Enhancement of Stability of Bubbles to Disproportionation Using Hydrophilic Silica Particles Mixed with Surfactants or Proteins - Food Colloids: Self-Assembly and Material Science - page 359

particles were a gift from Professor Binks (University of Hull). Water from a
Milli-Q system (Millipore, Watford, UK), free from surface-active impurities
and with a conductivity of less than 10 7 Scm 1 , was used throughout.
Didodecyldimethylammonium bromide (DDAB) (98%), bovine b-lactoglobulin
(b-LG, crystallized and lyophilized, with

90% protein content, lot no.

B

114H7055), L -a-phosphatidylcholine (lecithin) from frozen egg-yolk ( 99%),

imidazole and sodium chloride buffer salts were from Sigma (Poole, Dorset).
The b-casein was obtained as a freeze-dried sample from the Hannah Research
Institute (Ayr, Scotland). The protein gave clear sharp peaks when analysed
by fast protein liquid chromatography (FPLC) using a Mono-Q ion-
exchange column with a linear gradient. 9 The buffer solution consisted of
0.05 mol dm 3 imidazole + 0.05 mol dm 3 NaCl, adjusted to pH ¼ 7by
addition of HCl.
Detailed descriptions of the experimental techniques and apparatus to
determine bubble stability have been given elsewhere. 2,10,11 A wide range of
concentrations of silica particles, DDAB, protein and lecithin were screened in
order to test the relative contributions of the particles + amphiphilic molecules
to the stability of the bubbles to disproportionation. The experiments involved
measuring the change in radius of individual bubbles and also the fraction of
surviving bubbles, F, as a function of time at the A-W interface. The quantity
F is defined as the ratio of the number of bubbles still visible after a certain time
to the number of bubbles present at the start (immediately after their forma-
tion). The particle dispersions were prepared, shaken for 15 s to produce the
bubbles/foam and then transferred into an observation chamber. 10 The A-W
interface was viewed from above through the upper window of the chamber
using a microscope, video camera and a video recorder.
The contact angle y was measured between a drop of DDAB solution and a
flat surface formed of fumed silica particles. The flat surface was created by
compressing the silica powder in a specially adapted hydraulic press at pressures
up to 1250 bar.

B

24.3 Results and Discussion

24.3.1 Fumed Silica Particles + DDAB
DDAB is a cationic surfactant of very low critical micelle concentration (cmc).
It adsorbs strongly to the negatively charged surface of silica. 14 At monolayer
coverage it renders the surface hydrophobic by adsorbing with the head group
down on the surface.
Figure 1 illustrates the most stable systems obtained at different fumed silica
particle +DDAB mixtures. The fraction of surviving bubbles, F, is plotted as a
function of time following the creation of the bubbles. When the DDAB
concentration was low (1.7 10 6 mol dm 3 ), the particle concentration
needed for reasonable bubble stability was 2.2 wt%. The DDAB concentra-
tions giving the most stable systems with 3.0 and 3.5 wt% silica particles were