When Like Destabilizes Like: Inverted Solvent Effects in Apolar Nanoparticle Dispersions (original) (raw)
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Molecular dynamics simulation of the forces between colloidal nanoparticles in n-decane solvent
The Journal of Chemical Physics, 2007
Molecular-dynamics is utilized to simulate solvation forces between two nanoparticles immersed in two different solvents: Lennard-Jones spheres and and n-decane. Three different sizes and shapes of solvophilic nanoparticles are investigated. Nanoparticles in the Lennard-Jones liquid exhibit solvation forces that oscillate between attraction and repulsion as the nanoparticle separation increases. The magnitude of these solvation forces increases with particle size and depends on particle shape, consistent with the Derjaguin approximation. When n-decane is the solvent, the solvation forces are negligible for small nanoparticles, with sizes comparable to the end-to-end distance of all-trans decane. The solvation forces oscillate between attraction and repulsion for sufficiently large nanoparticles in decanehowever the Derjaguin approximation does not appear to be effective at describing the dependence of nanoparticles forces on nanoparticle size and shape when decane is the solvent. For both the Lennard-Jones and n-decane solvents, it is apparent that the force profiles are influenced by the surface roughness of the nanoparticles. These factors should be taken into account in efforts to engineer colloidal suspensions.
Granular Matter, 2008
Molecular-dynamics is utilized to simulate solvation forces between two nanoparticles immersed in two different solvents: Lennard-Jones spheres and and n-decane. Three different sizes and shapes of solvophilic nanoparticles are investigated. Nanoparticles in the Lennard-Jones liquid exhibit solvation forces that oscillate between attraction and repulsion as the nanoparticle separation increases. The magnitude of these solvation forces increases with particle size and depends on particle shape, consistent with the Derjaguin approximation. When n-decane is the solvent, the solvation forces are negligible for small nanoparticles, with sizes comparable to the end-to-end distance of all-trans decane. The solvation forces oscillate between attraction and repulsion for sufficiently large nanoparticles in decanehowever the Derjaguin approximation does not appear to be effective at describing the dependence of nanoparticles forces on nanoparticle size and shape when decane is the solvent. For both the Lennard-Jones and n-decane solvents, it is apparent that the force profiles are influenced by the surface roughness of the nanoparticles. These factors should be taken into account in efforts to engineer colloidal suspensions.
The Journal of Physical Chemistry B, 2012
Structural estimators for the entropy are combined with an analysis of the different contributions to the energy of solvation to understand the molecular basis of the thermodynamics of solvation of passivated nanoparticles. Molecular dynamics simulations of thiolated gold clusters in ethane are performed over a wide range of densities close to the critical isotherm. The entropic changes associated with solvent reorganization around the passivated nanoparticle are estimated from the nanoparticle−solvent pair correlation function, while the entropy of the ligand shell is estimated from the covariance in the positional fluctuations of the ligand atoms. The ligand-shell entropy (S L) is shown to be fairly insensitive to variations in solvent density ranging from vacuum to twice the critical density (ρ c). In contrast, the entropy change due to solvent reorganization (ΔS ns ord) shows a minimum around the critical point where the solvent excess shows a maximum. Combining the entropic estimates with the nanoparticle−solvent interaction energies, the free energy of solvation is shown to decrease with density once the critical point is crossed in a manner qualitatively consistent with available experimental data. The results suggest that such an approach to obtain structural insights into the thermodynamics of solvation of passivated nanoparticles could be useful in understanding the stability of nanoparticle dispersions of widely varying chemistries. This study also demonstrates that the theoretical analysis of solvation and self-assembly developed in the context of biomolecular hydration can be very usefully extended to understand the behavior of inorganic nanoparticle dispersions.
Industrial & Engineering Chemistry Research, 2010
A thermodynamic model for a mixture of alkanethiol-coated nanoparticles (NPs) and low molecular weight (non-polymeric) solvent is developed, and calculations of liquid-liquid phase equilibrium for different values of NP core radius, alkanethiol chain length, solvent molar volume and alkanethiol-solvent interaction parameter, are presented. The model takes into account the swelling of the organic coronas and the dispersion of particles with swollen coronas in the solvent. The energetic interaction between alkyl chains and solvent is considered, both within the corona and between the outer alkyl segments and free solvent. Swelling involves mixing of alkanethiol chains and solvent in the corona and stretching of the organic chains. Dispersion considers an entropic contribution based on Carnahan -Starling equation of state and an enthalpic term calculated considering the surface contacts between alkyl segments placed in the external boundary of the corona and the molecules of free solvent. Two different kinds of phase equilibrium are found. One of them, observed at high values of the interaction parameter, is the typical liquid-liquid equilibrium for compact NPs in a poor solvent where a complete phase separation is observed when cooling (increasing the interaction parameter). The second liquid-liquid equilibrium is observed at low values of the interaction parameter, where swelling of coronas is favored. In this region two different phases co-exist, one more concentrated in NPs that exhibit relatively compact coronas and the other one more diluted in NPs with extended coronas. In diluted solutions of NPs the deswelling of the fully extended coronas takes place abruptly in a very small temperature range, leading to a solution of compact NPs. This critical transition might find practical applications similar to those found for the abrupt shrinkage of hydrogels at a critical temperature. 3
Can Near-Critical Solvents Drive Colloidal Self-Assembly?
In 1978 Fisher and de Gennes predicted the existence of long-ranged solvent-mediated (SM) interactions between two colloidal particles suspended in a near-critical binary solvent. The range of these (universal) SM forces, often referred to as critical Casimir forces, is set by the correlation length of the solvent which diverges on approaching its critical point. The remarkable sensitivity of SM interactions to the temperature and composition of the solvent sparked recent interest, driven by prospects of unparalleled control of colloidal self-assembly in a tunable, reversible, and in-situ fashion. Here we determine both the effective SM pair interactions and the full phase diagram of Brownian discs suspended in an explicit two-dimensional supercritical binary liquid mixture. The SM pair interactions are most attractive at off-critical compositions of the solvent, and combined with the SM many-body interactions they drive colloidal gas-liquid and fluid-solid phase transitions in a surprisingly large regime away from criticality. Our simulation study, supported by a mean-field theory, provides a fresh perspective on colloidal self-assembly mediated by solvent critical fluctuations, and opens new avenues for controlling and manipulating this process.
Solvent-driven interactions between hydrophobically-coated nanoparticles
Soft matter, 2015
Interaction between hydrophobically-coated gold nanoparticles suspended in oil is usually described as the combination of strong attractive van der Waals attraction between the gold cores and interaction between the ligands. The latter interaction is expected to be purely repulsive if the suspending medium is a good solvent for the ligands or partially attractive for a bad solvent. By measuring the structure factor of interacting gold nanoparticles in various solvents, we show that the chemical affinity of the ligand with the solvent is not the only parameter that controls the interaction between the ligands and that the solvent conformation (small rigid or long flexible molecules) also plays a crucial role. Gold nanoparticles covered with hexanethiol or dodecanethiol thus undergo a larger attraction in n-dodecane or n-hexadecane compared to toluene or cyclohexane. As a consequence, self-assembly of these nanoparticles into superlattices appears at a much lower volume fraction than ...