Ionic Regulation Mechanisms of Surfactant Desorption from the Spherical Micelles

doi:10.19596/j.cnki.1001-246x.8701

Abstract

Abstract:

A coarse-grained molecular dynamics method coupled with umbrella sampling is developed to study the desorption process of SDS and CTAC surfactants from spherical micelles. The influence mechanisms of aggregation number, salt species and concentrations on the desorption process of surfactant from spherical micelles are revealed. We find that the radius and eccentricity of spherical micelles both increase with aggregation number rising, and the effects of salt concentration depend on the ionic radius and adsorption characteristics of the counterions. Larger salicylate ions with stronger adsorption ability have more significant effect on the micellar structures. Based on umbrella sampling method, we obtain the desorption free energy and time of surfactants from micelles. we find that the desorption free energy and desorption time of surfactant both show a non-monotonic variation with the increase of aggregation number and salt concentration, whose mechanisms are attributed to the electrostatic shielding induced by ion adsorption on micelle surface. Moreover, we reveal that the free energy plays a leading role in surfactant desorption process. Combing the theory of micelle thermodynamics, we further develop the critical micelle concentration prediction method, and obtain the distribution range of micelle size under the critical micelle concentration of surfactants.

Key words: micelle, surfactant desorption, ionic regulation, molecular dynamics, free energy calculation

CLC Number:

O469

Boyao WEN, Genying GAO, Xi LU, Songtao GUAN, Zhengyuan LUO, Bofeng BAI. Ionic Regulation Mechanisms of Surfactant Desorption from the Spherical Micelles[J]. Chinese Journal of Computational Physics, 2024, 41(2): 193-202.

Figures/Tables 8

Fig.1 Spherical micelle model and coarse-grained representations of molecules and their Martini bead types

Fig.2 PMF profiles and sampling histogram of surfactant adsorption and desorption processes at or from the spherical micelle (CTAC/NaSal system, N = 80, R = 1.0)

Table 1 Structural properties of SDS spherical micelles (N = 60, R = 0)

	R_g/nm	R_s/nm
Our simulation results	1.55±0.05	2.00±0.06
Ref.[30]	1.55±0.30	1.98±0.27
Ref.[31]	1.50	1.96
Ref.[32]	1.58	1.90
Ref.[33]	1.64	1.99
Ref.[34]	1.60±0.06	1.97±0.08
Ref.[35]	1.54	2.00

Fig.3 Micelle radius of spherical micelle with different aggregation number and salt-to-surfactant concentration ratio (a) SDS/NaCl system; (b) CTAC/NaSal system

Fig.4 Eccentricities e of spherical micelles with different aggregation number and salt-to-surfactant concentration ratio (a) SDS/NaCl system; (b) CTAC/NaSal system

Fig.5 Desorption free energy of surfactant from spherical micelle with different aggregation number and salt-to-surfactant concentration ratio (a) aggregation number; (b) salt-to-surfactant concentration ratio on desorption free energy and net charge of micelle

Fig.6 Mean first-passage time profiles and desorption time of SDS molecules from the interfaces (a) mean first-passage time of SDS molecules from the oil-water interfaces with different adsorbed numbers; (b) desorption time of SDS molecules from interfaces with different adsorbed numbers; (c) effects of ionic species and concentrations on the desorption time of SDS molecules

Fig.7 Predicted CMC values of SDS and CTAC surfactants with different aggregation numbers

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