140 JOURNAL OF COSMETIC SCIENCE
PHOTOSTABILITY STUDY
The selection of ACN as the solvent for the positive control was based both on its use as
a diluent and mobile phase component of the HPLC method, as well as on other AVB
photostability studies described in the literature.31 It was demonstrated that AVB is
predominantly found in the enol form, and after exposure to radiation it presents photo-
instability converting into the keto form. In addition to exposure time and concentration,
photo-instability was also dependent on the type of solvent. In methanol, chloroform, and
acetone, there was no significant photoisomerization of AVB under the conditions tested.32,33
In agreement with these findings, our photostability study showed significant differences
between exposed and unexposed samples and between samples of NLC containing AVB
when compared to samples of AVB in ACN (positive control). Formulations containing
free AVB after UVA irradiation demonstrated a 50–70% decrease in the filter content,
while encapsulated AVB showed a maximum 5% decrease. Figure 7 shows the evaluation
AVB-NLC and AVB in ACN exposed to UVA radiation for a period of 24 hours. Under
the experimental conditions, the samples showed significant differences (p 0.05) when
compared.
In Figure 7, it is possible to observe that the AVB in ACN showed a decrease in its content,
reaching 50% in 24 hours. On the other hand, in both the NLCs containing AVB exposed
or not to UV radiation, the AVB content was maintained during the period of 24 hours,
which suggests that the AVB presented a photostable behavior when encapsulated in NLC.
As observed in previous works,32 the presence of cinnamic-acid derivate filters can increase
the formulation photostability. Since some filters affect the stability of others, the resulting
stability may chance in different filter combinations, and this could be the case of carnauba
wax, which presents in its composition p-methoxycinnamic diesters.32,34 Other works
have shown that when avobenzone sunscreens were nanoencapsulated in nanocapsules or
nanoemulsion, photodegradation was lower than when compared to traditional emulsions.35
Figure 6. Average diameter, PdI and zeta potential of AVB-NLC (F3) evaluated during 30d at room
temperature. Values represent mean ± S.D (n =3).

141 ImprovedPROVED AVB PhotostabilityOTOSTABILITY UsingING NLCs
In addition to photostability, the increase in the photoprotective properties of creams based
on lipid nanoparticles versus conventional creams has already been described.36,37 Liposomes
and lipid nanoparticles have increased AVB photoprotection by 40%.38 The increased UV
protection is attributed to the synergistic effect of the lipid matrix. Isopropyl myristate has
been shown to be the most appropriate lipid-excipient for ensuring AVB photostability.30
Similarly, the entrapment of avobenzone in lipid microparticles have been demonstrated as
an effective strategy to reduce the photo-instability of AVO after irradiation.39 Normally
nanoparticles act as UV physical filters, due to their efficient light scattering properties,
which promotes a synergistic UV protection effect with the encapsulated UV chemical
filter. Additionally, depending on their composition, lipid components may present
antioxidant activity and add to the photostabilisation effect of the encapsulated material.39
Understanding photodegradation pathways when using photosensitive filters is a crucial
step in the design of photoprotective formulations. The combination of several strategies,
including encapsulation, antioxidants, photostabilizing substances, and suppressors can be
used as effective tools for photoprotection.39-41
SKIN RETENTION AND PERMEATION
Recently, the Food and Drug Administration has evidenced that various UV filters
can diffuse into systemic circulation and cause harm.42 The lipophilic character and
permeability properties of AVB, such as its lipophilicity, could favor its diffusion through
biological membranes. AVB pKa is between 9.60–9.80. At both physiological pH values
and cutaneous pH, there will be a clear dominance of AVB in the nonionized state, which
is predictive of high permeation across biological barriers.43,44
The effectiveness of a sunscreen depends on its ability to keep UV filter molecules in the
outermost region of the skin. The ideal sunscreen should have low permeation properties
and high accumulation capacity in this region.39 Here, we demonstrated that encapsulation
in NLCs promoted greater skin retention of AVB, showing a significantly (p 0.05) higher
retention on the skin surface when compared to the concentration that penetrated the
stratum corneum (Figure 8).
Figure 7. Photostability of free and NLC-encapsulated AVO (F3). Samples were exposed for 24h and subjected
to UVA radiation (3,025 mW/cm²) for 24h in a photostability chamber.