Stabilizing properties of fucoidan for the alumina suspension containing the cationic surfactant

Carbohydrate Polymers 245 (2020) 116523

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Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol

Stabilizing properties of fucoidan for the alumina suspension containing the
cationic surfactant

T

Jakub Matusiaka,*, Elżbieta Grządkaa, Anna Bastrzykb
a

Department of Radiochemistry and Environmental Chemistry, Faculty of Chemistry, Institute of Chemical Sciences, Maria Curie-Sklodowska University, M. CurieSklodowska Sq. 3, 20-031 Lublin, Poland
b
Department of Process Engineering and Technology of Polymer and Carbon Materials, Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrzeze
Wyspianskiego 27, 50-370 Wrocław, Poland

A R T I C LE I N FO

A B S T R A C T

Keywords:
Adsorption
Biopolymer
Aluminium(III) oxide
Interface
Electrosteric
Turbidimetry

The paper presents the influence of fucoidan (FD) on stability of alumina suspensions in the presence of cationic
surfactant hexadecyltrimethylammonium bromide (CTAB). The research results show that fucoidan adsorbs on
the alumina surface and that the adsorption decreases in the CTAB presence. This is due to formation of the
polymer-surfactant complexes characterized by lower affinity for the alumina surface than pure fucoidan. The
complex formation was confirmed by the tensiometric studies where the increase of the CTAB/FD surface
tension in comparison to pure CTAB was observed. It was established that fucoidan possesses great stabilizing
efficiency regardless of pH. Furthermore, stability of the fucoidan/alumina system increased after CTAB addition
due to the presence of non-adsorbed complexes between the alumina particles. The results indicate that fucoidan
could be successfully used as a stabilizer of colloidal suspensions where the presence of surfactant is required,
that is in cosmetic and pharmaceutical industries.

1. Introduction

stabilizing properties of the fucoidan/alumina system. Therefore to get
a full picture of the system stability a lot of measurements have to be
conducted including the adsorption of the polymer on the solid surface,
stability studies, determination of the interactions between the surfactant and the polymer, and the electrokinetic properties. Multiple
methods can be used to characterize the interactions between the
polymer and the surfactants. Among others, those are nuclear magnetic
resonance spectroscopy (Grządka, Matusiak, & Stankevič, 2019), smallangle neutron and X-ray scattering (Penfold et al., 1997; Shtykova
et al., 2000), isothermal titration calorimetry (Skvarnavičius,
Dvareckas, Matulis, & Petrauskas, 2019) and surface tension measurements (Touhami, Rana, Neale, & Hornhof, 2001). In the case of the
adsorption and stability such techniques as UV–vis, FT-IR (Chiem,
Huynh, Ralston, & Beattie, 2006), QCM-D (Krasowska, Zawała,
Bradshaw-Hajek, Ferri, & Beattie, 2018), ellipsometry (Fujiwara., 2007)
and turbidimetry (Ostolska & Wiśniewska, 2014) are used. Another
factor influencing stability of colloidal systems is the conformation of
adsorbed macromolecules added to the system. Due to the complex
interactions between the macromolecules and the solid surface, adsorbed polymer chains can be characterized by different configurations

(Gregory & Barany, 2011). Such configuration has a direct influence on
the stabilizing potential of used polymers. When the polymer

Induced stability is a vast subject in the case of different colloidal
systems (Studart, Amstad, & Gauckler, 2007). Since such systems are
not naturally stable, it is very important to increase their stability
(Singh, Menchavez, Takai, Fuji, & Takahashi, 2005). In terms of industrial applications stability of a new product determines not only its
sensory properties but also the economic value of the final product. The
consumer demands for organic, non-toxic and environmentally friendly
products generate the urge to study new possible substances that can
permanently replace their synthetic substitutes. In the case of beauty,
cosmetic, and pharmaceutical industries such seemingly basic research
leads to the development of more sophisticated ones as well as to the
whole new field of R&D. The suspensions containing different oxides,
stabilizers, bioactive substances, and surfactants are commonly used to
care and treat human skin and health. In such complicated systems,
each component can interact with others changing the properties of the
whole system. This is of great significance to the interactions between
the polymers (stabilizers) and the surfactants. Such interactions influence not only the adsorption of the polymers on the oxide surface but
also the long-time system stability. Hence the authors infer that the
presence of the cationic surfactant influences the adsorption and

⁎

Corresponding author.
E-mail addresses: (J. Matusiak), (E. Grządka), (A. Bastrzyk).

/>Received 13 February 2020; Received in revised form 21 April 2020; Accepted 25 May 2020
Available online 03 June 2020
0144-8617/ © 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license

( />

Carbohydrate Polymers 245 (2020) 116523

J. Matusiak, et al.

pharmaceutical use.

concentration is low and the number of the active centres on the surface
is not limited, the polymer chains usually form the flat configurations
on the solid surface, where so called trains structures predominate.
Between two trains, segments called loops can be observed. Loops are
attached to the solid surface by trains, and most of their segments is
present in the bulk phase. At the end of each trains, a free chains extending towards the bulk of the solution can be observed. They are
called tails (Nylander, Samoshina, & Lindman, 2006). For small solid
particles more extended configurations were observed, whereas for
larger ones where the surface was not limited the flat trains conformation predominated (Chodanowski & Stoll, 2001). Moreover, with
the increasing polymer concentration the configuration changes from
the flat to the more extended one (Nylander et al., 2006). The conformation of the polymers on the solid surface can be also confirmed
using the zeta potential measurements. The formation of the extended
adsorption layer containing numerous loops and tails leads to the decrease of the zeta potential (Ostolska & Wiśniewska, 2015).
To confirm the Author’s hypothesis ternary system composed of
aluminium(III) oxide acting as a solid substance, fucoidan as a polymeric stabilizer,
and the cationic surfactant
hexadecyltrimethylammonium bromide (CTAB) as a surface active agent was
used. Fucoidan is a natural sulfated polysaccharide present in different
species of brown seaweeds. Since its discovery by Kylin in 1913 (Kylin,
1913), it has been extensively studied for ages. Just like in case of other
marine polysaccharides both the composition and the structure vary
according to the species, season, and harvest location of the plant.

According to Fletcher et al. the basic structure of fucoidan is composed
of sulfated fucose backbone, however, it can contain other sugars such
as galactose, uronic acid and xylose (Fletcher, Biller, Ross, & Adams,
2017). Rioux et al. observed that the molecular weight of fucoidan can
also vary significantly from low molecular weight to high polymeric
structures of 1600 kDa (Rioux, Turgeon, & Beaulieu, 2007). Fucoidan is
a highly bioactive substance with potentially positive health effects
such as anticancer, anticoagulant and antithrombotic, antivirus, antitumor and immunomodulatory activities (Zong, Cao, & Wang, 2012; Li,
Lu, Wei, & Zhao, 2008; Ale, Mikkelsen, & Meyer, 2011). What is particularly important for skin care products is that fucoidan has also the
antioxidant and anti-inflammatory properties (Li et al., 2008;
Wijesekara, Pangestuti, & Kim, 2011; Wang, Zhang, Zhang, & Li, 2008).
The non-clay materials such as mineral oxides are a group of ingredients used in cosmetic and pharmaceutical industry in such products as creams, emulsions, pastes etc. (Carretero & Pozo, 2010;
Morganti, 2010). Aluminium(III) oxide, also called alumina, is one of
such materials used in cosmetic industry. FDA (Food and Drug Administration) confirmed that alumina is a safe material for the use in
contact with soft tissues, bones, and internal organs. According to
Becker et al. alumina is used industrially in the leave-on products such
as nail polish, around the eye cosmetics and skin care products (Becker
et al., 2016).
The lack of papers concerning the use of fucoidan as a stabilizer for
different colloidal systems is observed. The novelty proposed by the
Authors is the use of natural stabilizer possessing potential health
promoting properties. This approach reduces the need for the use of
other synthetic stabilizers and increases the product applicability. The
main goal of this study was to investigate the influence of fucoidan on
stability of the alumina suspensions in the presence and absence of the
cationic surfactant CTAB. To characterize the types of stability of the
fucoidan/CTAB/alumina systems the adsorption measurements
(UV–vis, FT-IR) were conducted. Stability of the studied systems was
measured by means of the turbidimetric method using the TSI index.
Moreover, the influence of pH on the adsorption, stability and electrokinetic properties was also studied. The obtained results indicate

greatly stabilizing properties of fucoidan for the alumina suspensions in
the presence and absence of the cationic surfactant and the whole
studied pH range (3–7) which enables its use as an efficient stabilizer
for the colloidal suspensions with the possible cosmetic and

2. Materials and methods
2.1. Materials
Fucoidan (Fucus serratus) sulfated L-fucose algal polysaccharide was
obtained from Carbosynth Limited (CAS No 9072-19-9). Its chemical
formula is (C6H9O3SO3)n. The molecular weight of this compound declared by the manufacturer equals 1 705 000 Da. However, this value
was also verified using the static light scattering method (Zetasizer
NanoZS, Malvern Instruments). In this technique the scattering intensity of a number of concentrations of the sample (2000−4000 ppm)
was measured and used to construct the Debye plot created by measuring the scattered light at a single angle (173°) at multiple sample
concentrations with water as a standard. From this plot the average
molecular weight (Mw) (obtained from the intersection point between
the obtained Debye plot and the ordinate axis) and the second virial
coefficient (A2) (determined from the slope of the Debye plot) were
calculated. According to the measurements the molecular weight of
fucoidan equals 1 730 000 Da whereas the A2 coefficient 0.000321 cm3
mol g−2. Moreover, the content of sulfate groups in the fucoidan was
estimated by means of the barium sulfate precipitation method using
the barium chloride-gelatin reagent (Dodgson & Price, 1962). The exact
procedure altogether with the calibration curve has been included in
the Appendix A. The sulfate content obtained with this method equalled
5.96 %, which agrees with the literature data (Lim & Mustapha, 2017).
Hexadecyltrimethylammonium bromide is a cationic quaternary
ammonium surfactant (CAS No 57-09-0). It was obtained from Fluka
Analytical. The chemical formula of this compound is CH3(CH2)15N(Br)
(CH3)3 whereas its molecular weight equals 364.45 Da. The concentration of the surfactant in all conducted measurements was 0.002
%. Such concentration did not exceed the critical micelle concentration

(CMC) of CTAB (0.00092 mol dm−3 = 0.0337 %) (Li, Zhang, Zhang, &
Han, 2006).
Aluminum oxide (Al2O3) (CAS No 1344-28-1) was used as the adsorbent. The oxide was washed with doubly distilled water until its
conductivity was lower than 2 μS cm−1. The XRF measurements
(Epsilon 5, PANalitycal) confirmed that the adsorbent is free of impurities and the studied sample contained over 99.6 % of Al2O3. In
order to get full characteristics of the adsorbent the SEM images
(Quanta 3D FEG, FEI), the low-temperature adsorption-desorption of
nitrogen BET (ASAP 2420, Micromeritics Inc.) and the dynamic light
scattering (Zetasizer Nano ZS, Malvern Instruments) methods were
used. The structure of measured alumina is presented in Fig. 1. The
specific surface area of Al2O3 equals 171 m2/g, the adsorption average
pore width equals 15.67 nm whereas the pore volume is 0.67 cm3/g.
The average particle size of alumina calculated by intensity equals
172.4 nm.
2.2. Methods
2.2.1. Stability measurements
The turbidimetric method is one of the most precise methods to
determine stability of colloidal suspensions. The methodology of stability measurements was previously described (Matusiak, Grządka, &
Bastrzyk, 2018). 0.005 g of alumina was dispersed in 10 cm−3 of a
solution containing NaCl (0.01 mol dm−3). The suspensions were
treated by ultrasounds for 30 s. In the case of the samples containing
fucoidan (20−1000 ppm) and FD + CTAB (100 ppm + 0.002 %), the
substances were added after the ultrasound treatment was applied. pH
of the suspensions was adjusted to 7 using either the HCl or NaOH
solutions.
2.2.2. Adsorption measurements
The amount of fucoidan adsorption on the Al2O3 surface was
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4 cm–1 and the mirror velocity 2.5 kHz at room temperature using the
Bio-Rad Excalibur FT-IR 3000 MX spectrometer and the MTEC Model
300 photoacoustic cell. The cell was purged with dry helium prior to
the measurements. The spectra were normalized as MTEC carbon black
was used as a standard. The samples of fucoidan, alumina and fucoidan
adsorbed on alumina were directly placed in a 10 mm stainless steel cup
(sample thickness was lower than 6 mm). For each spectrum the interferograms of 1024 scans were averaged. Smoothing functions were
not used. All spectral measurements were performed at least three
times.
2.2.5. Electrokinetic properties
The surface charge density and the point of zero charge (pHpzc) of
alumina (0.1 g) in the presence and absence of fucoidan (200−800
ppm) and CTAB (0.002 %) was determined using the potentiometric
titration method. The background electrolyte used in this study was
0.01 mol dm−3 NaCl. The measurements were conducted in a thermostated Teflon vessel with a shaker to which an automatic burette
(Dosimat 665, Methrom) and a pH-meter were connected. The whole
process was controlled by the Titr_v3 computer program written by W.
Janusz.
The zeta potential of the alumina suspensions was calculated from
the electrophoretic mobility measurements and the Smoluchowski
equation (Zetasizer Nano ZS, Malvern Instruments). The sample of
alumina (0.01 g) was added to 100 cm3 of the background electrolyte
solution (NaCl, 0.01 mol dm−3) and treated with ultrasounds (3 min).
Next in some systems fucoidan (1−100 ppm) or fucoidan (10 ppm) and
CTAB (0.002 %) were added. The obtained data are presented as the
zeta potential curves versus pH. Moreover, pHiep (the isoelectric point)

was also estimated. All measurements were performed five times and
the average values are reported.

Fig. 1. SEM morphology of alumina; magnitude 100 000×.

calculated from the concentration difference before and after the adsorption measurements using a calibration curve. The first step of the
measurements was preparation of the polymer/electrolyte/metal oxide
suspensions in the same systems additionally enriched by CTAB. A
portion of alumina (0.05 g) was added to 10 cm3 of solution containing
fucoidan (the concentration range from 20 to 600 ppm), NaCl (0.01 mol
dm−3), doubly-distilled water and in some systems CTAB (0.002 %).
The pH of the obtained suspensions was adjusted to 7 using HCl or
NaOH, respectively (pH meter, Beckman φ 360). The suspensions were
shaken for 18 h in the water bath at the temperature 25 °C, 120 rpm
(OLS26, Grant) to achieve the adsorption-desorption equilibrium. To
determine the fucoidan adsorption on the alumina surface the spectrophotometric method was used (Albalasmeh, Berhe, & Ghezzehei,
2013). 3 cm3 of H2SO4 (98 %) was added to 1 cm3 of supernatant obtained after centrifugation of a suspension using a high-speed centrifuge
(310b Mechanika Precyzyjna). The samples absorbance was measured
at 315 nm with a UV–vis spectrophotometer (Cary 100, Varian Instruments). Doubly-distilled water was used as the reference. All measurements were done four times and the average values are reported.

3. Results and discussion
Adsorption of polysaccharides on the solid surface is a process
governed by multiple factors among which pH, type and concentration
of the used electrolyte as well as the presence of surfactants play a
major role (Grządka & Matusiak, 2017; Grządka, Matusiak, &
Paszkiewicz, 2018). One should be aware that while discussing the
polymer adsorption on the oxide surface, the state of the solid surface is
particularly important. Since the number of the adsorption centres on
the solid surface is constant, pH is the main parameter controlling the
adsorption of the ionic polysaccharides on the metal oxide. The protonation/deprotonation reactions taking place on the solid surface involve some changes in the concentration of the surface groups responsible for the interactions with the adsorbate. For aluminium(III)

oxide the isoelectric point is close to pH = 9 which agrees with the
literature data (Kosmulski, 2018). In such case alumina is positively
charged at pH lower than pHiep and negatively charged above pH = 9.
Thus, the adsorption of anionic fucoidan below pHiep is most likely
electrostatically driven (Liu, Zhang, & Laskowski, 2000). Fig. 2 represents the fucoidan adsorption on the alumina surface depending on
suspension pH.
As one can see, the fucoidan adsorption on Al2O3 is the highest at
the acidic pH (high concentration of the positively charged surface
groups), but it decreases with the increasing pH. This is a result of the
electrostatic interactions between the negatively charged polymer
groups and the positively charged solid surface groups. These interactions are the strongest at low pH values. Since the concentration of the
sulfate groups present in the polymer chain is not high, besides for the
electrostatic interactions most likely the hydrogen bonding and hydrophobic interactions take place in the adsorption mechanism (Merta,
Tammelin, & Stenius, 2004). The studies presented by Indest et al.
showed that the ionic strength influences the adsorption of fucoidan on
PET-chitosan films (Indest et al., 2009). This is in agreement with the
general opinion that the adsorption of the ionic polymers increases with

2.2.3. Surface tension measurements
To analyze the interactions between fucoidan and CTAB the surface
tension measurements of the surfactant without any additives as well as
in the presence of fucoidan were conducted (Theta Optical
Tensiometer, KSV Instruments). The pendant-drop method was used to
estimate the critical micelle concentration of pure CTAB as well as the
critical association concentrations of this cationic surfactant in the
presence of fucoidan (200 ppm). The analysis of the obtained data is
based on fitting the equations to drop profiles for the pendant drop
using the Young-Laplace equation:

1

1⎞
Δp = γ ⎛
+
R2 ⎠
⎝ R1
⎜

⎟

(1)

where: Δp (Pa) is the difference in pressures of the fluids across the
interface, γ (N/m) is the interfacial tension of the fluid pair, and R1 and
R2 (m) are the radii of curvature of the interface in the orthogonal directions.
All measurements were made at least five times and the average
values are reported.
2.2.4. FT-IR spectroscopy
FT-IR/PAS spectroscopy was used as the complementary technique
for confirmation of fucoidan adsorption on the alumina surface. The
spectra were collected in the 4000−400 cm–1 range with the resolution
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J. Matusiak, et al.

Fig. 2. The influence of pH on fucoidan adsorption (200 ppm) on the alumina
surface; 0.01 mol dm−3 NaCl.

Fig. 4. The FT-IR/PAS spectra of (a) fucoidan (FD), (b) post-adsorption and (c)
pre-adsorption alumina.

Comparing the spectra before and after adsorption, it is clear that
fucoidan adsorbs on the alumina surface. At 2950−2850 cm−1 and
1415 cm−1 the C–H stretching vibrations can be observed. Clearly
visible in the FD spectrum (Fig. 4a) and slightly covered by the eOH
vibrations (1640 cm−1) (Fig. 4b) a band attributed to the stretching of
dissociated carboxylic group COO− (1609 cm−1) can be observed. This
band indicates that uronic acid is present in the fucoidan chain which is
in agreement with the literature data (Marinval et al., 2016). At 1148
and 1077 cm−1 the bands responsible for the C–O stretching vibrations
are visible, whereas at 1746 cm-1 stretching of C]O is observed.
However, one should keep in mind that the absorption band of the
carbonyl group is one of the strongest known in IR spectroscopy
(Socrates, 2001). Thus, the band intensity is not strictly correlated with
the concentration of the C]O groups. The bands unique for fucoidan
visible at 1234–1251 and 819–850 cm–1 corresponds to the S]O
asymmetric stretching vibrations of sulfate groups and C–O–S vibrations, respectively (Chale-Dzul, Moo-Puc, Robledo, & Freile-Pelegrín,
2015; Wang et al., 2010; Shanura Fernando et al., 2017).
One of the most common methods used to study the interactions
between surfactants and polymers is the measurement of surface tension. It can be observed that the surface tension in the CTAB/FD system
increases in comparison to the pure CTAB solution (Fig. 5).
Such observation indicates the presence of interactions between the
polymer and the surfactant. There are several points on the surface
tension isotherm of surfactant solution that can be observed after the
addition of the polymer. The critical association concentration (T1,
CAC) is observed when the interactions between the surfactant and the
polymer start. In this region the CTAB molecules start to bind to the FD
chains, resulting in the formation of polymer-surfactant complexes (Bit,

Ali, Debnath, & Saha, 2010). Considering the fact that CAC is always
lower than the critical micelle concentration (CMC) (Thalberg,
Lindman, & Karlström, 1990), in the CTAB/FD system the interactions
start at a very low surfactant concentration, indicating non-cooperative

Fig. 3. The influence of the cationic surfactant CTAB on the fucoidan (FD)
adsorption on the alumina surface; pH = 7, 0.01 mol dm−3 NaCl.

the increasing salt concentration or ionic strength (Chibowski, OpalaMazur, & Patkowski, 2005). The ionic strength used in the following
measurements (IS = 0.01 NaCl) is low, so it doesn’t have a high influence on the fucoidan conformation on the alumina surface. To discuss the conformation of fucoidan on the alumina surface, different
factors must be examined. The used adsorbent is nano-alumina with the
mean particle size of 172.4 nm. The studies of Chodanowski pointed out
that in case of small solid particles, more extended configuration of the
polymer is observed (Chodanowski & Stoll, 2001). This agrees with the
presented adsorption data. It can be observed that the adsorption increases steadily with the increasing concentration, whereas the plateau
is not reached (Fig. 3). Considering the fact, that the polymer is unable
to adsorb at the inner pores of alumina, the most probable structure of
the adsorbed fucoidan layer is the conformation rich in loops and tails,
rather than flat trains.
The presence of cationic surfactant also influences the polymers
adsorption on the solid surface. The interactions between those molecules can lead to the formation of polymer-surfactant complexes. In
such case the polymer adsorption in the presence of the surfactant can
either increase or decrease. There are a few explanations for this phenomenon. If the surfactant interacts with the polymer resulting in the
formation of complexes, and such complexes are more likely to remain
in the bulk of the solution, the lower adsorption is observed (Fig. 3).
In this case some of the fucoidan macromolecules interact with the
CTAB molecules resulting in the formation of non-adsorbing complexes.
The adsorption decrease is due to the fact that fewer fucoidan chains
are adsorbed on the alumina surface and thus the adsorption is smaller.
On the contrary if the surfactant is not present in the system, the free

polymer chains can only interact with the surface which results in a
higher adsorption.
FT-IR/PAS spectroscopy was used as a complementary method to
confirm the fucoidan (FD) adsorption on the alumina surface. Fig. 4
shows the spectra of pure FD, post-adsorption, and pre-adsorption
alumina.

Fig. 5. The changes of CTAB surface tension in the presence of fucoidan (FD,
200 ppm); T = 25 °C.
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binding of surfactant molecules to the polymer chain (Khan &
Brettmann, 2015). When the polymer chains become saturated with the
surfactant molecules, the critical saturation concentration point (T2)
can be observed. However, sometimes this point is hard to quantify
experimentally. A further increase of the surfactant concentration leads
to the situation where the polymer chain is saturated and it is more
thermodynamically favourable for the surfactant to form micelles
(Goddard, 1986). It is known that the polymer and the surfactant will
interact in solutions, mostly by hydrophilic, hydrophobic and electrostatic interactions. The association between the surfactant head groups
and the polyelectrolytes is driven by the entropic gains whereas enthalpic contributions should be also mentioned (Bai, Santos, Nichifor,
Lopes, & Bastos, 2004). In the case of the CTAB/FD system the electrostatic interactions are most likely responsible for the initial contact
because of the strength of the electrostatic forces whereas the hydrophobic effect occurs in the micelle formation stage (Khan & Brettmann,
2015). The end of the interactions between CTAB and FD can be observed in the final stage of the surface tension isotherm.
A system is considered stable when the sum of the repulsion forces is

equal to the attractive ones. Keeping that in mind there are different
factors influencing stability of the colloidal systems, among which pH
and the presence of other molecules can be mentioned. Fig. 6a shows
the influence of the fucoidan concentration on stability of the alumina
suspensions.
The TSI values change from 0 to 100 being the lowest for the most
stable and the highest for the completely unstable systems. As one can
see, the alumina suspension itself is rather unstable (change of TSI from
9.64 in the first hour to 68.75 in the fifteenth hour). The reason for this
is the low efficiency of the electrostatic mechanism responsible for
stabilization of “hard” colloidal particles. When two particles contact,
their electrical double layers overlap which results in the repulsion of
solid particles. However, the efficiency of such mechanism decreases in
time when particles aggregate and destabilization occurs (Tadsors,
2012). The addition of the adsorbing anionic polymer clearly influences
the system stability. It can be observed that stability increases greatly
with the concentration. Such situation occurs when the oppositely
charged polymer chains adsorb on the solid surface, creating a compact
layer of macromolecules. This process is called electrosteric stabilization for ionic polymers and steric stabilization for non-ionic ones. At pH
= 7 the alumina surface is still positively charged whereas the fucoidan
chains possess the anionic charge in terms of sulfate (–O–SO3−) and
carboxyl dissociated groups (–C–O–O−). This leads to the formation of
the fucoidan layer, increasing the output of the repulsive forces in the
system. The largest system stability is observed for the concentration of
fucoidan equal 1000 ppm. In this case TSI increases from 2.89 in the
first hour to 26.43 in the fifteenth hour. For the alumina suspensions
without the polymer the difference in TSI from the first to the last
measurements is 59.11 whereas for the alumina/fucoidan 1000 ppm
system it is only 23.54. As pH has a great effect on the protonation/
deprotonation reactions on the solid surface, its influence the fucoidan/

alumina system stability was also studied (Fig. 6b). As follows from the
measurements pH has an insignificant effect on the fucoidan/alumina
system stability. This is consistent with the adsorption data that the
highest stability is observed at pH = 3 where the alumina surface
possesses the largest positive charge and fucoidan adsorption is the
highest. Stability decreases slightly with the increase of pH due to the
formation of less packed polymer adsorption layer.
The final factor influencing the fucoidan/alumina system stability is
the presence of the cationic surfactant CTAB (Fig. 6c). The presence of
CTAB increases the stabilization efficiency of fucoidan for the alumina
suspension. As indicated by the adsorption and surface tension data
(Figs. 3 and 5) fucoidan forms complexes with CTAB characterized by
lower affinity for the alumina surface than pure fucoidan. Even though
the adsorption layer in the presence of CTAB is less packed, the stability
increase is still observed. This is because the formed FD-CTAB complexes stabilize the alumina particles by orientating between them,

Fig. 6. The influence of the fucoidan concentration (a), pH (b), and CTAB (c) on
stability of the alumina suspensions. The used conditions: pH = 7, 0.01 mol
dm−3 NaCl (a); 100 ppm FD, 0.01 mol dm−3 NaCl (b); pH = 7, 0.002 % CTAB,
0.01 mol dm−3 NaCl (c).

which results in the reduced attraction.
Fig. 7a presents the influence of fucoidan concentration and Fig. 7b
the presence of CTAB on the zeta potential of the alumina suspension in
the presence of 0.01 mol dm−3 NaCl as a background electrolyte. As
can be seen the alumina surface is positively charged up to pH = 9 and
starts to be negative above this value. This indicates that the isoelectric
point (pHiep) of this oxide is located near pH = 9, being consistent with
the literature data (Kosmulski, 2001). However, the presence of negatively charged fucoidan causes the decrease of the zeta potential as well
as the shift of the pHiep towards lower pH values. There are a few

reasons for this effect. One is an anionic character of this polysaccharide. Negatively charged groups from the fucoidan macromolecules present in the diffused part of the electric double layer cause
the observed decrease. Taking into account the fact that the electrokinetic mobility recalculated to the zeta potential is measured on the
slipping plane, the shift of this plane towards the bulk solution caused
by the fucoidan adsorption is also responsible for the zeta potential
decrease. Another observation from Fig. 7a is that the higher concentrations of fucoidan are, the larger decrease is observed. This is a
consequence of higher fucoidan adsorption on the alumina surface occurring at higher concentration of this polysaccharide as well as a larger
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Fig. 7. The Influence of fucoidan (FD) concentration (a) and the presence of CTAB (0.002 %) (b) on the zeta potential of alumina; 0.01 mol dm−3 NaCl.

Fig. 8. The Influence of fucoidan (FD) concentration (a) and the presence of CTAB (0.002 %) (b) on the surface charge of alumina; 0.01 mol dm−3 NaCl.

background electrolyte. The first observation from the obtained data is
that pHpzc for pure alumina is situated near pH = 8.5 which indicates
that below this value the number of positively charged surface groups is
the largest, whereas above this point the concentration of negatively
charged groups is dominant. This phenomenon changes a little bit after
the addition of anionic fucoidan. When the adsorption of polyanion
occurs, the presence of negatively charged groups originating from the
polysaccharide dissociation are present in the compact part of the
electrical double layer. Their presence causes the decrease in the surface charge density. The higher the fucoidan concentration, the lower
the values of the surface charge density are. Another very important
conclusion from Fig. 8b is that the presence of cationic CTAB has an
insignificant effect on the value of the surface charge density of the
alumina/fucoidan system. As follows the CTAB molecules do not occur

in large numbers in the compact part of the electric double layer but
they are present in the upper, diffused layer, being consistent with the
previous findings (Matusiak, Grządka, Paszkiewicz, & Patkowski,
2019).

number of negatively charged groups present in the diffused part of the
electrical double layer. Moreover, the decrease of the zeta potential in
the system containing the polymer in comparison to the system without
it can be also attributed to the conformation of fucoidan on the alumina
surface. The adsorption measurements indicate that the adsorbed fucoidan chains are extending towards the bulk solution, rather than
forming the flat adsorption layer. With the increasing concentration the
adsorption increases, whereas the zeta potential decreases (Fig. 7a).
The shift of the slipping plane towards the bulk solution can be caused
by the conformation of adsorbed fucoidan. For flat conformation lower
decrease of zeta potential would be observed. The zeta potential decreases greatly when the fucoidan concentration increases slightly, only
from 1 ppm to 10 ppm. This can be attributed to the increasing number
of the negatively charged fucoidan groups in the diffused part of the
electrical double layer. However, keeping in mind the adsorption
measurements it can be assumed that the formed adsorption layer is
rich in loops and tails structures, which results in the shift of the slipping
plane and decrease of the zeta potential. Fig. 7b presents the influence
of CTAB on the zeta potential of the 10 ppm fucoidan/alumina system.
This is clearly visible that the addition of the cationic surfactant causes
the increase of the zeta potential values compared to the system
without CTAB. As follows the present cationic groups of the surface
active agent have so large influence on the zeta potential that despite an
obvious shift of the slipping plane accompanying the fucoidan adsorption on alumina in the presence of CTAB, the zeta potential increase
takes place.
The electrokinetic properties of the compact part of the electric
double layer can be analysed by means of the surface charge density

measurements. Fig. 8a presents the effect of fucoidan concentration and
Fig. 8b the presence of CTAB on the surface charge density of the
alumina suspension in the presence of 0.01 mol dm-3 NaCl as a

4. Conclusions
The presented study confirms high stabilizing properties of fucoidan
for the alumina suspensions in all studied pH range. According to the
authors’ best knowledge the lack of such studies on the use of fucoidan
as a stabilizer of colloidal suspensions is observed. Thus, the obtained
results are innovative and very promising in the field of colloidal stability and the use of natural polysaccharides in this field. The adsorption and zeta potential studies allowed to establish that the adsorbed
fucoidan layer is probably composed of extended structures, such as
loops and tails. The surface tension measurements confirmed the electrostatic complex formation between fucoidan and CTAB surfactant.
6


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J. Matusiak, et al.

Moreover, it was found out that in the presence of CTAB stability of the
fucoidan/alumina system slightly increased due to the formation of
non-adsorbed polymer-surfactant complexes orienting between the
solid particles, which resulted in lower attraction and slower aggregation. The results throw new light on the use of fucoidan as a natural
stabilizer of the colloidal suspensions, particularly in the cosmetic and
pharmaceutical industries.

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CRediT authorship contribution statement
Jakub Matusiak: Funding acquisition, Project administration,
Conceptualization, Resources, Methodology, Investigation, Data curation, Visualization, Formal analysis, Writing - original draft, Writing review & editing. Elżbieta Grządka: Supervision, Conceptualization,
Methodology, Investigation, Writing - review & editing. Anna
Bastrzyk: Investigation, Resources.
Acknowledgements
The Authors would like to acknowledge that this research was financially funded by the National Science Centre, Poland [grant number
2017/27/N/ST4/02259]. Moreover, the work was supported by subsidy from the Polish Ministry of Science and Higher Education for the
Faculty of Chemistry of Wroclaw University of Science and Technology.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
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