The temperatures corresponding to maxima in δCp(sln; T) depend on the concentration of CTAB(aq). Similar extrema are observed for CTAB solutions in D2O, but in aqueous solutions the extrema become less intense when either hexanol or KBr is added, the maxima being completely lost in the latter case. A pattern in δCp(sln; T) similar to that shown by CTAB(aq) is observed for aqueous solutions containing tetradecyltrimethylammonium bromide (TDTAB). At fixed total concentration, 0.02 mol dm–3, the temperatures at maxima in δCp(sln; T) show a simple dependence on the molar ratio CTAB to TDTAB in mixed solutions of the two surfactants. The dependence on temperature of the relaxational (configurational) contribution of the solute Cpm(sln; config) to the molar heat capacity of the solution, ΔCpm(sln; T) is analysed in terms of both single and coupled equilibria. The results are discussed in terms of changes in the shape of the micelles and in the extent of counterion binding resulting from an increase in temperature.

Transfer chemical potentials for ions, solubilities of salts and kinetics of reactions involving inorganic complex ions at ambient pressure and 298.2 K in binary aqueous mixtures containing ethanol and propan-2-ol

Solubilities of several complex salts at ambient temperature and pressure are reported for aqueous solutions and for solutions in binary aqueous organic mixtures. The organic cosolvents are ethanol and propan-2-ol. These and earlier published solubilities are analysed using the TATB assumption which sets the transfer chemical potentials of Ph4As+ ions equal to those of Ph4B– ions. The calculations yield transfer parameters for various ions including complex metal ions. Derived transfer parameters for ions are compared for a range of solvent systems comprising binary aqueous mixtures, including those systems where the organic cosolvents are methanol, ethanol, propan-2-ol and acetone. These transfer parameters are used in an analysis of kinetic data describing chemical reaction between hydroxide ions and iron(II) complex cations, [Fe(phen)3]2+ in aqueous solutions. Definitions of standard state chemical potentials for solutes and solvents are considered with reference to descriptions of composition of these systems using the mole fraction scale.

Formation of vesicular bilayers in aqueous solutions containing mixtures of dialkyldimethylammonium bromides

DSC scans for vesicles in aqueous systems produced by mixtures of dihexadecyldimethylammonium (DHAB where R = C16H33 in R2N+Me2Br−) and dioctadecyldimethylammonium bromides (DOAB where R = C18H37 in R2N+Me2Br−) show for most mixtures two melting points corresponding to gel to liquid-crystal transitions which depend on the composition of the vesicular system. The gel to liquid-crystal transitions occur over a wider temperature range than for vesicular systems prepared using a single dialkyldimethylammonium surfactant. The resulting temperature-mole fraction diagram is similar to that observed for continuous solid solutions. The overall pattern shows that in a given solution two types of domains in the vesicular bilayer systems can co-exist having slightly different compositions and hence melting temperatures.

Gel to liquid crystal transitions for vesicles in aqueous solutions prepared using mixtures of sodium dialkylphosphates (R1O)(R2O)PO2–Na+ and (R3O)2PO2–Na+, where R1= C10H21, R2= C14H29 or C18H37 and R3= C12H25, C14H29, C16H33 or C18H37

The scans recorded by differential scanning microcalorimetry (DSC) for aqueous solutions containing vesicles prepared from mixtures of two sodium dialkylphosphates are complicated where the first surfactant anion (R1O)(R2O)PO2–Na+ has alkyl groups R1 and R2 with different chain lengths, and where the second surfactant anion (R3O)2PO2– has two alkyl groups R3 with the same chain length. For mixed solutions where R1= C10H21, R2= C14H29 or C18H37 and R3= C12H25, C14H29, C16H33 or C18H37, the DSC traces can be understood in geometric terms. Where the chains can be assembled into bilayers with modest mismatch, the DSC traces show well resolved features. With increase in mismatch, the complexities of the DSC traces are consistent with the presence of bilayers in which the domains differ in composition. Poor chain packing and consequent weak intravesicular van der Waals forces between the alkyl chains favour low temperatures for gel to liquid crystal transitions.

Differential scanning microcalorimetric study of sodium Di-n-dodecylphosphate vesicles in aqueous solution

The scans recorded by differential scanning calorimetry are reported for aqueous solutions containing vesicles formed by sodium di-n-dodecylphosphate (DDP). The gel-to-liquid–crystal transition occurs near 35 °C, the melting temperature Tm. The dependences of heat capacity on temperature near Tm are analysed to yield related enthalpies of transition and patch numbers which record the number of DDP monomers which melt co-operatively. The dependences of the enthalpy of transition and patch number offer indications of vesicle size and the tightness of packing of monomers within the vesicles.

ENTHALPIES OF INTERACTION BETWEEN DIMETHYLDIOCTADECYLAMMONIUM BROMIDE VESICLES IN AQUEOUS-SOLUTION AND EITHER DIPICOLINATE OR SULFATE ANIONS

Injection of small aliquots of dipicolinate anions (sodium salt) into an aqueous solution containing dimethyl-dioctadecylammonium bromide (DOAB) vesicles is endothermic at 50 °C, becoming first more and then less endothermic. The injection process is effectively athermal for solutions containing more than equimolar amounts of DOAB and dipicolinate anions. A similar pattern is observed when small aliquots of sodium sulfate(aq) are injected into DOAB(aq). The overall patterns of enthalpy changes are attributed to the vesicle–dianion interaction which is exothermic and head-group dehydration with bromide ion displacement which is endothermic. Neverthless, a complexity emerges if the solutions include a buffer which turns out to play a less than passive role. This conclusion is supported by differential scanning microcalorimetry for DOAB(aq) in the presence and absence of HEPES buffer.

Effects of added urea and alkylureas on gel to liquid-crystal transitions in DOAB vesicles

The gel to liquid-crystal transition for vesicles in aqueous solution formed by dimethyldi-n-octadecylammonium bromide (DOAB) occurs at 44.7°C. Moreover, the shapes of the scans recorded by a sensitive DSC microcalorimeter are very similar when the vesicular solutions are prepared starting with solid DOAB and comparable amounts of either solid urea or solid alkylureas. Therefore, the DOAB vesicles in aqueous solution accommodate this class of solutes without marked changes in the melting temperature and the enthalpy of the transition. The contrast with effects of added surfactants and simple organic solutes such as THF and ethanol is particularly significant.

Phase transitions in the bilayers of vesicles formed from binary mixtures of symmetric di-n-alkylphosphates in aqueous solutions

Vesicles in aqueous solutions were prepared from binary equimolar mixtures of di-n-alkyl-phosphates (sodium and potassium), (R1O)2PO2–M+ and (R2O)2PO2–M+. When the number of carbon atoms in R1 and R2 differs by two and when R1 or R2= C12H25, C14H29, C16H33 and C18H37 the membranes undergo well defined gel to liquid crystal transitions at characteristic temperatures Tm. The recorded Tms are intermediate between the melting temperatures for vesicles prepared from the respective single di-n-alkylphosphates. Furthermore, the extrema recorded by differential scanning microcalorimetry show that the vesicle membrane is made up of domains that differ in composition. For those vesicles produced from di-n-alkylphosphates where the number of carbon atoms in R1 and R2 differs by more than two the plots recorded by the scanning microcalorimeter are complex. The scans show many extrema, suggesting that the bilayers are formed from many domains having different compositions. In all cases, the scan patterns are essentially repeated through several heat–cool–heat…cycles. The temperatures Tm are increased relative to those of the component surfactants when K+ and Na+ salts are mixed, showing that the counter cations play an important role in determining the thermotropic properties of the vesicles reflecting the importance of electrical interactions in determining the packing within the bilayers.

Hexadecyltrimethylammonium chloride and bromide in aqueous solutions containing urea and several alkylureas: A differential scanning calorimetric study

With increase to the concentration of urea over the range 0 ⩽[urea]/mol dm–3⩽ 3.0, the extremum in the differential heat capacity, δCp, of hexadecyltrimethylammonium bromide (CTAB; aq; 0.02 mol dm–3; reference, water) moves from 45 °C to lower temperatures and the associated enthalpy change characterising cluster reorganisation decreases. Addition of ethylurea to this CTAB solution produces a more dramatic shift in the extremum; the effect is even more striking when 1, 3-diethylurea is added. Addition of other alkylureas including, 1,1-diethylurea both shifts the extremum in δCp and reduces its magnitude. A similar pattern emerges for aqueous solutions containing hexadecyltrimethylammonium chloride (CTAC). The effect of these ureas on CTAB(aq) is attributed to adsorption and penetration of the alkylureas into the CTAB micelles and clusters.