Conformations of hydrophobic chains at liquid/gas interface and their implications on surfactant adsorption (original) (raw)
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Surfactant Adsorption at the Solid—Liquid Interface—Dependence of Mechanism on Chain Length
The Journal of Physical Chemistry, 1964
At low concentrations, alkylammoniuni ions affect the {-potential of quartz in nearly the same way as do sodium ions; at a certain concentration of alkylammonium ions there is a sudden change in the electrokinetic potential. This has been attributed to the formation of "hemimicelles" or two-dimension aggregates of the long chain ions. As in micelle formation, the cohesive or van der Waals interaction between hydrocarbon chains reduces the work of bringing the polar groups together into aggregates at the solid-liquid interface. We have measured the l-potential-concentration curves for quartz in the presence of alkylammonium acetates of chain length from 10 to 18 carbons. From the variation with chain length of the concentration of surfactant a t zero {-potential, it is possible to determine the value of the van der Waals cohesive energy. Our value of 0.97kT or 580 cal./ mole, in good agreement with literature values, substantiates the hemimicelle hypothesis and strengthens the validity of electrokinetic techniques.
Journal of Colloid and Interface Science, 1986
Adsorption isotherm equations for surfactant or collector adsorption, such as the Stern-Langmuir equation and the regular behavior model are briefly discussed. It is concluded that for a correct description of the adsorption process, chain length effects must be taken into account far more stringently than has been the practice to date. To this end a general model for the adsorption of flexible long-chain surfactants with a charged head group is developed on the basis of the polymer adsorption theory of Scheutjens and Fleer, in which electrostatic interactions are incorporated. The general model is applied for two specific adsorbed chain conformations: fiat adsorption and end-on or perpendicular adsorption. These conformations represent the extremes for adsorption of a surfactant onto a hydrophobic and a hydrophilic surface, respectively. The isotherm equations quantitatively show how the chain length effects contribute to the adsorption free energy. In the case of pseudoideal behavior, which is observed if the solvent is poor, both equations reduce to the Stern-Langmuir isotherm. For r = 1 the equation reduces to the regular behavior model. Moreover, the general form of the equations is the same as that obtained in the case of regular behavior. The form of the master equation suggests that it will be quite useful in describing surfactant adsorption. A brief discussion is given of Traube's rule and the hemi-micelle concept in the light of the newly developed model.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1993
A comparison of experimental data with self-consistent field lattice calculations and molecular dynamics simulations has shown that the latter two approaches are able to predict in a qualitative sense the relation between the structure of a surfactant and its interfacial tension at an oil/water interface. Micelles can also be observed in the simulations and in the self-consistent field calculations. Advantages and disadvantages of the simulations and the self-consistent field calculations are discussed and it is concluded that current theoretical models provide reasonable descriptions of complex colloidal systems.
Langmuir, 2009
The effect of adding an alcohol to surfactant systems depends much on the alcohol chain length. Investigations on the effect of alcohols in micellar systems point out that medium-chain alcohols are appreciably incorporated in the micellar phase whereas short-chain alcohols are localized mainly in the aqueous phase. Nonetheless, penetration of the hydrocarbon chain of alcohols in the micellar shell has been experimentally observed for the entire homologous series of linear 1-alcohols. We present a thermodynamic model in which the alcohol molecules play two roles: cosurfactant and cosolvent. The cosurfactant effect of the alcohols is included by assuming that the alcohol molecules are nonionic surfactants. The cosolvent effect is modeled by accounting for the changes in the free energy to relocate the surfactant tail from the solvent to the aggregate core. The effects of short-chain alcohols in the macroscopic interfacial tension and dielectric constant of the solvent medium are also taken into account. For short-chain alcohols the partition coefficient of the alcohols between water and liquid hydrocarbons provides knowledge of the fraction of the molecules that participate in each function. Our proposed thermodynamic model improves the modeling of the effect of short-and medium-chain alcohols in self-assembly of molecules that are of increasing importance in modern scientific research and technological processes.
Journal of Molecular Liquids, 2021
In the present study, we investigated the effect of the number and position of functional groups, and the length of the main chain and side chain in organic surfactant on adsorption behavior and interfacial heat transfer between silica surface and alkane solvent by non-equilibrium molecular dynamics simulation, where the surfactants were primary/secondary alcohol, monohydric/dihydric alcohol, and linear/ branched alcohol. The results showed a similar adsorption behavior for all the surfactant types, where hydroxyl functional (AOH) groups adsorbed onto the silica surface and alkyl chain was in contact with the solvent, which produced a heat path from silica via surfactant to solvent. The number of adsorbed AOH groups did not directly translate to significantly decreased thermal boundary resistance due to the adsorption structure. Coulomb interaction enabled the closer distance between primary terminal AOH groups in surfactant and silica surface, which enhanced the solid-surfactant intermolecular heat transfer. However, Coulomb interaction contributed less to shorten the molecular distances between secondary AOH groups in surfactant and silica surface, which was connected to less efficient heat transfer from silica to surfactant and thereby did not enhance the interfacial heat transfer as much as surfactants with terminal AOH. The increase in terminal AOH groups in the surfactant molecules could not significantly reduce thermal boundary resistance, although the adsorption amount of AOH was distinctly greater than that of surfactants with single AOH. The side chain in surfactant enabled the efficient surfactant-solvent intermolecular heat transfer but related to the desorption of surfactant when decreasing the temperature. Thus branched-chain dihydric alcohol performed better than other surfactants on reducing thermal boundary resistance when the interfacial temperature was high enough to maintain the sufficient adsorption amount. We considered such reverse temperature-sensitive surfactant has a great potential application to fulfill multiple needs for heat dissipation of electronic devices, especially the high temperature operation. The new insights obtained in the present study were a step towards a molecular structure design of surfactant enhancing solid-liquid interfacial heat transfer.
Journal of Physical Chemistry B, 2006
Interactions between surfactants, and the resultant ordering of surfactant assemblies, can be tuned by the appropriate choice of head-and tailgroups. Detailed studies of the ordering of monolayers of long-chain n-alkanoic and n-alkanol monolayers at the water-vapor interface have demonstrated that rigid-rod all-trans ordering of the tailgroups is maintained upon replacing the alcohol with a carboxylic acid headgroup. In contrast, at the water-hexane liquid-liquid interface, we demonstrate that substitution of the -CH 2 OH with the -COOH headgroup produces a major conformational change of the tailgroup from disordered to ordered. This is demonstrated by the electron density profiles of triacontanol (CH 3 (CH 2 ) 29 OH) and triacontanoic acid (CH 3 (CH 2 ) 28 COOH) monolayers at the water-hexane interface, as determined by X-ray reflectivity measurements. Molecular dynamics simulations illustrate the presence of hydrogen bonding between the triacontanoic acid headgroups that is likely responsible for the tail ordering. A simple free energy illustrates the interplay between the attractive hydrogen bonding and the ordering of the tailgroup.
The Journal of Chemical Physics, 2001
Molecular dynamics simulations are used to study the formation of gaseous and liquid expanded phases of surfactants on a liquid/vapor interface. Both insoluble and soluble surfactants are considered, modeled as freely jointed chains in a monatomic solvent with appropriate Lennard–Jones interactions. For both insoluble and soluble cases our results indicate that the surface tension as a function of coverage shows a plateau close to the clean interface value until a critical surface concentration, beyond which the surface ...
Langmuir, 2004
The self-assembly of nonionic surfactants in bulk solution and on hydrophobic surfaces is driven by the same intermolecular interactions, yet their relationship is not clear. While there are abundant experimental and theoretical studies for self-assembly in bulk solution and at the air-water interface, there are only few systematic studies for hydrophobic solid-water interfaces. In this work, we have used optical reflectometry to measure adsorption isotherms of seven different nonionic alkyl polyethoxylate surfactants (CH3(CH2)I-1(OCH2CH2)JOH, referred to as CIEJ surfactants, with I) 10-14 and J) 3-8), on hydrophobic, chemically homogeneous self-assembled monolayers of octadecyltrichlorosilane. Systematic changes in the adsorption isotherms are observed for variations in the surfactant molecular structure. The maximum surface excess concentration decreases (and minimum area/molecule increases) with the square root of the number of ethoxylate units in the surfactant (J). The adsorption isotherms of all surfactants collapse onto the same curve when the bulk and surface excess concentrations are rescaled by the bulk critical aggregation concentration (CAC) and the maximum surface excess concentration. In an accompanying paper we compare these experimental results with the predictions of a unified model developed for selfassembly of nonionic surfactants in bulk solution and on interfaces.
Adsorption of interacting long-chain surfactant molecules: Isotherm equations
Journal of Colloid and Interface Science, 1988
A recently introduced lattice model for the adsorption of long-chain flexible ionic surfactants is briefly explained and generalized to describe the adsorption of both ionic and nonionic surfactants. To obtain analytical isotherm equations the general model is simplified by considering a stepwise segment density profile and a conformation in which m of the r segments are thought to be adsorbed in the first layer, whereas the remaining r -m segments protrude in the solution phase as a tail. Segment-segment and segment-solvent interactions are incorporated using the Flory-Huggins approach. The isotherm equations derived for both types of surfactants are very similar. Apart from the adsorption energy they show the effects of train size, chain length, solvent quality, and the specific nature of the head segments. Approximate expressions are given for the conformational entropy loss upon adsorption. For chains of one segment the equations reduce to the classical models. The isotherm equation for nonionic surfactants is an improvement of Kronberg's adsorption model.
1997
In the present report molecular dynamics (MD) simulations are used to study the dependence of the superficial energy and entropy of a model heptane/water system as a function of surfactant concentration. For that purpose the total energy of three model cells representing the heptane and water surfactant solutions, and the heptane/water interface, had been followed as a function of temperature for different nonylphenol triethoxylated concentrations. It was found that the surface free energy changes linearly with temperature but presents a minimum with respect to surfactant concentration. That minimum has been studied under the scope of a simple theoretical model which was previously developed to relate molecular structure to interfacial properties. The minimum value of the interfacial energy is caused by optimum surfactant-solvent interaction energies. These energies account for a decrease of the interfacial tension with respect to surfactant concentration at constant temperature and influence its reduction with respect to temperature at constant surfactant concentration. As expected however, detailed variation of the interfacial tension for temperatures close to the solvent boiling points cannot be reproduced using constant density models, neither for the clean heptane/water system nor for the ternary heptane/water/ surfactant system. For this last case, the appropriate consideration of the surfactant excluded volume was found to be very important. The effects of excluded volume corrections for the adequate MD simulation of surfactant molecules at interfaces within the present framework are separately discussed in the second part of this series.