Production of Micro- and Nanoporous,

Superhydrophobic layers from Sol-Gels

Neil J. Shirtcliffe, Paul Roach, Glen McHale, Michael Newton, Carole C. Perry

Silicate Sol Gels

Partially & fully hydrolysed silicates can link together

Sol-Gel= preparation of oxide materials from solution

Solvent creates porous structure unless complete separation occurs

Usually organosilicon compounds hydrolysed to form intermediates

Hydroxide and organic groups usually present until thermally treated

Mechanism

O RSi

CH3

OO

R

R OH2O RSi

CH3

OOH

R+ HCl+

OHSi

CH3

OO

R

R NH4

+ OSi

CH3

OO

R

R

OH+ +

OH Si

CH3

OO

R

ROSi

CH3

OO

R

R

O Si

CH3

OO

R

RSi

CH3

OO

R

R

+

Acid Hydrolysis

Base Catalysed Gelation

O RSi

CH3

OOH

ROSi

CH3

OO

RSi

CH3

OO

R

R

R

H+

2

Sol-Gel Phase Separation

As the chains grow they become more hydrophobic and eventually phase separate to form a bicontinuous structure

OH Si

CH3

OO

R

ROSi

CH3

OO

R

R

+

Polar O Si

CH3

OO

R

RSi

CH3

OO

R

R

Less Polar

Bi-continuous Structure

M. Stockenhueber

Shrinkage

Shrinkage caused by surface tension of drying solvent, varies with pore size. Means that total pore volume decreases with pore size.

Can be overcome by supercritical drying, this is technically easy but takes time=money

P=2γlv/r

r

Size Of Domains

•Rate of phase separation

•Polarity of solvent

•Starting material

•Temperature

•Rate of hardening

•Starting material

•Time in acid

•Temperature

•Shrinkage

•Surface tension

•Contact angle of solvent “r”

•Pore Size “r”

•Strength

•(Temperature/pH=coarsening)

Super-hydrophobicity

Max. Angle 120°

Super-hydrophobicity

Lotus LeafBarthlott&Neinhuis

Flat Teflon

Sol-Gel Foams

Materials

Sample Temp./°°°° C Post treatment

Advancing Angle Θ/°

Receding Angle Θ/°

Hysteresis ∆Θ/°

Flat 300 None 107 87 20

Flat 400 None 90 69 21

Flat 500 None 81 67 14

Flat 550 None 54 31 12

Foam1 300 None 153 137 16

Foam1 300 Abraded 156 152 4

Foam2 300 None 155 149 6

Foam1 400 None Absorbed Absorbed -

Advancing and receding contact angles of drops of water on organo-silica flat surfaces and foams. Foam 1- MTEOS sol-gel produced using 1.1 M ammonia: Foam 2- sol-gel produced using MTEOS and 2.2 M ammonia.

Effect of Heating

3500 3000 2500 2000 1500 1000-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

300º C

110º C225º C

400º C

500º C

Abs

orba

nce

Wavenumber/cm-1

100 200 300 400 500 600 7000.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

No

rma

lise

d M

ass

Temperature/° C

Pore Size/Ammonia

10 µm

MTEOS with 1.1 M ammonia, heated to 300° C MTEOS with 2.2 M ammonia, heated to 300° C

Contact Angle/Ammonia

130

135

140

145

150

155

160

165

170

0 1 2 3 4

Ammonia Concentration / M

Pore Size/End Group

10 µmMTEOS with 1.1 M ammonia, heated to 300° C PTEOS with 22 M ammonia, heated to 300° C

O RSi

CH2

OO

R

R

CH2

CH3

O RSi

CH3

OO

R

RO RSi

OO

R

R

H

Sol-Gel Films

Properties

Thermally insulatingWaterproof or water absorbingGas permeableUseful thickness depends on pore size

Recipe

CH3CH

CH3

OHN CH3

CH3

CH

O

DMF IPA

Water Contact Angle/Hydrolysis Time

144

146

148

150

152

154

156

158

20 40 60 80 100 120 140

Hydrolysis Time / mins

1

2

3

Thin Films Look Like Trees

and have lower contact angles

Conclusion

•Can produce layers of sol-gel that are very hydrophobic and porous.•Can vary pore size using hydrolysis time, solvent polarity and monomer•Can vary hydrophobicity by thermal treatment•With supercritical drying, will be attempting to decouple pore size and pore fraction•With suitable coupling agents have coated glass and gold