UV can, of course, pass energy onto other materials, increasing their energy and creating a number of possible responses. Of these responses, the ones most relevant to my investigation are…
Excitement - When electrons become excited from absorbing a certain frequency of UV
Photodisassociation - where an electron absorbs so much light that the atom is belongs to no longer functions properly, causing molecules to break apart
Ionisation - when energy absorbed from a very high-energy photon causes an atom to completely lose an electron
If a molecule achieves photodisassociation (or ionisation) then the light energies being absorbed correspond to UV light. It would also be a higher energy UV than that required for excitement. Using an equation made by Max Plank, it's possible to find the maximum energy a molecule may absorb when exposed to UV.
E = h v
E = energy, h = Planks constant, 6.63 x 10 J Hz
Since V is frequency and we're dealing with wavelength values, I also substituted the equation C (speed of light) = λV to get E = h (C/λ)
Considering UV is between 200-400nm its energies vary from…
Upper = 6.63 x 10 x (3 x 10/200 x 10) = 9.94 x 10
Per Mole = 9.94 x 10 x 6.02 x 10 = 598 KJ/mol
Lower = 6.63 x 10 x (3 x 10/400 x 10) = 4.97 x 10
Per Mole = 4.97 x 10 x 6.02 x 10 = 299 KJ/mol
Because I'm more interested in the damaging UV, I will take the upper limit of energy and say that bonds with enthalpies of ≈ 600 KJ/mol will be best at absorbing UV. Some bonds that are found within this region are
C=C bonds - 612 KJ/mol
F=H Bonds - 568 KJ/mol
Note: in reality, the highest probable UV energy is for 280nm
= 6.63 x 10 x (3 x 10/280 x 10) = 7.10 x 10
Per Mole = 7.10 x 10 x 6.02 x 10 = 427 KJ/mol
Because UVc doesn't reach earth's surface!
The fact that C=C makes an entrance suggests that conjugated systems will absorb UV well, this is reflected in natures own 'light grabber' chlorophyll (3)…
Above is chlorophyll a, notice the extended delocalisation due to C=C bonds.
Active Ingredients In Sunscreens
From looking at the active ingredients in sunscreens, I found that a lot contained a benzene ring (sometimes two). This is significant because of the delocalisation of electrons that occurs on benzene (shown below).
Active ingredients included 2-ethylhexyl-p methoxycinnamate, octocrylene and octyl triazone (all containing benzene rings) and octyl methoxycinnamate, which has extended delocalisation on the COOH group. I also noticed some UV reflectors such as Titanium Dioxide present, especially in the higher protection sunscreens. These heavy metal oxides reflect light because of a lack of outer shell electrons, meaning absorption occurs very little (if at all).
Risk Assessment
●Ultraviolet exposure: I will be using a UV lamp, emitting both UVa and UVb rays
● UVa:
Can Cause skin cancer
Can Cause photo-ageing
Can Cause skin Erythema
● UVb:
Can Cause skin cancer
Can lead to development of cataracts
Can cause skin erythema
● Sunscreens:
May contain nut derivatives that can cause adverse reactions/allergies
● Liquid Paraffin 50% in White Soft Paraffin
Flammable
Mild irritant
Mildly toxic
● Aqueous cream
Mild toxicity if ingested
On rare occasions causes skin irritation
● Safety Precautions
Wear UV protective glasses
Avoid prolonged direct exposure to UV
If people coming into contact with sunscreens have any nut allergies then take extra care with handling etc
Avoid naked flames when working with paraffin
If anything is accidentally swallowed then seek medical attention
Method
Apparatus…
● Sunscreens, factor 4, 8 and 12
● Aqueous Cream
● Paraffin mix (½ liquid paraffin, ½ white soft paraffin)
● Microscope Slides (all 1.07mm thick)
● Cover Slips
● Retort Stands
● UV Lamp
● UV Detector
● Stop Clock
● UV Protective Sunglasses
● Metre Rule
● Micrometer
Method:
Measurements will be made regarding UV intensity in Wm.
The actual experiment will take place in a dark room (help avoid interference from background UV), with the UV source clamped vertically in a retort stand and aligned with the UV detector (clamped in another stand). In between the source and detector will be another stand, holding the slide, test material and coverslip so that the detector is as close as possible to the slide and is directly behind the 'test area' (bit covered in sunscreen). All slides will be checked with a micrometer to see if they're the same thickness, then approx 0.05g of substance is spread over a 1cm area, the coverslip applied and then the thickness will be checked again with a micrometer to insure that the test is for the same 'dosage' of sunscreen over a given area. Each experiment will be timed for 300 seconds, with regular readings being taken and recorded. Aside from varying the substance being tested, I will also use different distances between the UV source and slide to get a more comprehensive set of results. Other instruments such as a UV spectrometer and a more powerful pinpoint UV source would have been used if they were available and would result in a much more accurate set of results. The distances from the detector I will use are 5 - 30cm at 5cm intervals, again, this is limited because of the space available and because my UV source isn't very strong, more would be better (though maybe not necessary). Below is a diagram of my test setup
Results