ISSN 1061-933X, Colloid Journal, 2009, Vol. 71, No. 3, pp. 391–396. © Pleiades Publishing, Ltd., 2009.

Spreading Behaviour of Aqueous Trisiloxane Solutions

over Hydrophobic Polymer Substrates1
Caleb Chong Wei Pinga, N. A. Ivanovaa, V. M. Starova, N. Hilalb, and D. Johnsonb
aDepartment of Chemical Engineering, Loughborough University, LE11 3TU, UK
bSchool of Chemical, Environmental and Mining Engineering, University of Nottingham, UK
e-mail: V.M.Starov@lboro.ac.uk
Received June 10, 2008

Abstract—The kinetics of spreading of aqueous trisiloxane solutions over different solid hydrophobic sub-
strates has been investigated experimentally. Two pure trisiloxane surfactants with 6 and 8 oxyethylene groups
at concentrations close to the critical aggregation concentration and the critical wetting concentration were used
in the spreading experiments. Three hydrophobic substrates (Teflon AF, Parafilm, and polystyrene) having dif-
ferent surface properties were used. It was found that the spreading behaviour depends on the hydropho-
bic/roughness properties of substrates. The rapid spreading and complete wetting were observed for both trisi-
loxane surfactant solutions at the critical wetting concentration on a substrate with a moderate hydrophobicity.
For both highly hydrophobic Teflon AF and Parafilm substrates only partial wetting was found. The experi-
ments have shown that the spreading behaviour over all substrates proceeds at two stages. At the critical aggre-
gation concentration for both trisiloxanes on all substrates the time lag of the spreading was detected.
DOI: 10.1134/S1061933X09030156

1 INTRODUCTION surface roughness and local tension gradients lead to an

Trisiloxane surfactants consist of non-polar trisilox- asymmetric drop shape and formation of fingers and
ane headgroups and the polar group, which is a dendrites.
poly(ethylene oxide) chain. Trisiloxanes are com- It was shown in [4] that the spreading rate of T8
monly denoted as M(D’En)M, where M stands for the aqueous solution at concentrations above the CWC
trimethylsiloxy group (CH3)3SiO–, the term D' stands reached the maximum on a hydrophobized rough gold-
for the –Si(CH3)(R)–, where R is an alkylene spacer coated quartz crystal substrate, where a pure water con-
attached to the silicon, and En stands for polyoxyeth- tact angle is cos θ = 0.4. On substrates with higher
ylene –(CH2CH2O)nH– [1]. For simplicity, trisiloxane
with n oxyethylene groups is referred to below as Tn. The degree of hydrophobicity ( cos θ = −0.4) or with lower
special feature of trisiloxane surfactants is to promote degree of hydrophobicity ( cos θ = 1) the spreading rate
spreading of aqueous solutions over hydrophobic sub- was substantially lower as compared with above men-
strates that has substantial importance in many techno- tioned case of intermediate hydrophobicity. The reduc-
logical applications such as coatings, paintings, agro- tion in the spreading rate of aqueous trisiloxane solu-
chemicals [2]. tions at concentration above the CWC on substrates
Despite of intensive investigations of spreading of with extremely low or high surface energy was also
trisiloxane solutions undertaken during the past decade, reported in [5]. The maximum spreading rate was found
the information on influence of the surface energy and on substrates characterized by water contact angles 90°
chemistry of substrates on spreading behaviour is still and 79°. It was also found that on highly hydrophobic
limited. surfaces rapid spreading is observed for 1 wt % solu-
tions of the relatively short-chained poly-oxyethylene
Spreading of aqueous trisiloxane (T6 and T8) solu- (T5 and T6) derivatives. On slightly polar surfaces,
tions on a rough graphite substrate was reported in [3].
At the critical wetting concentration (CWC) the transi- dilute 0.1 wt % solution of the longer-chained (T8)
tion from the partial wetting to the complete wetting derivative spread faster. However, the results were
was found. Three regimes of the spreading dynamics obtained at trisiloxane concentrations much higher than
were observed: (i) early stage where wetting diameter both the critical aggregation concentration (CAC) and
is proportional to tn with n is in the range 0.12–0.22; CWC.
(ii) during the second stage the exponent increased up This work aims to investigate the spreading behav-
to 0.58; (iii) during the last stage of the spreading the iour of droplets of aqueous trisiloxane surfactants solu-
tions at concentrations 1 CAC and 1 CWC over differ-
1The article is published in the original. ent hydrophobic solid surfaces.

391
392 PING et al.

Table 1. Macroscopic contact angle of water, film thickness, thickness of polymeric films and their roughness are
and roughness factor of polymeric substrates summarized in Table 1. The latter Table shows that the
surface of Teflon AF coating is substantially smoother
Polymeric Water contact Thickness of RMS roughness
substrate angle, deg polymer film, µm (50 × 50 µm2)
and more hydrophobic as compared with the other
polymer surfaces.
Teflon AF 117 ± 1 0.86 ± 0.03 <5 Aqueous solutions of surfactants were prepared
Polystyrene 85 ± 3 1.9 ± 0.2 > 30 with ultra pure water (18.2 MΩ cm) obtained from a
Parafilm 106 ± 2 – ~ 60 Millipore filter. The concentrations of solutions under
investigation were around CAC at which trisiloxanes
form aggregates/vesicles [1, 6], and CWC [6]. The
Table 2. Values of the CAC, CWC, and surface tension at CAC CAC, CWC and surface tension values at CAC for T6
for trisiloxanes [6] and T8 trisiloxanes are presented in Table 2.
All experiments were conducted in a closed cham-
Surfactant CAC, mol/l CWC, mol/l γ at CAC, mN/m ber at 22 ± 1°C and 60 ± 5% humidity. A droplet of the
surfactant solution of 2.6 ± 0.3 µl was deposited on a
polymer coated wafer with a precision syringe. The
T6 9.6 × 10–5 5.52 × 10–4 – smallness of the droplet size allowed neglecting the
gravity action.
T8 10.9 × 10–5 7.9 × 10–4 22 The side view of the spreading process was captured
using a CCD camera at a rate of 30 frames per second.
Slowly spreading droplets were observed for at least
EXPERIMENTAL 100 s, and fast spreading droplets over 5 s. Captured
images were then analysed automatically by using Drop
Non-ionic trisiloxanes tracking and evaluation analysis software developed by
[(CH3)3SiO]2Si(CH3)(CH2)3(OCH2CH2)nOH, denoted Micropore Technologies, UK for monitoring the time
as Tn, with n = 6 and 8 were provided by Dow Corn- evolution of the diameter, height, radius of curvature, and
ing Corp. and used without further purification. contact angle of spreading droplets. The experiments
Poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole- were repeated at least four times to control the reproduc-
co-tetrafluoroethylene] denoted as Teflon AF, polysty- ibility for each solution. Contact angles were averaged
rene standard with molecular weight 105, and Parafilm® with accuracy 1–3°. The change of droplet volume
“M” (for short Parafilm) were purchased from Sigma- caused by evaporation was found less than 5%.
Aldrich, UK.
Teflon AF (0.5 g) was dissolved in 200 ml of Fluo-
rinert F75 solvent, and polystyrene (1 g) was dissolved RESULTS AND DISCUSSION
in 100 ml of dichloromethane solvent. Hydrophobic Figures 1–3 summarize the time evolution of a
polymer substrates were formed by coating silicon diameter of droplets base of T6 and T8 trisiloxane solu-
wafer pieces with Teflon AF or polystyrene solutions. tions at CAC and CWC concentrations spread over
The silicon wafer pieces measuring around 1 × 1 cm2 polymer hydrophobic substrates. It was found that the
were carefully cleaned according to the following pro- spreading process proceeds in two stages over all poly-
tocol: 30 min ultrasonication in isopropyl alcohol, then styrene, Parafilm and Teflon AF substrates. In case of
rinsing in distilled water and soaked in chromic acid for concentrations closed to the CAC, the droplets of both
1 h, intensive rinse in distilled water and de-ionized trisiloxanes start spreading in about 0.3 s after the dep-
water, and then dried in a strong air jet. Then silicon osition of droplets on any polymer substrates investi-
wafer pieces were placed in a covered dish and polymer gated (see curves 1, 2 in Figs. 1–3). Note, in experi-
solution was deposited on each piece. After this proce- ments on spreading over Teflon AF, concentration of T8
dure, the silicon wafer pieces were left for 24 h in order solution was about 1.5 CAC, hence, the time lag in this
to evaporate the solvent. case was slightly shorter than for 1 CAC solutions
Parafilm substrates were prepared in the following (Fig. 3).
way. Parafilm pieces measuring around 1 × 1 cm2 were At concentration C ≈ CAC, the droplets of both T6
cut and attached with a small pure water droplet to the and T8 solutions spread at the first stage for 18–20 s
silicon wafers pre-cleaned according to the above pro- over all substrates. After that, over both Parafilm and
cedure. Teflon AF substrates, the spreading proceeded rather
Surface topology of polymer-coated silicon wafers slower: the second stage started, which was lasted up to
was inspected using atomic force microscopy. The 100 s (Figs. 2, 3). In the case of polystyrene surface, the
thicknesses of Teflon AF and polystyrene films on spreading rate at the second stage was drastically
wafers were measured by using SG-Certus film thick- increased (see Fig. 1).
ness measurement system (Scalar Technologies, UK). At high concentration (~1 CWC), the droplets of
The data of the macroscopic contact angle of water, both trisiloxane solutions start spreading over all inves-

COLLOID JOURNAL Vol. 71 No. 3 2009

 SPREADING BEHAVIOUR OF AQUEOUS TRISILOXANE SOLUTIONS 393

Diameter, mm
4.0
3.8
3.6
4
3.4
1
3.2 3
3.0
2.8 2

2.6
2.4
2.2
2.0
0.01 0.1 1 10 100
Time, s

Fig. 1. Spreading kinetics of the droplets of (1, 3) T6 and (2, 4) T8 solutions at concentrations (1, 2) 1 CAC and (3, 4) 1 CWC over
polystyrene substrate.

Diameter, mm
4.0
3.8
3.6
3.4 4
3.2 3
3.0
2.8 1
2.6 2
2.4
2.2
2.0
0.01 0.1 1 10 100
Time, s

Fig. 2. Spreading kinetics of the droplets of (1, 3) T6 and (2, 4) T8 solutions at concentrations (1, 2) 1 CAC and (3, 4) 1 CWC over
Parafilm substrate.

tigated polymer substrates immediately after the depo- the first stage took few seconds, and the time evolution
sition. In this case, the spreading was rather fast and the of the droplet base diameter was much slower as com-
process of spreading was completed in 5 s on polysty- pared with polystyrene and Parafilm substrates.
rene substrate, and in about 25 s on Teflon AF and Para-
film substrates. For the droplets of T6 solution on Para- The dependences of the diameter droplet base on the
film and polystyrene substrates, the first stage took less spreading time, D(t), were fitted using a power law Di(t)
than 0.5 s, and then the spreading develops during the n
second faster stage. For T8 solution, the first stage was = A i t i for each stage (Fig. 4), where D(t) is the diame-
shorter than for T6 solution. In the case of smooth and ter of the droplet base, n is the fitted exponent, and A is
highly hydrophobic Teflon AF substrates (see Fig. 3), the prefactor; i—is the subscript that corresponds to the

COLLOID JOURNAL Vol. 71 No. 3 2009

394 PING et al.

Diameter, mm
3.0

2.8

4
2.6
3 2

2.4
1

2.2

2.0
0.01 0.1 1 10 100
Time, s

Fig. 3. Spreading kinetics of the droplets of (1, 3) T6 and (2, 4) T8 solutions at concentrations (1, 2) 1 CAC and (3, 4) 1 CWC over
Teflon AF substrate.

Diameter, mm
4.0
3.8
3.6
3.4 3
3.2
3.0
2.8
2
2.6 Time lag
2.4
1
2.2
2.0
0.01 0.1 1 10 100
Time, s

Fig. 4. Dependencies of the droplet diameter on spreading time for T6 and T8 solutions at different concentrations over various poly-
mer substrates: 1—T6 (CAC), Teflon AF; 2—T8 (CAC), polystyrene; 3—T8 (CWC), Parafilm. Solid lines fit the first and second
stages according to a power law with the corresponding values of exponents shown in Table 3.

first or the second stages. The values of fitting exponent ues of n2 exponent at the second stage are varying sub-
for all experimental data are presented in Table 3. stantially and depend on concentration of trisiloxanes
Table 3 shows that the values of n1 exponent corre- and the properties of the substrates. The lowest values of
sponding to the first stage for both trisiloxanes, and both n2 correspond to highly hydrophobic and rather smooth
concentrations and for all substrates used are quite simi- Teflon AF substrates (see Table 1). The highest n2 values
lar and ranging between 0.03 and 0.06 for Teflon/Para- fit the spreading curves for low hydrophobic and more
film and polystyrene, correspondingly. However, the val- rough polystyrene substrates (see Table 1). Moreover,

COLLOID JOURNAL Vol. 71 No. 3 2009

 SPREADING BEHAVIOUR OF AQUEOUS TRISILOXANE SOLUTIONS 395

for Teflon AF (cases of C = CAC and C = CWC) and Table 3. Values of the fitted exponent n for trisiloxane solutions
Parafilm (C = CAC) the values of n2 for both trisiloxane at different concentrations on different polymer substrates
solutions are less than the values of n1. The latter means
that in these both cases the spreading is mostly completed T6 T8
during the first stage where n1 = 0.03–0.06 that corre- Substrate
Concen-
sponds to a partial wetting. tration n1 n2 n1 n2
Indeed, Fig. 5 shows the time dependences of the 1st stage 2nd stage 1st stage 2nd stage
cosine of contact angle for the CAC solutions spreading
over hydrophobic surfaces, the droplets of both T6 and Polystyrene CAC 0.051 0.093 0.041 0.070
T8 solutions did not wet completely the Teflon AF and
Parafilm surfaces; they wet only partially these polymer θw = 85° CWC 0.060 0.160 0.034 0.100
surfaces. The final contact angles reached by droplets
of trisiloxane solutions were found in the range 70–60° Parafilm CAC 0.032 0.012 0.037 0.021
on Teflon AF and Parafilm (see Fig. 5). In the case of
the low hydrophobic and quite rough polystyrene sub- θw = 106° CWC 0.044 0.122 0.046 0.082
strates, where n2 > n1 the both trisiloxane solutions at
C = CAC showed an effective wetting behaviour Teflon AF CAC 0.032 0.017 0.033 0.017
(Fig. 5). However, the spreading process is rather slow,
e.g. according to our observations of T8 solution the θw = 117° CWC 0.047 0.007 0.026 0.009
contact angle went down to 20° in more than 100 s.
Table 3 shows that at CWC the exponents n2 for T6
and T8 solutions on polystyrene and Parafilm substrates spreading over different polymer substrates. The drop-
are ranging from 0.08 to 0.16 that it close to Tanner’s lets of both T6 and T8 solutions completely wet Parafilm
law of spreading (n = 0.1) [7]. The latter means that in and polystyrene, and only on Teflon AF substrate the
this case there is capillary regime of the spreading, partial wetting behaviour of the droplets was observed.
which corresponds to the complete wetting case. In Note, in Fig. 6 the values of final contact angles of
Figure 6 the cosine of contact angle on the spreading droplets on polystyrene and Parafilm are presented at
time is presented for the CWC trisiloxane solutions

cos θ
0.8
1
2
0.7 3
4
5
0.6 6

0.5

0.4

0.3

0.2

0.1

0
0.01 0.1 1 10 100
Time, s
–0.1

Fig. 5. Time evolution of cosine of the contact angle of the droplets of (1–3) T6 and (4–6) T8 solutions spreading over different
polymer surfaces: 1, 4—Teflon AF; 2, 5—Parafilm; 3, 6—polystyrene. Concentration of solutions is about 1 CAC.

COLLOID JOURNAL Vol. 71 No. 3 2009

396 PING et al.

cosθ
1.2
1
2
3
1.0 4
5
6
0.8

0.6

0.4

0.2

0
0.01 0.1 1 10 100
Time, s

Fig. 6. Time evolution of cosine of the contact angle of the droplets of (1–3) T6 and (4–6) T8 solutions spreading over different
polymer surfaces: 1, 4—Teflon AF; 2, 5—Parafilm; 3, 6—polystyrene. Concentration of solutions is about 1 CWC.

the moments when the contact angle was still measur- loxane solutions at both critical concentrations—CAC
able. and CWC. On highly hydrophobic Teflon AF substrates
It was also found (Figs. 5 and 6) that the values of ini- only partial wetting took place.
tial contact angle of droplets of both trisiloxane solutions
depend on the hydrophobicity of substrates. On the ACKNOWLEDGMENTS
Teflon AF surface, the initial contact angle took values
close to 90° and its values decreased down to 60–50° for The research was supported by Engineering and
low hydrophobic Parafilm and polystyrene substrates. Physical Sciences Research Council, UK.

REFERENCES
CONCLUSIONS
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The kinetic of spreading of aqueous T6 and T8 trisi- muir, 1994, vol. 10, p. 1724.
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