Trends Biomater. Artif. Organs. Vol. 16 (1) pp 46-51 (2002)                             


Jai Paul Singhal and Alok R. Ray

Centre for Biomedical Engineering
Indian Institute of Technology
New Delhi - 110 016
All India Institute of Medical Sciences
New Delhi - 110 029

Antibacterial activity has been imparted to nylon-6 by adsorption of iodine. Iodine adsorption has been carried out by
two ways: a) dipping nylon-6 fibers into solution of iodine in acetone and b) exposing nylon-6 fibers to iodine vapours.
The amount of iodine adsorbed has been found to be very high as compared to theoretical amount of iodine required to
form single layer on the surface of fibers. Iodine adsorption from solution in acetone does not follow Langmuir
adsorption, but adsorption in vapour phase appears to follow pseudo-Langmuir adsorption isotherm. In case of
adsorption of iodine from vapour phase, equilibrium is established between nylon-iodine complex and iodine vapours.
However, in case of adsorption from solution, no such equilibrium appears to be established. This may be due to
change in nature and ratio of different iodine species in the nylon-iodine complex. Possible mechanism of iodine
adsorption has been discussed.

INTRODUCTION                                                  additional iodine to give triiodide or pentaiodide
          Iodine is well known for its antibacterial
properties [1] and is used to impart                                  The present study addresses the
antibacterial activity to polymers [2-4].                     adsorption of iodine on nylon both from solution
Polymer-iodine complexes (i.e. iodophors) are                 and vapour phase and role of solvent on the
widely used as antiseptic products during                     adsorption behaviour of iodine.
surgical interventions [5-7]. The deficiency of               EXPERIMENTAL
iodine in human beings causes goiter, a                       Materials
disease prevalent in hills of India. A controlled                       Un-pigmented multifilament braided
release formulation of iodine can be used for                 nylon-6 (nylon-6) fibers (denier 44) were
eradication of the iodine deficiency [8,9]. Iodine            obtained from Modipon Limited, India.
has also been used to remove biological                       Analytical grade acetone (BDH, India) and
contaminations from water [10]. Further,                      Iodine (E. Merck, India) were used without
controlled release of iodine in water is likely to            purification.
improve its effectiveness as water disinfectant.
The antibacterial activity of iodophors is exerted            Methods
by free iodine released from the complex that                 Iodine doping using solution
subsequently binds to bacteria. So the                                Nylon fibers were doped with iodine by
effectiveness of iodophor depends upon the                    immersing in acetone solution of iodine for 48
availability of free iodine that is directly related          hours     at    room      temperature.   Iodine
to the nature of iodine interaction with the                  concentration in the solution was varied from
polymer. In iodophors, iodine may exist in                    0.6 to 3.7 g/dl in the present study. The fibers
various forms like 2, I-, I3-, I5-[11-13]. Nylon-6
                       I                                      were removed from iodine solution, washed
also forms iodophor by adsorbing iodine either                twice with pure acetone, dried and weighed.
from its solution or vapours phase. Nylon-6 can               Iodine content of the fibers was estimated from
adsorb Iodine either in molecular form or ionic               the resulting weight increase of the fibers as
form. The iodide ions can form complex with                   shown below.
                                Increase in weight of Nylon fibers
 Percent iodine uptake =                                                   X 100          - (1)
                                Original weight of Nylon fibers
Iodine doping from vapours                              adsorption of Iodine from the aqueous solution
                                                        in KI [14].
          Nylon-6 fibers were iodized by
                                                                  All the repeat units of nylon can form
exposure to iodine vapours for period of eight
                                                        adduct with iodine. However, adduct formation
hours. The experiment was carried out in a
                                                        is restricted by geometry of nylon material. It
tube with constriction towards lower side.
                                                        had been observed that iodine moles adsorbed
Iodine was taken in the lower portion and nylon
                                                        per mole of repeat units remains for less than
fibers in the upper portion. Iodine was
                                                        unity (fig. 3). Although adsorption does not
vapourized by heating in water bath. System
                                                        follow Langmuir equation, but iodine may be
was cooled to room temperature and fibers
                                                        adsorbed as molecular I or other forms such
were washed with acetone to remove the                       -
                                                        as I, I3- etc. If iodine has been adsorbed as
loosely held iodine from the surface of fibers.
                                                        molecular iodine, following equilibrium will be
Fibers were dried and weighed. Iodine uptake
was calculated from the increase in the weight
of fibers using equation (1).
                                                            Nylon-6 + I2                             Nylon –
RESULTS AND DISCUSSION                                  Iodine adduct
Adsorption of Iodine from solution                           (N6)
          It was observed from fig. 1 that nylon-6                          [NI]
adsorbs iodine from its solution and the amount           KNI =                           - (3)
                                                                            [N6] [I2]
of iodine adsorbed increased with the increase
in concentration of iodine in the solution.             Where KNI is the equilibrium constant, [N6] and
However, increase in iodine adsorbed was not            [NI] are the absorption sites and complex
linear initially. Sukawa et al [14] have reported       formed per mole of nylon repeat unit at
linear increase in adsorption of iodine on nylon-       equilibrium, and [I2] is the concentration of
6 from aqueous solution of iodine containing            iodine in the solution. If ‘a’ and ‘r’ are the initial
potassium iodide. This difference of adsorption         number of adsorption sites and complex formed
may be due to different solvent. The adsorption         per mole of nylon repeat unit, after substitution
behaviour of solute is generally explained by           equation (3) can be rewritten as
the empirical Freundlich adsorption isotherm,                               r
                                                          KNI =                           - (4)
x / m = k . cn           - (2)                                              (a-r) [I2]

where x/m is the amount of solute adsorbed by                  1         1                 1          1
a given weight of adsorbent (micromoles/gm), c                      =               x             +       - (5)
                                                               r         KNI . a           [I2]       a
is the molar concentration of solution, and k
and n are the constants for the given adsorbent
                                                         Since KNI and a are constant for a given
and solute. The values of k and n are
                                                         system, so plot of 1/r vs 1/[I2] should be linear.
determined by plotting log (x/m) vs log c. From
                                                         It was found that in the present system of
fig. 2 values of k and n are found to be 60400
                                                         iodine adsorption on nylon from iodine solution
and 1.5625 respectively. According to Langmuir
                                                         in acetone, plot of 1/r against 1/[I2] is not linear
theory of unimolecular adsorption, value of n
                                                         initially (fig. 4). Similar observation has also
should be between zero and one. Since the
                                                         been made for adsorption of iodine from
value of n is found to be more than one, it
                                                         aqueous solution [14]. This deviation has been
seems that iodine adsorption on nylons does
                                                         attributed to either a large value of `a' with small
not follow unimolecular adsorption. This
                                                         value of KNI. This deviation may be due to
observation is similar to that made for
                                                         change in value of 'a' and 'K NI'. The change in
                                                         number of adsorptions sites, 'a' and 'K NI' may
be caused by change in structure of the iodine
nylon adduct. Iodine is known to form complex
with   various    polymers     through    lone

                                                        Adsorption of Iodine from vapours
pair of electrons [15]. It appears that iodine                   Fig. 6 shows that nylon also adsorbs
adsorption on nylon is not a simple physical            iodine in vapour phase. It has been observed
adsorption but a chemisorption. Several steps           that the weight of iodine adsorbed on nylon
might be involved in the iodine adsorption on           fibers from vapour phase also does not
nylons (fig. 5). In the first step iodine might         increase linearly (fig. 6). Freundlich empirical
interact with lone pair of electron on oxygen of        equation for adsorption of gases on a solid
amide group. Yamamoto et al. [16,17] has                surface is given as
reported that nylon-iodine adduct may undergo
a transition to form iodide ion. Subsequently,          x/m = k pn   - (6)
iodide ion may take up more iodine molecules
to form polyiodide (I3-, I5-) ions [11-13]. This will   where p is the pressure of gas adsorbed.
increase the number of adsorptions sites and            Pressure of iodine vapours is directly
shift the equilibrium. Hence, both 'a' and 'K NI'       proportional to the amount of iodine vapourized.
will                 be                    affected.    So, the equation may be written as

                                                        x/m = k' Mn - (7)

                                                        where M is the number of iodine moles
                                                        vaporized and k' is the new constant. The
                                                        values of k' and n for iodine adsorption from
                                                        vapour phase has been found to be 40000 and
                                                        0.59 respectively using plot of log (x/m) vs log
                                                        M (fig. 7). Unlike adsorption from solution, the
                                                        value of n has been found to be less than unity.
                                                        It appears that iodine adsorption follows
                                                        Langmuir unimolecular adsorption theory.
                                                        However, theoretical calculations shows that
                                   Adsorption of lodine on Nylon-6                                49

the quantity of iodine required to form a single

                                                      oncluded that iodine absorption follows pseudo-
                                                      Langmuir adsorption isotherm. It has also been
                                                      observed that the number of iodine moles per
                                                      mole of repeat unit remains for less than unity
                                                      (fig. 8). Attempt has been to find equilibrium
layer on the surface of nylon fibers is much          constant ‘KNI‘ from equation (5) by taking
less than that actually adsorbed. So it may be        number of moles of iodine [M] vaporized in
                                                      place      of    iodine   concentration     [I2].
50                                                 Jai Paul Singhal et. al.


From fig. 9, the value for equilibrium constant,                           Solvent plays an important role not
KNI has been found to be 9.69 mol-1.                             only in determining the extent of iodine
                                                                 adsorption on nylon, but also on adsorption
         These observations indicate that the                    behaviour and iodine-nylon interactions. Solvent
iodine adsorption from vapours is different from                 facilitates the change in nature of iodine
that observed in case of adsorption from                         species adsorbed on nylon.
solution. Solvent appears to play an important
role by facilitating change in nature of iodine                  Acknowledgments
adsorbed        and     affecting   iodine-nylon
interactions. In vapour phase, Iodine is                                This work has been supported by
adsorbed on nylon mainly in molecular form. In                   Board of Research in Nuclear Sciences,
the presence of solvent, iodine changes to ionic                 Department of Atomic Energy, Government of
form, I-, that may form complex with more                        India.
iodine molecules to form polyiodide ions like I3-,
I5- etc.


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