Hernán E. Ríos

Behavior of low-solubility detergents

Abstract The solubility behavior in water of different salts of cetyltrimethyl ammonium (CTA + ), such as iodide (CTAI), thiocyanate (CTASCN), and perchlorate (CTAClO 4 ), is reported. The order of solubilities is CTASCN > CTAI > CTAClO 4 . Ion-pair association was established by using the Kraus-Bray conductimetric method. Results indicate that CTAI is 50% and CTASCN is about 30% associated in water solutions. CTAClO 4 has solubility too low to measure by the conductimetric method. In the presence of CTAB and CTAOTOS micelles, the conductivity of these systems is constant as a function of detergent concentration above the micellar concentration, indicating that the dissolved monomers are incorporated into the micellar phase. This produces an increase in the micellar size and in the degree of dissociation. Finally, micellar features of the series (CTAF, CTACl, CTAB, and CTAI) can be related to the radii of the hydrated counterions.

Adsorption of Poly(Mono‐N‐Alkylmaleate‐Alt‐N‐Vinyl‐2‐Pyrrolidone) Sodium Salts at Air/Water Interface

The surface properties of poly(mono‐n‐alkylmaleate‐alt‐N‐vinyl‐2‐pyrrolidone) sodium salts derivatives with n=8, 10, 12, 16, and 18 carbon atoms in the side chain were analyzed. The surface tension behavior suggests that the adsorption at the surface is competitive with the polyelectrolyte “micellization” in the bulk. In fact, polyelectrolytes containing shorter side chains are more active at the surface than are polyelectrolytes with larger lateral chains. The contributions to the standard free energy of the adsorption process, related with their efficiency and effectiveness, were +1.07 kJ and −0.61 kJ per mole of methylene group, respectively. A positive value for the incremental free energy suggests that the gradual addition of methylene groups to the polyelectrolyte side chain actually stabilizes the polyelectrolyte in the bulk solution, whereas a negative value makes the packing process of the polyelectrolyte hydrophobic tails more spontaneous in an arrangement normal to the surface.

Surface properties of poly(N-monoalkylmaleamic acid-alt-styrene) sodium salts: effect of the molecular weight and the side chain length.

The surface properties of poly(N-monoalkylmaleamic acid-alt-styrene) sodium salts are studied as a function of the molecular weight and the size of the linear alkyl lateral chain of the polyelectrolyte. The experimental results are well described by the Gibbs-Szyszkowski treatment. Both the surface tension behavior and the standard free energy of adsorption depend on the polyelectrolyte side chain and on the average molecular weight, M(w). An M(w)-dependent contribution to the free energy of adsorption ranging from -1.21 to -1.05 kJ for mole of methylene groups is found. The area covered by monomer units increases with M(w) and the sizes of side chains are similar to those reported in small-molecule systems. The nature of the functional group amide in the side chain has practically no effect on the surface properties as compared with the ester group in this kind of polyelectrolytes.

Cooperative Association of p-Alkylbenzene Sulfonates Sodium Salts to Poly-2-(dimethylamino) Ethylmethacrylate-N-alkyl Quaternized at the Water/Chloroform Interface

The association of p-alkyl benzene sulfonate sodium salts with several poly-2-(dimethylamino) ethylmethacrylate-N-alkyl quaternized bromides at the water/chloroform interface was studied. High association percentages were found which increase with both the size of the sulfonate molecule and the length of the polyelectrolyte side chain: octyl, decyl, dodecyl and tetradecyl. The results fit well to the Hills equation for the association of anionic surfactant to polyelectrolytes. Benzene sulfonate shows an anticooperative behavior for the association, whereas 4-methyl benzene sulfonate and 4-ethylbenzene sulfonate are increasingly cooperative for the association. From the association constants, the standard free energies of transfer from water to the interfacial polyelectrolytes were determined. Their values were a linear function of the number of carbon atoms of the aliphatic residue of the benzene sulfonate molecule ranging from −15.7 kJ/mol of CH2 groups for the octyl polyelectrolyte to −18.9 kJ/mol of CH2 groups for the tetradecyl derivative. These values are similar to those reported for the association of some of these sulfonates molecules with ammonium type cationic micelles. The results for the incremental free energy of transfer by mol of methylene group from water to micelles are in the same order of magnitude as those reported for p-alkyl phenoxides and p-alkylbenzoates to hexadecyltrimethyl ammonium bromide micelles. The results shows that amphipathic counterions as small as 4-ethylbenzenesulfonate are enough to induce a cooperative effect in their association to cationic polyelectrolytes.

Effect of phenols on the potentiometric response of a nitrate-ion-selective electrode

The effect of phenols containing different electron-withdrawing substituents on the potentiometric responses of several liquid PVC membranes containing the complex trioctylmethylammonium-nitrate, TOMA(+)NO(3)(-), is analyzed. The results make it possible to separate these phenols into two groups; those phenols containing electron-releasing groups, which produce almost Nernstian slopes, and those containing electron-withdrawing substituents, which generate sub-Nernstian slopes. The highly negative standard free energy of transfer of the aryl phenolic group from water to a cationic polar head suggests that these phenols are mainly located in the membrane phase associated with TOMA(+) via a cation-pi interaction. It seems that the strength of this interaction, and hence of the nitrate dissociation, is affected by the presence of phenols in an extension which correlates well with the kind of phenol present in these membranes and, consequently, with the type of their potentiometric responses.

Maleic acid-alt-styrene copolymer as compatibilizer for poly(ethylene oxide)-poly(styrene) blends

Maleic acid-alt-styrene (MAaS) copolymer with number-average molecular weight [Mbar] n = 2500 was used as a compatibilizer in blends of poly(ethylene oxide) (PEO) and poly(styrene) (PS). PEO with weight-average molecular weight [Mbar] w = 105 (PEO100) and two PS samples with [Mbar] w = 9 × 104 and 4 × 105, respectively (PS90 and PS400, respectively) were used. A depression of the melting temperature T m of PEO in blends containing MAaS relative to pure PEO and PEO/PS blends was observed. The melting enthalpy ΔH m for the PEO/PS blends containing MAaS was lower than those of pure PEO and PEO/PS blends without compatibilizer. The crystallization kinetics of PEO and the blends were studied by differential scanning calorimetry (DSC) at different crystallization temperatures T c. Flory-Huggins interaction parameters χ12 for the blends were estimated. Their values are in good agreement with those obtained for similar systems and suggest that the free energy of mixing ΔG mix should be negative. Polarized optical microscopy shows differences in the macroscopic homogeneity of the blends containing compatibilizer that could be attributed to a compatibilization process.

Preferential Adsorption Phenomena in Polyelectrolytes in Methanol/Water Solution: Effect of the Counterion

The preferential adsorption behaviour of nitrate and chloride salts of poly(N,N-dimethyl-N-(2-hydroxypropyl)ammonium) in solutions of methanol/ water mixtures is analysed. At low methanol content both polyions show an adsorption of methanol which is 2·5 times larger for chloride polyion than that for nitrate salt. In the former case, the amount of methanol adsorption is enough to produce an important change in the average distances between charges. This change induces an anomalous behaviour of the conductimetric interaction parameter with solvent composition. This effect is not observed in the case of nitrate polyion and the trend of the conductimetric interaction parameter correlates well with the nature of the medium. This effect is attributed to the low methanol adsorption in this case.

Association of 4-n-Alkylbenzene Sulfonates with Hydrophobically Modified Poly (Diallyldimethylammonium) Chlorides

The association of poly (diallyldimethylammonium) chloride (PDDAC) hydrophobically modified with octyl, decyl, and dodecyl chlorides, with 4-n-alkylbenzene sulfonates, was studied in aqueous solution by ultrafiltration. Experimental data agree well with the Hills equation for the association process. The association degree increases with the size of the sulfonate anion and decreases as the polyelectrolyte side chains length increases. This kind of polyelectrolyte is able to form hydrophobic microdomains lowering the availability of association sites due to a hypercoiling process. The association process was, in all cases, cooperative, having a maximum with the octyl derivative. The association degrees were significantly higher than those found with PDDAC itself. The standard free-energy contribution to the association process was between −19 and −20 kJ/mol of benzene sulfonate. These values are comparable with those recently found for a family of hydrophobic cationic polyelectrolytes at the water/chloroform interface. Therefore, in terms of the association process, it does not matter if the polyelectrolyte is adsorbed at a liquid/liquid interface or if it is soluble in a pure phase. The contribution of the methylene groups was very small, ranging from −0.20 to −0.50 kJ/mol. This reveals that the main interaction governing the association is the aromatic ring–ammonium group interaction.

Poly(complexes) Stabilized by Acid-Base Interactions: Poly(4-vinylpyridine) and Poly(ethylene-co-maleic acid)

Abstract Interpolymer complex (IC) formation between poly(eth) lene-co-maleic acid) and poly(4-vinylpiridine) (P4VPy) has been observed and characterized. Data from viscosity, infrared spectroscopy and thermogravimetric analysis show the presence of an interpolymer complex depending on the composition and pH of the solution. The driving force for poly(complex) formation seems to be the specific acid-base interactions between free carboxylic groups of the copolymer and the pyridine nitrogen of P4VPy.

INTERMOLECULAR ASSOCIATION OF AMPHIPATHIC POLYELECTROLYTES IN AQUEOUS SOLUTIONS

The intermolecular association of amphipathic polyelectrolytes is studied using fluorescence and conductivity methods. This type of interaction is evidenced by the decrease in the average distances between charges as the polyelectrolyte side chain increases in length. This modify the dissociation degree and consequently, the linear charge density parameter. These distances were calculated with the Manning counterion condensation theory for the conductivity of polyelectrolyte solutions. The determination of the ratio I1/I3 of the fluorescence bands of pyrene with polymer content reveals the formation of hydrophobic microdomains at very low concentrations, smaller than the required concentration to produce a significant change in the average distances between charges. These distances and also the I1/I3 ratio vary with polymer concentration and their values were dependent on the size of the side chain in the polyelectrolyte. Finally, the experimental behavior of the solution viscosity and electrical conductivity of polyelectrolytes, which increase drastically with dilution, can be explained as a continuous change in the average distances between charges which produce conformational changes.