Phase behavior of grafted chain molecules: Influence of head size and chain length

Stadler, Christoph; Schmid, Friederike

doi:10.1063/1.478934

Cited by 25 publications

(21 citation statements)

References 45 publications

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“…At low temperatures, this constraint results in negative surface pressures indicative of the metastability of the system. To alleviate this problem, one should execute simulations in the N⌸T ensemble, as recently considered by Stadler et al ͓5,6͔. Although the majority of the work presented here was performed using the HS model, in which the water subphase was replaced by a hard attractive surface, we have also utilized the ͑9-3͒ potential model to account for surfactantwater interactions. However, using this model, it was found that the monolayer would buckle, akin to the phenomenon termed ''collapse,'' forming bilayer or multilayered structures at temperatures and densities where one would expect to find the liquid phase for a Langmuir monolayer.…”

Section: Discussionmentioning

confidence: 99%

Monte Carlo studies of model Langmuir monolayers

et al. 2001

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This paper examines some of the basic properties of a model Langmuir monolayer, consisting of surfactant molecules deposited onto a water subphase. The surfactants are modeled as rigid rods composed of a head and tail segment of diameters sigma(hh) and sigma(tt), respectively. The tails consist of n(t) approximately 4-7 effective monomers representing methylene groups. These rigid rods interact via site-site Lennard-Jones potentials with different interaction parameters for the tail-tail, head-tail, and head-head interactions. In a previous paper, we studied the ground-state properties of this system using a Landau approach. In the present paper, Monte Carlo simulations were performed in the canonical ensemble to elucidate the finite-temperature behavior of this system. Simulation techniques, incorporating a system of dynamic filters, allow us to decrease CPU time with negligible statistical error. This paper focuses on several of the key parameters, such as density, head-tail diameter mismatch, and chain length, responsible for driving transitions from uniformly tilted to untilted phases and between different tilt-ordered phases. Upon varying the density of the system, with sigma(hh)=sigma(tt), we observe a transition from a tilted (NNN)-condensed phase to an untilted-liquid phase and, upon comparison with recent experiments with fatty acid-alcohol and fatty acid-ester mixtures [M. C. Shih, M. K. Durbin, A. Malik, P. Zschack, and P. Dutta, J. Chem. Phys. 101, 9132 (1994); E. Teer, C. M. Knobler, C. Lautz, S. Wurlitzer, J. Kildae, and T. M. Fischer, J. Chem. Phys. 106, 1913 (1997)], we identify this as the L'(2)/Ov-L1 phase boundary. By varying the head-tail diameter ratio, we observe a decrease in T(c) with increasing mismatch. However, as the chain length was increased we observed that the transition temperatures increased and differences in T(c) due to head-tail diameter mismatch were diminished. In most of the present research, the water was treated as a hard surface, whereby the surfactants are only allowed to move within the plane of this surface. However, we have also utilized a more realistic model for the surfactant-water interactions, developed by Karaborni and Toxvaerd, in order to examine the role which the coupled effects of head group size and head group-subphase interactions plays in determining tilt ordering and on the stability of the monolayer. It is found that increasing the head diameter results in a widening of the air-water interface and an associated destruction of orientational order. Furthermore, the onset of capillary waves at lower temperatures for larger head diameters implies that the L2-L1 phase boundary for acids and acetates should move to lower temperatures relative to the L'(2)/Ov-L1 phase boundary for alcohols and esters. This feature has yet to be seen in experimental studies.

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Section: Discussionmentioning

confidence: 99%

Monte Carlo studies of model Langmuir monolayers

et al. 2001

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“…The aim is to reproduce the main features of the generic structure and phase behavior observed experimentally. Here we used the molecular model developed previously in a series of papers by the German group at Mainz [27][28][29][30][31][32][33][34][35]. For completeness, a brief explanation of the molecular potentials is given and their corresponding algebraic expressions are written down.…”

Section: Molecular Model and Simulations Detailsmentioning

confidence: 99%

“…In this paper we present the results of Monte Carlo (MC) simulations of a version of the CGMM's of Langmuir monolayers developed previously [28][29][30][31][32][33][34][35]. In the present model we have incorporated the long-range interactions due to the Coulomb forces among the ionic hydrophilic molecular heads.…”

Section: Introductionmentioning

confidence: 99%

Phase behavior of Langmuir monolayers with ionic molecular heads: Molecular simulations

González-Castro

Ramı́rez-Santiago

2015

Phys. Rev. E

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We carried out Monte Carlo simulations in the N,Π,T ensemble of a Langmuir monolayer coarse-grained molecular model. Considering that the hydrophilic groups can be ionized by modulating acid-base interactions, here we study the phase behavior of a model that incorporates the short-range steric and long-range ionic interactions. The simulations were carried out in the reduced temperature range 0.1≤T*<4.0, where there is a competition of these interactions. Different order parameters were calculated and analyzed for several values of the reduced surface pressure in the interval, 1≤Π*≤40. For most of the surface pressures two directions of molecular tilt were found: (i) towards the nearest neighbor (NN) at low temperatures, T*<0.7, and most of the values of Π* and (ii) towards next-nearest neighbors (NNN) in the temperature interval 0.7≤T*<1.1 for Π*<25. We also found the coexistence of the NN and NNN at intermediate temperatures and Π*>25. A low-temperature reentrant disorder-order-disorder transition in the positions of the molecular heads and in the collective tilt of the tails was found for all the surface pressure values. It was also found that the molecular tails arranged forming "rotating patterns" in the temperature interval, 0.5

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“…The brushes formed by homopolymers were the subjects of many theoretical studies [4][5][6]. Heteropolymeric (amphiphilic) brushes also became recently subjects of simulation studies by means of Monte Carlo [7][8][9][10] and molecular dynamics methods [11][12][13]. Structure and morphology of polymer brush formed by amphiphilic chains were also recently studied by means of Monte Carlo simulations [14].…”

Section: Introductionmentioning

confidence: 99%