Hydration Structure of Partially Hydrolyzed Preformed Particle Gel†

doi:10.7503/cjcu20140908

Abstract

Abstract:

Partially hydrolyzed preformed particle gel(PPG) was widely used in petroleum industry for enhancing oil recovery. Its aggregation and swelling in solution can effectively block the high permeability areas. In this work, a series of molecular dynamics simulations was conducted to investigate the swelling and hydration of PPG. After a 20 ns simulation, the volume and radius of gyration of the particle increased rapidly. The one reason was speculated as the strong hydration of hydrophilic groups of PPG(i.e. —COO^- and —CONH₂) in solution. It was shown that the structure of water was strongly modified by the presence of polymer. Then this polymer hydrophilic group induced modification was characterized from dynamic, structure and hydrogen bond aspects to gain a fully understanding of the hydration of PPG on molecular level. The structure of the hydration shell was investigated by spatial distribution function(SDF), radial distribution function(RDF) and distribution of dipole. The RDF and dipole distribution both indicated that O(COO^-) induced a more ordered and densely packed hydration shell than O(CONH₂) and N(CONH₂), which means that O(COO^-) has a very strong hydration ability. The dynamics of water around hydrophilic groups, as manifested in the translational and rotational diffusion and residence time of water is slowed down in the presence of the hydrophilic group. The electrostatic interaction and hydrogen bonds between water and hydrophilic group were speculated to account for their slow mobility. The hydrogen bonds in the vicinity of hydrophilic side groups also become stronger and longer lived by calculating the hydrogen bond residence time. The hydrogen bond network formed by hydration layer water molecules also stabilized the hydration shell according to the dipole reorientation residence time. In brief, the negatively charged center atoms of hydrophilic groups of PPG induced a highly ordered, tightly packed hydration shell around them and bound them with hydrogen bonds and electrostatic attraction which strengthened its hydration ability.

Key words: Preformed particle gel, Molecular dynamic simulation, Swelling, Hydration structure, Enhanced oil recovery

CLC Number:

TrendMD:

MA Ying, ZHANG Heng, YUAN Shiling. Hydration Structure of Partially Hydrolyzed Preformed Particle Gel^†[J]. Chem. J. Chinese Universities, 2015, 36(2): 386.

Figures/Tables 13

Fig.1 Scheme of cross-linked partially hydrolyzed polyacrylamide and conformations of simulation (A) Scheme of cross-linked partially hydrolyzed polyacrylamide; (B) initial conformation of mutually linked polymer network for molecular dynamics(MD) simulation with enlarged hydrophobic linker(in CPK model) and hydrophilic chains.

Table 1 Force field parameters for PPG used in this work*

Group	σ/nm	ε/(kJ·mol^-1)	q/e
CH₃	0.379	0.753	0
CH₂	0.395	0.586	0
CH	0.423	0.544	0
C(CONH₂)	0.336	0.406	0.38
O(CONH₂)	0.263	1.725	-0.38
N(CONH₂)	0.298	0.877	-0.48
H(CONH₂)	0	0	0.24
C(COO^-)	0.336	0.406	0.27
O(COO^-)	0.263	1.725	-0.635

Fig.2 Conformation change of PPG during swelling (A) Radius of gyration; (B) volume; (C) hydrophilic and hydrophobic solvent access surface area; (D) expansion ratio.

Fig.3 Mean square displacement-time curves of water around certain atoms of hydrophilic groups

Table 2 Dynamic properties of water around certain atoms of hydrophilic groups

Certain atom of hydrophilic groups	10⁵ Diffusion coefficient, D/(cm²·s^-1)	Residence time, τ_r/ps
O(COO^-)	2.95	13.78
O(CONH₂)	3.25	6.58
N(CONH₂)	3.50	10.71
Bulk water^[33]	3.58	4.40

Fig.4 Residence time of water around certain atoms of hydrophilic groups (A) Illustration of definition of Pi, Pi(t1)=1 when water molecule in the first shell at t0 and t1. The location of water molecule during t0—t1 does not count; (B) time correlation functions of water around certain atoms of hydrophilic groups.

Fig.5 Spatial distributions of water and ions around certain atoms of hydrophilic groups(A, C) Spatial distribution of water and Na+ around —COO-; (B, D) spatial distribution of water around CONH2

Fig.6 Radial distribution functions between certain atoms of hydrophilic groups and water molecules and scheme of hydrogen bond structure in hydration shells around them (A) gCOO-OW(r); (B) gN(CONH2)-OW(r); (C) gO(CONH2)-OW(r).

Fig.7 Definitions of α(A) and dipole orientation distribution of water molecules in the first hydration shell of certain atoms of hydrophilic groups(B)

Table 3 Hydrogen bond properties around certain atoms of hydrophilic groups

Certain atom of hydrophilic groups	Number of H bonds	Life time/ps	Dipole reorientation residence time, τ_μ/ps
O(COO^-)	1.64	10.20	6.17
O(CONH₂)	1.33	1.75	4.68
N(CONH₂)	1.14	1.11	4.28

Fig.8 Time correlation function CHB(t) for hydrogen bond formed between certain atoms of hydrophilic groups and neighbor water molecules

Fig.9 Illustration of angle between water dipole vectors(A) and reorientation autocorrelation functions of water around certain atoms of hydrophilic groups(B)

Fig.10 Scheme of PPG and surrounding hydrogen bonds network formed by water molecules in hydration shell

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