No. Form W-8 certifies your tax status for US withholding taxes only.  When you complete Form W-8 you are required to certify your country of tax residence and to establish whether you qualify for a reduced rate of withholding on your account.  The US does not tax short-term or long-term capital gains for nonresidents, therefore there is no withholding on capital gains.  

IB withholds on dividends including qualified interest income (known as interest-related dividends) and short-term capital gain distributions from mutual funds.  The withholding rate may be reduced by an applicable tax treaty.   Stamp duty tax will be collected for all London Stock Exchange shares purchased in real-time.

Circular 230 Notice: These statements are provided for information purposes only, are not intended to constitute tax advice which may be relied upon to avoid penalties under any federal, state, local or other tax statutes or regulations, and do not resolve any tax issues in your favor.