The Kirkwood-Buff integrals of some binary aqueous alcohol mixtures are computed from the available vapor pressure measurements and compared with previous results as well as small angle neutron scattering experiments. The emphasis of the present report is on accuracy of the results that can be achieved by these two different types of measurements. This seems to be needed, mainly in view of the discrepancies between the various published results, as shown herein. It is argued that agreement in peak positions is more important than that in magnitude. In general, very good agreement is obtained by both methods, and sources of disagreements are discussed. The issue of the computer simulations of aqueous systems and the problematics related to correlations, microheterogeneity, and consequently the Kirkwood-Buff integrals are equally discussed herein.

We report on the phase transitions, solution structure, and consequent effect on the photophysical properties of poly[3-(6-trimethylammoniumhexyl)thiophene] bromide (P3TMAHT) in aqueous sodium dodecylsulfate (SDS). Polythiophene was mixed with SDS or deuterated SDS to form P3TMAHT(SDS)x complex (x = the molar ratio of surfactant over monomer units) in D2O and studied by small-angle neutron and X-ray scattering (SANS/SAXS) and optical spectroscopy. At room temperature, P3TMAHT forms charged aggregates with interparticle order. The addition of SDS eliminates the interparticle order and leads to rod-like (x = 1/5) or sheet-like polymer−SDS aggregates (x = 1/2 to 1) containing rod-like (x = 1/5 to 1/2) or sheet-like (x = 1/2 to 1) polymer associations. Partial precipitation occurs at the charge compensation point (x = 1). Ellipsoidal particles without interparticle order, reminiscent of SDS micelles modified by separated polymer chains, occur for x = 2 to 5. Free SDS micelles dominate for x = 20. Structural transitions lead to a concomitant variation in the solution color from red (P3TMAHT) to violet (x = 1/5 to 1) to yellow (x > 2). The photoluminescence fingerprint changes progressively from a broad featureless band (x = 0) through the band narrowing and appearance of vibronic structure (x = 1/5 to 1) to the return to a blue-shifted broad emission band (x = 5). The polymer stiffness reaches a maximum for x = 1, which leads to minimization of the Stokes shift (0.08 eV). This work gives fundamental information upon how surfactant complexation can influence both the solution structure and photophysical properties of a water-soluble polythiophene.