Aqueous Suspensions Of Polymethylsiloxane
Aqueous suspensions of polymethylsiloxane
Yu. N. Shevchenko1, V. M. Gun’ko2, V. V. Turov2, V. I. Zarko2
1“KREOMA-PHARM”, 3 st. Radischeva, 03680 Kiev, Ukraine and
2Institute of Surface Chemistry, 31 Prospect Nauki, 03680 Kiev, Ukraine
Polymethylsiloxane (PMS) xerogels are hydrophobic and are practically not wetted by water (Q > 95o), however, hydrogels of PMS can hold large amounts of water and the corresponding aqueous suspensions possess a marked durability and stability. The states of the interfacial water in such systems can significantly differ from those near unmodified silica surfaces. Clearly, the characteristics of the interfacial water impact strongly the physicochemical properties of the PMS hydrogels.
The aim of this work was to study the aqueous suspensions of PMS, especially the particle size distributions and characteristics of the bound water layers depending on the concentration of PMS (CPMS) and estimation of contribution of inter-particle interactions to Gibbs free energy changes at the PMS/water interfaces, using photon correlation spectroscopy (Zetasizer 3000, Malvern Instr.) and 1H NMR (WP-100 SY (Bruker) NMR spectrometer) with freezing out of the bulk water.
The concentration of the bound water and free surface energy change by several times with decreasing CPMS in the aqueous suspensions of PMS (Table1), which suggest that the structure of the hydrogen bond network at the interface depends strongly on CPMS. A major change in the free surface energy occurs on alterations of CPMS between 2 and 5 wt.%, which is supposedly due to strong inter-particle interactions appearing during concentrating suspension, which influences the structure of disturbed interfacial water layers.
The magnitude of the free surface energy of PMS xerogel is significantly larger than that for hydrogels of PMS and close to that for fumed silica. This testifies that significant reconstruction of the surfaces during drying of PMS hydrogel, which impacts the structure of the interfacial water. Maxima of -d(ΔG(х))/dx for PMS xerogel is observed at x = 2.5 nm (approximately 8 monolayers of water), which corresponds to strong distortion
Table 1 Characteristics of Bound Water Layers for Hydrogels and Xerogel of PMS
CPMS
wt.%
ΔGs
kJ/mol
ΔGw
kJ/mol
CH2Os
mg/g
CH2Ow
mg/g
gS
mJ/mol
1.25
4
0.7
140
120
145
2.5
4
1.0
130
300
141
5
2.4
1.0
105
125
66
10
3
1.2
70
60
59
28
(Хerogel)
4
0.8
150
300
235
Notes. СН2Оs and СН2Оw denote the concentrations of strongly and weakly bound waters respectively, DGs and DGw are maximal decrease of the free energy of strongly and weakly bound waters, gS is the free surface energy of the adsorbent in the aqueous media.
in the structure of the boundary water layer along larger distance than that for PMS hydrogels at CPMS
Dependence of effective minimal and maximal PMS particle sizes Def on CPMS shows the availability of both very tiny and large particles over a broad CPMS range. It should be noted that a decrease in maximal and minimal Def is observed with decreasing CPMS. For maximum dispersed diluted aqueous suspensions of PMS, monomodal particle size distribution is found and the value of effective hydrodynamic diameter Def (geometrical diameter plus double shear layer) of PMS particles does not change practically with CPMS and equals to 7-8 nm. One can assume that this Def corresponds to the size of primary PMS particles and subsequent decomposition of them is possible on destroy of the chemical bonds but not inter-particle bonds characteristic for larger aggregates of primary particles.
The photon correlation spectroscopy investigations of Def(pH) at CPMS = 0.0078 and 0.0312 wt.% show that the Def value depends slightly on pH at the minimal PMS concentration (at pH
Thus, the performed investigations show that strong swarms of PMS particles do not form and aggregates and agglomerates can be easily decomposed to the sizes close to 10 nm. The particle size distributions in diluted aqueous suspensions are monomodal at the level of primary particles at Def
It was shown that the structural properties of the aqueous suspensions of PMS, the particle size distribution, the free surface energy and the characteristics of the interfacial water layers depend on the concentration of PMS, and some of these values increase but others decrease nonlinearly with CPMS, which suggest that non-additive enhancement of contribution of inter-particle interaction between PMS swarms with CPMS resulting in entirely stricturization of the system at relatively low CPMS. Notice that in the case of highly dispersed fumed silica possessing primary particle sizes of 5-12 nm close to those of PMS, the aqueous suspensions at the same concentrations of the solid phase have lower viscosity but they scatter the light significantly stronger than PMS hydrogels, which can be explained by features of the PMS particle structure with large contribution of 2d-motifs.
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