567 research outputs found
Theoretical description of mixed film formation at the air/water interface : carboxylic acids–fatty amines
Thermodynamic parameters of mixed monolayer formation of aliphatic amines CnH2n+1NH2 and carboxylic acids CnH2n+1COOH (n = 6–16) are calculated using the quantum chemical semiempirical PM3 method. Four types of mixed dimers and tetramers amine–acid are considered. The total contribution of interactions between the hydrophilic parts of amine and acid into clusterization Gibbs energy is slightly lower than the corresponding interactions for individual surfactants. It suggests a synergetic interaction between the regarded amphiphilic compounds as proved by experimental data in the literature. Two types of competitive film formation are possible: mixed 2D film 1, where the molecules of the minor component are single distributed among the molecules of the prevailing second component (mixture of components on molecular level), and 2D film 2 with a domain structure comprised of pure component “islands” linked together. The dependence of the Gibbs energy of clusterization per monomer for 2D film 1 on the component mole fraction shows that the maximum synergetic effect is typical for the case that both surfactants have the same even number of carbon atoms in the hydrocarbon chain and form an equimolar mixture. Formation of 2D film 1 is more preferable than that of 2D film 2, if the difference of the hydrocarbon chain lengths is not larger than 5 methylene units. The limiting mole fraction of carboxylic acids in such mixed monolayers is 66.7%
Cooperative effects in surfactant adsorption layers at water/alkane interfaces
In the present work, the properties of dodecyl dimethyl phosphine oxide (C12DMPO) at the water/decane interface are studied and compared with those obtained earlier at the interface to hexane. To simulate the interfacial behavior, a two-component thermodynamic model is proposed, which combines the equation of state and Frumkin isotherm for decane with the reorientation model involving the intrinsic compressibility for the surfactant. In this approach, the surface activity of decane is governed by its interaction with C12DMPO. The theory predicts the influence of decane on the decrease of the surface tension at a very low surfactant concentration for realistic values of the ratio of the adsorbed amounts of decane and surfactant. The surfactantrsquo;s distribution coefficient between the aqueous and decane phases is determined. Two types of adsorption systems were used: a decane drop immersed into the C12DMPO aqueous solution, and a water drop immersed into the C12DMPO solution in decane. To determine the distribution coefficient, a method based on the analysis of the transfer of C12DMPO between water and decane is also employed
Application of axisymmeric profile analysis for measuring surface tension at harmonic surface oscillations
Тензіометрія аналізу профілю як розвиток стандартного аналізу осесиметричної форми краплі
використана для вимірювання поверхневого натягу рідин методом форм крапель і бульбашок різних
конфігурацій. Алгоритм швидкого пошуку застосований для розрахунку ортогональної відстані
між точками експериментального профілю і теоретичним профілем. Встановлена прийнятна
стабільність результатів в діапазоні до 60% зміни об'єму краплі або бульбашки. За результатами
вимірювань оцінені параметри вязкоеластичності.Тензиометрия анализа профиля как развитие стандартного анализа осесимметричной формы
капли использована для измерения поверхностного натяжения житкостей методом форм капель и
пузырьков различных конфигураций. Алгоритм быстрого поиска применен для расчета
ортогонального расстояния между точками экспериментального профиля и теоретическим
профилем. Установлена приемлемая стабильность результатов в диапазоне до 60% изменения
объема капли или пузырька. По результатам измерений оценены параметры вязкоэластичности.The profile analysis tensiometry as a development of the standard analysis of axissymmetric drop shape
was used for measuring the surface tension of drops and bubbles of different configurations. A “Quick
Search” algorithm was applied to calculate the orthogonal distance between the experimental profile
points and the theoretical profile. An acceptable stability of the results was established in a range for up to
60% of drop/bubble volume changes. The parameters of the surface visco-elasticity were estimated on the
basis of these surface tension measurements
Effect of Impurities in Description of Surface Nanobubbles
Surface nanobubbles emerging at solid-liquid interfaces of submerged
hydrophobic surfaces show extreme stability and very small (gas-side) contact
angles. In a recent study Ducker (W. A. Ducker, Langmuir 25, 8907 (2009).)
conjectured that these effects may arise from the presence of impurities at the
air-water interface of the nanobubbles. In this paper we present a quantitative
analysis of this hypothesis by estimating the dependence of the contact angle
and the Laplace pressure on the fraction of impurity coverage at the liquid-gas
interface. We first develop a general analytical framework to estimate the
effect of impurities (ionic or non-ionic) in lowering the surface tension of a
given air-water interface. We then employ this model to show that the
(gas-side) contact angle and the Laplace pressure across the nanobubbles indeed
decrease considerably with an increase in the fractional coverage of the
impurities, though still not sufficiently small to account for the observed
surface nanobubble stability. The proposed model also suggests the dependencies
of the Laplace pressure and the contact angle on the type of impurity
β-Lactoglobulin Adsorption Layers at the Water/Air Surface: 5. Adsorption Isotherm and Equation of State Revisited, Impact of pH
The theoretical description of the adsorption of proteins at liquid/fluid interfaces suffers
from the inapplicability of classical formalisms, which soundly calls for the development of more
complicated adsorption models. A Frumkin-type thermodynamic 2-D solution model that accounts
for nonidealities of interface enthalpy and entropy was proposed about two decades ago and has been
continuously developed in the course of comparisons with experimental data. In a previous paper
we investigated the adsorption of the globular protein β-lactoglobulin at the water/air interface
and used such a model to analyze the experimental isotherms of the surface pressure, Π(c), and the
frequency-, f-, dependent surface dilational viscoelasticity modulus, E(c)f
, in a wide range of protein
concentrations, c, and at pH 7. However, the best fit between theory and experiment proposed
in that paper appeared incompatible with new data on the surface excess, Γ, obtained from direct
measurements with neutron reflectometry. Therefore, in this work, the same model is simultaneously
applied to a larger set of experimental dependences, e.g., Π(c), Γ(c), E(Π)f
, etc., with E-values
measured strictly in the linear viscoelasticity regime. Despite this ambitious complication, a best
global fit was elaborated using a single set of parameter values, which well describes all experimental
dependencies, thus corroborating the validity of the chosen thermodynamic model. Furthermore, we
applied the model in the same manner to experimental results obtained at pH 3 and pH 5 in order to
explain the well-pronounced effect of pH on the interfacial behavior of β-lactoglobulin. The results
revealed that the propensity of β-lactoglobulin globules to unfold upon adsorption and stretch at the
interface decreases in the order pH 3 > pH 7 > pH 5, i.e., with decreasing protein net charge. Finally,
we discuss advantages and limitations in the current state of the mode
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