Sharepoint Power Shell Create a Site Collection in a Specific Data Base
By default, Site collections are created on the web application database. If you want to create a Site Collection in a diferent database, you can use the following Script:
#Create the Database
New-SPContentDatabase -Name New_Content_DB -WebApplication http://myWebApplication
#Create the Site Collection
New-SPSite http://myWebApplication/newSite -OwnerAlias <owner account name including domain name e.g.domain\username> -ContentDatabase New_Content_DB -Name <Name of your site collection> -Description <Description of site collection> -Template “STS#0”
You should also configure your WebApplication Paths to be able to create your site collection in a specific url.