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Monomer exchange and concentration fluctuations in poly(ethylene glycol) monoalkyl ether/water mixtures. Toward a uniform description of acoustical spectra
No full textThe ultrasonic absorption spectra between 100 kHz and 2 GHz of a variety of poly(ethylene glycol) monoalkyl ether/water (CiEj/H2O) mixtures are discussed to indicate common trends in the nonionic surfactant solutions. The systems considered in this study include 13 surfactants with different alkyl chains and/or number of ethylene glycol groups, extending from C2E1 to C12E5. Despite a multitude of different shapes of the ultrasonic spectra, depending on the CiEj molecule, the temperature, and the mixture composition, it is found that all spectra can be analytically represented by a linear superposition of some relaxation spectral terms. Particularly interesting are the term that is related to the monomer exchange between micelles and the suspending phase and the terms that reflect the fluctuations in the local concentration near a critical demixing point. The parameters of the latter reveal a mechanism of fluctuation controlled monomer exchange. Its relaxation rate is largely controlled by diffusion; its amplitude is correlated with the monomer concentration of surfactants. The treatment includes solutions of low molecular weight CiEj (i less than or equal to 4) which are not normally considered micellar systems, for which (rather high) critical micelle concentration values may be derived from the ultrasonic absorption data
Monomer exchange and concentration fluctuations of micelles. Broad-band ultrasonic spectrometry of the system triethylene glycol monohexyl ether/water
No full textAt four temperatures between 17.5 and 40 degrees C and several surfactant concentrations, the ultrasonic absorption spectra in the frequency range from 100 kHz to 2 GHz, the sound velocities, and the shear viscosities have been measured for aqueous solutions of triethylene glycol monohexyl ether. At solute concentrations smaller than or close to the critical micelle concentration (cmc), the broad-band spectra reveal one relaxation region reflecting the formation/decay kinetics of oligomeric species. The spectra of the more concentrated solutions show two different relaxations, one subject to a small the other to a broad distribution of relaxation times. The former relaxation is due to the monomer exchange between micelles and the suspending liquid. The parameters of this process largely follow the predictions of the Teubner-Kahlweit theory, which is based on the Aniansson-Wall model of micelle kinetics. Close to the cmc, however, the principal relaxation rate shows a different behavior, which is assumed to also result from the action of oligomeric species. The relaxation time distribution is considered a consequence of fluctuations in the local micelle concentration. The second relaxation, which extends over a significantly broader frequency range, can be well represented by the Bhattacharjee-Ferrell model of critical concentration fluctuations, For the solution of critical composition, the relaxation rates of local energy fluctuations derived from the ultrasonic spectra are confirmed by the (static) shear viscosity data. We discuss these fluctuations as to be due to variations in the local concentration of micelles
Monomer exchange and concentration fluctuations in poly(ethylene glycol) monoalkyl ether/water mixtures. Dependence upon. nonionic surfactant composition
No full textThe ultrasonic absorption spectra between 10(5) Hz and 2 x 10(9) HZ. the sound velocities at different frequencies, and the shear viscosities of aqueous solutions of the following nonionic surfactants have been measured: C6E4, C6E5, C7E3, C8E4, C8E5, C10E4, and C12E5. Most CiEj/water systems have been considered at different temperatures between 12 and 40 degrees C and/or at different concentrations (0.01 mol/L less than or equal to c < 0.33 mol/L). The results are compared to those for the triethylene glycol monohexyl ether/water (C6E3/H2O) system reported previously. At solute concentrations around the critical micelle concentration the ultrasonic spectra show one relaxation term due to the formation/decay kinetics of oligomeric species. At higher surfactant content much more complicated spectra reveal the simultaneous presence of various processes. All systems are subject to local fluctuations in the micelle concentration which are described by the Bhattacharjee-Ferrell model here. The spectra for the shorter surfactants C6E4, C6E5, C7E3, and C8E5, as those for C6E3, in addition to the Bhattacharjee-Ferrell contribution exhibit a lower frequency Hill-type relaxation term which is attributed to the monomer exchange process. With the C8E4/water and the C10E4/water systems the monomer exchange equilibrium is reflected by two or one relaxation terms with discrete relaxation time, respectively. In the C12E5/water mixture, as a result of the small monomer concentration, contributions from the exchange process are missing in the spectra. The long chain micelle solutions (C8E5, C10E4, C12E5) also show contributions from an ultrahigh-frequency relaxation with relaxation time at around 0.25 ns. it is attributed to the chain rotational isomerization. The parameters of the different molecular mechanisms are discussed, particularly in view of the simultaneous action of the monomer exchange and the fluctuations in the micelle concentration. Also evaluated is the background contribution to the ultrasonic spectra, yielding a shear viscosity relaxation in the megahertz frequency range
Dynamic scaling of the critical ultrasonic attenuation in binary liquids: Evidence from broadband spectrometry
No full textBetween 200 kHz and 500 MHz, the ultrasonic attenuation spectrum of the ethanol/dodecane mixture of critical composition has been measured at various temperatures near the critical one. The measured broadband spectra are evaluated in light of the Ferrell-Bhattacharjee dynamic scaling theory. The experimental findings largely fit the theoretical spectral function if suitable crossover corrections are made using dynamic lightscattering data. The measured scaling function follows the predictions of the recent Folk-Moser renormalization-group theory even better. Combining our ultrasonic attenuation coefficient: data with density data from the literature, the asymptotic critical behavior of the thermal expansion coefficient and of the specific heat has been determined and found consistent with the two-scale factor universality. The sonic attenuation data have also been evaluated to yield the adiabatic coupling constant g of the dynamic scaling theory, The unusually small absolute value [g] =0.1 resulted, in fairly good agreement with \g\ = 0.13 as following from thermodynamic data PACS number(s): 62.60.+v, 64.60.Ht, 43.35.+d
Monomer exchange kinetics, dynamics of concentration fluctuations, and chain isomerization of nonionic surfactant/water systems. Evidence from Broadband Ultrasonic Spectra
No full textUltrasonic absorption spectra, measured between 0.1 and 2000 MHz, are discussed for a variety of poly(ethylene glycol) monoalkyl ether/water (CiEj/H2O) mixtures. Depending on the temperature, the surfactant concentration, and on the length of the hydrophobic (C-i) as well as the hydrophilic part (E-j) of the surfactant molecules, the spectra reveal a multitude of shapes. The set of spectra, however, can be consistently described con sidering (i) a relaxation term representing the monomer exchange of the micellar solutions, (ii) another one that reflects the local fluctuations in the surfactant concentration, and, with several systems, (iii) additional terms due to CiEj associations or conformational isomerizations. The parameters of these terms are discussed in the light of relevant models. Evidence is presented for a more general view of a fluctuation controlled monomer exchange mechanism that combines aspects of both theoretical models, the micelle formation/decay kinetics and dynamics of local concentration fluctuations
The binary system triethylamine-water near its critical consolute point: An ultrasonic spectrometry, dynamic light scattering, and shear viscosity study
No full textUltrasonic attenuation spectra between 100 kHz and 500 MHz, mutual diffusion coefficients and shear viscosities of the triethylamine/water mixture of critical composition have been measured at various temperatures near the critical one. The broadband ultrasonic spectra reveal two relaxation terms with discrete relaxation time and a term that is subject to a broad relaxation time distribution. The former have been discussed to be due to a protolysis reaction and a structural isomerization. The latter term has been evaluated in the light of the Bhattacharjee-Ferrell dynamic scaling theory, relating the sonic spectrum to fluctuations in the local mixture concentrations. The relaxation rate of the Bhattacharjee-Ferrell term follows power law behavior. However, its amplitude (Gamma(0)=45x10(9) s(-1)) is considerably smaller than that derived from the dynamic light scattering and shear viscosity measurements (Gamma(0)=96x10(9) s(-1)). This result is assumed to be due to a shear viscosity relaxation. Using density and heat capacity measurements from the literature, the adiabatic coupling constant g of the triethylamine/water system has been derived from the amplitude of the Bhattacharjee-Ferrell term in the ultrasonic spectra and from a thermodynamic relation as well. Again, a discrepancy is found. The ultrasonic spectra yield g=0.19, whereas g=0.98 follows otherwise. This difference in the g values is taken as an indication of the limitations of the Bhattacharjee-Ferrell model. It had been derived assuming a small amplitude in the singular part of the heat capacity, a precondition which is clearly not fulfilled with the triethylamine/water system. (C) 2002 American Institute of Physics
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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