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Results of dynamic light scattering experiments are reported which demonstrate that the mutual diffusion coefficient of 2-butoxyethanol/water mixtures of noncritical composition exhibits a characteristic temperature and composition dependence: The diffusion coefficients of the two phases of noncritical composition coexisting along the liquid/liquid coexistence curve decrease by an order of magnitude in the temperature range (Tp−Tc)<10 K approaching the lower critical point (Tp, temperature of phase separation; Tc, critical temperature). In the homogeneous 1-fluid phase region of the phase diagram, the D vs (T−Tp) curves of mixtures of noncritical composition are shifted to smaller values of D with decreasing values of ‖y−yc‖ (y, mass fraction of 2-butoxyethanol; yc, critical mass fraction). These findings are discussed on the basis of the model of regular mixtures.

Dynamic light scattering experiments and measurements of surface tension and viscosity of aqueous solutions of poly(2-vinylpyridine) quaternized with ethyl bromide are carried out as function of the charge density and polyelectrolyte concentration (Mw ) 2.9 × 10 5 , Mw/Mn ) 1.11; Re, degree of quaternization, 0.09 e Re e 0.46). The polyelectrolytes are dissolved in an aqueous solution of potassium bromide (cs ) 1 × 10 -2 c + , c + ) 1 mol dm -3 ). Their concentrations c jp are in the range 0.1c j + e c jp e 27c j + (c j + ) 1 g dm -3 ). The reduced variable Λ ()Recm/cs) is used to characterize the composition of the solutions (cm, molar volume concentration of the repeat unit of the polymer backbone). Monomodal and bimodal dynamic modes characterized by the apparent diffusion coefficients Ds and Df (index s, slow; index f, fast) are found under the following conditions: monomodal, 0.03 e Λ e 0.1; bimodal, 0.1 e Λ e 8. A third dynamic mode characterized by the apparent diffusion coefficient Dint (index int, intermediate) appears at Λ J 1 with small values of Re in the range 0.09 e Re e 0.15 (Ds < Dint < Df). It is assumed that the third mode reflects dynamic properties of hydrophobic domains formed by uncharged segments of the polymer backbone at low values of Re. Hydrophobic interactions show up also in surface tension and viscosity measurements. A model proposed by Lee and Schurr (J. Polym. Sci. 1975, 13, 873) describes the experimental Df data of the fast diffusive mode with two fit parameters (Zeff, effective number of charges of a polyion; Dp, diffusion coefficient of a polyion). Zeff is smaller by a factor of 5-7 than the value calculated on the basis of Re.

In a recent publication on critical opalescence it was shown that, for a binary liquid mixture of critical concentration, measurements of the angular dependence of scattered intensity at small temperature distances above the critical temperature can be related to the range of molecular forces.' This concept has been applied to polymer solutions. The range of molecular forces and the critical temperature of solutions of polystyrene of different molecular weights in cyclohexane have been reported.2 It was found that the range of molecular forces I was strikingly small. Thus, we investigated the critical opalescence of polystyrene in ethylcyclohexane in order to see whether the smallness of 2 is characteristic only of the system polyst yrene-cyclohexane.The scattering formula of solutioiis of critical concentration at small temperature distances above the critical temperature is stated in a simplified form in eq. (1) := C(T/T,)/[(AT/TJ + (81r2/3) (Z2/X2) sin ' ( 0 / 2 ) ](1)

The water spider Argyroneta aquatica lives under water, diving to various depths from time to time. At rest, it breathes air trapped within its diving bell with a hydrophilic surface. Outside their diving bell water spiders trap air on their abdomen under a layer of hydrophobic hair. Is the structure of the layer of hair trapping a volume of air on the abdomen of the water spider Argyroneta aquatica under water related to its observed diving depth (of the order of decimetre)? A positive answer to this question is given, based on the law of Laplace in combination with information obtained from SEM- photographs of the abdomen of a water spider.

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